Adaptive remodeling of renal Na+ and K+ transport during pregnancy

Low serum carnitine level is associated with increased urinary carnitine excretion in late pregnancy

BACKGROUND

Carnitine is essential for fatty acid metabolism. Free carnitine (FCA) is excreted in the urine in the glomerulus, but is partly reabsorbed by a carnitine transporter. The mechanism underlying the decrease in serum carnitine level during pregnancy is unclear.

OBJECTIVE

To investigate whether low carnitine level is associated with increased renal excretion in pregnant women.

METHODS

We recruited 43 healthy pregnant and 25 non-pregnant women. Total carnitine (TCA) and FCA levels were measured using the enzymatic cycling method, and the acylcarnitine (ACA) level was calculated. Fractional excretion (FE) was calculated as carnitine clearance divided by creatinine clearance.

RESULTS

The mean TCA, FCA, and ACA levels were lower at 12 weeks of gestation in pregnant than non-pregnant women (P < .001); the levels decreased further at 36 weeks, reaching 39%, 36%, and 52% of those in non-pregnant women, respectively (P < .001). The FEs were 3-4-fold higher in pregnant women than non-pregnant women. Pregnant women had a lower serum FCA/TCA ratio than non-pregnant women (0.788 ± 0.098 vs 0.830 ± 0.074, respectively; P < .05), whereas the urine FCA/TCA ratio was similar between the groups.

CONCLUSION

Low carnitine level is associated with increased renal excretion during late pregnancy.

Sex differences in renal transporters: assessment and functional consequences

Mammalian kidneys are specialized to maintain fluid and electrolyte homeostasis. The epithelial transport processes along the renal tubule that match output to input have long been the subject of experimental and theoretical study. However, emerging data have identified a new dimension of investigation: sex. Like most tissues, the structure and function of the kidney is regulated by sex hormones and chromosomes. Available data demonstrate sex differences in the abundance of kidney solute and electrolyte transporters, establishing that renal tubular organization and operation are distinctly different in females and males. Newer studies have provided insights into the physiological consequences of these sex differences. Computational simulations predict that sex differences in transporter abundance are likely driven to optimize reproduction, enabling adaptive responses to the nutritional requirements of serial pregnancies and lactation - normal life-cycle changes that challenge the ability of renal transporters to maintain fluid and electrolyte homeostasis. Later in life, females may also undergo menopause, which is associated with changes in disease risk. Although numerous knowledge gaps remain, ongoing studies will provide further insights into the sex-specific mechanisms of sodium, potassium, acid-base and volume physiology throughout the life cycle, which may lead to therapeutic opportunities.

Potassium homeostasis: sensors, mediators, and targets

Transmembrane potassium (K) gradients are key determinants of membrane potential that can modulate action potentials, control muscle contractility, and influence ion channel and transporter activity. Daily K intake is normally equal to the amount of K in the entire extracellular fluid (ECF) creating a critical challenge - how to maintain ECF [K] and membrane potential in a narrow range during feast and famine. Adaptations to maintain ECF [K] include sensing the K intake, sensing ECF [K] vs. desired set-point and activating mediators that regulate K distribution between ECF and ICF, and regulate renal K excretion. In this focused review, we discuss the basis of these adaptions, including (1) potential mechanisms for rapid feedforward signaling to kidney and muscle after a meal (before a rise in ECF [K]), (2) how skeletal muscles sense and respond to changes in ECF [K], (3) effects of K on aldosterone biosynthesis, and (4) how the kidney responds to changes in ECF [K] to modify K excretion. The concepts of sexual dimorphisms in renal K handling adaptation are introduced, and the molecular mechanisms that can account for the benefits of a K-rich diet to maintain cardiovascular health are discussed. Although the big picture of K homeostasis is becoming more clear, we also highlight significant pieces of the puzzle that remain to be solved, including knowledge gaps in our understanding of initiating signals, sensors and their connection to homeostatic adjustments of ECF [K].

