FUNCTION OF THE SARCOPLASMIC-RETICULUM AND EXPRESSION OF ITS CA-2+-ATPASE GENE IN PRESSURE OVERLOAD-INDUCED CARDIAC-HYPERTROPHY IN THE RAT

The reduction in Ca2+ concentration during diastole and relaxation occurs differently in normal hearts and in hypertrophied hearts secondary to pressure overload. We have studied some possible molecular mechanisms underlying these differences by examining the function of the sarcoplasmic reticulum and the expression of the gene encoding its Ca2+-ATPase in rat hearts with mild and severe compensatory hypertrophy induced by abdominal aortic constriction. Twelve sham-operated rats and 31 operated rats were studied 1 month after surgery. Eighteen animals exhibited mild hypertrophy (left ventricular wt/body wt < 2.6) and 13 animals severe hypertrophy (left ventricular wt/body wt > 2.6). During hypertrophy we observed a decline in the function of the sarcoplasmic reticulum as assessed by the oxalate-stimulated Ca2+ uptake of homogenates of the left ventricle. Values decreased from 12.1 ± 1.2 nmol Ca2+/mg protein/min in sham-operated rats to 9.1 ± 1.5 and 6.7 ± 1.1 in rats with mild and severe hypertrophy, respectively (p < 0.001 and p < 0.001, respectively, vs. shams). This decrease was accompanied by a parallel reduction in the number of functionally active Ca2+-ATPase molecules, as determined by the level of Ca2+-dependent phosphorylated intermediate: 58.8 ± 7.4 and 48.1 ± 13.5 pmol P/mg protein in mild and severe hypertrophy, respectively, compared with 69.7 ± 8.2 in shams (p < 0.05 and p < 0.01, respectively, vs. shams). Using S1 nuclease mapping, we observed that the Ca2+-ATPase messenger RNA (mRNA) from sham-operated and hypertrophied hearts was identical. Finally, the relative level of expression of the Ca2+-ATPase gene was studied by dot blot analysis at both the mRNA and protein levels using complementary DNA clones and a monoclonal antibody specific to the sarcoplasmic reticulum Ca2+-ATPase. In mild hypertrophy, the concentrations of Ca2+-ATPase mRNA and protein in the left ventricle were unchanged when compared with shams (mRNA, 93.8 ± 10.6% vs. sham, NS; protein, 105.5 ± 14% vs. sham, NS). In severe hypertrophy, the concentration of Ca2+-ATPase mRNA decreased to 68.7 ± 12.9% and that of protein to 80.1 ± 15.5% (p < 0.001 and p < 0.05, respectively), whereas the total amount of mRNA and enzyme per left ventricle was either unchanged or slightly increased. The slow velocity of relaxation of severely hypertrophied heart can be at least partially explained by the absence of an increase in the expression of the Ca2+-ATPase gene and by the relative diminution in the density of the Ca2+ pumps. We cannot, however, exclude that another mechanism could also be involved in the depression of the sarcoplasmic reticulum function of moderately hypertrophied hearts.