Role of CA(2+)-activated K+ channels in the regulation of basilar arterial tone in spontaneously hypertensive rats

BK Channels in Tail Artery Vascular Smooth Muscle Cells of Normotensive (WKY) and Hypertensive (SHR) Rats Possess Similar Calcium Sensitivity But Different Responses to the Vasodilator Iloprost

It has been reported that, in the spontaneously hypertensive rat (SHR) model of hypertension, different components of the G-protein/adenylate cyclase (AC)/Calcium-activated potassium channel of high conductance (BK) channel signaling pathway are altered differently. In the upstream part of the pathway (G-protein/AC), a comparatively low efficacy has been established, whereas downstream BK currents seem to be increased. Thus, the overall performance of this signaling pathway in SHR is elusive. For a better understanding, we focused on one aspect, the direct targeting of the BK channel by the G-protein/AC pathway and tested the hypothesis that the comparatively low AC pathway efficacy in SHR results in a reduced agonist-induced stimulation of BK currents. This hypothesis was investigated using freshly isolated smooth muscle cells from WKY and SHR rat tail artery and the patch-clamp technique. It was observed that: (1) single BK channels have similar current-voltage relationships, voltage-dependence and calcium sensitivity; (2) BK currents in cells with a strong buffering of the BK channel activator calcium have similar current-voltage relationships; (3) the iloprost-induced concentration-dependent increase of the BK current is larger in WKY compared to SHR; (4) the effects of activators of the PKA pathway, the catalytic subunit of PKA and the potent and selective cAMP-analogue Sp-5,6-DCl-cBIMPS on BK currents are similar. Thus, our data suggest that the lower iloprost-induced stimulation of the BK current in freshly isolated rat tail artery smooth muscle cells from SHR compared with WKY is due to the lower efficacy of upstream elements of the G-Protein/AC/BK channel pathway.

Alcohol and pregnenolone interaction on cerebral arteries through targeting of vascular smooth muscle Ca2+- and voltage-gated K+ channels of big conductance

Despite the significant number of people who may be taking pregnenolone supplements while drinking alcohol (ethanol), the widely documented cerebrovascular actions of pregnenolone and ethanol, and the critical dependence of cerebrovascular function on cerebral artery diameter, there are no studies addressing the effect of pregnenolone + ethanol in combination on cerebral artery diameter. We investigated this by evaluating the effect of this combination on middle cerebral artery diameter in male and female C57BL/6J mice, both in vivo and in vitro. The use of de-endothelialized, in vitro pressurized middle cerebral artery segments allowed us to conduct a concentration-response study of constriction induced by pregnenolone ± ethanol, in which drug action could be evaluated independently of circulating and endothelial factors. In both male and female animals, pregnenolone at lower concentrations (≤1 μM) was found to synergize with 50 mM ethanol to cause vasoconstriction. In both sexes, this synergism was lost as one or both vasoconstrictors approached their maximally effective concentrations (75 mM and 10 μM for ethanol and pregnenolone, respectively), whether this was evaluated in vitro or in vivo using a cranial window. Vasoconstriction by pregnenolone + ethanol was abolished by 1 μM paxilline, indicating BK channel involvement. Moreover, cell-free recordings of BK channel activity in cerebral artery myocyte membranes showed that 10 μM pregnenolone and pregnenolone +50 mM ethanol reduced channel activity to an identical extent, suggesting that these drugs inhibit cerebrovascular BK channels via a common mechanism or mechanisms. Indeed, pregnenolone was found to disrupt allosteric coupling to C a 2 + -driven gating, as previously reported for ethanol.

