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Modulation of excitation–contraction coupling by isoproterenol in cardiomyocytes with controlled SR Ca 2+ load and Ca 2+ current trigger

2004; Wiley; Volume: 556; Issue: 2 Linguagem: Inglês

10.1113/jphysiol.2003.055384

1469-7793

Kenneth S. Ginsburg, Donald M. Bers,

Neuroscience and Neural Engineering

Cardiac Ca 2+ 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 2+ current ( I Ca ), SR Ca 2+ uptake and SR Ca 2+ loading (which independently increase SR Ca 2+ release). We measured I Ca and SR Ca 2+ release ± 1 μ m isoproterenol (ISO; isoprenaline) 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 2+ load and I Ca were adjusted by conditioning. We thus evaluated PKA effects on SR Ca 2+ release at constant SR Ca 2+ load and I Ca trigger (with constant unitary I Ca ). The amount of SR Ca 2+ release increased as a function of either I Ca or SR Ca 2+ load, but ISO did not alter the relationships (measured as gain or fractional release). This was true over a wide range of SR Ca 2+ load and I Ca . However, the maximal rate of SR Ca 2+ release was ∼50% faster with ISO (at most loads and I Ca levels). We conclude that the isolated effect of PKA on SR Ca 2+ release is an increase in maximal rate of release and faster turn‐off of release (such that integrated SR Ca 2+ release is unchanged). The increased amount of SR Ca 2+ release normally seen with ISO depends primarily on increased I Ca trigger and SR Ca 2+ load, whereas faster release kinetics may be the main result of RyR phosphorylation.