Relation of NADH/NAD to contraction in vascular smooth muscle

The relationship of NADH/NAD to O2 consumption with respect to the different phases of contraction in vascular smooth muscle in response to a maximal depolarizing concentration of KCl was investigated. The NADH bound to cellular proteins could be distinguished from free NADH in whole tissue homogenates. Evidence suggested that the NADH was bound to pyruvate dehydrogenase and perhaps to other dehydrogenases since binding paralleled the changes in the activity of pyruvate dehydrogenase with contraction. The measured changes in NADH were attributed to that within the mitochondrial compartment since the contribution of reducing equivalents within the cytoplasmic compartment was negligible. During the phase of contraction in which force was initially being generated and at which O2 consumption was the highest, there was a net increase in NADH/NAD. After stable isometric force was maintained, at which time O2 consumption had returned to slightly above the basal pre-contraction level, there was a net decrease in NADH/NAD. Previous evidence indicates the phosphorylation potential (ATP/ADP) may decrease during this phase of contraction. It is concluded that contraction of vascular smooth muscle is accompanied by a changing pool of reducing equivalents. Factors which govern O2 consumption may change during the different phases of muscle contraction.