Intestinal ion transport in NKCC1-deficient mice

Abstract

The Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) located on the basolateral membrane of intestinal epithelia has been postulated to be the major basolateral Cl(-) entry pathway. With targeted mutagenesis, mice deficient in the NKCC1 protein were generated. The basal short-circuit current did not differ between normal and NKCC1 -/- jejuna. In the -/- jejuna, the forskolin response (22 microA/cm(2); bumetanide insensitive) was significantly attenuated compared with the bumetanide-sensitive response (52 microA/cm(2)) in normal tissue. Ion-replacement studies demonstrated that the forskolin response in the NKCC1 -/- jejuna was HCO(3)(-) dependent, whereas in the normal jejuna it was independent of the HCO(3)(-) concentration in the buffer. NKCC1 -/- ceca exhibited a forskolin response that did not differ significantly from that of normal ceca, but unlike that of normal ceca, was bumetanide insensitive. Ion-substitution studies suggested that basolateral HCO(3)(-) as well as Cl(-) entry (via non-NKCC1) paths played a role in the NKCC1 -/- secretory response. In contrast to cystic fibrosis mice, which lack both basal and stimulated Cl(-) secretion and exhibit severe intestinal pathology, the absence of intestinal pathology in NKCC1 -/- mice likely reflects the ability of the intestine to secrete HCO(3)(-) and Cl(-) by basolateral entry mechanisms independent of NKCC1.

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

“…A similar situation occurs in CFTR knockout mice (379), suggesting that the basolateral Na (123,277), and gastrointestinal secretory problems ending in death due to cecum and colonic blockade (123,157). Although BSC2/NKCC1 knockout mice exhibit a clear decrease in Cl Ϫ influx in respiratory epithelium, the absence of a respiratory phenotype appears to be due to compensation by other anion transporters (158).…”

“…These observations, however, have not been consistently observed because increased mortality due to intestinal problems was not present in BSC2/NKCC1-null mice done by other groups (78,306). The difference appears to be adaptability of intestinal epithelium, which in the absence of Cl Ϫ secretion exhibits an increase in HCO 3 Ϫ secretion (157). Finally, within the gastrointestinal system, as revealed by Evans et al (114) BSC2/NKCC1 knockout mice also exhibited severe salivation impairment (Fig.…”

Electrolyte Transport in the Mammalian Colon: Mechanisms and Implications for Disease

“…This points to the existence of additional basolateral uptake mechanisms, such as coupled Cl Ϫ /HCO 3 Ϫ and Na ϩ /H ϩ exchange, that might contribute to Cl Ϫ secretion. In addition, part of the residual cAMP activated shortcircuit current is probably caused by secretion of HCO 3 Ϫ , whose uptake does not require transport by NKCC1 (231).…”

Decreased blood pressure and vascular smooth muscle tone in mice lacking basolateral Na+-K+-2Cl−cotransporter

“…NKCC1 is expressed in most tissues, including heart (10, 42), vascular smooth muscle (39), and endothelial cells (37). Although the roles of NKCC1 in transepithelial ion transport and in the regulation of cell volume and intracellular ion concentrations are well established (9,14,15,17,(19)(20)(21)24, 36, 37), its cardiovascular functions are not well understood. Vasoactive agents alter the activity of NKCC1 in vascular smooth muscle (2,39,47) and in endothelial cells (6,27,35).…”

“…NKCC1-deficient (Nkcc1 Ϫ/Ϫ ) mice exhibit reduced epithelial chloride secretion (15,17,19,20), male sterility (40), and both profound deafness and a balance defect (9,15). In our own study (15), we also reported a significant reduction in mean arterial pressure (MAP) of anesthetized Nkcc1 Ϫ/Ϫ mice, measured using a femoral artery cath-eter; however, another group of investigators (40) observed no significant difference in systolic blood pressure of awake Nkcc1 Ϫ/Ϫ and wild-type (Nkcc1 ϩ/ϩ ) mice, measured using a tail-cuff apparatus.…”