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This study investigated the interaction between L-type Ca2+ current (ICaL) and Ca2+ release from the sarcoplasmic reticulum (SRCR) in whole cell voltage-clamped guinea pig ventricular myocytes. Quasiphysiological cation solutions (Nao+:KI+) were used for most experiments. In control conditions, there was no obvious interaction between ICaL and SRCR. In isoproterenol, activation of ICaL from voltages between -70 and -50 mV reduced the amplitude and accelerated the decay of the current. Short (50 ms), small-amplitude voltage steps applied 60 or 510 ms before stimulating ICaL inhibited and facilitated the current, respectively. These changes were blocked by ryanodine. Low-voltage activated currents such as T-type Ca2+ current, TTX-sensitive ICa (ICaTTX), or “slip mode” Ca2+ conductance via INa+ were not responsible for low-voltage SRCR. However, L-type Ca2+ currents could be distinguished at voltages as negative as -45 mV. It is concluded that in the presence of isoproterenol, Ca2+ release from the SR at negative potentials is due to activation of L-type Ca2+ channels.

Inactivation of the L-type Ca2+current ( ICaL) was studied in isolated guinea pig ventricular myocytes with different ionic solutions. Under basal conditions, ICaLof 82% of cells infused with Cs+-based intracellular solutions showed enhanced amplitude with multiphasic decay and diastolic depolarization-induced facilitation. The characteristics of ICaLin this population of cells were not due to contamination by other currents or an artifact. These phenomena were reduced by ryanodine, caffeine, cyclopiazonic acid, the protein kinase A inhibitor H-89, and the cAMP-dependent protein kinase inhibitor. Forskolin and isoproterenol increased ICaLby only ∼60% in these cells. Cells infused with either N-methyl-d-glucamine or K+-based intracellular solutions did not show multiphasic decay or facilitation under basal conditions. Isoproterenol increased ICaLby ∼200% in these cells. In conclusion, we show that multiphasic inactivation of ICaLis due to Ca2+-dependent inactivation that is reversible on a time scale of tens of milliseconds. Cs+seems to activate the cAMP-dependent protein kinase pathway when used as a substitute for K+in the pipette solution.