High Salt Upregulates Ca2+-Sensing Receptor Expression and Ca2+-Induced Relaxation of Contracted Mesenteric Arteries from Dahl Salt-Sensitive Rats

High Ca2+ lowers blood pressure in hypertension, but the mechanism is not clear. The missing link may be the perivascular sensory nerve Ca2+-sensing receptor (CaSR) that mediates a vasodilator system after activation by interstitial Ca2+ Our results show that high salt increased CaSR expression in mesenteric arteries as well as Ca2+ relaxation of contracted mesenteric arteries from salt-sensitive (SS) rats. The CaSR was expressed as a doublet (≈120-150 kDa) in arteries from animals fed a high-salt diet for 1-4 weeks. The higher molecular weight glycosylated protein increased in arteries from SS animals; however, expression of the low molecular mass high-mannose protein decreased over 4 weeks of feeding the diet. In tissues from salt-resistant (SR) rats, the diet decreased CaSR expression after 4 weeks. Ca2+ relaxation of mesenteric arteries under phenylephrine tone increased in SS rats but decreased in arteries from SR rats fed the high-salt diet. Ca2+-activated K+ channels have a larger role in Ca2+ relaxation of arteries in SR than SS rats. The data suggest that high salt epigenetically regulates the receptor at the translational level in vivo and that the in vitro effect of Ca2+ is on receptor trafficking and signaling. In conclusion, upregulated expression of the CaSR in salt sensitivity increased receptor-mediated vascular relaxation. These findings show that CaSR signaling may compensate for changes in the vasculature in salt-sensitive hypertension. SIGNIFICANCE STATEMENT: The perivascular sensory nerve Ca2+-sensing receptor (CaSR) mediates Ca2+ relaxation of isolated mesenteric arteries under tension. This receptor may therefore play a significant role in relaxation of resistance arteries in vivo, thus explaining the blood pressure-lowering effect of dietary Ca2+. The present studies describe the effect of high salt-induced upregulation of the CaSR in salt-sensitive rats and the roles played by Ca2+-activated K+ channels and nitric oxide in Ca2+ responses.

Celastrol Dilates and Counteracts Ethanol-Induced Constriction of Cerebral Arteries

The increasing recognition of the role played by cerebral artery dysfunction in brain disorders has fueled the search for new cerebrovascular dilators. Celastrol, a natural triterpene undergoing clinical trials for treating obesity, exerts neuroprotection, which was linked to its antioxidant/anti-inflammatory activities. We previously showed that celastrol fit pharmacophore criteria for activating calcium- and voltage-gated potassium channels of large conductance (BK channels) made of subunits cloned from cerebrovascular smooth muscle (SM). These recombinant BK channels expressed in a heterologous system were activated by celastrol. Activation of native SM BK channels is well known to evoke cerebral artery dilation. Current data demonstrate that celastrol (1-100 µM) dilates de-endothelialized, ex vivo pressurized middle cerebral arteries (MCAs) from rats, with EC₅₀ = 45 µM and maximal effective concentration (E_max)= 100 µM and with MCA diameter reaching a 10% increase over vehicle-containing, time-matched values (P < 0.05). A similar vasodilatory efficacy is achieved when celastrol is probed on MCA segments with intact endothelium. Selective BK blocking with 1 μM paxilline blunts celastrol vasodilation. Similar blunting is achieved with 0.8 mM 4-aminopirydine, which blocks voltage-gated K⁺ channels other than BK. Using an in vivo rat cranial window, we further demonstrate that intracarotid injections of 45 μM celastrol into pial arteries branching from MCA mimics celastrol ex vivo action. MCA constriction by ethanol concentrations reached in blood during moderate-heavy alcohol drinking (50 mM), which involves SM BK inhibition, is both prevented and reverted by celastrol. We conclude that celastrol could be an effective cerebrovascular dilator and antagonist of alcohol-induced cerebrovascular constriction, with its efficacy being uncompromised by conditions that disrupt endothelial and/or BK function. SIGNIFICANCE STATEMENT: Our study demonstrates for the first time that celastrol significantly dilates rat cerebral arteries both ex vivo and in vivo and both prevents and reverses ethanol-induced cerebral artery constriction. Celastrol actions are endothelium-independent but mediated through voltage-gated (K_V) and calcium- and voltage-gated potassium channel of large conductance (BK) K⁺ channels. This makes celastrol an appealing new agent to evoke cerebrovascular dilation under conditions in which endothelial and/or BK channel function are impaired.

