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

Nitric oxide-induced biphasic mechanism of vascular relaxation via dephosphorylation of CPI-17 and MYPT1

Nitric oxide (NO) from endothelium is a major mediator of vasodilatation through cGMP/PKG signals that lead to a decrease in Ca(2+) concentration. In addition, NO-mediated signals trigger an increase in myosin light chain phosphatase (MLCP) activity. To evaluate the mechanism of NO-induced relaxation through MLCP deinhibition, we compared time-dependent changes in Ca(2+), myosin light chain (MLC) phosphorylation and contraction to changes in phosphorylation levels of CPI-17 at Thr38, RhoA at Ser188, and MYPT1 at Ser695, Thr696 and Thr853 in response to sodium nitroprusside (SNP)-induced relaxation in denuded rabbit femoral artery. During phenylephrine (PE)-induced contraction, SNP reduced CPI-17 phosphorylation to a minimal value within 15 s, in parallel with decreases in Ca(2+) and MLC phosphorylation, followed by a reduction of contractile force having a latency period of about 15 s. MYPT1 phosphorylation at Ser695, the PKG-target site, increased concurrently with relaxation. Phosphorylation of RhoA, MYPT1 Thr696 and Thr853 differed significantly at 5 min but not within 1 min of SNP exposure. Inhibition of Ca(2+) release delayed SNP-induced relaxation while inhibition of Ca(2+) channel, BK(Ca) channel or phosphodiesterase-5 did not. Pretreatment of resting artery with SNP suppressed an increase in Ca(2+), contractile force and phosphorylation of MLC, CPI-17, MYPT1 Thr696 and Thr853 at 10 s after PE stimulation, but had no effect on phorbol ester-induced CPI-17 phosphorylation. Together, these results suggest that NO production suppresses Ca(2+) release, which causes an inactivation of PKC and rapid CPI-17 dephosphorylation as well as MLCK inactivation, resulting in rapid MLC dephosphorylation and relaxation.

Ca2+-dependent rapid Ca2+ sensitization of contraction in arterial smooth muscle

Ca(2+) ion is a universal intracellular messenger that regulates numerous biological functions. In smooth muscle, Ca(2+) with calmodulin activates myosin light chain (MLC) kinase to initiate a rapid MLC phosphorylation and contraction. To test the hypothesis that regulation of MLC phosphatase is involved in the rapid development of MLC phosphorylation and contraction during Ca(2+) transient, we compared Ca(2+) signal, MLC phosphorylation, and 2 modes of inhibition of MLC phosphatase, phosphorylation of CPI-17 Thr38 and MYPT1 Thr853, during alpha(1) agonist-induced contraction with/without various inhibitors in intact rabbit femoral artery. Phenylephrine rapidly induced CPI-17 phosphorylation from a negligible amount to a peak value of 0.38+/-0.04 mol of Pi/mol within 7 seconds following stimulation, similar to the rapid time course of Ca(2+) rise and MLC phosphorylation. This rapid CPI-17 phosphorylation was dramatically inhibited by either blocking Ca(2+) release from the sarcoplasmic reticulum or by pretreatment with protein kinase C inhibitors, suggesting an involvement of Ca(2+)-dependent protein kinase C. This was followed by a slow Ca(2+)-independent and Rho-kinase/protein kinase C-dependent phosphorylation of CPI-17. In contrast, MYPT1 phosphorylation had only a slow component that increased from 0.29+/-0.09 at rest to the peak of 0.68+/-0.14 mol of Pi/mol at 1 minute, similar to the time course of contraction. Thus, there are 2 components of the Ca(2+) sensitization through inhibition of MLC phosphatase. Our results support the hypothesis that the initial rapid Ca(2+) rise induces a rapid inhibition of MLC phosphatase coincident with the Ca(2+)-induced MLC kinase activation to synergistically initiate a rapid MLC phosphorylation and contraction in arteries with abundant CPI-17 content.

Native-type DHP-sensitive calcium channel currents are produced by cloned rat aortic smooth muscle and cardiac α1subunits expressed inXenopus laevisooeytes and are regulated by α2- and β-subunits

Native tissue-like L-type voltage-dependent calcium channels (L-VDCC's) were expressed by in vitro transcribed cRNA injection of rat aorta or rabbit cardiac alpha 1 subunit into Xenopus laevis oocytes. Co-injection of VSM-alpha 1 with the cloned skeletal muscle beta-subunit (SK-beta) of the L-type VDCC significantly increased the expressed peak current amplitude without significant changes in kinetics. Similar results were obtained by co-injection of cardiac alpha 1 (DSHT-alpha 1) the cloned skeletal alpha 2-subunit (SK-alpha 2) or with SK-beta. The oocytes co-expressing cRNA's retained L-type VDCC pharmacology.

