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AbstractExocytosis from nerve terminals is triggered by depolarization-evoked Ca*+ entry, which also activates calmodulin and stimulates protein phosphorylation. Ba2+ is believed to replace Ca2+ m triggering exocytosis without activation of calmodulin and can therefore be '+ used to unravel aspects of presynaptic function. We have analysed the cellular actions of Ba in relation to its effect on transmitter release from isolated nerve terminals. Barium evoked specific release of amino acid transmitters, catecholamines and neuropeptides (EC,, 0.2-0.5 mM), similar to K'/Ca" -evoked release both in extent and kinetics. Ba2 +-and Ca2+ -evoked release were not additive. In contrast to Ca'+, Ba2+ triggered release which was insensitive to trifluoperizine and hardly stimulated protein phosphorylation. These observations are in accordance with the ability of Ba2+ to replace CaZf m exocytosis without activating calmodulin. Nevertheless, calmodulin appears to be essential for regular (Cazf -triggered) exocytosis, given its sensitivity to trifluoperizine.Both Ba2+-and Ca'+-evoked release were blocked by okadaic acid. Furthermore, anti-calcineurin antibodies decreased Ba'+-evoked release. In conclusion, BaZf replaces Ca'+/calmodulin in the release of the same transmitter pool. Calmodulin-dependent phosphorylation appears not to be essential for transmitter release. Instead, our data implicate both CaZf -dependent and -independent dephosphorylation in the events prior to neurotransmitter exocytosis.Keywords: Exocytosis; Ba*+; Ca'+-dependent release; Calmodulin; Okadaic acid, Phosphatase
IntroductionThe regulated exocytosis of neurotransmitters and neuropeptides is triggered by depolarization evoked Ca*+ entry. Although this process is reasonably specific for Ca2+, several divalent cations with similar physico-chemical properties, such as Ba'+, Sr2+ and Pb2+, were found to evoke transmitter release as well (Zengel and Magleby, 1980;Augustine and Eckert, 1984;Heldman et al., 1989;Shao and Suszkiw, 1991;McMahon and Nicholls, 1993;Sihra et al., 1993). Among these, Ba2+ is the best documented and probably the most effective. However, Ba2+ was found to be ineffective in binding and activating the