Signalling pathway for histamine activation of non-selective cation channels in equine tracheal myocytes

1.The signalling pathway underlying histamine activation of non-selective cation channels was investigated in single equine tracheal myocytes. Application of histamine (100 mu M) activated the transient calcium-activated chloride current I-Cl(Ca) and sustained, low amplitude nonselective cation current (I-Cat). The H-1 receptor antagonist pyrilamine (10 mu M) blocked activation of I-Cl(Ca) and I-Cat. Simultaneous application of histamine (100 mu M) and caffeine (8 mM) during H-1 receptor blockade activated I-Cl(Ca), but not I-Cat. Neither the H-2 receptor antagonist cimetidine (20 mu M) nor the H-3 receptor antagonist thioperamide (20 mu M) prevented activation of I-Cl(Ca) and I-Cat. 2. Intracellular dialysis of anti-G alpha(1)/G alpha(o) antibodies completely blocked activation of I-Cat by histamine, whereas I-Cl(Ca) was not affected. By contrast, anti-Ga-q/Ga-11 antibodies greatly inhibited I-Cl(Ca) but did not alter activation of I-Cat. 3. 1-Oleoyl-2-acetyl-sn-glycerol (OAG, 20-100 mu M) did not induce any current or affect currents activated by histamine or methacholine (mACH). Simultaneous application of OAG and caffeine activated I-Cl(Ca), but not I-Cat, indicating that a rise in [Ca2+](i) and stimulation of diacylglycerol-sensitive protein kinase C (PKC) is not sufficient to activate I-Cat. The phospholipase C inhibitor U73122 (2 mu M) blocked histamine activation of I-Cl(Ca) and I-Cat, but simultaneous exposure of myocytes to histamine and caffeine restored both I-Cl(Ca) and I-Cat in the presence of U73122. 4. Histamine and mACH activated currents with equivalent I-V relationships. The currents activated by these agonists were not additive; following activation of I-Cat by mACH, histamine failed to induce an additional membrane current. Similarly, mACH did not induce an additional current after full activation of I-Cat by histamine. 5. We conclude that H-1 histamine receptors activate I-Cat through coupling to G(1)/G(o) proteins. Activation of I-Cat also requires intracellular calcium release, mediated by H-1 receptors coupling to G(q)/G(11) proteins. This coupling is analogous to the activation of I-Cat by costimulation of M-2 and M-3 receptors.