Depolarization Of Smooth Muscle Cells Research Articles

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.

High K+-induced membrane depolarization attenuates endothelium-dependent pulmonary vasodilation

Impairment of endothelium-dependent pulmonary vasodilation has been implicated in the development of pulmonary hypertension. Pulmonary vascular smooth muscle cells and endothelial cells communicate electrically through gap junctions; thus, membrane depolarization in smooth muscle cells would depolarize endothelial cells. In this study, we examined the effect of prolonged membrane depolarization induced by high K(+) on the endothelium-dependent pulmonary vasodilation. Isometric tension was measured in isolated pulmonary arteries (PA) from Sprague-Dawley rats, and membrane potential was measured in single PA smooth muscle cells. Increase in extracellular K(+) concentration from 4.7 to 25 mM significantly depolarized PA smooth muscle cells. The 25 mM K(+)-mediated depolarization was characterized by an initial transient depolarization (5-15 s) followed by a sustained depolarization that could last for up to 3 h. In endothelium-intact PA rings, ACh (2 microM), levcromakalim (10 microM), and nitroprusside (10 microM) reversibly inhibited the 25 mM K(+)-mediated contraction. Functional removal of endothelium abolished the ACh-mediated relaxation but had no effect on the levcromakalim- or the nitroprusside-mediated pulmonary vasodilation. Prolonged ( approximately 3 h) membrane depolarization by 25 mM K(+) significantly inhibited the ACh-mediated PA relaxation (-55 +/- 4 vs. -29 +/- 2%, P < 0.001), negligibly affected the levcromakalim-mediated pulmonary vasodilation (-92 +/- 4 vs. -95 +/- 5%), and slightly but significantly increased the nitroprusside-mediated PA relaxation (-80 +/- 2 vs. 90 +/- 3%, P < 0. 05). These data indicate that membrane depolarization by prolonged exposure to high K(+) concentration selectively inhibited endothelium-dependent pulmonary vasodilation, suggesting that membrane depolarization plays a role in the impairment of pulmonary endothelial function in pulmonary hypertension.

Mechanisms that regulate [Ca2+]i following depolarization in rat systemic arterial smooth muscle cells.

1. We have used the patch-clamp technique in combination with fluorimetric recording to study the mechanisms that regulate intracellular Ca2+, [Ca2+]i, following depolarization in cells isolated from the rat femoral artery. 2. Depolarization to 0 mV from a holding potential of -70 mV increased [Ca2+]i. Little Ca2+ release from sarcoplasmic reticulum, SR, was detected during depolarization since application of 30 microM ryanodine, a Ca2+-release inhibitor, had no significant effect on total Ca2+ buffering power. 3. Upon repolarization to -70 mV, 7 out of 13 cells showed three phases of Ca2+ removal; an initial rapid first phase, a slow second phase, and a faster third phase. Six cells, in which Ca2+ recovered quickly, lacked the third phase. The third phase was also absent in cells treated with a SR Ca2+-pump inhibitor, cyclopiazonic acid. 4. The peak first-phase Ca2+ removal rate observed upon repolarization to -70 mV was significantly reduced in cells treated with a mitochondrial Ca2+ uptake inhibitor, carbonyl cyanide m-chlorophenylhydrazone. However, an ATP-synthase inhibitor, oligomycin B, had no significant effect. 5. The Ca2+ removal rate was little affected by clamping the cell at +120 mV rather than -70 mV, suggesting that Ca2+ removal processes are largely voltage independent. Also, little inward current was associated with Ca2+ clearance, indicating that Ca2+ removal does not involve an electrogenic process. 6. Our results suggest that Ca2+-induced Ca2+ release contributes little to the elevation of Ca2+ in these cells. The SR Ca2+ pump may contribute to Ca2+ removal over a low [Ca2+]i range in cells where [Ca2+]i remains high for long enough, while mitochondrial Ca2+ uptake may be important when [Ca2+]i is high.

