Single nisoldipine-sensitive calcium channels in smooth muscle cells isolated from rabbit mesenteric artery

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Comparative studies of AJG049, a novel Ca2+ channel antagonist, on voltage-dependent L-type Ca2+ currents in intestinal and vascular smooth muscle

BACKGROUND AND PURPOSE

Antagonists of Ca2+ channels reduce contraction of intestinal smooth muscle but also affect vascular smooth muscle. We have therefore examined the effects of AJG049, a newly synthesized antagonist for regulation of gut motility, on voltage-dependent L-type Ca2+ channels, in vascular and intestinal smooth muscle, comparing AJG049 with two other Ca2+ channel antagonists, verapamil and diltiazem.

EXPERIMENTAL APPROACH

Affinities of AJG049 for various types of voltage-dependent Ca2+ channels were examined by binding studies. Effects of AJG049 on voltage-dependent inward Ca2+ (or Ba2+) currents (ICa or IBa) in dispersed smooth muscle cells from guinea-pig ileum, colon and mesenteric artery were measured using conventional whole-cell configurations.

KEY RESULTS

In binding studies, AJG049 showed a high affinity for the diltiazem-binding site of L-type Ca2+ channels. In whole-cell configuration, AJG049 suppressed ICa in ileal myocytes, with concentration-, voltage-and use-dependencies. AJG049 shifted the steady-state inactivation curve of ICa to the left. The order of potency to inhibit ICa in ileal myocytes was AJG049>verapamil>diltiazem. AJG049 also suppressed IBa in guinea-pig mesenteric arterial myocytes, showing concentration- and voltage-dependencies and the potency order for this action was also AJG049>verapamil>diltiazem. For the relative ratio of Ki values between ileal and mesenteric arterial myocytes, the order was AJG049>diltiazem>>verapamil.

CONCLUSIONS AND IMPLICATIONS

These results show that AJG049 inhibits L-type Ca2+ channels mainly through the diltiazem-binding site(s). From our results, AJG049 showed a little selectivity for these Ca2+ channels in intestinal smooth muscle.

Endothelium-dependent blunting of myogenic responsiveness after chronic hypoxia

Blunted agonist-induced vasoconstriction after chronic hypoxia is associated with endothelium-dependent vascular smooth muscle (VSM) cell hyperpolarization and decreased vessel-wall Ca(2+) concentration ([Ca(2+)]). We hypothesized that myogenic vasoconstriction and pressure-induced Ca(2+) influx would also be attenuated in vessels from chronically hypoxic (CH) rats. Mesenteric resistance arteries isolated from CH [barometric pressure (BP), 380 Torr for 48 h] or normoxic control (BP, 630 Torr) rats were cannulated and pressurized. VSM cell resting membrane potential was recorded at intraluminal pressures of 40-120 Torr under normoxic conditions. VSM cells in vessels from CH rats were hyperpolarized compared with control rats at all pressures. Inner diameter was maintained for vessels from control rats, whereas vessels from CH rats developed less tone as pressure was increased. Pressure-induced increases in vessel-wall [Ca(2+)] were also attenuated for arteries from CH rats. Endothelium removal restored myogenic constriction to vessels from CH rats and normalized VSM cell resting membrane potential and pressure-induced Ca(2+) responses to control levels. Myogenic constriction and pressure-induced vessel-wall [Ca(2+)] increases remained blunted in the presence of nitric oxide (NO) synthase inhibition for arteries from CH rats. We conclude that blunted myogenic reactivity after chronic hypoxia results from a non-NO, endothelium-dependent VSM cell hyperpolarizing influence.

Dihydropyridine-sensitive Ca(2+) channels in human glomerular mesangial cells

In mesangial cells (MC), the response of intracellular Ca(2+) concentration ([Ca(2+)](i)) to a contractile agonist is biphasic with a large, transient increase in [Ca(2+)](i) followed by a smaller but sustained elevation as Ca(2+) flows into the cell from the extracellular fluid. It has been postulated that membrane depolarization precedes opening of Ca(2+) channels in the plasmalemmal membrane. However, a role for voltage-gated Ca(2+) channels (VGCC) in human MC has been controversial, and their existence has not been verified with single-channel analysis. We used fura 2 fluorescence and patch-clamp techniques to determine the properties of the Ca(2+) entry pathway responsible for the sustained response of [Ca(2+)](i) in human MC. We found that ANG II at 10 nM, 100 nM, and 1 microM increased [Ca(2+)](i) to sustained levels of 22%, 35%, and 49%, respectively, above baseline. The sustained response to 1 microM ANG II was attenuated by diltiazem and was reduced to a value less than baseline in the absence of external Ca(2+). None of the peak responses (due to release of intracellular stores of Ca(2+)) were affected by removal of external Ca(2+) or addition of diltiazem. Upon elevating the extracellular [K(+)] from 5 mM to 75 mM, [Ca(2+)](i) reached a sustained level of 48% greater than baseline. This effect of high K(+) was attenuated by either Ca(2+) removal or addition of diltiazem. In the presence of 75 or 140 mM K(+), the dihydropyridine agonist BAY K 8644 (1 microM and 10 microM) initiated sustained [Ca(2+)](i) responses averaging 18% and 25%, respectively, greater than baseline. With <10 nM Ca(2+) in the external solution, BAY K 8644 did not significantly affect [Ca(2+)](i). In separate patch-clamp experiments, barium-selective channels were found in cell-attached patches with 90 mM BaCl(2) and 10 microM BAY K 8644 in the pipette solution. The single-channel conductance was 11.2 pS, and the open probability increased steeply at membrane potentials between -30 mV and 0 mV. It is concluded that human glomerular MC contain dihydropyridine-sensitive Ca(2+) channels responsible for the voltage-regulated entry of Ca(2+) into the cell during an agonist-induced contraction.

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.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Different potencies of dihydropyridine derivatives in blocking T-type but not L-type Ca2+ channels in neuroblastoma-glioma hybrid cells

Evidence has accumulated that classic L-type Ca2+ channel blockers with a dihydropyridine structure also inhibit T-type Ca2+ channels in certain types of central and peripheral neurons and in smooth muscle cells, albeit with a lower potency. Thus beneficial therapeutic effects of dihydropyridines in cardiovascular and neurological diseases may not only be associated with L-type but also with T-type Ca2+ channel blockade. Little is known about the exact order of potency of dihydropyridine derivatives at T-type Ca2+ channels. Here we investigate the efficacy and potency of four therapeutically used compounds, i.e. nifedipine, nimodipine, nicardipine, niguldipine, in the neuroblastoma-glioma cell line NG108-15. For comparative purposes the Ca2+ channel agonist Bay K 8644 was included. Ca2+ channel currents were measured with the whole-cell voltage clamp technique. Subtype Ca2+ channel currents were separated by clamp protocol and selective blockers. T-type Ca2+ channel currents were inhibited with decreasing potency in the order niguldipine > nicardipine > nimodipine > nifedipine (IC50-values 244 nM, 2.5 microM, 9.8 microM, 39 microM), whereas L-type Ca2+ channel currents were blocked with similar potency (IC50 for nicardipine 75 nM). Bay K 8644 increased T-type Ca2+ channel current at nanomolar concentrations (i.e. 95 +/- 16% increase by 300 nM). T-type Ca2+ channel block was completely reversible with exception of the block by niguldipine. Our results indicate a variability of two orders of magnitude in potency of T-type Ca2+ channel block by the dihydropyridine derivatives investigated. It is speculated that the relation between the L- and T-type Ca2+ channel block may determine the therapeutic profile of a dihydropyridine derivative.

