Large conductance calcium‐activated non‐selective cation channel in smooth muscle cells isolated from rat portal vein.

Abstract

1. Membrane currents of isolated smooth muscle cells from rat portal vein were studied using the whole-cell patch-clamp technique. 2. In addition to the Ca(2+)-dependent Cl- current, single-channel activities were recorded in response to external applications of 10 mM-caffeine, in the absence of EGTA in the pipette solution. The conductance of this novel type of channel was around 200 pS for membrane potentials ranging from -100 to +60 mV. 3. The single-channel activities were also induced by external application of noradrenaline (10(-5)M) or acetylcholine (10(-5)M), by Ca2+ entry through voltage-dependent Ca2+ channels, and by intracellular application of ryanodine (10(-5)M). The caffeine-activated single-channel currents disappeared when 10 mM-EGTA was added to the pipette solution or after replacement of external Ca2+ with Ba2+. These results show that these channels are Ca2+ dependent. 4. Alterations of the Cl- equilibrium potential did not produce any change in the reversal potential of the caffeine-activated single-channel current indicating that it was not carried by Cl- ions. The value of the reversal potential was about +10 mV, irrespective of the CsCl-, KCl- or NaCl-containing solutions used to fill the pipette. This observation indicates that the channel was equally permeable to these monovalent cations. 5. Caffeine activated single-channel currents when cells were bathed in 90 mM-Ba2+ plus 1 mM-Ca(2+)- or 91 mM-Ca(2+)-containing solutions, showing that divalent cations permeate the channels. The permeability for Ca2+ over Na+ was high as the ratio PCa/PNa was estimated to be 21. 6. Single-channel activities induced by caffeine were not modified by Ca2+ channel blockers such as dihydropyridines and phenylalkylamines. 7. It is concluded that portal vein smooth muscle cells possess Ca(2+)-activated nonspecific channels which are highly permeable to Ca2+ ions.