A small-conductance charybdotoxin-sensitive, apamin-resistant Ca(2+)-activated K+ channel in aortic smooth muscle cells (A7r5 line and primary culture)

Vasopressin stimulates action potential firing by protein kinase C-dependent inhibition of KCNQ5 in A7r5 rat aortic smooth muscle cells

[Arg(8)]-vasopressin (AVP), at low concentrations (10-500 pM), stimulates oscillations in intracellular Ca(2+) concentration (Ca(2+) spikes) in A7r5 rat aortic smooth muscle cells. Our previous studies provided biochemical evidence that protein kinase C (PKC) activation and phosphorylation of voltage-sensitive K(+) (K(v)) channels are crucial steps in this process. In the present study, K(v) currents (I(Kv)) and membrane potential were measured using patch clamp techniques. Treatment of A7r5 cells with 100 pM AVP resulted in significant inhibition of I(Kv). This effect was associated with gradual membrane depolarization, increased membrane resistance, and action potential (AP) generation in the same cells. The AVP-sensitive I(Kv) was resistant to 4-aminopyridine, iberiotoxin, and glibenclamide but was fully inhibited by the selective KCNQ channel blockers linopirdine (10 microM) and XE-991 (10 microM) and enhanced by the KCNQ channel activator flupirtine (10 microM). BaCl(2) (100 microM) or linopirdine (5 microM) mimicked the effects of AVP on K(+) currents, AP generation, and Ca(2+) spiking. Expression of KCNQ5 was detected by RT-PCR in A7r5 cells and freshly isolated rat aortic smooth muscle. RNA interference directed toward KCNQ5 reduced KCNQ5 protein expression and resulted in a significant decrease in I(Kv) in A7r5 cells. I(Kv) was also inhibited in response to the PKC activator 4beta-phorbol 12-myristate 13-acetate (10 nM), and the inhibition of I(Kv) by AVP was prevented by the PKC inhibitor calphostin C (250 nM). These results suggest that the stimulation of Ca(2+) spiking by physiological concentrations of AVP involves PKC-dependent inhibition of KCNQ5 channels and increased AP firing in A7r5 cells.

Apamin-Sensitive K + Currents Mediate Arachidonic Acid-Induced Relaxations of Rabbit Aorta

Arachidonic acid induces an endothelium-dependent relaxation of the rabbit aorta that is blocked by lipoxygenase inhibitors. The cellular vasodilatory mechanisms activated by arachidonic acid metabolites remain undefined. In rabbit thoracic aortic rings pretreated with indomethacin (10 μmol/L) and contracted with phenylephrine, arachidonic acid (0.1 to 100 μmol/L) induced concentration-dependent relaxations. Maximal relaxations averaged 45±3% and were inhibited by increasing extracellular K + (30 mmol/L, 15±5%; P <0.001) or incubation with apamin (100 nmol/L, 26±7%; P <0.05) but not incubation with charybdotoxin (100 nmol/L, 41±5%). In aortic strips with an intact endothelium that were treated with phenylephrine, arachidonic acid (10 μmol/L) increased the membrane potential from −28.7±1.3 to −37.8±3.0 mV ( P <0.01). Preincubation with apamin did not alter basal membrane potential but inhibited arachidonic acid-induced hyperpolarization (−31.5±1.5 mV). Incubation of rabbit aortic segments with apamin or charybdotoxin did not alter [ 14 C]arachidonic acid metabolism. Whole-cell outward K + currents from isolated rabbit aortic smooth muscle cells averaged 43.0±4.8 pA/pF at 60 mV and were significantly decreased to 35.7±4.2 pA/pF by apamin ( P <0.001). Subsequent addition of charybdotoxin further decreased maximal currents to 14.4±2.3 pA/pF. Addition of 11,12,15-trihydroxyeicosatrienoic acid increased the outward whole-cell K + current. In inside-out patches of aortic smooth muscle, apamin inhibited the calcium activation (100 to 300 nmol/L; P <0.001) of a small-conductance K + channel (≈24 pS). These results suggest that arachidonic acid induces endothelium-dependent hyperpolarization and relaxation of rabbit aorta through activation of smooth muscle, apamin-sensitive K + currents.

