SEARCH · PubMed Health

Results for “Calcium Channels, T-Type”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 271 records · Page 15Linked to original sources

PubMed1994

Effects of pioglitazone on calcium channels in vascular smooth muscle.

Pioglitazone, an insulin-sensitizing, antidiabetic agent, has blood pressure-lowering effects in insulin-resistant hypertensive rats and attenuates growth factor-induced increases of intracellular Ca2+ in rat aortic vascular smooth muscle cells. To determine whether modulation of voltage-dependent Ca2+ channels plays a role in this association, we investigated the effects of pioglitazone on voltage-dependent current in cultured rat aortic (a7r5) and freshly dissociated rat tail artery vascular smooth muscle cells. Both cell types were studied with whole-cell patch-clamp techniques. Current through L-type Ca2+ channels was elicited with a voltage ramp in the presence of Ba2+ substituted for Ca2+. T-type Ca2+ current was studied using a two-pulse protocol that enabled the isolation of transient current. In a7r5 vascular smooth muscle cells, 2-minute application of pioglitazone (5 and 10 mumol/L) reduced L-type current by 7.9 +/- 1.0% (n = 8) (mean +/- SEM, number of cells) and 14.5 +/- 3.0% (n = 9) (P < .01, two-tailed paired t test), respectively. In contrast, 2-minute application of pioglitazone had no significant effect on T-type Ca2+ current. In freshly dissociated tail artery vascular smooth muscle cells, 2-minute application of 10 mumol/L pioglitazone had an insignificant effect (4.8 +/- 5.6% reduction); however, 25 mumol/L pioglitazone reduced L-type current by 27.3 +/- 7.2% (n = 5) (P < .01). Two-minute application of 0.1% or 0.2% dimethyl sulfoxide (vehicle) alone had no significant effects on currents in either type of vascular smooth muscle cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1988

Phorbol ester increases the dihydropyridine-sensitive calcium conductance in a vascular smooth muscle cell line.

In vascular smooth muscle, phorbol esters cause a slowly developing contraction and an associated transmembrane calcium flux, both of which are inhibited by dihydropyridine calcium channel antagonists. In the A7r5 cultured vascular cell line, we used the whole-cell voltage-clamp technique to identify voltage-dependent calcium conductances and investigate the effect of phorbol esters on that conductance having characteristic dihydropyridine sensitivity (slowly inactivating, high-threshold, "L-type"). With barium as the charge carrier, large-amplitude (100-800 pA) inward currents of two types were characterized by their kinetics and voltage dependence. With holding potential--80 mV, a rapidly inactivating, low-threshold current ("T-type") was activated by depolarizations above-40 mV and was maximal at -10 mV. With holding potential -30 mV, this component was inactivated, and a second slowly inactivating, high-threshold current was activated above -10 mV and was maximal at +10 to +20 mV. These currents are similar to the T-type and L-type currents previously described in vascular smooth muscle cells. When added to the bath, the active phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (100 nM) increased the slowly inactivating (L-type) current by 32 +/- 20% (n = 8, +/- SD). Phorbol-12,13-dibutyrate (100 nM) caused a similar effect, but the inactive phorbol, 4-alpha-phorbol (100 nM), did not. We conclude that at least two distinct calcium conductances are expressed in A7r5 vascular smooth muscle cells, and that the dihydropyridine-sensitive calcium conductance is acutely modulated by phorbol esters, presumably acting through stimulation of protein kinase C. Such modulation may play a role in increasing transmembrane calcium influx mediated by agonist-receptor interactions that lead to activation of protein kinase C and may help to sustain or amplify calcium-dependent cell responses.

Animals↗

PubMed1990

Reduced L-type calcium current in the embryonic chick heart with persistent truncus arteriosus.

Calcium currents were examined in an experimental model in the embryonic chick heart with a congenital malformation known as persistent truncus arteriosus. This is a severe defect characterized by failure of conotruncal and aorticopulmonary septation of the embryonic heart tube. As a result, no separation into the aortic and pulmonary arteries occurs, and there is a common outflow tract. The hearts with persistent truncus arteriosus had a 26% greater ventricular to whole embryo weight, which indicated that the ventricles were enlarged. Both the low-threshold T-type (ICa.T) and the 1,4-dihydropyridine-sensitive L-type (ICa.L) Ca2+ currents were present in the ventricular myocytes from hearts at day 11 of incubation. However, day 11 hearts with persistent truncus arteriosus showed a twofold reduction in the peak magnitude of ICa.L at a test potential of + 10 mV without a concomitant reduction in the number of L channels detected by 1,4-dihydropyridine antagonist [(+)[3H]PN200-110] and agonist (Bay K 8644) receptor binding. The results indicated that an L channel regulatory mechanism other than protein synthesis was affected. These changes are consistent with responses to conditions of excessive hemodynamic burden that have been characterized in adult hearts.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1990

Actions of Ca2+ antagonists on two types of Ca2+ channels in rat aorta smooth muscle cells in primary culture.

