Rapid changes in polyphosphoinositide metabolism associated with the response of Dunaliella salina to hypoosmotic shock.

Abstract

The inositol phospholipids phosphatidylinositol, phosphatidylinositol 4-phosphate (PIP), and phosphatidylinositol 4,5-bisphosphate (PIP2) comprise 14.8, 1.2, and 0.3 mol %, respectively, of Dunaliella salina phospholipids.In isolated plasma membrane fractions, PIP and PIPz are highly concentrated, together comprising 9.5 mol % of plasmalemma phospholipids.The metabolism of these inositol phospholipids and phosphatidic acid (PA) is very rapid under normal growth conditions.Within 5 min after introduction of 32Pi into the growth medium, over 75% of lipid-bound label was found in these quantitatively minor phospholipids.Within 2 min after a sudden hypoosmotic shock, the levels of PIPz and PIP dropped to 65 and 79%, respectively, of controls.Within the same time frame, PA rose to 141% of control values.These data suggest that a rapid breakdown of the polyphosphoinositides may mediate the profound morphological and physiological changes which allow this organism to survive drastic hypoosmotic stress.In contrast to hypoosmotic shock, hyperosmotic shock induced a rise in PIP2 levels to 131% of control values, whereas the level of PA dropped to 56% of controls after 4 min.These two different types of osmotic stress affect inositol phospholipid metabolism in a fundamentally opposite manner, with only hypoosmotic shock inducing a net decrease in polyphosphoinositides.The inositol phospholipids play a fundamental role in transducing many extracellular signals that lead to alterations of intracellular physiology.These signals trigger phospholipase C-catalyzed hydrolysis of the inositol phospholipids, yielding the polar inositol phosphates, which elevate cytoplasmic calcium levels, and the nonpolar 1,2-diacylglycerols, which activate protein kinase C (1).Recent evidence indicates that the elevation of intracellular calcium, potentially initiated by inositol phosphates, is critical in cell volume regulation following hypoosmotic stress in urinary bladder epithelial cells (2).We have undertaken a systematic study of inositol phospholipid metabolism following hypoosmotic shock in a different organism, the unicellular green alga, Dunuliella salina.Because Dumliella will grow in saline conditions ranging from 0.5 to 5 M NaCl (3) and is very responsive to osmotic stimuli, it would appear to possess extremely effective mechanisms for tolerating osmotic stress.Dunuliella can rapidly expand the surface area of its plasma-