Focal release of cytotoxic proteins by eosinophils onto the target surface plays an important role in parasite killing. Degranulation was stimulated by intracellular application of calcium and guanosine 5 -3-O-(thio)triphosphate via the recording patch pipette or via streptolysin-O permeabilization. Exocytotic fusion was monitored by capacitance measurements, whereas release of fluorescent weak bases, which accumulate selectively within eosinophil granules, was followed by fluorescence imaging. Several distinct types of granule fusion events were directly observed by simultaneous capacitance and fluorescence measurements. These are fusion of a single granule with the plasma membrane, intracellular granule-granule fusion, fusion of large compounds of pre-fused granules with the plasma membrane (compound exocytosis), and sequential fusion of granules to granules previously fused to the plasma membrane. Extensive granule-granule fusion was also observed by electron microscopy of permeabilized cells. All these fusion mechanisms contribute to focal release. The coexistence of distinct modes of exocytosis suggests that their regulation may modulate effector functions of eosinophils during helminth infection and allergic response.

1. A novel slowly activating voltage-dependent K+ current was observed in isolated nerve terminals from rat neurohypophysis using the whole-cell configuration of the patch-clamp technique. 2. The activation kinetics of the slow current could be fitted assuming Hodgkin-Huxley-type kinetics, an exponential, n, of 1P3 and activation time constants decreasing from 4 s at -50 mVto 07 sat +40 mV.3. A positive shift of reversal potential was observed when [K+] was increased in the bath solution. The current is carried mainly but not exclusively by K+ ions.4. When intracellular free [Mg2+] was low (-60 /LM), average current density was 74 pA pF-1 at membrane potentials around 0 mV. In 83 % of nerve terminals current amplitude was >20 pA pF'.5. The slow current was never observed when the pipette contained 4-6 mm free Mg2+. At a physiological level of free Mg2+ (0.5 mM) the average current density was 16 pA pF1.6. When nerve terminals were analysed after patch-clamp experiments for vasopressin content by immunodetection, no difference in current amplitude was found between the terminals containing vasopressin and all analysed terminals. 7. The voltage dependence of activation was fitted by a Boltzmann equation giving a halfactivation potential of -37 mV and a slope factor of about 9 mV. 8. Tail current deactivation kinetics was biexponential with time constants of 0-12 and 1.5 s.Kinetics was dependent on the duration of the activating pulse. 9. Noise analysis of the slow current indicated a single-channel current of 0 33 pA at +6 mV, corresponding to a single-channel conductance of 4-3 pS.10. This is the first demonstration of a current similar to the slow K+ current, 'KS' in a neurone, suggesting that a protein similar to the IK,-inducing channel protein ISK (minK) may be present in peptidergic nerve terminals. 11. The activation properties are consistent with a role of the slow current in inhibition of excitability, at least at the level of the nerve terminal.