Cardiac Ca transients are enhanced by cAMP-dependent protein kinase (PKA). However, PKA-dependent modulation of ryanodine receptor (RyR) function in intact cells is difficult to measure, because PKA simultaneously increases Ca current (I_Ca), SR Ca uptake and SR Ca loading (which independently increase SR Ca release). We measured I_Ca and SR Ca release ±1 mM isoproterenol (ISO) in voltage-clamped ventricular myocytes of rabbits and transgenic mice (expressing only non-phosphorylatable phospholamban). This mouse model helps control for any effect of ISO-enhanced SR uptake on observed release, but the two species produced essentially identical results. SR Ca load and I_Ca were adjusted by conditioning (with constant unitary I_Ca). We thus evaluated PKA effects on SR Ca release at constant SR Ca load and I_Ca trigger. The amount of SR Ca release increased as a function of either I_Ca or SR Ca load, but ISO did not alter the relationships (measured as gain or fractional release). This was true over a wide range of SR Ca load and I_Ca. However, the maximal rate of SR Ca release was ~50% faster with ISO (at most loads and I_Ca levels). We conclude that the isolated effect of PKA on SR Ca release is an increase in maximal rate of release and faster turn-off of release (such that integrated SR Ca release is unchanged). The increased amount of SR Ca release normally seen with ISO depends primarily on increased I_Ca trigger and SR Ca load, whereas faster release kinetics may be the main result of RyR phosphorylation.