Mechanisms of blockade of two types of Ca2+ channels by the organic Ca2+ antagonists, nicardipine, diltiazem, verapamil, and flunarizine, were examined in rat aorta smooth muscle cells in primary culture by using the whole-cell voltage-clamp method. T-type Ca2+ current (T-type ICa) was isolated by an internal perfusion of 5 mM F-, which irreversibly suppressed the L-type ICa, without affecting T-type ICa. L-type ICa was isolated by setting a holding potential at -60 mV, at which most of the T-type Ca2+ channels were inactivated. L-type ICa is halved by 0.1 microM nicardipine, 3.0 microM diltiazem, 0.6 microM verapamil, and 0.1 microM flunarizine, whereas T-type ICa is halved by the same drugs at 0.6, 30, 30, and 0.1 microM, respectively. Diltiazem and verapamil accelerated the decay of L-type ICa and cumulatively blocked L-type ICa during repetitive step depolarizations elicited every 30 seconds ("use-dependent block"). Diltiazem and verapamil neither changed the decay of T-type ICa nor showed a use-dependent block of T-type ICa. Nicardipine and flunarizine blocked both L- and T-type ICa from the first depolarization step after drug treatment ("tonic block") and shifted their steady-state inactivation curves to the left. The estimated binding constants of nicardipine and flunarizine for the inactivated state of T-type Ca2+ channels (48 and 19 nM, respectively) were smaller than those for the resting state of L-type Ca2+ channels (160 and 90 nM, respectively). A low concentration (0.1 microM) of nicardipine initially potentiated T-type ICa and then reduced it. We conclude from these results that 1) nicardipine and flunarizine block not only the resting state but, more preferentially, the inactivated state of both the L- and T-type Ca2+ channels; 2) verapamil and diltiazem preferentially act on the open state of the L-type Ca2+ channel and on the resting and inactivated state of the T-type Ca2+ channel; and 3) the T-type Ca2+ channel of the rat aorta smooth muscle cells appears to be more sensitive to nicardipine and flunarizine than does the L-type Ca2+ channel at around the resting membrane potential.

Animals↗

PubMed1992

Cholesterol increases the L-type voltage-sensitive calcium channel current in arterial smooth muscle cells.

To determine whether membrane free cholesterol affects calcium currents in vascular smooth muscle cells, whole-cell patch clamp recordings were made before and after cholesterol enrichment of cells by exposure to cholesterol-rich liposomes. Exposure to cholesterol-rich liposomes resulted in a gradual increase in the L-type current over 20 hours and a plateau (73 +/- 7% increase over basal) between 20 and 32 hours. This effect was associated with a rightward shift in the inactivation potential and a decrease in the sensitivity to (-)-PN-202-791, a dihydropyridine antagonist. There was no change in the maximum L-type current stimulated by (+)-PN-202-791, a dihydropyridine agonist. Liposome exposure caused a small, transient increase in the T-type current (peak effect, 20 minutes). We conclude that membrane cholesterol has important effects on the L-type calcium current in vascular smooth muscle cells, which is most likely due to an alteration in channel functional state rather than an increase in channel expression.

Animals↗

PubMed1995

Diminished Ca2+ and Ba2+ currents in myocytes surviving in the epicardial border zone of the 5-day infarcted canine heart.

