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

Abstract

Changes in contractile and relaxation properties of heart muscle in the cardiac hypertrophy induced by pressure overload have been attributed to alterations in intracellular Ca 2+transport as well as the phenotypic and quantitative changes in contractile protein. However, contradictory data have been reported regarding Ca 2+uptake, release and storage by the sarcoplasmic reticulum (SR). The purpose of this study was to evaluate the changes in SR Ca 2+-ATPase, ryanodine receptor, calsequestrin and α-actin gene expression, and the changes in Ca 2+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 Ca 2+uptake capacity were then measured as a function of the severity of cardiac hypertrophy. Ca 2+-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. Ca 2+uptake capacity showed similar changes along with a positive correlation with Ca 2+-ATPase mRNA level ( r=0.67, P<0.001). In contrast, the level of calsequestrin mRNA expression was unaltered and that of α-actin was markedly increased over a range of severity of cardiac hypertrophy. These findings suggest that the expression of sarcoplasmic reticulum genes for Ca 2+uptake and release is up- or downregulated dependent on the degree of pressure overload. The gene for the SR Ca 2+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 Ca 2+uptake and release proteins to those encoding contractile proteins could significantly contribute to the slowed contractile and relaxation properties seen in pressure-overloaded hearts.