Differential participation of calcium-activated potassium channel in endothelium-dependent hyperpolarization-type relaxation in superior mesenteric arteries of spontaneously hypertensive rats

The purpose of this study was to determine the relationship of K_Ca channels to endothelium-dependent hyperpolarizing factor (EDHF)-mediated relaxation induced by acetylcholine (ACh) in the superior mesenteric arteries of 7-month-old spontaneously hypertensive rats (SHR). Upon inhibition of nitric oxide synthase and cyclooxygenase, ACh-induced EDHF-mediated relaxation was found to be weaker in SHR than in age-matched Wistar Kyoto rats (WKY). These relaxations in both group were attenuated by combined treatment with small-conductance and intermediate-conductance Ca²⁺-activated K⁺ channels (SK_Ca and IK_Ca) inhibitors, with the exception of relaxation resistant to inhibition of these channels in SHR (vs. WKY). Treatment with large-conductance Ca²⁺-activated K⁺ channels (BK_Ca) inhibitor specifically attenuated relaxation in SHR, but not in WKY. Protein expression of IK_Ca and SK_Ca in the arteries did not differ between the 2 groups, whereas ratio of sloβ1 subunit to α subunit of BK_Ca was increased in SHR (vs. WKY). These results suggest that EDHF-mediated relaxations in superior mesenteric arteries are impaired in SHR, and utilize components of BK_Ca in addition to SK_Ca/IK_Ca channel activities, that the increased participation of BK_Ca may be attributable to alterations in α and sloβ1 subunit ratio, and that components unrelated to K_Ca activity may also contribute to the difference between SHR and WKY arteries.

Alterations in vasoconstrictor responses to the endothelium-derived contracting factor uridine adenosine tetraphosphate are region specific in DOCA-salt hypertensive rats

Uridine adenosine tetraphosphate (Up(4)A) has been recently identified as a novel and potent endothelium-derived contracting factor and contains both purine and pyrimidine moieties, which activate purinergic P2X and P2Y receptors. The present study was designed to compare contractile responses to Up(4)A and other nucleotides such as ATP (P2X/P2Y agonist), UTP (P2Y(2)/P2Y(4) agonist), UDP (P2Y(6) agonist), and α,β-methylene ATP (P2X(1) agonist) in different vascular regions [thoracic aorta, basilar, small mesenteric, and femoral arteries] from deoxycorticosterone acetate-salt (DOCA-salt) and control rats. In DOCA-salt rats [vs. control uninephrectomized (Uni) rats]: (1) in thoracic aorta, Up(4)A-, ATP-, and UTP-induced contractions were unchanged; (2) in basilar artery, Up(4)A-, ATP-, UTP- and UDP-induced contractions were increased, and expression for P2X(1), but not P2Y(2) or P2Y(6) was decreased; (3) in small mesenteric artery, Up(4)A-induced contraction was decreased and UDP-induced contraction was increased; expression of P2Y(2) and P2X(1) was decreased whereas P2Y(6) expression was increased; (4) in femoral artery, Up(4)A-, UTP-, and UDP-induced contractions were increased, but expression of P2Y(2), P2Y(6) and P2X(1) was unchanged. The α,β-methylene ATP-induced contraction was bell-shaped and the maximal contraction was reached at a lower concentration in basilar and mesenteric arteries from Uni rats, compared to arteries from DOCA-salt rats. These results suggest that Up(4)A-induced contraction is heterogenously affected among various vascular beds in arterial hypertension. P2Y receptor activation may contribute to enhancement of Up(4)A-induced contraction in basilar and femoral arteries. These changes in vascular reactivity to Up(4)A may be adaptive to the vascular alterations produced by hypertension.

Mitochondrial monoamine oxidase-A-mediated hydrogen peroxide generation enhances 5-hydroxytryptamine-induced contraction of rat basilar artery

BACKGROUND AND PURPOSE

We evaluated the role(s) of monoamine oxidase (MAO)-mediated H₂O₂ generation on 5-hydroxytryptamine (5-HT)-induced tension development of isolated basilar artery of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats.

EXPERIMENTAL APPROACH

Basilar artery (endothelium-denuded) was isolated for tension measurement and Western blots. Enzymically dissociated single myocytes from basilar arteries were used for patch-clamp electrophysiological and confocal microscopic studies.

