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

Aim In 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. Methods Rat soleus muscles were stimulated at 60 Hz in standard Krebs‐Ringer buffer, and contraction force and velocity were measured. Tetanic [Ca ++ ] i was in some experiments either lowered by addition of 10 μmol/L dantrolene or use of submaximal stimulation (30 Hz) or increased by addition of 2 mmol/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 . Results Compared 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 13 ± 1% ( 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). Conclusion Contrary 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.