| 12654897 |
Gomez T, Medina V, Ramirez CM, Dopido R, Lorenzo A, Diaz M: Regulation of L-alanine transport systems A and ASC by cyclic AMP and calcium in a reptilian duodenal model. J Exp Biol. 2003 May;206(Pt 9):1589-98.
The regulation of neutral amino acid transport by cyclic AMP (cAMP) and calcium across the isolated duodenum of the lizard Gallotia galloti has been studied under short-circuit conditions. Active L-alanine transport was stimulated by forskolin, theophylline and dibutyryl cyclic AMP (db-cAMP). All these agents increased transmural potential difference (PD) and short-circuit current (I (sc)) in a manner consistent with the activation of a chloride secretory pathway. Both forskolin and theophylline increased intracellular cAMP levels in the lizard duodenal mucosa. Addition of calcium ionophore A23187 rapidly reduced mucosa-to-serosa L-alanine fluxes and diminished net L-alanine transport. Despite the reduction of alanine fluxes by A23187, transepithelial PD and I (sc) values were increased by the ionophore. Analyses of the responses of isolated transport pathways indicated that the Na (+)-independent L-alanine transport system was unaffected by db-cAMP or calcium ionophore. By contrast, Na (+)-dependent transport activities were profoundly modified by these agents. Thus, while system A [alpha-methylamino-isobutiric acid (MeAIB)-transporting pathway] was stimulated by increased calcium, system ASC activity was nearly abolished. Calcium ionophore also potentiated the electrogenic response of system A. Forskolin strongly stimulated system ASC activity but left system A activity unchanged. Activation of system ASC by forskolin was clearly electroneutral, as pre-incubation of the tissues with the chloride channel blocker diphenylamine-2-carboxilic acid (DPC) completely prevented forskolin-induced transepithelial electrical responses. It is concluded that intracellular messengers cAMP and calcium oppositely modulate active Na (+)-dependent (L)-alanine transport in the lizard intestine. The different sensitivity exhibited by individual transport pathways may well account for the changes observed in overall alanine transport. |
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