The ATP4− Receptor-Operated Channel (P2Z Class) of Human Lymphocytes Allows Ba2+ and Ethidium+ Uptake: Inhibition of Fluxes by Suramin

It is now recognized that extracellular ATP can open a receptor-operated ion channel in a variety of cell types. In human lymphocytes this P2Z purinergic channel conducts Na+, K+, Rb+, Li+, and Ca2+ but its permeability to larger cations is not known. Fluorometric measurements were used to show that ATP4− induced the entry of Sr2+ (87 Da) and Ba2+ (137 Da) into human lymphocytes loaded with fura-2. Flow cytometry was used to show that ATP4− induced the entry of ethidium+ (314 Da) but that the larger propidium2+ cation (414 Da) was excluded. ATP4− induced entry of both Ba2+ and ethidium+ showed features previously demonstrated for smaller cation permeants: (i) inhibition by amiloride analogs, (ii) sigmoid dependence of flux on ATP concentrations, and (iii) inhibition by extracellular Na+ ions. Specific inhibitors of L-type voltage-gated Ca2+ channels (nisoldipine and diltiazem) had no effect on ATP4−-induced Ba2+ influx. Suramin and reactive blue 2, which are recognized antagonists of ATP-operated purinergic receptors in other tissues, inhibited ATP-induced uptake of ethidium+ in lymphocytes with K1/2 of 61 and 69 μM, respectively. However, hexamethylene amiloride was a more potent inhibitor of ATP-induced ethidium+ uptake with a K1/2 of 13 μM. These data show that the ATP4− receptor-operated channel of human lymphocytes allows influx of cations as large as Ba2+ or ethidium+ and that this influx is inhibited by suramin and reactive blue 2.