Effect of protein kinase C activation on agonist-mediated phosphositide metabolism in rabbit retinal cells.
4?-Phorbol 12-myrisate 13-acetate (PMA), a tumour-promoting phorbol ester, and 1-oleoly-2-acetylglycerol (OAG), a synthetic diacylglycerol, induced an inhibition of muscarinic and ?(1)-adrenergic receptor-mediated stimulation of PIP(2) breakdown and IPs accumulation in both rabbit retinal slices and primary retinal cultures. Furthermore, an increase in [Ca(2+)](i), mediated by activation of these receptors in 3-5 and 25-30 day old rabbit retinal cultures, was also inhibited by PMA. Neither PMA nor OAG had an effect on the serotonin-mediated PIP(2) breakdown, IPs accumulation or Ca(2+) mobilization. Although A23187 also stimulated IPs formation by acting directly on phospholipase C, PMA had no effect. Maximal inhibition of the carbachol- and noradrenaline-mediated responses was achieved with a 15 min preincubation with PMA at concentrations of 0.1 and 0.01 ?M in retinal slices and primary retinal cultures, respectively. Neither PMA nor OAG influenced the basal levels of phosphoinositides, IPs or [Ca(2)](i). In addition, the inactive phorbol ester, 4?-phorbol 12,13-didecanoate, had no effect on any of the agonist-induced responses. Staurosporine, a potent inhibitor of protein kinase C, significantly attenuated the inhibitory effects exerted by PMA and OAG. These results suggest that calcium- and phospholipid-dependent protein kinase, which is activated by either PMA or OAG, exert inhibitory effects on muscarinic and ?(1)-adrenergic responses. This modulatory feedback "down regulation" role by PKC does not, however, affect serotonergic mediated responses, and thus exhibits a certain selectivity about the site of action. The possible mechanism(s) by which PKC induces its actions are discussed.
Ghazi
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Nuffield Laboratory of Ophthalmology, University of Oxford, Walton Street, Oxford OX2 6AW, England.
Osborne
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