Ba2+ replaces Ca2+/calmodulin in the activation of protein phosphatases and in exocytosis of all major transmitters

• Published in: European journal of pharmacology 1995-11, Vol.291 (3), p.387-398
• Main Authors: VERHAGE, M, HENS, J. J. H, DE GRAAN, P. N. E, BOOMSMA, F, WIEGANT, V. M, LOPES DA SILVA, F. H, GISPEN, W. H, GHIJSEN, W. E. J. M
• Format: Article
• Language: English
• Subjects: Animals; Barium - pharmacology; Biological and medical sciences; Calcium - physiology; Calmodulin - physiology; Central nervous system; Central neurotransmission. Neuromudulation. Pathways and receptors; Dopamine - metabolism; Dopamine Antagonists - pharmacology; Enkephalin, Methionine - metabolism; Ethers, Cyclic - pharmacology; Exocytosis - drug effects; Fundamental and applied biological sciences. Psychology; gamma-Aminobutyric Acid - metabolism; Glutamic Acid - metabolism; In Vitro Techniques; Male; Neurotransmitter Agents - metabolism; Norepinephrine - metabolism; Okadaic Acid; Phosphoprotein Phosphatases - antagonists & inhibitors; Phosphoprotein Phosphatases - metabolism; Phosphotransferases - metabolism; Rats; Rats, Wistar; Sincalide - metabolism; Synaptosomes - drug effects; Synaptosomes - metabolism; Trifluoperazine - pharmacology; Vertebrates: nervous system and sense organs
• ISSN: 0922-4106; 0014-2999
• DOI: 10.1016/0922-4106(95)90081-0

Exocytosis from nerve terminals is triggered by depolarization-evoked Ca2+ entry, which also activates calmodulin and stimulates protein phosphorylation. Ba2+ is believed to replace Ca2+ 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 Ba2+ in relation to its effect on transmitter release from isolated nerve terminals. Barium evoked specific release of amino acid transmitters, catecholamines and neuropeptides (EC50 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 Ca2+, 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 Ca2+ 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, Ba2+ replaces Ca2+/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.