The Ca(++)-channel blocker Ro 40-5967 blocks differently T-type and L-type Ca++ channels.

The effects of Ro 40-5967, a nondihydropyridine Ca++ channel blocker, on low-voltage activated (T-type) and high-voltage activated (L-type) Ca++ channels were compared. L-type barium currents were measured in Chinese hamster ovary cells stably transfected with the alpha 1 subunit of the class Cb Ca++ channel. T-type barium currents were investigated in human medullary thyroid carcinoma cells. The Ba++ currents of human medullary thyroid carcinoma cells were transient, activated at a threshold potential of -50 mV with the maximum at -14 +/- 3.2 mV and blocked by micromolar Ni++. The T- and L-type current inactivated with time constants of 33.4 +/- 4.1 and 416 +/- 26 msec at maximum barium currents, respectively. Ro 40-5967 inhibited reversibly the T- and L-type currents with IC50 values of 2.7 and 18.6 microM, respectively. The inhibition of the L-type current was voltage-dependent, whereas that of the T-type current was not. Ro 40-5967 blocked T-type current already at a holding potential of -100 mV. The different types of block, i.e., voltage-dependent vs. tonic block, may contribute to the pharmacological profile of Ro 40-5967 in intact animals.

Relation between veratridine reaction dynamics and macroscopic Na current in single cardiac cells.

Veratridine modification of Na current was examined in single dissociated ventricular myocytes from late-fetal rats. Extracellularly applied veratridine reduced peak Na current and induced a noninactivating current during the depolarizing pulse and an inward tail current that decayed exponentially (tau = 226 ms) after repolarization. The effect was quantitated as tail current amplitude, Itail (measured 10 ms after repolarization), relative to the maximum amplitude induced by a combination of 100 microM veratridine and 1 microM BDF 9145 (which removes inactivation) in the same cell. Saturation curves for Itail were predicted on the assumption of reversible veratridine binding to open Na channels during the pulse with reaction rate constants determined previously in the same type of cell at single Na channels comodified with BDF 9145. Experimental relationships between veratridine concentration and Itail confirmed those predicted by showing (a) half-maximum effect near 60 microM veratridine and no saturation up to 300 microM in cells with normally inactivating Na channels, and (b) half-maximum effect near 3.5 microM and saturation at 30 microM in cells treated with BDF 9145. Due to its known suppressive effect on single channel conductance, veratridine induced a progressive, but partial reduction of noninactivating Na current during the 50-ms depolarizations in the presence of BDF 9145, the kinetics of which were consistent with veratridine association kinetics in showing a decrease in time constant from 57 to 22 and 11 ms, when veratridine concentration was raised from 3 to 10 and 30 microM, respectively. As predicted for a dissociation process, the tail current time constant was insensitive to veratridine concentration in the range from 1 to 300 microM. In conclusion, we have shown that macroscopic Na current of a veratridine-treated cardiomyocyte can be quantitatively predicted on the assumption of a direct relationship between veratridine binding dynamics and Na current and as such can be successfully used to analyze molecular properties of the veratridine receptor site at the cardiac Na channel.

Mutually exclusive action of cationic veratridine and cevadine at an intracellular site of the cardiac sodium channel.

