Vascular smooth muscle energetics.

Abstract

ATP utilization (delta approximately P) during an isometric contraction has been studied in terms of both measurements of oxygen consumption and lactate production as well as of the tissue nucleotide and metabolite levels. The contribution of breakdown of preformed ATP and phosphocreatine (PCr) pools to delta approximately P during contraction is minor compared to that made by metabolic synthesis of ATP. For tonic vascular smooth muscle (VSM), in fact, no change in ATP or PCr from resting levels can be measured. In contrast to amphibian skeletal muscle, a P:O of 3 can be demonstrated in VSM. In both tonic and phasic VASM, delta approximately P is biphasic with contraction duration, attaining a maximal value before that of isometric force and declining to a steady-state value approximately 60% of the maximal suprabasal rate during the maintenance of constant isometric force. The steady-state rate of ATP utilization per unit force maintained increases with extracellular Ca2+. Both the pre-steady-state temporal dependence and the steady-state dependence on Ca2+ are consistent with the hypothesis that myosin phosphorylation modulates the cross-bridge cycle rates. VSM metabolism, when viewed in terms of ATP synthesis, is primarily oxidative. However, even under fully oxygenated conditions, lactate is the major end product of glucose catabolism. Recent work has shown that aerobic lactate production is specifically coupled to Na-K transport in many, but not all, vascular tissues. Oxidative metabolism, on the other hand, is strongly related to active isometric force. The biochemical basis of this functional compartmentation was investigated at the level of substrate specificity.(ABSTRACT TRUNCATED AT 250 WORDS)