Ventricular arrhythmias frequently occur in patients suffering from ischemic heart disease. In a canine model developed to understand the pathoelectrophysiological mechanisms of the ischemia-related arrhythmias, electrical stimulation can initiate and terminate reentrant ventricular tachyarrhythmias, which arise in surviving subepicardial muscle fibers (epicardial border zone [EBZ] fibers) of the left ventricle 5 days after coronary artery occlusion. Both the structural and electrical changes occurring in the EBZ provide the important substrate for generation of reentrant ventricular tachyarrhythmias. In this study, we tested the hypothesis that abnormalities exist in the electrophysiological properties of macroscopic Ca2+ currents in myocytes isolated from the EBZ of the 5-day infarcted canine heart (IZs). We recorded the T-type (ICa,T) and L-type (ICa,L) Ca2+ currents by using the whole-cell voltage-clamp technique with either Ca2+ or Ba2+ (5 mmol/L) as the charge carrier and under experimental conditions (Na(+)- and K(+)-free solutions, 10 mmol/L intracellular EGTA) that eliminated contamination by other currents. When Ca2+ served as the charge carrier, the density of peak ICa,T in IZs (0.89 +/- 0.5 pA/pF, n = 28) was similar to that in myocytes from normal noninfarcted hearts (NZs) (1.1 +/- 0.5 pA/pF, n = 32). Although no changes existed in the properties of ICa,T, dramatic changes occurred in the density and function of ICa.L in IZs compared with NZs. Density of peak ICa,L at a holding potential of -40 mV (8-second clamp-step interval) was significantly reduced in IZs (4.6 +/- 1.5 pA/pF, n = 40) compared with NZs (7.2 +/- 1.6 pA/pF, n = 53). The reduction in peak ICa,L density was not attributable to altered steady state inactivation relations or a delay in recovery of ICa,L from inactivation. The time course of decay of peak ICa,: was described by a biexponential function in both cell types, with the fast and slow time constants (tau 1 and tau 2, respectively) of decay being significantly faster in IZs (tau 1 12.3 +/- 3.6 ms; tau 2, 55.1 +/- 31.1 ms) than in NZs (tau 1, 16.1 +/- 4.1 ms; tau 2, 85.2 +/- 51.7 ms). In addition, rapid clamp stimulation (at 1-s intervals) of cells produced a larger frequency-dependent decrease of peak ICa,L density in IZs than NZs, suggesting that at more physiologically relevant rates, little ICA.L may be activated. Finally, a significant reduction and acceleration of decay of the ICa,L persisted even when Ca2+ was substituted by equimolar Ba2+ as the charge carrier.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1990

Dopamine inhibits two characterized voltage-dependent calcium currents in identified rat lactotroph cells.

The effects of dopamine (DA) on voltage-dependent Ca2+ currents were investigated in cultured rat lactotroph cells using the patch clamp recording technique. Each recorded cell was identified by the reverse hemolytic plaque assay. In the whole-cell configuration, two types of Ca2+ currents, L and T, were characterized on the basis of their kinetics, voltage sensitivity, and pharmacology. The L component had a threshold of -25 mV, showed little inactivation during a 150-msec voltage step, and was maximal at +10 mV. Cadmium ions (100 microM) significantly reduced its amplitude (75%). The T component was activated at a membrane potential close to -50 mV, was maximal at -10 mV, and showed a voltage-dependent inactivation between -90 and -30 mV. It was quickly inactivated during a maintained depolarization (time constant, 27 ms at -30 mV) and was strongly reduced (80%) by nickel ions (100 microM). Bath application of DA (10 nM) caused a markedly general depression of inward Ca2+ currents, acting differently on the T- and L-type currents. DA application shifted the voltage-dependence of the L-type current activation toward depolarization values (8 mV) without modifying its time- and voltage-dependent inactivation. In contrast, DA enhanced the inactivation of the T-type current by accelerating its time-dependent inactivation (25% decrease in the time constant of inactivation) and by shifting the voltage-dependence of the T-type current inactivation toward hyperpolarizing values (-63 mV in control vs. -77 mV in the presence of DA). These effects of DA were dose-dependent and involved the activation of a D2 receptor type. They were mimicked by bromocriptine application (10 nM), whereas sulpiride (100 nM) blocked the DA-evoked response. The D1 antagonist SCH 23390 was ineffective up to 100 microM. All of these DA-induced modifications in Ca2+ currents were abolished using a GTP-free pipette solution or after pretreatment of cells with pertussis toxin, suggesting that DA can regulate the function of Ca2+ channels through GTP-binding proteins (G-proteins). Our results show that DA acts simultaneously by reducing both voltage-dependent Ca2+ currents on lactotroph cells. Thus, DA reduces the entry of Ca2+ ions across the surface membrane and thereby influences electrical activity and the cytosolic free Ca2+ concentration involved in both basal and evoked PRL release.

Animals↗

PubMed1991

Reevaluation of the electrophysiological actions of thyrotropin-releasing hormone in a rat pituitary cell line (GH3).