Veratridine modification of Na current was examined in single dissociated ventricular myocytes from late-fetal rats by applying pulses to -30 mV for 50 ms every 2 or 5 s from a holding potential of -100 mV (20 degrees C) and measuring amplitude, Itail, and time constant, tau tail, of the post-repolarization inward tail current induced by the alkaloid. Increasing the pH of a 30 microM veratridine superfusate from 7.3 to 8.3 (which increases the fraction of uncharged veratridine molecules from 0.5 to 5% while decreasing that of protonated molecules from 99.5 to 95%) increased Itail by a factor of 2.5 +/- 0.5 (mean +/- SEM; n = 3). Switching from 100 microM veratridine superfusate at pH 7.3 to 10 microM at pH 8.3 did not affect the size of Itail (n = 4). Intracellular (pipette) application of 100 microM veratridine at pH 7.3 or 8.3 produced small Itail's suggesting transmembrane loss of alkaloid. If this was compensated for by simultaneous extracellular application of 100 microM veratridine at a pH identical to intracellular pH, Itail (measured relative to the maximum amplitude induced by a combination of 100 microM veratridine and 1 microM BDF 9145 in the same cell) at pHi 7.3 did not significantly differ from that at pHi 8.3 (84 +/- 4 vs. 70 +/- 6%; n = 3 each). Results from six control cells and five cells subjected to extra- and/or intracellularly increased viscosity by the addition of 0.5 or 1 molal sucrose showed that increasing intracellular viscosity 1.6- and 2.5-fold increased tau tail 1.5- and 2.3-fold, respectively, while a selective 2.5-fold increase of extracellular viscosity did not significantly affect tau tail.(ABSTRACT TRUNCATED AT 250 WORDS)

[Effects of benzyltetrahydropalmatine on action potentials of myocardium and transmembrane K+ and Ca2+ currents in Purkinje fibers].

Standard microelectrode and two-microelectrode voltage clamp techniques were used to study the effects of benzyltetrahydropalmatine (BTHP) on action potentials of isolated myocardium and transmembrane K+ and Ca2+ currents in Purkinje fibers. The effect of BTHP 3-100 mumol/L consisted of prolongation of the action potential duration and reduction of delayed rectifier current (Ik) in concentration-dependent manner. At concentration above 200 mumol/L, the contractile force of the isolated myocardium was depressed and in voltage clamp experiments the slow inward current (Isi) was reduced. These results suggest that the inhibition of Ik induced by BTHP was in relation to its anti-arrhythmic action.

[Effects of dauricine on slow action potentials in myocardium].

The effects of dauricine were examined on the slow action potentials induced by both high K+ (24 mmol/L) or TTX (40 mumol/L) in guinea pig papillary muscles and in sinoatrial node cells of rabbit. Dauricine 40 mumol/L decreased the maximal upstroke velocity (Vmax) of slow action potentials and prolonged the action potential duration at 50% repolarization (APD50). Bay K 8644 (50 nmol/L) increased the amplitude and Vmax of slow action potentials, which were antagonized by dauricine (40 mumol/L). An inhibitory influence of dauricine on the action potential amplitude was observed in papillary muscles superfused with Tyrode's solution containing high K+ and in rabbit sinoatrial node cells, but it was not with TTX. The resting potentials of the 3 slow action potentials were not affected by dauricine. The results suggest that dauricine possesses calcium-antagonistic effect.

[The mechanism of antiarrhythmic action of 7-bromoethoxybenzene-tetrahydropalmatine].

The mechanism of antiarrhythmic action and the electrophysiologic effects of 7-bromoethoxybenzene-tetrahydroptamine (EBP) have been studied using conventional microelectrode technique. The effect of EBP on the membrane ISi and I(X) currents were investigated in the canine cardiac purkinje fibres using the double microelectrode voltage clamp methods. EBP was shown to increase the duration of action potential at 20 and 90% of repolarization of isolated guinea pig papillary muscles. However, the amplitude of action potential, the resting potential and the overshoot and maximum rate of 0 phase depolarization (Vmax) remained unchanged. The membrane potential was held at 40 mV and command potential at -15 mV, 0.3 Hz 500 ms. After 10 min of perfusion of EBP 30 mumol/L, the slow inward current (ISi) was reduced markedly. When the holding potential was held at -20 mV, and the command potential at +10 mV, EBP (10-100 mumol/L) exerted a depressed effect on the delayed (outward) rectifier current (I(X)) in a dose-dependent manner. It may be concluded that the antiarrhythmic effects of EBP is mainly related to decrease the automaticity and prolong the duration of action potential at 20-90% of repolarization of isolated guinea pig papillary muscles.