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The double electrode voltage-clamp technique was used to study voltage-dependent Ca 2+channels in Pleurodelesoocytes. From a holding potential of −80 mV, Ba-current ( IBa) (recorded in Cl-free solution, Ba 2+= 40 mM) activated at −36.7 ± 4 mV, peaked at −11.6 ± 4 mV and reversed at 55 ± 7 mV ( n= 24). This current activated slowly (rise time was 0.98 ± 0.2 s; n= 14 at −10 mV) and was not inactivated. Cadmium (Cd 2+, 500 μM) completely inhibited IBa. The effect of Cd 2+was dose-dependent (EC 50= 37 ± 5 μM; n= 5). Moreover, IBawas insensitive to ω-conotoxin (10 μM) but interestingly this IBa. displayed dihydropyridine (DHP) sensitivity. Bay K 8644 (5 μM), a DHP activator, increased the peak current amplitude in a dose-dependent manner (EC 50= 5.9 ± 0.6 μM; n= 10) and shifted the threshold and the maximum of current/voltage relationship towards negative potentials by − 10 mV. Nifedipine (5 μM), a DHP antagonist, decreased IBaby 80% at HP of −80 mV (EC 50= 1.2 ± 0.2 μM; n= 6). We concluded that Pleurodelesoocytes possess High-Voltage Activated Ca 2+channels with properties similar to L-type Ca 2+channels.

The double electrode voltage‐clamp technique was used to study voltage‐dependent Ca2+channels in Pleurodelesoocytes. From a holding potential of −80 mV, Ba‐current (IBa) (recorded in Cl‐free solution, Ba2+= 40 mM) activated at −36.7 ± 4 mV, peaked at −11.6 ± 4 mV and reversed at 55 ± 7 mV (n= 24). This current activated slowly (rise time was 0.98 ± 0.2 s; n= 14 at −10 mV) and was not inactivated. Cadmium (Cd2+, 500 μM) completely inhibited IBa. The effect of Cd2+was dose‐dependent (EC50= 37 ± 5 μM; n= 5). Moreover, IBawas insensitive to ω‐conotoxin (10 μM) but interestingly this IBa. displayed dihydropyridine (DHP) sensitivity. Bay K 8644 (5 μM), a DHP activator, increased the peak current amplitude in a dose‐dependent manner (EC50= 5.9 ± 0.6 μM; n= 10) and shifted the threshold and the maximum of current/voltage relationship towards negative potentials by − 10 mV. Nifedipine (5 μM), a DHP antagonist, decreased IBaby 80% at HP of −80 mV (EC50= 1.2 ± 0.2 μM; n= 6). We concluded that Pleurodelesoocytes possess High‐Voltage Activated Ca2+channels with properties similar to L‐type Ca2+channels.

In various species, including humans, 5-hydroxytryptamine (5-HT) has been shown to exert positive chronotropic and inotropic cardiac effects through different types of receptors. The goal of the present study was to investigate the regulation by 5-HT of voltage-gated Ca2+ channels in human atrial myocytes and to characterize the receptor involved. Cardiomyocytes isolated enzymatically and mechanically were voltage-clamped using the whole-cell configuration of the patch-clamp technique. Extracellular perfusion of 5-HT increased Ca2+ current (ICa) amplitude with a EC50 (0.1 microM) similar to that observed with isoprenaline. The effects of 5-HT were blocked by the addition of protein kinase A inhibitor in the pipette. In addition, the effects of 5-HT, isoprenaline, and intracellular cAMP on ICa were not additive. These results support the hypothesis that the inotropic effect of 5-HT in human atrial myocytes is related to an increase of ICa via an elevation of intracellular cAMP levels and stimulation of cAMP-dependent protein kinase. The effects of 5-HT were not blocked by antagonists of 5-HT1 (methiothepin), 5-HT2 (ketanserin), or 5-HT3 (ICS 205-930 at a low concentration) receptors. The benzamide derivatives renzapride and zacopride and the azabicyclobenzimidazolone derivative BIMU 8 increased ICa, but less efficiently than did 5-HT or 5-methoxytryptamine. Moreover, ICS 205-930 at high concentrations (greater than 1 microM) completely antagonized the effects of 5-HT. Thus, the pharmacology of the 5-HT receptor involved in an increase of ICa in human atrial myocytes resembles that recently described for the 5-HT4 receptor. In atrial myocytes dissociated from rat, rabbit, guinea pig, or frog, 5-HT at high concentrations had no effect on Ca2+ currents, suggesting that the distribution of 5-HT4 receptors in cardiac tissues is species dependent.

Two kinds of Ca2+channel activities have been recorded in unfertilized Pleurodelesoocytes during the resting season: the previously described L‐type and a transient one. The transient Ba current (IBa‐t) exhibited a voltage threshold of −36±7 mV, peaked at −18±8 mV and reversed around +50 mV. It showed a fast monoexponential decay with an inactivation time course of 31.4 ± 1.7 ms at −20 mV. IBa‐twas insensitive to nifedipine and ω‐conotoxin‐GVIA but blocked by Ni2+(50 μM). Moreover, Cd2+also reduced IBa‐tbut was less efficient thane of Ni2+. When using Ca2+as the charge carrier, the amplitude and the decay of ICa‐twere largely similar to those of IBa‐t. These data demonstrated that the population of Ca2+channels could be seasonally modulated in Pleurodelesoocytes.