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

Purpose: Muscarinic receptor mediated contractions of the detrusor rely on Ca influx through voltage-gated Ca channels but to our knowledge the mechanism linking stimulation of M3Rs to the activation of voltage dependent Ca 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 Ca influx and contraction. We investigated whether TRPC4 channels are involved in cholinergic mediated contractions of the detrusor.

Materials And Methods: Isometric tension recordings were made on strips of murine detrusor and intracellular Ca measurements were made on isolated detrusor myocytes using confocal microscopy. Transcriptional expression of TRPC and IPR subtypes in intact detrusor strips and isolated detrusor myocytes was assessed using reverse transcriptase-polymerase chain reaction.

Results: 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 IPR inhibitor 2-APB. Carbachol evoked reproducible Ca responses in isolated detrusor myocytes, consisting of an initial Ca transient followed by Ca oscillations. ML204 inhibited the initial Ca transient whereas 2-APB inhibited the Ca oscillations. Reverse transcriptase-polymerase chain reaction experiments showed that TRPC4β, TRPC6 and IPR1 were selectively expressed in isolated detrusor myocytes. Control experiments demonstrated that ML204 did not affect L-type Ca or BK current amplitude, caffeine induced Ca transients or KCl induced contractions of the detrusor.

Conclusions: Muscarinic receptor mediated contractions of the detrusor involve the activation of TRPC4β channels.