Ouabain-insensitive active sodium transport in rat jejunum: evidence from ATPase activities, Na uptake by basolateral membrane vesicles and in vitro transintestinal transport

The second sodium pump: from the function to the gene

Transepithelial Na(+) transport is mediated by passive Na(+) entry across the luminal membrane and exit through the basolateral membrane by two active mechanisms: the Na(+)/K(+) pump and the second sodium pump. These processes are associated with the ouabain-sensitive Na(+)/K(+)-ATPase and the ouabain-insensitive, furosemide-inhibitable Na(+)-ATPase, respectively. Over the last 40 years, the second sodium pump has not been successfully associated with any particular membrane protein. Recently, however, purification and cloning of intestinal α-subunit of the Na(+)-ATPase from guinea pig allowed us to define it as a unique biochemical and molecular entity. The Na(+)- and Na(+)/K(+)-ATPase genes are at the same locus, atp1a1, but have independent promoters and some different exons. Herein, we spotlight the functional characteristics of the second sodium pump, and the associated Na(+)-ATPase, in the context of its role in transepithelial transport and its response to a variety of physiological and pathophysiological conditions. Identification of the Na(+)-ATPase gene (atna) allowed us, using a bioinformatics approach, to explore the tertiary structure of the protein in relation to other P-type ATPases and to predict regulatory sites in the promoter region. Potential regulatory sites linked to inflammation and cellular stress were identified in the atna gene. In addition, a human atna ortholog was recognized. Finally, experimental data obtained using spontaneously hypertensive rats suggest that the Na(+)-ATPase could play a role in the pathogenesis of essential hypertension. Thus, the participation of the second sodium pump in transepithelial Na(+) transport and cellular Na(+) homeostasis leads us to reconsider its role in health and disease.

Response of rainbow trout gill Na+-ATPpase to T(3) and NaCl administration

The effect of the administration of commercial diets supplemented with 9 mg kg(-1) 3,5,3'-triiodo-l-thyronine (T(3)) or 10% (w/w) NaCl was evaluated on the ouabain-insensitive Na+-ATPase activity in rainbow trout gill microsomes. The trial, carried out following the seasonal trend from March to mid-May, included a treatment phase in freshwater and a subsequent transfer to brackish water (22 per thousand salinity) where trout were not treated. pH dependence, apparent Km values for Mg(2+) and Na+, and Hill coefficients evaluated throughout the trial for Na+-ATPase were generally not affected by the treatments and habitat change. In comparison with the control group, in both treated groups, Na+-ATPase activity was lower during the freshwater phase and higher after brackish-water transfer. As compared with untreated trout, gill (Na++K+)-ATPase activity during the freshwater phase was stimulated by NaCl treatment and also by T(3) treatment after transfer to brackish water. The results indicate that NaCl and T(3) administration act differently on the two ATPase activities involved in Na+ regulation and suggest a prevalent role of Na+-ATPase activity in hypoosmotic conditions.

Effects of heat stress on Na+,K(+)-ATPase, Mg(2+)-activated ATPase, and Na(+)-ATPase activities of broiler chickens vital organs

Na+,K(+)-ATPase, Mg(2+)-activated ATPase, and Na(+)-ATPase activities of brain, heart, kidney, and small and large intestinal mucosa of broiler chickens exposed to heat stress (41 degrees C, 65% relative humidity for 6 h) and thermoneutral (25 degrees C, 65% relative humidity) conditions were determined. Brain and kidneys were found to have significantly higher Na+,K(+)-ATPase activities than those of heart and intestinal mucosa. Mg(2+)-activated ATPase and Na(+)-ATPase activities in the intestinal mucosa were higher than those of brain, kidneys, and heart under thermoneutral conditions. While there was a significant inhibition of total ATPase, Na+,K(+)-ATPase, Mg(2+)-activated ATPase, and Na(+)-ATPase activity of small and large intestinal mucosa of broiler chickens exposed to heat stress, the inhibitory effect was limited to total ATPase and Na+,K(+)-ATPase enzymes in kidneys. Heat stress produced a significant increase only in Mg(2+)-activated ATPase activity of the heart, without a remarkable change in all forms of ATPase activity in the brain. Heat stress significantly decreased the ratio of Na+,K(+)-ATPase to Mg(2+)-ATPase in the heart, kidneys, and small and large intestinal mucosa. The percentage of Na(+)-ATPase in Na+,K(+)-ATPase of brain, heart, and kidneys did not significantly change during heat stress, but the ratio in small and large intestinal mucosa increased significantly during heat stress. The severe disturbances in both serum electrolytes and acid-base balance observed in previous heat stress studies could partly be mediated by direct or indirect effects of heat stress on Na+,K(+)-ATPase, Mg(2+)-activated ATPase, and Na(+)-ATPase activities of kidneys, and small and large intestinal mucosa of broiler chickens.

Cl/HCO3 exchange in the basolateral membrane domain of rat jejunal enterocyte

Basolateral membrane vesicles isolated from rat jejunal enterocyte and well purified from brush border contamination were tested to examine Cl and HCO3 movements. Uptake experiments provided no evidence for a coupling between Na and HCO3 fluxes; K-HCO3 and K-Cl cotransports also could be excluded. Transport studies revealed the presence of a Cl/HCO3 exchanger accepting other anions and inhibitable by the disulfonic stilbenes SITS and DIDS. We can exclude that the evidenced HCO3-dependent Cl uptake is due to brush border contamination, since in jejunal brush border membranes this mechanism, if present, has a very low transport rate. Besides the Cl/HCO3 antiporter, a Cl-conductive pathway seems to exist in jejunal basolateral membranes.

Ouabain-insensitive Na-ATPase activity in homogenates from different animal tissues

1. Two Na(+)-stimulated ATPase activities were determined in gill homogenates from squid, shrimp and teleost fish; in kidney slice homogenates from teleost fish, bullfrog, toad, iguana, chicken, duck, rat, pig and cow, as well as in homogenates from rat small intestinal cells, brain cortex and liver slices. The two Na(+)-stimulated ATPase activities, the Na- and the Na,K-ATPase, showed a different behavior toward K+ and ouabain. 2. The ouabain-insensitive, K(+)-independent, Na-ATPase activity for all the studied homogenates was completely inhibited by 2 mM furosemide. 3. An increase in cell volume of the kidney, brain cortex and liver slice preparations, as well as of the rat small intestinal cells, produced a concomitant increase of the ouabain-insensitive Na-ATPase.

Aging and ATPase activities in rat jejunum

In addition to the well-known (Na,K)-ATPase activity, an ouabain-insensitive Na-ATPase has been evidenced in the basolateral membrane of intestinal and renal cells from different mammals. Basolateral membranes of jejunal enterocytes from rats of different ages, i.e., very young, young, adult and old were separated by self-orienting, Percoll-gradient centrifugation. The total protein content and both Na- and (Na,K)-ATPase activities in initial homogenate and final pellets were analyzed. The dry weight of homogenate and pellet was also determined. The two ATPase activities and the protein content of the basolateral membrane fraction decrease with age when referred to the dry weight of the pellet. This diminution is also evident in the initial homogenate. The activation curve of Na-ATPase, hyperbolic in shape, gives Km and Vmax values unaffected by aging. The same behaviour is true for the kinetic parameters of (Na,K)-ATPase, which has a sigmoidal velocity curve. From these results, it seems that both Na- and (Na,K)-ATPase have the same characteristics in the basolateral membrane of the enterocyte throughout the life span of the animal, but they decrease quantitatively with aging.