Biochemical, autoradiographic and functional studies on a unique glutamate binding site in adrenal gland

L-Glutamate is known to function as a major excitatory neurotransmitter in the mammalian central nervous system, and recent reports suggest the existence of receptors for glutamate in several peripheral tissues. In the present study, the characteristics of the binding of [3H]L-glutamate to sections of bovine adrenal gland were studied, and the localisation of this binding was investigated in adrenal glands from cow, dog, rat and guinea pig. In addition, the effects of glutamate on catecholamine release from the perfused isolated bovine adrenal gland were investigated. Binding of [3H]L-glutamate to slide-mounted sections of bovine adrenal gland was of high affinity (Kd 0.4 microM), rapid, saturable, reversible, stereospecific and to a single population of sites. The pharmacological profile of this binding site appeared to be unique, and did not correspond to any of the central receptor subtypes for glutamate so far identified. In the adrenal gland of the cow, rat and guinea pig, the binding density of [3H]L-glutamate was higher in cortex than medulla, while this pattern was reversed in the canine adrenal gland. Glutamate had no effect on the basal secretion of noradrenaline or adrenaline from the perfused isolated bovine adrenal gland, and neither glutamate nor the glutamate receptor antagonist kynurenate altered the nicotine-stimulated release of these catecholamines. These results suggest the existence of a novel peripheral binding site for glutamate in the adrenal gland. The differential autoradiographic localisation of this binding site in the adrenal glands of the various species studied may reflect different functional properties of glutamate in these species, and suggests possible roles for glutamate in the modulation of adrenal function.