Bay K 8644 reveals two components of L-type Ca2+ channel current in clonal rat pituitary cells

Whole-cell L-type Ca2+ channel current was recorded in GH3 clonal rat pituitary cells using Ba2+ as a charge carrier. In the presence of the dihydropyridine agonist Bay K 8644, deactivation was best described by two exponential components with time constants of approximately 2 and approximately 8 ms when recorded at -40 mV. The slow component activated at more negative potentials than the fast component: Half-maximal activation for the slow and fast components occurred at approximately -15 and approximately 1 mV, respectively. The fast component was more sensitive to enhancement by racemic Bay K 8644 than the slow component: ED50fast = approximately 21 nM, ED50slow = approximately 74 nM. Thyrotropin-releasing hormone (TRH; 1 microM) inhibited the slow component by approximately 46%, whereas the fast component was inhibited by approximately 22%. TRH inhibition of total L-current showed some voltage dependence, but each Bay K 8644-revealed component of L-current was inhibited in a voltage-independent manner. Therefore, the apparent voltage dependence of TRH action is derived from complexities in channel gating rather than from relief of inhibition at high voltages. In summary, Bay K 8644-enhanced L-currents in GH3 cells consist of two components with different sensitivities to voltage, racemic Bay K 8644, and the neuropeptide TRH.

Subunit-dependent modulation of recombinant L-type calcium channels. Molecular basis for dihydropyridine tissue selectivity

At least four calcium channel subtypes (P, T, N, and L) have now been classified on the basis of their biophysical and/or pharmacological properties. L-type channels, a channel family particularly important to physiological function of the cardiovascular system, are identified by their slow voltage- and calcium-dependent inactivation as well as their sensitivity to dihydropyridine (DHP) calcium channel antagonists. In this study, we report the results of experiments in which we have measured the DHP modulation of recombinant calcium channel activity in cells transfected with alpha 1 subunits of cardiac and smooth muscle L-type calcium channels. We find subunit-dependent differences in the voltage and concentration dependence of channel modulation. Our results provide evidence for a molecular basis for DHP sensitivity of heart and smooth muscle calcium channels and, additionally, indicate that, even within one family of calcium channels, slight differences in channel structure can cause marked differences in channel pharmacology.

Two high-voltage-activated, dihydropyridine-sensitive Ca2+ channel currents with distinct electrophysiological and pharmacological properties in cultured rat aortic myocytes

In smooth muscle cells, essentially two distinct types of voltage-gated Ca2+ channels have been shown, on the basis of their distinct electrophysiological and pharmacological properties, to coexist. Here we report that, in addition to a dihydropyridine (DHP)-sensitive, low-voltage-activated Ba2+ current (IBa,LVA), two types of high-voltage-activated Ba2+ currents with distinct waveforms were recorded in whole-cell clamped aortic myocytes; these were referred to as IBa,HVA1 and IBa,HVA2. They were investigated in cells where no IBa,LVA was detectable. IBa,HVA1 had a slow, monoexponential decay. In contrast, the decay of IBa,HVA2 was much faster and biexponential. In addition, IBa,HVA2 had more negative ranges of activation and steady-state inactivation than IBa,HVA1 and was more sensitive to the DHP antagonist nicardipine (concentrations for half maximum inhibition 0.2 microM and 2 microM, respectively). When using the physiological ion Ca2+ as the charge carrier, the decay of HVA1 currents was not altered, whereas both time constants of HVA2 current decay were accelerated five-fold. Moreover, permeability ratios (ICa/IBa) were also significantly different (0.2 and 0.6 for HVA1 and HVA2 respectively). IBa,HVA1 and IBa,HVA2 are consistent either with the existence and activation of two functionally distinct subtypes of the so-called "DHP-sensitive L-type" Ca2+ channel or with different gating behaviours of a single type of channel. Potentially, they may serve distinct biological functions and constitute distinct targets for neurotransmitters and drugs.

Dihydropyridine-sensitive calcium channels expressed in canine colonic smooth muscle cells

Experiments were performed to identify and characterize the types of calcium channels that regulate inward calcium current in canine colonic smooth muscle. Freshly dispersed smooth muscle cells from the circular layer of the canine proximal colon were used. Single-channel currents were measured with 80 mM Ba2+ as the charge carrier. Small-conductance (10 +/- 2 pS, EBa = 46 +/- 11 mV, n = 9) and large-conductance (21 +/- 1 pS, EBa = 52 +/- 3 mV, n = 19) single-channel currents were observed during depolarizing voltage steps positive to -30 mV. Both types of single-channel currents were inhibited by the addition of 10(-6) M nifedipine to the bath solution. The smaller current was infrequently observed and therefore was not further characterized. Open probability (P(o)) of the larger current amplitude was strongly dependent on voltage. Activation curves were well described by a Boltzmann function with half activation occurring at 4 mV, and a 5-mV increase in membrane potential resulted in an e-fold increase in P(o). BAY K 8644 (1 microM) shifted the activation curve to the left while nifedipine (1 microM) resulted in a right shift. Molecular analysis showed that only the C class of Ca2+ channel alpha 1-subunit is expressed in this tissue. Furthermore, only a single splice variant (rbc-II) was observed. The results suggest that a single class of dihydropyridine-sensitive calcium channels regulates inward calcium current in canine colonic smooth muscle cells.

Potential-dependent inward currents in single isolated smooth muscle cells of the rat ileum

1. Calcium (ICa) and sodium (INa) currents were studied in single smooth muscle cells freshly isolated from both the newborn (1-3 days old) and adult rat ileum, using the patch-clamp technique (whole-cell configuration). 2. Under conditions when INa was blocked, two components of ICa, low-voltage activated or ICa,low and high-voltage activated or ICa,high, were observed in the newborn rat ileal cells. ICa,high and ICa,low have differing voltage ranges of activation and steady-state inactivation and time courses of recovery from inactivation. Potential dependence of ICa,low was much steeper and shifted toward negative membrane potential than that for ICa,high (slope factors and the potential of half-maximal inactivation were 13.6 and -60.6 and 8.8 and -49 mV for ICa,low and ICa,high, correspondingly). 3. Nifedipine at the high concentration of 30 microM exerted no effect on ICa,low and only slightly suppressed ICa,high, decreasing its peak to 0.81 +/- 0.04 (n = 7) at the holding potential of -80 mV and to 0.66 +/- 0.05 (n = 3) at -50 mV. ICa,high was suppressed significantly by Cd2+ ions, while ICa,low was more sensitive to Ni2+ ions. 4. Results presented here suggest that the properties of high-voltage-activated (HVA) Ca2+ channels in the rat small intestine are quite different to those described for L-type Ca2+ channels found in other smooth muscles. It is proposed that HVA Ca2+ channels are similar to N-type Ca2+ channels. 5. Comparison of Ca2+ currents in newborn and adult rat ileal cells showed that the contribution of ICa,low to the net Ca2+ current was negligible in adults, whereas the properties of HVA Ca2+ channels were similar in the neonatal and adult animals. 6. INa, studied in nominally Ca(2+)-free physiological salt solution, activated in the voltage range between -50 and -40 mV and reached its peak at -10 mV. INa was blocked in a dose-dependent manner by TTX with an apparent dissociation constant of 4.5 nM. 7. INa decay was monoexponential in the voltage range studied and its time constant decreased monotonically with membrane depolarization from 4.7 +/- 0.2 ms (n = 6) at -30 mV to 0.51 +/- 0.03 ms (n = 7) at 20 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage-dependent calcium current and the effects of adrenergic modulation in rat aortic smooth muscle cells

Smooth muscle cells from rat aorta were cultured in defined, serum-free medium and studied using whole-cell patch-clamp techniques. Under conditions designed to isolate currents through Ca channels, step depolarizations produced inward currents which were fast in onset and inactivated rapidly, with little sustained inward current being observed. Both Ni and Cd blocked these currents, with Ni being effective at 50 microM. Removal of external Na or addition of 1 microM tetrodotoxin had no effect. Peak inward currents were attained at about -15 mV, with half-maximal activation at -41 mV using -80 mV holding potentials. The transient inward currents were reduced by depolarized holding potentials, with half-maximal steady-state inactivation at -48 mV. In three of the 98 cells studied, small maintained inward currents were observed with a -40 mV holding potential. The Ca channel antagonist nicardipine (5 microM) blocked the transient inward current while neither of the dihydropyridine Ca channel agonists S(+)202 791 and (-)BAY K 8644 produced a significant augmentation of sustained inward current. At 10 microM, both noradrenaline and adrenaline but not phenylephrine decreased the peak inward current. This inhibition was unaffected by a variety of adrenoceptor antagonists and was also observed when internal solutions having high Ca buffering capacity were used, but was absent when GDP-beta-S instead of GTP was included in the pipette solution. The main conclusions from this study are that under our cell culture conditions, rat aortic smooth muscle cells possess predominantly a transient, low-threshold-activated inward Ca current and that this Ca current is inhibited by certain adrenoceptor agonists but with a quite atypical adrenoceptor antagonist pharmacology.