Apamin inhibits NO-induced relaxation of the spontaneous contractile activity of the myometrium from non-pregnant women

There is now considerable evidence for the involvement of K+ channels in nitric oxide (NO) induced relaxation of smooth muscles including the myometrium. In order to assess whether apamin-sensitive K+ channels play a role in NO - induced relaxation of the human uterus, we have studied the effect of specific blockers of these channels on the relaxation of myometrium from non-pregnant women. In vitro isometric contractions were recorded in uterine tissues from non-pregnant premenopausal women who had undergone hysterectomy. Apamin (10 nM) and scyllatoxin (10 nM) did not alter spontaneous myometrial contractions. However, 15-min pretreatment of the myometrium strips with apamin completely inhibited relaxation caused by diethylamine-nitric oxide (DEA/NO). The pretreatment with scyllatoxin significantly reduced (about 2.6 times) maximum relaxation of the strips induced by DEA/NO (p < 0.05). These results strongly suggest that, beside Ca2+ and voltage dependent charybdotoxin-sensitive (CTX-sensitive) K+ channels, apamin-sensitive K+ channels are also present in the human non-pregnant myometrium. These channels offer an additional target in the development of new tocolytic agents.

Potassium channels and vascular proliferation

Potassium channels are currently the focus of much attention because of their recently discovered role in the regulation of vascular smooth muscle growth. Dramatic alterations in the expression and activity of K+ channels causing marked changes in the cell's electrical properties accompany enhanced growth of smooth muscle cells (SMCs). These findings indicate that alterations in K+ channel function are important for SMC proliferation. However, the mechanisms by which changes in K+ channel activity influence cellular growth pathways are poorly understood. The emergent electrical properties caused by modulation of K+ channels are associated with marked differences in the spatial and temporal organization of Ca2+ signaling. Thus, changes in K+ channel function may represent a universal mechanism by which Ca2+ signals are targeted towards activation of gene expression and cell growth. As enhanced growth of smooth muscle underlies many cardiovascular diseases and clinical pathologies, the identification of an important role for K+ channels in SMC proliferation indicates a new source of therapeutic targets to regulate proliferative vascular disorders.

Molecular cloning and characterization of the intermediate-conductance Ca(2+)-activated K(+) channel in vascular smooth muscle: relationship between K(Ca) channel diversity and smooth muscle cell function

Recent evidence suggests that functional diversity of vascular smooth muscle is produced in part by a differential expression of ion channels. The aim of the present study was to examine the role of Ca(2+)-activated K(+) channels (K(Ca) channels) in the expression of smooth muscle cell functional phenotype. We found that smooth muscle cells exhibiting a contractile function express predominantly large-conductance ( approximately 200 pS) K(Ca) (BK) channels. In contrast, proliferative smooth muscle cells express predominantly K(Ca) channels exhibiting a much smaller conductance ( approximately 32 pS). These channels are blocked by low concentrations of charybdotoxin (10 nmol/L) but, unlike BK channels, are insensitive to iberiotoxin (100 nmol/L). To determine the molecular identity of this K(+) channel, we cloned a 1.9-kb cDNA from an immature-phenotype smooth muscle cell cDNA library. The cDNA contains an open reading frame for a 425 amino acid protein exhibiting sequence homology to other K(Ca) channels, in particular with mIK1 and hIK1. Expression in oocytes gives rise to a K(+)-selective channel exhibiting intermediate-conductance (37 pS at -60 mV) and potent activation by Ca(2+) (K(d) 120 nmol/L). Thus, we have cloned and characterized the vascular smooth muscle intermediate-conductance K(Ca) channel (SMIK), which is markedly upregulated in proliferating smooth muscle cells. The differential expression of these K(Ca) channels in functionally distinct smooth muscle cell types suggests that K(Ca) channels play a role in defining the physiological properties of vascular smooth muscle.

Fast BK-type channel mediates the Ca(2+)-activated K(+) current in crayfish muscle

The role of the Ca(2+)-activated K(+) current (I(K(Ca))) in crayfish opener muscle fibers is functionally important because it regulates the graded electrical activity that is characteristic of these fibers. Using the cell-attached and inside-out configurations of the patch-clamp technique, we found three different classes of channels with properties that matched those expected of the three different ionic channels mediating the depolarization-activated macroscopic currents previously described (Ca(2+), K(+), and Ca(2+)-dependent K(+) currents). We investigated the properties of the ionic channels mediating the extremely fast activating and persistent I(K(Ca)). These voltage- and Ca(2+)-activated channels had a mean single-channel conductance of approximately 70 pS and showed a very fast activation. Both the single-channel open probability and the speed of activation increased with depolarization. Both parameters also increased in inside-out patches, i.e., in high Ca(2+) concentration. Intracellular loading with the Ca(2+) chelator bis(2-aminophenoxy) ethane-N, N,N',N'-tetraacetic acid gradually reduced and eventually prevented channel openings. The channels opened at very brief delays after the pulse depolarization onset (<5 ms), and the time-dependent open probability was constant during sustained depolarization (< or =560 ms), matching both the extremely fast activation kinetics and the persistent nature of the macroscopic I(K(Ca)). However, the intrinsic properties of these single channels do not account for the partial apparent inactivation of the macroscopic I(K(Ca)), which probably reflects temporal Ca(2+) variations in the whole muscle fiber. We conclude that the channels mediating I(K(Ca)) in crayfish muscle are voltage- and Ca(2+)-gated BK channels with relatively small conductance. The intrinsic properties of these channels allow them to act as precise Ca(2+) sensors that supply the exact feedback current needed to control the graded electrical activity and therefore the contraction of opener muscle fibers.