KEY RESULTS

Under resting tension, 5-HT elicited a concentration-dependent tension development with a greater sensitivity (with unchanged maximum tension development) in SHR compared with WKY (EC(50) : 28.4 ± 4.1 nM vs. 98.2 ± 9.4 nM). The exaggerated component of 5-HT-induced tension development in SHR was eradicated by polyethylene glycol-catalase, clorgyline and citalopram whereas exogenously applied H₂O₂ enhanced the 5-HT-elicited tension development in WKY. A greater protein expression of MAO-A was detected in basilar arteries from SHR than in those from WKY. In single myocytes and the entire basilar artery, 5-HT generated (clorgyline-sensitive) a greater amount of H₂O₂ in SHR compared with WKY. Whole-cell iberiotoxin-sensitive Ca(2+) -activated K(+) (BK(Ca) ) amplitude measured in myocytes of SHR was approximately threefold greater than that in WKY (at +60 mV: 7.61 ± 0.89 pA·pF(-1) vs. 2.61 ± 0.66 pA·pF(-1) ). In SHR myocytes, 5-HT caused a greater inhibition (clorgyline-, polyethylene glycol-catalase- and reduced glutathione-sensitive) of BK(Ca) amplitude than in those from WKY.

CONCLUSIONS AND IMPLICATIONS

5-HT caused an increased generation of mitochondrial H₂O₂ via MAO-A-mediated 5-HT metabolism, which caused a greater inhibition of BK(Ca) gating in basilar artery myocytes, leading to exaggerated basilar artery tension development in SHR.

Loss of cerebrovascular Shaker-type K(+) channels: a shared vasodilator defect of genetic and renal hypertensive rats

The cerebral arteries of hypertensive rats are depolarized and highly myogenic, suggesting a loss of K(+) channels in the vascular smooth muscle cells (VSMCs). The present study evaluated whether the dilator function of the prominent Shaker-type voltage-gated K(+) (K(V)1) channels is attenuated in middle cerebral arteries from two rat models of hypertension. Block of K(V)1 channels by correolide (1 micromol/l) or psora-4 (100 nmol/l) reduced the resting diameter of pressurized (80 mmHg) cerebral arteries from normotensive rats by an average of 28 +/- 3% or 26 +/- 3%, respectively. In contrast, arteries from spontaneously hypertensive rats (SHR) and aortic-banded (Ao-B) rats with chronic hypertension showed enhanced Ca(2+)-dependent tone and failed to significantly constrict to correolide or psora-4, implying a loss of K(V)1 channel-mediated vasodilation. Patch-clamp studies in the VSMCs of SHR confirmed that the peak K(+) current density attributed to K(V)1 channels averaged only 5.47 +/- 1.03 pA/pF, compared with 9.58 +/- 0.82 pA/pF in VSMCs of control Wistar-Kyoto rats. Subsequently, Western blots revealed a 49 +/- 7% to 66 +/- 7% loss of the pore-forming alpha(1.2)- and alpha(1.5)-subunits that compose K(V)1 channels in cerebral arteries of SHR and Ao-B rats compared with control animals. In each case, the deficiency of K(V)1 channels was associated with reduced mRNA levels encoding either or both alpha-subunits. Collectively, these findings demonstrate that a deficit of alpha(1.2)- and alpha(1.5)-subunits results in a reduced contribution of K(V)1 channels to the resting diameters of cerebral arteries from two rat models of hypertension that originate from different etiologies.

Enalapril treatment alters the contribution of epoxyeicosatrienoic acids but not gap junctions to endothelium-derived hyperpolarizing factor activity in mesenteric arteries of spontaneously hypertensive rats

