ENDOTHERMIC FORCE GENERATION IN FAST AND SLOW MAMMALIAN (RABBIT) MUSCLE-FIBERS

Isometric tension responses to rapid temperature jumps (T-jumps) of 3- 7 degrees C were examined in single skinned fibers isolated from rabbi t psoas (fast) and soleus (slow) muscles. T-jumps were induced by an i nfrared laser pulse (wavelength 1.32 mu m, pulse duration 0.2 ms) obta ined from a Nd-YAG laser, which heated the fiber and bathing buffer so lution in a 50-mu l trough. After a T-jump, the temperature near the f iber remained constant for similar to 0.5 s, and the temperature could be clamped for longer periods by means of Peltier units assembled on the back trough wall. A T-jump produced a step decrease in tension in both fast and slow muscle fibers in rigor, indicating thermal expansio n. In maximally Ca-activated (pCa similar to 4) fibers, the increase o f steady tension with heating (3-35 degrees C) was approximately sigmo idal, and a T-jump at any temperature induced a more complex tension t ransient than in rigor fibers. An initial (small amplitude) step decre ase in tension followed by a rapid recovery (tau(1); see Davis and Har rington, 1993) was seen in some records from both fiber types, which p resumably was an indirect consequence of thermal expansion, The net ri se in tension after a T-jump was biexponential, and its time course wa s characteristically different in the two fibers, At similar to 12 deg rees C the reciprocal time constants for the two exponential component s (tau(2) and tau(3), respectively) were similar to 70 . s(-1) and sim ilar to 15 . s(-1) in fast fibers and similar to 20 . s(-1) and simila r to 3 . s(-1) in slow fibers, In both fibers, tau(2) (''endothermic f orce generation'') became faster with an increase in temperature, Furt hermore, tau(3) was temperature sensitive in slow fibers but not in fa st fibers. The results are compared and contrasted with previous findi ngs from T-jump experiments on fast fibers. It is observed that the fa st/slow fiber difference in the rate of endothermic force generation ( three- to fourfold) is considerably smaller than the reported differen ces in the ''phosphate release steps'' (>30-fold).