Analysis of inhibitory and excitatory actions of dopamine on chemoreceptor discharges of carotid body of cat in vivo.

Chemoreceptor discharges were recorded from the carotid sinus nerve of the cat in vivo, and their frequency was used as an index of receptor activity. The effects of dopamine on chemoreceptor activity were analyzed in normal and ischemic carotid bodies. Intra-arterial injections of dopamine (DA) induced various patterns of chemoreceptor responses; simple inhibition, inhibition followed by excitation, and simple excitation, depending upon doses and the time interval between two injections. After a large dose of DA, a previous inhibitory response to DA was converted into an excitatory one. Pretreatment of the animal with reserpine (0.1 mg/kg, i.v.) or 5-hydroxydopamine (5 mg/kg, i.v.) also converted the inhibitory response to DA into the excitatory one. After haloperidol (0.1 mg/kg, i.v.), the inhibitory response to DA was completely blocked, and DA induced a dose-dependent increase in chemoreceptor discharges. After 1-hr ischemia of the carotid body, DA induced only the inhibitory response, which was blocked by haloperidol, but did not produce any excitatory responses. Results indicate that DA exerts a self-blocking action on inhibitory dopaminergic receptors which are possibly located on the nerve ending, and that DA, also acting directly on the glomus cell, would produce the chemoreceptor excitation.