Calcium Regulation of Tension Redevelopment Kinetics with 2-Deoxy-ATP or Low [ATP] in Rabbit Skeletal Muscle

The correlation of acto-myosin ATPase rate with tension redevelopment kinetics (ktr) was determined during Ca+2-activated contractions of demembranated rabbit psoas muscle fibers; the ATPase rate was either increased or decreased relative to control by substitution of ATP (5.0 mM) with 2-deoxy-ATP (dATP) (5.0 mM) or by lowering [ATP] to 0.5 mM, respectively. The activation dependence of ktr and unloaded shortening velocity (Vu) was measured with each substrate. With 5.0 mM ATP, Vu depended linearly on tension (P), whereas ktr exhibited a nonlinear dependence on P, being relatively independent of P at submaximum levels and rising steeply at P > 0.6–0.7 of maximum tension (Po). With dATP, Vu was 25% greater than control at Po and was elevated at all P > 0.15Po, whereas Po was unchanged. Furthermore, the Ca+2 sensitivity of both ktr and P increased, such that the dependence of ktr on P was not significantly different from control, despite an elevation of Vu and maximal ktr. In contrast, lowering [ATP] caused a slight (8%) elevation of Po, no change in the Ca+2 sensitivity of P, and a decrease in Vu at all P. Moreover, ktr was decreased relative to control at P > 0.75Po, but was elevated at P < 0.75Po. These data demonstrate that the cross-bridge cycling rate dominates ktr at maximum but not submaximum levels of Ca2+ activation.