Phosphatidylinositol 4,5-bisphosphate hydrolysis in turkey erythrocytes is regulated by P2y purinoceptors.

When intact [3H]inositol-loaded turkey erythrocytes were stimulated with the purinergic agonist ADP, there was a rapid increase (2.5-fold after 30 sec) in the intracellular content of [3H]inositol 1,4,5-trisphosphate, followed by increases in the levels of [3H]inositol bisphosphate and [3H]inositol 1,3,4,5-tetrakisphosphate (4-fold and 5-fold, respectively, after 3 min). [3H]inositol monophosphate levels did not rise in the first 3 min of ADP stimulation but increased slowly thereafter, demonstrating that the primary response of turkey erythrocytes to purinergic stimulation is hydrolysis of phosphatidylinositol 4,5-bisphosphate. Inositol phosphate accumulation was evoked by a P2y purinoceptor, as indicated by the rank order of potencies of a variety of purinergic agonists. 2-Methylthioadenosine 5'-triphosphate was the most potent agonist tested, with an EC50 value of 0.36 microM. High performance liquid chromatography analysis demonstrated the presence of three distinct inositol tetrakisphosphate isomers in [3H]inositol-loaded turkey erythrocytes, inositol 1,3,4,6-tetrakisphosphate [Ins(1,3,4,5)P4], inositol 1,3,4,6-tetrakisphosphate [Ins(1,3,4,6)P4], and inositol 3,4,5,6-tetrakisphosphate. Prolonged stimulation with adenosine 5'-O-(2-thiodiphosphate), a nonhydrolyzable analogue of ADP, resulted in a 60-fold increase in the level of [3H]Ins(1,3,4,5)P4, whereas a substantial rise in the [3H]Ins(1,3,4,6)P4 fraction was also seen. These results indicate that turkey erythrocytes represent a valuable model system for studies of purinoceptor function as well as fundamental aspects of cell surface receptor-regulated phosphoinositide metabolism.