Ba2+ replaces Ca2+/calmodulin in the activation of protein phosphatases and in exocytosis of all major transmitters.
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.
Verhage
M
M
Rudolf Magnus Institute for Neurosciences, Department of Medical Pharmacology, University of Utrecht, Netherlands.
Hens
J J
JJ
De Grann
P N
PN
Boomsma
F
F
Wiegant
V M
VM
da Silva
F H
FH
Gispen
W H
WH
Ghijsen
W E
WE
D000818:Animals; D001464:Barium / Q000494:pharmacology*; D002118:Calcium / Q000502:physiology*; D002147:Calmodulin / Q000502:physiology*; D004298:Dopamine / Q000378:metabolism; D018492:Dopamine Antagonists / Q000494:pharmacology; D004744:Enkephalin, Methionine / Q000378:metabolism; D004988:Ethers, Cyclic / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects; D018698:Glutamic Acid / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D018377:Neurotransmitter Agents / Q000378:metabolism*; D009638:Norepinephrine / Q000378:metabolism; D019319:Okadaic Acid; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010770:Phosphotransferases / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012844:Sincalide / Q000378:metabolism; D013574:Synaptosomes / Q000187:drug effects* / Q000378:metabolism; D014268:Trifluoperazine / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D002147:Calmodulin; D018492:Dopamine Antagonists; D004988:Ethers, Cyclic; D018377:Neurotransmitter Agents; D019319:Okadaic Acid; D014268:Trifluoperazine; D001464:Barium; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D004744:Enkephalin, Methionine; D010770:Phosphotransferases; D010749:Phosphoprotein Phosphatases; D012844:Sincalide; D002118:Calcium; D004298:Dopamine; D009638:Norepinephrine