RESUMO
The AE2 Cl-/HCO3- exchanger is expressed in numerous cell types, including epithelial cells of the kidney, respiratory tract, and alimentary tract. In gastric epithelia, AE2 is particularly abundant in parietal cells, where it may be the predominant mechanism for HCO3- efflux and Cl- influx across the basolateral membrane that is needed for acid secretion. To investigate the hypothesis that AE2 is critical for parietal cell function and to assess its importance in other tissues, homozygous null mutant (AE2(-/-)) mice were prepared by targeted disruption of the AE2 (Slc4a2) gene. AE2(-/-) mice were emaciated, edentulous (toothless), and exhibited severe growth retardation, and most of them died around the time of weaning. AE2(-/-) mice exhibited achlorhydria, and histological studies revealed abnormalities of the gastric epithelium, including moderate dilation of the gastric gland lumens and a reduction in the number of parietal cells. There was little evidence, however, that parietal cell viability was impaired. Ultrastructural analysis of AE2(-/-) gastric mucosa revealed abnormal parietal cell structure, with severely impaired development of secretory canaliculi and few tubulovesicles but normal apical microvilli. These results demonstrate that AE2 is essential for gastric acid secretion and for normal development of secretory canalicular and tubulovesicular membranes in mouse parietal cells.
Assuntos
Proteínas de Transporte de Ânions , Antiporters , Proteínas de Membrana/genética , Proteínas de Membrana/fisiologia , Acloridria/genética , Alelos , Animais , Northern Blotting , Western Blotting , Sobrevivência Celular , Antiportadores de Cloreto-Bicarbonato , Células Epiteliais/metabolismo , Epitélio/metabolismo , Mucosa Gástrica/metabolismo , Vetores Genéticos , Concentração de Íons de Hidrogênio , Membranas Intracelulares/metabolismo , Camundongos , Camundongos Mutantes , Camundongos Transgênicos , Microscopia Confocal , Microscopia Eletrônica , Microscopia de Fluorescência , Mutação , Células Parietais Gástricas/metabolismo , Fenótipo , RNA Mensageiro/metabolismo , Proteínas SLC4A , TransgenesRESUMO
The apical Na+-H+ exchanger NHE3 plays an important role in fluid reabsorption in the proximal tubule. However, whether its deletion alters the salt and water transport in the distal nephron remains unknown. To answer these questions, wild-type (Nhe3+/+) and NHE3 null mice (Nhe3-/-) were placed in metabolic cages and their water balance and urine osmolality were examined. Nhe3-/- mice demonstrated a significant polydipsia (P < 0.03) and polyuria (P < 0.04), with a lower urine osmolality (P < 0.003) as compared to Nhe3+/+ mice. Northern hybridization and immunoblotting studies indicated that the mRNA expression and protein abundance of the collecting duct (CD) water channel AQP2 decreased by 52 % (P < 0.0003) and 73 % (P < 0.003) in the cortex, and by 53 % and 54 % (P < 0.002) in the inner medulla (IM) of Nhe3-/- vs. Nhe3+/+ mice. The expression of AQP2 in the outer medulla (OM) remained unchanged. Further, the mRNA expression and protein abundance of the medullary thick ascending limb (mTAL) apical Na+-K+-2Cl- cotransporter (NKCC2) decreased by 52 % (P < 0.02) and 44 % (P < 0.01), respectively, in the OM of Nhe3-/- vs. Nhe3+/+ mice. The circulating plasma levels of vasopressin as well as the mRNA expression of vasopressin prohormone were significantly increased in Nhe3-/- vs. Nhe3+/+ mice (P < 0.05). Studies in mice treated with acetazolamide indicated that increased bicarbonate and fluid delivery to distal nephron did not alter the expression of NKCC2 in mTAL and decreased AQP2 protein only in OM but not in the cortex or IM. In conclusion, mice lacking the apical NHE3 have impairment in their water balance and urine osmolality, which correlates with the downregulation of AQP2 expression. These defects occur despite increased circulating levels of vasopressin. We propose that an ADH-independent mechanism is responsible for the downregulation of AQP2 and the resulting polyuria in NHE3 null mice.