TTX-sensitive Na(+) and nifedipine-sensitive Ca(2+) channels in rat vas deferens smooth muscle cells

The inward currents in single smooth muscle cells (SMC) isolated from epididymal part of rat vas deferens have been studied using whole-cell patch-clamp method. Depolarising steps from holding potential -90 mV evoked inward current with fast and slow components. The component with slow activation possessed voltage-dependent and pharmacological properties characteristic for Ca(2+) current carried through L-type calcium channels (I(Ca)). The fast component of inward current was activated at around -40 mV, reached its peak at 0 mV, and disappeared upon removal of Na ions from bath solution. This current was blocked in dose-dependent manner by tetrodotoxin (TTX) with an apparent dissociation constant of 6.7 nM. On the basis of voltage-dependent characteristics, TTX sensitivity of fast component of inward current and its disappearance in Na-free solution it is suggested that this current is TTX-sensitive depolarisation activated sodium current (I(Na)). Cell dialysis with a pipette solution containing no macroergic compounds resulted in significant inhibition of I(Ca) (depression of peak I(Ca) by about 81% was observed by 13 min of dialysis), while I(Na) remained unaffected during 50 min of dialysis. These data draw first evidence for the existence of TTX-sensitive Na(+) current in single SMC isolated from rat vas deferens. These Na(+) channels do not appear to be regulated by a phosphorylation process under resting conditions.

Regional difference in sympathetic neurotransmitter- and Ca2+ channel-mediated responses in rat vas deferens

1. Pharmacological properties were compared in the epididymal and prostatic portions of the rat vas deferens. 2. Contractile response to norepinephrine (NE) was larger in the epididymal portion, in spite of the smaller diameter in this region. In contrast, contraction evoked by ATP or alpha,beta-methylene ATP (alpha,beta-mATP) was larger in the prostatic portion. The sensitivities to NE but not to alpha,beta-mATP are different in these portions. 3. ATP or NE facilitated the contraction induced by the other agonist, suggesting that they cooperatively elicit smooth muscle contraction. This cooperation was observed in both portions. 4. Neither the contraction elicited by the addition of Ca2+ to smooth muscle depolarized with 63.7 mM extracellular K+ nor the relaxation by nifedipine of the depolarized smooth muscle precontracted with 1.8 mM extracellular Ca2+ was regionally different. However, Bay k 8644 elicited contraction only in the epididymal portion. A combination of 5 mMK+ with Bay k 8644 also caused oscillatory contraction in the prostatic portion. 5. A radioligand binding study was performed using the microsomal fraction prepared separately from these portions. Both the dissociation constant and the maximum binding for 3H-nimodipine were smaller in the epididymal fraction than in the prostatic fraction. 6. These results suggest that 1. the NE-elicited contraction is more pronounced in the epididymal portion, 2. the purinoceptor-mediated contractions along the vas deferens are less heterogeneous, and 3. although the sensitivity to Bay k 8644 is higher in the epididymal portion, Ca2+ channel-mediated responses are not regionally different when they are fully activated.

Ca(2+)-induced inhibition of 45Ca2+ influx and Ca2+ current in smooth muscle of the rat vas deferens

The present study investigates how changes in intracellular Ca2+ concentration modulate the influx of 45Ca2+ in isolated rat vasa deferentia. Raising extracellular K+ concentration ([K+]0) to > or = 32 mM increased 45Ca2+ influx during the 1st min in solutions containing 0.03-1.5 mM extracellular Ca2+ concentration ([Ca2+]0). During the 6th min in [K+]0 > or = 50 mM, 45Ca2+ influx was less than during the 1st min. This decline in 45Ca2+ influx occurred for [Ca2+]0 > or = 0.4 mM. Procaine potentiated K(+)-stimulated 45Ca2+ influx in 1.5 mM [Ca2+]0 and eliminated the decline of 45Ca2+ influx in low [Ca2-]0. Ryanodine and norepinephrine reduced K(+)-stimulated 45Ca2+ influx. 45Ca2+ content changed with time in accordance with the changes observed in 45Ca2+ influx. In isolated cells, voltage-dependent inward currents inactivated more rapidly with 1.5 mM Ca2+ as the charge carrier than with 1.5 mM Ba2+, and the steady-state inactivation relationship was shifted in the hyperpolarizing direction. Inward current was reduced with either caffeine, ryanodine, or norepinephrine. The inhibitory effects of norepinephrine were abolished by depletion of intracellular Ca2+ stores. These results are compatible with the hypothesis that K(+)-stimulated 45Ca2+ influx declines with time due to Ca(2+)-induced inhibition of Ca2- channels. Ca(2+)- and inositol 1,4,5-trisphosphate-induced releases of Ca2+ from the sarcoplasmic reticulum appear to play an important role in this process.

