Phosphoinositide metabolism of rat pineal gland in a low ionic strength medium.

Pineal glands were incubated in the presence of 32P orthophosphate. When all NaCl in a conventional incubation medium was replaced by isotonic sucrose, i.e. when the ionic strength of the medium was decreased, there was a marked increase in 32P labelling of phosphatidylinositol (PI) and phosphatidic acid (PA). The 32P labelling of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) was not affected. No net synthesis of PI was observed. The increased labelling of PI therefore represents an increase in the turnover of PI. The 32P labelling of PI was observed also in media where NaCl was replaced by fructose or mannitol, but not in media, where NaCl was replaced by choline chloride. The effect depends on the concentration of the HEPES buffer and was not found in the medium with a bicarbonate buffer. 32P labelling of PI was not blocked by alpha 1 adrenergic blockers, phentolamine and prazosin, and did not depend on the presence of Ca2+ in the incubation medium. The effect was blocked by a Ca2+ channel blocker, MnCl2. Only 32P labelling of PI and not that of phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) was increased during prolonged incubation in the sucrose medium. It is suggested that a decrease in the charge distribution across the plasma membrane as a result of the absence of most monovalent cations is responsible for the increased metabolism of phosphatidylinositol.