| Abstract: | We have investigated the effects of norepinephrine (NE) and acetylcholine (ACh) on prostaglandin (PGE2 and 6 keto-PGF1α) production by rabbit iris, measured by radioimmunoassay (RIA), and the type of phospholipase activated by NE in irides in which phosphatidylinositol (PI) was doubly prelabeled with [3H] myo-inositol and [1-14C] arachidonic acid (14C-AA), quantitated by radiometric and chromatographic methods. PGE2 output in 60 min (3.6 μg/g tissue) was 2.6 times greater than 6 keto-PGF1α. PG production is time-dependent and it is stimulated by NE and ACh in a dose-dependent manner. The Ne- and ACh-induced release of PGE2, measured by RIA, is mediated through α1-adrenergic and muscarinic cholinergic receptors, respectively, and it requeires Ca2+ for maximal stimulation. Studies on the mechanism of AA release PI in irides doubly prelabeled with 14C-AA and [3H] myo-inositol revelased the following: (a) Both Ne and ACh increased the breakdown of PI, and this was accompanied by a significant increase in the release of AA and consequently PGE2. The stimulatory effects of NE and ACh are mediated through α1-adrenergic and muscainic cholinergic receptors respectively. (b) The NE-induced formation of 3H-lyso PI and the NE-induced metabolism of 14C-1,2-diacyl-glycerol (DG) are time-dependent. Two pathways for AA release from PI are probably operaitve in the iris: (a) An indirect release by PI-specific phospholipase C which producers DG, followed by the actions of DG- and monoacylglycerol lipases on DG to release AA. (b) A direct release by phospholipase A2. Whether lyso PI is a product of the polypholipids such as prosphatidylcholine and phosphatidylethanolamine could also serve as a source for AA in PG synthesis. In conclusion, the data presented provide evidence that in the iris the neurotransmitter-stimulated release of PG and AA, from phosphoinositides, for PG synthesis is coupled to the activation of α1-adrenergic and muscarinic cholinergic receptors. |
