1. The relationship between energy turnover and mechanical performance was investigated in chemically skinned single fibres from rabbit psoas muscle at 15°C, pH = 741, with MgATP, 5 mm; free Mg2", 1 mM; ionic strength, 200 mm and sarcomere length, 2-4 jum by measuring force production and myofibrillar ATP turnover during isometric contractions as well as during repetitive changes in length. ATP hydrolysis was stoichiometrically coupled to the breakdown of NADH, which was measured photometrically via the absorption of near UV light at 340 nm. 2. Force and ATPase activity were measured during square-wave length changes of different amplitudes (1-10 % of the fibre length, Lo) and different frequencies (2-5-167 Hz).The average force during the length changes 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' per myosin head. ATP turnover increased monotonically with increasing amplitude as well as with increasing frequency until saturation was reached. The greatest 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 shortening velocity in a hyperbolic fashion. The ATP turnover increased with ramp velocity up to 0 5 Lo s' and stayed almost constant (at 2-2 times the isometric value) for larger velocities. 4. Isometric force and ATPase activity both decreased as the calcium concentration was decreased. They did not vary in proportion at low Ca2" concentrations, but this could largely be accounted for by the presence of a residual, Ca2"-dependent, membrane-bound ATPase. At high calcium concentrations ATPase activity during square-wave length changes was higher than the isometric value, but at low calcium concentrations (pCa > 641), the ATPase activity during the length changes decreased below the isometric value and reached a minimum of 40 % of the isometric level. 5. ATPase activity and average force obtained during changes in length show a high, movement protocol-independent correlation. During the length changes the rate of ATP turnover divided by the average force level (tension cost) was larger than the isometric tension cost. The largest value found, for 10 % length changes at 23 Hz, was 17 times the tension cost under isometric conditions. 6. The effect of the length changes on energy turnover and of the variation with amplitude, frequency and calcium concentration can be understood in a simple three-state crossbridge model, consisting of a detached, a non-or low-force-producing, and a forceproducing state. In this model, length changes enhance cross-bridge detachment from both attached states, and the calcium concentration determines which of their counteracting contributions to overall ATP turnover prevails.During muscle contraction, energy turnover associated with and breakage of bonds, the cross-bridge cycle, is driven by the contractile...

Inorganic phosphate (Pi) release was determined by means of a fluorescent Pi-probe in single permeabilized rabbit soleus and psoas muscle fibers. Measurements of Pi release followed photoliberation of approximately 1.5 mM ATP by flash photolysis of NPE-caged ATP in the absence and presence of Ca2+ at 15 degrees C. In the absence of Ca2+, Pi release occurred with a slow rate of 11 +/- 3 microM . s-1 (n = 3) in soleus fibers and 23 +/- 1 microM . s-1 (n = 10) in psoas fibers. At saturating Ca2+ concentrations (pCa 4.5), photoliberation of ATP was followed by rapid force development. The initial rate of Pi release was 0.57 +/- 0.05 mM . s-1 in soleus (n = 13) and 4.7 +/- 0.2 mM . s-1 in psoas (n = 23), corresponding to a rate of Pi release per myosin head of 3.8 s-1 in soleus and 31.5 s-1 in psoas. Pi release declined at a rate of 0.48 s-1 in soleus and of 5.2 s-1 in psoas. Pi release in soleus was slightly faster in the presence of an ATP regenerating system but slower when 0.5 mM ADP was added. The reduction in the rate of Pi release results from an initial redistribution of cross-bridges over different states and a subsequent ADP-sensitive slowing of cross-bridge detachment.

The interpretation of skinned fibre experiments in the presence of MgADP and the absence of CP is complicated by intracellular adenine nucleotide gradients caused by ATP hydrolysis and diffusion of the reactants (e.g. Cooke & Pate, 1985). In order to minimize the intracellular concentration gradients, isolated Triton-permeabilized myocytes were used in this study. In addition, model calculations were employed to evaluate the influence of the adenine nucleotide concentration profiles inside the cardiomyocytes.

While AMP hydrolysis to adenosine is prominent in early ischemia and acts to preserve cellular energy potential, during a second ischemic period, nucleotides are conserved by the stable inhibition of AMP hydrolysis. Furthermore, during 10% flow conditions, nucleotides are conserved, possibly via an IMP-accumulatory pathway.