Novel blockade of Ca2+ current by quinacrine in smooth muscle cells of the guinea pig

Effects of quinacrine on voltage-dependent Ca2+ channel current (ICa) were examined using whole cell voltage clamp in single smooth muscle cells isolated from vas deferens and urinary bladder and single cardiac myocytes from ventricle of the guinea pig. When ICa was elicited by depolarization from a holding potential of -60 to 0 mV for 150 msec every 15 sec in vas deferens myocytes, external application of quinacrine reduced the amplitude of ICa in a concentration-dependent manner in a range of 0.1 approximately 30 microM, and the IC50 of quinacrine was 1.3 microM. The block was at least partly removed by washout. The block of ICa by 1 microM quinacrine in vas deferens myocytes greatly depended upon the activation potentials but only slightly on the holding potentials. Use-dependent development of the block was also observed. Addition of 300 microM quinacrine to the pipette-filling solution did not significantly affect ICa. The IC50 of quinacrine for ICa block in urinary bladder myocytes was 1.1 microM and comparable to that in vas deferens. On the other hand, IC50 for the block of ICa elicited by depolarization from -45 to 0 mV in cardiac ventricular myocytes was 5.6 microM. It is concluded that quinacrine is a potent blocker of L-type Ca2+ channels in two types of smooth muscle myocytes and that the potency appeared to be approximately five times higher than that in cardiac myocytes. The action of quinacrine may be due to the direct block of Ca2+ channels from outside of the cell membrane.

Ca2+ currents in cerebral artery smooth muscle cells of rat at physiological Ca2+ concentrations

Single Ca2+ channel and whole cell currents were measured in smooth muscle cells dissociated from resistance-sized (100-microns diameter) rat cerebral arteries. We sought to quantify the magnitude of Ca2+ channel currents and activity under the putative physiological conditions of these cells: 2 mM [Ca2+]o, steady depolarizations to potentials between -50 and -20 mV, and (where possible) without extrinsic channel agonists. Single Ca2+ channel conductance was measured over a broad range of Ca2+ concentrations (0.5-80 mM). The saturating conductance ranged from 1.5 pS at 0.5 mM to 7.8 pS at 80 mM, with a value of 3.5 pS at 2 mM Ca (unitary currents of 0.18 pA at -40 mV). Both single channel and whole cell Ca2+ currents were measured during pulses and at steady holding potentials. Ca2+ channel open probability and the lower limit for the total number of channels per cell were estimated by dividing the whole-cell Ca2+ currents by the single channel current. We estimate that an average cell has at least 5,000 functional channels with open probabilities of 3.4 x 10(-4) and 2 x 10(-3) at -40 and -20 mV, respectively. An average of 1-10 (-40 mV and -20 mV, respectively) Ca2+ channels are thus open at physiological potentials, carrying approximately 0.5 pA steady Ca2+ current at -30 mV. We also observed a very slow reduction in open probability during steady test potentials when compared with peak pulse responses. This 4-10-fold reduction in activity could not be accounted for by the channel's normal inactivation at our recording potentials between -50 and -20 mV, implying that an additional slow inactivation process may be important in regulating Ca2+ channel activity during steady depolarization.

Modulation of calcium channel currents by arachidonic acid in single smooth muscle cells from vas deferens of the guinea-pig

1. Effects of arachidonic acid (AA) on voltage-dependent Ca channel currents were investigated by whole-cell-clamp methods in single smooth muscle cells freshly isolated from vas deferens of the guinea-pig. 2. Ca channel current was decreased by application of 1-30 microM AA in a concentration-dependent manner. When Ca2+ or Ba2+ was the charge carrier, Ca channel current (ICa or IBa) was reduced by AA to a similar extent (IC50 = 10 and 6 microM, respectively). Addition of 15 mM BAPTA to the pipette solution did not affect the reduction of IBa by 10 microM AA. 3. The effect of AA on IBa was not prevented by internal application of 1 mM nordihydroguaiaretic acid (NDGA) and 1 mM indomethacin (Indo). When the pipette solution contained 0.1 mM guanosine-5'-triphosphate (GTP), IBa was decreased slightly but significantly by application of 30 microM prostaglandin F2 alpha (PGF2 alpha) but not by PGE2. This effect of PGF2 alpha was irreversible or not observed when the pipette solution contained 0.3 mM guanosine-5'-(3-thiotriphosphate) (GTP gamma S) or both GTP or guanosine-5'-O-(2-thiodiphosphate) (GDP beta S), respectively. 4. External application of 100 units ml-1 superoxide dismutase slightly but significantly attenuated the inhibition of IBa by 1-30 microM AA. Intracellular application of 1 mM GDP beta S or 0.3 mM GTP gamma S did not significantly change the effect of AA. Intracellular application of 0.1 mM 1-(5-isoquinolinesulphonyl)-2-methylepiperazine (H-7) also did not change the effect of AA. 5. These results indicate that the decrease in Ca channel currents in vas deferens smooth muscle cells is mainly due to AA itself, as opposed to its metabolites. The effect of AA may be due to AA itself, as opposed to its metabolites. The effect of AA may be due to its direct action on Ca channels or membrane phospholipids, but may not be mediated by activation of GTP binding proteins or protein kinase C. The inhibition of Ca channel current by AA may be partly induced by superoxide radicals derived from AA oxidation. PGF2A also reduces Ca channel currents but probably by a separate mechanism via activation of a GTP binding protein.

