Effects of serotonin on Na+-H+ exchange and intracellular calcium in isolated chicken enterocytes

The effects of serotonin or 5-hydroxytryptamine (5-HT) on Na absorption and intracellular free Ca (Cai) in isolated chicken enterocytes was examined. The rate of initial 22Na uptake was inhibited by 50% after 90 s of stimulation with 5-HT (10(-5) M), an effect that was not additive with amiloride (10(-3) M) and was transient (less than 10 min). 5-HT similarly decreased intracellular pH (pHi) in cells loaded with the pH indicator carboxyfluorescein, an effect that was also transient, not additive with amiloride, and Na dependent. The ED50 of this effect was approximately 10(-8) M. 5-HT also stimulated a transient increase in Cai as determined by quin2 fluorescence. A maximal increase of 60 nM occurred significantly before the peak change in pHi, but the total duration of response was similar in each case. In the absence of extracellular Ca, the 5-HT effects on pHi and Cai still persisted. In cells loaded with the Ca-buffering agent MAPTAM, the 5-HT (10(-5) M) inhibitory effect of 22Na influx was partially inhibited. We conclude that 5-HT directly inhibits Na absorption by isolated enterocytes by releasing endogenous Ca, which subsequently causes an inhibition of amiloride-sensitive Na+-H+ exchange.