Characterization of Muscarinic Receptor-Mediated Cationic Currents in Longitudinal Smooth Muscle Cells of Mouse Small Intestine

Abstract

Abstract.: 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 (mIcat) sensitive to cytosolic Ca2+ concentration ([Ca2+]i) and that the initial peak and oscillatory components are contaminated by Ca2+-activated Cl− currents. Under conditions of [Ca2+]i buffered to 100 nM, the mIcat response to cumulative carbachol applications was inhibited competitively by an M2-selective antagonist but non-competitively by an M3-selective one. Also it was severely reduced by pertussis toxin (PTX) treatment or a phospholipase C (PLC) inhibitor. Comparative analysis of mIcat 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, mIcat arises mainly via an M2/M3 synergistic mechanism involving PTX-sensitive G-proteins and PLC activity in the absence of current modulation by [Ca2+]i changes, as described for guinea-pig ileal mIcat. The channels underlying mIcat are also indistinguishable in gating properties between both types of myocytes. Keywords:: muscarinic receptor, receptor-oparated cation channel, purtussis toxin, phospholipase C, intestinal smooth muscle