Properties of inactivation of calcium channel currents in smooth muscle cells of rabbit portal vein

With 10 mM Ba2+ as the charge carrier, inactivation of Ca2+ channel currents could be subdivided into at least two exponentials in smooth muscle cells dispersed from the rabbit portal vein by use of the whole-cell configuration of the patch-clamp technique: fast and slow inactivation. All characteristics of inactivation were independent of the size of the currents. Step changes in the holding potential unveiled an extremely slow recovery and an onset of inactivation of the order of several minutes. Steady-state inactivation critically depended on the duration of the pre-steps. Inactivation curves obtained under steady-state conditions showed as shift by approximately 25 mV towards negative potentials by comparison with curves obtained using 1-s pre-pulses. This shift greatly reduced the window current. Recovery from inactivation studied with double-pulse protocols could be classified into at least two exponentials. The contribution of the slow recovery was accentuated at negative holding potentials. Recovery from inactivation critically depended on the duration of the conditioning voltage step, and was also dependent on the duration of the pre-step: its voltage dependence disappeared when pre-pulses longer than 2 s were applied. Onset of inactivation was composed of at least two exponentials: the fast component was accelerated at less negative pre-step potentials. We propose that several inactivated states are involved in Ca2+ channel inactivation. Transitions between these states are voltage dependent and voltage independent.

Blockade by calmodulin inhibitors of Ca2+ channels in smooth muscle from rat vas deferens

1. Effects of three compounds which are used as calmodulin inhibitors (trifluoperazine, W-7 and calmidazolium) on Ca2+ channels were investigated in smooth muscle from rat vas deferens. 2. All three calmodulin inhibitors relaxed the smooth muscle precontracted by a high concentration of KCl (63.7 mM). The order of potency for the relaxation was trifluoperazine > W-7 > calmidazolium. 3. In binding studies using a microsomal fraction of vas deferens, all these calmodulin inhibitors displaced specific [3H]-nimodipine binding. Trifluoperazine and W-7 inhibited the binding at concentrations that relaxed the smooth muscle whereas calmidazolium inhibited at concentrations much lower than those necessary for muscle relaxation. 4. Ba2+ current flowing through voltage-gated Ca2+ channels was measured under whole-cell voltage-clamp conditions in isolated smooth muscle cells. The Ba2+ current was suppressed by the three calmodulin inhibitors in the concentration-range where inhibition of [3H]-nimodipine binding was observed. Neither voltage-dependence nor the inactivation time course of Ba2+ current were affected by these compounds. 5. The results suggest that the calmodulin inhibitors directly block Ca2+ channels in the smooth muscle cells. The channel inhibition by trifluoperazine and W-7, but perhaps not that by calmidazolium, may be responsible for the muscle relaxation observed with these compounds.

Low dihydropyridine receptor density in vasa deferentia of castrated rats

Radioligand binding studies in crude membrane preparations of vasa deferentia of normal rats, with the 1,4-dihydropyridine (+)-[3H]-PN200-110 (isradipine) showed typical saturation isotherms. The binding exhibited a KD of 259 +/- 60 pM and Bmax of 144 +/- 20 fmol mg-1 protein. The low KD and the stereoselective displacement of (+)-[3H]-PN200-110 binding by (+)- and (-)-PN200-110 and by nifedipine suggests that these tissues contain dihydropyridine receptors probably coupled to voltage-sensitive, L-type calcium channels. In membrane preparations from vasa deferentia from rats castrated 30 days previously the maximum specific binding was 25 +/- 10 fmol mg-1 protein, representing only 11% of total binding; thus, the calculation of reliable KD values was not feasible. These findings suggest that a testicular hormone, possibly testosterone, plays an important role in the regulation of dihydropyridine-sensitive, voltage-dependent calcium channels in the rat vas deferens.

