Effect of carbachol on phospholipase C-mediated phosphatidylinositol 4,5-bisphosphate hydrolysis, and its modulation by isoproterenol in rabbit corneal epithelial cells.

The effects of carbachol (CCh) on phospholipase C(PLC)-mediated phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and its modulation by isoproterenol were investigated in SV40-adenovirus transformed rabbit corneal epithelial cells (RCEC). When examined under light microscope, these cells exhibited a cobblestone-like appearance typical of the corneal epithelial cells grown in primary culture. Addition of CCh (0.1 mM) for 30 min to RCEC, prelabeled with 32Pi, decreased the radioactivity in phosphatidylinositol 4-phosphate and PIP2 by 15 and 27%, respectively, and concomitantly increased the radioactivity in phosphatidylinositol and phosphatidic acid by 14 and 38%, respectively. When the concentration of CCh was increased to 1 mM, the changes in radioactivity were even more pronounced. Addition of CCh (0.1 mM) to the cells, prelabeled with myo[3H]inositol, increased the accumulation of [3H]inositol 1,4,5-trisphosphate ([3H]InsP3) by 115%, indicating stimulation of PLC-mediated PIP2 hydrolysis. Similar increases were also observed in [3H]InsP1 and [3H]InsP2. The effects of CCh on inositol phosphate accumulation were time- and dose-dependent, and were inhibited by atropine (10 microM), suggesting that the observed effects of CCh were mediated by activation of muscarinic cholinergic receptors. The effects of CCh were antagonized more potently by 4-diphenylacetoxy N-methyl-piperidine than by pirenzepine, indicating that the muscarinic receptors involved in PLC activation are probably of M3 type. By Western immunoblotting analysis with various anti-PLC antibodies, the RCEC were shown to contain PLC gamma 1 and PLC delta 1 in the soluble fraction and PLC beta 1 in the microsomal fraction. Addition of isoproterenol to RCEC, increased cAMP both in a time- and dose-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)