Background: Formation of an ileostomy results in losses of intestinal salt and water. In ileal mucosa, Na+ absorption is mediated, in part, by Na+/H+ exchangers (isoforms NHE2 and NHE3)located in the apical membrane of the enterocyte. It is not known whether diversion of luminal contents alters expression of NHE isoforms in ileal mucosa. Aim: to evaluate whether luminal Na+ or nutrients influence NHE expression in a rat model of ileal diversion.
Methods: Adult Sprague- Dawley rats underwent sham operation (SHAM)or loop ileostomy (ILEO), whick left 5 cm of terminal ileum defunctionalized. Rats were sacrificed at 8 or 21 days after operation and mucosa was harvested. Northern analysis was used to assess mRNA levels for NHE2, NHE3 and Na/K ATPase (a housekeeping transporter.
Results: As shown in the Table, mRNA levels of NHE-3 were decreased most markedly. Subsequent studies focused on responses of NHE3 mRNA levels to the infusion of Na+ or organic nutrients into the diverted ileal segment. First, groups of ILEO rats (n=5 ea) received 2x daily infusion of 3 ml solutions containing normal saline (NS,308 mOsm) or mannitol (MAN, 308 mOsm). Compared to a control ILEO group (100±8%), NHE-3 mRNA levels increased significantly in response to NS (140±16%, p<0.01)but not in response to MAN (105±5%,p>0.05). In further studies, groups of ILEO rats (n=5) received 2x daily infusion of the enterocyte nutrient glutamine (25mM in NS) or the colonocyte nutrient butyrate (25mM in NS). Compared to animals receiving NS alone (100±11%), NHE3 mRNA levels were markedly elevated in response to glutamine (174±20%, p<0.02) but not in response to butyrate (127±12%,p>0.05).
Conclusions: Luminal diversion selectively attenuates NHE3 gene expression in ileal mucosa. Luminal Na+ preserves, and with glutamine, may enhance mRNA levels of NHE-3 in ileal mucosa. The NHE-3 isoform may prove to be a suitable target for therapies aimed at improving Na+ absorption by ileal mucosa after diversion or massive resection of upstream regions of intestine.
8 day ILEO (n=10 ea) 21 day ILEO (n=5 ea)
Na/K NHE2 NHE3 Na/K NHE2 NHE3
100±8 100±3 100±6 100±4 100±2 100±5
93±4 105±5 83±5* 101±5 89±4* 32±6*
Densitometry normalized to GAPDH, means±SE, p<0.05 by ANOVA