Effects of exercise metabolites on adrenergic vasoconstriction in the gracilis muscle of the dog.

Comparable and submaximal responses to sympathetic stimulation and to intra-arterial injections and infusions of norepinephrine were elicited in the isolated dog gracilis muscle, perfused with blood at constantflow. The changes in potassium concentration, osmolality, Po2 and inorganic phosphate concentration were determined during exercise. Blood was perfused to the muscle with the concentrations of each of these parameters varied to include that seen in exercise. Hypoxia (average Po2 5.4 mm Hg) produced a marked vasodilatation, and responses to norepinephrine and sympathetic stimulation were both increased to the same extent, due to the lowered vascular tone. Increases in osmolality (up to 50 mOsmol/kg) caused little antagonism of adrenergic responses, but higher increases caused a significant and equal inhibition of responses to norepinephrine and sympathetic stimulation. Increases in phosphate concentration (plus 41.6 mM) had little effect either on muscle resistance or on adrenergic responses. Increases in potassium concentration (plus10 mM) completely abolished responses to sympathetic stimulation, whereas those to norepinephrine were unaffected. This prejunctional effect may result from inhibition of sympathetic neurotransmission due to depolarization of adrenergic nerve terminals.