Effects of castration and diabetes mellitus on cholinergic responsiveness and muscarinic receptors in the rat vas deferens

Studies were carried out to compare the effects of diabetes mellitus and castration on the muscarinic responsiveness of rat vasa deferentia. One and two months after castration or after streptozotocin-induced diabetes, there were significant decreases in serum testosterone levels, accompanied by significant decreases in vas deferens weights, protein contents, and protein concentrations. However, contractile responses of vasa deferentia from streptozotocin-diabetic or castrated rats to carbachol were significantly increased compared to controls. These changes were accompanied by increases in Kd and decreases in Bmax (pm/tissue) for 3H-quinuclidinyl benzilate (QNB) binding. Contractile responses and QNB binding were also studied in the spontaneously diabetic BB/W or rat, a less severely diabetic model than the streptozotocin-diabetic rat. Ninety days after the onset of diabetes in the BB rat, there was an increase in the contractile response to carbachol. There were qualitatively similar but smaller changes in vas deferens weights, protein contents, protein concentrations, and QNB binding than in the streptozotocin-diabetic rats. The data show that diabetes and castration increased the contractile responses of vasa deferentia despite decreases in protein content and in the number of muscarinic receptors. The results suggest that muscarinic receptor-effector coupling is more efficient in vasa deferentia from castrated or diabetic rats, resulting in a greater contractile response to cholinergic agonists.