Role of the sarcoplasmic reticulum in regulating the activity-dependent expression of the glycogen phosphorylase gene in contractile skeletal muscle cells

Nerve-evoked contractile activity in skeletal muscle regulates transcript a nd protein levels of many metabolic genes in a coordinate fashion, includin g the muscle isozyme of glycogen phosphorylase (MGP). Cellular signaling me chanisms mediating the activity-dependent modulation of MGP transcript leve ls were investigated in a spontaneously contractile rat skeletal muscle cel l line (Rmo). Mechanisms regulating MGP mRNA levels in Rmo myotubes were co mpared with those previously shown to modulate the gene encoding the a subu nit of the acetylcholine receptor (alpha AChR). Reducing the resting membra ne potential from -78 to -30 mV, either eleclrochemically (KCI) or by incre asing Na permeability (veratridine): (1) prevented activation of transverse tubules, (2) impeded calcium release by the sarcoplasmic reticulum (SR), a nd (3) blocked Rmo contractility. MGP mRNA levels decreased to 30% of contr ol levels and alpha AChR levels increased to 350% following 24 h of depolar ization. Differing mechanisms appear to mediate this voltage-dependent regu lation of MGP and alpha AChR. Inhibition of SR calcium efflux selectively d ecreased MGP mRNA levels by 30-50% when using dantrolene, thapsigargin, or a dose of ryanodine shown to inactivate Ca2+-induced SR Ca2+ release (CICR) . By contrast blockade of voltage sensors in transverse tubules with nifedi pine, a dihydroaminopyridine (DHAP) antagonist, selectively increased alpha AChR mRNA levels by twofold. These data indicate that the voltage-dependen t regulation of AChR gene expression differs from that modulating the MGP g ene. KCI-induced depolarization and dantrolene both inhibit pulsatile SR Ca 2+ efflux in Rmo myotubes, but by differing mechanisms. Depolarization and dantrolene comparably reduced MGP mRNA levels and decreased MCP transcript stability from a t(1/2) of 24 h to 14.5 and 16 h, respectively. Reduced tra nscript stability can account for the observed reduction in mRNA levels of MGP in noncontractile Rmo myotubes and could be a significant regulatory me chanism in skeletal muscle that coordinates the activity-dependent expressi on of MGP with other glycogenolytic genes. (C) 2000 Wiley-Liss, Inc.