Comparative analysis of the isoform expression pattern of Ca2+-regulatory membrane proteins in fast-twitch, slow-twitch, cardiac, neonatal and chronic low-frequency stimulated muscle fibers

Although all muscle cells generate contractile forces by means of organized filament systems, isoform expression patterns of contractile and regulator y proteins in heart are not identical compared to developing, conditioned o r mature skeletal muscles. In order to determine biochemical parameters tha t may reflect functional variations in the Ca2+-regulatory membrane systems of different muscle types, we performed a comparative immunoblot analysis of key membrane proteins involved in ion homeostasis. Cardiac isoforms of t he al-dihydropyridine receptor, Ca2+-ATPase and calsequestrin are also pres ent in skeletal muscle and are up-regulated in chronic low-frequency stimul ated fast muscle. In contrast, the cardiac RyR2 isoform of the Ca2+-release channel was not found in slow muscle but was detectable in neonatal skelet al muscle, Upregulation of RyR2 in conditioned muscle was probably due to d egeneration-regeneration processes. Fiber type-specific differences were al so detected in the abundance of auxiliary subunits of the dihydropyridine r eceptor, the ryanodine receptor and the Ca2+-ATPase, as well as triad marke rs and various Ca2+-binding and ion-regulatory proteins. Hence, the variati on in innervation of different types of muscle appears to have a profound i nfluence on the levels and pattern of isoform expression of Ca2+-regulatory membrane proteins reflecting differences in the regulation of Ca2+-homeost asis. However, independent of the muscle cell type, key Ca2+-regulatory pro teins exist as oligomeric complexes under native conditions. (C) 2000 Elsev ier Science B.V. All rights reserved.