Reduction in endothelium-derived hyperpolarizing factor (EDHF)-mediated dilatory function in large, elastic arteries during hypertension is reversed after blood pressure normalization. We investigated whether similar mechanisms occurred in smaller mesenteric resistance arteries from aged Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHRs), and SHRs treated with the angiotensin-converting enzyme inhibitor, enalapril, using immunohistochemistry, serial-section electron microscopy, electrophysiology and wire myography. Unlike the superior mesenteric artery, EDHF relaxations in muscular mesenteric arteries were not reduced in SHRs, although morphological differences were found in the endothelium and smooth muscle. In WKY rats, SHRs and enalapril-treated SHRs, relaxations were mediated by small-, large-, and intermediate-conductance calcium-activated potassium channels, which were distributed in the endothelium, smooth muscle, and both layers, respectively. However, only WKY hyperpolarizations and relaxations were sensitive to gap junction blockers, and these arteries expressed more endothelial and myoendothelial gap junctions than arteries from SHRs. Responses in WKY rats, but not SHRs, were also reduced by inhibitors of epoxyeicosatrienoic acids (EETs), 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE) and miconazole, although sensitivity to EET regioisomers was endothelium-independent in all rats. Enalapril treatment of SHRs reduced blood pressure and restored sensitivity to 14,15-EEZE, but not to gap junction blockers, and failed to reverse the morphological changes. In conclusion, the mechanisms underlying EDHF in muscular mesenteric arteries differ between WKY rats and SHRs, with gap junctions and EETs involved only in WKY rats. However, reduction of blood pressure in SHRs with enalapril restored a role for EETs, but not gap junctions, without reversing morphological changes, suggesting a differential control of chemical and structural alterations.

Effects of K+ channel modulators on oscillatory contractions in sinoaortic denervated rat aortas

Sinoaortic denervated (SAD) rats present arterial pressure lability without sustained hypertension. We investigated the relation between sinoaortic denervation and the occurrence of oscillatory contractions in SAD rat aortas, as well as the effect of various K(+) channel modulators on these oscillations. Aortas were removed and concentration-effect curves to phenylephrine (0.01 to 10 muM) were constructed in arteries from SAD and Sham-operated rats in order to verify the occurrence of oscillations. We also evaluated the effects of various K(+) channel modulators on these oscillations. Only SAD rat aortas exhibited oscillatory contractions. Tetraethylammonium increased the frequency (28.5+/-3.5 to 41.5+/-4.5 counts/5 min) and amplitude (0.435+/-0.07 to 0.630+/-0.09 g) of the oscillations. Apamin and 4-aminopyridine did not alter the oscillations. Barium chloride converted the oscillatory contractions to a tonic contraction. Pinacidil rapidly blocked the oscillatory contractions and glibenclamide evoked reduction in amplitude from 0.410+/-0.07 to 0.180+/-0.06 g. Iberiotoxin increased the frequency of oscillatory contractions (from 28.0+/-3.5 to 51.5+/-7.5 counts/5 min) but decreased the amplitude (from 0.410+/-0.08 to 0.195+/-0.2 g). Our results demonstrate that SAD rat aortas exhibit oscillatory contractions and K(+) channels, mainly K(ATP) and BK(Ca), play a dominant role in these oscillations.

ACE inhibition restores the vasodilator potency of the endothelium-derived relaxing factor, L-S-nitrosocysteine, in conscious Spontaneously Hypertensive rats

OBJECTIVE

The major aim of this study was to determine whether the angiotensin converting enzyme (ACE) inhibitors, captopril or enalapril, restore the diminished vasodilator potency of the endothelium-dependent agonist, acetylcholine (ACh), and the endothelium-derived relaxing factor (EDRF), L-S-nitrosocysteine (L-SNC), in conscious Spontaneously Hypertensive (SH) rats.

METHODS

The hemodynamic responses elicited by i.v. injections of ACh, L-SNC, and nitric oxide donors such as MAHMA NONOate, were determined in SH rats treated for 7 days with captopril, enalapril, or the direct vasodilator hydralazine. The effects of captopril, enalapril or hydralazine on oxidant stress levels in blood serum and aorta of WKY and SH rats were also determined.

RESULTS

Captopril, enalapril and hydralazine elicited equivalent falls in mean arterial pressure and systemic vascular resistances in SH rats. ACh- and L-SNC-induced vasodilation were increased in captopril- or enalapril-treated SH rats such that the responses were equal to those in normotensive Wistar Kyoto rats. The attenuated responses of ACh and L-SNC in SH rats were not improved by hydralazine. The vasodilator effects of MAHMA NONOate, which were substantially augmented in SH rats, were not affected by captopril, enalapril or hydralazine. The levels of oxidant stress were markedly reduced in captopril- or enalapril-treated but not hydralazine-treated SH rats.