Voltage-dependent calcium current in dissociated smooth muscle cells of the buccal mass of Aplysia

Isolated smooth muscle cells of the buccal mass of Aplysia contracted in response to depolarization elicited by a patch electrode in whole-cell configuration. With cesium-containing pipet solution and tetraethylammonium and 4-aminopyridine in the external solution depolarization elicited inward current. The voltage-dependent inward current was blocked completely by lanthanum (10 mmol.l-1), inhibited 80-90% by nifedipine (1 mumol.l-1), and was dependent upon extracellular calcium. These results showed that the voltage-dependent inward current was due to activation of voltage-dependent calcium channels (VDCaCH). Minimal depolarization to begin activating VDCaCH was -60 to -30 mV. Inward current peaked within 8 ms and then decreased rapidly to a lower level of relatively non-inactivating current. The initial peak current could be mostly inactivated by a depolarization to -20 mV for 500 ms. Nifedipine reduced both the peak current and the relatively non-inactivating current. Nifedipine inhibited high potassium-elicited contractions of both intact and dissociated muscle. These results suggested that VDCaCH mediates calcium influx which triggers contraction in molluscan smooth muscle fibers.

Calyculin A increases voltage-dependent inward current in smooth muscle cells isolated from guinea pig taenia coli

The effects of a potent phosphatase inhibitor, calyculin A (CL-A), on inward currents in guinea pig taenia coli smooth muscle cells were examined. CL-A increased the inward current, and this effect of CL-A was inhibited by a protein kinase C inhibitor, H-7, and by nifedipine. Phorbol 12,13-dibutyrate, an activator of protein kinase C, also increased the inward current and this effect was antagonized by H-7. These results suggest that in guinea pig taenia coli smooth muscle cells CL-A may facilitate the opening of the L-type Ca2+ channels through the protein kinase C-dependent phosphorylation system.

Receptor-activated calcium influx in human airway smooth muscle cells

1. Fluorescence measurements of intracellular calcium concentrations ([Ca2+]i) were made on cultured human airway smooth muscle cells using the dye Fura-2. The response to either histamine (100 microM) or bradykinin (1 microM) was biphasic, with a transient increase in [Ca2+]i followed by a sustained [Ca2+]i increase lasting many minutes. The average steady-state (plateau) [Ca2+]i following agonist activation was 267 +/- 5 nM, whereas the average basal [Ca2+]i was 148 +/- 4 nM. 2. The sustained rise in [Ca2+]i required the continued presence of either histamine or bradykinin and was dependent on extracellular Ca2+. The magnitude of the transient rise in [Ca2+]i was not dependent on extracellular Ca2+. Sustained, receptor-activated rises in [Ca2+]i were rapidly abolished by chelation of extracellular Ca2+, or addition of non-permeant polyvalent cations, whereas these agents had minor effects in the absence of agonist. These data indicate that the sustained increase in [Ca2+]i was dependent on receptor-activated Ca2+ influx. 3. Receptor-activated Ca2+ influx was not affected by treatment with organic Ca2+ channel antagonists (nifedipine (10 microM), nisoldipine (10 microM) or diltiazem (10 microM] or agonists (Bay K 8644 (500 nM to 10 microM) or Bay R 5417 (500 nM]. The magnitude of the sustained rise was also not affected by pre-treatment with ouabain (100 microM) indicating little involvement of Na(+)-Ca2+ exchange in the influx mechanism. 4. Receptor-activated Ca2+ influx could be completely inhibited by several polyvalent cations (Co2+, Mn2+, Ni2+, -Cd2+ or La3+). Quantitative estimates of the potency of block were obtained for Ni2+ and La3+. These measurements indicate that the pKi for Ni2+ was 3.6 and for La3+ was 3.5. 5. Both Mn2+ and Co2+ ions caused a time-dependent quench of intracellular Fura-2; however, permeation of neither ion was increased following receptor activation, indicating that the influx pathway is not permeable to these cations. 6. Fura-2 was used to monitor the rate of Ba2+ entry into airway smooth muscle cells by monitoring the Ca(2+)-Fura-2 and Ba(2+)-Fura-2 isosbestic points as well as the 340 and 380 nm signals. Cell activation did not increase the rate of Ba2+ entry indicating that the Ca2+ influx pathway was poorly permeant to Ba2+ ions. Ba2+ (2 mM) was able to inhibit Ca2+ entry as shown by its effects on the Ba(2+)-independent, Ca(2+)-dependent wavelength (371 nm). 7. The voltage dependence of Ca2+ influx was examined before and after agonist-induced activation. The effect of KCl-induced depolarization prior to cell activation was to cause a slight increase in [Ca2+]i.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple types of Ca2+ channels in visceral smooth muscle cells

Single-channel currents were recorded from two classes of Ca2+ channels in visceral smooth muscle cells isolated from the stomach of the toad, Bufo marinus: a class of small-conductance channels (approximately 11 pS) and a class of large-conductance channels (approximately 26 pS). Small-conductance channels were present in a majority of patches and gave rise to a slowly inactivating current (t1/2 approximately 250 ms at 0 mV). Openings of large-conductance channels could be unequivocally resolved only in the presence of the dihydropyridine Ca2+ agonist Bay K 8644. Two subtypes of the large-conductance channels were found--those with a very slow rate of decay (greater than 500 ms) and those with a faster one (less than 100 ms). Large-conductance channels resemble L-type Ca2+ channels of other preparations. Small-conductance channels do not fit unambiguously into the other existing categories (i.e., N or T). Correspondence between single-channel and macroscopic Ca2+ currents is discussed.

