Cav1.3 L-Type Calcium Channels-Mediated Ryanodine Receptor Dependent Calcium Release Controls Heart Rate

Abstract

Pacemaker activity of the sino-atrial node (SAN) controls heart rate. However, the mechanism underling SAN pacemaker activity is not completely understood and in particular, the respective physiological importance of ion channels and ryanodine receptor (RyR)-dependent Ca2+ release in pacemaking is hotly debated. We have investigated Ca2+ handling in SAN pacemaker cells of wild-type (WT) and mice lacking L-type Cav1.3 (Cav1.3-/-) channels. In isolated Cav1.3-/- SAN cells the frequency of Ca2+ transients was reduced by 45% compared to WT pacemaker cells. Loss of Cav1.3 channels also blunted by about 47% the positive chronotropic effect induced by 0.01 μM isoproterenol (ISO). Furthermore, in Cav1.3-/- pacemaker cells, local Ca2+ release (LCR) occurring during the diastolic phase was reduced by 71%. In SAN cells from mice in which L-type Cav1.2 channels have been rendered insensitive to dihydropyridines (Cav1.2DHP-/-), application of 0.3 μM isradipine decreased diastolic LCR by 78 %, thus showing that Cav1.3 channels are major regulators of RyR-dependent LCR during the diastolic phase. In individual cells of isolated intact SAN, pacemaking of Cav1.3-/- cells was characterized by reduced (37%) frequency of Ca2+ transients and an increase in Ca2+ waves. Normal pacemaking in Cav1.3-/- isolated SAN cells and intact tissue could be observed only after direct activation of RyR-dependent Ca2+ release by low doses of caffeine (200 μM). Experiments with high doses of caffeine (10 mM) in Cav1.3-/- cells, showed that the reduction in diastolic LCR and in the frequency Ca2+ transients could not be ascribed to a decrease in sarcoplasmic reticulum (SR) Ca2+ content. Our results show that in SAN pacemaker cell, LCR Ca2+ release is tightly controlled by Cav1.3 channels and that such a control is critical for promoting the formation of whole-cell Ca2+ transients.Support: FWF (P20670, P22528), ANR-06-PHYSIO-004-01.