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Latrotoxin (LTX, 1–3 nm) caused a gradual increase of the supontaneous catecholamine release rate in bovine adrenal chromaffin cells superfused with normal Krebs–Hepes solution containing 2.5 mm Ca2+. Ca2+ removal abolished this effect. LTX enhanced also the secretory responses to high K+ (35 or 70 mm) and to acetylcholine (ACh, 30 μm). The application of Ca2+ pulses to cells previously superfused with a 0 Ca2+ solution (Krebs–Hepes deprived of CaCl2) induced secretory responses that gradually reached 400–800 nA of catecholamines, provided that LTX was present. The responses to ACh or 35 mm K+ pulses (in the presence of Ca2+) were also enhanced by LTX, from around 100–200 nA to over 1000 nA. Though such enhancement remained in the presence of Ca2+ channel blockers, it disappeared upon the lowering of [Na+]0 or in electroporated cells. Using protocols similar to those of secretion, LTX did not enhance basal 45Ca2+ uptake, whole‐cell Ca2+ currents or basal [Ca2+]j. In fact, LTX attenuated the K+‐ or ACh‐evoked increases in 45Ca2+ uptake and [Ca2+]1. It is proposed that the secretory response to brief periods of Ca2+ reintroductions is triggered by local subplasmalemmal Ca2+1 tranMents, produced by the Na+‐Ca2+ exchanger of the plasma membrane working in the reverse mode. This situation might be physiologically reproduced during ACh stimulation of chromaffin cells, which is followed by the firing of Na+‐dependent action potentials.

1 The effects of R56865 (a new class of cardioprotective agent which prevents Na+ and Ca2+ overload in cardiac myocytes) on catecholamine release, whole-cell current through Ca2+ channels (IBa) and cytosolic Ca2+ concentrations, [Ca2+]j, have been studied in bovine chromaffin cells.2 R56865 caused a time-and concentration-dependent blockade of catecholamine release from superfused cells stimulated intermittently with 5 s pulses of 59 mM K+. After 5 min superfusion, a 3 fLM concentration inhibited secretion by 20%; the blockade increased gradually with perfusion time, to reach 85% after 40 min. The IC50 to block secretion after 5 min periods of exposure to increasing concentrations of R56865 was around 3.1 fiM. The blocking effects of R56865 were reversible after 5-15 min wash out. In high Ca2+ solution (10 mM Ca25), the degree of blockade of secretion diminished by 20% in comparison with 1 mM Ca2+. 3 In electroporated cells, R56865 (10 I1M) did not modify the secretory response induced by the 2+ application of 10 pM free Ca 4 R56865 blocked the peak IBa current in a concentration-and time-dependent manner; its IC50 was very similar to that obtained for secretion (3 tLM). The compound not only reduced the size of the peak current but also promoted its inactivation; when the effects of R56865 were measured at the most inactivated part of the current, its IC50 was 1 tLM. Both the inactivation and the reduction of the peak currents were reversible upon washing out the drug.

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