AJP - Heart and Circulatory Physiology, Vol 263, Issue 4 1208-H1212, Copyright © 1992 by American Physiological Society
Muscarinic cholinergic receptors in canine adrenal gland
J. R. Tobin, M. J. Breslow and R. J. Traystman
Department of Anesthesiology and Critical Care Medicine, Johns Hopkins Medical Institutions, Baltimore, Maryland 21205.
Muscarinic cholinergic receptor stimulation has been reported to modulate
adrenal catecholamine and steroid secretion and influence medullary
vascular tone. The present study was undertaken to localize and
characterize muscarinic cholinergic receptor binding sites in canine
adrenal medulla and cortex. Binding studies using semipurified membranes
demonstrated specific, saturable binding of
L-[benzilic-4,4'-3H(N)]-quinuclidinyl benzilate ([3H]QNB) in medulla, with
a dissociation constant (KD) equal to 152 pM and estimated maximal binding
sites (Bmax) equal to 179 fmol/mg protein. Displacement of [3H]QNB by
cholinergic agents in order of decreasing potency was atropine,
pirenzepine, oxotremorine, pilocarpine, and carbamylcholine, suggesting the
receptor was M1 subtype, similar to sympathetic ganglion muscarinic
receptors. Autoradiographic imaging studies demonstrated highly specific,
homogeneous binding throughout the medulla. No specific binding was found
in adrenal cortex. These results suggest that muscarinic effects on blood
flow may be neurally mediated, rather than the result of stimulation of
vascular muscarinic receptors. In medulla, muscarinic receptors are evenly
distributed and modulate catecholamine secretion in all cells rather than
subpopulations of cells as in other species. Absence of [3H]QNB binding in
cortex argues against muscarinic receptor modulation of cortical steroid
secretion.