Endothelium-derived hyperpolarizing factor--a critical appraisal

Endothelium-derived hyperpolarizing factor is defined as that substance which produces vascular smooth muscle hyperpolarization which cannot be explained by nitric oxide or by a cyclo-oxygenase product such as prostacyclin. The possibility that the factor is an epoxyeicosatrienoic acid or a cannabinoid agonist such as anandamide continues to be investigated, but definitive evidence in favour of either is lacking. The sensitivity of EDHF-mediated responses to charybdotoxin, to apamin or to mixtures of these two toxins may indicate the opening of more than one smooth muscle K-channel, but the possibility that these are located on the vascular endothelium is discussed.

Involvement of voltage-dependent potassium channels in the EDHF-mediated relaxation of rat hepatic artery

1. In the rat hepatic artery, the acetylcholine-induced relaxation mediated by endothelium-derived hyperpolarizing factor (EDHF) is abolished by a combination of apamin and charybdotoxin, inhibitors of small (SKCa) and large (BKCa) conductance calcium-sensitive potassium (K)-channels, respectively, but not by each toxin alone. The selective BKCa inhibitor iberiotoxin cannot replace charybdotoxin in this combination. Since delayed rectifier K-channels (KV) represent another target for charybdotoxin, we explored the possible involvement of KV in EDHF-mediated relaxation in this artery. 2. The KV inhibitors, agitoxin-2 (0.3 microM), kaliotoxin (0.3 microM), beta-dendrotoxin (0.3 microM), dofetilide (1 microM) and terikalant (10 microM), each in combination with apamin (0.3 microM) had no effect on the EDHF-mediated relaxation induced by acetylcholine in the presence of N omega-nitro-L-arginine (0.3 mM) and indomethacin (10 microM), inhibitors of nitric oxide (NO) synthase and cyclo-oxygenase, respectively (n = 2-3). Although the KV inhibitor margatoxin (0.3 microM) was also without effect (n = 5), the combination of margatoxin and apamin produced a small inhibition of the response (pEC50 and Emax values were 7.5 +/- 0.0 and 95 +/- 1% in the absence and 7.0 +/- 0.1 and 81 +/- 6% in the presence of margatoxin plus apamin, respectively; n = 6; P < 0.05). 3. Ciclazindol (10 microM) partially inhibited the EDHF-mediated relaxation by shifting the acetylcholine-concentration-response curve 12 fold to the right (n = 6; P < 0.05) and abolished the response when combined with apamin (0.3 microM; n = 6). This combination did not inhibit acetylcholine-induced relaxations mediated by endothelium-derived NO (n = 5). 4. A 4-aminopyridine-sensitive delayed rectifier current (IK(V)) was identified in freshly-isolated single smooth muscle cells from rat hepatic artery. None of the cells displayed a rapidly-activating and -inactivating A-type current. Neither charybdotoxin (0.3 microM; n = 3) nor ciclazindol (10 microM; n = 5), alone or in combination with apamin (0.3 microM; n = 4-5), had an effect on IK(V). A tenfold higher concentration of ciclazindol (0.1 mM, n = 4) markedly inhibited IK(V), but this effect was not increased in the additional presence of apamin (0.3 microM; n = 2). 5. By use of membranes prepared from rat brain cortex. [125I]-charybdotoxin binding was consistent with an interaction at a single site with a KD of approximately 25 pM. [125I]-charybdotoxin binding was unaffected by iberiotoxin (0.1 microM, n = 6), but was increased by apamin in a concentration-dependent manner (Emax 43 +/- 10%, P < 0.05 and pEC50 7.1 +/- 0.2; n = 7-8). Agitoxin-2 (10 nM) displaced [125I]-charybdotoxin binding by 91 +/- 3% (n = 6) and prevented the effect of apamin (1 microM; n = 6). 6. It is concluded that the EDHF-mediated relaxation in the rat hepatic artery is not mediated by the opening of either KV or BKCa. Instead, the target K-channels for EDHF seem to be structurally related to both KV and BKCa. The possibility that a subtype of SKCa may be the target for EDHF is discussed.

