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 S₁ 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 Ca²⁺-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), Ca²⁺-ATPase mRNA represents only 44.6% the amount observed in young adults (1-2 months). In the fetal-neonatal period the increase in Ca²⁺-ATPase mRNA concentration is parallel to the increase in myosin heavy chain mRNA concentration, but during senescence the two mRNAs do not evolve in parallel. The concentration of calsequestrin mRNA also peaks at day 4 but is equally abundant in fetal, adult, and senescent rats, which supports and extends previous observations showing that, during maturation of SR, calsequestrin is synthesized earlier than ATPase. Our results indicate that maturation of SR, which is in part responsible for changes in relaxation rate, does not involve a Ca²⁺-ATPase and calsequestrin isoform switch but, rather, changes in the concentration of single isoforms.