CONCLUSIONS

The finding that the ACE inhibitors improved the vasodilator potencies of L-SNC and the EDRF released by ACh in SH rats, suggests that the diminished vasodilator potency of these compounds was due to augmented ACE activity, which increased oxidant stress levels. This study provides the first evidence to support the concept that ACE inhibition lowers arterial pressure in SH rats, at least in part, by restoring the vasodilator potency of endothelium-derived L-SNC.

The vasodilator potency of the endothelium-derived relaxing factor, L-S-nitrosocysteine, is impaired in conscious spontaneously hypertensive rats

OBJECTIVE

This study compared the hemodynamic responses elicited by the endothelium-derived relaxing factor (EDRF), L-S-nitrosocysteine (L-SNC), the non-prostanoid EDRF released by acetylcholine (ACh) and nitric oxide (NO)-donors such as MAHMA NONOate, in conscious spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats.

METHODS

The depressor and/or vasodilator responses elicited by intravenous injections of ACh, L-SNC and MAHMA NONOate were determined in adult WKY and SH rats before and after intravenous injection of the NO synthesis inhibitor, N(G)-nitro-L-arginine methylester (L-NAME), or the cyclooxygenase inhibitor, indomethacin.

RESULTS

The responses elicited by ACh and L-SNC were smaller in SH than in WKY rats whereas the responses elicited by MAHMA NONOate were augmented in SH rats. The ACh-induced responses were not diminished after injection of L-NAME in WKY or SH rats. Indomethacin did not affect the responses to any of the vasodilator agents in WKY or SH rats. Addition of L-SNC to whole blood or thoracic aortae from SH rats yielded similar amounts of NO to those of WKY rats.

CONCLUSIONS

The vasodilator potencies of ACh and L-SNC were diminished whereas that of NO was augmented in SH rats. The loss of potency of L-SNC in SH rats was not obviously due to differences in decomposition to NO or the overactivity of cyclooxygenase factors. This study provides the first evidence that diminished endothelium-dependent vasodilation in SH rats may involve a loss of vasodilator potency of endogenous L-SNC.

Relative contribution of Rho kinase and protein kinase C to myogenic tone in rat cerebral arteries in hypertension

Arterial smooth muscle constriction in response to pressure, i.e., myogenic tone, may involve calcium-dependent and calcium-sensitization mechanisms. Calcium sensitization in vascular smooth muscle is regulated by kinases such as PKC and Rho kinase, and activity of these kinases is known to be altered in cardiovascular disorders. In the present study, we evaluated the relative contribution of PKC and Rho kinase to myogenic tone in cerebral arteries in hypertension. Myogenic tone and arterial wall calcium in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) were measured simultaneously, and the effect of PKC and Rho kinase inhibitors on myogenic tone was evaluated. SHR arteries showed significantly greater myogenic tone than WKY arteries. Pressure/wall tension-arterial wall calcium curves showed a hyperbolic relation in WKY rats, but the curves for SHR arteries were parabolic. Myogenic tone was decreased by the Rho kinase inhibitors Y-27632 and HA-1077, with a significantly greater effect in SHR than in WKY arteries. Reduction in myogenic tone produced by the PKC inhibitor bisindolylmaleimide I in WKY and SHR arteries was significantly less than that produced by Rho kinase inhibition. The pressure-dependent increase in myogenic tone was significantly decreased by Y-27632, and the decrease was markedly greater than that produced by bisindolylmaleimide I in SHR arteries. In WKY arteries, the pressure-dependent increase in myogenic tone was decreased to a similar extent by Y-27632 and bisindolylmaleimide I. These results suggest greater myogenic tone with increased calcium sensitization in SHR arteries, largely because of Rho kinase activation, with a minor contribution of PKC activation.

