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Phorbol Myristate Acetate Inhibition of Phospholipase C Activation by Carbachol in Slices of Rat Brain Cortex Is a Delayed and Indirect Effect
Author(s) -
Lee HorngMo,
Fain John N.
Publication year - 1991
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1991.tb02040.x
Subject(s) - carbachol , ionomycin , endocrinology , phorbol , medicine , protein kinase c , chemistry , muscarinic acetylcholine receptor , phospholipase c , cholinergic , depolarization , cytosol , biology , biochemistry , stimulation , receptor , signal transduction , enzyme
We examined the effect of phorbol esters on phos‐pholipase C activation in rat brain cortical slices and membranes. There was little effect of concurrent addition of phorbol 12‐myristate 13‐acetate (PMA) with carbachol on phos‐phoinositide breakdown due to carbachol over a 1‐h incubation of brain slices. However, if slices were preincu‐bated for 3 h with 1 μ M . PMA. or 200 μ M /sphingosine before addition of carbachol, there was a 35–50% inhibition of phosphoinositide breakdown. There was also a marked loss of protein kinase C (PKC) activity from both cytosol and membranes after a 3‐h exposure to PMA. The loss in responsiveness to the muscarinic agonists in slices was not reflected in carbachol‐stimulated phospholipase C activation using isolated membranes. However, the decrease in car‐bachol‐induced phosphoinositide breakdown seen in slices after a 3‐h exposure to PMA was abolished if the extracellular K + concentration was elevated from 5.9 to 55 m M Because elevation of the K + level induces depolarization and increases Ca 2+ entry, we examined the effect of ionomycin, a Ca 2+ ionophore. Ionomycin potentiated the effects of carbachol on phosphoinositide breakdown but was unable to reverse the effects of a 3‐h incubation with PMA. Because apamin, an inhibitor of Ca 2+ ‐dependent K + channels, mimicked the effects of exposure to PMA for 3 h, it is possible that these channels are involved in muscarinic cholinergic regulation of phosphoinositide breakdown in rat brain slices. These results support the hypothesis that prolonged PMA treatment in rat brain cortex has no direct effect on phospholipase C activation by muscarinic cholinergic stimulation.