Role of enhanced arachidonate availability through phospholipase A₂ pathway in mediation of increased prostaglandin synthesis by glomeruli from diabetic rats

Prostaglandin E₂ (PGE₂) production by superfused glomeruli from rats made diabetic for 2 wk by streptozocin injection is twofold higher than that by glomeruli from normal rats. The higher rates of PGE₂ production by glomeruli from diabetic rats are associated with higher levels of labeled free arachidonate in glomeruli prelabeled with [³H]arachidonate, both basally and after stimulation with Ca²⁺ ionophore A23187. The difference between release of labeled arachidonate from phospholipids of diabetic versus normal glomeruli is likely underestimated by measurements of arachidonate alone due to more rapid incorporation of released arachidonate into triacylglycerol of diabetic glomeruli. A23187 induced a fall in labeled phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol in glomeruli that had been prelabeled with [³H]arachidonate and also induced a reduction in the mass of these phospholipids. Consistent with the higher levels of labeled arachidonate, the reduction in both labeled phospholipids and phospholipid mass with A23187 was greater in glomeruli from diabetic than normal rats. Furthermore, the reduction in labeled phospholipids and phospholipid mass with A23187 was largely (62-80%) accounted for by a fall in phosphatidylcholine plus phosphatidylethanolamine in glomeruli from both normal and diabetic rats. These results suggest a primary role for phospholipase A₂ in A23187 actions on glomerular arachidonate release in normal rats and for the higher levels of arachidonate found in glomeruli from diabetic rats. Nevertheless, A23187 also stimulated the production of inositol phosphates - a measure of cellular phospholipase C activity. The increases in inositol phosphate production induced by A23187 were greater in glomeruli from diabetic than normal rats. However, the difference in inositol phosphate production between the two groups was abolished by indomethacin, suggesting that they were secondary to the enhanced prostanoid production by the diabetic glomeruli. Studies of phospholipase A₂ and C activities in subcellular fractions of glomerular homogenates also supported a role for enhanced phospholipase A₂ activity in the mediation of increased availability of arachidonate for prostaglandin synthesis in glomeruli from diabetic versus normal rats. Thus, phospholipase A₂ activity was twofold higher in glomeruli from diabetic than normal rats, whereas phospholipase C activity was not different between the two groups. Treatment of diabetic rats with insulin prevented the rise in glomerular phospholipid A₂ otherwise seen in these rats.