Effect of an intracellular calcium chelator on the regulation of electrically evoked [3H]-noradrenaline release from rat hippocampal slices.

1. The electrically (3 Hz, 5 min) evoked [3H]-noradrenaline ([3H]-NA) release from rat hippocampal slices was reduced by prior treatment of the slices with 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetraacetomethylester (BAPTA/AM) in a concentration-(10 to 500 microM) dependent manner (40% at 30 microM). Reduction of medium calcium from 1.3 to 0.5 mM caused a larger (70%) decrease. BAPTA free acid (100 mM), a non-permeable Ca(2+)-chelator had no significant effect. 2. Basal [3H]-noradrenaline release was reduced by BAPTA/AM in a concentration-dependent manner (50% at 30 microM), but reduction of external Ca2+ from 1.3 to 0.5 mM did not alter basal release. 3. About 10% of total [3H]-NA in the slices was released at 3 Hz stimulation in 1.3 mM Ca2+ buffer. Addition of the alpha 2-adrenoceptor antagonist, idazoxan (1 microM), increased electrically evoked [3H]-NA release to 26% but stimulated release was not altered by the adenosine A1-receptor antagonist, 8-cyclopentyl theophylline (8-CPT) (1 microM). 4. Evoked release was reduced by the alpha 2-receptor agonist, UK 14,304, in a concentration-dependent manner in the presence of 8-CPT (1 microM). The magnitude of this effect was not altered by the treatment of slices with 30 microM BAPTA/AM. 5. The adenosine A1 receptor agonist, N6-cyclohexyl adenosine (CHA) (1 microM) inhibited electrically evoked [3H]-NA release by about 40% in the presence of idazoxan (1 microM). The effect of CHA was not significantly altered by treatment of slices with BAPTA/AM. 7. The present results show that spontaneous [3H]-NA release is affected by reduction of intracellular Ca2+, but not by reduction of extracellular Ca2+ or by the presynaptic agonists or w-conotoxin. By contrast, electrically evoked release was affected more strongly by alterations of extracellular Ca2+ than by buffering intracellular Ca2+. The reduction of electrically evoked [3H]-NA release by agonists at the adenosine Al-receptor and a2-adrenoceptor is probably mediated through the control of Ca2+ entry via membrane ion channels or at a low affinity Ca2'-site governing evoked release.