Impaired cardiac performance in heterozygous mice with a null mutation in the sarco(endo)plasmic reticulum Ca2+-ATPase isoform 2 (SERCA2) gene

The sarco(endo)plasmic reticulum Ca2+-ATPase isoform 2 (SERCA2) gene encodes both SERCA2a, the cardiac sarcoplasmic reticulum Ca2+ pump, and SERCA2b, which is expressed in all tissues. To gain a better understanding of the physiological functions of SERCA2, we used gene targeting to develop a mouse in which the promoter and 5' end of the gene were eliminated. Mating of heterozygous mutant mice yielded wild-type and heterozygous offspring; homozygous mutants were not observed. RNase protection, Western blotting, and biochemical analysis of heart samples showed that SERCA2 mRNA was reduced by approximately 45% in heterozygous mutant hearts and that SERCA2 protein and maximal velocity of Ca2+ uptake into the sarcoplasmic reticulum were reduced by approximately 35%. Measurements of cardiovascular performance via transducers in the left ventricle and right femoral artery of the anesthetized mouse revealed reductions in mean arterial pressure, systolic ventricular pressure, and the absolute values of both positive and negative dP/dt in heterozygous mutants. These results demonstrate that two functional copies of the SERCA2 gene are required to maintain normal levels of SERCA2 mRNA, protein, and Ca2+ sequestering activity, and that the deficit in Ca2+ sequestering activity due to the loss of one copy of the SERCA2 gene impairs cardiac contractility and relaxation.     

Inhibition of skeletal muscle sarcoplasmic reticulum Ca2+-ATPase by nitric oxide

The effects of nitric oxide on the activities of thapsigargin-sensitive sarcoplasmic reticulum Ca2+-ATPase (SERCA) and Ca2+ uptake by sarcoplasmic reticulum (SR) membranes prepared from white skeletal muscle of rabbit femoral muscle were studied. Pretreatment of the SR preparations with nitric oxide at concentrations of up to 250 microM for 1 min decreased the SERCA activity concentration dependently, and also decreased their Ca2+ uptake. Both these effects of nitric oxide were reversible. Inhibitors of guanylyl cyclase and protein kinase G (PKG) had no significant effect on the nitric oxide-induced inhibitions of SERCA and Ca2+ uptake. Moreover, dithiothreitol did not reverse the inhibitory effects of nitric oxide on SERCA and Ca2+ uptake. These findings suggest that nitric oxide inhibits SERCA, mainly SERCA 1, of rabbit femoral skeletal muscle by an action independent of the cyclic GMP-PKG system or oxidation of thiols, and probably by a direct action on SERCA protein.