Chloride channel blockers inhibit myogenic tone in rat cerebral arteries

Nitric oxide contracts longitudinal smooth muscle of opossum oesophagus via excitation-contraction coupling

1. The effects of sodium nitroprusside (SNP) and diethylenetriamine/nitric oxide adduct (DETA/NO), putative nitric oxide (NO) donors, on opossum oesophageal longitudinal smooth muscle were investigated using isometric tension and intracellular micro-electrode recordings. 2. SNP produced concentration-dependent contractions of oesophageal longitudinal smooth muscle with an EC(50) of 239.6 +/- 78.2 microM (mean +/- S.E.M., n = 10). Maximal contraction induced by SNP (1 mM) was about 75.5 +/- 8.5 % (n = 10) of the 60 mM KCl-induced contraction. The SNP-induced contraction was resistant to tetrodotoxin (TTX; 1 microM), but abolished by nifedipine (1 microM), as well as by niflumic acid (300 microM) and 9-anthroic acid (9-AC; 1 mM), Ca(2+)-activated Cl(-) channel blockers. 3. DETA/NO at concentrations of 100 and 500 microM induced 83.1 +/- 24.4 and 104.1 +/- 34.9 % of the 60 mM KCl-induced contraction (n = 4), respectively, which was abolished by nifedipine (1 microM), niflumic acid (300 microM) and 9-AC (1 mM). 4. Pre-application of 1H-[1,2,4]oxidiazolo[4,3,-alpha]quinoxalin-1-one (ODQ) (10 microM), a guanylate cyclase inhibitor, significantly inhibited the SNP-induced contraction, whereas 8-bromo-cGMP (1 mM), a membrane-permeable analogue of cGMP, mimicked the SNP-induced contraction. 5. Intracellular recordings revealed that SNP (300 microM) depolarized resting membrane potentials (RMPs) and increased the frequency of spontaneous spike-like action potentials. However, these electrical alterations were eliminated by pretreatment with niflumic acid (300 microM). 6. These results suggest that NO produces an excitation-contraction coupling in opossum oesophageal longitudinal smooth muscle via a cGMP-dependent signalling pathway. This contraction depends on extracellular Ca(2+) entry through activation of L-type Ca(2+) channels.

Tamoxifen activates smooth muscle BK channels through the regulatory beta 1 subunit

Estrogen (17beta-estradiol; 17betaE) and xenoestrogens, estrogenic compounds that are not steroid hormones, have non-genomic actions at plasma membrane receptors unrelated to the nuclear estrogen receptor. The open probability (P(o)) of large conductance Ca(2+)/voltage-sensitive k(+)(BK) channels is increased by 17betaE through the regulatory beta1 subunit. The pharmacological nature of the putative membrane binding site is unclear. We probed the site by determining whether tamoxifen ((Z)-1-(p-dimethylaminoethoxy-phenyl)-1,2-diphenyl-1-butene; Tx), a chemotherapeutic xenoestrogen, increased P(o) in clinically relevant concentrations (0.1-10 microm). In whole cell patch clamp recordings on canine colonic myocytes, which express the beta1 subunit, Tx activated charybdotoxin-sensitive K(+) current. In single channel experiments, Tx increased the NP(o) (P(o) x number channels; N) and decreased the unitary conductance (gamma) of BK channels. Tx increased NP(o) (EC(50) = 0.65 microm) in excised membrane patches independent of Ca(2+) changes. The Tx mechanism of action requires the beta1 subunit, as Tx increased the NP(o) of Slo alpha expressed in human embryonic kidney cells only in the presence of the beta1 subunit. Tx decreased gamma of the alpha subunit expressed alone, without effect on NP(o). Our data indicate that Tx increases BK channel activity in therapeutic concentrations and reveal novel pharmacological properties attributable to the alpha and beta1 subunits. These data shed light on BK channel structure and function, non-genomic mechanisms of regulation, and physiologically and therapeutically relevant effects of xenoestrogens.

The pathobiology of pulmonary hypertension. Smooth muscle cells and ion channels

Chronic hypoxic pulmonary arterial hypertension, APAH, and PPAH are characterized by vasoconstriction and vascular remodeling and are associated with decreased Kv currents in PA smooth muscle cells. Although Kv2.1 is less well studied, it seems that Kv1.5 is particularly important in the pulmonary circulation in animals and humans because it has been implicated in physiologic phenomena (HPV) and all of the aforementioned pulmonary hypertensive disorders. This occurrence is perhaps because of the fact that it controls Em in the PA smooth muscle cells and it has a short turnover half time. It is also certain that the pathogenesis of PPAH is multifactorial and not a result of a single abnormality. The recently discovered "PPAH gene" in chromosome 2q in patients with familial PPAH (6%-12% of patients) does not seem to encode for any Kv channels. Kv1.5 abnormalities, however, are likely to be a strong predisposing factor that, in association with others such as endothelial dysfunction, [figure: see text] anorexigen use, or viral infections, will initiate a process that eventually leads to PPAH. The selective Kv1.5 down-regulation leaves wide open the door to replacement gene therapy in pulmonary hypertension research.

Measurement of chloride flux associated with the myogenic response in rat cerebral arteries

1. Self-referencing ion-selective (SERIS) electrodes were used to measure the temperature and pressure dependence of Cl(-) efflux, during myogenic contraction of pressurized rat cerebral resistance arteries. 2. At room temperature (18-21 degrees C), a small, pressure-independent Cl(-) efflux was measured. On warming to 37 degrees C, arteries developed pressure-dependent myogenic tone, and this was associated with a pressure-dependent increase in Cl(-) efflux (n = 5). 3. Both myogenic tone and the pressure- and temperature-dependent Cl(-) efflux were abolished on application of 10 microM tamoxifen, a Cl(-) channel blocker (IC(50) 3.75 +/- 0.2 microM). Tamoxifen (10 microM) also prevented contraction to 60 mM K(+), suggesting non-specific effects of tamoxifen (n = 5). 4. Myogenic tone was abolished by 2 microM nimodipine, but Cl(-) efflux was unaffected. In the presence of nimodipine, 10 microM tamoxifen still abolished pressure- and temperature-dependent Cl(-) efflux (n = 3). 5. In summary, a Cl(-) efflux can be measured from rat cerebral arteries, with a temperature dependence that is closely correlated with myogenic contraction. We conclude that Cl(-) efflux through Cl(-) channels contributes to the depolarization associated with myogenic contraction.

Invited review: arteriolar smooth muscle mechanotransduction: Ca(2+) signaling pathways underlying myogenic reactivity

The smooth muscle of arterioles responds to an increase in intraluminal pressure with vasoconstriction and with vasodilation when pressure is decreased. Such myogenic vasoconstriction provides a level of basal tone that enables arterioles to appropriately adjust diameter in response to neurohumoral stimuli. Key in this process of mechanotransduction is the role of changes in intracellular Ca(2+). However, it is becoming clear that considerable complexity exists in the spatiotemporal characteristics of the Ca(2+) signal and that changes in intracellular Ca(2+) may play roles other than direct effects on the contractile process via activation of myosin light-chain phosphorylation. The involvement of Ca(2+) may extend to modulation of ion channels and release of Ca(2+) from the sarcoplasmic reticulum, alterations in Ca(2+) sensitivity, and coupling between cells within the vessel wall. The purpose of this brief review is to summarize the current literature relating to Ca(2+) and the arteriolar myogenic response. Consideration is given to coupling of Ca(2+) changes to the mechanical stimuli, sources of Ca(2+), involvement of ion channels, and spatiotemporal aspects of intracellular Ca(2+) signaling.

Contribution of chloride channel activation to the elevated muscular tone of the pulmonary artery in monocrotaline-induced pulmonary hypertensive rats

In monocrotaline-treated rat pulmonary artery from which endothelium was removed, greater spontaneous muscular tone was observed under resting conditions than in vehicle-treated artery. The aim of the present study was to show the possible contribution of Cl- channels in the mechanism of the elevated tone. Verapamil almost completely inhibited the elevated spontaneous muscular tone by decreasing [Ca2+]i. The elevated muscular tone was also inhibited by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS), a Cl- channel inhibitor. After the inhibition of muscular tone by DIDS, verapamil did not induce further relaxation. Quantitative RT-PCR analysis indicated that the mRNA levels of ClC3 and Ca2+-activated Cl- channels did not change in the pulmonary hypertensive pulmonary artery from those of vehicle-treated rats. These results suggest that the elevated muscular tone observed in the monocrotaline-induced hypertensive pulmonary artery is due to membrane depolarization of smooth muscle cells and that this phenomenon might be mediated by the activation of DIDS-sensitive Cl- channels.

