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

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Abstract
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. For the PTU treated papillary muscles the cycling rate (±SEM) during force maintenance was 2·71 (±0±44) cycles s−1 while in the myofibril at high and low ionic strength it was 0·97 (±0·19) and 0·34 (±0·14) cycles s−1, respectively. We suggest that this difference may be a result of reduced cycling rate in myofibrillar preparations caused by a disorganization of the filament lattice as a result of loss of the sarcolemma and when unrestrained sarcomere shortening occurs. Similar to the results found in the rabbit (with low V1/V3 ratios) the economy of force maintenance was substantially increased in the PTU (low V1/V3) treated rat hearts. Analysis of this increase in economy indicates that it resulted from a decrease in the myosin crossbridge cycling rate associated with an increase in the ‘on’ time (period during which the crossbridge is connected to actin and developing force). In the normal heart preparations studies were carried out at a lower temperature (21 vs 11°C) to see if decreasing the cycling rate by means of a temperature change would increase the economy of force maintenance and if the Q10 for the cycling rate and ‘on’ time were identical. Force maintenance at the lower temperature was more economical than at the higher temperature while the Q10 for cycling rate and ‘on’ time were 1·7 and 2·7, respectively.

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