TEMPORAL DIFFERENCES IN THE INDUCTION OF DIHYDROPYRIDINE RECEPTOR SUBUNITS AND RYANODINE RECEPTORS DURING SKELETAL-MUSCLE DEVELOPMENT

The expression of the dihydropyridine (DHP) and ryanodine receptors in skeletal muscle was investigated during development of rat myotubes i n culture as well as during embryonic and postnatal development in the rat. Through the use of specific gene probes, antibodies and radiolig and binding ([H-3]PN 200-110 (DHP) and [H-3]ryanodine), we identified a significant difference between the time course of appearance of the DHP receptor and the ryanodine receptor during muscle development. Alt hough the number of DHP receptors dramatically increased at early stag es of development (up to day 7 in tissue culture and day 20 postnatal) , increase in the ryanodine receptor density occurred comparatively la ter at day 10 in culture and day 30 postnatal. This process was associ ated with parallel changes in the expression of the mRNA encoding the alpha(1), alpha(2) and beta subunits of the DHP receptor and the skele tal muscle ryanodine receptor. The genes encoding the DHP receptor sub units were activated in a temporally distinct manner during developmen t. The increase in beta subunit transcript appeared and plateaued firs t, at the onset of myoblast fusion and day 16 embryonic. This was foll owed closely by an increase in expression of the mRNAs for alpha(1) an d alpha(2) subunits which coincided with the sharp rise in the DHP rec eptor density. Ryanodine receptor gene expression was induced well aft er the DHP receptor gene expression had plateaued. The temporal appear ance of the polypeptides comprising the DHP receptor subunits and the ryanodine receptor paralleled the induction of the genes encoding thes e receptors. These results imply that gene expression is a major mecha nism that contributes to the regulation of DHP and ryanodine receptor numbers during muscle development. The temporal differences in the ind uction of the genes encoding the DHP receptor subunits and the ryanodi ne receptor suggests that these genes are under the control of distinc t endogenous factors. These differences in expression of the DHP recep tor and the ryanodine receptor may contribute to the different mechani sms of excitation-contraction coupling in immature versus adult skelet al muscle.