The electrophysiological actions of TRH were examined in the clonal pituitary cell line GH3 with the use of the perforated patch variation of the standard whole cell patch-clamp technique. The action of TRH on spontaneously spiking cells was to cause a brief hyperpolarization (first phase action), followed by a period during which action potential behavior was significantly modified (second phase action). The modifications during second phase action included a reduction in the slope of the up-stroke, a reduced peak potential, an increase in duration, and a depolarizing shift of the after-hyperpolarization. The modification of voltage- and calcium-dependent conductances that underlie these changes were investigated in voltage clamp experiments. During first phase action TRH was found to increase calcium-dependent potassium current. During second phase action TRH was found to significantly reduce the L-type calcium current (35%), with no alteration in the T-type calcium current. The second phase action of TRH on calcium-dependent potassium conductance was complex. First, a decrease was observed. This was followed by an increase that did not become fully manifest until after TRH was washed from the cell. TRH caused no change in voltage-dependent potassium current. These results indicate that the second phase action of TRH on action potential behavior in GH3 cells is mediated by a reduction in L-type calcium current and alterations in the behavior of calcium-dependent potassium currents, but not through changes in voltage-dependent potassium currents.

Action Potentials↗

PubMed1992

Fast Na+ channels and slow Ca2+ current in smooth muscle from pregnant rat uterus.

Smooth muscle cells normally do not possess fast Na+ channels, but inward current is carried through two types of Ca2+ channels: slow (L-type) Ca2+ channels and fast (T-type) Ca2+ channels. Using whole-cell voltage clamp of single smooth muscle cells isolated from the longitudinal layer of 18-day pregnant rat uterus, depolarizing pulses, applied from a holding potential of -90 mV, evoked two types of inward current, fast and slow [8]. The fast inward current decayed within 30 ms, depended on [Na]o, and was inhibited by TTX (K0.5 = 27 nM). The slow inward current decayed slowly, was dependent on [Ca]o, and was inhibited by nifedipine. These results suggest that the fast inward current is a fast Na+ channel current, and that the slow inward current is a Ca2+ slow channel current. A fast-inactivating Ca2+ channel current was not evident. Thus, the ion channels which generate inward currents in pregnant rat uterine cells are TTX-sensitive fast Na+ channels and dihydropyridine-sensitive slow Ca2+ channels. The number of fast Na+ channels increased during gestation. The averaged current density increased from 0 on day 5, to 0.19 on day 9, to 0.56 on day 14, to 0.90 on day 18, and to 0.86 pA/pF on day 21. This almost linear increase occurs because of an increase in the fraction of cells which possess fast Na+ channels, and it is suggested that the fast Na+ current may be involved in spread of excitation. The Ca2+ channel current density also was higher during the latter half of gestation. These results indicate that the fast Na+ channels and Ca2+ slow channels in myometrium become more numerous as term approaches, and may facilitate parturition. Isoproterenol (beta-agonist) did not affect either ICa(s) or INa(f), whereas Mg2+ (K0.5 of 12 mM) and nifedipine (K0.5 of 3.3 nM) depressed ICa(s). Oxytocin had no effect on INa(f) and actually depressed ICa(s) to a small extent. Therefore, the tocolytic action of beta-agonists cannot be explained by an inhibition of ICa(s), whereas that of Mg2+ can be so explained. The stimulating action of oxytocin on uterine contractions is not due to stimulation of ICa(s). Figure 11 summarizes the possible mechanisms by which uterine contractility can be modulated. In contrast to vascular smooth muscle, neither ISO nor adenosine, which produce elevation of cyclic AMP, affected ICa and INa. Therefore, no arrow can be drawn between cA-PK/cG-PK and the Ca2+ slow channel.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine↗

PubMed1992

Interaction between stannous chloride and calcium channel blockers in frog neuromuscular transmission.

We have investigated the interactions between stannous chloride (SnCl2) and calcium (Ca) channel blockers on endplate potentials (e.p.p.) and on miniature endplate potentials (m.e.p.p.) to determine which type of channel (among L-, N-, and T-type) participates in the SnCl2-induced increase in Ca entry into motor nerve terminals. The e.p.p. amplitude augmented by 30 microM SnCl2 was decreased by cumulative addition of 10 microM CdCl2 or 0.5 microM omega-conotoxin but not by 10 microM NiCl2 or 5 microM nicardipine. The SnCl2 (30 microM)-induced rise in m.e.p.p. frequency in high-potassium medium was reduced by 0.5 microM omega-conotoxin but not by 5 microM nicardipine. These results suggest that activation of the N-type Ca channel is involved in the SnCl2-induced increase in Ca entry into the nerve terminals.

