Unidirectional uptake of substrates at the blood side of secretory epithelia: stomach, salivary gland, pancreas.

This paper reviews studies of cellular uptake of substrates by the gastric mucosa, salivary gland, and pancreas by single-circulation, multiple tracer dilution techniques. The application of this methodology to secretory organs in vivo has permitted the characterization of transport phenomena at the blood-tissue interface of resting and secreting epithelia. To estimate uptake of a test molecule after an intraarterial injection, its venous concentration profile (30 samples in 1 in) was compared with that of 1) an intravascular marker or 2) a diffusible molecule that remains confined to the extracellular space. Among the molecules investigated were 86Rb, 57Co-labeled cyanocobalamin, 125I-labeled insulin, [3H]ouabain, [3H]dopamine, [3H]norepinephrine, and a wide range of labeled amino acids. High tracer uptake (80%) was measured that could be inhibited by specific unlabeled competitors. Unidirectional influx was saturable and Michaelis-Menten kinetic constants could be estimated. The ultimate objective was to identify various transport systems and/or receptors at the basolateral side of these epithelia. However, nerve terminals in the interstitium could be the major site for the uptake of catecholamines.