Respiratory effects of pressor and depressor agents in conscious rats

We hypothesized that the respiratory baroreflex in conscious rats is either more transient, or has a higher pressure threshold than in other species. To characterize the effect of arterial pressure changes on respiration in c onscious rats, ventilation ((V) over dot) was measured by the plethysmograp hic technique during injections, or infusions, of presser and depressor age nts. Bolus injections of angiotensin II (Ang II) or arginine vasopressin (A VP), transiently increased mean arterial pressure (MAP; mean +/- SE) 43 +/- 6 and 28 +/- 5 mmHg (1 mmHg = 133.3 Pa), respectively, and immediately red uced tidal volume (Vt) and, in the case of AVP, (V) over dot. In contrast, by 10 min of a sustained elevation of MAP (40 +/- 3 mmHg) with infusion of Ang II, Vt, f, and (V) over dot were not different from control levels. Bol us injection of sodium nitroprusside (SNP) to lower MAP (-28 +/- 3 mmHg) im mediately increased breathing frequency (f) and (V) over dot, whereas susta ined infusion of SNP to lower MAP (-21 +/- 3 mmHg) did not change f or (V) over dot at 10 and 20 min. In conscious rats, both injection and infusion o f the presser agent PE (+40 to 50 mmHg) stimulated f and (V) over dot; this contrasted with anesthetized rats where PE inhibited f and (V) over dot, a s reported by others. In conscious rats, respiratory responses associated w ith baroreflexes adapt rapidly and, in the case of PE, can be overridden by some other mechanism.