Age-related changes in sarcoplasmic reticulum Ca2+-ATPase and α-smooth muscle actin gene expression in aortas of normotensive and spontaneously hypertensive rats

Abstract

The expression of the sarcoplasmic reticulum (SR) Ca2+-ATPase gene and the SR Ca2+ pump function were investigated in thoracic aortas of 5- and 17-week-old normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). The relative level of the two isoforms of SR Ca2+-ATPase mRNA expressed in the aorta (i.e., SERCA 2a and SERCA 2b) was determined by quantitative S1 nuclease protection analysis and normalized to the level of α-smooth muscle (α-Sm) actin mRNA. The level of α-Sm actin mRNA itself was normalized to the level of 18S ribosomal RNA using slot-blot hybridization assays. Total SR Ca2+ pump activity was estimated by measuring the rate of oxalate-supported Ca2+ uptake in homogenates. At 5 weeks, the amount of SERCA 2a and SERCA 2b mRNA, normalized to 18S ribosomal RNA, and the ratio of α-Sm actin mRNA to 18S RNA were identical in SHR and WKY rats. The Ca2+ pump activity was similar in the two strains of rats at 5 weeks. From 5 to 17 weeks, the amount of SERCA 2a mRNA increased in both strains while the level of SERCA 2b mRNA remained constant. The Ca2+ pump activity was unchanged in SHRs and tended to decrease in WKY rats. Accordingly, the change in the ratio of the SR Ca2+-ATPase mRNA isoforms does not appear to influence SR function. The level of α-Sm actin mRNA and SERCA 2a mRNA increased in parallel from 5 to 17 weeks in both strains. The regulation of the expression of SERCA 2a and α-Sm actin mRNA seems to be coordinated, whereas that of SERCA 2b and α-Sm actin mRNA appears to be unrelated. When related to α-Sm actin mRNA, the level of total SR Ca2+-ATPase mRNA at 17 weeks in thoracic aortas was lower in SHR than in WKY rats.