Putative roles for lysophospholipids as mediators and lipoxygenase-mediated metabolites of arachidonic acid as potentiators of stimulus-secretion coupling: dual mechanisms of p-hydroxymercuribenzoic acid-induced insulin release.

This paper explores the mechanism whereby insulin (I) secretion is stimulated by p-hydroxymercuribenzoic acid (PHMB), an agent which inhibits the esterification of arachidonic acid (AA) into phospholipids in intact rat islets. An effect of PHMB on I release could be seen even at substimulatory glucose concentrations (0-1.7 mM) and was resistant to blockade of energy flux using antimycin A, or of glucose metabolism using mannoheptulose. It was, however, inhibited by Ni++, Co++, La , replacement of chloride in the buffer by the impermeant anion isethionate or reduced ambient temperature (16 degrees C), but not by extracellular Ca++ depletion or 8-(N,N-diethylamino) octyl 3,4,5-trimethoxy-benzoate hydrochloride (a putative stabilizer of intracellular Ca++ stores); thus PHMB's effect may require the translocation of membrane-associated Ca++ stores, leading to exocytotic hormone release. Although PHMB increases the accumulation of lipoxygenase-derived metabolites of AA, I secretion at 1.7 mM glucose unexpectedly was resistant to cyclooxygenase or lipoxygenase inhibition and could not be reproduced by exogenous AA (0.18 through 262 microM). However, it could be mimicked closely by exogenous lysophosphatidylcholine, which also shared with PHMB an identical profile of reversibility and pharmacologic inhibitability. Lysophospholipid (lyso-PL)-induced I release could not be attributed to detergent effects because, for example, it occurred in the absence of significant 51Cr release. The lyso-PL effect demonstrated structural specificity (lysophosphatidyl-ethanolamine and lysophosphatidylserine being essentially inactive) and was specific for lyso-PLs as neither phosphatidylcholine itself nor glycerophosphorylcholine (the deacylation product of lysophosphatidylcholine) had any effect. In contrast to the effects of lyso-PLs, the energy-dependent effects of glucose (16.7 mM) or the amino acid alpha-ketoisocaproic acid (15 mM) on I release were abrogated by inhibitors of phospholipases or lipoxygenase. This effect of phospholipase inhibition could be circumvented by exogenous lyso-PLs. We conclude that lyso-PLs (generated by energy-dependent phospholipid deacylation or by inhibition of reacylation) may be true mediators of I release, whereas the role of concomitantly generated oxygenation products of AA is restricted to the modulation of stimulated release.