Voltage-dependent sodium and potassium, but no calcium conductances in DDT1 MF-2 smooth muscle cells

Voltage-dependent inward and outward membrane currents were investigated in the DDT1 MF-2 smooth muscle cell line using the whole-cell patch-clamp technique. Application of a pulse protocol with subsequent depolarizing voltage steps elicited an inactivating inward current and a non-inactivating outward current. The outward current was activated at membrane potentials more positive than -35 mV, with tau act = 30 -40 ms. The outward current was blocked by tetraethylammonium (NEt4Cl) and 3,4-aminopyridine in a dose-dependent manner (EC50 of 5 mM and 0.5 mM, respectively). The amplitude of the outward current was linked to the potassium equilibrium potential (Vek), and tail currents reversed near Vek. The outward current was completely abolished when intracellular potassium was substituted by 106 mM caesium and 20 mM NEt4Cl. The inward current was activated at potentials more positive than -30 mV with tau act of 1.6-2.5 ms, and with tau inact of 1.7-3.0 ms. Steady-state inactivation was 50% at a holding potential of -40 mV. The inward current was blocked by tetrodotoxin (EC50 of 0.15 microM) and dependent on the reversal potential for sodium. Voltage-dependent calcium currents could not be detected. Further, the cytoplasmic free calcium concentration, as measured using Indo-1 fluorescence, was not changed during high-potassium (40 mM)-induced depolarization. In contrast, contraction of freshly obtained hamster vas deferens tissue elicited by high-potassium(40 mM)-induced depolarization was largely inhibited by diltiazem (20 microM). These findings showed that voltage-dependent calcium channels are not functional in DDT1 MF-2 smooth muscle cells in contrast to freshly obtained Syrian hamster vas deferens smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel currents in isolated smooth muscle cells from the basilar artery of the guinea pig

Ca2+ channel currents were studied in smooth muscle cells from the basilar artery of the guinea pig using the whole-cell patch-clamp technique. 10 mM Ba2+ as the charge carrier and strong buffering of intracellular Ca2+ with EGTA (pCai = 8). Cell capacitance was 18.8 +/- 6.6 pF (n = 96) and maximum current density at +10 to +20 mV (holding potential less than -55 mV), measured early in dialysis, was -14.8 +/- 4.9 pA/pF (n = 83). Currents reversed at approximately +95 mV and, at more positive potentials, outward Cs+ currents were recorded that were blocked by either external Cd2+ or Ca2+. One component of current was identified that had properties consistent with L-type channels. On the basis of measurements of tail currents, its threshold for activation was -15 mV, its voltage dependence of activation was steep and it was half-activated at +8.5 mV. It inactivated very slowly at +15 mV (2787 +/- 511 ms) and it deactivated rapidly (251 +/- 55 microseconds) at -55 mV. It was quickly lost during dialysis and was largely blocked by 1 nM nifedipine (1-s pulses, holding potential = -55 mV). A second component, termed B-type current, was identified that had properties inconsistent with those of T-type channels. On the basis of tail currents, its threshold for activation was -30 mV, its voltage dependence of activation was less steep and it was half-activated at +33.7 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual action of FRC8653, a novel dihydropyridine derivative, on the Ba2+ current recorded from the rabbit basilar artery

Actions of FRC8653 on the macroscopic and unitary Ba2+ currents were studied using the rabbit basilar artery. Application of (+/-)-FRC8653 (less than 1 microM) increased the amplitude of the inward current when depolarization pulses more negative than -10 mV were applied but inhibited it when depolarization was more positive than 0 mV (in each case from a holding potential of -80 mV). At a holding potential of -40 mV, (+/-)-FRC8653 (greater than 0.1 nM) consistently inhibited the inward current. (-)-FRC8653 (greater than 1 nM) inhibited the amplitude of the inward current evoked by a depolarizing pulse more positive than -10 mV (the holding potential being -80 mV). At the holding potential of -80 mV, but not at -40 mV, (+)-FRC8653 (1 microM) enhanced the current amplitude evoked by a depolarizing pulse more negative than -10 mV but inhibited the current evoked by a pulse more positive than 0 mV. (+/-)-FRC8653 shifted the voltage-dependent inhibition curves to the left, and the slope of the curve became steeper (test pulse of +10 mV). Two types of single Ca2+ channel currents (12 and 23 pS) were recorded from the basilar artery by the cell-attached patch-clamp method. Opening of the 12-pS channel occurred with a depolarizing pulse (-20 mV) from a holding potential of -80 mV, but not from one of -60 mV. (+)-FRC8653 activated, and (-)-FRC8653 inhibited, the 23-pS channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channels, potassium channels, and voltage dependence of arterial smooth muscle tone

Resistance arteries exist in a maintained contracted state from which they can dilate or constrict depending on need. In many cases, these arteries constrict to membrane depolarization and dilate to membrane hyperpolarization and Ca-channel blockers. We discuss recent information on the regulation of arterial smooth muscle voltage-dependent Ca channels by membrane potential and vasoconstrictors and on the regulation of membrane potential and K channels by vasodilators. We show that voltage-dependent Ca channels in the steady state can be open and very sensitive to membrane potential changes in a range that occurs in resistance arteries with tone. Many synthetic and endogenous vasodilators act, at least in part, through membrane hyperpolarization caused by opening K channels. We discuss evidence that these vasodilators act on a common target, the ATP-sensitive K (KATP) channel that is inhibited by sulfonylurea drugs. We propose the following hypotheses that presently explain these findings: 1) arterial smooth muscle tone is regulated by membrane potential primarily through the voltage dependence of Ca channels; 2) many vasoconstrictors act, in part, by opening voltage-dependent Ca channels through membrane depolarization and activation by second messengers; and 3) many vasodilators work, in part, through membrane hyperpolarization caused by KATP channel activation.

Endothelin augments unitary calcium channel currents on the smooth muscle cell membrane of guinea-pig portal vein

1. The effects of endothelin (ET) on the Ca2+ channel current in smooth muscle cells of the guinea-pig portal vein were investigated using the patch-clamp technique with whole-cell and cell-attached configurations. 2. ET augmented the macroscopic Ba2+ current in a dose-dependent manner; this effect was inhibited by nifedipine or Cd2+. Augmentation of the inward current by ET did not depend on the amplitude of the depolarizing pulse. Further, when the membrane potential was held at -60 mV, ET increased the amplitude of the Ba2+ inward current measured at the peak and end of the depolarizing pulse to the same extent. 3. By contrast, when the membrane potential was held at -80 mV, depolarizing pulses to potentials more negative than 0 mV produced greater augmentation of the inward current than did those more positive than 0 mV. Moreover, when a depolarizing pulse to below 0 mV was applied, ET increased the peak amplitude of the inward current more than the amplitude measured at the end of pulse. 4. Using the patch-clamp technique with cell-attached configuration, two types of unitary Ba2+ current with conductances of 22 and 12 pS were obtained in 50 mM-Ba2+ solution. Nifedipine inhibited both types of unitary channel current, but the sensitivity of the 22 pS Ca2+ channel to nifedipine was 20-fold higher than the 12 pS Ca2+ channel. 5. Bath application of ET prolonged the mean open time, reduced the number of sweeps in which no Ca2+ channel was opened ('blank' sweep), and increased the number of channel openings evoked by each depolarizing pulse without changes of conductance. As a consequence, ET increased the open probability of both channels. 6. Augmentation of the 12 pS channels by ET was seen only in the early phase of a depolarizing pulse (57 ms from the onset of 170 ms pulse), while augmentation of the 22 pS channels was seen during the entire period of a depolarizing pulse. 7. When ET was added to the pipette solution, the activity of both Ca2+ channels was increased. However, this effect was less frequently observed than when ET was applied in the bath. 8. These results suggest that ET augments both the nifedipine-sensitive and resistant Ca2+ channels in the smooth muscle cell membrane of the guinea-pig portal vein, but in different ways. Presumably, ET acts indirectly on the voltage-dependent Ca2+ channel.

Effects of pinaverium on voltage-activated calcium channel currents of single smooth muscle cells isolated from the longitudinal muscle of the rabbit jejunum

1. Smooth muscle cells of the longitudinal muscle of the rabbit jejunum were dispersed by enzyme treatment and recordings of membrane current were made in the whole-cell mode by patch clamp technique. The action of pinaverium bromide on the voltage-dependent inward current of single isolated smooth muscle cells was studied in solutions containing normal concentrations of calcium or high concentrations of barium at room temperature. 2. Pinaverium reduced the voltage-dependent inward current with an IC50 of 1.5 microM. This IC50 is similar to those of verapamil, diltiazem and flunarizine on these cells as described by others. Occasionally evidence of a potentiating action of pinaverium on the inward current was seen. 3. Repetitive stimulation of the cells did not increase blockade of inward current by pinaverium unlike the use-dependent blockade seen with verapamil, methoxyverapamil, and diltiazem in these and in other smooth muscle cells. 4. The inactivation of inward current was studied by holding at various potentials for 2 or 10 s before evoking inward current. The voltage at which current was 50% available was changed very little by pinaverium although other calcium entry blockers, for example the dihydropyridines, have been reported to produce appreciable negative shifts which indicate considerable voltage-dependence of their blockade. This may indicate that pinaverium has similar affinities for the closed available and inactivated calcium channel states so that blockade is not appreciably voltage-dependent.

