A new type of ATP-activated calcium channels in rat macrophage plasma membranes

To study mechanisms of receptor-operated Ca2+ influx in non-excitable cells, membrane currents of rat peritoneal macrophages were recorded using whole-cell cell-attached and outside-out configurations of the patch clamp technique. Under whole-cell recording conditions, ATP applied in micromolar concentrations elisited an inward current response when the bath solution contained Ba2+, Ca2+ or Na+ as the only permeant cations. Increasing the Mg2+ concentration had an inhibitory effect on the ATP-induced inward current indicating that the active form of ATP responsible for the cation entry is ATP(4-). The nucleotide potency order was ATP > ATP gamma S > ADP. UTP was completely ineffective (n=19). The data obtained are consistent with the ATP receptor being of the P-2Z type. The macrophage plasma membrane was impermeamble to Tris, during the ATP-induced current at ATP(4-) concentrations varying from 0.07 to 500 mu M. At higher concentrations, ATP produced a large inward steady-state current, which could be attributed to membrane permeabilization. Activity of single channels was recorded when ATP was applied to the external surface of the patch membrane both in cell-attached and outside-out experiments. A specific property of the channels appeared to be the existence of at least four conductance sublevels. With 105 mM Ba2+ as a permeant cation the conductance sublevels were 3.5, 7, 10 and 15 pS. With 10 mM Ca2+ the sublevel conductances were equal to 4, 9, 13 and 17 pS. Extrapolated reversal potential values estimated from current-voltage curves for predominant conductance levels were equal to +40 and +26 mV for 105 mM Ba2+ and 10 mM Ca2+ respectively. The permeability ratios fell in the sequence: P-Ca : P-Ba : P-K = 71 : 29 : 1. Thus, ATP-activated channels in the macrophage membrane are rather selective for divalent vs. monovalent cations, with a predominant permeability for Ca2+.