High energy phosphate utilization for work production and tension maintenance in frog muscle

Frog (Rana esculenta, L.) gastrocnemii (161 pairs) or sartorii (8 pairs) were stimulated by intermittent tetani at 10 degrees C (20 Hz, supramaximal intensity, N2 atmosphere) isometrically (IM) or isotonically (IT) for various durations (6-30 s) at different tensions (0.05-1.00 Po = maximal IM tension at resting length, lo). The energy expenditure (E) was measured from ATP and phosphocreatine breakdown and the high energy phosphate equivalent of lactic acid production. Both in IM and IT conditions, E was found to be a linear function of the summated tetanus duration (t): E = a + b t, where b is the energy cost of tension maintenance. For the gastrocnemius, both in IM and IT, b was independent of the tension developed and equal to 0.45 mumol approximately P.g-1.s-1, whereas for the sartorius b was tension-dependent varying between 0.58 and 0.28 mumol approximately P.g-1.s-1 for Po and 0.18 Po, respectively. The constancy of the b value in muscles with pennate structure may be tentatively attributed, at least in part, to the greater internal energy dissipation, regardless of the tension developed. The term a of the above equation is due to all time-independent processes of muscle contraction, i.e.: (1) activation energy; (2) internal work and (3) external work (in IT only). Based on the measured value of a and on the work performed, the mechanical equivalent of approximately P splitting was calculated as 17.7 kJ.mol-1, a figure close to that previously obtained on frog sartorius (16.7 kJ.mol-1) and dog gastrocnemius (19.2 kJ.mol-1).(ABSTRACT TRUNCATED AT 250 WORDS)