Physiological determinants of Qmax in contracting canine skeletal muscle in situ.

Canine skeletal muscle preparations in situ contain varying mixtures of fast and slow fatigue resistant fibers. All vessels on the arterial side of the vascular bed in these preparations from the conduit arteries to the arterioles contribute to the vascular resistance. The vascular beds autoregulate at rest and during contractions, indicating that the effects of the surgery are minimal. Stimulation frequency and pattern, i.e., twitch or brief tetanic contractions, the force exerted, and the length at which the muscle contracts, combine to determine the metabolic rate, which in turn, establishes the blood flow at a given perfusion pressure. When blood flow is normalized for oxygen uptake, the ratio is similar for twitch and rhythmic brief tetanic contractions. The ratio stays constant over a wide range of frequencies even though the flow increases and is greater than the theoretical minimum set by the maximum arteriovenous oxygen concentration difference. The infusion of vasodilators such as adenosine results in higher flows than those obtained with 6 Hz stimulation alone, indicating the presence of a "pharmacological reserve". The peak blood flow measured with isometric twitch contractions at 6 Hz in constant pressure pump perfused muscles at the optimum length for tension development averages 19.7 +/- 1.1 ml.kg-1.s-1. The pharmacological maximum at 6 Hz averages 25.9 +/- 1.6 ml.kg-1.s-1 with an observed maximum of 33.2 ml.kg-1.s-1.