HISTAMINE ACTIVATES CL- AND K-PIG TRACHEAL MYOCYTES - CONVERGENCE WITH MUSCARINIC SIGNALING PATHWAY( CURRENTS IN GUINEA)

1. We investigated the effects of histamine on membrane currents and c ontractile state of isolated guinea-pig tracheal myocytes using perfor ated patch and whole-cell recording techniques. The effects of histami ne were compared to those of acetylcholine (ACh) and caffeine. 2. Duri ng voltage clamp (V(hold) = -60 mV), histamine elicited contraction an d an inward current (I(hist)) which was often followed by current osci llations. I(hist) had a reversal potential (V(rev)) of - 9 +/- 3 mV. 3 . I(hist) was dependent on the Cl- gradient and was antagonized by the Cl- channel blocker niflumic acid. V(rev) was more positive (+ 2 +/- 1 mV) when K+-selective currents were blocked by Cs+ and TEA. When all external Na+ was replaced with N-methyl-D-glucamine, there was a smal l reduction in the amplitude of I(hist). 4. The histamine-induced curr ent was similar to that elicited by ACh and by caffeine with respect t o time course, amplitude, and current-voltage relationship. Responses to histamine and to ACh were non-additive, consistent with a convergen ce of histaminergic and cholinergic signalling pathways. I(hist) was a ntagonized by the H-1 histaminergic receptor antagonist astemizole, bu t not by atropine. 5. When recorded using the perforated patch configu ration, I(hist) could be elicited repeatedly for more than 30 min. Whe n cells were studied in the whole-cell configuration using a pipette s olution containing 0-025 mm EGTA, the amplitude of I(hist) was initial ly the same as that obtained using perforated patch but then decreased ; the time required for the responses to decrease to 50 % (t1/2) was 8 .2 +/- 1.0 min. When 1 mm EGTA was included in the pipette solution (w hole-cell configuration), the initial response to histamine was signif icantly decreased in size and t1/2 was reduced to 3.3 +/- 0.7 min. 6. The characteristics of the signalling pathway were examined in cells s tudied using the whole-Cell configuration with 0.025 mm EGTA in the re cording pipette. Heparin significantly reduced t1/2 to 4.3 +/- 0.8 min . GTPgammaS elicited inward current and oscillations; both effects wer e enhanced by histamine. GTPgammaS also reduced t1/2 to 1.4 +/- 0.1 mi n. Pertussis toxin did not alter the amplitude or time course of I(his t). 7. We conclude that in guinea-pig tracheal myocytes, binding of hi stamine to H-1 receptors leads to release of Ca2+ from intracellular s tores and subsequent activation of Cl- and K+ conductances as well as contraction. Furthermore, we demonstrate that ACh elicits similar phys iological responses due to a convergence of the histaminergic and musc arinic signalling pathways.