Physiological doses of [Ile5]- and [Val5]-Angiotensin II fail to increase plasma catecholamines at the peak pressor response in rats.

Pressor responses to [Ile5]-Angiotensin II and [Val5]-Angiotensin II were measured following single intravenous doses of 0.0075, 0.075, 0.375, 0.75 and 7.5 micrograms/kg b.w. in anesthetised rats. [Val5]-Angiotensin II was significantly (10 per cent) more potent than [Ile5]-angiotensin II when log dose was plotted against the increase in carotid arterial pressure. Doses of 0.075 microgram/kg b.w. or more of each of the two peptides gave a full pressor response within 30 seconds. However, only at the highest and, probably, nonphysiological dose of [Ile5]-Angiotensin II (7.5 micrograms/kg b.w.) was there a significant increase in noradrenaline and adrenaline concentrations measured using a radioenzymatic assay. [Val5]-Angiotensin II was more potent than [Ile5]-Angiotensin II insofar as plasma noradrenaline increased significantly (p < 0.05) following doses of 0.75 and 7.5 micrograms/kg b.w.; adrenaline at the higher dose only. Plasma dopamine was unresponsive to both peptides.