Expression of sarcoplasmic reticulum Ca2+-ATPase and calsequestrin genes in rat heart during ontogenic development and aging

Little is known concerning the molecular mechanisms responsible for changes in sarcoplasmic reticulum (SR) function during ontogenic development and aging except that the amount of SR Ca2'-ATPase mRNA varies in these conditions. The aim of the present work was to determine whether SR maturation requires expression of specific isoforms and synchronous accumulation of mRNAs encoding proteins located in SR. Thus, we have studied expression of SR Ca2'-ATPase and calsequestrin genes in the rat at different developmental stages from 14 fetal days to 24 months of age. Analysis of alternative splicing of the major Ca2'-ATPase gene expressed in heart by nuclease S, mapping led us to conclude that the Ca2'-ATPase gene expressed in heart was not differentially spliced during ontogenic development and senescence. A single calsequestrin mRNA isoform was also detected in rat heart whatever the developmental stage. The amount of specific mRNA was then measured by dot blot and normalized to 18S ribosomal RNA or to myosin heavy chain mRNA. The amount of Ca'4-ATPase mRNA relative to 18S RNA increases substantially at the end of fetal life and in the early postnatal period (9.5±0.5 % in the 14-15-day fetus versus 99±7 % in the 4-day-old rat). A stable high level is observed during adulthood. In aged rats (24 months), Ca24-ATPase mRNA represents onl