Mechanisms of coronary artery depolarization by uridine triphosphate

We sought to define the basic mechanisms by which pyrimidine nucleotides constrict rat coronary resistance arteries. Uridine triphosphate (UTP) caused a dose-dependent constriction in coronary arteries stripped of endothelium. UTP also depolarized and increased cytosolic Ca2+ in coronary smooth muscle cells. Nisoldipine, an antagonist of voltage-operated Ca2+ channels, blocked the rise in cytosolic Ca2+ and reduced UTP-induced vasoconstriction by approximately 75% which suggests a prominent role for depolarization in this constrictor response. The ionic basis of UTP-induced depolarization was subsequently explored in coronary smooth muscle cells using whole-cell patch-clamp electrophysiology. In the absence of K+ and with CsCl in the pipette, UTP (40 microM) activated a sustained inwardly rectifying current (-0.66 +/- 0.10 pA/pF at -60 mV). A 100 mM reduction in bath Na+ shifted the reversal potential of this current (from -2 +/- 1 to -28 +/- 4 mV) and reduced the magnitude (from -2.26 +/- 0.61 to -0.51 +/- 0.11 pA/pF). In addition to activating a depolarizing cation current, UTP inhibited hyperpolarizing outward currents. Specifically, UTP inhibited ATP-sensitive and voltage-dependent K+ currents yet had no effect on inwardly rectifying and Ca2+-activated K+ channels. This study indicates that electromechanical coupling is integral to pyrimidine-induced constriction in coronary resistance arteries.

Chloride in smooth muscle

Interest in the functions of intracellular chloride expanded about twenty years ago but mostly this referred to tissues other than smooth muscle. On the other hand, accumulation of chloride above equilibrium seems to have been recognised more readily in smooth muscle. Experimental data is used to show by calculation that the Donnan equilibrium cannot account for the chloride distribution in smooth muscle but it can in skeletal muscle. The evidence that chloride is normally above equilibrium in smooth muscle is discussed and comparisons are made with skeletal and cardiac muscle. The accent is on vascular smooth muscle and the mechanisms of accumulation and dissipation. The three mechanisms by which chloride can be accumulated are described with some emphasis on calculating the driving forces, where this is possible. The mechanisms are chloride/bicarbonate exchange, (Na+K+Cl) cotransport and a novel entity, "pump III", known only from own work. Their contributions to chloride accumulation vary and appear to be characteristic of individual smooth muscles. Thus, (Na+K+Cl) always drives chloride inwards, chloride/bicarbonate exchange is always present but does not always do it and "pump III" is not universal. Three quite different biophysical approaches to assessing chloride permeability are considered and the calculations underlying them are worked out fully. Comparisons with other tissues are made to illustrate that low chloride permeability is a feature of smooth muscle. Some of the functions of the high intracellular chloride concentrations are considered. This includes calculations to illustrate its depolarising influence on the membrane potential, a concept which, experience tells us, some people find confusing. The major topic is the role of chloride in the regulation of smooth muscle contractility. Whilst there is strong evidence that the opening of the calcium-dependent chloride channel leads to depolarisation, calcium entry and contraction in some smooth muscles, it appears that chloride serves a different function in others. Thus, although activation and inhibition of (Na+K+Cl) cotransport is associated with contraction and relaxation respectively, the converse association of inhibition and contraction has been seen. Nevertheless, inhibition of chloride/bicarbonate exchange and "pump III" and stimulation of (K+Cl) cotransport can all cause relaxation and this suggests that chloride is always involved in the contraction of smooth muscle. The evidence that (Na+K+Cl) cotransport more active in experimental hypertension is discussed. This is a common but not universal observation. The information comes almost exclusively from work on cultured cells, usually from rat aorta. Nevertheless, work on smooth muscle freshly isolated from hypertensive rats confirms that (Na+K+Cl) cotransport is activated in hypertension but there are several other differences, of which the depolarisation of the membrane potential may be the most important.Finally, a simple calculation is made which indicates as much as 40% of the energy put into the smooth muscle cell membrane by the sodium pump is necessary to drive (Na+K+Cl) cotransport. Notwithstanding the approximations in this calculation, this suggests that chloride accumulation is energetically expensive. Presumably, this is related to the apparently universal role of chloride in contraction.

Characterization of stretch-activated cation current in coronary smooth muscle cells

Stretch-activated ion currents were recorded from vascular smooth muscle (VSM) after enzymatic isolation of single cells from porcine coronary arterioles. Patch pipettes were used to record whole cell current and control cell length. Under voltage clamp in physiological saline solution, an inward cation current (I(CAT)) was activated by 105--135% longitudinal stretch. I(CAT) coincided with an increase in intracellular Ca(2+) concentration. Under current clamp, membrane depolarization was induced by stretch. The magnitude of I(CAT) varied from -0.8 to -6.9 pA/pF at a holding potential of -60 mV. I(CAT) was graded with stretch, inactivated on release, and could be repeatedly induced. A potassium current (I(K)) activated in unstretched cells by depolarization was also enhanced by stretch. In Ca(2+)-free bath solution, stretch-induced enhancement of I(K) was blocked, but I(CAT) was still present. Hexamethyleneamiloride (50 microM), a reputed inhibitor of mechanosensitive channels, blocked I(CAT) and the stretch-induced increase in I(K) but not basal I(K). Grammostolla spatulata venom (1:100,000) blocked basal I(K), blocked stretch-induced increases in I(K), and blocked I(CAT). Iberiotoxin, a specific Ca(2+)-activated K(+) channel blocker, did not alter I(CAT) but blocked the stretch-induced increase in I(K) and increased the magnitude of stretch-induced depolarization. We concluded that longitudinal stretch directly activates a cation current and secondarily activates a Ca(2+)-activated K(+) current in isolated coronary myocytes. Although these two currents would partially counteract each other, the predominance of I(CAT) at physiological potentials is likely to explain the depolarization and contraction observed in intact coronary VSM during pressure elevation.

Chloride channels and their functional roles in smooth muscle tone in the vasculature

Although evidence of important contributions by Cl- channels to agonist-induced currents have been reported in vascular smooth muscle cells, the functional roles played by Cl- channels in the smooth muscle contraction and in setting the membrane potential remain essentially obscure. All of the admittedly few papers published have focused on the physiological roles of Cl- channels in the contraction and membrane depolarization elicited by agonists. At present, it seems likely that in vascular cells: a) Cl- conductance contributes to membrane depolarization, with the subsequent contraction being due to Ca2+ release from the intracellular store sites, and b) Cl- movements through the membrane of the Ca2+ store sites also regulate Ca2+ release and Ca2+ uptake from/into the store sites. As a Ca2+-dependent Cl- current is most easily demonstrated under quasi-physiological conditions (by the perforated patch-clamp method), the contribution made by Cl- channels to smooth muscle function may be more important than previously thought. The development of the new, selective Cl--channel blockers as well as the identification and gene engineering of the channel molecules are essential if we are to advance our knowledge of the physiology and pharmacology of the Cl- channels residing in vascular smooth muscle cells.

Distribution of chloride channel-2-immunoreactive neuronal and astrocytic processes in the hippocampus

The chloride homeostasis of neurons and non-neuronal cells is maintained in part by a voltage-sensitive inwardly rectifying chloride conductance through the chloride channel-2. This channel is activated by hyperpolarization and extracellular hypotonicity. In the present study, hippocampal sections were immunostained for chloride channel-2, and somata and dendrites of both pyramidal and non-pyramidal cells were found to be immunoreactive. In addition, glial processes in the vicinity of small blood vessels were also immunostained, whereas the neuropil of strata pyramidale and lacunosum-moleculare contained chloride channel-2-positive punctate structures. Electron microscopy and double immunostaining using antibodies against chloride channel-2 and glial fibrillary acidic protein confirmed that the dense network of chloride channel-2-positive processes corresponds to the end feet of astrocytes. The distribution of chloride channel-2-immunoreactive astrocytes was inhomogeneous throughout the hippocampus: strata oriens, pyramidale and lacunosum-moleculare of CA1-CA3 and the outer molecular layer of the dentate gyrus contained the majority of immunoreactive end feet, whereas the other layers showed sparse labeling. Subcellular studies demonstrated that, in addition to astrocytes, chloride channel-2 was localized in the membrane of dendrites, dendritic spines, cell bodies and axon initial segments of neurons, frequently close to, or within active zones of, symmetrical synapses.Thus, chloride channel-2 appears to be involved in transmembrane chloride movements associated with GABAergic synaptic transmission. The specific laminar distribution of chloride channel-2-positive astroglial processes coinciding with that of GABAergic axon terminals suggests that the network of astrocytes may be able to siphon and deliver Cl(-) ions to layers with intense GABAergic transmission, thereby increasing the efficacy of GABA(A) receptor-mediated inhibition.

