Endothermic force generation in skinned cardiac muscle from rat.
Isometric tension responses to rapid temperature jumps (T-jumps) of 2-6 degrees C were examined in skinned muscle fibre bundles isolated from papillary muscles of the rat heart. T-jumps were induced by an infra-red laser pulse (wave length 1.32 microm, pulse duration 0.2 ms) obtained from a Nd-YAG laser, which heated the fibres and bathing buffer solution in a 50 microl trough; the increased temperature by laser pulse was clamped at the high temperature by a Peltier system (see Ranatunga, 1996). In maximally Ca2+ -activated (pCa ca. 4.5) fibres, the relationship between tension and temperature was non-linear, the increase of active tension with temperature being more pronounced at lower temperatures (below ca. 20 degrees C). A T-jump at any temperature (range 3-35 degrees C) induced an initial step decrease of tension of variable amplitude (Phase 1), probably due to thermal expansion, and it was followed by a tension transient which resulted in a net rise of tension above the pre-T-jump level. The rate of net rise of tension (Phase 2b or endothermic force generation) was 7-10/s at ca. 12 degrees C and its Q10 was 6.3 (below 25 degrees C). In cases where the step decrease of tension in Phase 1 was prominent, an initial quick tension recovery phase (Phase 2a, 70-100/s at 12 degrees C) that did not contribute to a rise of tension above the pre-T-jump level, was also seen. This phase (Phase 2a) appeared to be similar to the quick tension recovery induced by a small length release and its rate increased with temperature with a Q10 of 1.8. In some cases where Phase 2a was present, a slower tension rise (Phase 3) was seen; its rate (ca. 5/s) was temperature-insensitive. The results show that the rate of endothermic force generation in cardiac fibres is clearly different from that of either fast-twitch or slow-twitch mammalian skeletal muscle fibres; implication of such fibre type-specific differences is discussed in relation to the difficulty in identifying the biochemical step underlying endothermic cross-bridge force generation.
Ranatunga
K W
KW
Department of Physiology, School of Medical Sciences, University of Bristol, UK. k.w.Ranatunga@bristol.ac.uk
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001831:Body Temperature; D002118:Calcium / Q000494:pharmacology; D004734:Energy Metabolism / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000502:physiology*; D010210:Papillary Muscles / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
D000255:Adenosine Triphosphate; D002118:Calcium