Although all muscle cells generate contractile forces by means of organized filament systems, isoform expression patterns of contractile and regulatory proteins in heart are not identical compared to developing, conditioned or mature skeletal muscles. In order to determine biochemical parameters that 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 the al-dihydropyridine receptor, Ca2+-ATPase and calsequestrin are also present in skeletal muscle and are up-regulated in chronic low-frequency stimulated fast muscle. In contrast, the cardiac RyR2 isoform of the Ca2+-release channel was not found in slow muscle but was detectable in neonatal skeletal muscle, Upregulation of RyR2 in conditioned muscle was probably due to degeneration-regeneration processes. Fiber type-specific differences were also detected in the abundance of auxiliary subunits of the dihydropyridine receptor, the ryanodine receptor and the Ca2+-ATPase, as well as triad markers and various Ca2+-binding and ion-regulatory proteins. Hence, the variation in innervation of different types of muscle appears to have a profound influence on the levels and pattern of isoform expression of Ca2+-regulatory membrane proteins reflecting differences in the regulation of Ca2+-homeostasis. However, independent of the muscle cell type, key Ca2+-regulatory proteins exist as oligomeric complexes under native conditions. (C) 2000 Elsevier Science B.V. All rights reserved.
