[Contractile characteristics of single skinned rat soleus muscle fibers under gravitational load: effects of a calcium-binding agent].
Excessive intracellular calcium accumulation is believed to trigger the development of functional and structural changes in muscle fibers under microgravity conditions. The hypothesis was testified in the 14-day hindlimb suspension study with the application of a Ca(2+)-binding agent (10% EGTA). Twenty one rats were divided into 3 groups: cage controls (7), hindlimb-suspended rats that received intraperitoneal injections of saline (7), and hindlimb-suspended rats with EGTA treatment. Whereas the diameter of muscle fibers of unloaded rat soleus muscle was 20% less than in the control group (and there were no significant differences between rats with injections of EGTA and without them), the decrease of maximal tension was more pronounced (more than 50%). This discrepancy resulted in a decrease of maximal specific tension. The value of absolute tension in rats treated with placebo was by 52%, and in EGTA-treated rats by 41% less than in the control group. Thus, there were no significant differences in specific tension between this group and the control group. Obviously, the injections of EGTA prevented the effects of those mechanisms that induce a decline of tension in muscle fibers but are not linked with the reduction of fiber size. The Ca/tension curve in hindlimb-suspended saline-treated rats shifted to the right so that the pCa thresholds changed from 6.85 +/- 0.03 in cage controls to 6.70 +/- 0.04 (p < 0.05), which indicates that myofibrils of unloaded soleus are less sensitive to Ca2+. At the same time, the pCa threshold in EGTA-treated hindlimb-suspended rats was 6.93 +/- 0.02. It is concluded that chronic binding of excess calcium results in an increase in Ca sensitivity indices.
SRC Institute of Biomedical Problems, Russian Academy of Sciences, Khoroshevskoe Shosse 76a, Moscow, 123007 Russia.
K S
KS
Litvinova
I B
IB
Kozlovskaia
T L
TL
Nemirovskaia
B S
BS
Shenkman
D000818:Animals; D002118:Calcium / Q000737:chemistry*; D002614:Chelating Agents / Q000737:chemistry / Q000494:pharmacology*; D004533:Egtazic Acid / Q000737:chemistry / Q000494:pharmacology*; D019417:Hindlimb Suspension; D008297:Male; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar
D002614:Chelating Agents; D004533:Egtazic Acid; D002118:Calcium