Characterization of divalent cation-induced [3H]acetylcholine release from EGTA-treated rat hippocampal synaptosomes

Calcium-naive synaptosomes were used to assess the effects of divalent cations on [3H]acetylcholine release from rat hippocampal homogenates. Following equilibration with calcium-free buffer (containing 10 microM EGTA), calcium reversibly increased [3H]acetylcholine efflux (up to five-fold) while causing no measurable efflux of lactate dehydrogenase. When substituted for calcium, strontium and barium behaved similarly although barium exhibited three-fold greater efficacy. In the presence of elevated potassium, 4-aminopyridine or tetraethylammonium, the secretagogue efficacy of calcium (but not barium) was markedly increased. The release-promoting effects of both cations were inhibited by lanthanum, magnesium, cadmium, and omega-conotoxin but were insensitive to nifedipine and cobalt (both 10 microM). In addition, stimulation of muscarinic cholinergic autoreceptors substantially inhibited both calcium and barium-evoked [3H]acetylcholine release. Taken together, these results indicate that cation-evoked transmitter release from calcium-naive synaptosomes is subject to normal neuroregulatory mechanisms and therefore should be useful for investigating presynaptic modulation of neuronal exocytosis.

Inhibition of Ca-channels by diazepam compared with that by nicardipine in pheochromocytoma PC12 cells

The effects of diazepam on voltage-gated Ca channels were studied in PC12 pheochromocytoma cells using whole-cell voltage-clamp techniques. An inward current activated by a depolarizing voltage step to +10 mV from a holding potential of -60 mV in 10.8 mM Ba was larger than that activated in 10.8 mM Ca. The Ba current was completely blocked by a low concentration of Cd (30 microM) and was also sensitive to nicardipine (100 nM to 10 microM). Diazepam (1-100 microM) inhibited the Ba current in a concentration-dependent manner. Neither diazepam nor nicardipine affected the current-voltage relationship or the dependence on holding potentials of the Ba current. Both slightly accelerated the inactivation time course of the Ba current. When diazepam was applied to the cells in combination with nicardipine, the observed inhibition agreed with a value predicted assuming independent blockade by diazepam and by nicardipine. These results suggest that diazepam inhibits Ca channels in a manner similar to nicardipine, but that the binding sites for diazepam are different from those for nicardipine.

Effects of 2,3-butanedione monoxime on whole-cell Ca2+ channel currents in single cells of the guinea-pig taenia caeci

1. The inhibitory actions of cadmium (Cd2+), nifedipine and 2,3-butanedione monoxime (BDM) on whole-cell Ca2+ channel currents in single cells of the guinea-pig taenia caeci were investigated using a single-electrode whole-cell voltage-clamp technique. 2. Calcium channel currents were isolated using pipette solutions containing Cs+, tetraethylammonium and ATP (3 mM). Ca2+ or Ba2+ (7.5 mM) in the bathing solution acted as the charge carrier during inward current flow. Ca2+ channel currents in 7.5 mM-Ba2+ (IBa) were recorded at potentials positive to -40 mV, were maximal near 0 mV and reversed near +60 mV. Ca2+ channel activation showed a sigmoidal relationship with potential, which was half-maximal at -13 mV. 3. Both the inward and outward flow of current was depressed and eventually blocked by 0.3-100 microM-Cd2+, 0.1-10 microM-nifedipine and 2-20 mM-BDM. Half-maximal blockade of IBa at 0 mV was achieved with approximately 3 microM-Cd2+, 1 microM-nifedipine and 10 microM-BDM. Steady-state activation curves were not affected by Cd2+ or BDM, but were shifted in the hyperpolarizing direction by nifedipine at concentrations > 1 microM. 4. Calcium channel currents in single cells and K+ contractures in intact strips were both blocked in a voltage-dependent manner. Steady-state inactivation curves (f infinity (V)) for IBa were shifted 20 mV in the hyperpolarizing direction by 0.3 microM-nifedipine and 4 mV by 10 mM-BDM. From these shifts a dissociation binding constant to inactivated Ca2+ channels for nifedipine was estimated as 78 nM, and for BDM, 5 mM. 5. At 10 microM Cd2+ produced a 43 +/- 6% (n = 3) block of the inward current at 0 mV when Ca2+ (7.5 mM) was the charge carrier (ICa), compared with the 36 +/- 3% block of IBa induced by 1 microM-Cd2+, consistent with the suggestion that Ca2+, Ba2+ and Cd2+ compete for the same binding site. In contrast, nifedipine (1 microM) and BDM (10 mM) blocked ICa more effectively than IBa. 6. Bay K 8644 (1.0 microM) increased Ca2+ channel currents two- to fourfold at all potentials due to a shift, of approximately 10 mV in the negative direction, of their activation curve and an equal shift in the positive direction of their inactivation curve. BDM (5-10 mM) could antagonize the action of Bay K 8644, shifting both curves back towards their control.(ABSTRACT TRUNCATED AT 400 WORDS)