Interaction of BKCa channel modulators with adrenergic agonists in the rat aorta is influenced by receptor reserve

Our main objective was to study the interaction of BKCa channel modulators with adrenergic agonists UK 14304 and noradrenaline (NA), acting on alpha1-adrenoceptors, in the rat aorta and how this is affected by receptor reserve. NA and UK 14304 evoked concentration-dependent contractions of the rat aorta. UK 14304 was a partial agonist relative to NA in this preparation. The BK(Ca) channel blocker tetraethylammonium (TEA, 1 mM) and opener NS 1619 (3 x 10(-5) M) modulated NA- and UK 14304-induced contractions, and were more effective on UK 14304-induced contractions. TEA (1 mM) increased the maximum response to NA and UK 14304 by about 13% and 300%, respectively, while NS 1619 (3 x 10(-5) M) reduced the maximum response to UK 14304 by about 81% compared to 31% for noradrenaline. The effect of TEA on the noradrenaline concentration-response curve was increased after treatment of the aorta with phenoxybenzamine (PBZ), an irreversible alpha1-adrenoceptor antagonist, to reduce receptor reserve. We concluded that the interaction of BKCa channel modulators with alpha1-adrenergic agonists in the rat aorta was influenced by receptor reserve.

Vasomotion: cellular background for the oscillator and for the synchronization of smooth muscle cells

1. Vasomotion is the oscillation of vascular tone with frequencies in the range from 1 to 20 min(-1) seen in most vascular beds. The oscillation originates in the vessel wall and is seen both in vivo and in vitro. 2. Recently, our ideas on the cellular mechanisms responsible for vasomotion have improved. Three different types of cellular oscillations have been suggested. One model has suggested that oscillatory release of Ca2+ from intracellular stores is important (the oscillation is based on a cytosolic oscillator). A second proposed mechanism is an oscillation originating in the sarcolemma (a membrane oscillator). A third mechanism is based on an oscillation of glycolysis (metabolic oscillator). For the two latter mechanisms, only limited experimental evidence is available. 3. To understand vasomotion, it is important to understand how the cells synchronize. For the cytosolic oscillators synchronization may occur via activation of Ca2+-sensitive ion channels by oscillatory Ca2+ release. The ensuing membrane potential oscillation feeds back on the intracellular Ca2+ stores and causes synchronization of the Ca2+ release. While membrane oscillators in adjacent smooth muscle cells could be synchronized through the same mechanism that sets up the oscillation in the individual cells, a mechanism to synchronize the metabolic-based oscillators has not been suggested. 4. The interpretation of the experimental observations is supported by theoretical modelling of smooth muscle cells behaviour, and the new insight into the mechanisms of vasomotion has the potential to provide tools to investigate the physiological role of vasomotion.

Calcium influx pathways in rat CNS pericytes

In central nervous system (CNS), pericytes have been proposed to play a role in broad functional activities including blood-brain barrier, microcirculation, and macrophage activity. However, contractile responses and Ca2+ signaling in CNS pericytes have not been elucidated. The aim of the present study is to investigate contractility and Ca2+ influx pathway in CNS pericytes. CNS pericytes were cultured from rat brain. Contraction of the pericytes in response to various stimuli was evaluated by the change in surface area measured by a light microscope with a digital camera. Reverse transcription and polymerase chain reaction (RT-PCR) was performed to examine the expression of mRNA of alpha-smooth muscle actin. Intracellular Ca2+ was measured using fura-2 fluorescence spectroscopy. A23187 (Ca2+ ionophore), high external K+ (4 x 10(-2) mol/l), endothelin-1, and serotonin induced contraction of CNS pericytes. RT-PCR analysis revealed the expression of alpha-smooth muscle actin in CNS pericytes. Cytosolic Ca2+ ([Ca2+]i) increased after application of high concentration of external K+, tetraethylammonium, and charybdotoxin, which was inhibited by nicardipine and removal of external Ca2+. Angiotensin-II, serotonin, acetylcholine, ATP, and endothelin-1 caused biphasic response in [Ca2+]i. In response to these agents, [Ca2+]i rapidly increased and then decayed to a relatively constant Ca2+ plateau. The Ca2+ plateau was partially inhibited by nicardipine and completely abolished by omission of external Ca2+. After intracellular Ca2+ store was depleted by the removal of external Ca2+ and addition of thapsigargin, reapplication of external Ca2+ evoked increases in [Ca2+]i. These results indicate that CNS pericytes express mRNA of alpha-smooth muscle actin and possess contractile ability. In CNS pericytes, resting membrane potential is regulated by large conductance Ca2+-activated K+ channels and Ca2+ enters into the cells via L-type voltage-dependent Ca2+ channels, agonist-activated Ca2+ permeable channels, and capacitative Ca2+ entry pathways.