A temporal dissociation of energy liberation and high energy phosphate splitting during shortening in frog skeletal muscles.

Abstract

A B S T R A C T Measurements of the time course of high energy phosphate splittingand energy liberation were performed on rapidly shortening Rana pipiens skeletal muscles. In muscles contracting 30 times against small loads (<0.02 Po), the ratio of explained heat + work (H + W) (calculated from the measured high energy phosphate splitting) to observed H + W (from myothermal and mechanical measurements) was 0.68 ~ 0.08 and is in agreement with results obtained in isometric tetani of R. pipiens skeletal muscle. In lightly afterloaded muscles which were tetanized for 0.6 s and whose metabolism was arrested at 3.0 s after the beginning of stimulation, a similar ratio of explained H + W to observed H + W was obtained. However, in identical contractions in which metabolism was arrested at 0.5-0.75 s after the beginning of stimulation, the ratio of explained H + W to observed H + W declined significantly to values ranging from 0.15 to 0.40. These results suggest that rapid shortening at the beginning of contraction induces a delay between energy production and measurable high energy phosphate splitting. This interpretation was tested and confirmed in experiments in which one muscle of a pair contracted isometrically while the other contracted against a small afterioad. The afterload and stimulus pattern were arranged so that at the time metabolism was arrested, 0.5 s after the beginning of stimulation, the total energy production by both muscles was the same. Chemical analysis revealed that the isotonically contracting muscle split only 25% as much high energy phosphate as did the isometrically contracting muscle.Identification of the chemical sources of the energy liberated during contraction is important in understanding the molecular mechanism of muscle contraction. In frog muscle contracting at 0°C, the primary energy-yielding reaction is the splitting of phosphocreatine (PC), which occurs subsequent to ATP hydrolysis (Carlson et al., 1963(Carlson et al., , 1967Marechal and Mommaerts, 1963;Wilkie, 1968 duced in proportion to PC hydrolysis and that the ratio of H + W to APC is approximately -11 kcal/mol (Carlson et al., 1963(Carlson et al., , 1967Wilkie, 1968). This result is independent of the type of contraction-relaxation cycle. Isometric and isotonic twitches and tetani lead to the same conclusion. Since the molar enthalpy change, M-/, for PC splitting is -8.1 kcal/mol (Woledge, 1972), the above experiments suggest that only a fraction, approximately 0.74, of the observed H + W during contraction can be explained by PC splitting. This fraction is called the explained enthalpy fraction.To further elucidate the relation of metabolism to energy production it is necessary to extend the above experiments to the time course of single isometric and isotonic contractions. As a muscle presumably returns to its precontraction state after a contraction-relaxation cycle, the above results do not imply that during a single cycle the same relation of energy liberation to metabolism will be observed. Homsher et al. (1975) have...

High‐energy phosphate metabolism and energy liberation associated with rapid shortening in frog skeletal muscle

“…The idea that less enthalpy might be produced in the post-shortening period than could be explained by the simultaneous chemical reactions was predicted by Davies (1963) and received support from the results of Rall et al (1976). This result has now been demonstrated directly.…”

“…The present design has another advantage. The heat production in excess of that expected from PCr splitting which has been observed during rapid shortening (Rall et al 1976) occurred very early in the tetanus, when a similar 'unexplained' heat production is seen in isometric tetani (Gilbert, Kretzschmar, Wilkie & Woledge, 1971;Curtin & Woledge, 1979;Homsher, Kean, Wallner & Garibian-Sarian, 1979). This ambiguity may be removed by delaying the start of shortening until after 2 sec of isometric contraction, since heat plus work production 424 ENERGETICS OF MUSCLE SHORTENING from sources other than PCr splitting is insignificant after this time in R. pipiens muscles (as calculated by Curtin & Woledge (1978) from the results of Homsher, Rall, Wallner & Ricchiuti (1975) (Rall et al 1976).…”

“…In direct comparisons between rapidly shortening and isometric contractions made by Kushmerick, Larson & Davies (1969) and Rall, Homsher, Wallner & Mommaerts (1976) both groups found that the extent ofPCr splitting in a brieflightly after-loaded contraction was less than that in an isometric contraction. In addition, Rall et al confirmed directly the conclusion of Kushmerick et al, that there is much more heat production during such shortening than can be accounted for by simultaneous PCr splitting.…”

Dependence of shortening heat on sarcomere length in frog muscle and fiber bundles.

“…The present study indicated that shortening heat represents the property of the cross-bridges. It has been shown, however, that the shortening heat of rapidly shortening muscles is not accompanied by high energy phosphate splitting (KUSHMERICK et al, 1969;RALL et al, 1976;HOMSHER et al, 1981). Thus shortening heat cannot originate simply from a higher turnover rate of cross-bridges.…”

“…Shortening heat presumably represents extra chemical reactions caused by the greater rate of cross-bridge turnover during shortening. It has been shown, however, that shortening heat in rapidly shortening muscles is not accompanied by simultaneous high energy phosphate splitting (KUSHMERICK et al,1969;RALL et al,1976;HOMSHER et a!.,1981). HoMSHER et al (1981) have shown that the unexplained enthalpy during rapid shortening is reversed in a post-shortening period, at the same time being accompanied by enthalpy production in excess of that in muscles without shortening.…”