Effect of beta-adrenergic agonists on apnea reflexes in newborn lambs

The laryngeal chemoreflex and the trigeminal diving reflex were studied in unanesthetized newborn lambs. Water stimulation of the laryngeal chemoreflex resulted in apnea, bradycardia, hypertension, and blood flow redistribution in the dive pattern. This response was significantly reduced after treatment with beta-adrenergic agonists, e.g., terbutaline. The response to laryngeal saline stimulation was not significantly altered by beta-adrenergic agonists. A similar response to trigeminal dive reflex stimulation elicited through cooling of the snout was also significantly reduced by terbutaline. Propranolol, a beta-adrenergic antagonist, reversed the terbutaline-induced effect on the laryngeal chemoreflex response. Stimulation of the superior laryngeal nerve resulted in a reflex response comparable to that from laryngeal water stimulation. The reflex response was also attenuated by terbutaline, which indicates that the action of terbutaline is not on the laryngeal chemoreceptors. A possible direct effect from beta-adrenergic agonists on the respiratory center is suggested by a latency of 15-30 min before the reflex response was reduced after intravenous but not intrathecal administration. An effect of terbutaline via the arterial chemoreceptors is also possible.