Blockade of chloride channels reveals relaxations of rat small mesenteric arteries to raised potassium

1. Raised extracellular K(+) relaxes some arteries, and has been proposed as Endothelium-Derived Hyperpolarizing Factor (EDHF). However, relaxation of rat small mesenteric arteries to K(+) is highly variable. We have investigated the mechanism of K(+)-induced dilatation and relaxation of pressurized arteries and arteries mounted for measurement of isometric force. 2. Raising [K(+)](o) from 5.88 - 10.58 mM did not dilate or relax pressurized or isometric arteries. Relaxation to raised [K(+)](o) was revealed in the presence of 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB); this effect of NPPB was concentration-dependent (IC(50): 1.16 microM). 3. Relaxations to raised [K(+)](o) in the presence of NPPB, were abolished by 30 microM Ba(2+) or endothelial-denudation. Acetycholine (10 microM) relaxed endothelium-intact arteries in presence of raised [K(+)](o) NPPB and Ba(2+). 4. Relaxations to raised [K(+)](o) were revealed in hyperosmotic superfusate (+60 mM sucrose). These relaxations were abolished by 30 microM Ba(2+). In the presence of raised [K(+)](o), 60 mM sucrose and 30 microM Ba(2+), 10 microM acetycholine still relaxed all arteries. 5. Fifty microM 18 alpha-glycyrrhetinic acid (18 alpha-GA), a gap junction inhibitor, depressed relaxations to both 10 microM acetylcholine and raised [K(+)](o), in the presence of 10 microM NPPB. 6. In summary, blockade of a volume-sensitive Cl(-) conductance in small rat mesenteric arteries, using NPPB or hyperosmotic superfusion, reveals a endothelium-dependent, Ba(2+) sensitive dilatation or relaxation of rat mesenteric arteries to raised [K(+)](o). We conclude that inwardly rectifying potassium channels on the endothelium underlie relaxations to raised [K(+)](o) in rat small mesenteric arteries.

Opposing roles of K(+) and Cl(-) channels in maintenance of opossum lower esophageal sphincter tone

The ionic basis underlying the maintenance of myogenic tone of lower esophageal sphincter circular muscle (LES) was investigated in opossum with the use of standard isometric tension and conventional intracellular microelectrode recordings in vitro. In tension recording studies, nifedipine (1 microM) reduced basal tone to 27.7 +/- 3.8% of control. The K(+) channel blockers tetraethylammonium (TEA, 2 mM), charybdotoxin (100 nM), and 4-aminopyridine (4-AP, 2 mM) enhanced resting tone, whereas apamin and glibenclamide were without affect. Cl(-) channel blockers DIDS (500 microM) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (500 microM), as well as niflumic acid (0.1-300 microM), decreased basal tone, but tamoxifen was without effect. Intracellular microelectrode recordings revealed ongoing, spontaneous, spike-like action potentials (APs). Nifedipine abolished APs and depolarized resting membrane potential (RMP). Both TEA and 4-AP significantly depolarized RMP and augmented APs, whereas niflumic acid dose-dependently hyperpolarized RMP and abolished APs. These data suggest that, in the opossum, basal tone is associated with continuous APs and that K(+) and Ca(2+)-activated Cl(-) channels have important opposing roles in the genesis of LES tone.

Effects of mibefradil and nifedipine on arteriolar myogenic responsiveness and intracellular Ca(2+)

1. Ca(2+) entry mechanisms underlying spontaneous arteriolar tone and acute myogenic reactivity remain uncertain. These studies aimed to compare the effects of nifedipine and the putative T-channel blocker, mibefradil, on arteriolar myogenic responsiveness and intracellular Ca(2+) (Ca(2+)(i)). 2. First order cremaster muscle arterioles (1A) were isolated from rats, cannulated, pressurized to 70 mmHg in the absence of intraluminal flow, and mechanical responses studied by video microscopy. The Ca(2+)(i) was measured using fluorescence imaging of Fura 2 loaded arterioles. 3. Both nifedipine and mibefradil showed dose-dependent inhibition of spontaneous myogenic tone (at 70 mmHg; pEC(50) 7.04+/-0.17 vs 6.65+/-0.20 respectively, n=6 for both, n.s.) and KCl-induced vasoconstriction (at 70 mmHg; pEC(50) 6.93+/-0. 38 vs 6.45+/-0.27 respectively, n=6 for both, n.s.). 4. In arterioles maintained at 50 mmHg, nifedipine (10(-7) and 10(-5) M) caused a concentration dependent reduction in Ca(2+)(i), however, mibefradil (10(-7) and 10(-5) M) had no effect. Furthermore nifedipine significantly attenuated the increase in Ca(2+)(i) associated with an acute pressure step (50 - 120 mmHg) whereas mibefradil was considerably less effective. 5. Mibefradil (10(-7) M) significantly attenuated contractile responses to 60 mM KCl without altering the KCl-induced increase in Ca(2+)(i), in contrast to nifedipine (10(-7) M) which reduced both Ca(2+)(i) and contraction. 6. Membrane potential of arterioles with spontaneous myogenic tone (70 mmHg) was -41.5+/-1. 0 mV. Nifedipine (10(-7) or 10(-5) M) had no effect on membrane potential, however mibefradil (10(-5) M) caused significant depolarization. 7. In summary, both mibefradil and nifedipine inhibit arteriolar spontaneous tone and acute myogenic reactivity. While there may be overlap in the mechanisms by which these agents inhibit tone, differences in effects on membrane potential and intracellular Ca(2+) levels suggest mibefradil exhibits actions other than blockade of Ca(2+) entry in skeletal muscle arterioles.

Dual modulation of swelling-activated chloride current by NO and NO donors in rabbit portal vein myocytes

1. The effects of authentic NO and the NO donor S-nitroso-N-acetylpenicillamine (SNAP) on swelling-activated chloride currents (Iswell) were investigated in freshly dispersed rabbit portal vein smooth muscle cells. Iswell was recorded with the perforated patch configuration of the whole-cell patch clamp technique. 2. In approximately 50 % of cells NO and SNAP inhibited the amplitude of Iswell by about 45 % in a voltage-independent manner. Iswell was also inhibited by an inhibitor of NO-sensitive guanylate cyclase (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and by KT5823, an inhibitor of cGMP-dependent protein kinase. 3. In other cells both NO and SNAP enhanced Iswell by about 40 % in a voltage-independent manner. A similar increase was produced by application of the cell-permeable cGMP analogue 8-bromo-guanosine 3', 5'-cyclic monophosphate (8-Br-cGMP). However, 8-Br-cGMP had no effect on current amplitude in cells pre-treated with KT5823. In contrast 8-Br-cGMP increased the amplitude of Iswell in cells which had been pre-treated with ODQ. 4. SNAP also modulated Iswell recorded in the conventional whole-cell configuration with internal solutions containing 10 mM EGTA to rule out any contribution from Ca2+-activated Cl- currents. 5. These data suggest that the amplitude of Iswell can be enhanced by NO via a cGMP-dependent phosphorylation and inhibited by NO in a cGMP-independent manner.

Influence of T-type Ca2+ (mibefradil) and Cl- (indanyloxyacetic acid 94) channel antagonists on α1-adrenoceptor mediated contractions in rat aorta

The effects of the T-type and L-type Ca2+ channel antagonists, mibefradil and nifedipine, respectively, and those of a Cl- channel antagonist, indanyloxyacetic acid 94, on mechanical responses elicited by selective activation of α1-adrenoceptors using cirazoline were examined in rat isolated aortic rings. The presence of mibefradil (300 nM), indanyloxyacetic acid, 94 (30 µM) and nifedipine (300 nM) alone inhibited mechanical responses elicited by cirazoline. The concentration-response curves to cirazoline were displaced to the right with significant increases in the EC50 and significant depressions of the maximal responses in the presence of the individual agents mibefradil, indanyloxyacetic acid 94, or nifedipine. A combination of mibefradil and indanyloxyacetic acid 94 further inhibited the mechanical activity produced by cirazoline. The further reduction in the maximal response to cirazoline, in the presence of mibefradil and nifedipine, was insignificant when compared with the effects of nifedipine alone. In addition, maximal mechanical responses produced by cirazoline were not significantly affected by a combination of nifedipine and indanyloxyacetic acid 94 when compared with either nifedipine alone or mibefradil and indanyloxyacetic acid 94 combined. Our current findings indicate that mibefradil, indanyloxyacetic acid 94, and nifedipine can inhibit cirazoline-induced contractions to a varying degree. Moreover, based on our present data it would be reasonable to suggest that the contribution of T-type versus L-type Ca2+ channels to contractile responses obtained with cirazoline are approximately 21% and 35%, respectively, of the Emax. It would appear that L-type Ca2+ channels play a greater role in processes that are involved in excitation-contraction coupling subsequent to stimulation of α1-adrenoceptors. In addition, Cl- channels also appear to be involved in the process of contraction following α1-adrenoceptor activation.Key words: T-type Ca2+ channels, L-type Ca2+ channels, Cl- channels, isolated aortic rings.