Adaptive Changes in single-nephron GFR, Tubular Morphology, and Transport in a Pregnant Rat Nephron: Modeling and Analysis

Normal pregnancy is characterized by massive increases in plasma volume and electrolyte retention. Given that the kidneys regulate homeostasis of electrolytes and volume, the organ undergoes major adaptations in morphology, hemodynamics, and transport to achieve the volume and electrolyte retention required in pregnancy. These adaptations are complex, sometimes counterintuitive, and not fully understood. In addition, the demands of the developing fetus and placenta change throughout the pregnancy. For example, during late pregnancy, K⁺ retention and thus enhanced renal K⁺ reabsorption is required despite many kaliuretic factors. The goal of this study is to unravel how known adaptive changes along the nephrons contribute to the ability of the kidney to meet volume and electrolyte requirements in mid- and late pregnancy. We developed computational models of solute and water transport in the superficial nephron of the kidney of a rat in mid- and late pregnancy. The mid-pregnant and late-pregnant rat superficial nephron models predict that morphological adaptations and increased activity of the sodium hydrogen exchanger 3 (NHE3) and epithelial sodium channel (ENaC) are essential for enhanced Na⁺ reabsorption observed during pregnancy. Model simulations showed that for sufficient K⁺ reabsorption, increased H ⁺-K ⁺-ATPase activity and decreased K⁺ secretion along the distal segments is required in both mid- and late-pregnancy. Furthermore, certain known sex differences in renal transporter pattern (e.g., the higher NHE3 protein abundance but lower activity in the proximal tubules of virgin female rats compared to male) may serve to better prepare the female for the increased transport demand in pregnancy.

Time course of renal sodium transport in the pregnant rat

Progressive sodium retention and cumulative plasma volume expansion occur to support the developing fetus during pregnancy. Sodium retention is regulated by individual tubular transporters and channels. An increase or decrease in any single transporter could cause a change in sodium balance. Understanding the time-course for changes in each sodium transporter during pregnancy will enable us to understand progressive sodium retention seen in pregnancy. Here, we examined the activity of the major apical sodium transporters found in the nephron using natriuretic response tests in virgin, early pregnant, mid-pregnant, and late pregnant rats. We also measured renal and serum aldosterone levels. We found that furosemide sensitive sodium transport (NKCC2) is only increased during late pregnancy, thiazide sensitive sodium transport (NDCBE/pendrin) is increased in all stages of pregnancy, and that benzamil sensitive sodium transport (ENaC) is increased beginning in mid-pregnancy. We also found that serum aldosterone levels progressively increased throughout gestation and kidney tissue aldosterone levels increased only during late pregnancy. Here we have shown progressive turning on of specific sodium transport mechanisms to help support progressive sodium retention through the course of gestation. These mechanisms contribute to the renal sodium retention and plasma volume expansion required for an optimal pregnancy.

[Sectio Caesarea under Gitelman Syndrome]

Gitelman syndrome is a rare inherited renal tubulopathy characterized by hypokalemia, hypomagnesemia and metabolic alcalosis. It is caused by a mutation in the SLC12A3 gene leading to a dysfunction of the thiazide-sensitive sodium chloride cotransporter and the magnesium transporters in the distal convoluted tubules. Only few reports of pregnant woman with Gitelman syndrome exist. Due to many unsolved questions about the impact on pregnancy and the maternal and fetal outcome, the obstetric and anesthesiological management remains a challenge. We discuss the case of a primary cesarean delivery in a 22-year-old woman with a new diagnosed Gitelman syndrome focusing on the anesthesiological approach.

HGF/c-Met signaling regulates early differentiation of placental trophoblast cells

Depletion of hepatocyte growth factor (HGF) or mesenchymal-epithelial transition factor (c-Met) in mice leads to fetal lethality and placental maldevelopment. However, the dynamic change pattern of HGF/c-Met signaling during placental development and its involvement in the early differentiation of trophoblasts remain to be elucidated. In this study, using in situ hybridization assay, we elaborately demonstrated the spatial-temporal expression of Hgf and c-Met in mouse placenta from E5.5, the very early stage after embryonic implantation, to E12.5, when the placental structure is well developed. The concentration of the soluble form of c-Met (sMet) in maternal circulation peaked at E10.5. By utilizing the induced differentiation model of mouse trophoblast stem cells (mTSCs), we found that HGF significantly promoted mTSC differentiation into syncytiotrophoblasts (STBs) and invasive parietal trophoblast giant cells (PTGCs). Interestingly, sMet efficiently reversed the effect of HGF on mTSC differentiation. These findings indicate that HGF/c-Met signaling participates in regulating placental trophoblast cell fate at the early differentiation stage and that sMet acts as an endogenous antagonist in this aspect.