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 GTP gamma S-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 (similar to 0.35 mu(-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 mu M 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.