External cadmium and internal calcium block of single calcium channels in smooth muscle cells from rabbit mesenteric artery

The patch clamp technique was used to record unitary currents through single calcium channels from smooth muscle cells of rabbit mesenteric arteries. The effects of external cadmium and cobalt and internal calcium, barium, cadmium, and magnesium on single channel currents were investigated with 80 mM barium as the charge carrier and Bay K 8644 to prolong openings. External cadmium shortened the mean open time of single Ca channels. Cadmium blocking and unblocking rate constants of 16.5 mM-1 ms-1 and 0.6 ms-1, respectively, were determined, corresponding to dissociation constant Kd of 36 microM at -20 mV. These results are very similar to those reported for cardiac muscle Ca channels (Lansman, J. B., P. Hess, and R. W. Tsien. 1986. J. Gen. Physiol. 88:321-347). In contrast, Cd2+ (01-10 mM), when applied to the internal surface of Ca channels in inside-out patches, did not affect the mean open time, mean unitary current, or the variance of the open channel current. Internal calcium induced a flickery block, with a Kd of 5.8 mM. Mean blocking and unblocking rate constants for calcium of 0.56 mM-1 ms-1 and 3.22 ms-1, respectively, were determined. Internal barium (8 mM) reduced the mean unitary current by 36%. We conclude that under our experimental conditions, the Ca channel is not symmetrical with respect to inorganic ion block and that intracellular calcium can modulate Ca channel currents via a low-affinity binding site.

(+)-PN200-110 and ouabain binding sites in purified bovine adrenomedullary plasma membranes and chromaffin cells

Bovine adrenal medulla plasma membranes were purified by a differential centrifugation procedure using sucrose and Urografin discontinuous density gradients; the membranes were enriched 10-12-fold in acetylcholinesterase activity and [3H]ouabain binding sites. Specific (+)-[3H]PN200-110 binding to these membranes amounted to 90% of total binding and was saturable and of high affinity (KD = 41 pM; Bmax = 119 fmol/mg of protein) with a Hill coefficient close to 1, a result suggesting the presence of a single, homogeneous population of dihydropyridine receptors. The association and dissociation rate constants were, respectively, 7.5 X 108 M-1 min-1 and 0.023 min-1. Unlabeled (+)-PN200-110 displaced (+)-[3H]PN200-110 binding with a potency 100-fold higher than (-)-PN200-110 (IC50,0.5 and 45nM, respectively). Although the two enantiomers of BAY K 8644 completely displaced (+)-[3H]PN200-110 binding, they exhibited no stereoselectivity (IC50, 69 and 83 nM,respectively). Whereas ( +/- )-nitrendipine very potently displaced (+)-[3H]PN200-110 binding (IC50 = 1.3 nM) verapamil and cinnarizine displaced the binding by only 30 and 40% at 1 microM, and diltiazem increased it by 20% at 10 microM. [3H]Ouabain bound to plasma membranes with a KD of 34 nM and a Bmax of 9.75 pmol/mg of protein, a figure 80-fold higher than the Bmax for (+)-PN200-110. [3H]Ouabain also bound to intact chromaffin cells with a Bmax of 244 fmol/10(6) cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Low-voltage-activated calcium current in rat aorta smooth muscle cells in primary culture

1. Electrical and pharmacological properties of the low-voltage-activated Ca2+ current (ICa, LVA) in rat aorta smooth muscle cells (SMC) in primary culture were examined, particularly in comparison with the high-voltage-activated Ca2+ current (ICa, HVA). Both types of Ca2+ currents were recorded in external solution containing 20 mM-Ca2+, using the whole-cell voltage-clamp technique. 2. ICa, LVA was evoked by step depolarizations to potentials more positive than -60 mV from a holding potential of -100 mV, and reached a peak in the current-voltage (I-V) relationship around -30 mV. ICa, HVA was activated at -20 mV, and reached a peak at +20 mV. 3. The intracellular dialysis of 5 mM-F- irreversibly suppressed ICa, HVA, with time, while it has little effect on the ICa, LVA. The ICa, LVA could be separated from the ICa, HVA by either selecting the holding and test potential levels or by perfusing intracellularly with F-. 4. The ratio of peak amplitude of Ba2+, Sr2+ and Ca2+ currents in the respective I-V relationship was 1.6:1.2:1.0 for high-voltage-activated Ca2+ channels and was 1.0:1.4:1.0 for low-voltage-activated ones. 5. The inactivation phase of ICa, HVA was fitted by a sum of double-exponential functions, the time constants of which were larger when the current was carried by Ba2+ than by Ca2+. The inactivation time course of ICa, LVA was fitted by a single-exponential function, and the time constant was practically the same when the current was carried by Ba2+ or by Ca2+. Activation and inactivation processes of ICa, LVA were potential-dependent. 6. The steady-state inactivation curve of ICa, LVA was fitted by the Boltzmann equation, having a mid-potential of -80 mV and a slope factor of 5.0. The recovery time course from steady-state inactivation was fitted by a sum of two exponential functions. The time constants of the faster phase were 230 and 380 ms, and those of slower phase were 2.8 and 1.8 s at the repolarization potentials of -120 and -100 mV, respectively. 7. The amplitude of ICa, LVA depended on the external Ca2+ concentration ([Ca2+]o), approaching saturation at 95 mM [Ca2+]o. 8. Various polyvalent cations blocked both types of Ca2+ current reversibly in the order (IC50 in M): La3+ (8 x 10(-8)) greater than Cd2+ (6 x 10(-6)) greater than Ni2+ (1 x 10(-5)) greater than Zn2+ (2 x 10(-5)) for ICa, HVA, and La3+ (6 x 10(-7)) greater than Zn2+ (3 x 10(-5)) greater than Cd2+ (4 x 10(-4)) greater than Ni2+ (6 x 10(-4)) for ICa, LVA.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation produced by nifedipine of the unitary Ba current of dispersed smooth muscle cells of the rabbit ileum

Using a whole-cell clamp technique on longitudinal smooth muscle cells of the rabbit ileum, it was found that the rate of decay of the macroscopic Ba current evoked by depolarization (to +20 mV) was not modified by changing the holding potential from -60 mV to -30 mV. Cadmium (20 microM) left only a small transient inward current. Using the patch clamp technique with cell-attached configuration, only one type of unitary Ba current, with conductance of 25 pS was obtained in 100 mM Ba solution. The conductance depended on the external Ba concentration, had a dissociation constant of 19 mM and maximum conductance of 42 pS, suggesting that the Ca channel is of the L-type. Depolarizing pulses (to 0 mV and 150 ms duration) delivered at a frequency of 0.5 Hz (high frequency) evoked a unitary Ba current with low open probability (p less than 0.3). However 65-75% of depolarizing pulses evoked no unitary Ba current ("blank" sweep). On the other hand, depolarizing pulses at 0.033 Hz (low frequency) reduced the number of "blank" sweeps (20-50%), and increased the fraction of sweeps with high open probability (p greater than 0.5). Thus, the channel activity may depend on the stimulus frequency. Nifedipine, in both stimulus conditions, reduced the open probability of the channel due to an increase in the fraction of "blank" sweeps to a greater extent than to a decrease in the time constant of open-times.

