Mucosal enterokinase activity was established at intervals throughout the small intestine in guinea-pigs; maximum activity was present in the duodenum and proximal jejunum in new born as well as adult animals. Transposition of 5 cm lengths of small gut from the high enterokinase containing proximal region to the distal intestine and vice versa showed that mucosal enterokinase activity in the transposed segments was little changed after several weeks of healthy life. Isolation of proximal jejunal loops from luminal continuity resulted in the fall of mucosal enterokinase activity to minimal levels within 16 hours. Low levels of mucosal enterokinase activity were identified in loops of both proximal and distal jejunum 12 weeks after isolation. Luminal perfusion studies in vivo in proximal jejunal loops 24 hours after isolation showed that mucosal enterokinase activity could be restored to near normal levels within four to six hours by luminal sodium in the presence of active pancreatic endopeptidases, oligopeptides, L-amino acids, or D-glucose but not D-amino acids or D-fructose. Near normal mucosal enterokinase activity persisted in the loops for as long as luminal perfusion with 144 mM sodium and L-lysine or trypsin was maintained (24 hours). The time course of the restoration of mucosal enterokinase activity was compatible with an initial precursor activation as well as biosynthesis. The requirement for luminal sodium appeared to be absolute regardless of the co-substrate and supports the conclusion that mucosal enterokinase activity is dependent on mediated sodium transport. The ability of proximal intestinal enterocytes to respond to sodium flux with an increase in enterokinase activity is a property determined in intrauterine life: distal intestinal enterocytes may have functioning structural genes for enterokinase but appear to be unable to respond.
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