Isoform-independent, heart-rate related variation in cardiac myofibrillar. Activity, Ca2+-Activated Mg2+-Atpase. William Rouslin and Charles W. Broge
APStracts 2:0375C, 1995.
In the present study we compared the activities of the cardiac myofibrillar Ca2+-activated Mg2+-ATPase and the content of cardiac muscle mitochondrial ATPase inhibitor protein (IF1) of several mammalian species covering broad ranges of body mass and heart rate, i.e. from beef cattle to mouse. The cardiac myofibrillar ATPase from each species was assayed over a range of pCa's at pH 7.4. While the cardiac myofibrillar ATPase from all species examined showed essentially identical Ca2+ concentration dependencies with the ATPase in each species activating steeply between pCa 6.5 and 5.5, the maximal ATPase specific activity reached varied considerably from species to species and this variation was largely independent of the predominat cardiac myosin ATPase isoform present. Thus, while adult beef cattle, pig, dog and rabbit all contain predominantly the slow cardiac myosin ATPase isoform, the cardiac myofibrillar ATPase specific activities of these four species varied over approximately a four-fold range. Moreover, there was a fairly smooth curvilinear relationship between maximum Ca2+-activated myofibrillar ATPase activity and median conscious heart rate for the slow cardiac myosin ATPase-possessing species examined. This smooth continuum also extended to include two species possessing the fast cardiac myosin ATPase isoform, rat and mouse. This relationship between myofibrillar ATPase activity and heart rate which appears to be applicable to a broad range of species suggests that the myofibrillar ATPase is specifically modeled or fine-tuned to the kinetic (heart rate) demand of each species and, within slow and fast heart-rate ranges, is essentially independent of myosin ATPase isoform per se. Only hearts containing predominantly the slow myosin ATPase isoform contained functional levels of IF1. Finally, while it has been reported that the ratio of myosin Ca2+-ATPase to actomyosin Mg2+-ATPase activity is a good index of the percent of the fast myosin ATPase in rabbit myofibrillar prepara-tions, we found that this relationship may be applicable to only some species. 

Received 19 May 1995; accepted in final form 12 October 1995.
APS Manuscript Number C291-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95