Muscarinic cation current and suppression of Ca2+ current in guinea pig ileal smooth muscle cells

Cationic current (I-cat) and inhibition of the voltage-dependent Ca2+ current (I-Ca) evoked by muscarinic receptor activation with carbachol were studied using whole-cell patch clamp technique in smooth muscle cells isolated from longitudinal muscle of guinea pig small intestine. With low buffering of [Ca2+](i) (0.1 mM BAPTA [1,2-bis-(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid] in pipette solution) I-cat and I-Ca inhibitory responses had a rapid onset to an initial peak followed by a sustained phase. The sustained phase of I-Ca suppression was bigger than in the case when [Ca2+](i) was clamped to 100 nM, but decreased with repeated stimulation. Upon repealed stimulation with 50 mu M carbachol in cells where [Ca2+](i) was clamped to 100 nM and when GTP was absent, I-cat amplitude decreased strongly and more substantially compared to I-Ca inhibition, but both responses declined only slightly when 1 mM GTP was present in the pipette solution. GDP-beta S (1 or 5 mM) in pipette solution or pre-treatment of cells with pertussis toxin (6 mu g/ml, for 4 h or longer) blocked I-cat more than I-Ca suppression by carbachol, whereas L-NAME (N-Omega-nitro-L-arginine methyl ester hydrochloride) (100 mu M in pipette solution) affected neither of them significantly. We conclude that the cationic current and the suppression of the voltage-dependent Ca2+ current evoked by muscarinic receptor activation are mediated by pertussis toxin-sensitive G-protein(s) but the latter response was less sensitive to blockade by GDP-beta S and to GTP deficiency in the cell. (C) 1998 Elsevier Science B.V.