Reduction of inositol triphosphate in retinal microvessels by glucose and restimulation by myo-inositol

Decreased myo-inositol uptake is associated with reduced bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol content in cultured neuroblastoma cells exposed to L-fucose

L-Fucose is a potent, competitive inhibitor of myo-inositol transport by cultured mammalian cells. Chronic exposure of neuroblastoma cells to L-fucose causes a concentration-dependent decrease in myo-inositol content, accumulation, and incorporation into phosphoinositides. In these studies, L-fucose supplementation of culture medium was used to assess the effect of decreased myo-inositol metabolism and content on bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol production. Chronic exposure of cells to 30 mM L-fucose caused a sustained decrease in bradykinin-stimulated, but not basal, 3H-inositol phosphate release and 32P incorporation into phosphatidylinositol in cells incubated in serum-free, unsupplemented medium. In addition, 32P incorporation into phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was not altered in L-fucose-conditioned cells. Acute exposure of cells to serum-free medium containing 30 mM L-fucose did not affect either basal or bradykinin-stimulated 32P incorporation into phosphatidylinositol. Basal diacylglycerol content was decreased by 20% in cells chronically exposed to 30 mM L-fucose, although analysis of the molecular species profile revealed no compositional change. Bradykinin stimulated diacylglycerol production in neuroblastoma cells by increasing the hydrolysis of both phosphoinositides and phosphatidylcholine. Bradykinin-stimulated production of total diacylglycerol was similar for control and L-fucose-conditioned cells. However, there was a decrease in the bradykinin-induced generation of the 1-stearoyl-2-arachidonoyl diacylglycerol molecular species in the cells chronically exposed to 30 mM L-fucose. This molecular species accounts for about 70% of the composition of phosphoinositides, but only 10% of phosphatidylcholine. The results suggest that a decrease in myo-inositol uptake results in diminished agonist-induced phosphatidylinositol synthesis and phosphoinositide hydrolysis in cultured neuroblastoma cells grown in L-fucose-containing medium.

Omega-3 fatty acid treatment fails to alter alpha 1-adrenergic receptor mediated phosphoinositide turnover in streptozotocin-induced diabetic rats

1. The effect of omega-3 fatty acid supplementation on alpha 1-adrenergic receptor-mediated phosphoinositide turnover in the hearts of 6-week streptozotocin diabetic rats was studied. 2. The diabetic state was characterized by decreased body weight gain, hypoinsulinemia and hyperglycemia. Omega-3 fatty acid treatment did not have any significant effects on the above parameters. 3. Plasma triglyceride and cholesterol levels were significantly higher in diabetic rats compared with controls. 4. Omega-3 fatty acid treatment reduced the plasma triglyceride levels in the diabetic rats, whereas plasma cholesterol levels remained unaffected. 5. Norepinephrine (10 and 100 microM) stimulated inositol monophosphate (IP1) formation was significantly decreased in the diabetic rat heart compared with controls. Omega-3 fatty acid treatment had no significant effect on the norepinephrine-mediated IP1 formation in these rats. 6. Inositol bisphosphate (IP2) formation in response to 100 microM of norepinephrine was significantly lower in the diabetic rat heart; which was also not affected by omega-3 fatty acid treatment.

Attenuation of phosphoinositidase activity and phosphatidylinositol bisphosphate level of bovine retinal capillary pericytes in high glucose

Both phosphoinositidase (PIase) and individual species of inositol phospholipid (IPL) of bovine retinal capillary pericytes (BRCP) were quantitatively determined. When glucose in growth medium was increased from 5- to 15- or 30 mM, PIase activity was attenuated to 82% or 55%, respectively. In contrast, when glucose (5-, 15-, 30 mM) was added to an enzyme extract from cells grown in the standard growth medium (5 mM glucose, 0.04 mM myo-inositol) the PIase activity was not changed, indicating that the reduced PIase activity was not due to the direct effect of glucose. When IPLs from BRCP were analysed by HPLC and TLC, we observed reduction of the total and newly formed IPLs including the substrate of PIase. Phosphatidylinositol bisphosphate (PIP2). Reduced levels of IPLs were associated with a decrease in myo-inositol and an increase in sorbitol. The changes in IPL metabolism were reversed by adding either free myo-inositol or AL1576, an aldose reductase inhibitor (ARI), to the high-glucose medium. However, the addition of myo-inositol to the growth medium with a standard concentration of glucose only caused a marked increase in phosphatidylinositol, but not in PIP or PIP2, while the supplement of AL1576 in the standard medium did not cause any changes in IPL formation. These findings suggest that the alteration in IPL metabolism in BRCP may be related to insufficient myo-inositol or activated sorbitol pathway under high-glucose conditions. Further explanation of the role of the altered hydrolysis of PIP2 triggered by PIase may provide clues to understanding of the mechanism of decreased pericyte viability in the presence of high glucose concentrations.