EDH: endothelium-dependent hyperpolarization and microvascular signalling

Endothelium-dependent hyperpolarizing factor (EDHF) is a powerful vasodilator influence in small resistance arteries and thus an important modulator of blood pressure and flow. As the name suggests, EDHF was thought to describe a diffusible factor stimulating smooth muscle hyperpolarization (and thus vasodilatation). However, this idea has evolved with the recognition that a factor can operate alongside the spread of hyperpolarizing current from the endothelium to the vascular smooth muscle (VSM). As such, the pathway is now termed endothelium-dependent hyperpolarization (EDH). EDH is activated by an increase in endothelial [Ca²⁺ ]_i , which stimulates two Ca²⁺ -sensitive K channels, SK_Ca and IK_Ca . This was discovered because apamin and charybdotoxin applied in combination blocked EDHF responses, but iberiotoxin - a blocker of BK_Ca - was not able to substitute for charybdotoxin. SK_Ca and IK_Ca channels are arranged in endothelial microdomains, particularly within projections towards the adjacent smooth muscle, which are rich in IK_Ca channels and close to interendothelial gap junctions where SK_Ca channels, are prevalent. K_Ca activation hyperpolarizes endothelial cells, and K⁺ efflux through them can act as a diffusible 'EDHF' by stimulating VSM Na⁺ ,K⁺ -ATPase and inwardly rectifying K channels (K_IR ). In parallel, hyperpolarizing current spreads from the endothelium to the smooth muscle through myoendothelial gap junctions located on endothelial projections. The resulting radial EDH is complemented by the spread of 'conducted' hyperpolarization along the endothelium of arteries and arterioles to affect conducted vasodilatation (CVD). Retrograde CVD effectively integrates blood flow within the microcirculation, but how the underlying hyperpolarization is sustained is unclear.

TRPM4 inhibitor 9-phenanthrol activates endothelial cell intermediate conductance calcium-activated potassium channels in rat isolated mesenteric artery

BACKGROUND AND PURPOSE

Smooth muscle transient receptor potential melastatin 4 (TRPM4) channels play a fundamental role in the development of the myogenic arterial constriction that is necessary for blood flow autoregulation. As TRPM4 channels are present throughout the vasculature, we investigated their potential role in non-myogenic resistance arteries using the TRPM4 inhibitor 9-phenanthrol.

EXPERIMENTAL APPROACH

Pressure and wire myography were used to assess the reactivity of rat arteries, the latter in combination with measurements of smooth muscle membrane potential. Immunohistochemistry (IHC) and endothelial cell (EC) calcium changes were assessed in pressurized vessels and patch clamp measurements made in isolated ECs.

KEY RESULTS

The TRPM4 inhibitor 9-phenanthrol reversibly hyperpolarized mesenteric arteries to circa EK and blocked α1 -adrenoceptor-mediated vasoconstriction. Hyperpolarization was abolished and vasoconstriction re-established by damaging the endothelium. In mesenteric and cerebral artery smooth muscle, 9-phenanthrol hyperpolarization was effectively blocked by the KCa 3.1 inhibitor TRAM-34. 9-Phenanthrol did not increase mesenteric EC [Ca(2+)]i , and Na(+) substitution with N-methyl-D-glucamine only increased the muscle resting potential by 10 mV. Immunolabelling for TRPM4 was restricted to the endothelium and perivascular tissue.

CONCLUSIONS AND IMPLICATIONS

These data reveal a previously unrecognized action of the TRPM4 inhibitor 9-phenanthrol - the ability to act as an activator of EC KCa 3.1 channels. They do not indicate a functionally important role for TRPM4 channels in the reactivity of non-myogenic mesenteric arteries.

β₁-Adrenoceptor stimulation suppresses endothelial IK(Ca)-channel hyperpolarization and associated dilatation in resistance arteries

BACKGROUND AND PURPOSE

In small arteries, small conductance Ca²⁺-activated K⁺ channels (SK(Ca)) and intermediate conductance Ca²⁺-activated K⁺ channels (IK(Ca)) restricted to the vascular endothelium generate hyperpolarization that underpins the NO- and PGI₂-independent, endothelium-derived hyperpolarizing factor response that is the predominate endothelial mechanism for vasodilatation. As neuronal IK(Ca) channels can be negatively regulated by PKA, we investigated whether β-adrenoceptor stimulation, which signals through cAMP/PKA, might influence endothelial cell hyperpolarization and as a result modify the associated vasodilatation.

EXPERIMENTAL APPROACH

Rat isolated small mesenteric arteries were pressurized to measure vasodilatation and endothelial cell [Ca²⁺]i , mounted in a wire myograph to measure smooth muscle membrane potential or dispersed into endothelial cell sheets for membrane potential recording.

KEY RESULTS

Intraluminal perfusion of β-adrenoceptor agonists inhibited endothelium-dependent dilatation to ACh (1 nM-10 μM) without modifying the associated changes in endothelial cell [Ca²⁺]i . The inhibitory effect of β-adrenoceptor agonists was mimicked by direct activation of adenylyl cyclase with forskolin, blocked by the β-adrenoceptor antagonists propranolol (non-selective), atenolol (β₁) or the PKA inhibitor KT-5720, but remained unaffected by ICI 118 551 (β₂) or glibenclamide (ATP-sensitive K⁺ channels channel blocker). Endothelium-dependent hyperpolarization to ACh was also inhibited by β-adrenoceptor stimulation in both intact arteries and in endothelial cells sheets. Blocking IK(Ca) {with 1 μM 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34)}, but not SK(Ca) (50 nM apamin) channels prevented β-adrenoceptor agonists from suppressing either hyperpolarization or vasodilatation to ACh.

CONCLUSIONS AND IMPLICATIONS

In resistance arteries, endothelial cell β₁-adrenoceptors link to inhibit endothelium-dependent hyperpolarization and the resulting vasodilatation to ACh. This effect appears to reflect inhibition of endothelial IK(Ca) channels and may be one consequence of raised circulating catecholamines.

Vascular hyperpolarization to β-adrenoceptor agonists evokes spreading dilatation in rat isolated mesenteric arteries

BACKGROUND AND PURPOSE

β-Adrenoceptor stimulation causes pronounced vasodilatation associated with smooth muscle hyperpolarization. Although the hyperpolarization is known to reflect K(ATP) channel activation, it is not known to what extent it contributes to vasodilatation.

EXPERIMENTAL APPROACH

Smooth muscle membrane potential and tension were measured simultaneously in small mesenteric arteries in a wire myograph. The spread of vasodilatation over distance was assessed in pressurized arteries following localized intraluminal perfusion of either isoprenaline, adrenaline or noradrenaline.

KEY RESULTS

Isoprenaline stimulated rapid smooth muscle relaxation associated at higher concentrations with robust hyperpolarization. Noradrenaline or adrenaline evoked a similar hyperpolarization to isoprenaline if the α(1)-adrenoceptor antagonist prazosin was present. With each agonist, glibenclamide blocked hyperpolarization without reducing relaxation. Focal, intraluminal application of isoprenaline, noradrenaline or adrenaline during block of α(1)-adrenoceptors evoked a dilatation that spread along the entire length of the isolated artery. This response was endothelium-dependent and inhibited by glibenclamide.

CONCLUSIONS AND IMPLICATIONS

Hyperpolarization is not essential for β-adrenoceptor-mediated vasodilatation. However, following focal β-adrenoceptor stimulation, this hyperpolarization underlies the ability of vasodilatation to spread along the artery wall. The consequent spread of vasodilatation is dependent upon the endothelium and likely to be of physiological relevance in the coordination of tissue blood flow.

Evidence both L-type and non-L-type voltage-dependent calcium channels contribute to cerebral artery vasospasm following loss of NO in the rat

We recently found block of NO synthase in rat middle cerebral artery caused spasm, associated with depolarizing oscillations in membrane potential (E_m) similar in form but faster in frequency (circa 1 Hz) to vasomotion. T-type voltage-gated Ca²⁺ channels contribute to cerebral myogenic tone and vasomotion, so we investigated the significance of T-type and other ion channels for membrane potential oscillations underlying arterial spasm. Smooth muscle cell membrane potential (E_m) and tension were measured simultaneously in rat middle cerebral artery. NO synthase blockade caused temporally coupled depolarizing oscillations in cerebrovascular E_m with associated vasoconstriction. Both events were accentuated by block of smooth muscle BK_Ca. Block of T-type channels or inhibition of Na⁺/K⁺-ATPase abolished the oscillations in E_m and reduced vasoconstriction. Oscillations in E_m were either attenuated or accentuated by reducing [Ca²⁺]_o or block of K_V, respectively. TRAM-34 attenuated oscillations in both E_m and tone, apparently independent of effects against K_Ca3.1. Thus, rapid depolarizing oscillations in E_m and tone observed after endothelial function has been disrupted reflect input from T-type calcium channels in addition to L-type channels, while other depolarizing currents appear to be unimportant. These data suggest that combined block of T and L-type channels may represent an effective approach to reverse cerebral vasospasm.

The changing natural history of spinal muscular atrophy type 1

BACKGROUND

Noninvasive ventilation has become increasingly available to spinal muscular atrophy (SMA) patients since the early 1990 s. This is expected to have improved survival for SMA type 1 patients.

OBJECTIVE

To assess whether there has been a change in survival in patients with SMA type 1 between 1980 and 2006.

METHODS

We used deidentified, family-reported data from participants in the International Spinal Muscular Atrophy Patient Registry and obtained additional clinical information through a mail-in questionnaire. One hundred forty-three patients with SMA type 1 were included in the analysis. Survival of patients born in 1995-2006 (n = 78) was compared with that of patients born in 1980-1994 (n = 65), using the Kaplan-Meier method and Cox proportional hazards models with age at death as the outcome.

RESULTS

Patients born in 1995 though 2006 had significantly increased survival compared with those born in 1980-1994 (log-rank test, p < 0.001). In a Cox model, patients born in 1995-2006 had a 70% reduction in the risk of death compared with those born in 1980-1994 (hazard ratio [HR] 0.3, 95% CI 0.2-0.5, p < 0.001) over a mean follow-up of 49.9 months (SD 61.1, median 22.0). However, when controlling for demographic and clinical care variables, year of birth was no longer significantly associated with age at death (HR 1.0, 95% CI 0.6-1.8, p = 0.9), whereas ventilation for more than 16 h/d, use of a mechanical insufflation-exsufflation device, and gastrostomy tube feeding showed a significant effect in reducing the risk of death.

CONCLUSION

Survival in spinal muscular atrophy type 1 patients has increased in recent years, in relation to the growing trend toward more proactive clinical care.

