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Abstract

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).