Effect of duration of stimulation and pH on energetics of rabbit rectococcygeus muscle.

The mechanical and energetic performance of the rabbit rectococcygeus preparation have been studied at 27 degrees C. Energy flux has been measured either myothermically (heat production) or by recording the O2 consumption and lactate production of the muscle. The maximal active-to-basal O2 consumption ratio was 2.42. The peak oxidative energy flux was 3.5 mW/g, and the peak initial rate was about 5.0 mW/g. We could find no evidence for an increment in lactate production associated with mechanical activity. The effects of increasing the stimulus duration on the "economy" of force maintenance was assessed by plotting the heat-to-tension ratio against duration. For stimulus periods up to 20 s there was no evidence of a change in the economy: the slope of the relationship was similar in myothermic and O2 consumption experiments with split muscles. When total heat production was plotted against the stress-time integral for stimulus periods up to 60 s at both pH 7.4 and 6.7, there was no change in energy usage in the longer duration tetani (2-60 s). However, the reduction in pH, which had minimal mechanical effects other than prolonging relaxation, did slow the rate of evolution of recovery heat (time constant doubled) and reduced the rate of O2 consumption. It is concluded that the rectococcygeus preparation cannot make use of the energy-saving mechanisms that operate in arterial and several invertebrate smooth muscles.