Muscarinic Receptor Induced Contractions of the Detrusor are Mediated by Activation of TRPC4 Channels.

Abstract

Muscarinic receptor mediated contractions of the detrusor rely on Ca²⁺ influx through voltage-gated Ca²⁺ channels but to our knowledge the mechanism linking stimulation of M3Rs to the activation of voltage dependent Ca²⁺ channels has not been established. TRPC4 channels are receptor operated cation channels that couple muscarinic receptor activation to depolarization of intestinal smooth muscle cells, voltage-activated Ca²⁺ influx and contraction. We investigated whether TRPC4 channels are involved in cholinergic mediated contractions of the detrusor. Isometric tension recordings were made on strips of murine detrusor and intracellular Ca²⁺ measurements were made on isolated detrusor myocytes using confocal microscopy. Transcriptional expression of TRPC and IP₃R subtypes in intact detrusor strips and isolated detrusor myocytes was assessed using reverse transcriptase-polymerase chain reaction. Cholinergic stimulation of the detrusor induced by electrical field stimulation or exogenous application of carbachol or neostigmine evoked contractions consisting of a transient plus a tonic response, which was blocked by ML204, an inhibitor of TRPC4 channels. A phasic oscillatory component was blocked by the IP₃R inhibitor 2-APB. Carbachol evoked reproducible Ca²⁺ responses in isolated detrusor myocytes, consisting of an initial Ca²⁺ transient followed by Ca²⁺ oscillations. ML204 inhibited the initial Ca²⁺ transient whereas 2-APB inhibited the Ca²⁺ oscillations. Reverse transcriptase-polymerase chain reaction experiments showed that TRPC4β, TRPC6 and IP₃R1 were selectively expressed in isolated detrusor myocytes. Control experiments demonstrated that ML204 did not affect L-type Ca²⁺ or BK current amplitude, caffeine induced Ca²⁺ transients or KCl induced contractions of the detrusor. Muscarinic receptor mediated contractions of the detrusor involve the activation of TRPC4β channels.