CALCIUM MODULATES THE INFLUENCE OF LENGTH CHANGES ON THE MYOFIBRILLARADENOSINE-TRIPHOSPHATASE ACTIVITY IN RAT SKINNED CARDIAC TRABECULAE

The relationship between adenosine triphosphate (ATP) turnover and mus cle performance was investigated in skinned cardiac trabeculae of the rat at different [Ca2+] and two different sarcomere lengths (1.8 mu m and 2.2 mu m) at 20 degrees C. ATP turnover was measured photometrical ly by enzymatic coupling of the regeneration of ATP to the oxidation o f reduced nicotineamide adenine dinucleotide. The trabeculae were stud ied under isometric conditions and when the length was altered repetit ively at a frequency of 23 Hz, with a square wave, by 5% of the initia l length. The isometric ATPase activity amounted to 0.48 mM/s. Isometr ic ATP turnover and force were proportional at different [Ca2+]. Durin g length changes at maximal activation (pCa 4.27) and 2.2 mu m sarcome re length, ATPase activity increased to up to 162% whereas at low [Ca2 +], ATPase activity decreased with respect to the isometric value at t hat pCa. At pCa 5.5, ATPase activity was reduced to 33%. These results indicate that during the length changes the apparent cross-bridge det achment rate is increased and the apparent attachment rate is decrease d. The findings suggest that the Fenn effect, i. e. the increase in en ergy turnover above the isometric value during shortening, is present in cardiac trabeculae at high levels of activation, but is absent or r eversed at lower levels of activity.