In vivo bronchodilator activity of vasoactive intestinal peptide in the cat

Vasoactive intestinal peptide (VIP) is an octacosapeptide that occurs widely in the animal kingdom, is distributed in many organs and tissues, and has a broad range of biologic actions. We evaluated the ability of VIP to produce bronchodilation in anesthetized, mechanically ventilated, atropinized, closed chest cats. Boluses of VIP injected intravenously (0.1 to 10 micrograms X kg-1) reversed in a dose-dependent manner the increase in lung resistance (RL) and the decrease in dynamic lung compliance (Cdyn) induced by an intravenously administered infusion of 5-hydroxytryptamine (5HT) (5 to 30 micrograms X kg-1 X min-1). Distribution of bronchodilator activity was assessed by using RL as an index of central airways caliber and dynamic elastance (Edyn; the inverse of Cdyn) as an index of tone in peripheral airways and lung parenchyma. Although all levels of the tracheobronchial tree responded to VIP, the predominant site of action appeared to be in central airways. A similar distribution and magnitude of relaxant effects were observed after the intravenous administration of prostaglandin E2 (PGE2) (0.1 to 10 micrograms X kg-1). Prostaglandin E2 also was effective in reversing 5HT-induced bronchoconstriction when given by ultrasonic nebulization, whereas VIP was not. Bronchodilation mediated by VIP persisted after pretreatment with indomethacin or propranolol. We conclude that VIP is a potent relaxant of feline airways smooth muscle in vivo and that this effect of the peptide occurs independent of prostaglandin production or beta adrenergic receptor activation.