Altered Ca2+ handling by ryanodine receptor and Na+-Ca2+ exchange in the heart from ovariectomized rats:: role of protein kinase A

Our previous study has demonstrated that ovariectomy ( Ovx) significantly increased the left ventricular developed pressure ( LVDP) and the maximal rate of developed pressure over time (+/- dP/dt(max)) in the isolated perfused rat heart and the effects were reversed by female sex hormone replacement. In the present investigation, we studied the effects of Ovx for 6 wk on Ca2+ homeostasis that determines the contractile function. Particular emphasis was given to Ca2+ handling by ryanodine receptor ( RyR) and Na+-Ca2+ exchange ( NCX). Ca-45(2+) fluxes via the RyR, NCX, and Ca2+-ATPase ( SERCA) were compared with their expression in myocytes from Ovx rats with and without estrogen replacement. Furthermore, we correlated the handling of Ca2+ by these Ca2+ handling proteins with the overall Ca2+ homeostasis by determining the Ca2+ transients induced by electrical stimulation and caffeine, which reveals the dynamic changes of cytosolic Ca2+ concentration ([Ca2+](i)) in the heart. In addition, we determined the expression and contribution of protein kinase A ( PKA) to the regulation of the aforementioned Ca2+ handling proteins in Ovx rats. It was found that after Ovx there were 1) increased Ca2+ fluxes via RyR and NCX, which were reversed not only by estrogen replacement, but more importantly by blockade of PKA; 2) an increased expression of PKA; and 3) no increase in expression of NCX and SERCA. We suggest that hyperactivities of RyR and NCX are a result of upregulation of PKA. The increased release of Ca2+ through RyR and removal of Ca2+ by NCX are believed to be responsible for the greater contractility and faster relaxation after Ovx.