Thromboxane A2 inhibition of SKCa after NO synthase block in rat middle cerebral artery

BACKGROUND AND PURPOSE

NO/prostanoid independent, EDHF-mediated hyperpolarization and dilation in rat middle cerebral arteries is mediated solely by endothelial cell IK(Ca). However, when the NO-pathway is also active, both SK(Ca) and IK(Ca) contribute to EDHF responses. As the SK(Ca) component can be inhibited by stimulation of thromboxane A(2) (TxA(2)) TP receptors and NO has the potential ability to inhibit thromboxane synthesis, we investigated whether TxA(2) might explain loss of functional input from SK(Ca) during NOS inhibition in cerebral arteries.

EXPERIMENTAL APPROACH

Rat middle cerebral arteries were mounted in a wire myograph. Endothelium-dependent responses to the PAR2 agonist, SLIGRL were assessed as simultaneous changes in smooth muscle membrane potential and tension.

KEY RESULTS

Responses were obtained in the presence of L-NAME as appropriate. Inhibition of TP receptors with either ICI 192,605 or SQ 29,548, did not affect EDHF mediated hyperpolarization and relaxation, but in their presence neither TRAM-34 nor apamin (to block IK(Ca) and SK(Ca) respectively) individually affected the EDHF response. However, in combination they virtually abolished it. Similar effects were obtained in the presence of the thromboxane synthase inhibitor, furegrelate, which additionally revealed an iberiotoxin-sensitive residual EDHF hyperpolarization and relaxation in the combined presence of TRAM-34 and apamin.

CONCLUSIONS AND IMPLICATIONS

In the rat middle cerebral artery, inhibition of NOS leads to a loss of the SK(Ca) component of EDHF responses. Either antagonism of TP receptors or block of thromboxane synthase restores an input through SK(Ca). These data indicate that NO normally enables SK(Ca) activity in rat middle cerebral arteries.

NO and KATP channels underlie endotoxin-induced smooth muscle hyperpolarization in rat mesenteric resistance arteries

1 Smooth muscle membrane potential and tension measurements were made in isolated mesenteric resistance arteries from rats exposed to bacterial endotoxin (lipopolysaccharide, LPS; 10 mg kg(-1), i.p.) for 3 h to mimic septic shock syndrome. 2 Over this period, rats developed an endotoxaemic response, assessed in vivo as a 41+/-4 mmHg drop in mean blood pressure, vascular hyporeactivity to noradrenaline (1 microg kg(-1), i.v.) and a significant increase in core body temperature. 3 In mesenteric small resistance arteries from these rats (o.d. 180 - 240 microm), phenylephrine (0.01-3 microm)-evoked contraction was not altered when compared with arteries from sham-operated animals, but the concentration-relaxation curve to acetylcholine (ACh; 0.01 - 3 microm) displayed a small, but significant, shift to the right. 4 The smooth muscle resting membrane potential (-70.3+/-1.6 mV) in arteries from LPS-treated rats was significantly greater than in control arteries (-55.4+/-1.2 mV), but in both cases the smooth muscle was depolarized to a similar potential by the application of N(omega)-nitro-L-arginine methyl ester (L-NAME; 0.3 mm; -54.1+/-2.3 vs -52.4+/-2.5 mV) or glibenclamide (10 microm; -55.0+/-2.1 vs -50.4+/-2.0 mV). 5 ACh (1 microm) elicited a maximal hyperpolarization, which ranged from -14.7+/-3.2 mV (in arteries from LPS-treated rats) to -20.6+/-2.4 mV (in arteries from sham-operated rats), and was not altered by the presence of L-NAME. Levcromakalim (1 microm) increased the smooth muscle membrane potential by around -24 mV in arteries from both sets of experimental animals. 6 These results indicate that at the level of the resistance vasculature, endotoxaemia is associated with pronounced smooth muscle hyperpolarization reflecting the action of NO on KATP channels. These changes were not associated with vascular hyporeactivity or depressed endothelial cell function in vitro, suggesting that mesenteric resistance arteries may not contribute to equivalent changes in vivo.

Thromboxane receptor stimulation associated with loss of SKCa activity and reduced EDHF responses in the rat isolated mesenteric artery

1. The possibility that thromboxane (TXA(2)) receptor stimulation causes differential block of the SK(Ca) and IK(Ca) channels which underlie EDHF-mediated vascular smooth muscle hyperpolarization and relaxation was investigated in the rat isolated mesenteric artery. 2. Acetylcholine (30 nm-3 microm ACh) or cyclopiazonic acid (10 microm CPA, SERCA inhibitor) were used to stimulate EDHF-evoked smooth muscle hyperpolarization. In each case, this led to maximal hyperpolarization of around 20 mV, which was sensitive to block with 50 nm apamin and abolished by repeated stimulation of mesenteric arteries with the thromboxane mimetic, U46619 (30 nm-0.1 microm), but not the alpha(1)-adrenoceptor agonist phenylephrine (PE). 3. The ability of U46619 to abolish EDHF-evoked smooth muscle hyperpolarization was prevented by prior exposure of mesenteric arteries to the TXA(2) receptor antagonist 1 microm SQ29548. 4. Similar-sized smooth muscle hyperpolarization evoked with the SK(Ca) activator 100 microm riluzole was also abolished by prior stimulation with U46619, while direct muscle hyperpolarization in response to either levcromakalim (1 microm, K(ATP) activator) or NS1619 (40 microm, BK(Ca) activator) was unaffected. 5. During smooth muscle contraction and depolarization to either PE or U46619, ACh evoked concentration-dependent hyperpolarization (to -67 mV) and complete relaxation. These responses were well maintained during repeated stimulation with PE, but with U46619 there was a progressive decline, so that during a third exposure to U46619 maximum hyperpolarization only reached -52 mV and relaxation was reduced by 20%. This relaxation could now be blocked with charybdotoxin alone. The latter responses could be mimicked with 300 microm 1-EBIO (IK(Ca) activator), an action not modified by exposure to U46619. 6. An early consequence of TXA(2) receptor stimulation is a reduction in the arterial hyperpolarization and relaxation attributed to EDHF. This effect appears to reflect a loss of SK(Ca) activity.

Small- and intermediate-conductance calcium-activated K+ channels provide different facets of endothelium-dependent hyperpolarization in rat mesenteric artery

Activation of both small-conductance (SKCa) and intermediate-conductance (IKCa) Ca2+-activated K+ channels in endothelial cells leads to vascular smooth muscle hyperpolarization and relaxation in rat mesenteric arteries. The contribution that each endothelial K+ channel type makes to the smooth muscle hyperpolarization is unknown. In the presence of a nitric oxide (NO) synthase inhibitor, ACh evoked endothelium and concentration-dependent smooth muscle hyperpolarization, increasing the resting potential (approx. -53 mV) by around 20 mV at 3 microM. Similar hyperpolarization was evoked with cyclopiazonic acid (10 microM, an inhibitor of sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA)) while 1-EBIO (300 microM, an IKCa activator) only increased the potential by a few millivolts. Hyperpolarization in response to either ACh or CPA was abolished with apamin (50 nM, an SKCa blocker) but was unaltered by 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (1 microM TRAM-34, an IKCa blocker). During depolarization and contraction in response to phenylephrine (PE), ACh still increased the membrane potential to around -70 mV, but with apamin present the membrane potential only increased just beyond the original resting potential (circa -58 mV). TRAM-34 alone did not affect hyperpolarization to ACh but, in combination with apamin, ACh-evoked hyperpolarization was completely abolished. These data suggest that true endothelium-dependent hyperpolarization of smooth muscle cells in response to ACh is attributable to SKCa channels, whereas IKCa channels play an important role during the ACh-mediated repolarization phase only observed following depolarization.

A mechanosensitive cation channel in endothelial cells and its role in vasoregulation

Ca2+ is an important intracellular second messenger in signal transduction of endothelial cells. It has long been recognized that a mechanosensitive Ca2+-permeable channel is present in vascular endothelial cells. The activity of this channel may increase intracellular Ca2+ level in endothelial cells. A recent finding is that the activity of this channel may be regulated by cGMP through a protein kinase G-dependent pathway. Inhibition of the channel by cGMP abolishes the Ca2+ influx elicited by flow. Several inhibitors of the cation channel including Gd3+, Ni2+, and SK&F-96365 also inhibit the Ca2+ influx due to flow stimulation. These data suggest that a mechanosensitive cation channel is the primary pathway mediating the flow-induced Ca2+ entry in vascular endothelial cells. Another important finding is that the opening of this mechanosensitive channel by KT5823 leads to endothelium-dependent vascular dilation. Therefore, it appears that this channel may play a crucial role in the regulation of vascular tone.

Evidence for a differential cellular distribution of inward rectifier K channels in the rat isolated mesenteric artery

The distribution of functionally active, inwardly rectifying K (K(IR)) channels was investigated in the rat small mesenteric artery using both freshly isolated smooth muscle and endothelial cells and small arterial segments. In Ca(2+)-free solution, endothelial cells displayed a K(IR) current with a maximum amplitude of 190 +/- 16 pA at -150 mV and sensitivity to block with 30 microM Ba(2+) (n = 7). In smooth muscle cells, outward K current was activated at around -47 +/- 3 mV, but there was no evidence of K(IR) current (n = 6). Furthermore, raising extracellular [K(+)] to either 60 or 140 mM, or applying the alpha(1)-adrenoceptor agonist phenylephrine (PE; 30 microM), failed to reveal an inwardly rectifying current in the smooth muscle cells, although PE did stimulate an iberiotoxin-sensitive outward K current (n = 4). Exogenous K(+) (10.8-16.8 mM) both relaxed and repolarized endothelium-denuded segments of the mesenteric artery contracted with PE. These effects were depressed by 100 microM ouabain but unaffected by either 30 microM BaCl(2) or 3 microM glibenclamide. These data suggest that functional, inwardly rectifying Ba(2+)-sensitive channels are restricted to the endothelial cell layer in the rat small mesenteric artery.

