Muscarinic regulation of neonatal rat bladder spontaneous contractions

In vitro preparations of whole urinary bladders of neonatal rats exhibit prominent myogenic spontaneous contractions, the amplitude and frequency of which can be increased by muscarinic agonists. The muscarinic receptor subtype responsible for this facilitation was examined in the present experiments. Basal spontaneous contractions in bladders from 1- to 2-wk-old Sprague-Dawley rats were not affected by M₂ or M₃ receptor antagonists. However, administration of 0.5 μM physostigmine, an anticholinesterase agent that increases the levels of endogenous acetylcholine, or 50-100 nM carbachol, a cholinergic agonist at low concentrations, which did not cause tonic contractions, significantly augmented the frequency and amplitude of spontaneous contractions. Blockade of M₂ receptors with 0.1 μM AF-DX 116 or 1 μM methoctramine or blockade of M₃ receptors with 50 nM 4-diphenylacetoxy-N-methylpiperidine methiodide or 0.1 μM 4-diphenylacetoxy-N-(2-chloroethyl)piperidine hydrochloride (4-DAMP mustard) reversed the physostigmine and carbachol responses. M₂ and M ₃ receptor blockade did not alter the facilitation of spontaneous contractions induced by 10 nM BAY K 8644, an L-type Ca²⁺ channel opener, or 0.1 μM iberiotoxin, a large-conductance Ca²⁺-activated K⁺ channel blocker. NS-1619 (30 μM), a large-conductance Ca ²⁺-activated K⁺ channel opener, decreased carbachol-augmented spontaneous contractions. These results suggest that spontaneous contractions in the neonatal rat bladder are enhanced by activation of M₂ and M₃ receptors by endogenous acetylcholine released in the presence of an anticholinesterase agent or a cholinergic receptor agonist.