Stimulation of the Na(+)-K(+) pump in cultured guinea pig airway smooth muscle cells by serotonin

The effect of 5-hydroxytryptamine (5-HT) or serotonin on Na(+)-K(+) pump activity of airway smooth muscle was investigated by measuring (86)Rb(+) uptake in cultured guinea pig tracheal smooth muscle cells. (86)Rb(+) 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 (86)Rb(+) uptake (EC(50) = 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 alpha-methyl-5-HT, but not by the 5-HT(1) agonist 5-carboxamidotryptamine, the 5-HT(1A/1B/2C) agonist 1-(3-chlorophenyl)piperazine, or the 5-HT(3) agonist 1-(3-chlorophenyl)biguanide. 5-HT-stimulated (86)Rb(+) 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-HT(3) antagonist Y25310. 5-HT-stimulated (86)Rb(+) 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.