The effect of denervation on incorporation of 32P and (3H)glycerol by the muscle membrane

Characteristics of sarcoplasmic reticulum from normal and denervated rat skeletal muscle

Denervation of rat skeletal muscle produces after 14 days a decrease in Ca2+ uptake of a heterogeneous population of sarcoplasmic-reticulum vesicles, when measured in the presence of oxalate. The Mg2+-dependent ATPase (Ca2+-independent) activity increased after the same period and the Ca2+ + Mg2+-dependent ATPase activity decreased. Concomitant with these changes, there was an increase in vesicle size and calcium content. The observations are discussed in terms of changes in altered membrane structure, manifested in the shift of the equilibrium of the ATPase from an enzyme involved in calcium transport to a phosphoenzyme giving rise to an increase in the Mg2+-dependent ATPase activity.

The effect of hydrolytic enzymes on the acetylcholine sensitivity of the skeletal muscle cell membrane

Isometric tension developed by rat soleus and extensor digitorum longus (EDL) muscles in response to acetylcholine (Ach) applied in vitro was recorded. Tension of contractures elicited in response to Ach increased after muscles had been incubated with phospholipase C, pepsin, or soluble fractions prepared from muscle homogenate. Using intracellular microelectrodes, resting membrane potential (RMP) and depolarisation in response to Ach added to the bathing medium were recorded in endplate-free regions of the muscle fibres. No significant change in RMP was observed in muscles incubated with soluble muscle fraction or phospholipase C, but depolarisation in response to Ach or carbachol was significantly increased. The time course for the increase in depolarisation and the contracture response to Ach was similar. When all available receptors were blocked with alpha-bungarotoxin prior to incubation so that no response to Ach could be elicited, with subsequent incubation in muscle soluble fraction or phospholipase C, both contractures and depolarisation in response to Ach returned. These results support the hypothesis that receptors, not previously available to interact with Ach or alpha-bungarotoxin were revealed following incubation.

Effect of increasing duration of denervation on the rate of entry of inorganic phosphate into rat gastrocnemius muscle

The increased inorganic phosphate flow, characteristic of denervated gastrocnemius muscle, is shown to have no direct relation with either the loss of muscle mass or with the concentrations of the acid-soluble phosphate fractions. It is shown to increase hyperbolically with the time elapsed since the nerve section. The asymptotic value reached after thirty days suggests the presence of a saturable mechanism.

The effect of muscle extracts on the contracture response of skeletal muscle to acetylcholine

Preincubation of normal rat soleus muscles in vitro with homogenates prepared from mixed leg muscles which had been denervated 4 days previously resulted in an increase in the contracture response to acetylcholine. After 30 min incubation a 1.5-fold increase was observed. Homogenates of normally innervated muscles did not increase the response. The active principles of the denervated muscles were found to reside in the "cytosol" fraction. An approximately 2-fold increase was observed upon incubation with the cytosol for 30 min; incubation for longer periods resulted in a subsequent decrease in the response. The effect of the denervated muscle cytosol was concentration-dependent and heat-labile. Normal muscle cytosol also increased the soleus muscle response to acetylcholine but this fraction was less effective than denervated muscle cytosol. The response of control muscles incubated in Krebs-Henseleit solution was found to decrease with time. Commercially obtained phospholipases C and D increased the response of normal soleus muscles approximatley 2-fold. Phospholipase A, lipase, trypsin, collagenase and a bacterial protease had no effect, lysozyme produced a small but consistent increase in the response to acetylcholine.