Mechanisms of respiratory response to isoproterenol in glomectomized cats

Effects of intravenous isoproterenol (2-3 micrograms) on arterial pressure, end-tidal CO2 partial pressure (PCO2), medullary extracellular fluid (ECF) pH, and phrenic activity were studied in 13 anesthetized paralyzed cats whose vagi and carotid sinus nerves were cut. The cats were servo-ventilated to keep PCO2 relatively constant. Injections of Ringer solution were without effect. Isoproterenol caused arterial pressure to fall, a transient small (1 Torr) increase of PCO2, increased venous CO2 return to the lungs, a medullary ECF acidosis, and a stimulation of respiration that continued to be elevated after arterial pressure, PCO2, and medullary ECF pH had returned to control. We show that the ECF acidosis is minimally due to the hypotension and to the small transient rise of PCO2. We also show that the respiratory response cannot be explained solely by the ECF acidosis. We conclude that, in addition to its known stimulation of peripheral chemoreceptors, isoproterenol causes medullary ECF to become acidic probably due to metabolic effects on neural tissue and has a separate direct stimulating effect on neurons in the brain.