Animals↗

PubMed1992

Fast Na+ channels and slow Ca2+ current in smooth muscle from pregnant rat uterus.

Smooth muscle cells normally do not possess fast Na+ channels, but inward current is carried through two types of Ca2+ channels: slow (L-type) Ca2+ channels and fast (T-type) Ca2+ channels. Whole-cell voltage clamp was done on single smooth muscle cells isolated from the longitudinal layer of 18-day pregnant rat uterus. Depolarizing pulses, applied from a holding potential of -90 mV, evoked two types of inward current, fast and slow. The fast inward current decayed within 30 ms, depended on [Na]o, and was inhibited by TTX (K0.5 = 27 nM). The slow inward current decayed slowly, was dependent on [Ca]o (or Ba2+), and was inhibited by nifedipine. These results suggest that the fast inward current is a fast Na+ channel current, and that the slow inward current is a Ca2+ slow channel current. A fast-inactivating Ca2+ channel current was not evident. We conclude that the ion channels which generate inward currents in pregnant rat uterine cells are TTX-sensitive fast Na+ channels and dihydropyridine-sensitive slow Ca2+ channels. The number of fast Na+ channels increased during gestation. The averaged current density increased from 0 on day 5, to 0.19 on day 9, 0.56 on day 14, 0.90 on day 18, and 0.86 pA/pF on day 21. This almost linear increase occurs because of an increase in the fraction of cells which possess fast Na+ channels. The Ca2+ channel current density also was higher during the latter half of gestation. These results indicate that the fast Na+ channels and Ca2+ slow channels in myometrium become more numerous as term approaches, and we suggest that the fast Na+ current may be involved in spread of excitation. Isoproterenol (beta-agonist) did not affect either ICa(s) or INa(f), whereas Mg2+ (K0.5 of 12 mM) and nifedipine (K0.5 of 3.3 nM) depressed ICa(s). Oxytocin had no effect on INa(f) and actually depressed ICa(s) (but not IBa) to a small extent. Therefore, the tocolytic action of beta-agonists cannot be explained by an inhibition of ICa(s), whereas that of Mg2+ can be so explained. The stimulating action of oxytocin on uterine contractions cannot be explained by a stimulation of ICa(s).

Animals↗

PubMed1991

Activation of N-type calcium channels by stannous chloride at frog motor nerve terminals.

Stannous chloride (SnCl2) increases the calcium (Ca) entry into motor nerve terminals through the voltage-dependent Ca channels. The present study has been conducted to determine which type of channel (i.e., L-, N-, or T-type) participates in the Ca entry increased by SnCl2 (0.1 mM)-induced enhancement of the inward Ca current was more strongly inhibited by CdCl2 (50 microM) or omega-conotoxin (0.1 microM) than by NiCl2 (50 microM) or nifedipine (10 microM), respectively. From the results obtained, it is concluded that SnCl2 increases the Ca entry into the nerve terminal by activating the N-type Ca channels.

Animals↗

PubMed1990

Blockade of low and high threshold Ca2+ channels by diphenylbutylpiperidine antipsychotics linked to inhibition of prolactin gene expression.

The effects of diphenylbutylpiperidine (DPBP) antipsychotics on Ca2+ currents and prolactin (PRL) synthesis were studied in rat pituitary growth hormone (GH) cell lines (GH3 and GH4C1). In whole-cell patch-clamp experiments, DPBPs including fluspirilene, penfluridol, and pimozide at concentrations ranging from 0.25 to 5 microM each blocked current through low threshold T-type as well as high threshold L-type channels. Each of the drugs preferentially blocked T-type current, and complete inhibition was observed at concentrations as low as 1 microM. Inhibition of L-type channels by DPBPS was enhanced at depolarized holding potentials and promoted by prolonged channel activation. At concentrations similar to those which blocked Ca2+ currents, each of the three DPBPs markedly reduced basal PRL production by GH cells. PRL synthesis stimulated by the dihydropyridine Ca2+ agonist R5417 or thyrotropin releasing hormone (TRH) was also inhibited. The inhibitory effects of the DPBPs were observed at the level of gene transcription. Penfluridol and fluspirilene inhibited basal, Ca2(+)- and TRH-stimulated expression of a fusion gene construct containing the 5'-flanking sequence of the rat PRL gene linked to the luciferase gene. The effect was concentration-dependent with the IC50 values for both drugs of less than 1 microM. Nimodipine also reduced basal, R5417, and TRH-stimulated expression of the reporter gene construct. Similar results were obtained with a reporter gene construct containing the 5'-flanking sequence of the rat GH gene. The GH luciferase construct was only slightly responsive to R5417 and TRH; however, these responses were reduced by fluspirilene and nimodipine at concentrations of less than 1 microM. These studies demonstrate that the DPBP antipsychotics block T- as well as L-type Ca2+ channels in GH cells and inhibit PRL production at the level of transcription. They also indicate that functioning Ca2+ channels are necessary for TRH-stimulated PRL gene transcription.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1986

