A myothermal analysis of the myosin crossbridge cycling rate during isometric tetanus in normal and hypothyroid rat hearts.

Hypertrophied rabbit heart papillary muscles (thyrotoxicosis), with a high V1/V3 myosin isoenzyme ratio and contractile protein ATPase activity, have a high velocity of unloaded shortening and a decrease in the myothermal economy of isometric twitch force development and dissipation; in hypertrophied hearts (pressure overload) with a low V1/V3 isoenzyme ratio and ATPase activity, the converse was found to be true (Am J Cardiol 1979; 44:947-953; Fed Proc 1982; 41:192-198). In the present study the confounding problem of internal shortening, which takes place during force development and dissipation in the isometric twitch, is minimized by carrying out measurements of the rate of heat liberation during the plateau phase of tetanic force maintenance. The studies are further extended to another species (rat) where the V1/V3 myosin isoenzyme ratio is altered by treating the animal with propyl thiouracil added to the drinking water (PTU); here the contractile protein alteration occurs with myocardial atrophy rather than hypertrophy. High resolution, rapid temperature measurements are made in tetanically stimulated isometrically contracting rat heart papillary muscles from normal (high V1/V3 ratio) and PTU treated (low V1/V3 ratio) rats to assess the relationship between contractile protein performance (crossbridge cycling rate) in the intact muscle and that under controlled conditions in isolated myofibrils. In papillary muscles from the normal heart the crossbridge cycling rate (+/- SEM) during force maintenance was 6.53 (+/- 1.73) cycles/second compared with 3.13 (+/- 0.24) and 0.53 (+/- 0.17) cycles s-1 in the myofibril at high and low ionic strength, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)