Role of Ca2+ Mobilization in Muscarinic Receptor-Mediated Membrane Depolarization in Guinea Pig Ileal Smooth Muscle Cells

In single smooth muscle cells dispersed from guinea pig ileum, the muscarinic agonist carbachol (CCh) at 2 μM produced an oscillatory or sustained type of depolarization and at 100 μM, the latter type depolarization. Depletion of internal Ca2+ stores blocked the oscillatory response, but not the sustained responses to 2 μM and 100 μuM CCh, although their decay after reaching the peak became faster. Blocking voltage-dependent Ca2+ channels (VDCCs) blocked both types of response to 2 μM CCh, but only slowed the initial rising phase of 100 μuM CCh responses. Combination of Ca2+ store depletion and VDCC blockade abolished the responses to 2 μM CCh again and decreased those to 100 μuM CCh in peak amplitude and persistency. Combination of Ca2+ store depletion with removal of extracellular Ca2+ markedly reduced or abolished the 100 μuM CCh responses. The results suggest that muscarinic depolarization of the ileal cells requires Ca2+ mobilization for its generation and persistence; at weak muscarinic stimulation, both Ca2+ entry via VDCCs and Ca2+ release from internal stores may contribute to the Ca2+ mobilization; and under strong muscarinic stimulation, Ca2+ entry pathways resistant to VDCC blockers may also contribute to it.

Protein kinase C-dependent inhibition of K+ currents in noradrenaline-induced depolarization of smooth muscle of guinea-pig vas deferens

Ionic mechanisms and signal transduction underlying noradrenaline (NA)-induced depolarization in single smooth muscle cells of guinea-pig vas deferens were studied. NA caused depolarization followed by action potentials through activation of α1-adrenoceptors. In the presence of nifedipine, no action potential was generated, and the magnitude of the depolarization depended on the concentration of NA (0.1-100 μm). NA, through α1-adrenoceptor activation, reduced the magnitude of membrane currents in response to voltage ramp pulses from -90 to -30 mV in a concentration-dependent manner. The reversal potential of the current inhibited by NA changed proportionally to the change in the equilibrium potential of K+, suggesting that NA inhibited K+ channel activity. Treatment of cells with GDPβS, an inhibitor of G proteins, or bisindolylmaleimide (BIM), a selective protein kinase C (PKC) inhibitor, prevented the NA inhibition of the currents. Application of 12-O-tetradecanoylphorbol 13-acetate (TPA), an activator of PKC, mimicked the effect of NA. It is suggested that in the smooth muscle of guinea-pig vas deferens, activation of α1-adrenoceptors and the subsequent activation of PKC led to inhibition of K+ currents, which is responsible for the depolarization induced by NA.

Protein Kinase C‐Dependent Inhibition of K+ Currents in Noradrenaline‐Induced Depolarization of Smooth Muscle of Guinea‐Pig vas Deferens

Ionic mechanisms and signal transduction underlying noradrenaline (NA)-induced depolarization in single smooth muscle cells of guinea-pig vas deferens were studied. NA caused depolarization followed by action potentials through activation of 1-adrenoceptors. In the presence of nifedipine, no action potential was generated, and the magnitude of the depolarization depended on the concentration of NA (0.1-100 micrometer). NA, through 1-adrenoceptor activation, reduced the magnitude of membrane currents in response to voltage ramp pulses from -90 to -30 mV in a concentration-dependent manner. The reversal potential of the current inhibited by NA changed proportionally to the change in the equilibrium potential of K+, suggesting that NA inhibited K+ channel activity. Treatment of cells with GDPS, an inhibitor of G proteins, or bisindolylmaleimide (BIM), a selective protein kinase C (PKC) inhibitor, prevented the NA inhibition of the currents. Application of 12-O-tetradecanoylphorbol 13-acetate (TPA), an activator of PKC, mimicked the effect of NA. It is suggested that in the smooth muscle of guinea-pig vas deferens, activation of 1-adrenoceptors and the subsequent activation of PKC led to inhibition of K+ currents, which is responsible for the depolarization induced by NA.

Parathyroid hypertensive factor inhibits voltage-gated K+ channels in vascular smooth muscle cells

Parathyroid hypertensive factor (PHF) has been implicated in regulation of vascular smooth muscle tone and pathogenesis of several forms of hypertension. Earlier studies have suggested that PHF enhances the actions of other vasoconstrictors, while it has no in vitro vasoconstrictor property of its own. PHF was previously found to enhance the L-type Ca channel currents and intracellular Ca responses to depolarization in vascular smooth muscle cells (VSMCs). The present study examined whether PHF might act on K channels in the plasma membrane of VSMCs. Primary cultured VSMCs from rat tail artery were used. The whole-cell version of the patch-clamp technique was used under conditions in which there was no contribution of Ca-activated K channels to the outward current. Both purified and semipurified PHF inhibited the delayed rectifier type potassium current in a dose-dependent manner. The effect was time dependent and was first significantly different from the control current after 30 min. The inhibition of the delayed rectifier K channel was associated with a time-dependent decrease in the resting membrane potential. Therefore, PHF may alter VSMC cellular Ca responses by reducing the membrane potential to a level closer to the activation potential of Ca channels.

