NORADRENALINE RELEASE AND THE EFFECT OF ENDOGENOUS ACTIVATION OF THE PHOSPHOLIPASE-C PROTEIN-KINASE-C SIGNALING PATHWAY IN RAT ATRIA

1 It has been proposed that protein kinase C (PKC) in sympathetic nerv es is activated during action-potential evoked release of noradrenalin e and helps maintain transmitter output. We studied this phenomenon fu rther in rat atria radiolabelled with [H-3]-noradrenaline. 2 Noradrena line release was elevated by continuous electrical stimulation of the atria for 10 min at either 5 or 10 Hz. Two inhibitors of PKC, polymyxi n B (21 mu M) and Ro 318220 (3 mu M), markedly inhibited the release o f noradrenaline but only at the higher stimulation frequency. 3 Furthe r experiments were conducted with 10 Hz stimulation but for shorter tr ain durations. In this case polymyxin B inhibited noradrenaline releas e during a 10 or 15 s train of impulses but not during a 5 s train. Th is suggests that PKC effects are induced during the stimulation train by some process. 4 The diacylglycerol kinase inhibitor R59949 (10 mu M ), which prevents the breakdown of diacylglycerol, enhanced noradrenal ine release elicited by stimulation at 10 Hz for 10 or 15 s. This effe ct was not seen if polymyxin B was present and suggests that diacylgly cerol is the endogenous activator of PKC. 5 The source of the diacylgl ycerol may be through phospholipase C pathways, since the phospholipas e C inhibitor U73122 (3 mu M) inhibited noradrenaline release at 10 Hz for 10 s and the effect was not seen if polymyxin B was also present. 6 It is unlikely that phospholipase D is the source of diacylglycerol . Although the phospholipase D inhibitor wortmannin (1 mu M) inhibited noradrenaline release, this effect was still observed in the presence of polymyxin B. Furthermore ethanol, which inhibits diacylglycerol fo rmation by phospholipase D, had no effect on noradrenaline release. 7 We therefore suggest that during a train of high frequency pulses phos pholipase C is activated and this results in the production of diacylg lycerol which in turn activates PKC. This enables the neurones to main tain transmitter release at a high level.