Analysis of responses to angiotensin IV in the pulmonary vascular bed of the cat.

Responses to angiotensin IV were investigated and compared with responses to angiotensin II in the pulmonary vascular bed of the intact-chest cat under constant flow conditions. Under low resting tone conditions, intralobar injections of angiotensin IV caused dose-related increases in lobar arterial pressure. Angiotensin II also increased lobar arterial pressure and was 100 fold more potent than angiotensin IV. Dose-response curves for both peptides were parallel, and the time-to-peak increase in lobar arterial pressure in response to angiotensin IV and angiotensin II was similar whereas the duration of the response to angiotensin IV was significantly shorter. Following administration of the angiotensin receptor subtype 1 (AT1) antagonist, DuP 532 (2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)biphe nyl-4- yl)-methyl]imidazole), responses to angiotensin IV and angiotensin II were reduced in a similar manner, whereas pulmonary pressor responses to serotonin were not altered. In contrast to the inhibitory effects of the angiotensin AT1 receptor antagonist, administration of PD 123,319 ((S)-1-[[4-(dimethylamino)-3-methyl-phenyl]methyl-5-(diphenylacetyl++ +)- 4,5,6,7-tetrahydro-1H-imidazol[4,5-c]pyridine-6-carboxylic acid), an angiotensin AT2 receptor antagonist, did not change responses to angiotensin II and angiotensin IV. The results of the present study demonstrate that angiotensin IV has significant vasoconstrictor activity in the pulmonary vascular bed, and suggest that pressor responses to angiotensin IV are mediated by the activation of angiotensin AT1 receptors. These data indicate that angiotensin IV is 100-fold less potent than angiotensin II and suggest that the hexapeptide may have a lower apparent affinity for the angiotensin AT1 receptor than does angiotensin II in the pulmonary vascular bed of the cat.