Phosphatidylinositol breakdown

When acetylcholine-stimulated breakdown of phosphatidylinositol was first observed in the avian salt gland, it seemed logical to assume that this would be by cleavage of phosphatidylinositol to form diglyceride and inositol phosphate (Hokin, M.R. Hokin, L.E., 1964). A Ca -dependent enzyme with phospholipase C type activity which catalyzed this reaction had been demonstrated by Kemp et al. (1961). More recently, Dawson et al. (1971) showed that the products are inositol 1,2-cyclic phosphate, inositol 1-phosphate and diglyceride. There is an enzyme with these properties in mouse pancreas. The enzyme is not activated by acetylcholine in soluble extracts, and we have found no evidence that it is concerned in acetylcholine-stimulated breakdown of phosphatidylinositol in the intact cell, nor that the function of phosphatidylinositol breakdown is to form inositol 1,2-cyclic phosphate, as has been suggested by Michell Lapetina (1972). 

When we examined the water-soluble products of stimulated phosphatidylinositol breakdown in pancreas we did not find any increase... 

The lipid-soluble product of stimulated phosphatidylinositol breakdown can appear as (18 0,20 4)phosphatidic acid or (18 0,20 4)... 

The hormone pancreozymin (cholecystokinin-pancreoz3miin) stimulates the protein secretory cycle in the exocrine pancreas (Harper Raper, 1943) it also elicits phosphatidylinositol breakdown, formation of phosphatidic acid, and increased turnover of the polar headgroups of these phosphatides. These effects are essentially the same as those observed in response to acetylcholine (Hokin, M.R.,... 

If the major site of acetylcholine-stimulated phosphatidylinositol breakdown in the pancreas is the rough endoplasmic reticular membranes rather than at the cell surface, then it is necessary to postulate that some second messenger system is needed to carry the information from the receptor at the cell surface to the site of the response. The nature of this is not known. The dibutyryl derivatives of cyclic AMP and cyclic GMP do not give rise to phosphatidylinositol breakdown in the pancreas nor does the influx of Ca ion... 

Acetylcholine and pancreozymin both stimulate zymogen extrusion in the pancreas. However, many lines of evidence have indicated that the phosphatidylinositol response is not closely related to the process of exocytosis in the pancreas or in other glands. One point of difference between stimulation of exocytosis and stimulation of phosphatidylinositol breakdown in mouse pancreas is that maximum extrusion of enzjnne occurs in vitro in response to O.lyM acetylcholine stimulation of H phosphatidylinositol breakdown increases linearly between concentrations of acetylcholine of O.lpM to lOOyM (Hokin, M.R. 1974). 

Table 4. Comparison of the stimulation by acetylcholine of protein synthesis and of phosphatidylinositol breakdown in mouse pancreas...

Table 5. Comparison of acetylcholine stimulation of ouabain sensi-tive respiration and of phosphatidylinositol breakdown in goose...

This "microsomal" fraction from salt gland consists largely of smooth membrane fragments which appear to be derived from the extensive plasma membrane of cell (Slautterback, Hokin, L.E. Hokin, M.R., unpublished). The major site of acetylcholine-stimulated phosphatidylinositol breakdown in the salt gland appears therefore to be this extensive plasma membrane, which contains the NaK-ATPase and carries out the secretory function of the gland. 

The NaK-ATPase activity which is initiated in response to acetylcholine can be monitored by measurement of the 3-fold increase in the rate of respiration which it evokes. This increase in the respiratory rate is blocked by ouabain, which specifically blocks NaK-ATPase activity, and by atropine, which specifically blocks acetylcholine receptors. In the salt gland, an almost maximal increase in secretory NaK-ATPase activity occurs in response to O.lyM acetylcholine and in this tissue, in contrast to the pancreas, the breakdown of phosphatidylinositol, as measured by the amount resynthesized after addition of atropine to stimulated tissue, also occurs almost maximally at this same low concentration of acetylcholine (Table 5). These observations lead me to propose that in the salt gland cell, the function of stimulated phosphatidylinositol breakdown, and of its resynthesis during the reversion to the unstimulated state, is to exert on-off control of the activity of the secretory NaK-ATPase molecules. These are "turned on" when this tissue is stimulated, and are "turned off when the tissue reverts to the non-secreting state. A simple mechanism for this might be that when the responsive lipid molecules are in the phosphatidylinositol form, an active site of the secretory NaK-ATPase is buried in a hydrophobic area of the membrane, and that when the phosphatidylinositol is broken down, this results in a membrane change... 

THE POSSIBLE INVOLVEMENT OF PHOSPHATIDYLINOSITOL BREAKDOWN IN THE MECHANISM OF STIMULUS-RESPONSE COUPLING AT RECEPTORS WHICH CONTROL CELL-SURFACE CALCIUM GATES... 

A variety of physiological stimuli which cause the opening of cell-surface Ca2+ gates also stimulate phosphatidylinositol breakdown. Studies of various receptors and tissues have shown that the breakdown of phosphatidylinositol is not caused by an increase in the intracellular Ca + concentration. It seems most likely that the function of stimulated PI breakdown lies in the coupling between activated cell-surface receptors and the opening of membrane Ca " " gates, and some possible mechanisms for this coupling have been briefly discussed. 

Marshall, PJ, Dixon, JF and Hokin, LE (1980) Evidence for a role in stimulus-secretion coupling of prostaglandins derived from release of arachidonoyl residues as a result of phosphatidylinositol breakdown. Proc Natl Acad Sci USA, 77, 3292-3296. 

Marshall, PJ, Boatman, DE and Hokin, LE (1981) Direct demonstration of the formation of prostaglandin E2 due to phosphatidylinositol breakdown associated with stimulation of enzyme secretion in the pancreas. J Biol Chem, 256, 844-847. 

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