Adaptation of the skeletal muscle calcium-release mechanism to weight-bearing condition

In the present study, we examined whether weight-bearing condition can regulate the sarcoplasmic reticulum (SR) Ca2+-release mechanism. Measurements of alpha(1)-subunit dihydropyridine (alpha(1)-DHP) and ryanodine receptor levels were made in hypertrophied fast-twitch plantaris muscles 5 wk after surgical removal of synergist muscles (increased weight bearing) and in atrophied slow-twitch soleus muscles (14 days of non-weight bearing) of the rat. Rates of AgNO3-induced SR Ca2+ release were measured with fura 2 as the Ca2+ indicator and pyrophosphate as the precipitating ion during vesicular Ca2+ loading. Ca2+-release rates were 38, 49, and 58% lower in vesicles from hypertrophied vs. control muscles at AgNO3 concentrations of 0.05, 0.5, and 5 mu M, respectively (control = 18.2 +/- 1.4 mu M . mg(-1). min(-1)). Western blots showed no differences in the relative expression of alpha(1)-DHP or ryanodine receptor when IIID5 (monoclonal) or GP3 (polyclonal) antibodies were used. There was also no difference in ryanodine (10 nM) binding in Ca2+-incubated SR vesicles from hypertrophied muscles, suggesting no difference in the number of channels. In contrast, expression of alpha(1)-DHP and ryanodine receptors was increased by 144 and 157% in non-weight-bearing soleus muscles, respectively. Scatchard analysis of DHP binding showed a 40% increase in maximum binding capacity and no change in the dissociation constant with non-weight-bearing muscles. The direction of modification of the SR Ca2+-release mechanism is opposite with increased and decreased weight bearing, but the mechanism by which this is achieved appears to be different.