Phosphorylase kinase isozymes and phosphorylase in denervated skeletal muscles

Reduced glycogen phosphorylase activity in denervated hindlimb muscles of rat is related to muscle atrophy and fibre type

Changes in the activity of muscle glycogen synthase or phosphorylase (GP) may be responsible for the deregulation of glycogen synthesis and storage which occurs in diabetes mellitus. To clarify the relationship between muscle atrophy, fibre type, insulin-stimulated glucose uptake and GP activity during insulin resistance, we used sciatic nerve severance to induce insulin resistance in rat hindlimb muscles and compared the above parameters in muscles with a range of fibre types. Changes were analysed by comparison with the contralateral hindlimb, which bears more weight due to denervation of the opposing limb, as well as the sham-operated and contralateral limb of a separate rat. Denervation caused a decrease in insulin-stimulated glucose uptake by 1 day after denervation and a decline of GP activity after 7 days in all muscles investigated. GP activity decreased by 73% in soleus, 36% in red gastrocnemius, 35% in tibialis and 13% in white gastrocnemius, which was related to the degree of muscle atrophy and inversely related to the overall GP activity in non-denervated muscles. GP activity in muscles of the contralateral limb from the denervated rat did not differ from either hindlimb of the sham-operated rat. We conclude that the fibre-type related reduction in insulin-stimulated glucose uptake of denervated muscle determines the change in its metabolism and it is this metabolic change which determines the mechanism, rate and degree of muscle atrophy, which is directly related to the decline in GP activity.

Nerve-dependent factors regulating transcript levels of glycogen phosphorylase in skeletal muscle

1. Muscle glycogen phosphorylase (MGP), the rate-limiting enzyme for glycogen metabolism in skeletal muscle, is neurally regulated. Steady-state transcript levels of the skeletal muscle isozyme of MGP decrease significantly following muscle denervation and after prolonged muscle inactivity with an intact motor nerve. These data suggest that muscle activity has an important influence on MGP gene expression. The evidence to this point, however, does not preclude the possibility that MGP is also regulated by motor neuron-derived trophic factors. This study attempts to distinguish between regulation provided by nerve-evoked muscle contractile activity and that provided by the delivery of neurotrophic factors. 2. Steady-state MGP transcript levels were determined in rat tibialis anterior (TA) muscles following controlled interventions aimed at separating the contributions of contractile activity from axonally transported trophic factors. The innervated TA was rendered inactive by daily epineural injections of tetrodotoxin (TTX) into the sciatic nerve. Sustained inhibition of axonal transport was accomplished by applying one of three different concentrations of the antimicrotubule agent, vinblastine (VIN), to the proximal sciatic nerve for 1 hr. The axonal transport of acetylcholinesterase (AChE) was assessed 7, 14, and 28 days after the single application of VIN. 3. MGP transcript levels normalized to total RNA were reduced by 67% in rat TA, 7 days after nerve section. Daily injection of 2 microg TTX into the sciatic nerve for 7 days eliminated muscle contractile activity and reduced MGP transcript levels by 60%. 4. A single, 1-hr application of 0.10% (w/v) VIN to the sciatic nerve reduced axonal transport but did not alter MGP transcript levels in the associated TA, 7 days after treatment. Application of 0.10% VIN to the sciatic nerve also did not affect IA sensory or motor nerve conduction velocities or TA contractile function. 5. Treatment of the sciatic nerve with 0.40% (w/v) VIN for 1 hr reduced axonal transport and decreased MGP transcript levels by 50% within 7 days, but also reduced sensory and motor nerve conduction velocities and depressed TA contractile function. 6. Myogenin, a member of a family of regulatory factors shown to influence the transcription of many muscle genes, including MGP, was used as a molecular marker for muscle inactivity. Myogenin transcript levels were increased following denervation and after treatment with TTX or 0.40% VIN but not after treatment with 0.10% VIN. 7. The results suggest that MGP transcript levels in TA are regulated predominantly by muscle activity, rather than by the delivery of neurotrophic factors. Intrinsic myogenic factors, however, also play a role in MGP expression, since denervation did not reduce MGP transcript levels below 30% of control TA. The dominant influence of activity in the regulation of MGP contrasts with the proposed regulation of oxidative enzyme expression, which appears to depend on both activity and trophic factor influences.

Protein phosphorylation in fast and slow chicken skeletal muscles: effect of denervation

We have identified proteins in adult chicken skeletal muscle whose phosphorylation can be used as markers for the mature fast and slow muscle phenotype. These include phosphorylase, phosphorylase kinase, and a cyclic adenosine 3',5'-monophosphate (cAMP)-stimulated, calmodulin-inhibited 28-kDa band (markers for fast muscle), a calmodulin-stimulated 50-kDa band, and two cAMP-stimulated bands at 44 and 46 kDa (markers for slow muscle), and the relative concentrations of the regulatory subunits of cAMP-dependent protein kinase (RI and RII). After denervation the pattern of phosphorylation in fast muscle changed to resemble that of slow muscle: phosphorylation of the fast phenotype markers decreased; the slow phenotype markers, barely detectable in normal fast muscle, appeared as significant phosphoproteins; and the concentration of RII increased with no change in RI. This is consistent with denervation-induced changes observed using other phenotypic markers and indicates the potential for using these phosphoprotein markers in studies of muscle development and pathophysiology.

Neural regulation of the formation of skeletal muscle phosphorylase kinase holoenzyme in adult and developing rat muscle

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, alpha4beta4gamma4delta4, 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 gamma subunit, and an abrupt decrease in gamma-subunit protein. The levels of the alpha- and beta-subunit proteins in the denervated muscles also declined rapidly, their time course of reduction being similar to that for the gamma-subunit protein, but they did not decrease to the same extent. In contrast with the rapid decline in gamma-subunit mRNA upon denervation, alpha- and beta-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 gamma subunit occurs only with concomitant expression of the alpha and beta subunits. This would ensure that free and unregulated, activated gamma 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.

Effect of denervation on the expression of glycogen phosphorylase in mouse skeletal muscle

After sciatectomy of the left hind-limb of C57BL/J mice, a denervation-induced muscular atrophy ensued and was accompanied by a decrease in the specific activity of glycogen phosphorylase to approx. 25% of control values. The cofactor of phosphorylase, pyridoxal 5'-phosphate, was used as a specific label in the determination of the degradation rate of the enzyme following nerve section. After a delay of 3-4 days, phosphorylase was degraded approx, twice as rapidly in the denervated gastrocnemius (0.20 day-1) as in the control muscle (0.12 day-1). The effect of denervation on phosphorylase mRNA was measured by quantitative Northern-blot analysis using a rat skeletal-muscle phosphorylase cDNA probe. After an initial rapid decline, phosphorylase mRNA levels stabilized in denervated muscle at 50% of the value measured in the contralateral control muscle.