Pulmonary vascular effects of vasoactive intestinal peptide in conscious newborn lambs

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Abstract
Vasoactive intestinal peptide (VIP) may be a neutrotransmitter in a peptidergic nervous system and is found in nerves of pulmonary blood vessels. Information regarding its pulmonary vascular effects is limited. VIP effect on pulmonary vascular tone was studied in the immature lamb lung. Normoxic and hypoxic unsedated newborn lambs with chronically implanted flow probes around the right and left pulmonary arteries were used. VIP was injected into 1 pulmonary artery only and direct effects of this peptide on the pulmonary vessels were determined by comparing the flow changes in the injected vs. the uninjected lung. VIP was a powerful pulmonary vasodilator with a threshold of 0.3 .mu.g/kg. It also was a systemic vasodilator (after 1 .mu.g/kg, aortic pressure fell 27% and cardiac output increased 29%, both P L 0.01), with a threshold of 0.1 .mu.g/kg. Pretreatment with propranolol (1 mg/kg i.v.) did not abolish pulmonary or systemic vasodilation after VIP. Pretreatment with indomethacin (3 mg/kg per day for 3 days) abolished VIP-induced pulmonary vasodilation but probably did not affect systemic vasodilation. VIP is apparently a powerful pulmonary vasodilator in the newborn lamb. This dilation can be blocked by the cyclooxygenase inhibitor indomethacin. VIP is also a powerful systemic vasodilator in the newborn lamb but this effect is not blocked by either propranolol or indomethacin.