Bay K 8644 increases resting Ca2+ spark frequency in ferret ventricular myocytes independent of Ca influx: contrast with caffeine and ryanodine effects

Bay K 8644, an L-type Ca2+ channel agonist, was shown previously to increase resting sarcoplasmic reticulum (SR) Ca2+ loss and convert post-rest potentiation to decay in dog and ferret ventricular muscle. Here, the effects of Bay K 8644 on local SR Ca2+ release events (Ca2+ sparks) were measured in isolated ferret ventricular myocytes, using laser scanning confocal microscopy and the fluorescent Ca2+ indicator fluo-3. The spark frequency under control conditions was fairly constant during 20 s of rest after interruption of electrical stimulation. Bay K 8644 (100 nmol/L) increased the spark frequency by 466+/-90% of control at constant SR Ca2+ load but did not change the spatial and temporal characteristics of individual sparks. The increase in spark frequency was maintained throughout the period of rest. The increase in Ca2+ spark frequency induced by Bay K 8644 was not affected by superfusion with Ca2+-free solution (with 10 mmol/L EGTA) but was suppressed by the addition of 10 micromol/L nifedipine (which by itself did not alter resting Ca2+ spark frequency). This suggests that the effect of Bay K 8644 on Ca2+ sparks is mediated by the sarcolemmal dihydropyridine receptor but is also independent of Ca2+ influx. Low concentrations of caffeine (0.5 mmol/L) increased both the average frequency and duration of sparks. Ryanodine (50 nmol/L) increased the spark frequency and also induced long-lasting Ca2+ signals. This may indicate long-lasting openings of SR Ca2+ release channels and a lack of local SR Ca2+ depletion. In lipid bilayers, Bay K 8644 had no effect on either single-channel current amplitude or open probability of the cardiac ryanodine receptor. It is concluded that Bay K 8644 activates SR Ca2+ release at rest, independent of Ca2+ influx and perhaps through a functional linkage between the sarcolemmal dihydropyridine receptor and the SR ryanodine receptor. In contrast, caffeine and ryanodine modulate Ca2+ sparks by a direct action on the SR Ca2+ release channels.