MYOFIBRILLAR ATPASE ACTIVITY AND MECHANICAL PERFORMANCE OF SKINNED FIBERS FROM RABBIT PSOAS MUSCLE

1. The relationship between energy turnover and mechanical performance was investigated in chemically skinned single fibres from rabbit psoa s muscle at 15 degrees C, pH = 7.1, with MgATP, 5 mM; free Mg2+, 1 mM; ionic strength, 200 mM and sarcomere length, 2.4 mu m by measuring fo rce production and myofibrillar ATP turnover during isometric contract ions as well as during repetitive changes in length. ATP hydrolysis wa s stoichiometrically coupled to the breakdown of NADH, which was measu red photometrically via the absorption of near UV light at 340 nm. 2. Force and ATPase activity were measured during square-wave length chan ges of different amplitudes (1-10 % of the fibre length, L(0)) and dif ferent frequencies (2.5-167 Hz). The average force during the length c hanges was less than the isometric value and decreased with increasing amplitude and frequency. At full activation (pCa 4.5), the isometric ATP turnover rate (+/- S.E.M.) was 2.30 +/- 0.05 s(-1) per myosin head . ATP turnover increased monotonically with increasing amplitude as we ll as with increasing frequency until saturation was reached. The grea test increase observed was 2.4 times the isometric value. 3. Force and ATPase activity were also determined for ramp shortenings followed by fast restretches. The average force decreased with increasing shorten ing velocity in a hyperbolic fashion. The ATP turnover increased with ramp velocity up to 0.5 L(0) s(-1) and stayed almost constant (at 2.2 times the isometric value) for larger velocities. 4. Isometric force a nd ATPase activity both decreased as the calcium concentration was dec reased. They did not vary in proportion at low Ca2+ concentrations, bu t this could largely be accounted for by the presence of a residual, C a2+-dependent, membrane-bound ATPase. At high calcium concentrations A TPase activity during square-wave length changes was higher than the i sometric value, but at low calcium concentrations (pCa > 6.1), the ATP ase activity during the length changes decreased below the isometric v alue and reached a minimum of 40 % of the isometric level. 5. ATPase a ctivity and average force obtained during changes in length show a hig h, movement protocol-independent correlation. During the length change s the rate of ATP turnover divided by the average force level (tension cost) was larger than the isometric tension cost. The largest value f ound, for 10 % length changes at 23 Hz, was 17 times the tension cost under isometric conditions. 6. The effect of the length changes on ene rgy turnover and of the variation with amplitude, frequency and calciu m concentration can be understood in a simple three-state crossbridge model, consisting of a detached, a non- or low-force-producing, and a force-producing state. In this model, length changes enhance cross-bri dge detachment from both attached states, and the calcium concentratio n determines which of their counteracting contributions to overall ATP turnover prevails.