The Role of H, K-atpase Isoforms in Gastrointestinal Function
Epithelial tissues mediate absorptive and secretory ion transport processes to maintain physiological equilibrium of ions and fluid. These processes are mediated in part by ion transport proteins expressed throughout the mammalian gastrointestinal tract and the renal nephron. Ion transporters regulate, either directly or indirectly, blood and extracellular fluid volume and composition, stomach acid secretion, digestion and nutrient absorption, and excretion of metabolic by-products. Many of the genes encoding ion transporters have been identified and characterized at the molecular level; however, the specific contribution of each transporter to epithelial ion transport during normal and pathophysiological states is incompletely understood. The Na+-K+-2Cl- cotransporter isoform 1 (NKCC1) mediates the uptake of Na+, K+, and Cl- from extracellular fluid. In secretory epithelial tissues, this uptake is important for the secretion of K+ or Cl- enriched fluid. To identify the specific contribution that NKCC1 makes to epithelial secretion, I have generated and analyzed a recombinant mouse line with a targeted disruption of the NKCC1 gene. This study tests the hypothesis that NKCC1 is required for proper epithelial Cl- and K+ secretion, and that altered Na+-K+-Cl- uptake at the basolateral membrane will perturb fluid secretion. My results demonstrate the critical role of NKCC1 in hearing and refutes a previous hypothesis that it is necessary for gastric acid secretion. An unexpected finding was that NKCC1 plays an important role in the regulation of blood pressure. Na+/H+ exchanger isoform 3 (NHE3) mediates the absorption of Na+ and HCO3- from, and secretion of H+ into, the lumen of the small intestine and proximal tubule of the kidney. Mice lacking NHE3 exhibit a mild phenotype associated with perturbations in Na+-fluid volume homeostasis and elevated serum aldosterone. I have tested the hypothesis that compensation for these perturbations involves transcriptional remodeling of the renal ion transport system. Data obtained from gene expression profiling experiments of kidney mRNA, through the use of cDNA microarray technology and Northern blot analysis, suggests that the kidney responds to perturbations in Na+-fluid volume homeostasis largely through post-transcriptional regulation of ion transport and renal hemodynamics, and not through transcriptional regulation of ion transporters.