Spontaneous and GABA-evoked chloride channels on pituitary intermediate lobe cells and their internal Ca requirements

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Abstract
On porcine intermediate lobe (IL) endocrine cells, spontaneously opening chloride channels have been studied and compared to GABA-A activated chloride channels. Elementary currents were recorded mainly from outside-out patches excised from IL cells maintained in culture for 1–4 weeks. Spontaneous inward currents were observed in Cs-loaded cells after replacing Na in the extracellular medium by the impermeant ion choline. This activity, at an internal calcium concentration of 10−8 M corresponded to a channel for chloride ions with a main conductance level of 26 pS, and substates around 11 pS. The sequence of permeabilities to halides was I>Br>Cl. These conductance characteristics were common to the GABA-operated channels which also showed a main conductance substate of 23–31 pS. The open time of the 26 pS level mostly encountered in spontaneous activity, was distributed along two modes: one, the most frequent, around 1 ms, and the other around 4 ms. This latter mode was the predominant one observed during GABA and isoguvacine applications but in addition a bursting activity of 19 ms duration was also seen. Specific GABA-A receptor antagonists (bicuculline and SR 42641, 1 μM) blocked activity evoked by GABA (1–10 μM), but did not affect spontaneous events. These spontaneous Cl events were only observed in a restricted range of internal Ca concentrations, i.e. between 1 nM and 0.1 μM, and were practically abolished at Cai 1 μM. The GABA-induced activity of Cl channels was also Ca-sensitive, being reduced when Cai reached 1 μM.