Melatonin-induced calcium signaling in clusters of human and rat duodenal enterocytes

The amount of melatonin present in enterochromaffin cells in the alimentary tract is much higher than that in the central nervous system, and melatonin acting at MT(2) receptors mediates neural stimulation of mucosal HCO(3)(-) secretion in duodenum in vivo. We have examined effects of melatonin and receptor ligands on intracellular free calcium concentration ([Ca(2+)](i)) signaling in human and rat duodenal enterocytes. Clusters of interconnecting enterocytes (10-50 cells) were isolated by mild digestion (collagenase/dispase) of human duodenal biopsies or rat duodenal mucosa loaded with fura-2 AM and attached to the bottom of a temperature-controlled perfusion chamber. Clusters provided viable preparations and respond to stimuli as a syncytium. Melatonin and melatonin receptor agonists 2-iodo-N-butanoyl-5-methoxytryptamine and 2-iodomelatonin (1.0-100 nM) increased enterocyte [Ca(2+)](i), EC(50) of melatonin being 17.0 +/- 2.6 nM. The MT(2) receptor antagonists luzindole and N-pentanoyl-2-benzyltryptamine abolished the [Ca(2+)](i) responses. The muscarinic antagonist atropine (1.0 microM) was without effect on basal [Ca(2+)](i) and did not affect the response to melatonin. In the main type of response, [Ca(2+)](i) spiked rapidly and returned to basal values within 4-6 min. In another type, the initial rise in [Ca(2+)](i) was followed by rhythmic oscillations of high amplitude. Melatonin-induced enterocyte [Ca(2+)](i) signaling as well as mucosal cell-to-cell communication may be involved in stimulation of duodenal mucosal HCO(3)(-) secretion.