Maximum speed of shortening and atpase activity in atrial and ventricular myocardium of hyperthyroid rats.. Bottinelli, R., M. Canepari, V. Cappelli, and C. Reggiani. Institute of Human Physiology. University of Pavia. Pavia. Italy
APStracts 2:0143C, 1995.
The kinetic properties of the myofibrillar system of atrial and ventricular myocardium of hyperthyroid rats were analyzed by determining ATPase activity and maximum shortening velocity. Hyperthyroidism was induced by daily subcutaneous injections of triiodothyronine (0.2 mg/Kg body weight) for 2 weeks. The treatment induced a marked atrial and ventricular hypertrohy and, in ventricular myocardium, an isomyosin shift towards a homogeneous V1 composition. Skinned trabeculae and purified myofibrils were prepared from atrial and ventricular myocardium. Enzymatic assays on the myofibrils showed that both Ca-stimulated ATPase activity and Ca-Mg dependent ATPase activity had equal values in atrial and ventricular myocardium. In skinned trabeculae during maximal Ca activations force-velocity curves were determined by load clamp manoeuvres and unloaded shortening velocity (Vo) was obtained with the slack test method. Both maximum shortening velocity extrapolated from the force- velocity curves (Vmax) and Vo were significantly higher (+68% and +52%, respectively) in atrial than in ventricular preparations. Developed tension was significantly greater in ventricular preparations. Maximum power output was not significantly different. Previous findings (Cappelli et al. Circ. Res. 1989, 65: 446-457) had led to the conclusion that variations in ATPase activity and shortening velocity of ventricular myocardium can be accounted for by changes in isomyosin composition. In this light the present results suggest that: a) ATPase activity is equal in atrial and ventricular myocardium as the two tissues contain the same myosin heavy chain isoform, b) the difference in maximum speed of shortening between atrium and ventricle might be due to the presence of tissue specific isoforms of myosin light chains. 

Received 13 June 1994; accepted in final form 9 March 1995.
APS Manuscript Number C332-4.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 28 March 1995.