G protein control of potassium channel activity in a mast cell line.

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Abstract
Using the patch-clamp technique, we studied regulation of potassium channels by G protein activators in the histamine-secreting rat basophilic leukemia (RBL-2H3) cell line. These cells normally express inward rectifier K+ channels, with a macroscopic whole-cell conductance in normal Ringer ranging from 1 to 16 nS/cell. This conductance is stabilized by including ATP or GTP in the pipette solution. Intracellular dialysis with any of three different activators of G proteins (GTP gamma S, GppNHp, or AlF-4) completely inhibited the inward rectifier K+ conductance with a half-time for decline averaging approximately 300 s after "break-in" to achieve whole-cell recording. In addition, with a half-time averaging approximately 200 s, G protein activators induced the appearance of a novel time-independent outwardly rectifying K+ conductance, which reached a maximum of 1-14 nS. The induced K+ channels are distinct from inward rectifier channels, having a smaller single-channel conductance of approximately 8 pS in symmetrical 160 mM K+, and being more sensitive to block by quinidine, but less sensitive to block by Ba2+. The induced K+ channels were also highly permeable to Rb+ but not to Na+ or Cs+. The current was not activated by the second messengers Ca2+, inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate, or by cyclic AMP-dependent phosphorylation. Pretreatment of cells with pertussis toxin (0.1 microgram/ml for 12-13 h) prevented this current's induction both by guanine nucleotides and aluminum fluoride, but had no effect on the decrease in inward rectifier conductance. Since GTP gamma S is known to stimulate secretion from patch-clamped rat peritoneal mast cells, it is conceivable that K+ channels become inserted into the plasma membrane from secretory granules. However, total membrane capacitance remained nearly constant during appearance of the K+ channels, suggesting that secretion induced by GTP gamma S was minimal. Furthermore, pertussis toxin had no effect on secretion triggered by antigen, and triggering of secretion before electrical recording failed to induce the outward K+ current. Finally, GTP gamma S activated the K+ channel in excised inside-out patches of membrane. We conclude that two different GTP-binding proteins differentially regulate two subsets of K+ channels, causing the inward rectifier to close and a novel K+ channel to open when activated.

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