Carbachol-Activated Monovalent Cation-Selective Channels in the Murine Small Intestine

Muscarinic receptor-activated cationic channels in murine ileal myocytes

BACKGROUND AND PURPOSE

There is little information about the excitatory cholinergic mechanisms of mouse small intestine although this model is important for gene knock-out studies.

EXPERIMENTAL APPROACH

Using patch-clamp techniques, voltage-dependent and pharmacological properties of carbachol- or intracellular GTPgammaS-activated cationic channels in mouse ileal myocytes were investigated.

KEY RESULTS

Three types of cation channels were identified in outside-out patches (17, 70 and 140 pS). The voltage-dependent behaviour of the 70 pS channel, which was also the most abundantly expressed channel (approximately 0.35 micro(-2)) was most consistent with the properties of the whole-cell muscarinic current (half-maximal activation at -72.3+/-9.3 mV, slope of -9.1+/-7.4 mV and mean open probability of 0.16+/-0.01 at -40 mV; at near maximal activation by 50 microM carbachol). Both channel conductance and open probability depended on the permeant cation in the order: Cs+ (70 pS) >Rb+ (66pS) >Na+ (47 pS) >Li+ (30 pS). External application of divalent cations, quinine, SK&F 96365 or La3+ strongly inhibited the whole-cell current. At the single channel level the nature of the inhibitory effects appeared to be very different. Either reduction of the open probability (quinine and to some extent SK&F 96365 and La3+) or of unitary current amplitude (Ca2+, Mg2+, SK&F 96365, La3+) was observed implying significant differences in the dissociation rates of the blockers.

CONCLUSIONS AND IMPLICATIONS

The muscarinic cation current of murine small intestine is very similar to that in guinea-pig myocytes and murine genetic manipulation should yield important information about muscarinic receptor transduction mechanisms.

Characterization of muscarinic receptor-mediated cationic currents in longitudinal smooth muscle cells of mouse small intestine

In mouse intestinal smooth muscle cells held at -50 mV, carbachol evoked an atropine-sensitive inward current in the intracellular presence of Cs(+). The current response consisted of an initial peak followed by a smaller plateau component on which oscillatory currents frequently arose. Results from various experimental procedures indicated that the inward current is a muscarinic receptor-operated cationic current (mI(cat)) sensitive to cytosolic Ca(2+) concentration ([Ca(2+)](i)) and that the initial peak and oscillatory components are contaminated by Ca(2+)-activated Cl(-) currents. Under conditions of [Ca(2+)](i) buffered to 100 nM, the mI(cat) response to cumulative carbachol applications was inhibited competitively by an M(2)-selective antagonist but non-competitively by an M(3)-selective one. Also it was severely reduced by pertussis toxin (PTX) treatment or a phospholipase C (PLC) inhibitor. Comparative analysis of mI(cat) in mouse and guinea-pig intestinal myocytes indicated that the underlying channels resemble between those myocytes in agonist sensitivity, current-voltage relationship, and unitary conductance. The results suggest that in mouse intestinal myocytes, mI(cat) arises mainly via an M(2)/M(3) synergistic mechanism involving PTX-sensitive G-proteins and PLC activity in the absence of current modulation by [Ca(2+)](i) changes, as described for guinea-pig ileal mI(cat). The channels underlying mI(cat) are also indistinguishable in gating properties between both types of myocytes.