Thyroid status and muscarinic receptor density and affinity in rat intestinal smooth muscle

Hypothyroid and hyperthyroid states are associated with constipation and diarrhea, respectively. To determine the possible involvement of the cholinergic system in these disorders we studied the effect of thyroid status (thyroidectomy or thyroxin injection for 10-20 weeks) on intestinal smooth muscle muscarinic receptors. Receptor density and affinity were determined in the ileum and colon with [3H]-quinuclidinyl benzilate (QNB), and compared with the responsiveness of isolated muscle strips to cumulative doses of bethanechol. Neither the affinity nor the density of muscarinic receptors was significantly altered by chronic hypo- or hyperthyroidism: KD approximately 1nM; Bmax approximately 200 fmole/mg protein. Similarly, there was no change in functional response of the ileum or colon to cholinergic stimulation. The results suggest that chronic hypothyroidism or hyperthyroidism do not alter muscarinic receptors in rat ileal or colonic muscle.