Central respiratory inhibition by angiotensin II in anesthetized cats

Observations were made on respiration, alveolar concentration of CO2, arterial blood pressure and heart rate in barbiturate-anesthetized cats subjected routinely to section of the vagus and carotid sinus nerves and maintained with pure O2 for inhalation. Bolus i.v. injection of angiotensin II (AII), 0.6 to 6.0 micrograms/kg, evoked a prompt shortlasting suppression of breathing, manifested mainly as a reduction in tidal volume susceptible to the development of tachyphylaxis if injections were spaced more closely than 30 min apart. Ablation of the area postrema failed to eliminate the respiratory effect of All. The response also persisted after midbrain transection, spinal cord transection at C8, dorsal rhizotomy of spinal segments C1 to C8 and interruption of cranial nerves V, VII, VIII, IX, X, XI and XII. No influence of All was evident on indirectly evoked tibialis muscle contractions and on pulmonary compliance. A reduction in phrenic nerve impulse traffic coincided with the respiratory response to All. Cranial i.a. injection of All initiated the respiratory effect in its first pass through the brain, before any change occurred in the blood pressure. The degree of All-induced respiratory inhibition was relatively constant at different magnitudes of breathing produced by CO2 inhalation or by electrical stimulation of the respiratory center in the medulla oblongata. It is concluded that All given by bolus intravascular injection causes respiratory inhibition through a direct central action at the final processing step in the CO2-tidal volume controller.