Background and purpose: In the ciliary muscle, the tonic component of the contraction produced by cholinergic agonists is highly dependent on Ca(2+) provided by influx through non-selective cation channels (NSCCs) opened by stimulation of M(3) muscarinic receptors. We examined effects of YM-254890 (YM), a G(q/11)-specific inhibitor, on contraction, NSCC currents and [Ca(2+)](i) elevation induced by carbachol (CCh).
Experimental approach: Isometric tension was recorded from ciliary muscle bundles excised from bovine eyes. In ciliary myocytes dispersed with collagenase and cultured for 1-5 days, whole-cell currents were recorded by voltage clamp and the intracellular free Ca(2+) concentration [Ca(2+)](i) was monitored using the Fluo-4 fluorophore. Existence and localization of M(3) receptors and the a subunit of G(q/11) (G alpha(q/11)) were examined by immunofluorescence microscopy using AlexaFluor-conjugated antibodies.
Key results: Both phasic and tonic components of contractions evoked by 2 mu M CCh were inhibited by YM (3-10 mu M) in a dose-dependent manner. In the cultured cells, CCh (0.05-10 mu M) evoked an NSCC current as well as an elevation of the [Ca(2+)](i). Both initial and sustained phases of these CCh-evoked responses were abolished by YM (3-10 mu M). Immunostaining of the cytoplasmic side of the plasma membrane of ciliary myocytes revealed a dense distribution of M(3) receptors and G alpha(q/11).
Conclusions and implications: The tonic as well as phasic component of the ciliary muscle contraction appears to be under control of signals conveyed by a G(q/11)-coupled pathway. YM is a useful tool to assess whether G(q/11) is involved in a signal transduction system.