Cardiovascular responses in vivo to angiotensin II and the peptide antagonist saralasin in rainbow trout Oncorhynchus mykiss.

The effects of [Asn1,Val5]-angiotensin II (AngII) and [Sar1,Val5, Ala8]-angiotensin II (saralasin) on dorsal aortic blood pressure, pulse pressure and heart rate were examined in rainbow trout in vivo. AngII when administered as a single dose of 25 microg kg-1 induced a biphasic response in blood pressure, with a significant hypertensive response during the initial 10 min, followed by a significant hypotension of 70-75 % compared with the initial blood pressure after 50 min and continuing until approximately 80 min post-injection. The co-administration of AngII (25 microg kg-1) and saralasin (50 microg kg-1) resulted in the same hypertensive response during the initial phase, but abolished the hypotensive effect of AngII. Heart rate was significantly increased in response to AngII, but the administration of AngII and saralasin together attenuated the increase by approximately 44 %. Stimulation of the endogenous renin-angiotensin system using a vasodilator, sodium nitroprusside, significantly increased drinking rate in rainbow trout fry, a response inhibited by saralasin, indicating a role for AngII-induced hypotension in drinking. For the first time, a decrease in blood pressure in response to AngII in vivo has been demonstrated in fish, and this is discussed in relation to homeostasis of blood pressure and a possible role in the control of drinking.