Calcium channel types in cardiac myocytes: modulation by dihydropyridines and beta-adrenergic stimulation.

We used the patch clamp technique to record unitary calcium (Ca2+) channel activity in freshly dissociated ventricular myocytes from adult guinea pigs. We found two types of Ca2+ channels with distinct permeation and gating properties and different sensitivity to pharmacological agents. One channel (T-type) requires negative membrane potentials to remove inactivation. It gives rise to a transient mean current and is not affected by dihydropyridines or isoproterenol. The other Ca2+ channel (L-type) has a larger unitary barium-conductance, activates at more positive potentials and its averaged current decays much more slowly. It shows a distinct gating pattern with different gating modes, the proportion of which is drastically altered by dihydropyridine Ca2+-channel agonists and antagonists. L-type channel activity is modulated by beta-adrenergic stimulation by a mechanism of action which differs from that of dihydropyridine Ca2+-channel agonists.

Animals↗

PubMed1989

Effects of McN-6186 on voltage-dependent Ca++ channels in heart and pituitary cells.

McN-6186 (N-[2-(3,5-dimethoxyphenyl)ethyl]-5-methoxy-alpha-methyl-2 -(phenylethynyl) benzeneethanamine hydrochloride is a compound structurally distinct from other Ca++ channel ligands. McN-6186 showed some stimulation of 1,4-dihydropyridine-sensitive Ca++ uptake in neonatal rat ventricular cells at concentrations of 1 and 3 nM. At higher concentrations McN-6186 inhibited this uptake in rat ventricular cells at concentrations approximately 100-fold less than those needed to block the corresponding Ca++ uptake in rat anterior pituitary (GH3) cells. McN-6186 (2 microM) inhibited L-type Ca++ channel current in neonatal rat ventricular cells in a voltage-dependent manner while having little or no effect on this current in GH3 cells. In some ventricular cells tested, the T-type Ca++ current was also blocked by 2 microM McN-6186. McN-6186 inhibited (+)-[3H]PN200-110 binding in rat cardiac membranes with an IC50 value of 1.45 X 10(-7) M and a shallow Hill slope (nH = 0.42). It is concluded that McN-6186 blocks L-type Ca++ channels in heart cells preferentially to those found in GH3 cells. Furthermore, McN-6186 may have other sites and mechanisms of action in addition to L-type Ca++ channel blockade.

DOM 2,5-Dimethoxy-4-Methylamphetamine↗

PubMed1995

Voltage-dependent blockade of diverse types of voltage-gated Ca2+ channels expressed in Xenopus oocytes by the Ca2+ channel antagonist mibefradil (Ro 40-5967).

