Cardiovascular responses to neurohormones in conscious chickens and ducks

Cardiovascular responses (cardiac frequency, fH; mean arterial pressure, Pa; ischiatic arterial blood flow; Qi; ischiatic vascular resistance, Ri) to 1-norepinephrine (NE), acetylcholine (ACH), [Asp1, Val5]-angiotensin II (ANG II), and arginine vasotocin (AVT) were studied in conscious chickens (Gallus gallus) and Pekin ducks (Anas platyrhynchos). Age-dependent changes, in the cardiovascular variables and the responses to the native avian neurohormones, were also studied in ducks. NE injection caused larger increases in Pa and Ri, and a greater associated fall in fH, in ducks than in chickens. The pressor effect of ANG II was more persistent and developed at a slower rate than the response to NE in both species, although the pressor effect was greater in ducks. Interestingly, ANG II injection caused a tachycardia in these baroreceptor-intact birds. ACH and AVT produced similar rapid falls in Pa and a corresponding tachycardia in both species, although the fH response was greater in ducks. ANG II rapidly increased NE concentration in the arterial plasma of adult ducks, while AVT increased epinephrine (E) concentration. Resting Pa and hematocrit were lower, and fH was higher, in immature ducks. Immature ducks also were less responsive than adults to the cardiovascular actions of NE, ACH, AVT, and ANG II. The results demonstrate differences in the cardiovascular responses to neurohormones in chickens and ducks consistent with a higher level of cardioinhibitory nervous tone and a greater sensitivity to sympathetic stimuli in the aquatic species, which increase during maturation.