Diurnal potassium excretory cycles in the rat

Effect of circadian on the activities of ion transport ATPases and histological structure of kidneys in mice

The impacts of unnatural every day cycles (circadian) for 60 days on the histological structure of kidneys and ATPase activities in MF1 mice were studied. The exposure times were 16 h dark, 16 h light, 24 h dark, and 24 h light, and control exposure times were 12 h dark followed by 12 h light. Our results showed an increase in the total ATPase activity of mice in all groups. Additionally, the activity of the enzyme Na⁺/K⁺-ATPase was increased after 24 h darkness, 24 h light, and 16 h light exposures compared to control. The enzyme Mg⁺²-ATPase activities of the groups were higher when exposed to 16 h light, 24 h light, 24 h darkness and 16 h darkness. The activities of total ATPase, Na⁺/K⁺-ATPase and Mg⁺²-ATPase in kidneys were increased in all groups after 24 h light, 24 h darkness, 16 h darkness and 16 h light exposures. Interestingly, the activity of V-type ATPase was reduced after 16 h darkness, 24 h darkness and 16 h light. Taking everything into account, changes in the day by day cycle prompt neurotic changes, enzymatic and histological changes in the kidneys of mice. More studies should be directed to explore the impacts of light and darkness that can prompt these progressions.

Glucagon actions on the kidney revisited: possible role in potassium homeostasis

It is now recognized that the metabolic disorders observed in diabetes are not, or not only due to the lack of insulin or insulin resistance, but also to elevated glucagon secretion. Accordingly, selective glucagon receptor antagonists are now proposed as a novel strategy for the treatment of diabetes. However, besides its metabolic actions, glucagon also influences kidney function. The glucagon receptor is expressed in the thick ascending limb, distal tubule, and collecting duct, and glucagon regulates the transepithelial transport of several solutes in these nephron segments. Moreover, it also influences solute transport in the proximal tubule, possibly by an indirect mechanism. This review summarizes the knowledge accumulated over the last 30 years about the influence of glucagon on the renal handling of electrolytes and urea. It also describes a possible novel role of glucagon in the short-term regulation of potassium homeostasis. Several original findings suggest that pancreatic α-cells may express a “potassium sensor” sensitive to changes in plasma K concentration and could respond by adapting glucagon secretion that, in turn, would regulate urinary K excretion. By their combined actions, glucagon and insulin, working in a combinatory mode, could ensure an independent regulation of both plasma glucose and plasma K concentrations. The results and hypotheses reviewed here suggest that the use of glucagon receptor antagonists for the treatment of diabetes should take into account their potential consequences on electrolyte handling by the kidney.

Role of circadian rhythms in potassium homeostasis

It has been known for decades that urinary potassium excretion varies with a circadian pattern. In this review, we consider the historical evidence for this phenomenon and present an overview of recent developments in the field. Extensive evidence from the latter part of the past century clearly shows that circadian potassium excretion does not depend on endogenous aldosterone. Of note is the recent discovery that the expression of several renal potassium transporters varies with a circadian pattern that appears to be consistent with substantial clinical data regarding daily fluctuations in urinary potassium levels. We propose the circadian clock mechanism as a key regulator of renal potassium transporters, and consequently renal potassium excretion. Further investigation into the regulation mechanism of renal potassium transport by the circadian clock is warranted to increase our understanding of the clinical relevance of circadian rhythms to potassium homeostasis.

Role of pituitary in K+ homeostasis: impaired renal responses to altered K+ intake in hypophysectomized rats

The kidneys maintain extracellular K⁺ homeostasis by altering K⁺ excretion to match K⁺ intake. Because this can occur without changes in plasma K⁺ concentrations ([K⁺]), how the kidneys sense K⁺ intake is unclear. We tested the hypothesis that the pituitary plays a critical role in signaling K⁺ intake to the kidneys. If this hypothesis is true, hypophysectomy would impair kidney responses to altered K⁺ intake. Hypophysectomized (Hypox) and sham-operated control rats (n = 8 each) were compared for their abilities to adjust K⁺ excretion during a transition from normal to reduced (to one-third of normal) K⁺ intake, followed by a reversal to normal K⁺ intake. Food was provided only at night, and renal K⁺ excretion was determined both for absorptive (night or feeding) and postabsorptive (day or nonfeeding) periods. In normal rats, both absorptive and postabsorptive renal K⁺ excretion were changed in parallel to the changes in K⁺ intake, indicating a rapid adaptation of normal kidneys to altered K⁺ intake. In Hypox rats, whereas absorptive renal K⁺ excretion was changed in response to changes in K⁺ intake, postabsorptive K⁺ excretion was not responsive (P < 0.001), indicating impaired renal responses to altered K⁺ intake. In addition, Hypox rats, compared with control rats, showed K⁺ intolerance (increases in plasma [K⁺]) upon feeding (i.e., K⁺ intake) at night or following an intravenous K⁺ infusion (P < 0.01), indicating an impairment of acute renal responses to K⁺ intake. These data support that the pituitary plays a key role in the signaling of K⁺ intake to the kidneys (and kidney responses to altered K⁺ intake).