Four different types of Ca2+ channel alpha 1 subunits, representing the major classes of voltage-gated Ca2+ channels, were individually coexpressed along with alpha 2/delta and beta 2b subunits in Xenopus oocytes. These subunits (and the encoded channel types and major tissues of origin) included alpha 1C (L-type, cardiac), alpha 1B (N-type, central nervous system), alpha 1A (P/Q-type, central nervous system), and alpha 1E (most likely R-type, central nervous system). Divalent cation currents through these channels (5 mM Ba2+) were evaluated with the two-microelectrode voltage-clamp technique. The expressed channels were compared with regard to their responses to a structurally novel, nondihydropyridine compound, mibefradil (Ro 40-5967). In the micromolar concentration range, this drug exerted clear inhibitory effects on each of the four channel types, reducing divalent cation current at all test potentials, with the non-L-type channels being more sensitive to inhibition than the L-type channels under fixed experimental conditions. For all channel types, mibefradil was a much more effective inhibitor at more depolarized holding potentials, suggesting tighter binding of the drug to the inactivated state than to the resting state. The difference in apparent affinities of resting and inactivated states of the channels, calculated based on a modulated receptor hypothesis, was 30-70-fold. In addition, the time course of decay of Ca2+ channel current was accelerated in the presence of drug, consistent with open channel block. The effect of increasing stimulation frequency was tested for L-type channels and was found to greatly enhance the degree of inhibition by mibefradil, consistent with promotion of block by channel opening and inactivation. Allowing for state-dependent interactions, the drug concentrations found to block L-, N-, Q-, and R-type channels by 50% are at least 10-fold higher than half-blocking levels previously reported for T-type channels in vascular smooth muscle cells under similar experimental conditions. This may help explain the ability of the drug to spare working myocardium (strongly negative resting potential, dominance of L-type channels in their resting state) while reducing contraction in blood vessels (presumably involving T-type channels or partially inactivated L-type channels). Thus, mibefradil is a new addition to the family of nonselective organic Ca2+ channel inhibitors, as exemplified by bepridil and fluspirilene, and may prove useful as an experimental tool for studying diverse physiological events initiated by Ca2+ influx. It complements classes of drugs with relatively selective effects on L-type channels, as exemplified by nifedipine and diltiazem.

Animals↗

PubMed1995

Resolution and pharmacological analysis of the voltage-dependent calcium channels of Drosophila larval muscles.

Voltage-dependent calcium channels play a role in many cellular phenomena. Very little is known about Ca2+ channels in Drosophila, especially those in muscles. Existing literature on neuronal Ca2+ channels of Drosophila suggests that their pharmacology may be distinct from that of vertebrate Ca2+ channels. This raises questions on the pharmacology and diversity of Ca2+ channels in Drosophila muscles. Here we show that the Ca2+ channel current in the body-wall muscles of Drosophila larvae consists of two main components. One component is sensitive to 1,4-dihydropyridines and diltiazem, which block vertebrate L-type Ca2+ channels. The second component is sensitive to amiloride, which blocks vertebrate T-type Ca2+ channels. In contrast to Drosophila brain membrane preparations in which a majority of the Ca2+ channels are phenylalkylamine-sensitive but dihydropyridine-insensitive, the major current in the muscles was dihydropyridine-sensitive but relatively less sensitive to verapamil. This might indicate an underlying tissue specific distribution of distinct subtypes of dihydropyridine/phenylalkylamine-sensitive Ca2+ channels in Drosophila. Low verapamil sensitivity of the dihydropyridine-sensitive current of Drosophila muscles also set it apart from the vertebrate L-type channels which are sensitive to 1,4-dihydropyridines, benzothiazepines as well as phenylalkylamines. The dihydropyridine-sensitive current in Drosophila muscles activated in a similar voltage range as the vertebrate L-type current. As with the vertebrate current, blockade by dihydropyridines was voltage dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Amiloride↗

PubMed1993

Comparison of effects of tetrandrine on ionic channels of isolated rat neurohypophysial terminals and Y1 mouse adrenocortical tumor cells.

UNLABELLED: The effects of tetrandrine on the voltage-gated Ca2+, K+, and Na+ channels of the isolated rat neurohypophysial nerve terminals and Y1 mouse adrenocortical tumor cells were studied using standard and perforated patch-clamp techniques. Among the inward ionic currents, the T-type Ca2+ channel current (I(Ca)) in the Y1 cell line was strongly inhibited (by 52.9%) by a low concentration (13 mumol.L-1) of externally applied tetrandrine. The L-type I(Ca) in the isolated nerve terminals was also strongly inhibited (by 54.2%) by externally applied tetrandrine 33 mumol.L-1, whereas the N-type I(Ca) and Na+ current were not significantly affected by the same dose of the alkaloid. While it had no effect on the fast, transient K+ channels, external application of tetrandrine 1 mumol.L-1 blocked a slowly-gating, maxi Ca(2+)-activated K+ (K+(Ca)) channel in the nerve terminals, decreasing its P(o) by 84.4%. CONCLUSIONS: (1) The order of the sensitivities of ionic channels to tetrandrine is K+(Ca) > T- > L- > N-type Ca2+ > Na+; (2) tetrandrine serves as a specific blocker of the slowly-gating K+(Ca) channel.

Adenoma↗