A comparative analysis of the time course of cardiac Ca2+ current response to rapid applications of beta-adrenergic and dihydropyridine agonists.

A fast perfusion system was used to analyze the kinetics of the response of L-type calcium current (ICa) to rapid exposures to beta-adrenergic or dihydropyridine agonists in whole-cell patch-clamped frog ventricular myocytes. The perfusion system was based on the lateral motion of an array of plastic capillary tubes from which solutions flowed at a velocity of approximately 5 cm/s. Movement from one capillary to the adjacent one occurred in < 20 ms and complete exchange of extracellular solution was achieved in < 50 ms as demonstrated by the block of ICa by fastflow application of Cd during a depolarizing pulse. Fastflow applications of increasing concentrations of isoprenaline (Iso) led to a dose-dependent stimulation of ICa at [Iso] > 1 nM. The response of ICa to Iso always started after a delay of several seconds. The delay duration decreased as [Iso] increased, and was typically approximately 3 s at 10 microM Iso. The rising phase of ICa increase was monophasic and independent of [Iso] > 100 nM. For short applications of Iso (8.8 s), half maximal and maximal stimulation of ICa occurred approximately 20 s and approximately 40 s after the beginning of Iso application, respectively. When Iso was applied during a depolarizing pulse (with Ba as the charge carrier), IBa never increased during that pulse. The kinetics of the ICa response to Iso were not affected by varying the voltage clamp protocols or the ionic composition of intracellular and extracellular solutions. In comparison with the effects of Iso, the stimulatory effect of the dihydropyridine agonist (-)Bay K 8644 on ICa was approximately 15 times faster: delay, half-time to maximal and time to maximal responses were 15 times shorter with (-)Bay K 8644 than with Iso. It is concluded that frog ventricular myocytes respond slowly to a quick application of beta-adrenergic agonists.