Inhibitory effects of diltiazem on vasoconstrictor responses in the cat

The effect of diltiazem on vasoconstrictor responses was investigated in the feline mesenteric vascular bed under conditions of controlled blood flow. Diltiazem inhibited vasoconstrictor responses to sympathetic nerve stimulation, tyramine and norepinephrine suggesting that responses to both nerve-released and exogenous norepinephrine are dependent in part on an extracellular source of calcium. The calcium entry antagonist inhibited vasoconstrictor responses to alpha 1 and to alpha 2 adrenoceptor agonists over a wide range of concentration. Diltiazem also inhibited mesenteric vasoconstrictor responses to angiotensin II, vasopressin, prostaglandin F2 alpha and KCl. The inhibitor effects of diltiazem on vasoconstrictor responses to nerve stimulation and the pressor agents were reversible, and all responses returned to control value 30 to 45 min after the infusion of the calcium entry antagonist. The present data suggest that the inhibitory effects of diltiazem on responses to sympathetic nerve stimulation are postjunctional in nature, as responses to nerve-released and exogenous norepinephrine and nonadrenergic pressor agents are reduced to a similar extent. The present results suggest that vasoconstrictor responses to neuronally released and exogenous norepinephrine, as well as agents which activate membrane receptors or depolarize vascular smooth muscle in the feline mesenteric vascular bed, are dependent in part on an extracellular source of calcium. The inhibitory effects of diltiazem on vasoconstrictor responses to sympathetic nerve stimulation and pressor hormones may be relevant to the antihypertensive actions of this calcium entry antagonist.