SELECTIVITY OF ATP-ACTIVATED GTP-DEPENDENT CA2-PERMEABLE CHANNELS IN RAT MACROPHAGE PLASMA-MEMBRANE()

Outside-out configuration of the patch clamp technique was used to tes t whether an intracellular application of G protein activator (GTP gam ma S) affects ATP-activated Ca2+-permeable channels in rat macrophages without any agonist in the bath solution. With 145 mM K+ (pCa 8.0) in the pipette solution, activity of channels permeable to a variety of divalent cations and Na+ was observed and general channel characterist ics were found to be identical to those of ATP-activated ones. Absence of extracellular ATP makes it possible to avoid the influence of ATP receptor desensitization and to study the channel selectivity using a number of divalent cations (105 mM) and Nat (145 mM) as the charge car riers, Permeability sequence estimated by extrapolated reversal potent ial measurements was: Ca2+ : Ba2+ : Mn2+ : Sr2+ : Na+ : K+ = 68 : 30 : 26 : 10 : 3.5 : 1. Slope conductances (in pS) for permeant ions rank as follows: Ca2+ : Sr2+ : Na+ : Mn2+ : Ba2+ = 19 : 18 : 14 : 12 : 10. Unitary Ca2+ currents display a tendency to saturate with the Ca2+ con centration increase with apparent dissociation constant (K-d) of 10 mM . No block of Na+ permeation by extracellular Ca2+ in millimolar range was found. The data obtained suggest that (i) activation of some G pr otein is sufficient to gate the channels without the ATP receptor bein g occupied, (ii) the ATP receptor activation results in the gating of a special channel with the properties that differ markedly from those of the receptor-operated or voltage-gated Ca2+-permeable channels on t he other cell types.