VENTILATORY EFFECTS OF ANGIOTENSIN AND VASOPRESSIN IN CONSCIOUS RATS

Angiotensin II (ANG II) stimulates ventilation (V), when ventilatory b aroreceptor reflexes are taken into account, and arginine vasopressin (AVP) causes baroreflex inhibition of Bin conscious and anesthetized d ogs. To study mechanisms of hormonal modulation of V, a conscious rat model was investigated. V and metabolism were measured during steady-s tate intravenous infusions of ANG II and AVP in Sprague-Dawley rats (m ean arterial pressure (MAP) increased 20 mmHg (1 mmHg = 133.3 Pa)). Th ese data were compared with observations during equal pressor infusion s of phenylephrine (PE), an agent classically used to study barorecept or reflexes. V, respiratory frequency (f), and tidal volume (Vt) were maintained during the increased MAP associated with ANG II infusions, a response identical with that reported in conscious dogs. However, un like dogs, AVP infusion did not depress V and metabolism in rats. PE i n conscious rats caused an unexpected increase in Vt and V in associat ion with increased metabolism. None of the presser agents affected bre ath timing when the latter was binned by breath f. Since there was no obvious baroreflex inhibition of V with AVP and PE, potential stimulat ory effects of ANG II on respiration could not be discerned. As well, the ventilatory baroreceptor pressure threshold may be higher or adapt ation of the reflex may be faster in conscious rats than in dogs.