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

Abstract

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 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 k(tr) and unloaded shortening velocity (Vu) was measured with each substrate. With 5.0 mM ATP, Vu depended linearly on tension (P), whereas k(tr) 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 k(tr) and P increased, such that the dependence of k(tr) on P was not significantly different from control, despite an elevation of Vu and maximal k(tr). 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, k(tr) 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 k(tr) at maximum but not submaximum levels of Ca(2+) activation.

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“…1: AM*ADP → AM*) are increased when dATP is used as a substrate for contraction (vs. ATP) (6). The increase in skeletal muscle contraction is relatively small compared with what we have reported for cardiac muscle containing either α-or β-myosin (5,7,8). Indeed, with only 2% dATP (98% ATP; 5 mM total NTP), there is significant augmentation of force in demembranated cardiac muscle at (submaximal) Ca 2+ concentrations that occur during a cardiac twitch (18).…”

“…Although most are not as effective as ATP, we found that 2-deoxy-ATP (dATP) is more effective than ATP as a substrate for contraction of demembranated cardiac muscle, augmenting both force and shortening at all levels of Ca 2+ -mediated contractile activation (5)(6)(7)(8). Our detailed biochemical and mechanical analysis suggests that dATP increases the rates of both myosin binding and product release (steps 2 and 3 in Fig.…”

“…We (5)(6)(7)(8) and others (9)(10)(11)(12)(13)(14)(15)(16)(17) have reported that striated muscle myosin can use most naturally occurring nucleotides to support cross-bridge cycling and contraction to varying degrees. Although most are not as effective as ATP, we found that 2-deoxy-ATP (dATP) is more effective than ATP as a substrate for contraction of demembranated cardiac muscle, augmenting both force and shortening at all levels of Ca 2+ -mediated contractile activation (5)(6)(7)(8).…”

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“…Solutions-Relaxing and activating solutions were prepared using a custom software package as previously described (23,27 Mechanical Force Measurements-Force measurements in permeabilized trabeculae were performed as described previously (23). Permeabilized trabeculae were mounted between a force transducer (model 403A, Aurora Scientific Inc.) and a torque motor (model 312-C, Aurora Scientific Inc.) for rapid adjustments of fiber length.…”

“…Force-pCa data were plotted by expressing submaximal force (P) at each pCa as a fraction of maximal force measured at pCa 4.5 (P o ), which is P/P o , where P/P o ϭ P rel . The rate of force redevelopment (k tr ) following a 20% release and re-stretch including a 1-ms 5% overshoot was measured during each activation after steadystate force had reached a plateau value in a given pCa solution as previously described (27).…”