Different pools of esterified arachidonic acid in rabbit kidney medulla: relationship to Ca2+-stimulated prostaglandin biosynthesis.

We investigated the effect of Ca2+ ions on renal medulla metabolism of endogenous esterified arachidonic acid in contrast to that of radioactive arachidonate incorporated into medullary lipids. Some striking differences between the release of unlabeled prostaglandin E2 and of 14C-labeled prostaglandin E2 and arachidonic acid were seen in incubations in absence or presence of Ca2+ ions. These differences indicated that exogenous [14C] arachidonate incubated with medulla slices is incorporated into both Ca2+-sensitive and Ca2+-insensitive lipid pools of esterified arachidonate and furthermore, the Ca2+-sensitive pool is itself heterogeneous and consists of at least 2 functionally different lipid pools of esterified arachidonate. The first Ca2+-sensitive pool is characterized by a higher arachidonate turnover rate and incorporates more rapidly added radioactive arachidonate. The acylhydrolase activity which releases arachidonate from this pool is not efficiently coupled to prostaglandin endoperoxide synthase. In contrast, the second Ca2+-sensitive lipid pool has a slower arachidonate turnover rate and, consequently, a slower incorporation of added 14C-acid. The acylhydrolase activity associated with this pool is more tightly coupled to prostaglandin endoperoxide synthase, so that a higher portion of released arachidonate is converted to prostaglandin E2. Studies on arachidonic acid metabolic transformations using exogenously radioactive free acid added to tissues should therefore be interpreted with caution because the results obtained may not reflect accurately the metabolic fate of endogenous, lipid-esterified arachidonate which is released and metabolized under physiological conditions in vivo.