Extracellular ATP stimulates an amiloride-sensitive sodium influx in human lymphocytes

Abstract

Extracellular ATP has been shown to increase the Na + permeability of human lymphocytes by 3 to 12-fold. The kinetics of this ATP-induced response were studied by measuring 22Na + influx into chronic lymphocytic leukemic lymphocytes incubated in low-sodium media without divalent cations. ATP-stimulated uptake of 22Na-ions was linear over 4 min incubation and this influx component showed a sigmoid dependence on ATP concentration. Hill analysis yielded a K 1 2 of 160 μ m and a n value of 2.5. The nucleotide ATP-γ-S (1–2 m m) gave 30% of the permeability increase produced by ATP, but UTP (2 m m) and dTTP (2 m m) had no effect on 22Na influx. The amiloride analogs 5-( N-ethyl- N-isopropyl) amiloride and 5-( N,N-hexamethylene) amiloride, which are potent inhibitors of Na +H + countertransport, abolished 72–95% of the ATP-stimulated 22Na + influx. However, the involvement of Na +H + countertransport in the ATP-stimulated Na + influx was excluded by three lines of evidence. Sodium influx was stimulated 7-fold by extracellular ATP but only 2.4-fold by hypertonic conditions which are known to activate Na +H + countertransport. Addition of ATP to lymphocytes produced no change in intracellular pH when these cells were suspended in isotonic NaCl media. Finally ATP caused a membrane depolarization of lymphocytes which is inconsistent with stimulation of electroneutral Na +H + exchange. These data suggest that ATP acts cooperatively to induce the formation of membrane channels which allow increased Na + influx by a pathway which is partially inhibited by amiloride and its analogs.