Multiple types of voltage-dependent Ca channels in mammalian intestinal smooth muscle cells

(1) Whole-cell and single channel recording techniques have been applied to smooth muscle cells isolated from guinea-pig taenia coli to examine whether multiple types of Ca channels exist. (2) Whole-cell recordings under physiological Ca concentration (1.8 mM) revealed two current components with fast and slow inactivating kinetics. The fast inactivating component was present when cells were held at very negative potentials (-80 mV). It was insensitive to the dihydropyridine (DHP) derivative, nifedipine. In contrast, the slow inactivating component was present at less negative holding potentials. It was blocked by nifedipine. (3) The two current components were found to have closely similar voltage dependencies for activation. (4) These results suggest that the fast inactivating decay of the Ca current was mediated not only by the entry of Ca into the cell but also by a voltage-dependent process via a different type of Ca channel with fast inactivating kinetics. (5) Recordings from cell-attached membrane patches with 100 mM external Ba clearly showed the existence of multiple types of Ca channels with different conductances. (6) The large conductance channels (30 pS) activated at more positive potentials (0 mV) and their averaged current decayed much more slowly. The DHP Ca antagonist, nifedipine, inhibited the large conductance channels increasing the proportion of blank sweeps and reducing the averaged current. On the other hand, the DHP Ca-agonist, BayK 8644, increased the average current by increasing the mean open-times of the large conductance channels. The presence of micromolar Cd in the patch pipettes produced a flickering block of the large conductance channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperpolarizing vasodilators activate ATP-sensitive K+ channels in arterial smooth muscle

Vasodilators are used clinically for the treatment of hypertension and heart failure. The effects of some vasodilators seem to be mediated by membrane hyperpolarization. The molecular basis of this hyperpolarization has been investigated by examining the properties of single K+ channels in arterial smooth muscle cells. The presence of adenosine triphosphate (ATP)-sensitive K+ channels in these cells was demonstrated at the single channel level. These channels were opened by the hyperpolarizing vasodilator cromakalim and inhibited by the ATP-sensitive K+ channel blocker glibenclamide. Furthermore, in arterial rings the vasorelaxing actions of the drugs diazoxide, cromakalim, and pinacidil and the hyperpolarizing actions of vasoactive intestinal polypeptide and acetylcholine were blocked by inhibitors of the ATP-sensitive K+ channels, suggesting that all these agents may act through a common pathway in smooth muscle by opening ATP-sensitive K+ channels.

Modulation of single slow (L-type) calcium channels by intracellular ATP in vascular smooth muscle cells

Involvement of ATP in the regulation of slow (L-type) Ca2+ channels of vascular smooth muscle cells was investigated by recording single Ca2+ channel currents (single-channel conductance of 18 pS) using a patch clamp technique. In the cell-attached configuration, intracellular composition was modified by permeabilizing the cell membrane with mechanical disruption at one end of the cell. Single cells were freshly isolated from guinea-pig portal vein by collagenase treatment. For the channel recordings, the pipette solution contained 100 mM Ba2+ and the bath contained K+-rich solution (with 5 mM EGTA) to depolarize the membrane to near 0 mV. The channel activity decreased usually within 3 min after permeabilizing the cell end and exposure to ATP-free bath solution. If ATP (1-5 mM) was applied to the bath (access to cell interior) before complete disappearance of channel activity, channel activity was partially recovered. ATP did not change the current amplitude (i) or the mean open time of the channels, whereas the number of channels available for opening and/or the probability of their being open (NPo) were increased by ATP. A non-hydrolyzable analogue of ATP, AMP-PNP, did not exert an ATP-like effect; ATP-gamma-S had a weak effect. With 1 microM Bay-K-8644 (Ca2+ channel agonist) in the pipette, the activity of the Ca2+ channel was high; such activity persisted for more than 10 min after permeabilizing the cell and exposing to ATP-free solution containing KCN (1 mM) and 2-deoxy-D-glucose (10 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channels reconstituted from the skeletal muscle dihydropyridine receptor protein complex and its alpha 1 peptide subunit in lipid bilayers

In the first part of this study, we show that sDHPR and pDHPR preparations reconstituted into lipid bilayers formed on the tips of patch pipettes exhibit two divalent cation-selective conductance levels of 9 and 20 pS, similar in single-channel conductance to VSCC reported in a variety of intact preparations (see Pelzer et al. and Tsien et al. for review). The larger conductance level is similar to the VSCC identified in intact rat t-tubule membranes and described in sDHPR and pDHPR preparations, and shares many properties in common with activity from L-type VSCC. It is sensitive to augmentation by the DHP agonist (+/-)-BAY K 8644 and cAMP-dependent phosphorylation, and to block by the phenylalkylamine (+/-)-D600 and the inorganic blocker CoCl2. Its open-state probability and open times are increased upon depolarization as expected for a voltage-dependent activation process. Upon depolarization beyond the reversal potential, however, open-state probability and open times decline again. A reasonable way to explain the bell-shaped dependence of open times and open-state probability on membrane potential is to assume voltage-dependent ion-pore interactions that produce closing of the channel at strong negative and positive membrane potentials. By contrast, the smaller conductance level may be similar to the 10.6-pS t-tubule VSCC described by Rosenberg et al. and may best be compared with T-type VSCC. It is largely resistant to augmentation by (+/-)-BAY K 8644 and cAMP-dependent phosphorylation or block by (+/-)-D600, but is sensitive to block by CoCl2. Its open times and open-state probability show a sole dependence on membrane potential where depolarization increases both parameters sigmoidally from close to zero up to a saturating level. Both elementary conductance levels do not exhibit significant inactivation over a wide potential range, which may suggest that skeletal muscle VSCC inactivation is either poorly or not voltage-dependent at all. This possibility seems in agreement with bilayer recordings on reconstituted intact t-tubule membranes and voltage-clamp recordings on intact fibers. It supports the idea that the decline of Ca2+ current in intact skeletal muscle fibers may be due to Ca2+ depletion from the t-tubule system and/or to inactivation induced by Ca2+ release from the sarcoplasmic reticulum. We consistently observe two conductance levels of 9 and 20 pS, either singly, or together in the same bilayer from solubilized DHPR samples and even highly purified DHPR preparations.(ABSTRACT TRUNCATED AT 400 WORDS)

Two types of calcium currents in single smooth muscle cells from rat portal vein

1. Using the whole-cell recording mode of the patch-clamp technique, we investigated the calcium currents in isolated cells from rat portal vein in short-term primary culture. 2. From a holding potential of -70 mV the cells presented two types of calcium currents with 5 mM-extracellular calcium: one type was activated by small depolarizations and inactivated quickly (fast calcium current), whereas the other required stronger depolarizations for activation and inactivated more slowly (slow calcium current). 3. Isradipine (PN 200-110) blocked the slow calcium current at concentrations 300 times lower than those used to block the fast inward current. The isradipine-induced inhibition was voltage-dependent for the slow calcium current and voltage-independent for the fast calcium current. 4. The slow calcium current was lost during internal perfusion with a 0.5 microM-Ca2+ containing solution, and during stimulation of the cell at high frequencies (0.1-0.2 Hz) within 5-10 min. The fast calcium current was unchanged under these experimental conditions. 5. Steady-state inactivation curves for both fast and slow calcium currents showed differences in their voltage dependence. Half-maximal and complete inactivations of the fast calcium current were obtained at -50 and -30 mV while those of the slow calcium current were obtained at -20 and +10 mV. 6. Studied with the two-pulse protocol, inactivation of the slow calcium current was dependent on both membrane potential and calcium influx while that of the fast calcium current appeared only dependent on membrane potential. 7. Two types of calcium currents, differing in potential dependence of inactivation, and in sensitivities to dihydropyridines, stimulation frequency and intracellular calcium concentration were identified in cultured smooth muscle cells isolated from portal vein.

