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The Journal of Experimental Biology 206, 1589-1598 (2003)
doi: 10.1242/jeb.00280

Regulation of L-alanine transport systems A and ASC by cyclic AMP and calcium in a reptilian duodenal model

Tomás Gómez, Virtudes Medina, Cristina M. Ramírez, Rosa Dópido, Antonio Lorenzo and Mario Díaz^*

Laboratorio de Fisiología Animal, Departamento de Biología Animal, Universidad de La Laguna, 38206 Tenerife, Spain

^* Author for correspondence (e-mail: madiaz{at}ull.es)

Accepted 4 February 2003

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 [-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.

Key words: system A, system ASC, neutral amino acid transport, L-alanine transport, intracellular transducers, cyclic AMP, calcium, Gallotia galloti

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