Swelling-activated cation channels mediate depolarization of rat cerebrovascular smooth muscle by hyposmolarity and intravascular pressure

1. Increases in intravascular pressure depolarize vascular smooth muscle cells. Based on the attenuating effects of Cl- channel antagonists, it has been suggested that swelling-activated Cl- channels may be integral to this response. Consequently, this study tested for the presence of a swelling-activated Cl- conductance in both intact rat cerebral arteries and isolated rat smooth muscle cells. 2. A 50 mosmol l-1 hyposmotic challenge (300 to 250 mosmol l-1) constricted rat cerebral arteries. This constriction contained all the salient features of a pressure-induced response including smooth muscle cell depolarization and a rise in intracellular Ca2+ that was blocked by voltage-operated Ca2+ channel antagonists. The hyposmotically induced depolarization was attenuated by DIDS (300 microM) and tamoxifen (1 microM), a response consistent with the presence of a swelling-activated Cl- conductance. 3. A swelling-activated current was identified in cerebral vascular smooth muscle cells. This current was sensitive to Cl- channel antagonists including DIDS (300 microM), tamoxifen (1 microM) and IAA-94 (100 microM). However, contrary to expectations, the reversal potential of this swelling-activated current shifted with the Na+ equilibrium potential and not the Cl- equilibrium potential, indicating that the swelling-activated current was carried by cations and not anions. The swelling-activated cation current was blocked by Gd3+, a cation channel antagonist. 4. Gd3+ also blocked both swelling- and pressure-induced depolarization of smooth muscle cells in intact cerebral arteries. 5. These findings suggest that swelling- and pressure-induced depolarization arise from the activation of a cation conductance. This current is inhibited by DIDS, tamoxifen, IAA-94 and gadolinium.

Role of Cl(-) channels in alpha-adrenoceptor-mediated vasoconstriction in the anesthetized rat

In vitro studies have provided evidence that Cl(-) ion currents are important for activation of vascular smooth muscle contraction. The stilbene, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), disrupts Cl(-) metabolism by blocking Cl(-) channels and by inhibiting Cl(-) bicarbonate exchange. The aims of this study were to: (i) characterize the hemodynamic responses produced by DIDS in pentobarbital anesthetized rats, and (ii) examine vasoconstrictor responses to norepinephrine before and after administration of DIDS. DIDS (2.5-50 micromol/kg, 92.5 micromol/kg total dose, i.v.) produced dose-dependent but transient reductions in mean arterial blood pressure and in hindquarter, renal and mesenteric vascular resistances. Prior to the administration of DIDS, norepinephrine (1. 0-5.0 microgram/kg, i.v.) produced dose-dependent increases in mean arterial pressure, renal resistance and mesenteric resistance, but decreases in hindquarter resistance that were inversely related to dose. After administration of DIDS, the peak pressor responses produced by norepinephrine were either slightly diminished (1.0, 2.5 microgram/kg) or unchanged (5.0 microgram/kg). Peak norepinephrine-induced changes in hindquarter and renal vascular resistance were unaffected by DIDS, while increases in mesenteric resistance were augmented. The total norepinephrine-induced increases in mean arterial pressure (mm Hgxs) were markedly reduced by DIDS. These effects of DIDS on norepinephrine-induced responses were similar, but not identical to those of the voltage-sensitive Ca(2+) channel blocker, nifedipine (500 nmol/kg, i.v.). These findings suggest that DIDS may interfere with norepinephrine-induced depolarization of resistance arteries, thereby preventing activation of voltage-sensitive Ca(2+) channels.

Role of intracellular Ca(2+) release in histamine-induced depolarization in rabbit middle cerebral artery

The role of Ca(2+) mobilization from intracellular stores and Ca(2+)-activated Cl(-) channels in caffeine- and histamine-induced depolarization and contraction of the rabbit middle cerebral artery has been studied by recording membrane potential and isometric force. Caffeine induced a transient contraction and a transient followed by sustained depolarization. The transient depolarization was abolished by ryanodine, DIDS, and niflumic acid, suggesting involvement of Ca(2+)-activated Cl(-) channels. Histamine-evoked transient contraction in Ca(2+)-free solution was abolished by ryanodine or by caffeine-induced depletion of Ca(2+) stores. Ryanodine slowed the development of depolarization induced by histamine in Ca(2+)-containing solution but did not affect its magnitude. In arteries treated with 1 mM Co(2+), histamine elicited a transient depolarization and contraction, which was abolished by ryanodine. DIDS and niflumic acid reduced histamine-evoked depolarization and contraction. Histamine caused a sustained depolarization and contraction in low-Cl(-) solution. These results suggest that Ca(2+) mobilization from ryanodine-sensitive stores is involved in histamine-induced initial, but not sustained, depolarization and contraction. Ca(2+)-activated Cl(-) channels contribute mainly to histamine-induced initial depolarization and less importantly to sustained depolarization, which is most likely dependent on activation of nonselective cation channels.

Histamine-induced depolarization: ionic mechanisms and role in sustained contraction of rabbit cerebral arteries

The role of membrane depolarization in the histamine-induced contraction of the rabbit middle cerebral artery was examined by simultaneous measurements of membrane potential and isometric force. Histamine (1-100 microM) induced a concentration-dependent sustained contraction associated with sustained depolarization. Action potentials were observed during depolarization caused by histamine but not by high-K(+) solution. K(+)-induced contraction was much smaller than sustained contraction associated with the same depolarization caused by histamine. Nifedipine attenuates histamine-induced sustained contraction by 80%, with no effect on depolarization. Inhibition of nonselective cation channels with Co(2+) (100-200 microM) reversed the histamine-induced depolarization and relaxed the arteries but induced only a minor change in K(+)-induced contraction. In the presence of Co(2+) and in low-Na(+) solution, histamine-evoked depolarization and contraction were transient. We conclude that nonselective cation channels contribute to histamine-induced sustained depolarization, which stimulates Ca(2+) influx through voltage-dependent Ca(2+) channels participating in contraction. The histamine-induced depolarization, although an important and necessary mechanism, cannot fully account for sustained contraction, which may be due in part to augmentation of currents through voltage-dependent Ca(2+) channels and Ca(2+) sensitization of the contractile process.

Endothelium modulates anion channel-dependent aortic contractions to iodide

Anion currents contribute to vascular smooth muscle (VSM) membrane potential. The substitution of extracellular chloride (Cl) with iodide (I) or bromide (Br) initially inhibited and then potentiated isometric contractile responses of rat aortic rings to norepinephrine. Anion substitution alone produced a small relaxation, which occurred despite a lack of active tone and minimal subsequent contraction of endothelium-intact rings (4.2 +/- 1.2% of the response to 90 mM KCl). Endothelium-denuded rings underwent a similar initial relaxation but then contracted vigorously (I > Br). Responses to 130 mM I (93.7 +/- 1.9% of 90 mM KCl) were inhibited by nifedipine (10(-6) M), niflumic acid (10(-5) M), tamoxifen (10(-5) M), DIDS (10(-4) M), and HCO(-)(3)-free buffer (HEPES 10 mM) but not by bumetanide (10(-5) M). Intact rings treated with N(omega)-nitro-L-arginine (10(-4) M) responded weakly to I (15.5 +/- 2.1% of 90 mM KCl), whereas hemoglobin (10(-5) M), indomethacin (10(-6) M), 17-octadecynoic acid (10(-5) M), and 1H-[1,2, 4]oxadiazole[4,3-a]quinoxalin-1-one (10(-6) M) all failed to augment the response of intact rings to I. We hypothesize that VSM takes up I primarily via an anion exchanger. Subsequent I efflux through anion channels having a selectivity of I > Br > Cl produces depolarization. In endothelium-denuded or agonist-stimulated vessels, this current is sufficient to activate voltage-dependent calcium channels and cause contraction. Neither nitric oxide nor prostaglandins are the primary endothelial modulator of these anion channels. If they are regulated by an endothelium-dependent hyperpolarizing factor it is not a cytochrome P-450 metabolite.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Bimodal control of a Ca(2+)-activated Cl(-) channel by different Ca(2+) signals

Ca(2+)-activated Cl(-) channels play important roles in a variety of physiological processes, including epithelial secretion, maintenance of smooth muscle tone, and repolarization of the cardiac action potential. It remains unclear, however, exactly how these channels are controlled by Ca(2+) and voltage. Excised inside-out patches containing many Ca(2+)-activated Cl(-) channels from Xenopus oocytes were used to study channel regulation. The currents were mediated by a single type of Cl(-) channel that exhibited an anionic selectivity of I(-) > Br(-) > Cl(-) (3.6:1.9:1.0), irrespective of the direction of the current flow or [Ca(2+)]. However, depending on the amplitude of the Ca(2+) signal, this channel exhibited qualitatively different behaviors. At [Ca(2+)] < 1 microM, the currents activated slowly upon depolarization and deactivated upon hyperpolarization and the steady state current-voltage relationship was strongly outwardly rectifying. At higher [Ca(2+)], the currents did not rectify and were time independent. This difference in behavior at different [Ca(2+)] was explained by an apparent voltage-dependent Ca(2+) sensitivity of the channel. At +120 mV, the EC(50) for channel activation by Ca(2+) was approximately fourfold less than at -120 mV (0.9 vs. 4 microM). Thus, at [Ca(2+)] < 1 microM, inward current was smaller than outward current and the currents were time dependent as a consequence of voltage-dependent changes in Ca(2+) binding. The voltage-dependent Ca(2+) sensitivity was explained by a kinetic gating scheme in which channel activation was Ca(2+) dependent and channel closing was voltage sensitive. This scheme was supported by the observation that deactivation time constants of currents produced by rapid Ca(2+) concentration jumps were voltage sensitive, but that the activation time constants were Ca(2+) sensitive. The deactivation time constants increased linearly with the log of membrane potential. The qualitatively different behaviors of this channel in response to different Ca(2+) concentrations adds a new dimension to Ca(2+) signaling: the same channel can mediate either excitatory or inhibitory responses, depending on the amplitude of the cellular Ca(2+) signal.