Actions of neurotransmitters and other messengers on Ca2+ channels and K+ channels in smooth muscle cells

Ion channels play key roles in determining smooth muscle tone by setting the membrane potential and allowing Ca2+ influx. Perhaps not surprisingly, therefore, they also provide targets for neurotransmitters and other messengers that act on smooth muscle. Application of patch-clamp and molecular biology techniques and the use of selective pharmacology has started to provide a wealth of information on the ion channel systems of smooth muscle cells, revealing complexity and functional significance. Reviewed are the actions of messengers (e.g., noradrenaline, acetylcholine, endothelin, angiotensin II, neuropeptide Y, 5-hydroxytryptamine, histamine, adenosine, calcitonin gene-related peptide, substance P, prostacyclin, nitric oxide and oxygen) on specific types of ion channel in smooth muscle, the L-type calcium channel, and the large conductance Ca(2+)-activated, ATP-sensitive, delayed rectifier and apamin-sensitive K+ channels.

Modulation of Ca(2+)-activated K+ current by isoprenaline, carbachol, and phorbol ester in cultured (and fresh) rat aortic vascular smooth muscle cells

1. Effects of isoprenaline (ISO), carbachol, and phorbol ester on the outward K+ currents in single cultured (or fresh) rat aortic vascular smooth muscle (A7r5 and A-10) cells were examined using a whole-cell voltage-clamp (at room temperature 22 degrees C). 2. With 10 mM EGTA in the pipette solution, the delayed rectifier K+ current (IK) was activated by Ca2+ at pCa 7 more than at pCa 10, and was TEA (10 mM) and apamin (200 nM) sensitive, which represents a Ca(2+)-activated K+ current (IKCa). 3. In cultured A7r5 cells, isoprenaline (1 and 5 microM) and carbachol (0.1 and 1 microM) inhibited IKCa. Phorbol ester, 4-beta-phorbol-12, 13-dibutyrate (PDB), at 0.1 and 1 microM also inhibited IKCa, and increased the inhibitory effects induced by isoprenaline (1 microM). 4. In fresh aortic cells, these drugs, at the same concentrations, also produced the similar effects. 5. In A-10 cells, PDB (1 microM) enhanced the transient outward current (4-AP-sensitive), but ISO (1 microM) inhibited the current. 6. These results suggest that the IKCa current would be inhibited by cyclic nucleotides (cAMP and cGMP) and also by PK-C stimulation, and thereby be directly contributed to excitation-contraction coupling of the vascular smooth muscle cells.

Calcium-activated potassium channels in adrenal chromaffin cells

Rat chromaffin cells express an interesting diversity of Ca(2+)-dependent K+ channels, including a voltage-independent, small-conductance, apamin-sensitive SK channel and two variants of voltage-dependent, large-conductance BK channels. The two BK channel variants are differentially segregated among chromaffin cells, such that BK current is completely inactivating in about 75-80% of rat chromaffin cells, while the remainder express a mix of inactivating and non-inactivating current or mostly non-inactivating BKs current. The single-channel conductance of BKi channels is identical to that of BKs channels. Although rates of current activation are similar in the two variants, the deactivation kinetics of the two channels also differ. Furthermore, BKi channels are somewhat less sensitive to scorpion toxins than BKs channels. The slow component of BKi channel deactivation may be an important determinant of the functional role of these channels. During blockade of SK current, cells with BKi current fire tonically during sustained depolarizing current injection, whereas cells with BKs current tend to fire only a few action potentials before becoming quiescent. The ability to repetitively fire requires functional BKi channels, since partial blockade of BKi channels by CTX makes a BKi cell behave much like a BKs cell. In contrast, the physiological significance of BKi inactivation may arise from the ability of secretagogue-induced [Ca2+]i elevations to regulate the availability of BKi channels during subsequent action potentials (Herrington et al., 1995). By reducing the number of BK channels available for repolarization, the time course of action potentials may be prolonged. This possibility remains to be tested directly. These results raise a number of interesting questions pertinent to the control of secretion in rat adrenal chromaffin cells. An interesting hypothesis is that cells with a particular kind of BK current may reflect particular subpopulations of chromaffin cells. These subpopulations might differ either in the nature of the material secreted from the cell (e.g., Douglass and Poisner, 1965) or in the responsiveness to particular secretagogues. The differences in electrical behavior between cells with BKi and BKs current suggest that the pattern of secretion that might be elicited by a single type of stimulus could differ. For BKi cells, secretion may occur in a tonic fashion during sustained depolarization, while secretion from cells with BKs current may be more phasic. In the absence of specific structural information about the domains responsible for inactivation of BKi channels, our understanding of the mechanism of inactivation remains indirect. BKi inactivation shares many features with N-terminal inactivation of voltage-dependent K+ channels. However, there are provocative differences between the two types of inactivation which require us to propose that the native inactivation domain of BKi channels may occlude access of permeant ions to the BK channel permeation pathway in a position at some distance from the actual mouth of the channel. Further understanding of the structural and mechanistic basis of inactivation of BKi channels promises to provide new insights into both the cytoplasmic topology of BK channels and the Ca(2+)- and voltage-dependent steps involved in channel activation.

