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) Ca(2+)-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 slowtwitch 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. Ca(2+)-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 microM, respectively (control = 18.2 +/- 1.4 microM.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 Ca(2+)-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 Ca(2+)-release mechanism is opposite with increased and decreased weight bearing, but the mechanism by which this is achieved appears to be different.
Kandarian
S C
SC
Department of Health Sciences, Boston University, Massachusetts 02215, USA.
Peters
D G
DG
Favero
T G
TG
Ward
C W
CW
Williams
J H
JH
D000222:Adaptation, Physiological; D000818:Animals; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D020746:Calcium Channels, L-Type; D005260:Female; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism; D018657:Muscle Fibers, Slow-Twitch / Q000378:metabolism; D009124:Muscle Proteins / Q000378:metabolism; D018482:Muscle, Skeletal / Q000378:metabolism*; D009133:Muscular Atrophy / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D019837:Ryanodine Receptor Calcium Release Channel; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D016474:Weight-Bearing / Q000502:physiology*
D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D009124:Muscle Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D002118:Calcium