Effects of fluoxetine, phentermine, and venlafaxine on pulmonary arterial pressure and electrophysiology.

The anorexic agents dexfenfluramine and fenfluramine plus phentermine have been associated with outbreaks of pulmonary hypertension. The fenfluramines release serotonin and reduce serotonin reuptake in neurons. They also inhibit potassium current (IK), causing membrane potential depolarization in pulmonary arterial smooth muscle cells. The recent withdrawal of the fenfluramines has led to the use of fluoxetine and phentermine as an alternative anorexic combination. Because fluoxetine and venlafaxine reduce serotonin reuptake, we compared the effects of these agents with those of phentermine and dexfenfluramine on pulmonary arterial pressure, IK, and membrane potential. Fluoxetine, venlafaxine, and phentermine caused minimal increases in pulmonary arterial pressure at concentrations < 100 microM but did cause a dose-dependent inhibition of IK. The order of potency for inhibition of IK at +50 mV was fluoxetine > dexfenfluramine = venlafaxine > phentermine. Despite the inhibitory effect on IK at more positive membrane potentials, fluoxetine, venlafaxine, and phentermine, in contrast to dexfenfluramine, had minimal effects on the cell resting membrane potential (all at a concentration of 100 microM). However, application of 100 microM fluoxetine to cells that had been depolarized to -30 mV by current injection elicited a further depolarization of >18 mV. These results suggest that fluoxetine, venlafaxine, and phentermine do not inhibit IK at the resting membrane potential. Consequently, they may present less risk of inducing pulmonary hypertension than the fenfluramines, at least by mechanisms involving membrane depolarization.

Spontaneous and neurally activated depolarizations in smooth muscle cells of the guinea-pig urethra.

1. Membrane potential recordings were made from longitudinal smooth muscle cells of the guinea-pig urethra using conventional microelectrode techniques. 2. Smooth muscle cells of the urethra developed spontaneous transient depolarizations (STDs) and slow waves. Single unit STDs had amplitudes of approximately 5 mV and slow waves seemed to occur as amplitude multiples of single unit STDs. 3. STDs and slow waves were abolished by niflumic acid or low chloride solution and also by cyclopiazonic acid (CPA), BAPTA or high concentrations of caffeine. Lower concentrations of caffeine abolished slow waves but not STDs. Nifedipine inhibited slow waves but not STDs. 4. When stochastic properties of STDs were examined, it was found that the intervals between occurrences were not well modelled by Poisson statistics, instead the STDs appeared to be clustered. 5. Transmural stimulation evoked excitatory junctional potentials (EJPs) and triggered slow waves which were abolished by either alpha,beta-methylene-ATP or tetrodotoxin. Evoked slow waves were also abolished by caffeine, co-application of caffeine and ryanodine or by CPA which left EJPs unaffected. 6. In conclusion, smooth muscle cells of urethra exhibit STDs which are clustered rather than random events, and are the result of spontaneous Ca2+ release from intracellular stores and subsequent activation of Ca2+-activated chloride channels. STDs sum to activate L-type Ca2+ channels which contribute to the sustained phase of slow waves. Stimulation of purinoceptors by neurally released ATP initiates EJPs and also causes the release of Ca2+ from intracellular stores to evoke slow waves.

Involvement of Ca2+ mobilizing mechanisms in muscarinic receptor-mediated depolarization in single smooth muscle cells of guinea-pig ileum.

Involvement of Ca2+ mobilizing mechanisms in muscarinic receptor-mediated depolarization in single smooth muscle cells of guinea-pig ileum.

Functional changes in human pial arteries (300 to 900 micrometer ID) within 48 hours of aneurysmal subarachnoid hemorrhage.