Dihydropyridine inhibition of single calcium channels and contraction in rabbit mesenteric artery depends on voltage

1. The effects of membrane potential and the dihydropyridine calcium channel inhibitor, nisoldipine, on single calcium channels in the presence of Bay K 8644 and contraction in the presence and absence of Bay K 8644 were examined in the rabbit mesenteric artery. 2. Membrane depolarization decreased the peak average single calcium channel current that could be elicited by a test pulse to 0 mV. The steady-state inactivation relationship could be described by the Boltzmann equation, [1 + exp[Vm-V0.5)/k)]-1, with a steepness factor, k, of 7.1 mV. Nisoldipine shifted the steady-state inactivation curve to more negative potentials by increasing the fraction of test pulses without openings. 3. The degree of nisoldipine inhibition of average single calcium channel currents increased with membrane depolarization. Depolarization of the holding potential from -100 to -55 mV decreased the concentration of nisoldipine needed for 50% inhibition (Kapp) from 12.1 to 1.9 nM in the presence of 1 microM-Bay K 8644. 4. Membrane depolarization by external potassium (K+) of the intact artery in the presence of nisoldipine decreased contractions evoked by depolarizing test pulses. The relationship between membrane potential and contraction could be empirically described by the Boltzmann equation, with a steepness factor, k, of 7.1 mV. Increasing the nisoldipine concentration from 0.25 to 2.0 nM shifted the mid-point of this relationship from -20.5 to -33.0 mV, without affecting the steepness factor. 5. Nisoldipine inhibition of contraction increased with membrane depolarization. Membrane depolarization from -68.6 to -30.0 mV decreased the Kapp of nisoldipine for contractions from 3.02 to 0.69 nM. Bay K 8644 (1 microM) elevated Kapp about 9.3-fold at 5 mM-K+. In the presence of Bay K 8644, membrane depolarization from -68.6 to -30.0 mV reduced Kapp from 28.4 to 4.0 nM. 6. In the presence of nisoldipine, the effect of membrane depolarization on the time course of development of inhibition was examined. In 3 nM-nisoldipine, after membrane depolarization with 20 mM-K+, the time course of development of inhibition of force could be described by a single exponential with a time constant of 16.5 min. Membrane depolarization to a more positive potential accelerated the development of inhibition. 7. The results were interpreted by a model in which nisoldipine binds with higher affinity to the inactivated state than to the resting state of calcium channels in the mesenteric artery. The approach presented here can be used to estimate the properties of steady-state calcium channel inactivation and dihydropyridine interactions in smooth muscle cells in the intact artery under physiological conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

ATP regulation of the slow calcium channels in vascular smooth muscle cells of guinea pig mesenteric artery

Effects of intracellularly perfused ATP, and extracellularly applied cyanide and 2-deoxy-D-glucose, on fast and slow Ca2+ channel currents of isolated single vascular smooth muscle cells were investigated by a whole-cell voltage-clamp method combined with an intracellular perfusion technique. Single smooth muscle cells were prepared by collagenase treatment from guinea pig small mesenteric arteries (diameter of less than 300 micron). With Cs+-rich solution in the pipette and isotonic Ba2+ solution (100 mM) in the bath, depolarizing pulses evoked two types of the Ca2+ channel current. Depolarizing pulses from the holding potential of -80 mV to over -30 mV evoked a fast Ca2+ channel current. This fast component was inhibited by shifting the holding potential in a positive direction. With a holding potential of -40 mV, the fast component was almost inhibited. In contrast, the slow current was evoked by command potentials to above -10 mV, and its full amplitude was preserved at the holding potential of -40 mV. Without ATP in the pipette, the fast current was dominant. Increase in the ATP concentration in the pipette (0.3 to 5 mM) enhanced the slow current but did not affect the fast current. Maximum enhancement of the slow current was observed at 5 mM ATP. Increase in ATP concentration, however, did not modify the shape of the current trace and the steady state inactivation curve of the slow current. Maximum amplitudes of the fast current and slow current recorded with 5 mM ATP averaged 17.4 pA (SD of 10.4 pA, n = 30; observed at -10 mV to +10 mV) and 141.8 pA (SD of 27.1 pA, n = 30; observed at +30 mV to +40 mV), respectively. Presence of CN- and 2-deoxy-D-glucose (without glucose) in the bath, and absence of ATP in the pipette, abolished the slow current within 10 minutes; in contrast, it took more than 10 minutes to depress the fast current. The inhibitory effect of CN- and 2-deoxy-D-glucose on the slow current was reduced by intracellular application of ATP. In summary, the activation of the slow Ca2+ channel required physiological concentration of ATP, whereas the fast channel current was preserved, even under ATP-free conditions. These results indicate that only the slow current is a metabolically dependent Ca2+ channel current in these vascular smooth muscle cells.

Calcium currents are altered in the vascular muscle cell membrane of spontaneously hypertensive rats

Calcium currents were recorded during whole-cell voltage clamp in cultured azygos venous muscle cells from 1-3-day-old normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Different holding potentials were used to separate total cell current into its transient (T) and sustained or long-lasting (L) components. In recordings from 30 WKY and 30 SHR vascular cells, total cell calcium current was the same between cells from normotensive (167 +/- 20 pA) and hypertensive (139 +/- 15 pA) rats. However, the relative proportion of T and L calcium currents was different between WKY and SHR cells. In WKY cells, the peak amplitude of the L current was less than that of the T current (42 +/- 30% of total current), whereas in SHR cells, the L current was greater (62 +/- 3% of total current). Calcium currents in vascular muscle cells from SHR were activated and inactivated at more positive potentials than in cells from WKY. This study directly compares transmembrane calcium current in isolated cells from WKY and SHR blood vessels and shows that the proportions of T and L calcium channels activated by depolarization are altered in this genetic model of hypertension.

Nisoldipine. A preliminary review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy in the treatment of angina pectoris, hypertension and related cardiovascular disorders

Nisoldipine is an orally administered calcium entry blocking drug structurally related to nifedipine. In limited clinical trials it has been shown to be effective and relatively well tolerated in the treatment of patients with chronic stable angina pectoris and mild to moderate essential hypertension. As for all dihydropyridine-calcium antagonists, its major properties include potent peripheral and coronary vasodilation and improvement in myocardial oxygen supply relative to demand. These actions occur without depression of cardiac conduction or left ventricular function. Short term clinical trials have shown nisoldipine to produce both symptomatic and objective improvements in patients with chronic angina of effort and have suggested a benefit in vasospastic angina. A small number of comparative trials indicate that nisoldipine is equally as effective as nifedipine. In addition, in combination with beta-adrenoceptor blockade nisoldipine appears to offer additional benefit compared with beta-blockade alone and is well tolerated. In patients with mild to moderate essential hypertension nisoldipine monotherapy, in 1 or 2 daily doses, has maintained blood pressure control and has also been a useful addition to diuretics and beta-adrenoceptor blocking drugs in patients with poorly controlled disease. Side effects appear to be dose related, generally mild and transient, and are primarily those resulting from potent peripheral vasodilation - headache, flushing and pretibial or ankle oedema. Although studies to date are promising, there are no published long term studies (greater than 1 year) of nisoldipine in comparison with other calcium entry blockers and other drugs currently in clinical use for the treatment of angina pectoris or hypertension. Until such studies are completed the exact place of nisoldipine in the treatment of these diseases remains to be established.

