Abstract

The effect of 5-hydroxytryptamine (5-HT) or serotonin on Na⁺-K⁺ pump activity of airway smooth muscle was investigated by measuring ⁸⁶Rb⁺ uptake in cultured guinea pig tracheal smooth muscle cells.⁸⁶Rb⁺ uptake consisted of three distinct components, one sensitive to ouabain, one to bumetanide, and one insensitive to either inhibitor. 5-HT induced a concentration-dependent increase in ouabain-sensitive ⁸⁶Rb⁺ uptake (EC₅₀ = 21 nM) but had no effect on bumetanide-sensitive uptake, suggesting that it stimulates the Na⁺-K⁺ pump but not the Na⁺-K⁺-Cl⁻ cotransporter. Ouabain-sensitive uptake also was stimulated by the 5-HT_2A/2C agonists 2,5-dimethoxy-4-iodoamphetamine and α-methyl-5-HT, but not by the 5-HT₁ agonist 5-carboxamidotryptamine, the 5-HT_1A/1B/2C agonist 1-(3-chlorophenyl)piperazine, or the 5-HT₃ agonist 1-(3-chlorophenyl)biguanide. 5-HT-stimulated⁸⁶Rb⁺ uptake was inhibited by the 5-HT_2A antagonists ketanserin and spiperone, but not by the 5-HT_1A antagonist NAN 190 or the 5-HT₃antagonist Y25310. 5-HT-stimulated ⁸⁶Rb⁺ uptake was inhibited by reducing extracellular Na⁺ concentration and by the Na⁺-H⁺ exchange inhibitors dimethylamiloride and 5-(N-methyl-N-isobutyl)-amiloride. These observations suggest that 5-HT stimulates the Na⁺-K⁺ pump of airway smooth muscle via 5-HT_2A receptors by a mechanism dependent on Na⁺ influx, possibly through the Na⁺-H⁺ exchanger. Because stimulation of the Na⁺-K⁺ pump produces hyperpolarization, this may represent a negative-feedback mechanism that opposes contraction in response to 5-HT.