Differential effects of PACAP and VIP on the pulmonary and hindquarters vascular beds of the cat

Responses to pituitary adenylate cyclase-activating polypeptide (PACAP), a novel peptide derived from ovine hypothalamus with 68% sequence homology with vasoactive intestinal polypeptide (VIP), were investigated in the pulmonary and hindquarters vascular beds of the anesthetized cat under conditions of controlled blood flow. Injection of the peptide into the perfused lung lobe under elevated tone conditions produced dose-dependent decreases in lobar arterial pressure that were accompanied by biphasic changes in systemic arterial pressure characterized by an initial decrease followed by a secondary increase in pressure. When compared with other vasodilator agents in the pulmonary vascular bed, the relative order of potency was isoproterenol > PACAP > acetylcholine > calcitonin gene-related peptide > VIP. In the hindquarters vascular bed, intra-arterial injections of PACAP produced biphasic changes in hindquarters perfusion pressure characterized by initial decreases followed by secondary increases, which were accompanied by biphasic changes in systemic arterial pressure. In terms of relative vasodilator activity in the hindlimb, the order of relative potency was isoproterenol > acetylcholine > calcitonin gene-related peptide > VIP > PACAP. PACAP was the only agent that caused a secondary vasoconstrictor response in the hindlimb and produced biphasic changes in systemic arterial pressure. D-Phe²-VIP, a VIP receptor antagonist, blocked the hindquarters vasodilation in response to VIP but had no effect on responses to PACAP. The present investigation shows that PACAP produces pulmonary vasodilation, as well as dilation, and vasoconstriction in the systemic (hindlimb) vascular bed. These data demonstrate that responses to PACAP and VIP are different and suggest that vascular responses induced by the two peptides in the cat may, in part, be mediated by different receptors.