The effect of protein kinase C activation on muscarinic-M3- and K(+)-evoked release of [3H]noradrenaline and increases in intracellular Ca2+ in human neuroblastoma SH-SY5Y cells.

Short-term pretreatment (9 min) with the phorbol ester 12-myristate 13-acetate (PMA) alone had no effect on the basal release of [3H]noradrenaline ([3H]NA), but enhanced K+ (100 mM)-, acetylcholine (0.1 mM)-, carbachol (1 mM)-, muscarine (1 mM)- and arecoline (1 mM)-evoked release by 2.3-, 6.4-, 3.0-, 2.0- and 2.0-fold respectively in SH-SY5Y cells. Maximum effects of PMA were observed after a 10 min preincubation at a concentration of 0.1 microM. There was a 4-fold decrease in the EC50 values (concentration required for 50% of maximal stimulation) observed for carbachol- and acetylcholine-evoked release of [3H]NA in the presence of PMA. The inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate did not alter the K(+)- or carbachol-evoked release of [3H]NA. The enhancement of release in the presence of PMA was more potently inhibited by the protein kinase C inhibitors RO 31-7549 [concentration required for 50% inhibition (IC50) = 0.18 microM/bd and RO 31-8220 (IC50 = 0.56 microM) than by either polymyxin-B or H-7. Furthermore, in the absence of PMA, both K(+)- and carbachol-evoked release was inhibited by these antagonists. Atropine, hexahydro-sila-difenidol and pirenzepine antagonized the PMA-enhanced carbachol-evoked release of [3H]NA, with Ki values of 2.75 +/- 0.25 nM, 2.6 +/- 0.64 nM and 294 +/- 17 nM respectively. These values were consistent with the coupling of an M3 muscarinic receptor to the release of [3H]NA in SH-SY5Y cells. Whereas pretreatment with PMA (5 min) enhanced M3-evoked release of [3H]NA, it decreased the muscarinic-agonist-evoked initial peak (greater than 85%) and plateau phase in intracellular Ca2+. These results suggest that noradrenaline release evoked by muscarinic agonists was triggered not only by relatively small changes in Ca2+ but also by activation of protein kinase C.