Inactivation kinetics and pharmacology distinguish two calcium currents in mouse pancreatic B-cells

Voltage-dependent calcium currents were studied in cultured adult mouse pancreatic B-cells using the whole-cell voltage-clamp technique. When calcium currents were elicited with 10-sec depolarizing command pulses, the time course of inactivation was well fit by the sum of two exponentials. The more rapidly-inactivating component had a time constant of 75 +/- 5 msec at 0 mV and displayed both calcium influx- and voltage-dependent inactivation, while the more slowly-inactivating component had a time constant of 2750 +/- 280 msec at 0 mV and inactivated primarily via voltage. The fast component was subject to greater steady-state inactivation at holding potentials between -100 and -40 mV and activated at a lower voltage threshold. This component was also significantly reduced by nimodipine (0.5 microM) when a holding potential of -100 mV was used, whereas the slow component was unaffected. In contrast, the slow component was greatly increased by replacing external calcium with barium, while the fast component was unchanged. Cadmium (1-10 microM) displayed a voltage-dependent block of calcium currents consistent with a greater effect on the high-threshold, more-slowly inactivating component. Taken together, the data suggest that cultured mouse B-cells, as with other insulin-secreting cells we have studied, possess at least two distinct calcium currents. The physiological significance of two calcium currents having distinct kinetic and steady-state inactivation characteristics for B-cell burst firing and insulin secretion 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)

A possible role for abscisic acid analogues as calcium channel blockers in mammalian smooth muscle

1. The abscisic acid (ABA) analogue SD217595 at 10 microM caused inhibition of K(+)-induced phasic and tonic contractions of rat bladder detrusor smooth muscle strips. 2. This attenuation of contraction was a dual effect: the inhibition of phasic contractions was irreversible whereas that of the tonic contractions was readily reversed. 3. The dual inhibition is possibly due to blockade of two subtypes of voltage-operated calcium channels with T- and L-type characteristics. 4. The inhibition of contraction induced by SD217595 is in stark contrast to the potentiation of smooth muscle contraction reported previously with the parent molecule ABA. 5. Two other ABA analogues studied (WL019376 and WL019377) showed neither inhibitory or excitatory effects upon K(+)-induced smooth muscle contraction after short exposures of 10 min.

An ATP-activated conductance in pheochromocytoma cells and its suppression by extracellular calcium

1. ATP-activated inward current in PC12 pheochromocytoma cells was characterized using the whole-cell voltage-clamp technique. 2. ATP (100 microM) applied extracellular elicited an inward rectifying current with a reversal potential of about +7 mV. The current was desensitized in seconds in spite of continued presence of ATP. 3. A comparison was made of the ability of ATP and its analogues. The order of potency in activating the inward current was ATP greater than ATP gamma S greater than ADP; AMP, adenosine and alpha, beta-methylene ATP were inactive at concentrations up to mM. 4. The ATP-activated current was also observed when external Na+ and Ca2+ were replaced with K+, TEA, Tris or glucosamine. The order of ion selectivity was Na+ greater than K+ greater than TEA not equal to Tris greater than glucosamine. 5. The ATP-activated current was also recorded in extracellular solutions containing Ca2+ as the only external cation. The amplitude increased as the concentration of Ca2+ was increased in the range between 1.8 and 16.2 mM. However, the current amplitude decreased at higher Ca2+ concentrations and the current was not recorded in 110 mM-Ca2+ solution. 6. In the presence of 140 mM-Na+ in the external solution, the current amplitude also decreased as the external Ca2+ concentration was increased (from 1.8 to 16.2 mM). 7. The results indicate that Ca2+ as well as monovalent cations permeate through the ATP-sensitive pathway and that Ca2+ blocks ion permeation, including its own permeation through the pathway. This regulation by extracellular Ca2+ is different to the ATP-activated current in smooth muscle cells.

