A COMPARATIVE-ANALYSIS OF THE TIME-COURSE OF CARDIAC CA2-ADRENERGIC AND DIHYDROPYRIDINE AGONISTS( CURRENT RESPONSE TO RAPID APPLICATIONS OFBETA)

A fast perfusion system was used to analyze the kinetics of the respon se of L-type calcium current (I(Ca)) to rapid exposures to beta-adrene rgic or dihydropyridine agonists in whole-cell patch-clamped frog vent ricular 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 th e adjacent one occurred in < 20 ms and complete exchange of extracellu lar solution was achieved in < 50 ms as demonstrated by the block of I (Ca), by fastflow application of Cd during a depolarizing pulse. Fastf low applications of increasing concentrations of isoprenaline (Iso) le d to a dose-dependent stimulation of I(Ca) at [Iso] > 1 nM. The respon se of I(Ca) to Iso always started after a delay of several seconds. Th e delay duration decreased as [Iso] increased, and was typically appro ximately 3 s at 10 muM Iso. The rising phase Of I(Ca) increase was mon ophasic and independent of [Iso] > 100 nM. For short applications of I so (8.8 s), half maximal and maximal stimulation of I(Ca) occurred app roximately 20 s and approximately 40 s after the beginning of Iso appl ication, respectively. When Iso was applied during a depolarizing puls e (with Ba as the charge carrier), I(Ba) never increased during that p ulse. The kinetics of the I(Ca) response to Iso were not affected by v arying the voltage clamp protocols or the ionic composition of intrace llular and extracellular solutions. In comparison with the effects of Iso, the stimulatory effect of the dihydropyridine agonist (-)Bay K 86 44 on I(Ca) was approximately 15 times faster: delay, half-time to max imal and time to maximal responses were 15 times shorter with (-)Bay K 8644 than with Iso. It is concluded that frog ventricular myocytes re spond slowly to a quick application of beta-adrenergic agonists.