Carotid body I 1-imidazoline receptors: Binding, visualization and modulatory function

Abstract

The carotid body is influenced by many neurotransmitter receptors. A novel receptor specific for imidazolines has been implicated in cardiorespiratory regulation in the brain. To test for both I 1-imidazoline and α 2-adrenergic receptors, which also recognize imidazolines, specific [ 125I] p-iodoclonidine binding to carotid body membranes was characterized. The specific α 2-agents epinephrine (100 μM) or SK&F 86466 (10 μM) inhibited only a portion of specific [ 125I] p-iodoclonidine binding in both cat and rabbit carotid bodies, indicating the presence of I 1-imidazoline as well as α 2-adrenergic sites. The distribution of [ 125I] p-iodoclonidine binding sites was visualized autoradiographically. The cat carotid body was intensely labeled by [ 125I] p-iodoclonidine, with both I 1-imidazoline and α 2-adrenergic sites expressed. The relevance of I 1-imidazoline receptors in modulation of chemosensory discharge was determined in seven cats after α 2-adrenergic blockade. Clonidine (100 μg/kg) facilitated chemosensory activity particularly under hypoxia. We conclude that I 1-imidazoline receptors are expressed within the carotid body and may potentiate chemosensory discharge, in contrast to the inhibitory action of α 2-adrenergic receptors.