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/F0 with increased curvature reflecting decreased a/F0 .

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/F0 of 13 ± 1% (P = < .001) and 20 ± 2% (P = < .001) respectively. In contrast, increasing tetanic [Ca++ ]i with caffeine increased the curvature (a/F0 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.