The whole-cell Ca2+ channel current in single smooth muscle cells of the guinea-pig ureter

1. Calcium channel currents were recorded in single, enzymatically isolated smooth muscle cells of the guinea-pig ureter using a single-electrode whole-cell voltage clamp technique. Calcium and barium currents through voltage-activated Ca2+ channels were recorded in cells dialysed with Cs(+)- or Na(+)-containing saline which suppressed K+ currents. 2. Inward currents in Ca2+ (1.5-7.5 mM) or Ba2+ (1.5-7.5 mM) were recorded at potentials positive to -50 to -30 mV. Inward currents were maximal at 0 mV in 1.5 mM-Ca2+ and at +10 mV in 7.5 mM-Ba2+. Current flow through Ca2+ channels in Cs(+)-filled cells (in 1.5 mM-Ca2+ or 7.5 mM-Ba2+) changed from inward to outward at potentials positive to +70 mV. In Na(+)-filled cells this reversal potential was between +50 and +60 mV. 3. Replacing Ca2+ or Ba2+ with Co2+ (1.5 mM) blocked all inward current flow through these Ca2+ channels; outward currents at potentials positive to +40 mV, however, were increased. Cadmium (100 microM) and nifedipine (0.1-10 microM) reduced both inward and outward current flow. 4. Calcium channel activation showed a sigmoidal relationship with membrane potential; the potential of half-maximal activation was -8.4 mV in 1.5 mM-Ca2+ and -10.8 mV in 7.5 mM-Ba2+. The maximum membrane conductance to Ca2+ (in 1.5 mM-Ca2+) was 2.57 nS/cell or approximately 0.05 mS/cm2. 5. Evidence for a voltage-dependent inactivation mechanism included (a) the time-dependent relaxation of the outward currents at potentials positive to the reversal potential and (b) a steady-state inactivation (f infinity (V] vs. membrane potential relationship (in 7.5 mM-Ba2+) which ranged between -80 and 0 mV, with a half-maximal availability at -40.5 mV. 6. The voltage dependencies of the inward current elicited from -80 and -30 mV were similar, suggesting that depolarization activated only L-type Ca2+ channels. 7. It was concluded that the processes controlling the time course of the Ca2+ current in single ureteral cells bathed in physiological concentrations of Ca2+ were mostly voltage-dependent.

Existence of muscarinic suppression of a K current in PC-12 pheochromocytoma cells

Muscarinic influence on membrane currents of PC-12 pheochromocytoma cells were investigated with whole cell voltage-clamp methods. An outward K current was observed when depolarizing voltage steps were applied to the cells. Methacholine (MCh, 300 microM), a selective agonist for muscarinic receptors, partially suppressed the K current, and the suppression was enhanced by removal of external Ca. The effect of MCh was antagonized by a low dose (100 nM) of atropine. Nicotine (10 microM) induced an inward current in these cells but did not affect the K current activated by depolarizing voltage steps. A Ba current flowing through voltage-gated Ca channels was not changed by MCh. The results indicate the existence of a MCh-sensitive K current in PC-12 cells and suggest that the membrane currents of these cells are modulated by cholinergic agents through muscarinic mechanisms in addition to well-known nicotinic mechanisms.

Characterization of macroscopic outward currents of canine colonic myocytes

Outward currents of colonic smooth muscle cells were characterized by the whole cell voltage-clamp method. Four components of outward current were identified: a time-independent and three time-dependent components. The time-dependent current showed strong outward rectification positive to -25 mV and was blocked by tetraethylammonium. The time-dependent components were separated on the basis of their time courses, voltage dependence, and pharmacological sensitivities. They are as follows. 1) A Ca2+-activated K current sensitive to external Ca2+ and Ca2+ influx was blocked by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (0.1 X 10(-3) M) and nifedipine (1 X 10(-6) and was increased by elevated Ca2+ (8 X 10(-6) M) and BAY K 8644 (1 X 10(-6) M). 2) A "delayed rectifier" current was observed that decayed slowly with time and showed no voltage-dependent inactivation. 3) Spontaneous transient outward currents that were blocked by ryanodine (2 X 10(-6) M) were also recorded. The possible contributions of these currents to the electrical activity of colonic muscle cells in situ are discussed. Ca2+-activated K current may contribute a significant conductance to the repolarizing phase of electrical slow waves.

Calcium channel current and its sensitivity to (+) isradipine in cultured pregnant rat myometrial cells. An electrophysiological and a binding study

