Regulation by isoproterenol of muscarinic acetylcholine receptor numbers and sensitivity in rat submandibular, but not lacrimal, glands

Abstract

Chronic administration of DL-isoproterenol, a β-adrenergic agonist, to male Sprague-Dawley rats increased submandibular gland weights by 3 to 4-fold. This increase resulted from a combination of hyperplasia and hypertrophy of secretory cells. Possible effects of this drug regimen on submandibular gland muscarinic acetylcholine receptors were examined by analysis of the binding of the cholinergic antagonist, L-quinuclidinyl [ 3H]benzilate, to receptors in gland homogenates. Parallel investigations of receptors in exorbital lacrimal glands, an organ that is not grossly affected by chronic isoproterenol treatment, were also carried out. [ 3H]QNB bound to submandibular receptors with a K d of 37.8±6.3 pM in control rats and 41.0±4.0 pM in isoproterenol-treated animals, a non-significant difference ( P > 0.05). In contrast, the maximal binding level ( B max) is isoproterenol-treated rats, 1.52±0.10 fmol/ μg DNA, was depressed by approx. 30% ( P<0.05) from that of 2.22±0.16 fmol/ μg DNA in control animals. In lacrimal glands, both K d (61.3±5.3 vs. 53.2±4.0 pM) and B max (1.74±0.24 vs. 1.78±0.17 fmol/ μg DNA) were unchanged by isoproterenol treatment. The affinity of glandular muscarinic receptors for cholinergic agonists was also examined by competition experiments using carbachol. This agonist inhibited [ 3H]QNB binding to receptors in homogenates from both glands in a dose-dependent fashion. Inhibition constant ( K i) for this interaction were similar in control and isoproterenol-treated lacrimal glands; 53.6±5.4 μM and 66.6±7.9 μM, respectively ( P>0.05). In submandibular glands, isoproterenol treatment elicited a highly significant ( P < 0.01) shift in K i from 17.3±1.4 μM to 68.3±5.2 μM. These results demonstrate that chronic administration of isoproterenol to rats results in a reduction in receptor numbers and a decrease in their sensitivity to cholinergic agonists in submandibular, but not lacrimal, glands.