Effects of platelet-activating factor on the release of arachidonic acid and prostaglandins by rabbit iris smooth muscle. Inhibition by calcium channel antagonists.
Addition of physiological concentrations (10(-12)-10(-8)M) of platelet-activating factor (PAF) to rabbit iris muscle induced a rapid release (in 15s) of prostaglandin (PG)E2 and 6-oxo-PGF1 alpha, measured by radioimmunoassay and rapid release of 14C-labelled arachidonate and PGE2 in muscle prelabelled with [14C]arachidonic acid, measured by radiochromatography. These PAF actions are concentration- and time-dependent. The effect of PAF on PG release is not mediated through the cyclo-oxygenase pathway. The studies on the properties and mechanism of arachidonate release from phosphatidylinositol and other phospholipids in prelabelled irides by PAF suggest the involvement of a phospholipase A2. This conclusion is supported by the findings: (a) that both the removal of arachidonate and formation of lysophosphatidylinositol, from phosphatidylinositol, by PAF occur concomitantly in a time-dependent manner, (b) that Ca2+ is required for the agonist-induced release of arachidonate and PGE2, and (c) that in contrast to the rapid release of [3H]myo-inositol phosphates by carbachol and other Ca2+-mobilizing agonists previously reported in the iris muscle [Akhtar & Abdel-Latif (1984) Biochem. J. 224, 291-300], PAF (10(-12)-10(-8)M) did not appreciably enhance the release of [14C]myo-inositol phosphates and 32P labelling of phosphatidate and phosphatidylinositol in this tissue. Ca2+-channel antagonists, such as nifedipine, verapamil, diltiazem and manganese inhibited PAF-induced arachidonate and PGE2 release in a dose-dependent manner. K+ depolarization, which causes influx of extracellular Ca2+ in smooth muscle, did not increase the release of arachidonate and PGE2. The ability of Ca2+ antagonists to inhibit arachidonate release by PAF in this tissue probably reflects interference with PAF binding to its receptor. The PAF-induced release of arachidonate and PGE2 occur independently of the cyclo-oxygenase and lipoxygenase pathways. Whether the PAF-induced release of arachidonate and PG in the iris muscle is involved in the pathogenesis of inflammatory and/or physiological reactions in the eye, and how much the inhibitory effects of Ca2+-entry blockers on the PAF actions contribute to the therapeutic use of these drugs, remain to be established.
D015121:6-Ketoprostaglandin F1 alpha / Q000378:metabolism; D000818:Animals; D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015232:Dinoprostone; D004305:Dose-Response Relationship, Drug; D005260:Female; D007498:Iris / Q000378:metabolism; D008297:Male; D008861:Microsomes / Q000378:metabolism; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism*; D010716:Phosphatidylinositols / Q000378:metabolism; D010972:Platelet Activating Factor / Q000378:metabolism*; D011453:Prostaglandins / Q000378:metabolism*; D011458:Prostaglandins E / Q000378:metabolism; D011817:Rabbits; D013997:Time Factors
D001095:Arachidonic Acids; D002121:Calcium Channel Blockers; D010716:Phosphatidylinositols; D010972:Platelet Activating Factor; D011453:Prostaglandins; D011458:Prostaglandins E; D016718:Arachidonic Acid; D015121:6-Ketoprostaglandin F1 alpha; D015232:Dinoprostone; D002118:Calcium