PKA activation increased calcium spark frequency 2- to 3-fold in wild-type mouse ventricular myocytes but had no effect in phospholamban-knockout or mutant myocytes.
PKA-dependent increases in myocyte calcium sparks are driven entirely by phospholamban phosphorylation and increased SR calcium load, rather than direct effects on the ryanodine receptor.
Ryanodine receptor (RyR) phosphorylation by protein kinase A (PKA) may be important in modulating resting sarcoplasmic reticulum (SR) Ca2+ release, especially in heart failure. However, clear cellular data on PKA-dependent modulation of cardiac RyRs is limited because of difficulty in distinguishing between PKA effects on RyR, phospholamban (PLB), and Ca2+ current. To clarify this, we measured resting Ca2+ sparks in streptolysin-O permeabilized ventricular myocytes from wild-type (WT) and PLB knockout (PLB-KO) mice and transgenic mice expressing only double-mutant PLB (PLB-DM) that lacks the regulatory phosphorylation sites (S16A/T17A). In WT myocytes, cAMP dramatically increased Ca2+ spark frequency (CaSpF) by 2- and 3-fold when Ca2+ was clamped at 50 and 10 nmol/L (and the SR Ca2+ content also rose by 40% and 50%). However, in PLB-KO and PLB-DM, neither CaSpF nor SR Ca2+ load was changed by the addition of 10 micromol/L cAMP (even with phosphatase inhibition). PKA activation also increased Ca2+ spark amplitude, duration, and width in WT, but not in PLB-KO or PLB-DM. RyR phosphorylation was confirmed by measurements of 32P incorporation on immunoprecipitated RyR. In intact resting myocytes, PKA activation increased CaSpF 2.8-fold in WT, but not in PLB-KO, confirming results in permeabilized myocytes. We conclude that the PKA-dependent increase in myocyte CaSpF and size is entirely attributable to PLB phosphorylation and consequent enhanced SR Ca2+ load. PKA does not seem to have any appreciable effect on resting RyR function in these ventricular myocytes. Moreover, the data provide compelling evidence that elevated intra-SR Ca2+ increases RyR gating independent of cytosolic Ca2+ (which was clamped).
Li et al. (Fri,) reported a other. cAMP / PKA activation vs. PLB-KO and PLB-DM myocytes was evaluated on Calcium spark frequency (CaSpF) and SR Ca2+ load. PKA activation increased calcium spark frequency 2- to 3-fold in wild-type mouse ventricular myocytes but had no effect in phospholamban-knockout or mutant myocytes.