Fig. 4. Effects of dibutyryl cyclic AMP (db-cAMP; 500 µmol l^–1) on isolated L-alanine transport systems across the duodenal mucosa of Gallotia galloti. (A) Effects on Na⁺-independent _L-alanine transport. (B) Effects on system A activity as determined by measuring the unidirectional and net MeAIB (-methylamino-isobutiric acid) fluxes (10 mmol l^–1) in Na⁺-containing solutions. (C) Effects on system ASC. System ASC transport activity was obtained by determining L-alanine unidirectional and net fluxes in the presence of a saturating concentration of MeAIB (20 mmol l^–1) in Na⁺-containing solutions. Effects of db-cAMP were measured 20 min after its addition to the bathing solutions. The concentration of L-alanine in the bathing solutions was 1 mmol l^–1. All experiments were performed under short-circuit conditions. Results are means ± S.E.M. for 16 duodenal segments under each condition. *P=0.05; **P=0.01 compared with control.