Cardiac calcium signalling pathologies associated with defective calmodulin regulation of type 2 ryanodine receptor

Cardiac ryanodine receptor (RyR2) is a homotetramer of 560 kDa polypeptides regulated by calmodulin (CaM), which decreases its open probability at diastolic and systolic Ca2+ concentrations. Point mutations in the CaM-binding domain of RyR2 (W3587A/L3591D/F3603A, RyR2(ADA)) in mice result in severe cardiac hypertrophy, poor left ventricle contraction and death by postnatal day 16, suggesting that CaM inhibition of RyR2 is required for normal cardiac function. Here, we report on Ca2+ signalling properties of enzymatically isolated, Fluo-4 dialysed whole cell clamped cardiac myocytes from 10-15-day-old wild-type (WT) and homozygous Ryr2(ADA/ADA) mice. Spontaneously occurring Ca2+ spark frequency, measured at -80 mV, was 14-fold lower in mutant compared to WT myocytes. I-Ca, though significantly smaller in mutant myocytes, triggered Ca2+ transients that were of comparable size to those of WT myocytes, but with slower activation and decay kinetics. Caffeine-triggered Ca2+ transients were about three times larger in mutant myocytes, generating three- to four-fold bigger Na+-Ca2+ exchanger NCX currents (I-NCX). Mutant myocytes often exhibited Ca2+ transients of variable size and duration that were accompanied by similarly alternating and slowly activating I-NCX. The data suggest that RyR2(ADA) mutation produces significant reduction in I-Ca density and I-Ca-triggered Ca2+ release gain, longer but infrequently occurring Ca2+ sparks, larger sarcoplasmic reticulum Ca2+ loads, and spontaneous Ca2+ releases accompanied by activation of large and potentially arrhythmogenic inward I-NCX.