Animal studies of cerebral arteries 2 to 3 days after experimental subarachnoid hemorrhage (SAH) provide evidence of arterial change such as hyperresponsiveness to contractile agonists. There is evidence that small arteries, as well as those large enough to be seen on angiography, may be involved. To directly test these possibilities, the contractile and dilator responses of pial artery segments taken from patients up to 48 hours after SAH were compared with those from patients having elective surgery for an aneurysm (Clip) and with those from normal brain vessels overlying tumors (controls). Segments were mounted on a resistance artery myograph for measurements of wall force changes. There were no differences in maximum contractility (Emax) of the 3 groups of segments. The responses of the SAH segments to K+ (30 mmol/L) were 60.7+/-4.6% of Emax (n [number of vessels]=18), which was significantly greater than those of controls (29.9+/-5% Emax) (n=20). Clip responses were the same as control. Contractions of SAH segments to norepinephrine (1 micromol/L) were 54.3+/-7.9% Emax (n=12), and these were significantly greater than those of controls (15.1+/-6.2% Emax) (n=25). All SAH segments showed spontaneous contractile activity of varying patterns. Spontaneous activity did not occur in the Clip group and occurred in only 50% of control segments. Dilation to acetylcholine was numerically less in SAH and Clip segments than in controls, but differences were not statistically significant. The change in agonist responsiveness could result from exposure to agents that damage the blood vessel wall, resulting in partial depolarization of endothelial and smooth muscle cells. Small human pial arteries are hyperresponsive to contractile agents and show spontaneous contractile activity within 48 hours of SAH. Such effects could result in narrowed resistance arteries and reduction in cerebral blood flow. These effects emphasize the wisdom of early therapeutic intervention.

Chronic blockade of nitric oxide synthase and endothelin receptors during pregnancy in the rat: effect on reactivity of the uterine artery in vitro.

To investigate the effects of chronic blockade of nitric oxide (NO) production and endothelin (ET-1) receptor antagonism on endothelial and vascular smooth muscle function of the uterine artery in vitro obtained from nonpregnant and pregnant rats. Pregnant or nonpregnant Wistar rats were either treated orally for up to 18 days with the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME), the ETA-/ET beta-receptor antagonist bosentan, or both, or they received no treatment (controls). Absolute contractile force as well as endothelium-dependent and -independent vascular reactivity of uterine arteries were determined in vitro. Isometric tension was recorded. ANOVA and the Mann-Whitney U test were used for statistical analysis. Pregnancy increased absolute tension (mN/mm) elicited in uterine arteries by ET-1 (P < .01), serotonin (P < .05), norepinephrine (P < .02), and KCl (P < .0001). Chronic treatment with L-NAME or L-NAME plus bosentan, but not with bosentan alone, reduced contractions to KCl in pregnant and nonpregnant rats (P < .005-.0001), while pregnancy-induced enhancement in tension development remained unchanged in all groups (P < .005). After exposure of uterine arteries to L-NAME in vitro, vascular sensitivity to ET-1 was augmented in uterine arteries of pregnant but not of nonpregnant animals (P < .05). L-NAME-pretreatment did not influence the pregnancy-induced increase of vascular sensitivity to acetylcholine but reduced maximal relaxation in nonpregnant animals (P < .05). In addition, pregnancy diminished sensitivity of uterine arteries to sodium nitroprusside (P < .002), which was abolished by chronically administered L-NAME. Bosentan had no influence on vasodilation in vitro. Neither endothelin-1 nor nitric oxide seem to contribute to the augmented tension to depolarization and receptor-operated stimulation of vascular smooth muscle cells in rat uterine arteries during pregnancy. In addition, pregnancy is associated with increased NO production in uterine arteries, as evidenced by augmented endothelium-dependent relaxations, increased NO release by endothelin-1, and decreased sensitivity to sodium nitroprusside.

A Role for Potassium Channels in Smooth Muscle Cells and Platelets in the Etiology of Primary Pulmonary Hypertension

Plasma serotonin levels are markedly elevated in patients with primary pulmonary hypertension (PPH) and platelet levels of serotonin are low. Furthermore, plasma serotonin levels remain elevated after bilateral lung transplantation, in the absence of any pulmonary hypertension. Dexfenfluramine can cause the anorexigen-induced form of PPH that is clinically and histologically indistinguishable from PPH. We find that dexfenfluramine releases serotonin from platelets and inhibits its reuptake. These observations suggest that serotonin might be involved in, or be a marker for, the mechanism responsible for both forms of PPH. Dexfenfluramine causes inhibition of voltage-sensitive potassium (Kv) channels, membrane depolarization, and calcium entry in pulmonary artery smooth muscle cells and vasoconstriction in isolated perfused rat lungs. We have recently found that dexfenfluramine also inhibits Kv channels in megakaryocytes, the stem cell for platelets. In smooth muscle cells, taken from the pulmonary arteries of PPH patients, Kv channels appear to be dysfunctional. The underlying defect in PPH is likely to be an abnormality of one or more Kv channels in both pulmonary artery smooth muscle cells and platelets. Relatively few patients exposed to dexfenfluramine develop PPH. The factors responsible for susceptibility might be a difference in expression of potassium channels and/or a decrease in the endogenous production of nitric oxide.

