Effects of manipulating tetanic calcium on the curvature of the force-velocity relationship in isolated rat soleus muscles

AimIn dynamically contracting muscles, increased curvature of the force-velocity relationship contributes to the loss of power during fatigue. It has been proposed that fatigue-induced reduction in [Ca++](i) causes this increased curvature. However, earlier studies on single fibres have been conducted at low temperatures. Here, we investigated the hypothesis that curvature is increased by reductions in tetanic [Ca++](i) in isolated skeletal muscle at near-physiological temperatures. MethodsRat soleus muscles were stimulated at 60Hz in standard Krebs-Ringer buffer, and contraction force and velocity were measured. Tetanic [Ca++](i) was in some experiments either lowered by addition of 10mol/L dantrolene or use of submaximal stimulation (30Hz) or increased by addition of 2mmol/L caffeine. Force-velocity curves were constructed by fitting shortening velocity at different loading forces to the Hill equation. Curvature was determined as the ratio a/F-0 with increased curvature reflecting decreased a/F-0. ResultsCompared to control levels, lowering tetanic [Ca++](i) with dantrolene or reduced stimulation frequency decreased the curvature slightly as judged from increase in a/F-0 of 131% (P=<.001) and 20 +/- 2% (P=<.001) respectively. In contrast, increasing tetanic [Ca++](i) with caffeine increased the curvature (a/F-0 decreased by 17 +/- 1%; P=<.001). ConclusionContrary to our hypothesis, interventions that reduced tetanic [Ca++](i) caused a decrease in curvature, while increasing tetanic [Ca++](i) increased the curvature. These results reject a simple causal relation between [Ca++](i) and curvature of the force-velocity relation during fatigue.