Apamin-sensitive K+ channels mediate an endothelium-dependent hyperpolarization in rabbit mesenteric arteries

1. Vascular endothelial cells release a variety of substances which affect the membrane potential and tone of underlying vascular smooth muscle. In the presence of N omega-nitro-L-arginine to inhibit nitric oxide synthase and indomethacin to inhibit cyclo-oxygenase, acetylcholine (ACh; EC50 approximately 1 microM) elicited the release of an endothelium-derived hyperpolarizing factor (EDHF) in rabbit mesenteric arteries. 2. The hyperpolarization due to EDHF was blocked by apamin (IC50 approximately 0.3 nM), and by other inhibitors of the apamin-sensitive K+ channel (10 nM scyllatoxin, 100 microM d-tubocurarine, 300 microM gallamine) in the presence of indomethacin and N omega-nitro-L-arginine. The hyperpolarization was not blocked by glibenclamide (5 microM), iberiotoxin (10 nM), tetraethylammonium (1 mM), barium (500 microM), 4-aminopyridine (500 microM), ouabain (10 microM), bumetanide (10 microM), or nimodipine (100 nM). 3. In the presence of apamin and N omega-nitro-L-arginine, but the absence of indomethacin, ACh triggered a hyperpolarization that was blocked by glibenclamide, an inhibitor of ATP-sensitive K+ (KATP) channels. A similar glibenclamide-sensitive hyperpolarization was caused by Iloprost, a stable analogue of prostacyclin. 4. In experiments which distinguished the effects of EDHF, prostanoids and nitric oxide, hyperpolarizations and/or relaxations triggered by ACh were antagonized by muscarinic antagonists, the relative potencies (atropine approximately 4-DAMP > pirenzepine) of which indicated that the release of all three endothelium-derived factors was mediated by M3 receptors. 5. Our results suggest that ACh stimulates M3 receptors on endothelial cells, triggering the release of nitric oxide and prostanoids, which hyperpolarize underlying smooth muscle by activation of KATP channels, and the release of an EDHF, which hyperpolarizes smooth muscle through the activation of apamin-sensitive K+ (KAS) channels.

Fast, persistent, Ca(2+)-dependent K+ current controls graded electrical activity in crayfish muscle

The early outward current in opener muscle fibres of crayfish (Procambarus clarkii) was studied using the two-electrode voltage-clamp technique. This current was abolished in Ca(2+)-free and 5 mM Cd2+ solutions, and was blocked by extra- or intracellular tetraethylammonium, indicating that it was a Ca(2+)-dependent K+ current [IK(Ca)]. IK(Ca) was voltage dependent, apamin insensitive and sensitive to charybdotoxin (CTX), which, in addition to its tetraethylammonium sensitivity, suggests that the channels mediating IK(Ca) behave in a BK type manner. IK(Ca) activation was extremely fast, reaching a maximum within 5 ms, and the inactivation was incomplete, stabilizing at a persistent steady-state. IK(Ca) was insensitive to intracellular ethylenebis(oxonitrilo)tetraacetate (EGTA), but was abolished by injection of the faster Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), suggesting that voltage-dependent Ca2+ channels and those mediating IK(Ca) should be clustered closely on the membrane. Under two-electrode current-clamp recording mode, low amplitude, graded responses were evoked under control conditions, whereas repetitive all-or-none spikes were elicited by application of CTX or after loading the cells with BAPTA. We conclude that IK(Ca) activates extremely quickly, is persistent and is responsible for the generation and control of the low amplitude, graded, active responses of opener muscle fibres.

