Phosphatidylinositol is essential determinant for [K.sup.+] permeability involved in gastric proton pumping

Gastric vesicles purified from acid-secreting rabbit stomach display [K.sup.+] permeability manifested by the valinomycin-independent proton pumping of [H.sup.+]-[K.sup.+]-ATPase as monitored by acridine orange quenching. This apparent [K.sup.+] permeability is attenuated by the treatment of the membrane with 5 mM [Mg.sup.2+], and this phenomenon has been attributed to membrane-bound phosphoprotein phosphatase. However, with the exception of the nonspecific inhibitor pyrophosphate, protein phosphatase inhibitors failed to inhibit the loss of [K.sup.+] permeability. Preincubation of the membrane with neomycin, a phospholipase C inhibitor, surrogated the effect of [Mg.sup.2+], whereas another inhibitor, U-73122, did not. Phosphatidylinositol 4,5-bisphosphate ([PIP.sub.2]) restored the attenuated [K.sup.+] permeability by treatment with either [Mg.sup.2+] or neomycin. Furthermore, either phosphatidylinositol bound to phosphatidylinositol transfer protein or phosphatidylinositol 4,5,6-trisphosphate ([PIP.sub.3]) surrogated the effect of [PIP.sub.2]. [Mg.sup.2+] and neomycin reduced [K.sup.+] permeability in the membrane as determined by [Rb.sup.+] influx and [K.sup.+]-dependent [H.sup.+] diffusion. Treatment with [Mg.sup.2+] reduced the contents of [PIP.sub.2] and [PIP.sub.3] in the membrane. These results suggest that [PIP.sub.2] and/or [PIP.sub.3] maintain [K.sup.+] permeability, which is essential for proton pumping in the apical membrane of the secreting parietal cell. hydrogen; potassium; adenosinetriphosphatase; neomycin