Abstract

A muscarinic cholinergic effect on thyrotropin-stimulated cyclic AMP accumulation in cultured porcine thyroid cells is characterized. The muscarinic agonists carbachol, acetylcholine, oxotremorine, and pilocarpine decreased the acute cyclic AMP response to stimulation with thyrotropin (20 mU/ml). A 50% decrease was obtained in the presence of 0.1 mM carbachol. Maximal effects were observed when cells were cultured in the presence of thyrotropin (100 microU/ml) or prostaglandin E2 (1 microM), whereas cells cultured in basal medium or in the presence of dibutyryl cyclic AMP showed a low response to carbachol. Evidence is reported suggesting that carbachol acutely decreases cyclic AMP synthesis. The properties of the muscarinic receptors present in thyroid cells were defined by using the binding of the muscarinic antagonist [3H]quinuclidinyl benzilate to cell homogenates. Scatchard plots revealed a single population of binding sites with a KD of 0.3 nM. The numbers of binding sites per cell after 4 days in culture were 880 in control cells, 4335 in thyrotropin (100 microU/ml)-treated cells, 5600 in prostaglandin (1 microM)-treated cells, and 1420 in dibutyryl cyclic AMP-treated cells. Therefore the sensitivity to carbachol appeared to be related to the number of antagonist binding sites and to be independent of the sensitivity of the cells to acute thyrotropin stimulation.