Action of (+) isradipine (PN 200-110), a dihydropyridine derivative, was investigated on the Ca channel current in cultured cells obtained from the longitudinal layer of the pregnant rat myometrium (18-19 days of gestation). Under our experimental conditions, the inward current was attributed to L-type inward current since: (i) equimolar replacement of Ba for Ca induced an increase in the peak current and a decrease in inactivation rate; (ii) residual inward currents were recorded at the end of the pulse; (iii) membrane potential for mid inactivation was about -40 mV; (iv) the voltage dependencies of the peak current elicited from holding potentials of -40 mV and -80 mV were similar. The inward current could be reduced with nanomolar concentrations of (+) isradipine when cells were depolarized by pulses to positive potentials. This was characterized by a pronounced initial blockade, but by no increased in blockade when pulses were repeatedly applied at a frequency of 0.05 Hz. Using the double pulse procedure we confirmed that (+) isradipine did not bind to the open-state of the Ca channels. Voltage-dependence of (+) isradipine blockade was assessed by determining the steady-state availability of the Ca channels. From the shift of the inactivation curve in the presence of (+) isradipine we calculated a (K)I value of 130 pM. Scatchard analysis of the specific binding of (+)[3H] isradipine resulted in a linear plot, thereby indicating specific binding to a single class of sites with a dissociation constant Kd of about 100 pM.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Endothelin activates the dihydropyridine-sensitive, voltage-dependent Ca2+ channel in vascular smooth muscle

Endothelin is a potent endothelium-derived vasoconstrictor peptide recently characterized from porcine and human vascular endothelial cells. Here we provide evidence that endothelin activates the dihydropyridine-sensitive, voltage-dependent Ca2+ channel in porcine coronary artery smooth muscle. The vasoconstrictor action of endothelin is efficiently antagonized by low doses of the dihydropyridine Ca2+-channel blocker nicardipine. Endothelin augments the Ca2+-induced contraction in a high-K+ depolarizing solution, markedly enhances high-threshold Ca2+-channel current on the whole-cell patch clamp recording, and causes a sustained increase in the intracellular Ca2+ that is largely dependent on extracellular Ca2+. These findings suggest that endothelin exerts its vasoconstrictor effect by either directly or indirectly activating the voltage-dependent Ca2+ channel.

Calcium current inactivation in insulin-secreting cells is mediated by calcium influx and membrane depolarization

Inactivation of voltage-dependent calcium currents was studied in single, dissociated insulin-secreting HIT cells voltage-clamped by the whole-cell patch-clamp method at room temperature. Na and K currents were suppressed by tetrodotoxin, tetraethylammonium, ATP, 4-aminopyridine and Cs. Ca currents activated in less than 10 ms by depolarizations beyond -50 mV from a holding potential of -100 mV and were identified, as in previous studies, by their sensitivity to divalent cation blockade and permeability to Ba as a charge carrier. Sustained depolarization revealed two kinetically distinct phases of inactivation: a rapid phase inactivated approximately 50% of the current in less than 100 ms while the remaining current was inactivated over the next 10-20 s. Rapid inactivation appeared to be due to Ca2+ influx since it was slowed markedly when Ba2+ was used as the current carrier, while the degree of inactivation increased and decreased with increasing depolarization in direct parallel with the U-shaped current-voltage relationship for inward Ca current. Slow inactivation appeared to be voltage-dependent since current could be inactivated (by approximately 20%) by 10 s long depolarizations to potentials below the threshold for activating Ca current, slow time constants of inactivation were voltage-dependent and slow inactivation persisted when Ca was replaced with Ba. Ca currents with low activation thresholds (in the -50 to -30 mV range) appeared to be preferentially inactivated by the rapid Ca-dependent mechanism. Recovery of slowly inactivated Ca current was very slow and currents inactivated by larger depolarizations required longer recovery time than those elicited by smaller depolarizations. Rapid and slow inactivation mechanisms may be important in understanding the fast spiking and slow plateau depolarizations seen in pancreatic B-cells exposed to stimulatory levels of glucose.

Characteristics of [3H]nimodipine binding to sarcolemmal membranes from rat vas deferens and its regulation by guanine nucleotide

The binding properties of a 1,4-dihydropyridine (DHP) calcium entry blocker, [3H]nimodipine, to a microsomal fraction from rat vas deferens was characterized. The specific binding was saturable, rapid and reversible. Scatchard analysis of the binding revealed a single binding site, and the dissociation constant and the maximum number of binding sites were 0.31 +/- 0.02 nM and 97.0 +/- 7.19 fmol/mg protein, respectively. Both the Kd value obtained from the kinetic study and the IC50 value from relaxation of the K+-depolarized organ were approximately 0.4 nM, indicating that the binding site is closely related to the functional Ca2+ channel. The specific [3H]nimodipine binding was displaced by DHP derivatives at low concentration and by verapamil at high concentration, but diltiazem had no effect on the binding. Calcium chelating agents decreased the [3H]nimodipine binding which was restored by adding Ca2+. 5'-Guanylylimidodiphosphate caused a rightward shift of the displacement curve for Bay K 8644 but not for nimodipine, suggesting the involvement of guanine nucleotide binding protein in the signal transduction between the DHP binding site and the Ca2+ channel.