Abstract
The unidirectional influx of methionine into the brush border epithelium of chicken jejunum has been studied. Tissues leached of Na + transport methionine from a medium devoid of Na + with reduced apparent affinity ( K t) and maximal flux ( J max). Addition of Na + to the medium during a 1-min incubation with substrate, or during a 30-min preincubation, restored K t but affected J max slightly. Theophylline was found to maintain J max in the absence of Na +. Essentially complete restoration of K t and J max could be attained when theophylline-treated tissue was exposed to Na + for 30 min. Influx from a Na + medium was unaffected by theophylline pretreatment in Na +-containing buffer. K t was increased without an effet upon J max when influx was studied from choline medium following preincubation in Na +. Modifiers of tissue cyclic AMP levels were investigated in conjunction with theophylline. Histamine and carbachol were found to inhibit theophylline-stimulated transport. Secretin was found to stimulate influx in Na +-leached tissue, but did not potentiate the theophylline effect. Amino acids in the incubation medium inhibited theophylline-stimulated influx, whereas preloaded lysine or methionine had no effect. The results are interpreted in terms of a model which envisions roles for cellular and external Na + and for cyclic AMP in the activation and regulation of amino acid transport in intestine.
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