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Effects of manipulating tetanic calcium on the curvature of the force‐velocity relationship in isolated rat soleus muscles
Author(s) -
Kristensen A. M.,
Nielsen O. B.,
Overgaard K.
Publication year - 2018
Publication title -
acta physiologica
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.591
H-Index - 116
eISSN - 1748-1716
pISSN - 1748-1708
DOI - 10.1111/apha.12977
Subject(s) - dantrolene , curvature , chemistry , caffeine , tetanic stimulation , endocrinology , medicine , stimulation , muscle contraction , calcium , biochemistry , biology , mathematics , long term potentiation , geometry , receptor , organic chemistry
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.