Increased renal α-epithelial sodium channel (ENAC) protein and increased ENAC activity in normal pregnancy

Abstract

West C, Zhang Z, Ecker G, Masilamani SM. Increased renal ␣-epithelial sodium channel (ENAC) protein and increased ENAC activity in normal pregnancy. Am J Physiol Regul Integr Comp Physiol 299: R1326 -R1332, 2010. First published August 4, 2010 doi:10.1152/ajpregu.00082.2010.-Pregnancy-mediated sodium (Na) retention is required to provide an increase in plasma volume for the growing fetus. The mechanisms responsible for this Na retention are not clear. We first used a targeted proteomics approach and found that there were no changes in the protein abundance compared with virgin rats of the ␤ or ␥ ENaC, type 3 Na ϩ /H ϩ exchanger (NHE3), bumetanide-sensitive cotransporter (NKCC2), or NaCl cotransporter (NCC) in mid-or late pregnancy. In contrast, we observed marked increases in the abundance of the ␣-ENaC subunit. The plasma volume increased progressively during pregnancy with the greatest plasma volume being evident in late pregnancy. ENaC inhibition abolished the difference in plasma volume status between virgin and pregnant rats. To determine the in vivo activity of ENaC, we conducted in vivo studies of rats in late pregnancy (days 18 -20) and virgin rats to measure the natriuretic response to ENaC blockade (with benzamil). The in vivo activity of ENaC (UNaV postbenzamil-UNaV postvehicle) was markedly increased in late pregnancy, and this difference was abolished by pretreatment with the mineralocorticoid receptor antagonist, eplerenone. These findings demonstrate that the increased ␣-ENaC subunit of pregnancy is associated with an mineralocorticoid-dependent increase in ENaC activity. Further, we show that ENaC activity is a major contributor of plasma volume status in late pregnancy. These changes are likely to contribute to the renal sodium retention and plasma volume expansion required for an optimal pregnancy. collecting duct; plasma volume expansion; sodium retention IN HUMANS AND RATS, PREGNANCY is accompanied by marked changes in cardiovascular function, renal function, and fluid homeostasis. These adaptations permit an increase in blood volume that will supply the growing uterus and fetus without the development of maternal hypertension.A normal healthy pregnancy is associated with a cumulative plasma volume expansion (PVE) (30 -50%) and avid sodium retention (2,26,27). Failure of this adaptation is associated with maternal morbidity/mortality and intrauterine growth restriction (3, 27). Despite the progressive PVE, there is no increase in maternal blood pressure due to a marked decrease in total peripheral vascular resistance (2, 7).Renal sodium excretion determines volume homeostasis and the progressive PVE of normal pregnancy must reflect net renal sodium retention. Alexander et al.(1) performed balance studies in the pregnant rat that demonstrated cumulative sodium retention during pregnancy. The reabsorption of sodium along the renal tubule is determined by the regulation of the individual tubular cotransporters and channels (16), such that a change in the activity of any transporter or channel...

Chronic vasodilation increases renal medullary PDE5A and α-ENaC through independent renin-angiotensin-aldosterone system pathways

“…We found no change in renal cortical NCC (an aldosterone responsive sodium transporter in the distal tubule) with NIF-induced chronic vasodilation in the present study, although Wang et al did report increased NCC using a much higher dose of NIF (37). However, in normal pregnancy, we also observed that cortical NCC was unchanged (38); thus, the chronic systemic vasodilation in virgin female rats in the present study recapitulates the changes seen in normal pregnancy (24,38). The renal sodium retention of normal pregnancy is accompanied by early and sustained activation of the RAAS (5, 17, 34), a physiological response to an "underfill" signal.…”

“…The main novel findings of this study are: 1) the apical sodium transporter profile of the distal nephron with chronic NIF-induced vasodilation is similar to the previously defined profile in pregnancy (38), with a selective increase in ␣-ENaC abundance in the medulla. 2).…”

“…ENaC is a heteromultimeric channel composed of three homologous subunits (␣, ␤, ␥), with ␣-ENaC being the rate-limiting subunit for channel formation (21). We also found that ENaC blockade with benzamil abolished the PVE in late pregnancy (38). These data taken together emphasize the importance of collecting duct adaptations in mediating the renal sodium retention and volume expansion of normal pregnancy.…”

“…In addition, we recently examined the protein profile of the major sodium transporters/channels along the renal tubule during pregnancy and found that the only protein consistently upregulated throughout gestation was the ␣-subunit of the epithelial sodium channel (␣-ENaC) (38). ENaC is a heteromultimeric channel composed of three homologous subunits (␣, ␤, ␥), with ␣-ENaC being the rate-limiting subunit for channel formation (21).…”

Physiological and Molecular Responses to Altered Sodium Intake in Rat Pregnancy

“…Thus, ENaC appears to be a central element in Na + regulation in pregnancy. Although its transcript regulation is mediated via aldosterone,30 further activating modifications are due to protein‐cleaving conditions.…”