Inositol 1,4,5-trisphosphate mediates adrenaline activation of K conductance in mouse peritoneal macrophages.

In mouse peritoneal macrophages, α-adrenoceptor stimulation evokes a Ca-dependent K current [ I(Adr)] [Hara et al. (1991) Pflügers Arch 419:371-379]. The roles of D- myo-inositol 1,4,5-trisphosphate (Ins P) and a GTP-binding protein (G protein) in I(Adr) were investigated with tight-seal whole-cell recordings and fura-2 fluorescence measurements. Intracellular injection of lns P (5-50 μM) evoked transient outward currents [ I(Ins P)] with or without damped oscillations in membrane currents at -40 mV. Dialysis with 0.2 mM guanosine 5′-[3-thio]triphosphate (GTP[ γS], a poorly hydrolysable GTP analogue) at -40 mV activated oscillatory outward currents or a slowly developing steady current on which such oscillations were superimposed after a delay of 10-90 s. I(Ins P) and the GTP[ γS]-induced current { I(GTP[ γS])} were accompanied by an increase in conductance. Reversal potentials of both responses closely depended on the extracellular K concentration. Fura-2 measurements revealed that I(Ins P) and I(GTP[ γS]) result from a rise in intracellular free Ca concentration ([Ca]). Removal of extracellular Ca did not abolish I(Ins P) and I(GTP[ γS]). Both were blocked by bath-applied charybdotoxin. Intracellular D- myo-inositol 1,3,4,5-tetrakisphosphate (Ins P, 50 μM) did not evoke any responses, whereas D- myo-inositol 2,4,5-trisphosphate [Ins P(2,4,5), 20 μM] elicited an outward current at -40 mV. I(Ins P) was completely blocked by prior dialysis with the Ins P receptor antagonist heparin (5 mg/ml). Inclusion of guanosine 5′-[2-thio] diphosphate (GDP[ βS], 2 mM) or heparin (5 mg/ml) together with GTP[ γS] in the patch pipette solution completely blocked I(GTP[ γS]). These results indicate that intracellular injection of Ins P or GTP[ γS] mimic I(Adr). Furthermore, intracellular dialysis with heparin (3 mg/ ml) or GDP[ βS] (2 mM) greatly accelerated a run-down of I(Adr). On the other hand, I(Adr) was markedly prolonged in a cell dialysed with GTP[ γS] (0.2 mM). Therefore, it is concluded that I(Adr) results from stimulation of α-adrenoceptor and Ins P formation via a G protein. [ABSTRACT FROM AUTHOR]