Ontogeny of Na/H antiporter activity in rabbit renal brush border membrane vesicles.

Abstract

The development of the Na/H antiporter was studied in renal brush border membrane vesicles (BBMV) from fetal and adult rabbits using isotopic and fluorescent techniques. The kinetics of the antiporter studied by 'Na' uptake revealed that the V.,. was only 25% of that in the adult; however, the K.'s for Na'were not significantly different. These data were confirmed by a fluorescent assay using the pH-sensitive probe, acridine orange: the V.. was significantly lower in the fetal BBMV. Conductive Na' movement was estimated from amiloride-insensitive 22Na' uptake and the rate of alkalinization induced by K+, an ion whose relative conductance was found to be similar to that of Na'. Although relative Na' conductance was significantly greater in fetal BBMV, the lower V.,,, in fetal vesicles could not be ascribed to this factor. Maternal administration of betamethasone (50 ,ug/kg intramuscularly) for 2 d before delivery significantly increased the V.., of the antiporter to levels observed in the adult; K. was unaffected. Na/K ATPase activity increased fourfold after betamethasone, but the specific activities offour brush border marker enzymes and the kinetics of Na'-glucose cotransport were unchanged. These data indicate that there is a developmental increase in brush border Na/H exchange which is the result of an increase in the number and/ or the turnover number of the carriers. Further, these data suggest that the postnatal increase in antiporter activity may be related to the surge in glucocorticoid concentration that occurs perinatally. (J. Clin.

Ontogeny of Na+/H+ Antiporter Activity in Rat Proximal Convoluted Tubules

“…There is also evidence that the developmental increase in proximal tubule Na ϩ /H ϩ antiporter activity is responsible for inducing the postnatal increase in basolateral Na ϩ -K ϩ -ATPase activity (14). We have previously demonstrated that there is a maturational increase in rabbit PCT Na ϩ /H ϩ antiporter activity (2,15), and others have confirmed this using rabbit BBMV (16). However, Na ϩ /H ϩ antiporter activity in rat renal BBMV was found to be higher in neonates than in adults (17), raising the possibility that our findings (2,15) and those of Beck et al (16) may be peculiar to the rabbit.…”

“…4-Acetamido-4Ј-isothiocyano-stilbene-2,2Ј disulfonic acid (SITS, 1 mM) was present in the bathing solution to inhibit the sodium bicarbonate cotransporter, a major regulator of intracellular pH i in PCTs (15,18). The bathing solution had a bicarbonate concentration of 5 mM and a pH of 6.6 to compensate for the cell alkalinization caused by the addition of bath SITS (16,18). The bathing solution was exchanged at a rate of 5 mL/min.…”

Maturational Effects of Glucocorticoids on Neonatal Brush-Border Membrane Phosphate Transport

“…Neonatal proximal tubules have a lower rate of glucose (3,4) and proline (38) transport than adult proximal tubules. Although prenatal administration of g1ucocorticoids has been shown to accelerate the maturation of the renal apical Na"/H+ antiporter (2, 7), basolateral Na(HC0 3 h symporter (2), and basolateral Na+-K+-ATPase activity (I, [8][9][10], it had no effect on glucose transport (2,7). In the present study, we measured Na-dependent glucose and proline transport as a control to determine whether there was a nonspecific inhibition in 88M transport in dexamethasone-treated neonates.…”

“…The cortex was isolated and homogenized with a Teflon-glass Potter Eljevhem homogenizer. 88M vesicles were then isolated by differential centrifugation and magnesium precipitation as previously described (4,7,20,21). The final 88M vesicle fraction was resuspended in the above buffer at an approximate concentration of 10 mg 88M protein/ml, for equilibrium binding studies.…”

“…Neonates given glucocorticoids have an increase in proximal tubule Na+-K+-ATPase activity (1, 3,8,9). Fetal rabbits whose mothers were given 50 j.lg/kg of betamethasone daily for 2 d before delivery had a higher rate of renal BBM vesicle Na"IH+ antiporter activity and renal Na+-K+-ATPase activity than neonatal vehicle-treated controls (7). Using a similar protocol, we have found that proximal convoluted tubules from neonates whose mothers were treated with dexamethasone 2 d before delivery had a higher rate of volume absorption, bicarbonate transport, a more lumen-negative transepithelial potential difference, and higher rates of apical Na"IH+ anti porter and basolateral Na(HC03h symporter activity than control neonates (2).…”