Impaired M ₃ and enhanced M ₂ muscarinic receptor contractile function in a streptozotocin model of mouse diabetic urinary bladder

Abstract

We investigated the contractile roles of M ₂ and M ₃ muscarinic receptors in urinary bladder from streptozotocin-treated mice. Wild-type and M ₂ muscarinic receptor knockout (M ₂ KO) mice were given a single injection of vehicle or streptozotocin (125 mg kg ⁻¹) 2–24 weeks prior to bladder assays. The effect of forskolin on contractions elicited to the muscarinic agonist, oxotremorine-M, was measured in isolated urinary bladder (intact or denuded of urothelium). Denuded urinary bladder from vehicle-treated wild-type and M ₂ KO mice exhibited similar contractile responses to oxotremorine-M, when contraction was normalized relative to that elicited by KCl (50 mM). Eight to 9 weeks after streptozotocin treatment, the EC ₅₀ value of oxotremorine-M increased 3.1-fold in urinary bladder from the M ₂ KO mouse ( N = 5) compared to wild type ( N = 6; P < 0.001). Analogous changes were observed in intact bladder. In denuded urinary bladder from vehicle-treated mice, forskolin (5 µM) caused a much greater inhibition of contraction in M ₂ KO bladder compared to wild type. Following streptozotocin treatment, this forskolin effect increased 1.6-fold ( P = 0.032). At the 20- to 24-week time point, the forskolin effect increased 1.7-fold for denuded as well as intact bladders ( P = 0.036, 0.01, respectively). Although streptozotocin treatment inhibits M ₃ receptor-mediated contraction in denuded urinary bladder, muscarinic contractile function is maintained in wild-type bladder by enhanced M ₂ contractile function. M ₂ receptor activation opposes forskolin-induced relaxation of the urinary bladder, and this M ₂ function is enhanced following streptozotocin treatment.