Ion channels and vascular tone

Ion channels in the plasma membrane of vascular muscle cells that form the walls of resistance arteries and arterioles play a central role in the regulation of vascular tone. Current evidence indicates that vascular smooth muscle cells express at least 4 different types of K(+) channels, 1 to 2 types of voltage-gated Ca(2+) channels, >/=2 types of Cl(-) channels, store-operated Ca(+) (SOC) channels, and stretch-activated cation (SAC) channels in their plasma membranes, all of which may be involved in the regulation of vascular tone. Calcium influx through voltage-gated Ca(2+), SOC, and SAC channels provides a major source of activator Ca(2+) used by resistance arteries and arterioles. In addition, K(+) and Cl(-) channels and the Ca(2+) channels mentioned previously all are involved in the determination of the membrane potential of these cells. Membrane potential is a key variable that not only regulates Ca(+2) influx through voltage-gated Ca(2+) channels, but also influences release of Ca(2+) from internal stores and Ca(2+)- sensitivity of the contractile apparatus. By controlling Ca(2+) delivery and membrane potential, ion channels are involved in all aspects of the generation and regulation of vascular tone.

Traumatic brain injury reduces myogenic responses in pressurized rodent middle cerebral arteries

Traumatic brain injury (TBI) reduces cerebral vascular pressure autoregulation in experimental animals and in patients. In order to understand better the mechanisms of impaired autoregulation, we measured myogenic responses to changes in intraluminal pressure in vitro in pressurized, rodent middle cerebral arteries (MCAs) harvested after TBI. In an approved study, male Sprague-Dawley rats (275-400 g) were anesthetized, intubated, ventilated with 2.0% isoflurane in O2/air, and prepared for fluid percussion TBI. The isoflurane concentration was reduced to 1.5%, and rats (n = 6 per group) were randomly assigned to receive sham TBI followed by decapitation 5 or 30 min later or moderate TBI (2.0 atm) followed by decapitation 5 or 30 min later. After decapitation, MCA segments were removed, mounted on an arteriograph, and pressurized. MCA diameters were measured as transmural pressure was sequentially reduced. MCA diameters remained constant or increased in the sham groups as intraluminal pressure was reduced from 100 to 40 mm Hg. In both TBI groups, diameter decreased with each reduction in pressure. In summary, MCAs removed from uninjured, isoflurane-anesthetized rats had normal vasodilatory responses to decreased intraluminal pressure. In contrast, after TBI, myogenic vasodilatory responses were significantly reduced within 5 min of TBI and the impaired myogenic responses persisted for at least 30 min after TBI.

Ca(2+)-dependent Cl(-) channels in mouse and rabbit aortic smooth muscle cells: regulation by intracellular Ca(2+) and NO

Ca(2+)-dependent Cl(-) (Cl(-)(Ca)) channels and their regulation by intracellular Ca(2+) concentration ([Ca(2+)](i)) and nitric oxide (NO) were characterized in mouse and rabbit aortic smooth muscle cells (SMC) using patch clamp and fura 2 imaging. Single channels (1. 8 pS) and whole cell Cl(-)(Ca) currents were activated by caffeine-induced Ca(2+) release. Single Cl(-)(Ca) channels were also activated by >/=200 nM Ca(2+) in inside-out membrane patches and remained active for >5 min in </=1 microM Ca(2+) but showed rapid rundown in 2 mM Ca(2+). Authentic NO or S-nitroso-N-acetylpenicillamine (SNAP) did not affect their activation or rundown in inside-out patches. In the whole cell, SNAP (100 microM) and 8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate (50 microM) did not affect Cl(-)(Ca) current, but at a higher concentration SNAP (1 mM) induced a sustained [Ca(2+)](i) rise, accompanied by a dramatic decrease in caffeine-induced Ca(2+) release and Cl(-)(Ca) current. These results indicate that 1) mouse and rabbit aortic SMC possess 1.8-pS Cl(-)(Ca) channels that are activated by Ca(2+) release from the stores, 2) both activation and rundown of single Cl(-)(Ca) channels depend on [Ca(2+)](i), and 3) NO does not affect Cl(-)(Ca) channels directly or via cGMP but can inhibit their activation indirectly by decreasing Ca(2+) release from the stores.

Minimal model of arterial chaos generated by coupled intracellular and membrane Ca2+ oscillators

We have developed a mathematical model of arterial vasomotion in which irregular rhythmic activity is generated by the nonlinear interaction of intracellular and membrane oscillators that depend on cyclic release of Ca2+ from internal stores and cyclic influx of extracellular Ca2+, respectively. Four key control variables were selected on the basis of the pharmacological characteristics of histamine-induced vasomotion in rabbit ear arteries: Ca2+ concentration in the cytosol, Ca2+ concentration in ryanodine-sensitive stores, cell membrane potential, and the open state probability of Ca2+-activated K+ channels. Although not represented by independent dynamic variables, the model also incorporates Na+/Ca2+ exchange, the Na+-K+-ATPase, Cl- fluxes, and Ca2+ efflux via the extrusion ATPase. Simulations reproduce a wide spectrum of experimental observations, including 1) the effects of interventions that modulate the functionality of Ca2+ stores and membrane ion channels, 2) paradoxes such as the apparently unpredictable dual action of Ca2+ antagonists and low extracellular Na+ concentration, which can abolish vasomotion or promote the appearance of large-amplitude oscillations, and 3) period-doubling, quasiperiodic, and intermittent routes to chaos. Nonlinearity is essential to explain these diverse patterns of experimental vascular response.

cAMP-dependent protein kinase is in an active state in rat small arteries possessing a myogenic tone

The hypothesis that cAMP-dependent protein kinase (protein kinase A; PKA) is in an active state in small arteries possessing a myogenic tone was investigated in pressurized rat tail small arteries. At a pressure of 80 mmHg, these vessels constricted to 71.6 +/- 1.0% (n = 32) of the diameter of the fully relaxed state. The PKA inhibitors Rp-8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphothioate (Rp-CPT-cAMPS) and N-(2-([3-(4-bromophenyl)-2-propenyl]amino)-ethyl)-5- isoquinolinesulfonamide HCl (H-89) constricted these vessels dose dependently. For example, 300 microM Rp-CPT-cAMPS and 9 microM H-89 reduced vessel diameter by 11.0 +/- 1.2% (n = 8) and 14.3 +/- 3.6% (n = 5), respectively. The cGMP-dependent protein kinase (protein kinase G; PKG) inhibitor Rp-8-bromo-beta-phenyl-1,N(2)-etheno-guanosine 3', 5'-cyclic monophosphothioate (Rp-8-Br-PET-cGMPS) did not alter vessel diameter up to a concentration of 10 microM. Neither endothelium removal nor inhibition of neural transmission affected the action of Rp-CPT-cAMPS. The effect of 300 microM Rp-CPT-cAMPS was reduced by 82% after pretreatment of the vessel with 100 nM iberiotoxin, a blocker of calcium-activated potassium (K(Ca)) channels. However, the effect of 300 microM Rp-CPT-cAMPS was not altered after pretreatment with 1 mM 4-aminopyridine, a blocker of delayed rectifier potassium channels, or 10 microM ryanodine, a blocker of ryanodine receptor-generated calcium sparks. In inside-out patch-clamp experiments on cells isolated from rat tail small arteries, 10 U/ml of the catalytic subunit of PKA together with 100 microM MgATP increased K(Ca) channel activity 30.1 +/- 9. 8-fold (n = 9). Additionally, neither inhibition of PKA or PKG nor moderate activation of PKA or PKG altered the vessel response to a pressure step from 80 to 120 mmHg. These results suggest that in rat tail small arteries possessing a myogenic tone 1) PKA is in an active state modulating the level of the myogenic tone, and 2) K(Ca) channels mediate, at least partly, this effect of PKA.