Activation of endothelial cell IK(Ca) with 1-ethyl-2-benzimidazolinone evokes smooth muscle hyperpolarization in rat isolated mesenteric artery

1. In rat small mesenteric arteries contracted with phenylephrine, 1-ethyl-2-benzimidazolinone (1-EBIO; 3-300 microM) evoked concentration-dependent relaxation that, above 100 microM, was associated with smooth muscle hyperpolarization. 2. 1-EBIO-evoked hyperpolarization (maximum 22.1+/-3.6 mV with 300 microM, n=4) was endothelium-dependent and inhibited by charybdotoxin (ChTX 100 nM; n=4) but not iberiotoxin (IbTX 100 nM; n=4). 3. In endothelium-intact arteries, smooth muscle relaxation to 1-EBIO was not altered by either of the potassium channel blockers ChTX (100 nM; n=7), or IbTX (100 nM; n=4), or raised extracellular K(+) (25 mM). Removal of the endothelium shifted the relaxation curve to the right but did not reduce the maximum relaxation. 4. In freshly isolated mesenteric endothelial cells, 1-EBIO (600 microM) evoked a ChTX-sensitive outward K-current. In contrast, 1-EBIO had no effect on smooth muscle cell conductance whereas NS 1619 (33 microM) stimulated an outward current while having no effect on the endothelial cells. 5. These data show that with concentrations greater than 100 microM, 1-EBIO selectively activates outward current in endothelial cells, which presumably underlies the smooth muscle hyperpolarization and a component of the relaxation. Sensitivity to block with charybdotoxin but not iberiotoxin indicates this current is due to activation of IK(Ca). However, 1-EBIO can also relax the smooth muscle by an undefined mechanism, independent of any change in membrane potential.

Involvement of cyclic GMP and potassium channels in relaxation evoked by the nitric oxide donor, diethylamine NONOate, in the rat small isolated mesenteric artery

The relative functional importance of potassium channels and cGMP-dependent pathways in the relaxation of vascular smooth muscle to the novel nitric oxide donor, diethylamine NONOate (DEA NONOate), was investigated in a resistance artery. The contribution from cGMP-dependent signalling pathways was examined by exposing arteries to 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a selective inhibitor of soluble guanylyl cyclase, while the contribution through potassium channels was assessed with different sub-type-selective potassium channel blockers. DEA NONOate (3 nM-10 microM) evoked sustained relaxation in isolated segments of the rat small mesenteric artery contracted with phenylephrine (pEC50=6.7+/-0.2; n=11). The relaxation was attenuated significantly by either ODQ (10 microM; pEC50=5.8+/-0.4; n=7) or charybdotoxin (ChTX; 50 nM; pEC50=6.3+/-0.2; n=4), a peptide blocker of large conductance, calcium-activated potassium channels (BK(Ca)). The inhibitory effects of ODQ and ChTX were additive (pEC50=5.1+/-0.4; n=9). The selective inhibitor of BK(Ca) channels, iberiotoxin (IbTX; 30 nM), and 4-aminopyridine (4-AP; 1 mM), an inhibitor of voltage-gated potassium channels (Kv), failed to modify DEA NONOate-evoked relaxation. However, in the combined presence of both ODQ and either IbTX or 4-AP the relaxation was attenuated significantly (n=3). The blocker of ATP-modulated potassium channels (K(ATP)), glibenclamide (10 microM), and of small conductance calcium-activated potassium channels (SK(Ca)), apamin (30 nM), each failed to affect ODQ-sensitive or -resistant relaxations to DEA NONOate (n=3). In conclusion, relaxation to DEA NONOate in the rat isolated, small mesenteric artery can occur via both cGMP-dependent (ODQ-sensitive) and -independent (ODQ-resistant) mechanisms. However, the contribution made to relaxation by potassium channels appears to be unmasked following pharmacological attenuation of cGMP-dependent signalling pathways. The inhibitory action of ChTX suggests part of the cGMP-insensitive component involves the activation of potassium channels, a suggestion supported by the inhibitory actions of 4-AP and IbTX in the absence of cGMP.

Properties of smooth muscle hyperpolarization and relaxation to K+ in the rat isolated mesenteric artery

Smooth muscle membrane potential and tension in rat isolated small mesenteric arteries (inner diameter 100-200 microm) were measured simultaneously to investigate whether the intensity of smooth muscle stimulation and the endothelium influence responses to exogenous K+. Variable smooth muscle depolarization and contraction were stimulated by titration with 0.1-10 microM phenylephrine. Raising external K+ to 10.8 mM evoked correlated, sustained hyperpolarization and relaxation, both of which were inhibited as the smooth muscle depolarized and contracted to around -38 mV and 10 mN, respectively. At these higher levels of stimulation, raising the K+ concentration to 13.8 mM still hyperpolarized and relaxed the smooth muscle. Relaxation to endothelium-derived hyperpolarizing factor, released by ACh, was not altered by the level of stimulation. In endothelium-denuded arteries, the concentration-relaxation curve to K+ was shifted to the right but was not depressed. In denuded arteries, relaxation to K+ was unaffected by the extent of prior stimulation and was blocked with 0.1 mM ouabain but not with 30 microM Ba2+. The ability of K+ to stimulate simultaneous hyperpolarization and relaxation in the mesenteric artery is consistent with a role as an endothelium-derived hyperpolarizing factor activating inwardly rectifying K+ channels on the endothelium and Na+-K+-ATPase on the smooth muscle cells.

Endothelial cell protein kinase G inhibits release of EDHF through a PKG-sensitive cation channel

The release of dilator agents from vascular endothelial cells is modulated by changes in cytosolic Ca(2+) concentration ([Ca(2+)](i)). In this study, we demonstrate the presence of a Ca(2+)-permeable cation channel in inside-out membrane patches of endothelial cells isolated from small mesenteric arteries. The activity of the channel is increased by KT-5823, a highly selective inhibitor of protein kinase G (PKG), while it is decreased by direct application of active PKG. Application of KT-5823 induces Ca(2+) influx in the endothelial cells isolated from small mesenteric arteries, and it also causes endothelium-dependent relaxations in isolated small mesenteric arteries. KT-5823-induced relaxations in small mesenteric arteries are greatly reduced by 35 mM K(+) or 50 nM charybdotoxin + 50 nM apamin, suggesting that endothelium-derived hyperpolarizing factor (EDHF) is the participating dilator. The involvement of EDHF is further supported by experiments in which the relaxations of small mesenteric arteries are shown to be accompanied by membrane repolarization. These data strongly argue for a major role of a PKG-sensitive cation channel in modulating the release of EDHF from endothelial cells in rat small mesenteric arteries.

Evidence for expression and function of phosphodiesterase type 5 (PDE-V) in rat resistance arteries

Evidence is provided for expression and a functional role for phosphodiesterase type V (PDE-V) in the rat isolated small mesenteric artery. The reverse transcription polymerase chain reaction (RT--PCR) demonstrated mRNA for PDE-V, while Western blotting and immunocytochemical studies showed corresponding protein expression. Smooth muscle relaxation to the nitric oxide donor, diethylamine NONOate (DEA NONOate; 1 nM - 10 microM; pEC(50)=6.7+/-0.3) was potentiated significantly by the specific inhibitor of PDE-V, 4-[[3,4-(methylenedioxy)benzyl]amino]-6-chloroquinazoline (MBCQ; 1 microM; pEC(50)=10.5+/-0.04). These data show that PDE-V is expressed in both the smooth muscle and endothelial cells of a resistance artery, and the enzyme can significantly influence nitric oxide-evoked vasorelaxation.

The involvement of intracellular Ca(2+) in 5-HT(1B/1D) receptor-mediated contraction of the rabbit isolated renal artery

5-Hydroxytryptamine(1B/1D) (5-HT(1B/1D)) receptor coupling to contraction was investigated in endothelium-denuded rabbit isolated renal arteries, by simultaneously measuring tension and intracellular [Ca(2+)], and tension in permeabilized smooth muscle cells. In intact arterial segments, 1 nM - 10 microM 5-HT failed to induce contraction or increase the fura-2 fluorescence ratio (in the presence of 1 microM ketanserin and prazosin to block 5-HT(2) and alpha(1)-adrenergic receptors, respectively). However, in vessels pre-exposed to either 20 mM K(+) or 30 nM U46619, 5-HT stimulated concentration-dependent increases in both tension and intracellular [Ca(2+)]. 1 nM - 10 microM U46619 induced concentration-dependent contractions. In the presence of nifedipine (0.3 and 1 microM) the maximal contraction to U46619 (10 microM) was reduced by around 70%. The residual contraction was abolished by the putative receptor operated channel inhibitor, SKF 96365 (2 microM). With 0.3 microM nifedipine present, 100 nM U46619 evoked similar contraction to 30 nM U46619 in the absence of nifedipine, but contraction to 5-HT (1 nM - 10 microM) was abolished. In permeabilized arterial segments, 10 mM caffeine, 1 microM IP(3) or 100 microM phenylephrine, each evoked transient contractions by releasing Ca(2+) from intracellular stores, whereas 5-HT had no effect. In intact arterial segments pre-stimulated with 20 mM K(+), 5-HT-evoked contractions were unaffected by 1 microM thapsigargin, which inhibits sarco- and endoplasmic reticulum calcium-ATPases. In vessels permeabilized with alpha-toxin and then pre-contracted with Ca(2+) and GTP, 5-HT evoked further contraction, reflecting increased myofilament Ca(2+)-sensitivity. Contraction linked to 5-HT(1B/1D) receptor stimulation in the rabbit renal artery can be explained by an influx of external Ca(2+) through voltage-dependent Ca(2+) channels and sensitization of the contractile myofilaments to existing levels of Ca(2+), with no release of Ca(2+) from intracellular stores.

An indirect influence of phenylephrine on the release of endothelium-derived vasodilators in rat small mesenteric artery

1. The possibility that stimulation of smooth muscle alpha(1)-adrenoceptors modulates contraction via the endothelium was examined in rat small mesenteric arteries. 2. N(omega)-nitro-L-arginine methyl ester, (L-NAME, 100 microM to inhibit NO synthase) increased contraction to single concentrations of phenylephrine (1 - 3 microM) by approximately 2 fold (from a control level of 14.2+/-3.0 to 34. 1+/-4.2% of the maximum contraction of the artery, n=20). The action of L-NAME was abolished by disrupting the endothelium. 3. The subsequent addition of apamin (to inhibit small conductance Ca(2+)-activated K(+) channels, 50 nM) further augmented phenylephrine contractions, in an endothelium-dependent manner, to more than 3 fold above control (50.4+/-5.3% of the maximum contraction, n=11). 4.Charybdotoxin (non-selective inhibitor of large conductance Ca(2+)-activated K(+) channels, BK(Ca), 50 nM) plus L-NAME augmented the level of phenylephrine contraction to 4 - 5-fold above control (64.1+/-3.1%, n=5), but this effect was independent of the endothelium. The potentiation of contraction by charybdotoxin could be mimicked with the selective BK(Ca) inhibitor, iberiotoxin,. 5. Apamin together with L-NAME and charybdotoxin further significantly increased the phenylephrine contraction by 5 - 6-fold, to 79.9+/-3.5% of the maximum contraction of the artery (n=13). 6. Phenylephrine failed directly to increase the intracellular Ca(2+) concentration in endothelial cells freshly isolated from the small mesenteric artery. 7. Stimulation of smooth muscle alpha(1)-adrenoceptors in the mesenteric artery induces contraction that is markedly suppressed by the endothelium. The attenuation of contraction appears to reflect both the release of NO from the endothelium and the efflux of K(+) from both endothelial and smooth muscle cells. This suggests that the release of NO and endothelium-derived hyperpolarizing factor can be evoked indirectly by agents which act only on the smooth muscle cells.

