Muscarinic cholinergic induced subsensitivity in rabbit iris-ciliary body. Effects on myo-inositol triphosphate accumulation, arachidonate release, prostaglandin synthesis, and contraction.

The relationship between desensitization of iris muscarinic acetylcholine receptors and the receptor-mediated phosphoinositide hydrolysis system was examined in rabbit eyes. Rabbits were treated topically in one eye, the other eye served as control, with 2% pilocarpine in three successive doses (2 X 50 microL) at 32, 24 and 6 hr prior to sacrifice. The polyphosphoinositide (PPI) response was measured in vitro in the iris both as loss of 32P-radioactivity from phosphatidylinositol 4,5-bisphosphate (PIP2) and as accumulation of myo-[3H]-inositol trisphosphate (IP3); arachidonic acid (AA) liberation was measured by radiochromatography; prostaglandin (PG) E2 release was measured by radiochromatography and radioimmunoassay; and myosin light chain (MLC) phosphorylation was measured with SDS-polyacrylamide gel electrophoresis. Desensitized tissues showed significant decreases in carbachol-induced PIP2 hydrolysis, IP3 accumulation, AA liberation, PGE2 synthesis and muscle contraction. These effects are specific to cholinergic receptors in the iris, since substance P-induced IP3 accumulation and PGE2 release, was not affected by the cholinergic desensitization. The cholinergic desensitization of the PPI response in the iris is reversible. These findings are in accord with our previous studies on alpha 1-adrenergic desensitization of the PPI response in this tissue, and they add further support to the hypothesis that changes in the activities of the receptor-mediated phosphoinositide hydrolysis system and its derived second messengers may underlie the mechanism of alpha 1-adrenergic and muscarinic cholinergic subsensitivity in the iris-ciliary body.