Effects of anion channel antagonists in canine colonic myocytes: comparative pharmacology of Cl-, Ca2+ and K+ currents

1. Volume-Sensitive, Outwardly Rectifying (VSOR) Cl- currents were measured in canine colonic myocytes by whole-cell patch clamp. Decreasing extracellular osmolarity 50 milliosmoles l-1 activated current that was carried by Cl- and 5 - 7 times greater in the outward direction. 2. Niflumic acid, an inhibitor of Ca2+-activated Cl- channels, did not inhibit VSOR Cl- current. Glibenclamide, an antagonist of CFTR, and anthracene-9-carboxylate (9-AC) inhibited current less than 25% at 100 microM. 3. DIDS (4, 4-diisothiocyanato-stilbene-2,2'disulphonate) inhibited VSOR Cl- current more potently than SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonate). IC50s were 0.84 and 226 microM, respectively. 4. VSOR Cl- current was strongly inhibited by tamoxifen ([Z]-1-[p-dimethylaminoethoxy-phenyl]-1,2-diphenyl-1-butene), an anti-oestrogen compound (IC50=0.57 microM). 5. Gd3+ antagonized VSOR Cl- current more potently than La3+. The IC50 for Gd3+ was 23 microM. In contrast, 100 microM La3+ inhibited current only 35+/-7%. 6. Antagonists of VSOR Cl- current had non-specific effects. These compounds blocked voltage-dependent K+ and Ca2+ currents in colonic myocytes. Tamoxifen (10 microM) and DIDS (10 microM) inhibited L-type Ca2+ current 87+/-7 and 31+/-5%, respectively. Additionally, in the presence of 300 nM charybdotoxin, tamoxifen (1 microM) and DIDS (10 microM) inhibited delayed rectifier K+ current 38+/-8 and 10+/-2%, respectively. 7. The pharmacology of VSOR Cl- channels overlaps with voltage-dependent cation channels. DIDS and tamoxifen inhibited VSOR Cl- equally. However, because DIDS had much less effect on L-type Ca2+ and delayed rectifier K+ channels than did tamoxifen, it might be useful in experiments to investigate the physiological and pathophysiological role of this conductance in whole tissues.

Signaling mechanisms underlying the vascular myogenic response

The vascular myogenic response refers to the acute reaction of a blood vessel to a change in transmural pressure. This response is critically important for the development of resting vascular tone, upon which other control mechanisms exert vasodilator and vasoconstrictor influences. The purpose of this review is to summarize and synthesize information regarding the cellular mechanism(s) underlying the myogenic response in blood vessels, with particular emphasis on arterioles. When necessary, experiments performed on larger blood vessels, visceral smooth muscle, and even striated muscle are cited. Mechanical aspects of myogenic behavior are discussed first, followed by electromechanical coupling mechanisms. Next, mechanotransduction by membrane-bound enzymes and involvement of second messengers, including calcium, are discussed. After this, the roles of the extracellular matrix, integrins, and the smooth muscle cytoskeleton are reviewed, with emphasis on short-term signaling mechanisms. Finally, suggestions are offered for possible future studies.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Cellular and molecular basis for electrical rhythmicity in gastrointestinal muscles

Regulation of gastrointestinal (GI) motility is intimately coordinated with the modulation of ionic conductance expressed in GI smooth muscle and nonmuscle cells. Interstitial cells of Cajal (ICC) act as pacemaker cells and possess unique ionic conductances that trigger slow wave activity in these cells. The slow wave mechanism is an exclusive feature of ICC: Smooth muscle cells may lack the basic ionic mechanisms necessary to generate or regenerate slow waves. The molecular identification of the components for these conductances provides the foundation for a complete understanding of the ionic basis for GI motility. In addition, this information will provide a basis for the identification or development of therapeutics that might act on these channels. It is much easier to study these conductances and develop blocking drugs in expression systems than in native GI muscle cells. This review focuses on the relationship between ionic currents in native GI smooth muscle cells and ICC and their molecular counterparts.

Contribution of Ca2+-activated K+ channels and non-selective cation channels to membrane potential of pulmonary arterial smooth muscle cells of the rabbit

1. Using the perforated patch-clamp or whole-cell clamp technique, we investigated the contribution of Ca2+-activated K+ current (IK(Ca)) and non-selective cation currents (INSC) to the membrane potential in small pulmonary arterial smooth muscle cells of the rabbit. 2. The resting membrane potential (Vm) was -39.2 +/- 0.9 mV (n = 72). It did not stay at a constant level, but hyperpolarized irregularly, showing spontaneous transient hyperpolarizations (STHPs). The mean frequency and amplitude of the STHPs was 5.6 +/- 1. 1 Hz and -7.7 +/- 0.7 mV (n = 12), respectively. In the voltage-clamp mode, spontaneous transient outward currents (STOCs) were recorded with similar frequency and irregularity. 3. Intracellular application of BAPTA or extracellular application of TEA or charybdotoxin suppressed both the STHPs and STOCs. The depletion of intracellular Ca2+ stores by caffeine or ryanodine, and the removal of extracellular Ca2+ also abolished STHPs and STOCs. 4. Replacement of extracellular Na+ with NMDG+ caused hyperpolarization Vm of without affecting STHPs. Removal of extracellular Ca2+ induced a marked depolarization of Vm along with the disappearance of STHPs. 5. The ionic nature of the background inward current was identified. The permeability ratio of K+ : Cs+ : Na+ : Li+ was 1.7 : 1.3 : 1 : 0. 9, indicating that it is a non-selective cation current (INSC). The reversal potential of this current in control conditions was calculated to be -13.9 mV. The current was blocked by millimolar concentrations of extracellular Ca2+ and Mg2+. 6. From these results, it was concluded that (i) hyperpolarizing currents are mainly contributed by Ca2+-activated K+ (KCa) channels, and thus STOCs result in transient membrane hyperpolarization, and (ii) depolarizing currents are carried through NSC channels.

Ca2+ sparks activate K+ and Cl- channels, resulting in spontaneous transient currents in guinea-pig tracheal myocytes

1. Local changes in cytosolic [Ca2+] were imaged with a wide-field, high-speed, digital imaging system while membrane currents were simultaneously recorded using whole-cell, perforated patch recording in freshly dissociated guinea-pig tracheal myocytes. 2. Depending on membrane potential, Ca2+ sparks triggered 'spontaneous' transient inward currents (STICs), 'spontaneous' transient outward currents (STOCs) and biphasic currents in which the outward phase always preceded the inward (STOICs). The outward currents resulted from the opening of large-conductance Ca2+-activated K+ (BK) channels and the inward currents from Ca2+-activated Cl- (ClCa) channels. 3. A single Ca2+ spark elicited both phases of a STOIC, and sparks originating from the same site triggered STOCs, STICs and STOICs, depending on membrane potential. 4. STOCs had a shorter time to peak (TTP) than Ca2+ sparks and a much shorter half-time of decay. In contrast, STICs had a somewhat longer TTP than sparks but the same half-time of decay. Thus, the STIC, not the STOC, more closely reflected the time course of cytosolic Ca2+ elevation during a Ca2+ spark. 5. These findings suggest that ClCa channels and BK channels may be organized spatially in quite different ways in relation to points of Ca2+ release from intracellular Ca2+ stores. The results also suggest that Ca2+ sparks may have functions in smooth muscle not previously suggested, such as a stabilizing effect on membrane potential and hence on the contractile state of the cell, or as activators of voltage-gated Ca2+ channels due to depolarization mediated by STICs.

Biophysical signals underlying myogenic responses in rat interlobular artery

-To assess cellular mechanisms mediating myogenic responses of interlobular artery (ILA), experiments were performed with the use of isolated perfused hydronephrotic kidneys. ILAs were divided into 3 groups according to their basal diameters: proximal (>60 microm), intermediate (40 to 60 microm), and distal (<40 microm) ILAs. Myogenic responses were obtained by stepwise increase in perfusion pressure. Greater myogenic responsiveness was observed in ILAs with smaller diameters. Diltiazem (10 micromol/L) inhibited myogenic responses of all segments of ILAs. Furthermore, gadolinium (10 micromol/L), a mechanosensitive cation channel blocker, abolished myogenic responses of distal but not proximal ILA. In contrast, 2-nitro-4-carboxyphenyl-N, N-diphenyl-carbamate (200 micromol/L), an inhibitor of phospholipase C, prevented myogenic responses of proximal but not distal ILA. Finally, basal proximal ILA diameters were increased by treatment with 50 nmol/L of staurosporine (P<0.05), and subsequent addition of thapsigargin (1 micromol/L) blocked myogenic contraction of proximal ILAs. Myogenic responses of intermediate ILAs exhibited characteristics between those of distal and proximal ILAs. Our data indicate that underlying mechanisms for myogenic responses differ in distinct segments of ILAs. The present results suggest that mechanosensitive cation channels are involved in myogenic constriction of distal ILAs. Finally, our findings provide evidence that the stimulation of phospholipase C mediates myogenic contraction of proximal ILAs.