Contribution of activation of K+ channels to glyceryl trinitrate-induced relaxation of rabbit aorta

1. Possible contribution of K+ channel opening to the relaxation by glyceryl trinitrate (GTN) was examined using isolated rabbit aorta. 2. While glibenclamide and apamin failed to affect relaxation by GTN, both charybdotoxin (ChTx) and iberiotoxin (IbTx) effectively attenuated GTN-induced relaxation. 3. The increase in cGMP produced by GTN was not attenuated by ChTx and IbTx. 4. The inhibitory effect of ChTx on GTN-induced relaxation was not reduced in the presence of zaprinast, indicating that cGMP but not GMP was responsible for activation of the K+ channel. 5. Okadaic acid, a selective inhibitor of protein phosphatase 2A, had no effect on the relaxation by GTN. These results indicate that, though small in degree, activation of a ChTx-sensitive K+ channel (large conductance Ca(2+)-activated K+ channel) is involved in the GTN-induced relaxation in rabbit aorta.

Identification of the Ca2+ current activated by vasoconstrictors in vascular smooth muscle cells

The noncontractile aortic cell line A7r5 was chosen to study the effect of the vasoconstrictor peptide vasopressin on transmembrane Ca2+ movements, using conventional whole-cell patch recording techniques. Conditions in which previously characterised vasoconstrictor-modulated currents were suppressed revealed a tiny inward current component (-18 +/- 2 pA, n = 50, at -61 mV in 110 mM CaCl2). The vasopressin-activated inward current was absent when Ca2+ was absent from the extracellular solution, and the current amplitude increased with [Ca2+] (0.01-110 mM), with an apparent dissociation constant for Ca2+ of 9.7 mM. It was highly selective for Ca2+ over monovalent cations (permeability ratio Ca/Cs greater than 17). It was not voltage gated, except that the current/potential characteristic showed some inwards rectification. Amplitudes of the evoked inward currents had the same order of magnitude in Sr2+ and Ca2+, whereas they were much smaller in Mn2+, suggesting that this pathway is highly permeable to Sr2+ but poorly permeable to Mn2+. Inward currents evoked in Ca2+ were inhibited by other cations with the following order of potency: La3+ > Cd2+ > Co2+ approximately Ni2+ approximately Mn2+. The channel producing this current corresponds most probably to the ionic pathway originally called the receptor-operated calcium channel, which produces a long-lasting, constrictor-induced plateau of increased intracellular free calcium concentration in smooth muscle.

The isoquinoline derivative LOE 908 selectively blocks vasopressin-activated nonselective cation currents in A7r5 aortic smooth muscle cells

The effect of (R,S)-(3,4-dihydro 6,7-dimethoxy-isoquinoline-1-yl)-2-phenyl- N,N-di-[2-(2,3,4-trimethoxyphenyl)ethyl]-acetamide (LOE 908), a cation channel blocker in HL-60 promyeloblasts, was studied in the A7r5 smooth muscle cell line from rat thoracic aorta, using the whole-cell patch-clamp technique. At a holding potential of -60 mV, application of vasopressin induced a nonselective cation conductance in voltage-clamped A7r5 cells. The current-voltage relation was linear, and currents reversed close to 0 mV regardless of the chloride gradient. The activation of the nonselective cation conductance by vasopressin was not affected by dialysing cells with Ca(2+)-free internal solution. LOE 908 blocked this current in a concentration-dependent manner with an IC50 of 560 nM, whereas dihydropyridine-sensitive Ba2+ current through voltage-dependent Ca2+ channels was blocked with an IC50 of 28 microM. Another organic blocker of receptor-mediated Ca2+ entry, 1-beta-[3-(4-methoxyphenyl)-propoxy]-4-methoxyphenethyl-1H-imidazole hydrochloride (SK&F 96365), blocked both, the vasopressin-induced nonselective conductance and the voltage-activated Ba2+ current with similar IC50 values of 13 microM and 8 microM, respectively. The rank order of potency of inorganic blockers on the vasopressin-induced inward current was Gd3+ > La3+ > Cd2+. Vasopressin-induced non-selective cation current was also observed in pertussis toxin-pretreated A7r5 cells but was completely abolished after infusion of the GDP analogue, guanosine 5'-O-[3-thio]diphosphate, from the patch pipette. Furthermore, vasopressin induced a transient outward current, suggesting a Ca(2+)-activated K(+)-current, which overlapped with the nonselective cation conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide decreases [Ca2+]i in vascular smooth muscle by inhibition of the calcium current

