Energy utilization by Limulus telson muscle at different sarcomere and A-band lengths

Phosphorylargnine utilization (delta PArg) was determined in isolated Limulus telson muscle during isometric and isovelocity contractions at long, intermediate, and short lengths when thick filaments were either long and staggered, long and aligned, or shortened. Muscles developed 30% of maximum force at a length of 0.5 Lo (Lo 7 micrometers sarcomere length) during isometric tetani of 5, 15, and 30 s; however, the rate of delta PArg was the same in both the 0.5 Lo and Lo groups. External force and delta PArg were both less during isometric tetani at lengths of 1.7 and 2.0 Lo compared with the values at Lo, as expected due to a reduction in thick-thin filament overlap. However, at lengths of 0.6 and 0.3 Lo delta PArg was the same as at Lo despite a decrease in external isometric tension. No significant delta PArg was measured when telson muscles shortened rapidly (0.8 Vmax) at long or short lengths indicating the thick filament shortening per se required little, if any, energy expenditure. The overall chemical efficiency of telson muscles for performing external work during slow isovelocity contractions was 31% in groups shortening equivalent distances (0.5 Lo) from starting lengths of 1.6 and 1.3 Lo and 15% in the group shortening from 1.1 Lo.