Prenatal programming of rat proximal tubule Na+/H+ exchanger by dexamethasone

Abstract

Prenatal administration of dexamethasone causes hypertension in rats when they are studied as adults. Although an increase in tubular sodium reabsorption has been postulated to be a factor programming hypertension, this has never been directly demonstrated. The purpose of this study was to examine whether prenatal programming by dexamethasone affected postnatal proximal tubular transport. Pregnant Sprague-Dawley rats were injected with intraperitoneal dexamethasone (0.2 mg/kg) daily for 4 days between the 15th and 18th days of gestation. Prenatal dexamethasone resulted in an elevation in systolic blood pressure when the rats were studied at 7-8 wk of age compared with vehicle-treated controls: 131 +/- 3 vs. 115 +/- 3 mmHg (P < 0.001). The rate of proximal convoluted tubule volume absorption, measured using in vitro microperfusion, was 0.61 + 0.07 nl.mm(-1).min(-1) in control rats and 0.93+ 0.07 nl.mm(-1).min(-1) in rats that received prenatal dexamethasone (P < 0.05). Na(+)/H(+) exchanger activity measured in perfused tubules in vitro using the pH-sensitive dye BCECF showed a similar 50% increase in activity in proximal convoluted tubules from rats treated with prenatal dexamethasone. Although there was no change in abundance of NHE3 mRNA, the predominant luminal proximal tubule Na(+)/H(+) exchanger, there was an increase in NHE3 protein abundance on brush-border membrane vesicles in 7- to 8-wk-old rats receiving prenatal dexamethasone. In conclusion, prenatal administration of dexamethasone in rats increases proximal tubule transport when rats are studied at 7-8 wk old, in part by stimulating Na(+)/H(+) exchanger activity. The increase in proximal tubule transport may be a factor mediating the hypertension by prenatal programming with dexamethasone.

Fig. 1

Effect of prenatal dexamethasone (Dex) on systolic blood pressure. Pregnant Sprague-Dawley rats received either vehicle or Dex (0.2 mg/kg body wt) intraperitoneally daily between the 15th and 18th days of gestation. Blood pressure was measured in trained rats at 8 wk of age. There were 17 measurements in each group. *P < 0.001.

Fig. 2

Effect of prenatal Dex on proximal tubular volume absorption (J_V). Volume absorption, an index of sodium transport, was measured using the isolated perfused tubule technique in control rats and those that received prenatal Dex. There were 6 experiments in each group. The rate of volume absorption was significantly higher in rats that received prenatal Dex (*P < 0.05).

Fig. 3

Effect of prenatal Dex on Na⁺/H⁺ exchanger activity. Na⁺/H⁺ exchanger activity was measured in perfused tubules using the pH-sensitive dye BCECF. The rate was measured as the rate of change in intracellular pH with luminal sodium removal (dpHi/dt). There were 8 measurements in the control group and 9 in the prenatal dexamethasone group. The rate of change in pHi in tubules from rats that received prenatal Dex was higher than in tubules from control rats (*P < 0.05).

Fig. 4

Effect of prenatal Dex on renal cortical NHE3 mRNA abundance. NHE3 mRNA was compared with 28S using real-time PCR. There were 19 experiments in each group. There was no effect of prenatal Dex on NHE3 mRNA abundance (P = 0.15).

Fig. 5

Effect of prenatal Dex on brush-border membrane vesicle (BBMV) NHE3 protein abundance. There were 10 experiments in each group. NHE3 protein abundance was higher in BBMV from rats that were administered prenatal Dex (*P < 0.05).