Postprandial Effects on ENaC-Mediated Sodium Absorption

Abstract

Recent studies have suggested that postprandial increases in insulin directly contribute to reduced urinary sodium excretion. An abundance of research supports the ability of insulin to augment epithelial sodium channel (ENaC) transport. This study hypothesized that ENaC contributes to the increase in renal sodium reabsorption following a meal. To test this, we used fasted or 4 hour postprandial Sprague Dawley rats to analyze ENaC expression and activity. We also assessed total expression of additional sodium transporters (Na+-Cl− cotransporter (NCC), Na+-K+-2Cl− cotransporter (NKCC2), and Na+-K+-ATPase (NKA)) and circulating hormones involved in the renin-angiotensin-aldosterone system (RAAS). We found that after carbohydrate stimulus, ENaC open probability increased in split-open isolated collecting duct tubules, while ENaC protein levels remained unchanged. This was supported by a lack of change in phosphorylated Nedd4-2, an E3 ubiquitin ligase protein which regulates the number of ENaCs at the plasma membrane. Additionally, we found no differences in total expression of NCC, NKCC2, or NKA in the postprandial rats. Lastly, there were no significant changes in RAAS signaling between the stimulated and fasted rats, suggesting that acute hyperinsulinemia increases ENaC activity independent of the RAAS signaling cascade. These results demonstrate that insulin regulation of ENaC is a potential mechanism to preserve sodium and volume loss following a meal, and that this regulation is distinct from classical ENaC regulation by RAAS.

Increased ENaC activity during kidney preservation in Wisconsin solution

“…For Western blot analysis, kidney cortical lysates were prepared as previously described [14]. Briefly, kidney tissue samples (15–25 mg) were pulse sonicated in Laemmli buffer with a protease inhibitor cocktail (Roche) to a final concentration of 25 mg/ml.…”

“…Patch clamp analysis was used to assess ENaC activity in freshly isolated, split-opened cortical collecting duct (CCD) tubules. CCDs were isolated from rat kidney cortex as described previously [14–16]. A similar approach was used for isolation of CCDs from dog’s kidneys.…”

Association of estimated plasma volume and weight loss after long‐term administration and subsequent discontinuation of the sodium‐glucose cotransporter‐2 inhibitor tofogliflozin

“…Then 2 weeks after stopping tofogliflozin, IRI was significantly increased from week 52 (Table 1). An acute increase in serum insulin levels might increase fluid volume 12 and reduce urinary sodium excretion 13 . Taken together, PV re‐accumulation by increased IRI due to discontinuation of tofogliflozin and the disappearance of tofogliflozin‐induced urinary excretion might cause the overshoot of ePV from baseline levels (Figure S1).…”

“…An acute increase in serum insulin levels might increase fluid volume 12 and reduce urinary sodium excretion. 13 Taken together, PV re-accumulation by increased IRI due to discontinuation of tofogliflozin and the disappearance of tofogliflozin-induced urinary excretion might cause the overshoot of ePV from baseline levels (Figure S1). A positive correlation between BW changes and fasting insulin levels after tofogliflozin is discontinued (Table S3) might support that possibility.…”

Postprandial effects on electrolyte homeostasis in the kidney

“…SGK1 is a key regulator of ENaC in a number of different signaling pathways, including aldosterone stimulation, and an increase in its expression and/or activity increases ENaC activity (4,11,25). To establish the potential involvement of ENaC or other distal nephron transporters in postprandial Na ϩ retention, we used fasted Sprague-Dawley rats that were given either a high carbohydrate meal supplement (Supplical) or nothing and examined distal nephron Na ϩ channel/transporter expression as well as ENaC activity in freshly isolated split-open collecting duct tubules 4 h after administration of the meal supplement (1). A key unique factor of this study is that we did not manipulate the insulin or glucose levels of the animals, that is, the rats used in the study were not treated with a chemical to eliminate ␤-cells or instrumented in any way.…”