Modulation of excitation-contraction coupling by isoproterenol in cardiomyocytes with controlled SR Ca2+ load and Ca2+ current trigger.
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 microm 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 approximately 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.
Ginsburg
Kenneth S
KS
Department of Physiology, Loyola University Chicago, Stritch School of Medicine, Maywood, IL 60153, USA.
Bers
Donald M
DM
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism; D007545:Isoproterenol / Q000494:pharmacology*; D051379:Mice; D008822:Mice, Transgenic; D009200:Myocardial Contraction / Q000187:drug effects* / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D000099211:Phospholamban
D000318:Adrenergic beta-Agonists; D002118:Calcium; D002135:Calcium-Binding Proteins; D007545:Isoproterenol; D000099211:Phospholamban