Endothelium derived relaxing factor (nitric oxide, or NO) activates cytoplasmic guanylate cyclase in vascular smooth muscle and decreases vascular tone through cGMP-dependent mechanisms that are not yet understood fully. In cultured vascular smooth muscle cells (A7r5 cell line) sodium nitroprusside (NP), a vasodilator that decomposes into nitric oxide, lowered [Ca2+]i in cells in which [Ca2+]i was elevated after depolarization. NP decreased current through voltage-gated calcium channels, but did not affect release of calcium from intracellular stores. Hemoglobin, a scavenger of NO, reversed the effect of NP on [Ca2+]i and 8-Br-cGMP, a membrane permeant form of cGMP, mimicked the effect of NP on [Ca2+]i and on calcium currents. Thus, the signal transduction mechanism of endothelium dependent relaxation of vascular smooth muscle involves a decrease in [Ca2+]i by inhibition of Ca2+ entry. Relaxation or vasodilation would then result from decreased activity of myosin light chain kinase, in addition to myosin light chain dephosphorylation.

The role of the membrane potential of endothelial and smooth muscle cells in the regulation of coronary blood flow

In the mammalian heart the supply of oxygen and energy-rich substrates through the coronary arterioles is continuously adapted to the variations of cardiac work. The coronary resistance arteries and the surrounding myocardium form a functional unit with multiple interactions between coronary endothelial cells, smooth muscle cells, perivascular nerves, and cardiac muscle cells. We describe the mechanisms underlying the electrical and chemical communication between the different cell types, the ionic channels contributing to the resting potential of endothelial and smooth muscle cells, and the mechanisms responsible for modulation of the resting potential. The main conclusion of our analysis is that the membrane potential of coronary endothelial and smooth muscle cells is one of the major determinants of coronary blood flow, and that modulation of the membrane potential provides a way to dilate or constrict coronary resistance arteries. It is proposed that the membrane potential of the myo-endothelial regulatory unit, i.e., of the endothelial cells and the underlying smooth muscle cells in the terminal arterioles, may function as an integrator of the numerous local and global vasodilator and constrictor signals that provide for the adaptation of coronary blood flow to the metabolic demands of the heart.

Cardiovascular effects of SCA40, a novel potassium channel opener, in rats

1. Experiments have been performed to investigate the cardiovascular actions in the rat of SCA40, a novel potassium channel opener which is a potent relaxant of guinea-pig airway smooth muscle in vivo and in vitro. 2. SCA40 (0.01-30 microM) caused a complete and concentration-dependent relaxation of rat isolated thoracic aorta contracted with 20 mM KCl but failed to inhibit completely the spasmogenic effects of 80 mM KCl. 3. The ATP-sensitive K(+)-channel blocker, glibenclamide (3 microM), failed to antagonize the relaxant action of SCA40 on 20 mM KCl-contracted rat isolated thoracic aorta. 4. SCA40 (0.001-100 microM) had dual effects on rat isolated atria. At low concentrations, SCA40 produced a concentration-dependent decrease in the rate and force of contractions. At higher concentrations (greater than 1 microM) SCA40 induced concentration-dependent increases of atrial rate and force. 5. In vivo, in normotensive Wistar rats, SCA40 elicited a dose-dependent (1-100 micrograms kg-1) decrease in mean arterial pressure which was accompanied by a moderate dose-dependent increase in heart rate. SCA40 (100 micrograms kg-1) had a slightly greater hypotensive effect than cromakalim (100 micrograms kg-1) but the duration of the hypotension was longer with cromakalim than with SCA40. 6. The hypotensive effect of SCA40 was not reduced by propranolol, atropine, NG-nitro-L-arginine methyl ester (L-NAME) or glibenclamide. 7. It is concluded that the mechanism by with SCA40 relaxes vascular smooth muscle in vitro and in vivo involves activation of K(+)-channels distinct from glibenclamide-sensitive ATP-sensitive K(+)-channels.

