CALCIUM REGULATION OF TENSION REDEVELOPMENT KINETICS WITH 2-DEOXY-ATPOR LOW [ATP] IN RABBIT SKELETAL-MUSCLE

The correlation of acto-myosin ATPase rate with tension redevelopment kinetics (k(tr)) was determined during Ca+2-activated contractions of demembranated rabbit psoas muscle fibers; the ATPase rate was either i ncreased 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, r espectively. The activation dependence of k(tr) and unloaded shortenin g velocity (V-u) was measured with each substrate. With 5.0 mM ATP, V- u depended linearly on tension (P), whereas k(tr) exhibited a nonlinea r dependence on P, being relatively independent of P at submaximum lev els and rising steeply at P > 0.6-0.7 of maximum tension (P-o). With d ATP, V-u was 25% greater than control at P-o and was elevated at all P > 0.15P(o), whereas P-o was unchanged. Furthermore, the Ca+2 sensitiv ity of both k(tr) and P increased, such that the dependence of k(tr) o n P was not significantly different from control, despite an elevation of V-u and maximal k(tr). In contrast, lowering [ATP] caused a slight (8%) elevation of P-o, no change in the Ca+2 sensitivity of P, and a decrease in V-u at all P. Moreover, k(tr) was decreased relative to co ntrol at P > 0.75P(o), but was elevated at P < 0.75P(o). These data de monstrate that the cross-bridge cycling rate dominates k(tr) at maximu m but not submaximum levels of Ca2+ activation.