Neural influences on the co-ordination of expression of the multiple subunits of skeletal muscle phosphorylase kinase and their assembly to form the holoenzyme complex, α4β4γ4δ4, have been examined during denervation and re-innervation of adult skeletal muscle and during neonatal muscle development. Denervation of the tibialis anterior and extensor digitorum longus muscles of the rat hindlimb was associated with a rapid decline in the mRNA for the γ subunit, and an abrupt decrease in γ-subunit protein. The levels of the α- and β-subunit proteins in the denervated muscles also declined rapidly, their time course of reduction being similar to that for the γ-subunit protein, but they did not decrease to the same extent. In contrast with the rapid decline in γ-subunit mRNA upon denervation, α- and β-subunit mRNAs stayed at control innervated levels for approx. 8–10 days, but then decreased rapidly. Their decline coincided very closely with the onset of re-innervation. Re-innervation of the denervated muscles, which occurs rapidly and uniformly after the sciatic nerve crush injury, produced an eventual slow and prolonged recovery of the mRNA for all three subunits and parallel increases in each of the subunit proteins. A similar co-ordinated increase of both subunit mRNA and subunit proteins of the phosphorylase kinase holoenzyme was observed during neonatal muscle development, during the period when the muscles were attaining their adult pattern of motor activity. The phosphorylase kinase holoenzyme remains in a non-activated form during all of these physiological changes, as is compatible with the presence of the full complement of the regulatory subunits. These data are consistent with a model whereby the transcriptional and translational expression of phosphorylase kinase γ subunit occurs only with concomitant expression of the α and β subunits. This would ensure that free and unregulated, activated γ subunit alone, which would give rise to unregulated glycogenolysis, is not produced. The data also suggest that control of phosphorylase kinase subunit expression and the formation of the holoenzyme in skeletal muscle is provided by the motor nerve, probably through imposed levels or patterns of muscle activity.
Phosphorylation and activation of red skeletal muscle phosphorylase kinase isozyme.