Properties of a Cl- current activated by cell swelling in rabbit portal vein vascular smooth muscle cells

In rabbit portal vein smooth muscle cells, application of a hypotonic external solution caused cell swelling and evoked an outwardly rectifying Cl- current. The hypotonicity-activated current was markedly reduced by the anti-estrogen tamoxifen (10 microM) and was inhibited by DIDS in a voltage-dependent manner [the concentration required to inhibit the current by 50% (IC50) at -50 and +100 mV was 21 and 5 microM DIDS, respectively]. Indanyloxyacetic acid 94 (IAA-94) and niflumic acid also inhibited the hypotonicity-activated current, with 50% inhibition produced at concentrations of approximately 200 and 100 microM, respectively. In isotonic conditions, application of tamoxifen and DIDS to cells decreased the holding current due to the inhibition of a resting conductance that was outwardly rectifying and reversed at the Cl- equilibrium potential. These data show that rabbit portal vein myocytes have a resting Cl- conductance that is enhanced by cell swelling; its possible physiological role is discussed.

Functional and molecular identification of a novel chloride conductance in canine colonic smooth muscle

Swelling-activated or volume-sensitive Cl- currents are found in numerous cell types and play a variety of roles in their function; however, molecular characterization of the channels is generally lacking. Recently, the molecular entity responsible for swelling-activated Cl- current in cardiac myocytes has been identified as ClC-3. The goal of our study was to determine whether such a channel exists in smooth muscle cells of the canine colon using both molecular biological and electrophysiological techniques and, if present, to characterize its functional and molecular properties. We hypothesized that ClC-3 is present in colonic smooth muscle and is regulated in a manner similar to the molecular entity cloned from heart. Indeed, the ClC-3 gene was expressed in colonic myocytes, as demonstrated by reverse transcriptase polymerase chain reaction performed on isolated cells. The current activated by decreasing extracellular osmolarity from 300 to 250 mosM was outwardly rectifying and dependent on the Cl- gradient. Current magnitude increased and reversed at more negative potentials when Cl- was replaced by I- or Br-. Tamoxifen ([Z]-1-[p-dimethylaminoethoxy-phenyl]-1,2-diphenyl-1-butene; 10 microM) and DIDS (100 microM) inhibited the current, whereas 25 microM niflumic acid, 10 microM nicardipine, and Ca2+ removal had no effect. Current was inhibited by 1 mM extracellular ATP in a voltage-dependent manner. Cl- current was also regulated by protein kinase C, as phorbol 12,13-dibutyrate (300 nM) decreased Cl- current magnitude, while chelerythrine chloride (30 microM) activated it under isotonic conditions. Our findings indicate that a current activated by hypotonic solution is present in colonic myocytes and is likely mediated by ClC-3. Furthermore, we suggest that the ClC-3 may be an important mechanism controlling depolarization and contraction of colonic smooth muscle under conditions that impose physical stress on the cells.

Cyclic GMP-dependent but G-kinase-independent inhibition of Ca2+-dependent Cl- currents by NO donors in cat tracheal smooth muscle

1. The effects of NO donors on Ca2+-dependent Cl- currents (ICl(Ca)) were investigated in freshly isolated cat tracheal myocytes using the whole-cell patch clamp technique. 2. With nystatin-perforated whole-cell recording, carbachol (CCh, >/= 1 microM) induced a transient inward current (ICCh) with a reversal potential of about -20 mV. Activation of ICCh probably occurred through the M3 muscarinic receptor, since nanomolar concentrations of 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) greatly inhibited this current, while 11-(2-(diethylamino)methyl)-1-piperidinylacetyl)-5, 11-dihydro-6H-pyrido (2,3beta) (1,4)benzodiazepine-6-one (AF-DX 116) or pirenzepine at concentrations of up to 1 microM were almost ineffective. 3. Chloride channel/transporter blockers such as DIDS (100 microM), anthracene-9-carboxylic acid (9-AC, 100 microM) and niflumic acid (100 microM) greatly inhibited ICCh, but cation channel blockers, such as nifedipine (10 microM), Zn2+ (500 microM) or Gd3+ (500 microM), were without effect. 4. Activation of ICCh was strongly attenuated by pretreatment with ryanodine (4 microM) plus caffeine (10 mM). Addition of neomycin (1 mM) into the bath or inclusion of heparin (3 mg ml-1) in the pipette abolished a substantial part of ICCh. These results suggest that ICCh is ICl(Ca), which is activated by inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ release. 5. The nitric oxide donor S-nitroso-N-acetyl penicillamine (SNAP) reduced the amplitude of ICCh dose dependently (IC50, approximately 10 microM). Similar inhibition was also exerted by other types of NO donor such as glyceryl trinitrate (GTN) and (+/-)-E-methyl-2-(E-hydroxyimitol)-5-nitro-6-methoxy-3- hexeneamide (NO-R). 6. SNAP-induced ICCh inhibition was effectively antagonized by Methylene Blue (1-100 nM), and mimicked by dibutyryl cGMP (db-cGMP) (0.5-1 mM), whereas two structurally distinct types of cGMP-dependent (G)-kinase inhibitor, N-(2-aminoethyl)-5-isoquinilinesulphonamide (H-8, 2.5 microM) and KT5823 (1 microM), failed to counteract the inhibitory effects of SNAP or db-cGMP. Another G-kinase-specific inhibitor Rp-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate (Rp-8-pCPT-cGMPS; 1 microM) itself caused a marked reduction in ICCh. 7. SNAP (100 microM) or db-cGMP (100 microM) exhibited no inhibitory actions, when caffeine (10 mM) or photolytically released IP3 were used instead of CCh to activate the inward current. 8. These results suggest that inhibition of ICCh by NO donors involves a cGMP-dependent but G-kinase-independent mechanism, which may operate at a site(s) between the muscarinic (M3) and IP3 receptors.

Mechanosensitive cation channels mediate afferent arteriolar myogenic constriction in the isolated rat kidney

1. In order to assess ionic mechanisms mediating renal afferent arteriolar myogenic constriction, experiments were performed using isolated perfused hydronephrotic rat kidneys. 2. Increasing pressure progressively constricted the afferent arteriole (-0.26 +/- 0.02% mmHg-1, n = 21, r = 0.97). Gadolinium (10 microM), a mechanosensitive cation channel blocker, abolished this myogenic constriction. However, high potassium media (30 mM) constricted the afferent arteriole in the presence of gadolinium. 3. Lowering extracellular sodium concentration gradually attenuated afferent arteriolar myogenic constriction. In the perfusate containing 50 mM sodium, the myogenic response was arrested. 4. Afferent arteriolar myogenic constriction was prevented in calcium-free perfusate or by the L-type calcium channel blocker diltiazem (10 microM). 5. Our present findings provide evidence that increasing pressure gates mechanosensitive cation channels on the afferent arteriole, thereby eliciting membrane depolarization and activating voltage-dependent calcium channels.

Streptomycin inhibition of myogenic tone, K+-induced force and block of L-type calcium current in rat cerebral arteries

1. Streptomycin has been demonstrated to inhibit mechanosensitive conductances in a wide variety of cell types, including muscle. The action of streptomycin on rat cerebral arteries that exhibit pressure-induced myogenic response was investigated. 2. Pressure-induced tone, measured using isobaric myography, in isolated pressurized cerebral arteries was reversibly and concentration-dependently inhibited by streptomycin with an IC50 of 2.6 mM. 3. Isometric K+-induced force, measured using isometric myography, is supported by voltage-gated Ca2+ entry. Streptomycin reversibly and concentration-dependently inhibited isometric force with an IC50 of 1.71 mM. 4. Voltage-gated macroscopic inward Ca2+ channel currents were recorded from freshly isolated rat basilar myocytes. These were reversibly and concentration-dependently inhibited by streptomycin with an IC50 of 1.79 and 0.47 mM when 10 and 1.8 mM CaCl2, respectively, was used as the charge carrier. 5. These data suggest that streptomycin inhibits myogenic tone and K+-induced isometric force largely by blockade of L-type, dihydropyridine-sensitive Ca2+ channels. In conclusion, streptomycin is not useful in the investigation of stretch-activated channels which may underlie the myogenic response of rat small cerebral arteries.

Nitric oxide suppresses a Ca(2+)-stimulated Cl- current in smooth muscle cells of opossum esophagus

Nitric oxide (NO) hyperpolarizes visceral smooth muscles. Using the patch-clamp technique, we investigated the possibility that NO-mediated hyperpolarization in the circular muscle of opossum esophagus results from the suppression of a Ca(2+)-stimulated Cl- current. Smooth muscle cells were dissociated from the circular layer and bathed in high-K+ Ca(2+)-EGTA-buffered solution. Macroscopic ramp currents were recorded from cell-attached patches. Contaminating K(+)-channel currents were blocked with tetrapentylammonium chloride (200 microM) added to all solutions. Raising bath Ca2+ concentration above 150 nM in the presence of A-23187 (10 microM) activated a leak current (IL-Ca) with an EC50 of 1.2 microM at -100 mV. The reversal potential (Erev) of IL-Ca (-8.5 +/- 1.8 mV, n = 8) was significantly different (P < 0.05) from Erev of the background current (+4.2 +/- 1.2 mV, n = 8). Equimolar substitution of 135 mM Cl- in the pipette solution with gluconate significantly shifted Erev of IL-Ca to +16.6 +/- 3.4 mV (n = 4) (P < 0.05 compared with background), whereas replacement of total Na+ with Tris+ suppressed IL-Ca but did not affect Erev (-15 +/- 3 mV, n = 3; P > 0.05). IL-Ca was inhibited by DIDS (500 microM). Diethylenetriamine-NO adduct (200 microM), a NO donor, and 8-bromo-cGMP (200 microM) suppressed IL-Ca by 59 +/- 15% (n = 5) and 62 +/- 21% (n = 4) at -100 mV, respectively. We conclude that in opossum esophageal smooth muscle NO-mediated hyperpolarization may be produced by suppression of a Ca(2+)-stimulated Cl(-)-permeable conductance via formation of cGMP.