The influence of phenylephrine outward potassium currents in single smooth muscle cells from the rabbit mesenteric artery

In mesenteric artery smooth muscle cells, depolarizing voltage steps activated outward K+ currents whose amplitude was decreased by about 20% with phenylephrine (1-10 microM: n = 14 cells). Attenuation of outward current was only partly dependent on [Ca2+]i, because it persisted, although reduced, with 10 mM BAPTA in the patch pipette and was abolished in the presence of 3 mM 3,4-diaminopyridine (n = 13). In outside-out patches, phenylephrine did not exert any direct effect on the unitary current amplitude or open probability of large conductance K+ channels. Outward current was significantly increased (>100% in both cases) by 10 mM caffeine, presumably owing to the release of internal Ca2+ stores. With 10 mM BAPTA in the pipette, the only response to caffeine was a small decrease (9 +/- 3.7%, n = 10) in the K+ current. These observations show that a minor effect of phenylephrine is to reduce outward K+ current (probably Kv) in mesenteric cells.

5-hydroxytryptamine stimulation of phospholipase D activity in the rabbit isolated mesenteric artery

1. The involvement of phospholipase D (PLD) in the 5-hydroxytryptamine 5-HT1B/5-HT1D-signalling pathway was assessed in the rabbit isolated mesenteric artery. 2. RT-PCR analysis of mesenteric smooth muscle cells revealed a strong signal corresponding to mRNA transcript for the 5-HT1B receptor. The PCR fragment corresponded to the known sequence for the 5-HT1B receptor. No signal corresponding to 5-HT1D mRNA was detected. 3. Neither 5-HT (3 microM) nor KCl (45 mM) individually stimulated any significant increase in the smooth muscle concentration of [33P]-PtdBut to reflect PLD activity. However, in the presence of KCl (45 mM), 5-HT evoked a concentration-dependent increase in [33P]-PtdBut, to a maximum of 84% with 5-HT (3 microM). 4. [33P]-PtdBut accumulation evoked by 5-HT in the presence of KCl was abolished in nominally calcium-free Krebs-Henseleit Buffer (KHB) or with the selective protein kinase C inhibitor, Ro-31 8220 (10 microM, 20 min). 5. 5-HT (3 microM) in the presence of KCl (45 mM) failed to increase either the accumulation of [33P]-phosphatidic acid in the presence of butanol, or total [3H]-inositol phosphates ([3H]-InsP) in the presence of LiCl (10 mM). 6. 5-HT (0.1-1 microM) abolished forskolin (1 microM) stimulated increases in cyclic AMP (15 fold increase), an action which was pertussis toxin-sensitive. 7. Therefore, in the presence of raised extracellular potassium 5-HT can stimulate PLD via 5-HT1B receptors in the rabbit mesenteric artery. This action requires extracellular calcium and the activation of protein kinase C. These characteristics are identical to the profile for 5-HT1B/5-HT1D-receptor evoked contraction in vascular smooth muscle cells, suggesting a role for PLD in this response to 5-HT.

Role of heterocellular Gap junctional communication in endothelium-dependent smooth muscle hyperpolarization: inhibition by a connexin-mimetic peptide

A synthetic connexin-mimetic peptide (Gap 27 peptide) was used to evaluate the contribution of gap junctional communication to smooth muscle responses mediated by the endothelium-dependent agonist acetylcholine (ACh) in rabbit mesenteric arteries. Hyperpolarizations and relaxations to 0.1 and 1 microM ACh observed in the presence of nitric oxide synthase and cyclooxygenase inhibition were markedly attenuated by the peptide at a concentration of 300 microM, whereas the hyperpolarizing response to levcromakalim, a KATP channel opener, was unaffected. The peptide also attenuated intercellular transfer of Lucifer yellow in confluent cultures of COS-7 cells, thus confirming its ability to modulate the permeability of gap junctions. The findings demonstrate that heterocellular gap junctional communication contributes to NO- and prostanoid-independent mechanisms of vasorelaxation that are widely attributed to an endothelium-derived hyperpolarizing factor.

K+ is an endothelium-derived hyperpolarizing factor in rat arteries

In arteries, muscarinic agonists such as acetylcholine release an unidentified, endothelium-derived hyperpolarizing factor (EDHF) which is neither prostacyclin nor nitric oxide. Here we show that EDHF-induced hyperpolarization of smooth muscle and relaxation of small resistance arteries are inhibited by ouabain plus Ba2+; ouabain is a blocker of Na+/K+ ATPase and Ba2+ blocks inwardly rectifying K+ channels. Small increases in the amount of extracellular K+ mimic these effects of EDHF in a ouabain- and Ba2+-sensitive, but endothelium-independent, manner. Acetylcholine hyperpolarizes endothelial cells and increases the K+ concentration in the myoendothelial space; these effects are abolished by charbdotoxin plus apamin. Hyperpolarization of smooth muscle by EDHF is also abolished by this toxin combination, but these toxins do not affect the hyperpolarizaiton of smooth muscle by added K+. These data show that EDHF is K+ that effluxes through charybdotoxin- and apamin-sensitive K+ channels on endothelial cells. The resulting increase in myoendothelial K+ concentration hyperpolarizes and relaxes adjacent smooth-muscle cells by activating Ba2+-sensitive K+ channels and Na+/K+ ATPase. These results show that fluctuations in K+ levels originating within the blood vessel itself are important in regulating mammalian blood pressure and flow.

Characteristics of single, large-conductance calcium-dependent potassium channels (BKCa) from smooth muscle cells isolated from the rabbit mesenteric artery

Smooth muscle cells isolated from the secondary and tertiary branches of the rabbit mesenteric artery contain large Ca2+-dependent channels. In excised patches with symmetrical (140 mm) K+ solutions, these channels had an average slope conductance of 235 +/- 3 pS, and reversed in direction at -6.1 +/- 0.4 mV. The channel showed K+ selectivity and its open probability (Po) was voltage-dependent. Iberiotoxin (50 nm) reversibly decreased Po, whereas tetraethylammonium (TEA, at 1 mm) reduced the unitary current amplitude. Apamin (200 nm) had no effect. The channel displayed sublevels around 1/3 and 1/2 of the mainstate level. The effect of [Ca2+] on Po was studied and data fitted to Boltzmann relationships. In 0.1, 0.3, 1.0 and 10 microM Ca2+, V1/2 was 77.1 +/- 5.3 (n = 18), 71.2 +/- 4.8 (n = 16), 47.3 +/- 10.1 (n = 11) and -14.9 +/- 10.1 mV (n = 6), respectively. Values of k obtained in 1 and 10 microM [Ca2+] were significantly larger than that observed in 0.1 microM [Ca2+]. With 30 microM NS 1619 (a BKCa channel activator), V1/2 values were shifted by 39 mV to the left (hyperpolarizing direction) and k values were not affected. TEA applied intracellularly, reduced the unitary current amplitude with a Kd of 59 mm. In summary, BKCa channels show a particularly weak sensitivity to intracellular TEA and they also display large variation in V1/2 and k. These findings suggest the possibility that different types (isoforms) of BKCa channels may exist in this vascular tissue.

Interactions between endothelium-derived relaxing factors in the rat hepatic artery: focus on regulation of EDHF

1. In rat isolated hepatic arteries contracted with phenylephrine, acetylcholine and the calcium ionophore A23187 each elicit endothelium-dependent relaxations, which involve both nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). However, the contribution of prostanoids to these responses, and the potential interaction between EDHF and other endothelium-derived relaxing factors have not been examined. 2. In the presence of the NO synthase inhibitor N(G)-nitro-L-arginine (L-NOARG, 0.3 mM) and a mixture of charybdotoxin (0.3 microM) and apamin (0.3 microM), inhibitors of the target potassium (K) channel(s) for EDHF, acetylcholine and A23187 each induced a concentration-dependent and almost complete relaxation, which was abolished in the additional presence of indomethacin (10 microM). Thus, in addition to EDHF and NO, a relaxing factor(s) generated by cyclo-oxygenase (COX) contributes to endothelium-dependent relaxation in the rat hepatic artery. 3. The resting membrane potentials of endothelium-intact and endothelium-denuded vascular segments were -57 mV and -52 mV, respectively (P>0.05). In intact arteries, the resting membrane potential was not affected by L-NOARG plus indomethacin, but reduced to -47 mV in the presence of charybdotoxin plus apamin. Acetylcholine and A23187 (10 microM each) elicited a hyperpolarization of 13 mV and 15 mV, respectively. The hyperpolarization induced by these agents was not affected by L-NOARG plus indomethacin (12 mV and 14 mV, respectively), but reduced in the presence of charybdotoxin plus apamin (7 mV and 10 mV, respectively), and abolished in the combined presence of charybdotoxin, apamin and indomethacin. 4. The NO donor 3-morpholino-sydnonimine (SIN-1) induced a concentration-dependent relaxation, which was unaffected by charybdotoxin plus apamin, but abolished by the selective soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ, 10 microM). SIN-1 (10 microM) did not alter the resting membrane potential in endothelium-denuded vascular segments. 5. The COX-dependent relaxation induced by acetylcholine was abolished following exposure to 30 mM KCl, but unaffected by glibenclamide (10 microM). The prostacyclin analogue iloprost induced a concentration-dependent relaxation, which was also abolished in 30 mM KCl and unaffected by the combined treatment with glibenclamide, charybdotoxin and apamin. Iloprost (10 microM) induced a glibenclamide-resistant hyperpolarization (8 mV with and 9 mV without glibenclamide) in endothelium-denuded vascular segments. 6. Exposure to SIN-1 or iloprost did not affect the EDHF-mediated relaxation induced by acetylcholine (i.e. in the presence of L-NOARG and indomethacin). Replacement of L-NOARG with the NO scavenger oxyhaemoglobin (10 microM) or the soluble guanylate cyclase inhibitor ODQ (10 microM) or methylene blue (10 microM), which all significantly inhibited responses to endothelium-derived NO, did not affect the acetylcholine-induced relaxation in the presence of indomethacin, indicating that endogenous NO also does not suppress EDHF-mediated responses. 7. These results show that, in addition to EDHF and NO, an endothelium-derived hyperpolarizing factor(s) generated by COX contributes significantly to endothelium-dependent relaxation in the rat heptic artery. Neither this factor nor NO seems to regulate EDHF-mediated responses. Thus, EDHF does not serve simply as a 'back-up' system for NO and prostacyclin in this artery. However, whether EDHF modulates the NO and COX pathways remains to be determined.