Ca2+ images and K+ current during depolarization in smooth muscle cells of the guinea-pig vas deferens and urinary bladder.

1. Electrical events and intracellular calcium concentration ([Ca2+]) imaged using fluo-3 and laser scanning confocal microscopy were simultaneously monitored in single smooth muscle cells freshly isolated from guinea-pig vas deferens or urinary bladder. 2. Images obtained every 8 ms, during stepping from -60 to 0 or +10 mV for 50 ms under voltage clamp, showed that a rise in [Ca2+] could be detected within 20 ms of depolarization in five to twenty small (< 2 micrometer diameter) 'hot spots', over 95 % of which were located within 1.5 micrometer of the cell membrane. Depolarization at 30 s intervals activated hot spots at the same places. 3. Cd2+ or verapamil abolished both hot spots and Ca2+-activated K+ current (IK,Ca). Caffeine almost abolished hot spots and markedly reduced IK,Ca. Cyclopiazonic acid, which raised basal global [Ca2+], decreased the rise in hot spot [Ca2+] and IK,Ca amplitude during depolarization. These results suggest that Ca2+ entry caused Ca2+-induced Ca2+ release (CICR). 4. Under voltage clamp, hot spot [Ca2+] closely paralleled the rise in IK,Ca and reached a peak within 20 ms of the start of depolarization, but the rise in global [Ca2+] over the whole cell area was much slower. Step depolarization to potentials positive to -20 mV caused hot spots to grow in size and coalesce, leading to a rise in global [Ca2+] and contraction. Ca2+ hot spots also occurred during the up-stroke of an evoked action potential under current clamp. 5. It is concluded that the entry of Ca2+ in the early stages of an action potential evokes CICR from discrete subplasmalemma Ca2+ storage sites and generates hot spots that spread to initiate a contraction. The activation of Ca2+-dependent K+ channels in the plasmalemma over hot spots initiates IK,Ca and action potential repolarization.

Influence of chronic hypoxia on the contributions of non-inactivating and delayed rectifier K currents to the resting potential and tone of rat pulmonary artery smooth muscle.

Exposing rats to chronic hypoxia increased the 4-aminopyridine (4-AP) sensitivity of pulmonary arteries. 1 mM 4-AP caused smooth muscle cell depolarization and contraction in arteries from hypoxic rats, but had little effect in age-matched controls. Chronic hypoxia downregulated delayed rectifier K+ current (IK(V)), which was nearly 50% blocked by 1 mM 4-AP, and non-inactivating K+ current (IK(N)), which was little affected by 1 mM 4-AP. The results suggest that IK(N) determines resting potential in control rats and that its downregulation following hypoxia leads to depolarization, which activates IK(V) and increases its contribution to resting potential. The hypoxia-induced increase in 4-AP sensitivity thus reflects a switch in the major K+ current determining resting potential, from IK(N) to IK(V). This has important implications for the actions and specificity of pulmonary vasodilator drugs.

Mechanisms of non-adrenergic non-cholinergic synaptic transmission in smooth muscle cells of the gastrointestinal tract

Non-adrenergic non-cholinergic (NANCh) inhibitory synaptic potentials in smooth muscle cells (SMC) of the gastrointestinal tract are of a complex transmitter and ion nature. A blocker of ATP receptors, suramin, blocks the fast component, while a blocker of NO synthase, L-NOARG, blocks the slow component of NANCh inhibitory synaptic potentials. In the presence of both suramin and L-NOARG, SMC respond to stimulation of the intramural plexus by generating a low-amplitude hyperpolarization, and VIP is likely to be the transmitter for this effect. Low-conductance Ca2+-dependent potassium channels are involved in generation of the fast component of NANCh inhibitory synaptic potentials, and these channels are effectively blocked by apamin. The slow component of this potential is generated by high-conductance Ca2+-dependent potassium channels. In the presene of both apamin and L-NOARG (or charibdotoxin), SMC respond to intramural stimulations with non-cholinergic excitatory synaptic potentials, and ATP application evokes depolarization. Both effects are blocked by suramin. In the presence of apamin, noradrenaline also evokes depolarization in SMC, and this effect, similarly to hyperpolarization under normal conditions, is blocked by phentolamine. Our studies allow us to suggest that in smooth muscles of the gastrointestinal tract there are two types of synaptic transmission: the excitatory cholinergic, adrenergic, and ATP-ergic transmission and the inhibitory adrenergic, ATP-ergic, NO-ergic, and VIP-ergic transmission.

