Augmentation of carotid body chemoreceptor responses by isoproterenol in the cat

The effects of intravenous injections of isoproterenol (0.5-2 microgram) on the responses of carotid body chemoreceptor afferents and on integrated phrenic activity were investigated in twelve anesthetized and three decerebrate, unanesthetized cats. All animals were paralyzed and artificially ventilated. Isoproterenol stimulated carotid chemoreceptor activity and this stimulation was augmented by both hypoxia and hypercapnia. Following an injection of isoproterenol, the ratio of the minute phrenic activity relative to mean carotid chemoreceptor activity was increased. Thus, the stimulation of inspiratory phrenic output exceeded the stimulation of the chemoreceptor afferent input, and the peripheral chemoreflex activity does not account for the entire ventilatory response. To distinguish between a direct effect of isoproterenol and a possible secondary effect mediated via an increased venous return and an increased PaCO2, the latencies of the response of carotid chemoreceptors to both isoproterenol and hypercapnia were compared before and after carbonic anhydrase inhibition by acetazolamide. After acetazolamide, the latency of the response to hypercapnia increased from 3.5 sec to 8 sec whereas the latency of response to isoproterenol increased less, from 4.7 sec to 6.3 sec. Thus, isoproterenol stimulation was not mediated by CO2-H+. Propanolol, which blocked the systemic vascular effect, only partially blocked the chemoreceptor stimulation caused by isoproterenol, indicting that the effect of isoproterenol on chemoreceptor activity was not due to systemic cardiovascular changes.