Coupling between L-type Ca2+channels (dihydropyridine receptors, DHPRs) and ryanodine receptors (RyRs) plays a pivotal role in excitation-contraction (E-C) coupling in cardiac myocytes, and Ca2+influx is generally accepted as the trigger of sarcoplasmic reticulum (SR) Ca2+release. The L-type Ca2+channel agonist BayK 8644 (BayK) has also been reported to alter RyR gating via a functional linkage between DHPR and RyR, independent of Ca2+influx. Here, the effect of rapid BayK application on resting RyR gating in intact ferret ventricular myocytes was measured as Ca2+spark frequency (CaSpF) by confocal microscopy and fluo 3. BayK increased resting CaSpF by 401±15% within 10 seconds in Ca2+-free solution, and depolarization had no additional effect. The effect of BayK on CaSpF was dose-dependent, but even 50 nmol/L BayK induced a rapid 245±12% increase in CaSpF. Nifedipine (5 &mgr;mol/L) had no effect by itself on CaSpF, but it abolished the BayK effect (presumably by competitive inhibition at the DHPR). The nondihydropyridine Ca2+channel agonist FPL-64176 (1 &mgr;mol/L) did not alter CaSpF (despite rapid and potent enhancement of Ca2+current,ICa). In striking contrast to the very rapid and depolarization-independent effect of BayK on CaSpF, BayK increasedICaonly slowly (&tgr;=18 seconds), and the effect was greatly accelerated by depolarization. We conclude that in ferret ventricular myocytes, BayK effects onICaand CaSpF both require drug binding to the DHPR, but postreceptor pathways may diverge in transmission to the gating of the L-type Ca2+channel and RyR.(Circ Res. 2000;87:106-111.)