Maturation of proximal tubule Na+/H+ antiporter activity in sheep during transition from fetus to newborn.
We have studied maturational changes in the kinetics of the proximal tubule Na+/H+ antiporter. Microvillus membrane vesicles were prepared from renal cortex of fetal and newborn lambs. Amiloride-sensitive uptake of 22Na+ by these vesicles was measured and Woolf-Augustinsson-Hofstee plots were used to determine the Michaelis constant (Km) and rate of maximal uptake (Vmax). Initial studies of fetal lambs at 130-132 days gestation (n = 5; term is 145 days) and 3- to 4-day-old lambs (n = 5) revealed no maturational change in Km (7.27 +/- 1.25 for fetuses and 9.01 +/- 1.03 mM for lambs); however, there was a 242% increase in Vmax (from 1.28 +/- 0.33 in the fetuses to 4.37 +/- 0.85 nmol.s-1.mg protein-1 in the lambs, P = 0.005). Further definition of the developmental change in Na+/H+ antiporter Vmax was obtained when 144-day-gestation fetuses (n = 5) were compared with 24-h-old sibling lambs (n = 5) that had been delivered by cesarean section at 144 days gestation. Again, no significant difference was seen in Na+/H+ antiporter Km (14.9 +/- 6.5 for fetuses and 12.5 +/- 3.4 mM for lambs); however, a significant increase in Na+/H+ antiporter Vmax occurred (from 1.41 +/- 0.51 in the fetuses to 3.32 +/- 0.37 nmol.s-1.mg protein-1 in the lambs, P < 0.01). This study shows that there is a maturational increase in renal cortical Na+/H+ antiporter Vmax during the transition from fetal to newborn life. This increase parallels the increase in renal tubular Na+ reabsorption that occurs at this time.
Guillery
E N
EN
Department of Pediatrics, University of Iowa, Iowa City.
Karniski
L P
LP
Mathews
M S
MS
Robillard
J E
JE
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D000831:Animals, Newborn; D001692:Biological Transport / Q000187:drug effects; D002462:Cell Membrane / Q000378:metabolism; D005333:Fetus; D005865:Gestational Age; D007672:Kidney Cortex / Q000196:embryology / Q000378:metabolism / Q000502:physiology*; D007687:Kidney Tubules, Proximal / Q000196:embryology / Q000378:metabolism / Q000502:physiology*; D007700:Kinetics; D008871:Microvilli / Q000378:metabolism; D008985:Monensin / Q000494:pharmacology; D012756:Sheep; D012964:Sodium / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers / Q000187:drug effects / Q000378:metabolism*
D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D008985:Monensin; D012964:Sodium