Cyclic GMP-induced reduction in cardiac myocyte function is partially mediated by activation of the sarcoplasmic reticulum Ca²⁺-ATPase

We tested the hypothesis that the mechanism through which cyclic GMP reduces cardiac function is mediated by activation of the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA). Cardiac myocytes were isolated from New Zealand white rabbits (n = 11). Individual ventricular cells were stimulated by electrical field stimulation. The maximal rate of cell shortening and percentage shortening were measured with a video edge detector. Thapsigargin (10^-8 mol/l) was used as a specific inhibitor of SERCA. When 8-bromo-cyclic GMP (8-Br-cGMP, 10^{-7, -6, -5} mol/l) was added to cells, the maximal rate of myocyte shortening (R_max, μm/s) and percentage shortening were both decreased in a concentration-dependent manner. R_max decreased 27% from 117 ± 12 at baseline to 85.2 ± 13 when 10^-5 mol/l of 8-Br-cGMP was present, and percent shortening was reduced 28% from 6.0 ± 0.5 to 4.3 ± 0.5%. Thapsigargin (10^-8 mol/l) increased the maximal rate of myocyte shortening and percent shortening. Addition of thapsigargin prior to 8-Br-cGMP reduced the negative effects of cGMP on myocyte function. The percent shortening decreased only 11% and R_max decreased 14% with 10^-5 mol/l 8-Br-cGMP, which was not significant. Cyclopiazonic acid, another SERCA inhibitor, was also used to test whether 8-Br-cGMP reduced myocyte function through SERCA. The results were similar to those when thapsigargin was used. These results indicated that the cyclic GMP-induced reduction in cardiac myocyte function was partially mediated through the action of the sarcoplasmic reticulum Ca²⁺-ATPase.