Nitric oxide (NO)-induced activation of large conductance Ca2+-dependent K+ channels (BK(Ca)) in smooth muscle cells isolated from the rat mesenteric artery

1. To assess the action of nitric oxide (NO) and NO-donors on K+ current evoked either by voltage ramps or steps, patch clamp recordings were made from smooth muscle cells freshly isolated from secondary and tertiary branches of the rat mesenteric artery. 2. Inside-out patches contained channels, the open probability of which increased with [Ca2+]i. The channels had a linear slope conductance of 212+/-5 pS (n = 12) in symmetrical (140 mM) K+ solutions which reversed in direction at 4.4 mV. In addition, the channels showed K+ selectivity, in that the reversal potential shifted in a manner similar to that predicted by the Nernst potential for K+. Barium (1 mM) applied to the intracellular face of the channel produced a voltage-dependent block and external tetraethylammonium (TEA; at 1 mM) caused a large reduction in the unitary current amplitude. Taken together, these observations indicate that the channel most closely resembled BK(Ca). 3. In five out of six inside-out patches, NO (45 or 67 microM) produced an increase in BK(Ca) activity. In inside-out patches, BK(Ca) activity was also enhanced in some patches with 100 or 200 microM 3-morpholino-sydnonimine (SIN-1) (4/11) and 100 microM sodium nitroprusside (SNP) (3/8). The variability in channel opening with the NO donors may reflect variability in the release of NO from these compounds. 4. In inside-out patches, 100 microM SIN-1 failed to increase BK(Ca) activity (in all 4 patches tested), while at a higher (500 microM) concentration SIN-1 had a direct blocking effect on the channels (n = 3). NO applied directly to inside-out patches increased (P < 0.05) BK(Ca) activity in two patches. 5. In the majority of cells (6 out of 7), application of NO (45 or 67 microM) evoked an increase in the amplitude of whole-cell currents in perforated patches. This action was not affected by the soluble guanylyl cyclase inhibitor, 1H-[1,2,4] oxadiazolo [4,3-a]quinoxalin-1-one (ODQ). An increase in whole-cell current was also evoked with either of the NO donors, SIN-1 or SNP (each at 100 microM). With SIN-1, the increase in current was blocked with the BK(Ca) channel blocker, iberiotoxin (50 nM). 6. With conventional whole-cell voltage clamp, the increase in the outward K+ current evoked with SIN-1 (50-300 microM) showed considerable variability. Either no effect was obtained (11 out of 18 cells), or in the remaining cells, an average increase in current amplitude of 38.7+/-10.2% was recorded at 40 mV. 7. In cell-attached patches, large conductance voltage-dependent K+ channels were stimulated by SIN-1 (100 microM) applied to the cell (n = 5 patches). 8. These data indicate that NO and its donors can directly stimulate BK(Ca) activity in cells isolated from the rat mesenteric artery. The ability of NO directly to open BK(Ca) channels could play an important functional role in NO-induced relaxation of the vascular smooth muscle cells in this small resistance artery.

Evidence that different mechanisms underlie smooth muscle relaxation to nitric oxide and nitric oxide donors in the rabbit isolated carotid artery

1. The endothelium-dependent relaxants acetylcholine (ACh; 0.03-10 microM) and A23187 (0.03-10 microM), and nitric oxide (NO), applied either as authentic NO (0.01-10 microM) or as the NO donors 3-morpholino-sydnonimine (SIN-1; 0.1-10 microM) and S-nitroso-N-acetylpenicillamine (SNAP; 0.1-10 microM), each evoked concentration-dependent relaxation in phenylephrine stimulated (1-3 microM; mean contraction and depolarization, 45.8+/-5.3 mV and 31.5+/-3.3 mN; n=10) segments of rabbit isolated carotid artery. In each case, relaxation closely correlated with repolarization of the smooth muscle membrane potential and stimulated a maximal reversal of around 95% and 98% of the phenylephrine-induced depolarization and contraction, respectively. 2. In tissues stimulated with 30 mM KCl rather than phenylephrine, smooth muscle hyperpolarization and relaxation to ACh, A23187, authentic NO and the NO donors were dissociated. Whereas the hyperpolarization was reduced by 75-80% to around a total of 10 mV, relaxation was only inhibited by 35% (n=4-7 in each case; P<0.01). The responses which persisted to ACh and A23187 in the presence of 30 mM KCl were abolished by either the NO synthase inhibitor L-NG-nitroarginine methyl ester (L-NAME; 100 microM) or the inhibitor of soluble guanylyl cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 microM; 10 min; n=4 in each case; P<0.01). 3. Exposure to ODQ significantly attenuated both repolarization and relaxation to ACh, A23187 and authentic NO, reducing the maximum changes in both membrane potential and tension to each relaxant to around 60% of control values (n=4 in each case; P<0.01). In contrast, ODQ almost completely inhibited repolarization and relaxation to SIN-1 and SNAP, reducing the maximum responses to around 8% in each case (n=3-5; P<0.01). 4. The potassium channel blockers glibenclamide (10 microM), iberiotoxin (100 nM) and apamin (50 nM), alone or in combination, had no significant effect on relaxation to ACh, A23187, authentic NO, or the NO donors SIN-1 and SNAP (n=4 in each case; P>0.05). Charybdotoxin (ChTX; 50 nM) almost abolished repolarization to ACh (n=4; P<0.01) and inhibited the maximum relaxation to ACh, A23187 and authentic NO each by 30% (n=4-8; P<0.01). Application of ODQ (10 microM; 10 min) abolished the ChTX-insensitive responses to ACh, A23187 and authentic NO (n=4 in each case; P<0.01 5. When the concentration of phenylephrine was reduced (to 0.3-0.5 microM) to ensure the level of smooth muscle contraction was the same as in the absence of potassium channel blocker, ChTX had no effect on the subsequent relaxation to SIN-1 (n=4; P>0.05). However, in the presence of tone induced by 1-3 microM phenylephrine (51.2+/-3.3 mN; n=4), ChTX significantly reduced relaxation to SIN-1 by nearly 50% (maximum relaxation 53.2+/-6.3%, n=4; P<0.01). 6. These data indicate that NO-evoked relaxation of the rabbit isolated carotid artery can be mediated by three distinct mechanisms: (a) a cyclic GMP-dependent, voltage-independent pathway, (b) cyclic GMP-mediated smooth muscle repolarization and (c) cyclic GMP-independent, ChTX-sensitive smooth muscle repolarization. Relaxation and repolarization to both authentic and endothelium-derived NO in this large conduit artery appear to be mediated by parallel cyclic GMP-dependent and -independent pathways. In contrast, relaxation to the NO-donors SIN-1 and SNAP appears to be mediated entirely via cyclic GMP-dependent mechanisms.

Evidence that anandamide and EDHF act via different mechanisms in rat isolated mesenteric arteries

The endogenous cannabinoid, anandamide, has been suggested as an endothelium-derived hyperpolarizing factor (EDHF). We found that anandamide-evoked relaxation in isolated segments of rat mesenteric artery was associated with smooth muscle hyperpolarization. However, although anandamide-evoked relaxation was inhibited by either charybdotoxin (ChTX) or iberiotoxin, inhibition of the relaxation to EDHF required a combination of ChTX and apamin. The relaxations induced by either anandamide or EDHF were not inhibited by the cannabinoid receptor (CB1) antagonist SRI41716A, or mimicked by selective CB1 agonists. Thus, anandamide appears to cause smooth muscle relaxation via a CB1 receptor-independent mechanism and cannabinoid receptor activation apparently does not contribute to EDHF-mediated relaxation in this resistance artery.

Nitric oxide is the mediator of both endothelium-dependent relaxation and hyperpolarization of the rabbit carotid artery

It is controversial whether the endothelial cell release of nitric oxide (NO) or a different factor(s) accounts for endothelium-dependent hyperpolarization, because in many arteries endothelium-dependent relaxation and hyperpolarization resists inhibitors of NO synthase. The contribution of NO to acetylcholine-induced endothelium-dependent hyperpolarization and relaxation of the rabbit carotid artery was determined by measuring NO with electrochemical and chemiluminescence techniques. In the presence of phenylephrine to depolarize and contract the smooth muscle cells, acetylcholine caused concentration-dependent hyperpolarization and relaxation which were closely correlated to the release of NO. N(omega)-nitro-L-arginine methyl ester (30 microM) partially reduced the release of NO and caused a similar reduction in smooth muscle cell relaxation and hyperpolarization. To determine if the residual responses were mediated by another endothelium-derived mediator or NO released despite treatment with N(omega)-nitro-L-arginine methyl ester, N(omega)-nitro-L-arginine (300 microM) was added. The combined inhibitors further reduced, but did not eliminate, NO release, smooth muscle relaxation, and hyperpolarization. Hyperpolarization and relaxation to acetylcholine remained closely correlated with the release of NO in the presence of the inhibitors. In addition, the NO donor, SIN-1, caused hyperpolarization and relaxation which correlated with the concentrations of NO that it released. These studies indicate that (i) the release of NO by acetylcholine is only partially inhibited by these inhibitors of NO synthase when used even at high concentrations, and (ii) NO rather than another factor accounts fully for endothelium-dependent responses of the rabbit carotid artery.

Lack of endothelin ETB receptor-mediated smooth muscle hyperpolarization in rat mesenteric resistance arteries

The presence of functional endothelin ETB receptors was investigated in rat isolated mesenteric resistance arteries. Neither endothelin-3 (0.1-100 nM) nor the endothelin ETB selective agonists sarafotoxin S6c and BQ 3020 (both 1-100 nM) induced any measurable hyperpolarization or relaxation in stimulated (alpha 1-adrenoceptor agonist; phenylephrine) or unstimulated arteries. In both cases, the subsequent addition of acetylcholine (1 microM) hyperpolarized the membrane potential by 10-20 mV and totally reversed any contraction which was present. These results indicate that the endothelin ETB-mediated vasodilatation observed in the intact mesenteric bed does not reflect hyperpolarization of smooth muscle cells in resistance arteries arising from the mesenteric artery.

