Ba 2+ replaces Ca 2+/calmodulin in the activation of protein phosphatases and in exocytosis of all major transmitters

Abstract

Exocytosis from nerve terminals is triggered by depolarization-evoked Ca 2+ entry, which also activates calmodulin and stimulates protein phosphorylation. Ba 2+ is believed to replace Ca 2+ in 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 2+ in relation to its effect on transmitter release from isolated nerve terminals. Barium evoked specific release of amino acid transmitters, catecholamines and neuropeptides (EC 50 0.2–0.5 mM), similar to K +/Ca 2+-evoked release both in extent and kinetics. Ba 2+- and Ca 2+-evoked release were not additive. In contrast to Ca 2+, Ba 2+ triggered release which was insensitive to trifluoperizine and hardly stimulated protein phosphorylation. These observations are in accordance with the ability of Ba 2+ to replace Ca 2+ in exocytosis without activating calmodulin. Nevertheless, calmodulin appears to be essential for regular (Ca 2+-triggered) exocytosis, given its sensitivity to trifluoperizine. Both Ba 2+- and Ca 2+-evoked release were blocked by okadaic acid. Furthermore, anti-calcineurin antibodies decreased Ba 2+-evoked release. In conclusion, Ba 2+ replaces Ca 2+/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 Ca 2+-dependent and -independent dephosphorylation in the events prior to neurotransmitter exocytosis.