CORRELATIONS BETWEEN MYOD, MYOGENIN, SERCA1, SERCA2 AND PHOSPHOLAMBANTRANSCRIPTS DURING TRANSFORMATION OF TYPE-II TO TYPE-I SKELETAL-MUSCLE FIBERS

Canine latissimus dorsi, composed predominantly of fast-twitch muscle fibers, were subjected to chronic 1 Hz neuromuscular stimulation for p eriods up to 42 days to induce changes in gene expression. This produc ed down regulation of SERCA1 (fast-twitch isoform of sarco(endo)plasmi c reticulum Ca2+-ATPase), a gene product of fast-twitch muscle, and up regulation of SERCA2 (slow-twitch isoform of sarco(endo)plasmic retic ulum Ca2+-ATPase) and phospholamban, products of genes expressed by sl ow-twitch muscles. To assess the involvement of MyoD and myogenin in t he regulation of the expression of these genes their levels were measu red during the stimulation period. The prompt, at 7 days, fall in SERC A1 mRNA preceded the fall in MyoD by about 7 days, suggesting that the decline in MyoD was not causally related to the decline in SERCA1. Th e prompt rise in SERCA2 mRNA at 7 days preceded the rise in myogenin b y 14 days. The rise in myogenin at 21 days did correlate with the simi lar rise in phospholamban mRNA.