Regulation of arterial diameter and wall [Ca2+] in cerebral arteries of rat by membrane potential and intravascular pressure

1. The regulation of intracellular [Ca2+] in the smooth muscle cells in the wall of small pressurized cerebral arteries (100-200 micron) of rat was studied using simultaneous digital fluorescence video imaging of arterial diameter and wall [Ca2+], combined with microelectrode measurements of arterial membrane potential. 2. Elevation of intravascular pressure (from 10 to 100 mmHg) caused a membrane depolarization from -63 +/- 1 to -36 +/- 2 mV, increased arterial wall [Ca2+] from 119 +/- 10 to 245 +/- 9 nM, and constricted the arteries from 208 +/- 10 micron (fully dilated, Ca2+ free) to 116 +/- 7 micron or by 45 % ('myogenic tone'). 3. Pressure-induced increases in arterial wall [Ca2+] and vasoconstriction were blocked by inhibitors of voltage-dependent Ca2+ channels (diltiazem and nisoldipine) or to the same extent by removal of external Ca2+. 4. At a steady pressure (i.e. under isobaric conditions at 60 mmHg), the membrane potential was stable at -45 +/- 1 mV, intracellular [Ca2+] was 190 +/- 10 nM, and arteries were constricted by 41 % (to 115 +/- 7 micron from 196 +/- 8 micron fully dilated). Under this condition of -45 +/- 5 mV at 60 mmHg, the voltage sensitivity of wall [Ca2+] and diameter were 7.5 nM mV-1 and 7.5 micron mV-1, respectively, resulting in a Ca2+ sensitivity of diameter of 1 mum nM-1. 5. Membrane potential depolarization from -58 to -23 mV caused pressurized arteries (to 60 mmHg) to constrict over their entire working range, i.e. from maximally dilated to constricted. This depolarization was associated with an elevation of arterial wall [Ca2+] from 124 +/- 7 to 347 +/- 12 nM. These increases in arterial wall [Ca2+] and vasoconstriction were blocked by L-type voltage-dependent Ca2+ channel inhibitors. 6. The relationship between arterial wall [Ca2+] and membrane potential was not significantly different under isobaric (60 mmHg) and non-isobaric conditions (10-100 mmHg), suggesting that intravascular pressure regulates arterial wall [Ca2+] through changes in membrane potential. 7. The results are consistent with the idea that intravascular pressure causes membrane potential depolarization, which opens voltage-dependent Ca2+ channels, acting as 'voltage sensors', thus increasing Ca2+ entry and arterial wall [Ca2+], which leads to vasoconstriction.

Functional and molecular expression of volume-regulated chloride channels in canine vascular smooth muscle cells

1. We examined the possibility of functional and molecular expression of volume-regulated Cl- channels in vascular smooth muscle using the whole-cell patch-clamp technique and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) on cells from canine pulmonary and renal arteries. 2. Decreasing external osmolarity induced cell swelling, which was accompanied by activation of Cl--dependent outward-rectifying membrane currents with an anion permeability sequence of SCN- > I- > Br- > Cl- > aspartate-. These currents were sensitive to block by DIDS, extracellular ATP and the antioestrogen compound tamoxifen. 3. Experiments were performed to determine whether the molecular form of the volume-regulated chloride channel (ClC-3) is expressed in pulmonary and renal arteries. Quantitative RT-PCR confirmed expression of ClC-3 in both types of smooth muscle. ClC-3 expression was 76.4 % of beta-actin in renal artery and 48.0 % of beta-actin in pulmonary artery. 4. We conclude that volume-regulated Cl- channels are expressed in vascular smooth muscle cells and exhibit functional properties similar to those found in other types of cells, presumably contributing to the regulation of cell volume, electrical activity and, possibly, myogenic tone.

Non-specificity of chloride channel blockers in rat cerebral arteries: block of the L-type calcium channel

1. The effects of chloride channel blockers on pressure-induced constriction, K(+)-induced force, and whole-cell calcium channel currents were tested in rat cerebral arteries using isobaric and isometric myography, and patch clamp. 2. Under isobaric conditions at 75 mmHg, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), a chloride channel blocker, reversibly depressed the myogenic constriction with an IC50 of 32.8 +/- 0.52 microM (mean +/- S.E.M., n = 5). Blockers of Ca(2+)-activated chloride channels, flufenamic acid (100 microM) and 9-anthracene chloride (9-AC; 1 mM), and the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel blocker, glibenclamide (100 microM), were without effect in this tissue (n = 3). 3. Under isobaric conditions at 20 mmHg, 37 degrees C, raising [K+]o to 45 mM induced a constriction which was unaffected by 100 microM NPPB (n = 4). In contrast, at 75 mmHg and 18-21 degrees C, 100 microM NPPB completely and reversibly blocked a 45 mM K(+)-induced constriction (n = 3). 4. Under isometric conditions, NPPB reversibly depressed a 45 mM K(+)-induced force with an IC50 of 10.0 +/- 0.76 microM (mean +/- S.E.M., n = 5). Indanyloxyacetic acid 94 (IAA-94), another chloride channel blocker, depressed the K(+)-induced force with an IC50 of 17.0 +/- 1.2 microM (mean +/- S.E.M., n = 4). 5. Using whole-cell patch clamp, 100 microM NPPB or 200 microM IAA-94 blocked calcium channel currents carried by 10 mM Ba2+ by 79.1 +/- 1.7 and 39.8 +/- 7.0%, respectively (mean +/- S.E.M., n = 6). 6. In summary, chloride channel blockers depress calcium channel currents in rat cerebral arteries, which could contribute to a reduction in myogenic contraction.

Calcium mobilization contributes to pressure-mediated afferent arteriolar vasoconstriction

Preglomerular responses to vasoactive agonists utilize calcium released from intracellular stores and activation of calcium influx pathways to elicit vasoconstriction. The current study was performed to determine the role of calcium release from intracellular stores on the afferent arteriolar response to increases in perfusion pressure. Experiments were performed, in vitro, using the blood perfused, juxtamedullary nephron technique combined with videomicroscopy. The response of afferent arterioles to 30 mm Hg increases in perfusion pressure was determined before and after depletion of intracellular calcium pools with a 10-minute preincubation with 1 micromol/L thapsigargin or 100 micromol/L cyclopiazonic acid. Afferent arteriolar diameter averaged 20.2+/-1.0 microm (n=19) at a control perfusion pressure of 100 mm Hg. Increasing perfusion pressure to 130 and 160 mm Hg reduced afferent caliber by 10.7+/-1.0% (P<.05 versus con) and by 24.7+/-1.6% (P<.05 versus diameter at 130 mm Hg); respectively. Thapsigargin significantly increased afferent diameter by 21+/-2% (n=6) at 100 mm Hg and prevented pressure-induced autoregulatory responses. Afferent diameter averaged 24.3+/-1.7, 24.5+/-1.8 and 24.3+/-1.8 microm at perfusion pressures of 100, 130 and 160 mm Hg; respectively. Cyclopiazonic acid treatment also inhibited autoregulatory behavior but did not alter resting vessel diameter. Afferent arteriolar diameter (n=6) averaged 21.4+/-1.9 microm at 100 mm Hg and 20.9+/-2.1 and 20.5+/-2.2 microm at 130 and 160 mm Hg; respectively. Additional studies were performed to assess the role of phospholipase C activity in pressure-mediated autoregulatory behavior of afferent arterioles. Step increases in perfusion pressure decreased afferent diameter by 10.7+/-3.8 and 21.7+/-4.1%; respectively. Administration of the phospholipase C inhibitor, U-73122, (5 micromoles/L) did not significantly alter baseline diameter but did attenuate the pressure-mediated vasoconstrictor response. Increasing perfusion pressure to 130 and 160 mm Hg reduced afferent diameter by only 6.5+/-1.5 and 10.0+/-2.0%; respectively. These data demonstrate that interruption of calcium mobilization with thapsigargin, cyclopiazonic acid, or phospholipase C inhibition markedly attenuates pressure-mediated afferent arteriolar vasoconstriction and suggests that autoregulatory adjustments in afferent arteriolar diameter involve calcium release from inositoltrisphosphate(IP3)-sensitive intracellular stores.