Chloride conductance activated by external agonists and internal messengers in rat peritoneal mast cells.

1. Stimulation of mast cells by externally applied secretagogues activated a slowly developing membrane current. With high external and low internal chloride (Cl‐) concentrations, the current reversed at about ‐40 mV, but when external Cl‐ was made equal to internal Cl‐, the reversal potential shifted to about 0 mV, demonstrating that the current carrier was Cl‐. 2. In addition to external agonists, internally applied cyclic AMP and high concentrations of intracellular calcium [Ca2+]i could also activate the Cl‐ current. However, elevated [Ca2+]i produced only slow and incomplete activation. This suggests that the Cl‐ current is not directly Ca2+ activated. Also, activation of Cl‐ current by external agonists and by cyclic AMP was unimpaired when [Ca2+]i was clamped to low levels with internal ethylene glycol bis‐N,N,N',N'‐tetraacetic acid (EGTA), indicating that elevated [Ca2+]i is not necessary for activation of the Cl‐ current. Although activation by cyclic AMP was faster than that produced by elevated [Ca2+]i, it still required tens of seconds; thus the effect of cyclic AMP was also likely to be indirect. 3. Internal guanosine 5'‐O‐(3‐thiotriphosphate) (GTP‐gamma‐S) could also activate the Cl‐ current, suggesting the involvement of a G protein in the control of the current. 4. The variance associated with the Cl‐ current was small, and noise analysis gave a lower limit of about 1‐2 pS for the single‐channel conductance. The Cl‐ current was reduced by 4,4'‐diisothiocyano‐2,2'‐stilbenedisulphonate (DIDS), and during DIDS blockade, the variance of the current increased. This suggests that DIDS enters and blocks the open channel. 5. Activation of the Cl‐ current would make the membrane potential negative following stimulation of a mast cell, thus providing a driving force for entry of external calcium via the stimulation‐induced influx pathways described in the preceding paper (Matthews, Neher & Penner, 1989).