Evidence that potassium channels make a major contribution to SIN-1-evoked relaxation of rat isolated mesenteric artery

1. The NO donor 3-morpholino-sydnonimine (SIN-1; 0.01-10 microM) evoked concentration-dependent relaxation of rat isolated mesenteric arteries pre-constricted with phenylephrine (1-3 microM). The relaxation to SIN-1 was not significantly different between endothelium-intact or denuded arterial segments or segments in which basal nitric oxide (NO) synthesis was inhibited (n = 8; P > 0.05). In contrast, the membrane permeable analogue of guanosine 3':5'-cyclic monophosphate (cyclic GMP), 8-Br-cyclic GMP (0.01-1 mM), was much less effective in relaxing intact than denuded arterial segments or intact arterial segments pre-incubated with NO synthase blockers (n = 4; P < 0.01). 2. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 microM; 10 min) alone, did not alter SIN-1-evoked relaxation in any tissues (n = 5; P > 0.05). However, in parallel experiments, ODQ almost completely inhibited both basal and SIN-1-stimulated production of cyclic GMP in both the presence and absence of NO synthase blockers (n = 6; P < 0.01) indicating that full relaxation to SIN-1 can be achieved in the absence of an increase in cyclic GMP. 3. Exposure of endothelium-intact arterial segments to the potassium channel blocker charybdotoxin (50 nM; 10 min), significantly inhibited SIN-1-evoked relaxation, reducing the maximum response by around 90% (n = 5; P < 0.01). In contrast, in arterial segments in which either the endothelial cell layer had been removed or basal NO synthesis inhibited, relaxation to SIN-1 was not reduced in the presence of charybdotoxin (n = 6; P > 0.05). However, in the presence of NO synthase blockers and L-arginine (300 microM) together, charybdotoxin did significantly inhibit SIN-1-evoked relaxation to a similar extent as intact tissues (maximum response induced by around 80%; n = 4; P < 0.01). 4. Pre-incubation with apamin (30 nM; 10 min) or glibenclamide (10 microM; 10 min) did not alter SIN-1-evoked relaxation of phenylephrine-induced tone in any tissues (n = 4 and n = 6, respectively; P > 0.05). However, in the presence of either ODQ and apamin, or ODQ and glibenclamide, SIN-1-evoked relaxation was significantly attenuated in intact arterial segments and segments in which NO synthesis was blocked. 5. Exposure of intact arterial segments to charybdotoxin and apamin, in the presence of NO synthase blockers, also significantly inhibited SIN-1-evoked relaxation, reducing the maximum response by around 80% (n = 4; P < 0.01). 6. Addition of superoxide dismutase (SOD; 30 u ml-1), potentiated relaxations to SIN-1 in all tissues, but did not alter the effects of charybdotoxin and ODQ and SIN-1-evoked relaxation. 7. These data show that although relaxation to the NO-donor SIN-1 is not significantly different between endothelium-intact and denuded arterial segments, the mechanisms which mediate SIN-1-evoked relaxation in the rat isolated mesenteric artery appear to be modulated by the basal release of endothelium-derived NO. In the presence of an intact endothelial cell layer, the major mechanism for SIN-1-evoked relaxation appears to be the activation of charybdotoxin-sensitive potassium channels. In contrast, when basal NO synthesis is inhibited, SIN-1 appears to cause full relaxation by both the activation of a charybdotoxin-sensitive pathway and the stimulation of soluble guanylyl cyclase.

Influence of contractile agonists on the mechanism of endothelium-dependent relaxation in rat isolated mesenteric artery

This study demonstrates directly that the relative contribution of nitric oxide (NO) and an NO synthase-independent repolarization to acetylcholine-evoked relaxation in rat isolated mesenteric resistance arteries is determined by the processes which mediate pre-contraction. Noradrenaline-induced contractions were reversed by acetylcholine via both NO and NO synthase-independent smooth muscle repolarization. In contrast, reversal of contractions to the thromboxane-mimetic, U46619, by acetylcholine was entirely mediated by the actions of NO, independently of a change in membrane potential.

Phospholipase A2 and protein kinase C contribute to myofilament sensitization to 5-HT in the rabbit mesenteric artery

1. Calcium (Ca2+, 0.1-100 microM) stimulated concentration-dependent contractions in small strips from the rabbit mesenteric artery in which the smooth muscle cells had been permeabilized with Staphylococcus aureus alpha-toxin. 2. 5-Hydroxytryptamine (5-HT) and phenylephrine, each in the presence of 10 microM guanosine 5'-triphosphate (GTP), concentration-dependently stimulated additional contractions in strips sub-maximally contracted by the presence of a buffered concentration of calcium (0.3 microM). All the additional contraction was abolished with the selective inhibitor of protein kinase C, Ro 31-8220 (10 microM). 3. Quinacrine (10-50 microM), an inhibitor of phospholipase A2, selectively inhibited the sensitization to 5-HT, but did not alter the sensitization to either phenylephrine or GTP. 4. Myofilament sensitization to calcium was mimicked by exogenous arachidonic acid (300 microM, in the presence of indomethacin, miconazole and BW755c) and the stable analogue of arachidonic acid, 5,8,11,14-eicosatetrayonic acid (ETYA, 100 microM), and in both cases did not require the additional presence of GTP. Ro 31-8220, but not quinacrine, reduced the sensitization to arachidonic acid by around 30%. 5. These results indicate that G protein-linked myofilament sensitization to calcium in the mesenteric artery that follows the activation of 5-HT receptors, but not alpha 1-receptors, involves phospholipase A2. The sensitization stimulated by each of these different receptors, and a component of the response to arachidonic acid, also appears to involve the activation of protein kinase C.

Characterization of muscarinic receptors mediating contractions of circular and longitudinal muscle of human isolated colon

1. The effects of seven muscarinic receptor antagonists were used to characterize the receptors which mediate carbachol-evoked contractions of intertaenial circular and taenial longitudinal muscle in human isolated colon. The effects of these antagonists were studied upon colon contractions induced by cumulatively added carbachol which had mean EC50 values of 11.7 +/- 2.3 microM (n = 8) and 12.6 +/- 2.3 microM (n = 8) respectively upon circular and longitudinal smooth muscle. 2. All antagonists displaced concentration-response curves to carbachol to the right in a parallel manner. The maximum concentration of each antagonist added (30 nM-10 microM) did not significantly suppress the maximum response. 3. In circular muscle, the M3 muscarinic receptor antagonists, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), hexahydrosiladiphenidol (HHSiD) and para-fluoro-hexahydrosiladiphenidol (p-F-HHSiD) inhibited responses with pA2 values of 9.41 +/- 0.23, 7.17 +/- 0.07, 6.94 +/- 0.18 respectively. The M2 muscarinic receptor antagonist, AF-DX 116, the M2/M4 muscarinic receptor antagonist, himbacine, and the M1 muscarinic receptor antagonist, pirenzepine, yielded pA2 values of 7.36 +/- 0.43, 7.47 +/- 0.14 and 7.23 +/- 0.48 respectively. The non-selective antagonist, atropine, had a pA2 of 8.72 +/- 0.28. 4. In longitudinal muscle 4-DAMP, HHSiD, p-F-HHSiD, AF-DX 116, himbacine and pirenzepine gave pA2 values of 9.09 +/- 0.16, 7.45 +/- 0.43, 7.44 +/- 0.21, 6.44 +/- 0.1, 7.54 +/- 0.40, 6.87 +/- 0.38 respectively. Atropine yielded a pA2 value of 8.60 +/- 0.08. 5. The pharmacological profile of antagonist affinities at the muscarinic receptor population responding to muscarinic agonist-evoked contraction is similar to that widely accepted as characterizing the activation of an M3 muscarinic receptor subtype, although pA2 values of some antagonists are lower than that seen in other investigations.

Specific accumulation of inositol 1,4,5-trisphosphate in rabbit basilar artery in response to noradrenaline but not 5-hydroxytryptamine

This study examined the ability of 5-hydroxytryptamine and noradrenaline to stimulate inositol 1,4,5-trisphosphate (IP3) mass accumulation in segments of the rabbit basilar artery. 5-Hydroxytryptamine (5-HT, 100 microM) failed to stimulate any significant accumulation of IP3 during the 5 min period following its application. In the presence of prazosin, 5-HT (300 microM) caused a rapid, transient decrease in IP3 accumulation which was significant after 5 s but had increased to pre-stimulation levels within 15 s. In contrast, noradrenaline (10 microM) stimulated a rapid, transient accumulation of IP3 which was significant after 5 s but had declined to basal levels after 60 s. In basilar artery segments bathed in Krebs solution containing 25.7 mM K+ (normal concentration 5.7 mM), the basal IP3 concentration was significantly elevated. The IP3 accumulation stimulated by either 5-HT or raised K+ was not reduced by the presence of the alpha 1-adrenoceptor antagonist, prazosin (0.1 microM). In the presence of raised K+, 5-hydroxytryptamine caused a rapid, transient inhibition of the K(+)-induced IP3 accumulation, which was maximal after 5 s but had increased to pre-stimulation levels within 30 s in the continued presence of 5-hydroxytryptamine. Noradrenaline did not affect the IP3 accumulation induced by raised extracellular [K+]. These results provide further evidence that IP3 is not involved in 5-hydroxytryptamine-induced smooth muscle contraction in the rabbit basilar artery, but support a role for this second messenger in the contraction induced in response to noradrenaline.

Characterisation of inositol 1,4,5-trisphosphate binding sites in rabbit aortic smooth muscle

The present study investigated the characteristics of D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) binding sites in crude membrane preparations of rabbit aortic smooth muscle. A particular aim was to demonstrate if increases in cytoplasmic cyclic guanosine 3':5' monophosphate (cGMP), which mediates the effect of nitrovasodilators, may cause smooth muscle relaxation in part by the displacement of Ins(1,4,5)P3 binding. Negligible Ins(1,4,5)P3 binding was observed at pH < 7, while maximum binding occurred over the pH range 8-9. Saturation analysis of isotopic dilution binding data revealed an apparently homogenous population of Ins(1,4,5)P3 binding sites with a KD of 4.02 +/- 0.53 nM and a Bmax of 27.7 +/- 4.6 fmol/mg protein. Heparin, an Ins(1,4,5)P3 receptor antagonist, inhibited binding with an IC50 of 11.43 +/- 2.81 micrograms/ml. The ability of other polyphosphate compounds to inhibit Ins(1,4,5)P3 binding in this preparation was also examined. D-myo-Inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), adenosine 5'-triphosphate (ATP) and guanosine 5'-triphosphate (GTP) inhibited Ins(1,4,5)P3 binding, although each was significantly less potent that Ins(1,4,5)P3. In contrast, cyclic guanosine 3':5' monophosphate (cGMP) did not significantly alter Ins(1,4,5)P3 binding in rabbit aortic smooth muscle. This observation suggests that competitive inhibition of Ins(1,4,5)P3 receptor binding is not an important consideration in cGMP-mediated vascular smooth muscle cell relaxation.

