Sarcoplasmic Reticulum Genes Are Upregulated in Mild Cardiac Hypertrophy But Downregulated in Severe Cardiac Hypertrophy Induced by Pressure Overload

Changes in contractile and relaxation properties of heart muscle in the cardiac hypertrophy induced by pressure overload have been attributed to alterations in intracellular Ca2+transport as well as the phenotypic and quantitative changes in contractile protein. However, contradictory data have been reported regarding Ca2+uptake, release and storage by the sarcoplasmic reticulum (SR). The purpose of this study was to evaluate the changes in SR Ca2+-ATPase, ryanodine receptor, calsequestrin andα-actin gene expression, and the changes in Ca2+uptake capacity in various degrees of hypertrophied hearts due to pressure overload. Cardiac hypertrophy was produced in rats by placing a constricting clip (0.80 mm) around the suprarenal abdominal aorta for 8 days. The mRNA levels and Ca2+uptake capacity were then measured as a function of the severity of cardiac hypertrophy. Ca2+-ATPase and ryanodine receptor mRNA levels were increased in mildly hypertrophied hearts but were diminished in severely hypertrophied hearts, showing a bimodal response to pressure overload. Ca2+uptake capacity showed similar changes along with a positive correlation with Ca2+-ATPase mRNA level (r=0.67,P<0.001). In contrast, the level of calsequestrin mRNA expression was unaltered and that ofα-actin was markedly increased over a range of severity of cardiac hypertrophy. These findings suggest that the expression of sarcoplasmic reticulum genes for Ca2+uptake and release is up- or downregulated dependent on the degree of pressure overload. The gene for the SR Ca2+storage protein, calsequestrin, might be under different control from these genes in pressure overload. Our findings suggest that the decrease in ratio of mRNAs encoding Ca2+uptake and release proteins to those encoding contractile proteins could significantly contribute to the slowed contractile and relaxation properties seen in pressure-overloaded hearts.