Endothelial and smooth muscle cell conduction in arterioles controlling blood flow.

We performed intracellular recording with Lucifer yellow dye microinjection to investigate the cellular pathway(s) by which constriction and dilation are conducted along the wall of arterioles (diameter 47 +/- 1 microns, n = 63) supplying blood flow to the cheek pouch of anesthetized hamsters. At rest, membrane potential (Em) of endothelial (-36 +/- 1 mV) and smooth muscle (-35 +/- 1 mV) cells was not different. Micropipette delivery of norepinephrine (NE) or phenylephrine (PE) produced smooth muscle cell depolarization (5-41 mV) and vasoconstriction (7-49 microns) at the site of release and along the arteriole with no effect on Em of endothelial cells. KCl produced conduction of depolarization and vasoconstriction with similar electrical kinetics in endothelial and smooth muscle cells. Acetylcholine triggered conduction of vasodilation (2-25 microns) and hyperpolarization (3-33 mV) along both cell layers; in smooth muscle, this change in Em was prolonged and followed by a transient depolarization. These cell-specific electrophysiological recordings uniquely illustrate that depolarization and constriction are initiated and conducted along smooth muscle, independent of the endothelium. Furthermore, conduction of vasodilation is explained by the spread of hyperpolarization along homologously coupled endothelial and smooth muscle cells, with distinctive responses between cell layers. The discontinuity between endothelium and smooth muscle indicates that these respective pathways are not electrically coupled during blood flow control.

Chloride channel blockers inhibit myogenic tone in rat cerebral arteries.

1. We have investigated the role of chloride channels in pressure-induced depolarization and contraction of cerebral artery smooth muscle cells. 2. Two chloride channel blockers, indanyloxyacetic acid (IAA-94) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), caused hyperpolarizations (10-15 mV) and dilatations (up to 90%) of pressurized (80 mmHg), rat posterior cerebral arteries. Niflumic acid, a blocker of calcium-activated chloride channels, did not affect arterial tone. 3. Dilatations to IAA-94 and DIDS were unaffected by potassium channel blockers, but were prevented by elevated potassium. IAA-94 and DIDS had no effect on membrane potential or diameter of arteries at low intravascular pressure, where myogenic tone is absent. Reduction of extracellular chloride (60 mM Cl-) increased the pressure-induced contractions. Removal of extracellular sodium did not affect the pressure-induced responses. 4. Our results suggest that intravascular pressure activates DIDS- and IAA-94-sensitive chloride channels to depolarize arterial smooth muscle, thereby contributing to the myogenic constriction.

O-107 - Ca2+ imaging and membrane currents during depolarization in smooth muscle cells

O-107 - Ca2+ imaging and membrane currents during depolarization in smooth muscle cells

Impairment of Kit-dependent development of interstitial cells alters contractile responses of murine intestinal tract.

We examined developmental changes in responses of the isolated segment of the ileum of BALB/c mice treated with a monoclonal antibody (ACK2) to the receptor tyrosine kinase (Kit) for 4 days postnatally to pharmacological agents in vitro. Rhythmic contraction and relaxation of the isolated ileum started to appear on day 4 postpartum, and the sensitivity to acetylcholine (ACh) decreased gradually after birth. Treatment with ACK2 induced augmentation of contractile responses and receptor sensitivity of the longitudinal muscle of the ileum to ACh, bradykinin, and prostaglandin F2 alpha. ACh induced larger depolarization in smooth muscle cells of the ileum in the ACK2-treated mice than in the control. Circular muscle responses to these substances, as measured by changes in intraluminal pressure, were not altered by ACK2 treatment. Results suggest that interstitial cells play an important role not only in the development of the pacemaking system of the small intestine but also in the functional development of the contractile properties of the intestinal smooth muscle.