Multiple pathways underlying endothelium-dependent relaxation in the rabbit isolated femoral artery

1. In isolated segments of the rabbit femoral artery stimulated with noradrenaline, both acetylcholine (1 nM-10 microM) and the calcium ionophore A23187 (1 nM-100 microM) evoked endothelium-dependent smooth muscle relaxation and hyperpolarization while bradykinin (0.01-100 nM) had no effect. 2. The nitric oxide synthase inhibitors, NG-nitro-L-arginine (L-NOARG; 100 microM; 20 min) or NG-nitro-L-arginine methyl ester (L-NAME; 100 microM; 20 min) each abolished the hyperpolarization and the majority of the relaxation to acetylcholine (maximal response reduced from 96.8 +/- 2.3% to 2.0 +/- 1.4%). 3. The potassium channel blocker, glibenclamide (10 microM; 10 min) also abolished the change in membrane potential to acetylcholine but did not modify the smooth muscle relaxation. 4. In contrast, neither L-NAME nor glibenclamide modified the comparable responses of the femoral artery to A23187, which were also unaffected by the cyclo-oxygenase inhibitor, indomethacin (10 microM). 5. In artery segments stimulated with potassium chloride (25 mM), the maximal change in tension and membrane potential evoked by A23187 (100 microM) was significantly reduced from 95.0 +/- 4.5% and 23.0 +/- 2.0 mV to 69.0 +/- 10.1% and 12.0 +/- 1.5 mV, respectively. Under these conditions L-NAME further reduced the relaxation but not the accompanying hyperpolarization to A23187. 6. Endothelium-denuded arterial segments sandwiched with endothelium-intact 'donor' segments gave qualitatively similar relaxant responses to those described above for acetylcholine and A23187. 7. Exogenous nitric oxide (0.5-10 microM) stimulated a transient relaxation in pre-contracted artery segments, which at concentrations above 5 microM was accompanied by smooth muscle hyperpolarization(maximum 8.5 +/- 3.2 mV; n = 4). The hyperpolarization but not the relaxation to nitric oxide was abolished by either glibenclamide or 25 mM potassium.8. These data indicate that in the femoral artery, acetylcholine-induced relaxation can be attributed solely to the release of nitric oxide from the endothelium, which then stimulates relaxation independently of a change in smooth muscle membrane potential. In contrast, both the relaxation and hyperpolarization evoked by A23187 appear to be mediated predominantly by nitric oxide-independent pathways which appear to involve a diffusible factor released from the endothelium. The results suggest that this diffusible hyperpolarizing factor can be released from endothelial cells in the femoral artery by A23187 but not by acetylcholine.

Endothelium-dependent hyperpolarization: a role in the control of vascular tone

Endothelial-dependent relaxation of vascular smooth muscle cells evoked by a number of agonists, including cholinomimetics and substance P, is often accompanied by an increase (repolarization and/or hyperpolarization) in the membrane potential. This change in membrane potential appears predominantly to reflect the action of an endothelial-derived hyperpolarizing factor (EDHF), which is distinct from NO (or endothelial-derived relaxing factor), and is discussed in this article by Chris Garland and colleagues. In large conducting arteries, EDHF may provide a secondary system to NO, which assumes primary importance in some disease states such as pulmonary hypertension and atherosclerosis. However, in small resistance arteries (100-300 microns), EDHF appears to be a major determinant of vascular calibre under normal conditions, and may therefore be of primary importance in the regulation of vascular resistance.

The relative importance of nitric oxide and nitric oxide-independent mechanisms in acetylcholine-evoked dilatation of the rat mesenteric bed

1. The relative contribution of nitric oxide (NO) to acetylcholine-induced smooth muscle relaxation was investigated in the rat perfused mesenteric vasculature and in isolated segments of second, third and fourth order arterial branches. 2. The EC50 values and maximal relaxation to acetylcholine were not significantly different in the sequential arterial branches, being approximately 0.05 microM and 85%, respectively. 3. The NO synthase inhibitor L-NG-nitro-L-arginine methyl ester (L-NAME; 100 microM) reduced acetylcholine-evoked endothelium-dependent dilatation and relaxation in the perfused mesenteric bed and in isolated arterial segments. The maximum response to acetylcholine in both preparations was reduced by between 35% to 40% while the EC50 values were increased by 5-6 fold. L-NAME had no effect on basal smooth muscle tone in either case. 4. In contrast, endothelium-dependent dilatation of the perfused mesenteric bed evoked by A23187 (0.002-20 nmol), was unaffected by exposure to L-NAME. The EC50 values and maximal responses elicited by A23187 (20 nmol) before and after exposure to L-NAME were 0.96 +/- 0.5 nmol and 67.0 +/- 7.0% (n = 4), and 0.7 +/- 0.4 nmol and 70.0 +/- 5.0% (n = 4; P > 0.01), respectively. 5. Perfusion of the isolated mesenteric bed with raised K(+)-Krebs buffer (25 mM) had no effect on basal tone, but reduced the amplitude of both acetylcholine- and A23187-evoked dilatation. The maximum responses to acetylcholine (2 micromol) and A23187 (20 nmol) were reduced from 67.5 +/- 7.3% and 65.4+/-8.2% to 18.9 +/-11.0% (n=5; P<0.01) and 13.5 +/-12.0% (n=4; P<0.01), respectively.6. Exposure of the mesenteric bed to L-NAME in the presence of raised K+-Krebs further reduced the maximal response elicited by acetylcholine to only 8.9 +/- 2.8% (n =4; P< 0.01).7. These results indicate that acetylcholine-evoked vasodilatation of the rat mesenteric vasculature is mediated by both NO-dependent and -independent mechanisms. The relative contribution made by these mechanisms does not appear to differ in sequential branches of the mesenteric artery. In contrast,A23187-evoked vasodilatation appears to be mediated predominantly by a NO-independent mechanism which is sensitive to increases in the extracellular potassium concentration and may reflect the action of endothelium-derived hyperpolarizing factor (EDHF).

Smooth muscle hyperpolarization and relaxation to acetylcholine in the rabbit basilar artery

Acetylcholine-evoked relaxation of noradrenaline-stimulated segments of the rabbit basilar artery was accompanied by a small, transient hyperpolarization of the smooth muscle cell membrane which was diminished by repeated exposure to the agonist. In the presence of glibenclamide (10 microM) or high concentrations of potassium chloride (65 mM), the acetylcholine-evoked smooth muscle hyperpolarization was abolished, whereas the relaxation response was unaffected. Nitric oxide (NO gas in solution; 0.5-15 microM) evoked dose-dependent relaxation in noradrenaline contracted arterial segments, but had no effect on the smooth muscle membrane potential, even at a saturated concentration (150 microM), which was 10 times higher than required to stimulate maximal relaxation. Additionally, NO-evoked relaxations were unaffected by glibenclamide (10 microM), but the responses were significantly attenuated in the presence of 65 mM potassium chloride. These data show that, as in the rabbit middle cerebral artery, acetylcholine-evoked hyperpolarization in the rabbit basilar artery is mediated by glibenclamide-sensitive potassium channels. However, in contrast to the middle cerebral artery and to other vessels such as the rat mesenteric artery, the change in smooth muscle membrane potential does not make an important contribution to the relaxation evoked either by this agonist or by NO.

Contribution of both nitric oxide and a change in membrane potential to acetylcholine-induced relaxation in the rat small mesenteric artery

1. Acetylcholine stimulated repolarization and relaxation in isolated segments of rat small mesenteric artery (D100 = 325 +/- 9 microns) in which the smooth muscle cells were depolarized and contracted by submaximal concentrations of noradrenaline (0.75-2.5 microM). There was no significant difference either in the time taken to initiate relaxation or hyperpolarization, or for these parameters to reach maximum in response to acetylcholine. 2. The nitric oxide synthase inhibitor, NG-nitro L-arginine methyl ester (L-NAME, 100 microM) reduced the pD2 for acetylcholine-induced relaxation from 7.5 to 7 and depressed the maximum relaxation from 89% to 68% in tissues stimulated with noradrenaline. The pD2 for smooth muscle repolarization in these experiments was also reduced (7.4 to 6.6) but the maximum change in membrane potential in response to acetylcholine was unaltered. The increase in potential now clearly preceded relaxation by 3.7 s (to initiation) and 4.7 s (to maximum). 3. In the presence of noradrenaline and a raised potassium concentration (25 mM), the repolarization to acetylcholine was markedly attenuated. Simultaneous tension measurements also revealed a marked reduction in the maximal relaxation to acetylcholine, but the pD2 was unchanged at 7.4. 4. The residual relaxation recorded in the absence of marked repolarization (in the presence of noradrenaline and 25 mM potassium) was abolished by the addition of 100 microM L-NAME. 5. Nitric oxide gas in solution (0.2-2.2 microM; NOg) relaxed artery segments precontracted with noradrenaline. The magnitude of relaxation to NOg was not altered in the presence of noradrenaline and 25 mM potassium. 6. These data provide additional evidence that acetylcholine-evoked endothelium-dependent increases in membrane potential provide a major mechanism for smooth muscle relaxation in the mesenteric artery.They also show that voltage-dependent and independent (initiated by NO) mechanisms can both contribute to relaxation, and suggest that NO may modulate the increase in membrane potential or the release of a hyperpolarizing factor.

Phorbol esters impair endothelium-dependent and independent relaxation in rat aortic rings

1. This study examined the ability of various nitro-vasodilators, 8-bromo cyclic guanosine 3':5' monophosphate (8-BrcGMP) and forskolin to relax rings of rat thoracic aorta pre-contracted with either noradrenaline (0.1 microM) or the protein kinase C activators, phorbol 12,13-dibutyrate (PDB, 0.1 microM) or phorbol 12-myristate 13-acetate (PMA, 0.5 microM). 2. In noradrenaline pre-contracted rings, acetylcholine (10 nM-10 microM), sodium nitroprusside (1 nM-0.5 microM), the calcium ionophore A23187 (10 nM-10 microM) and 8-BrcGMP (10 mM) totally reversed the smooth muscle contraction. In PDB-contracted aortic rings acetylcholine, sodium nitroprusside and 8-BrcGMP-induced relaxation was reduced compared to that in noradrenaline-contracted aortic rings, but A23187 and forskolin-induced relaxations were unaffected. Both acetylcholine and A23187-induced relaxations in PDB-contracted rings were abolished in the presence of the nitric oxide synthesis inhibitor N omega-nitro-L-arginine (NOLA, 100 microM). 3. Acetylcholine and sodium nitroprusside were even less potent in their ability to relax PMA-contracted aortic rings compared with noradrenaline and PDB-contracted rings. A23187-induced relaxation was also inhibited in PMA-contracted rings. 4. These results show that protein kinase C activation reduces the ability of agents which liberate nitric oxide to induce smooth muscle relaxation, and also inhibits the biochemical pathways which are subsequently activated by nitric oxide and lead to vascular smooth muscle relaxation.

