Chemical change and energy output during muscular contraction

Abstract

SUMMARY1. The production of heat and (internal) work and the changes in the amount of phosphocreatine (PCr), ATP, inorganic phosphate (Pi) and sometimes lactate have been measured from moment to moment during and after tetanic isometric contractions of isolated frog muscles at 0°C.2. Heat production was measured by thermopiles and a novel apparatus was employed for freezing the muscles rapidly at a chosen instant so as to halt the chemical processes before analysis.3. Using unpoisoned muscles in oxygen, it was shown that neither oxidative recovery processes nor glycolytic ones led to appreciable restitution of PCr or ATP during 15 see of contraction. However, clear signs of recovery processes could be seen within a minute. In our preparations artificial 'ageing' by storage at low temperature did not interfere with the capacity for glycolysis.4. Our clearest result was that the break-down of PCr was not nearly large enough to account for the rapid heat production during the first few sec of contraction. By the end of a 15 see tetanus as much as 10 mcal/g remained unaccounted for.5. The source of this heat is not clear. At no time is there any sign of net

Post-contractile phosphocreatine splitting in muscle as revealed by time-resolved 31P nuclear magnetic resonance.

“…6. The course of producing the delayed PCr breakdown is essentially similar to the course of the unexplained heat production (GILBERT et al, 1971;CURTIN and WOLEDGE, 1979). Several mechanisms could underlie the post-contractile ATP or PCr utilization.…”

“…One discrepancy (the unexplained heat) relates to the observation that the measured enthalpy production (heat+work) during contraction is considerably greater than that expected on the basis of the changes in the known chemical reactions and the best estimate of the molar enthalpies of these reactions WOLEDGE, 1978, 1979;HOMSHER and KEAN, 1978;GILBERT et al, 1971). The simplest hypothesis to explain this energy imbalance is that a hitherto unidentified exothermic reaction occurs during contraction.…”

Muscular contraction.

“…The energy which appears as heat is far from being accounted for by the measured changes in ATP and PCr concentrations in the muscle during the contraction (Kushmerick, Larson & Davies, 1969;Gilbert, Kretzschmar, Wilkie & Woledge, 1971). I The drastic fall in energy liberation when a muscle is stretched during contraction has been repeatedly confirmed both by myothermal methods (Abbott, Aubert & Hill, 1951;Abbott & Aubert, 1951;Hill & Howarth, A. F. HUXLEY 8 MUSCULAR CONTRACTION 1959;Wilkie, 1968) and by measurement of phosphorylcreatine utilization (Wilkie, 1968) and appearance of inorganic phosphate (Curtin & Davies, 1973).…”

Initial Heat Production in Isometric Frog Muscle at 15°C

“…That reaction was estimated to be endothermic under in vivo conditions, at 0°C with an enthalpic change of -3.7 kcal/mol (Woledge, 1970). Gilbert et al (1971) demonstrated a breakdown of PCr during relaxation in the range of 10 -7 to 10 -6 mol/ g after tetani at 0°C in frog muscle. The coupling of PCr utilization during relaxation to ATP formation via the Lohmann reaction would absorb about 1 mcal/g of heat.…”

“…The coupling of PCr utilization during relaxation to ATP formation via the Lohmann reaction would absorb about 1 mcal/g of heat. Gilbert et al (1971) show no increase in ATP in muscle after activity (in fact, they show a slight decrease), so the newly formed ATP must be consumed by reactions that are themselves endothermic or only slightly exothermic for a net cooling to be observed. Tension-time and feedback heats, thermoelastic heat, and degraded internal work all are very slight during late relaxation, and would therefore provide very little heat to cancel any endothermic processes.…”