Cardiovascular effects of angiotensin-II in chickens

1. Mature WL cockerels with permanent cannulae in brachial artery and vein were restrained in an isolated sling. Blood pressure (BP) and heart rate (HR) were continuously recorded. When the chickens were habituated to the sling, injections began. In each experiment the cockerels were injected intravenously 6 times at 6 min intervals. 2. In the first experiment 6 injections of 0.5 nmol[Aspl, Val5]ANG-II/kg body weight were given. 3. In the second experiment oxytocin (OT) antagonist ([d(CH2)5-O-Me-Tyr2,Thr4,Tyr9,Orn8]VT) at a dose of 2 nmol/kg, was injected for the first 3 and 0.5 nmol ANG-II/kg for the last 3 injections. Such OT-antagonist pretreatment completely abolishes the vasodepressor (VDP) response to neurohypophysial peptides in chickens. 4. Injections of ANG-II resulted in a biphasic effect on BP, an initial brief fall followed by a prolonged rise. During the hypotensive phase, tachycardia developed which turned into bradycardia as the hypertensive phase appeared. No tachyphylaxis of the VDP effect of ANG-II was evident with repeated injections. 5. OT-antagonist pretreatment had no effect on the VDP response to ANG-II. 6. These results suggest that, unlike relaxation of chicken aortic ring in in vitro preparations, there is no tachyphylaxis of the VDP response to ANG-II, in vivo. Furthermore, the neurohypophysial peptides are not involved in the VDP effect of ANG-II because pretreatment with an OT-antagonist had no effect on it. The baroreflex buffers the effects of ANG-II on vascular tone by affecting HR. 7. As ANG-II is secreted during hypovolaemia, the biphasic haemodynamic response peptides may have a compensatory role following volume contraction.