Importance of inositol (1,4,5)-trisphosphate, intracellular Ca2+ release and myofilament Ca2+ sensitization in 5-hydroxytryptamine-evoked contraction of rabbit mesenteric artery

1. Small strips from third-order branches of rabbit mesenteric artery (approximately 150-200 microM wide) contracted in response to noradrenaline (10 microM) or 5-hydroxytryptamine (5-HT; 10 microM) in oxygenated Krebs solution containing 2.5 mM Ca2+. In a Ca(2+)-free mock intracellular solution (0 Ca2+ plus 0.2 mM EGTA), noradrenaline (10 microM) and caffeine (10 mM) induced only a single, transient contraction in artery strips, while 5-HT (10 microM) failed to induce any response. 2. In strips of mesenteric artery which had been permeabilized with Staphylococcus alpha-toxin and bathed in Ca(2+)-free mock intracellular solution, noradrenaline (10 microM), caffeine (10 mM) and D-myo-inositol (1,4,5)-trisphosphate (IP3, 100 microM), but not 5-HT (10 or 100 microM) induced a transient contraction. In contrast to the non-permeabilized strips, contractions to noradrenaline, caffeine and IP3 were restored by prior incubation (10 min) in solution containing 0.08 microM Ca2+. The contractions to noradrenaline and IP3 in permeabilized muscle strips required the presence of 100 microM guanosine 5'-triphosphate (GTP), although in the absence of Ca2+. GTP alone did not induce contraction. 3. Exposure of permeabilized mesenteric artery strips to IP3 significantly reduced the subsequent contractile responses to caffeine. Contractile responses to caffeine and IP3 were abolished by the Ca(2+)-ATPase inhibitor, thapsigargin (1 microM). 4. Ca2+ (0.1-10 microM) induced concentration-dependent contraction in permeabilized artery strips. In strips which were submaximally contracted with 0.5 microM Ca2+/100 microM GTP, the subsequent addition of 5-HT (10 microM) stimulated further contraction. The protein kinase C inhibitor, H-7 (1 microM) abolished the 5-HT/GTP-induced contraction, but did not alter the contraction to Ca2+. 5. In non-permeabilized, endothelium-denuded segments of rabbit mesenteric artery bathed in Ca2+-replete Krebs solution, noradrenaline (10 microM) stimulated a rapid, transient accumulation of IP3. 5-HT(100 microM) failed to stimulate IP3 accumulation during exposure periods of up to 5 min. 5-HT (100 microM)did stimulate IP3 accumulation if the external K+ concentration was raised (to around 25 mM). This concentration of K+ alone did not stimulate IP3 production and the 5-HT-stimulated IP3 accumulation in the presence of elevated extracellular [K+] was abolished by the alpha l-adrenoceptor antagonist, prazosin(O.1 microM).6. These results suggest that intracellular Ca2+ release does not play an important role in 5-HT-induced smooth muscle contraction in the rabbit mesenteric artery. This is despite the fact that a significant intracellular Ca2+ pool is present in these cells, which can be discharged by either noradrenaline or IP3.However, 5-HT did stimulate smooth muscle contraction in the presence of raised intracellular calcium,suggesting that a component of the contraction to 5-HT will reflect an increase in myofilament Ca2+sensitivity, possibly due to the activation of protein kinase C.

Differential effects of acetylcholine, nitric oxide and levcromakalim on smooth muscle membrane potential and tone in the rabbit basilar artery

1. Endothelium-dependent hyperpolarization of smooth muscle cells in isolated, pre-contracted segments of rabbit basilar artery in response to acetylcholine (100 microM) was abolished in the presence of glibenclamide (10 microM). 2. Acetylcholine-evoked relaxation was unaffected by either glibenclamide or 65 mM potassium chloride, indicating that the change in membrane potential did not form an essential component of relaxation and that high concentrations of potassium did not inhibit the release or action of endothelium-derived relaxing factor in this vessel. 3. Saturated solutions of nitric oxide (NO) gas in solution (150 microM), which evoked maximal relaxation of arterial segments pre-contracted and depolarized by noradrenaline (10-100 microM), did not alter the membrane potential of either unstimulated or depolarized smooth muscle cells. 4. The potassium channel opener levcromakalim, evoked concentration-dependent relaxation and hyperpolarization in pre-constricted smooth muscle cells. The threshold concentrations for hyperpolarization and relaxation, the EC50 values and the maximally effective concentration of levcromakalim (around 30 nM, 150 nM and 10 microM, respectively) were not significantly different, and both components of the response were inhibited by glibenclamide (10 microM), indicating a close coupling between the two responses. 5. In the presence of 65 mM potassium chloride, the hyperpolarization to levcromakalim was abolished, while a small relaxation (25 +/- 4%) persisted, indicating an additional mechanism for relaxation to this agent. 6. These results show that different mechanisms underlie the relaxant action of potassium channel openers, NO and endothelium-derived factors in cerebral arteries and provide further evidence that in the basilar artery, in contrast to some other vessels, endothelium-dependent hyperpolarization to acetylcholine is not important for smooth muscle relaxation.

Ca2+ channel antagonists and inhibition of protein kinase C each block contraction but not depolarization to 5-hydroxytryptamine in the rabbit basilar artery

The Ca2+ channel antagonists nifedipine and verapamil each significantly inhibited (50-100%) the smooth muscle contraction induced in response to either 5-hydroxytryptamine (1 microM, 5-HT) or 20 mM K+ (K(+)-physiological salt solution) in the basilar artery. Simultaneous measurements of smooth muscle membrane potential showed that changes in potential were not modified at this time. A similar inhibitory action against the smooth muscle contraction but not the depolarization to 5-HT was obtained with the putative protein kinase C and phospholipase C inhibitors, 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (10 microM, H7) and 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (70 microM, NCDC). These data indicate that 5-HT-induced Ca2+ influx through voltage sensitive channels is important for smooth muscle contraction but not depolarization in the rabbit basilar artery.

Electrophysiology of cerebral blood vessels

In spite of the relatively large amount of in vitro and in vivo data indicating that, in a number of ways, cerebral arteries are pharmacologically different from peripheral arteries, the mechanisms responsible for these differences are far from clear. An understanding of these mechanisms is particularly important for a rational approach to the treatment of disorders of the cerebral circulation including migraine, hypertension and the responses of cerebral vessels to subarachnoid haemorrhage. This review outlines electrophysiological data which are available from cerebrovascular smooth muscle cells, including the possibility that inwardly-rectifying potassium channels, active at potentials close to the resting membrane potential, are intimately involved in the changes in smooth muscle tone which couple blood flow to regional changes in nerve cell activity. The membrane potential changes in response to perivascular nerve stimulation, noradrenaline, 5-hydroxytryptamine and endothelium-derived hyperpolarizing factor are also described, together with the underlying membrane mechanisms and their relationship to smooth muscle contraction and relaxation.

Endothelium-dependent relaxation to acetylcholine in the rabbit basilar artery: importance of membrane hyperpolarization

1. Muscarinic stimulation of isolated, preconstricted segments of the basilar artery, with either acetylcholine or carbachol, was followed by endothelium-dependent smooth muscle relaxation and membrane hyperpolarization. 2. Smooth muscle relaxation to acetylcholine was stimulated in the presence of lower concentrations than the associated hyperpolarization (EC50 values 3.2 microM and 31.6 microM, respectively), and was sustained during agonist application, while the hyperpolarization was relatively transient. 3. Repeated exposure to acetylcholine was associated with loss of membrane hyperpolarization, while smooth muscle relaxation was unaltered. Following a second exposure to 100 microM acetylcholine, mean hyperpolarization was markedly depressed from 8.5 to 2 mV, and subsequent exposures failed to induce any hyperpolarization. Relaxations with a similar amplitude and rate of development, were recorded with each subsequent addition of acetylcholine. 4. The competitive substrate inhibitors for nitric oxide synthase, L-NG-monomethyl arginine (100 microM L-NMMA) or L-NG-nitro arginine methyl ester (100 microM L-NAME), modified the form and amplitude of both the relaxation and the hyperpolarization to acetylcholine. In the majority of experiments, both the hyperpolarization and the relaxation were almost totally abolished. 5. Neither nitric oxide, applied directly in physiological salt solution, nor sodium nitroprusside, produced smooth muscle hyperpolarization except in high concentrations. Reproducible, small amplitude (around 2 mV) hyperpolarization followed the application of either NO gas (15 microM) or sodium nitroprusside (100 microM), both of which induced almost maximal smooth muscle relaxation. 6. These data show that muscarinic stimulation of endothelial cells in the rabbit basilar artery is followed by both smooth muscle hyperpolarization and relaxation. They indicate that nitric oxide is involved in both of these responses, but that the smooth muscle hyperpolarization is not an essential component of the relaxation.

Endothelium-dependent contractile responses to 5-hydroxytryptamine in the rabbit basilar artery

1 5-Hydroxytryptamine (5-HT) and 5-carboxamidotryptamine (5-CT) stimulated additional, endothelium-dependent contractions in rabbit isolated basilar arteries which had been submaximally contracted with either histamine or potassium chloride. 2 The additional contractions to 5-HT were not altered by the 5-HT2 antagonist, ketanserin (1 microM), but were abolished in the presence of the cyclo-oxygenase inhibitor indomethacin (3 microM). 3 The additional smooth muscle contraction stimulated by 5-HT was increased in the presence of the competitive substrate inhibitor for nitric oxide synthase, NG-nitro-L-arginine methyl ester (L-NAME, 100 microM). 4 Neither of the selective 5-HT agonists, 8-hydroxy-dipropylaminotetralin (8-OH DPAT) or alpha-methyl 5-HT stimulated endothelium-dependent contraction, but these agonists did reduce the rate at which histamine-induced tension spontaneously declined. This effect represented a direct action on the smooth muscle cells, as it was independent of the presence of endothelial cells. 5 Smooth muscle relaxation was not obtained in response to 5-HT, whether or not indomethacin was present to block endothelium-dependent contraction. None of the other selective 5-HT agonists, 5-CT, 8-OH DPAT or alpha-methyl 5-HT produced endothelium-dependent smooth muscle relaxation, when applied against a background of contraction. 6 These data show that endothelium-dependent smooth muscle contraction can be produced by stimulating 5-HT receptors in the partially contracted rabbit basilar artery. Similar contraction to 5-CT indicates an involvement by 5-HT1 receptors. The susceptibility of the contractions to indomethacin suggest they are mediated by a metabolite of arachidonic acid.