AJP - Regulatory, Integrative and Comparative Physiology, Vol 265, Issue 2 310-R319, Copyright © 1993 by American Physiological Society

ARTICLES

Influence of extent of muscle shortening and heart rate on work from frog heart trabeculae

D. A. Syme
Department of Ecology and Evolutionary Biology, University of California, Irvine 92717.

Shortening, lengthening, and net work done by frog (Rana pipiens) heart trabeculae were measured over a range of strain amplitudes (length change) and cycle frequencies. Net work, the product of muscle strain and force over a full lengthening/shortening cycle, increased with strain to strains well over 25% of the muscle's rest length, a value greater than optimum strains reported for most skeletal muscles. A distinct optimum strain for net work was not found. Maximum net work per cycle averaged 7.5 J/kg for ventricular muscle and 2.0 J/kg for atrial muscle. Isometric twitch stress was maximal at 0.4-0.6 Hz twitch frequency in ventricular trabeculae (average 51 kN/m2) and 0.6-1.4 Hz in the atrium (average 14 kN/m2). The twitch duration decreased with increasing twitch frequency. Shortening and net work were maximal at 0.7-Hz cycle frequency in ventricular trabeculae and 0.9 to 1.4 Hz in the atrium. The decline in work per cycle at slower frequencies was due in part to a decline in twitch force. Maximum power for the ventricle was approximately 5 W/kg and occurred at 0.8 Hz and 26% strain, and was 1/3 to 1/4 the power of most skeletal muscles studied at similar temperatures.

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