Field stimulation-induced noradrenaline release from guinea-pig atria is modulated by prejunctional alpha 2-adrenoceptors and protein kinase C.

Guinea-pig left atria were loaded with 10 microCi 7-[3H]noradrenaline, and noradrenaline release from sympathetic nerve endings was then elicited by refractory period field stimulation. When one pulse of 0.2 ms duration was applied during each refractory period, the resulting transmitter release was halved by 3 x 10(-7) mol/l of the alpha 2-adrenoceptor agonist clonidine and increased about 2.5-fold by either 3 x 10(-7) mol/l of the alpha 2-adrenoceptor antagonist idazoxan, 5 x 10(-3) mol/l of the potassium channel blocker tetraethylammonium chloride (TEA) or 3 x 10(-7) mol/l phorbol-12-myristate-13-acetate (PMA), an activator of protein kinase C (PKC). Phorbol-12-myristate-13-acetate-4-O-methylether, a compound which does not stimulate PKC, was ineffective. The stimulatory effect of PMA was antagonized by 7 x 10(-5) mol/l of the PKC inhibitor polymyxin B. No significant transmitter release was observed when either PMA or TEA was applied together with 10(-7) mol/l of the sodium channel blocker tetrodotoxin. Combinations of either idazoxan and TEA or PMA and TEA caused greater increases of the noradrenaline release than any individual drug given alone. Thus, different mechanisms of action seem to mediate the increase of noradrenaline release by action potential prolongation on the hand and activation of PKC or inhibition of alpha 2-adrenoceptors on the other hand. In contrast, the effects of idazoxan and PMA were not additive, which suggests a common mechanism of action. In atria pretreated for 10 min with 10(-4) mol/l N-ethylmaleimide, an alkylating agent which inactivates Gi-proteins neither idazoxan nor PMA caused a significant increase of the stimulation-induced transmitter release, while TEA was still effective. When a train of four pulses, lasting 0.05 ms each, was applied during each refractory period, the resulting transmitter release was not modified by idazoxan or PMA, but was significantly increased by TEA. From these results, a scheme is proposed which links the regulation of noradrenaline release by prejunctional alpha 2-adrenoceptors and protein kinase C via an influence on a common inhibitory Gi-protein.