Endothelin and vasopressin activate low conductance chloride channels in aortic smooth muscle cells

The non-contractile aortic smooth muscle cell line A7r5 was used to study the membrane events involved in the effect of vasoconstrictor peptides. Whole-cell voltage-clamp and membrane potential recording techniques were used to demonstrate the contribution of an increased Cl- conductance to the late depolarization induced by endothelin-1 and vasopressin. During cell-attached patch recording with N-methyl-D-glucamine in the pipette, bath application of endothelin or vasopressin induced single-channel inward currents in the following minutes. The current/potential (I/V) curve of the most frequently observed channel type--a small conductance Cl- (SCl) channel--reversed near the cell membrane potential and showed a single-channel conductance of 1.8 pS for inward currents. After patch excision in an extracellular solution containing CaCl2 (2 mM), the frequency of SCl channel openings increased. Patch excision in the absence of peptide stimulation also produced this channel activity. Replacement of CaCl2 by a Ca2+ chelator on the intracellular face of a patch reversibly inhibited the channel activity, indicating that these SCl channels are Ca(2+)-activated Cl- channels. The single-channel I/V characteristic showed outward rectification above +50 mV. An analysis of the gating kinetics of the SCl channel is given. Another channel type was recorded less frequently after peptide stimulation. It had a lower conductance (1.0-1.3 pS) and slower kinetics and was designated a very small conductance Cl- channel. It is concluded that activation of two types of Cl- channels (at least one of which is Ca2+ dependent) is involved in the late depolarization produced by vasoconstrictor peptides in vascular smooth muscle cells of the aortic cell line A7r5.

Maxi K+ channels are stimulated by cyclic guanosine monophosphate-dependent protein kinase in canine coronary artery smooth muscle cells

By using a patch clamp technique, we examined the effect of cyclic guanosine monophosphate (cGMP)-dependent protein kinase (G kinase) on Ca(2+)-activated maxi K+ channels in canine coronary artery smooth muscle cells. Maxi K+ channels (274 +/- 4 pS in symmetrical 140 mM KCl at 24-26 degrees C) were activated by cytoplasmic Ca2+ and were completely blocked by 100 nM charybdotoxin (CTX). G kinase (300 U/ml) added to the cytoplasmic face of the membrane patch shifted the voltage dependence of these channels by about 25 mV in the negative direction in the presence of 1 microM Ca2+, 50 microM cGMP and 1 mM magnesium adenosine triphosphate. At -50 mV and 1 microM Ca2+, G kinase treatment increased the mean number of open channels 4.5-fold compared with the control. alpha-Human atrial natriuretic peptide (ANP, 100 nM) reduced the isometric tension of coronary arterial rings elicited by 14 or 24 mM KCl, but failed to relax the artery contracted by 34 mM KCl. Addition of 100 nM CTX augmented tension development elicited by 24 mM KCl and totally prevented ANP from relaxing the arterial rings. These results indicate that G kinase-dependent protein phosphorylation activates maxi K+ channels in canine coronary smooth muscle, and further suggest that the G kinase-induced activation of maxi K+ channels may cause hyperpolarization and relaxation of coronary artery.

Evaluation of the relaxant effects of SCA40, a novel charybdotoxin-sensitive potassium channel opener, in guinea-pig isolated trachealis

1. Experiments have been performed in order to analyse the mechanism whereby SCA40, a new imidazo[1,2-a]pyrazine derivative relaxes airway smooth muscle. 2. SCA40 (0.01-10 microM) caused a complete and concentration-dependent relaxation of guinea-pig isolated trachea contracted with 20 mM KCl but failed to inhibit completely the spasmogenic effects of 80 mM KCl. 3. Quinine (30 microM) antagonized the relaxant activity of SCA40 in 20 mM KCl-contracted guinea-pig isolated trachea. The ATP-sensitive K(+)-channel blocker, glibenclamide (3 microM), did not antagonize the relaxant activity of SCA40 in either 20 mM KCl or 1 microM carbachol-contracted isolated trachea. 4. SCA40 (0.01-10 microM) and isoprenaline (0.1 nM-10 microM) caused a complete and concentration-dependent relaxation of guinea-pig isolated trachea contracted with carbachol 1 microM. 5. The large-conductance Ca(2+)-activated K(+)-channel blocker, charybdotoxin (60-180 nM), non-competitively antagonized the relaxant activity of isoprenaline on 1 microM carbachol-contracted trachea. The inhibition was characterized by rightward shifts of the isoprenaline concentration-relaxation curves with depression of their maxima. 6. The relaxant activity of SCA40 in 1 microM carbachol-contracted trachea was antagonized by charybdotoxin (60-600 nM) in an apparently competitive manner. The concentration-relaxation curves to SCA40 were shifted to the right with no significant alteration in the maximum response. 7. It is concluded that SCA40 is a novel potassium channel opener which is a potent relaxant of guinea-pig airway smooth muscle in vitro. The relaxant activity of SCA40 does not involve ATP-sensitive K+-channels but rather large-conductance Ca2'-activated K+-channels or other charybdotoxin sensitive K+-channels.