Tension and heat production during isometric contractions and shortening in the anterior byssus retractor muscle of Mytilus edulis.

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Abstract
Tension and heat production were measured during phasic isometric contractions and isovelocity shortening in the anterior byssus retractor muscle (ABRM) of M. edulis at 20.degree. C. Isometric tension at lo was 550 .+-. 40 mN/mm2 (SD for 173 observations in nine muscles), while the isometric maintenance heat rate was 1.0 .+-. 0.2 mW/g wet wt. (SD for 78 observations in 8 muscles). Isometric tension and heat production measured as functions of muscle length over a range of 0.79-1.14 lo and bore a linear relation to each other. The force-velocity relation determined in isovelocity releases imposed during tetanic stimulation fitted the Hill equation with parameters a/Po = 0.07 .+-. 0.01 and b/lo = 0.016 .+-. 0.0007 s-1 (SE from non-linear least-squares regression of the pooled data from 7 experiments). Heat production measured in the same experiments showed that shortening heat was produced with a shortening heat coefficient .alpha./Po of 0.15. Shortening heat did not appear to be force-dependent, and separate experiments confirmed that it was a linear function of the amount of shortening.