Noradrenaline contracts arteries by activating voltage-dependent calcium channels

Noradrenaline (NA) regulates arterial smooth muscle tone and hence blood vessel diameter and blood flow. NA apparently increases tone by causing a calcium influx through the cell membrane. Two calcium influx pathways have been proposed: voltage-activated calcium channels and NA-activated calcium-permeable channels that are voltage-insensitive. Although voltage-activated calcium channels have been identified in arterial smooth muscle, voltage-insensitive calcium channels activated by NA have not. We show here that NA contractions of rabbit mesenteric arteries increase with depolarization. The increase parallels the elevation of open-state probability (P0) of single, voltage-dependent calcium channels. The action of noradrenaline can be explained by NA-activating voltage-dependent calcium channels, rather than by opening a second type of channel. We show directly that NA increases the open-state probability of single calcium channels. Thus, in the presence of NA, calcium entry through voltage-dependent calcium channels can regulate smooth muscle tone at physiological membrane potentials. These results may have relevance to pathophysiological conditions such as hypertension.

Calcium currents in single isolated smooth muscle cells from the rabbit ear artery in normal-calcium and high-barium solutions

1. Ca2+ inward current was studied using the whole-cell patch clamp technique in single smooth muscle cells enzymatically isolated from the rabbit ear artery. Currents were studied in salt solutions containing either normal (1.5 mM) Ca2+ or high (110 mM) Ba2+. Outward currents were minimized by using a high-Na+ intracellular solution containing 10 mM-TEA. 2. In normal-Ca2+ solution, the threshold at which inward current could be evoked was -60 mV at a holding potential of -80 mV and -48 mV at a holding potential of -60 mV. At both holding potentials, the current showed little inactivation over 500 ms near threshold, and inactivated substantially but incompletely with larger depolarizations. In high-Ba2+ solution where currents were 5-10 times larger, current threshold, maximal peak amplitude and apparent reversal potential were shifted in a positive direction along the voltage axis. 3. Inward current in normal-Ca2+ solution was only fully available negative to -90 mV, was half-inactivated near -47 mV and was about 90% inactivated at -10 mV. A component of non-inactivating current was, however, present in both normal-Ca2+ and high-Ba2+ solutions even after conditioning pulses of 5 s duration to +30 mV. The inactivation-potential relationship was shifted in a positive direction in high-Ba2+ solution, and its position showed considerable variation between cells. 4. The inward current from a holding potential of -70 mV was reduced in normal-Ca2+ solution by about 50% by nifedipine (0.1 microM) in some cells at all test potentials, although at more-negative test potentials peak current was unaffected in some cells even at 10 microM. In some cells when the holding potential was -90 mV, the peak current amplitude was increased markedly by 10 microM-nifedipine. In high-Ba2+ solution, from negative holding potentials (-70 to -80 mV), nifedipine augmented peak current at test potentials negative to -10 mV and shifted the current threshold in a negative direction; current at potentials positive of -10 mV was reduced. Both effects were concentration dependent. The stimulatory effect of nifedipine was abolished and its inhibitory effect was enhanced when less-negative holding potentials were employed. Bay K 8644 (0.1 or 1 microM) approximately doubled current amplitude in normal-Ca2+ solution at a holding potential of -60 or -70 mV, and shifted maximal peak and threshold current in a negative direction. 5. Holding at more-negative potentials made available a component of current which activated and inactivated at more-negative potentials especially in high-Ba2+ solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Purified skeletal muscle 1,4-dihydropyridine receptor forms phosphorylation-dependent oligomeric calcium channels in planar bilayers

The purified 1,4-dihydropyridine receptor from skeletal muscle has been incorporated into planar bilayers, and its channel characteristics have been investigated. Conductances showed the characteristics of an L-type Ca2+ channel: divalent cation selectivity (PBa/PNa approximately equal to 30), blockage of Na+ conductance by micromolar Ca2+, and blockage of the Ca2+ channel by D890 and by Cd2+. The alpha 1 subunit of the receptor must be phosphorylated by the cAMP-dependent protein kinase to give channel activity. BAY K 8644 did not activate nonphosphorylated channels, and (+)-PN200-110 caused dramatic prolongation of mean open times when applied after phosphorylation. Channel properties were found to be dependent on association of receptor molecules in the bilayer. Single receptor molecules form channels of 0.9 pS (100 mM Ba2+) and show no voltage-dependent gating. Upon association, both voltage-dependent gating and higher conductance events are recovered; stabilized conductance levels assume values of even multiples of 0.9 pS, predominately 7.5 and 15 pS and multiples of these values up to 60 pS. Thus, individual channels become functionally coupled (synchronous opening and closing) with association, reinstating the characteristics of one larger unitary channel. It is concluded that the L-type Ca2+ channel represents an oligomer of 1,4-dihydropyridine-receptor protein complexes, each of which constitutes a channel, where the array of channels (oligochannel) opens and closes in concerted action.

Saturation of calcium channels in single isolated smooth muscle cells of guinea-pig taenia caeci

1. Calcium channel currents were recorded in Cs+-dialysed voltage-clamped single smooth muscle cells isolated from the guinea-pig taenia caeci to evaluate the current-carrying ability of Ca2+, Ba2+, Sr2+ and Mg2+ ions. 2. Ba2+ and Sr2+ ions, as well as Ca2+ ions, were able to carry an inward current through calcium channels. Calcium channel current was not observed when Mg2+ was the only divalent cation in the external solution. 3. Concentration dependences of calcium (ICa), barium (IBa) and strontium (ISr) currents were studied. It was found that currents through calcium channels saturated with increasing the extracellular concentration of a current carrier. Saturation of each current can be fitted with a Langmuir curve with apparent dissociation constants of 1.2 mM for Ca2+, 1.8 mM for Sr2+ and 9.6 mM for Ba2+ ions. 4. External Mg2+ ions reduced both ICa and IBa.IBa was depressed to a greater extent than ICa by Mg2+ ions. Reduction of ICa by Mg2+ ions seems to agree with competitive antagonism between Ca2+ and Mg2+ ions (Hagiwara & Takahashi, 1967). 5. When the external divalent cation concentration [( C2+]o) was changed, the current-voltage relationship of currents through calcium channels was shifted along the potential axis suggesting that activation gating of calcium channels was affected by [C2+]o. These voltage shifts can be fitted with the Gouy-Chapman theory supposing the density of surface charges near calcium channels to be 0.5 e nm-2 and including more potent binding of Ca2+ ions to surface charges than of Ba2+, Sr2+ and Mg2+ ions. 6. The changes in the Ca2+, Ba2+ and Sr2+ concentrations at the surface of the membrane were calculated. It was found that saturation of IBa can be explained by saturation of Ba2+ surface concentration while saturation of ICa and ISr cannot. 7. It was suggested that barium ions were able to carry the larger current through calcium channels in smooth muscle cells due to their much weaker binding within the calcium channel.

Fast and slowly inactivating components of Ca-channel current and their sensitivities to nicardipine in isolated smooth muscle cells from rat vas deferens

(1) Fast and slowly inactivating components of Ca-channel current were compared to clarify whether more than one type of Ca-channel exists in smooth muscle cells from rat vas deferens using the whole cell variant of the patch clamp technique. The pipette was filled with 150 mM Cs solution to eliminate outward current and Ba was used as the charge carrier for Ca-channel current. (2) When activated by a 5 s test pulse to O mV from a holding potential of -60 mV, the inactivation process of Ba-current was well fitted by the sum of two exponentials. The time constant of the faster inactivating component was 100-300 s and that of the slower inactivating component was 1.5-3 s. Steady-state inactivation curves of the fast- and slow-components were very similar. (3) The inward current activated at O mV from -80 mV was inactivated faster than that from -30 mV. The voltage-dependencies of the peak current from holding potentials of -30 mV and -80 mV were similar. Both had voltage threshold at -30 mV and were maximal at +10 mV. (4) Low concentrations of nicardipine (10(-9) to 10(-7) M) preferentially inhibited the slow component while higher concentration (10(-6) to 10(-5) M) were required to block the fast component. The current activated from a holding potential of -30 mV was almost fully suppressed by 10(-7) M nicardipine whereas that from -80 mV was blocked only slightly.(ABSTRACT TRUNCATED AT 250 WORDS)