Renal potassium physiology: integration of the renal response to dietary potassium depletion

Pharmacokinetic and Bioequivalent Study of Potassium Chloride Sustained-Release Tablet Under Different Dietary Conditions in Healthy Chinese Subjects

Potassium (K⁺ ) is an endogenous substance that is an essential dietary component. However, the interaction between dietary arrangements and specific effects of dietary K⁺ intake in bioequivalence studies remains unclear. To investigate the influence of dietary arrangement on the bioequivalence of potassium chloride (KCl) sustained-release tablets in healthy Chinese volunteers, the pharmacokinetics of KCl were compared in two open-label, single-center, randomized, two-period crossover studies with different dietary conditions. All volunteers received an oral dose of 6 g of KCl sustained-release tablets under fasting conditions, with different dietary arrangements. Urine samples were collected on baseline days and 48 hours after tablet consumption. Inductively coupled plasma-optical emission spectrometry was used to measure the concentration of K⁺ in the urine samples. Pharmacokinetic parameters were analyzed using Phoenix WinNonlin software in a noncompartmental model. In either clinical trial, no significant differences were observed in the maximal rate of urinary excretion and cumulative urinary excretion from 0 to 24 hours of K⁺ between the reference and test drugs. The bioequivalence studies of both KCl sustained-release tablet formulations were successfully conducted under different dietary conditions.

Ion Transporters and Osmoregulation in the Kidney of Teleost Fishes as a Function of Salinity

Euryhaline teleosts exhibit major changes in renal function as they move between freshwater (FW) and seawater (SW) environments, thus tolerating large fluctuations in salinity. In FW, the kidney excretes large volumes of water through high glomerular filtration rates (GFR) and low tubular reabsorption rates, while actively reabsorbing most ions at high rates. The excreted product has a high urine flow rate (UFR) with a dilute composition. In SW, GFR is greatly reduced, and the tubules reabsorb as much water as possible, while actively secreting divalent ions. The excreted product has a low UFR, and is almost isosmotic to the blood plasma, with Mg²⁺, SO₄^2–, and Cl^– as the major ionic components. Early studies at the organismal level have described these basic patterns, while in the last two decades, studies of regulation at the cell and molecular level have been implemented, though only in a few euryhaline groups (salmonids, eels, tilapias, and fugus). There have been few studies combining the two approaches. The aim of the review is to integrate known aspects of renal physiology (reabsorption and secretion) with more recent advances in molecular water and solute physiology (gene and protein function of transporters). The renal transporters addressed include the subunits of the Na⁺, K⁺- ATPase (NKA) enzyme, monovalent ion transporters for Na⁺, Cl^–, and K⁺ (NKCC1, NKCC2, CLC-K, NCC, ROMK2), water transport pathways [aquaporins (AQP), claudins (CLDN)], and divalent ion transporters for SO₄^2–, Mg²⁺, and Ca²⁺ (SLC26A6, SLC26A1, SLC13A1, SLC41A1, CNNM2, CNNM3, NCX1, NCX2, PMCA). For each transport category, we address the current understanding at the molecular level, try to synthesize it with classical knowledge of overall renal function, and highlight knowledge gaps. Future research on the kidney of euryhaline fishes should focus on integrating changes in kidney reabsorption and secretion of ions with changes in transporter function at the cellular and molecular level (gene and protein verification) in different regions of the nephrons. An increased focus on the kidney individually and its functional integration with the other osmoregulatory organs (gills, skin and intestine) in maintaining overall homeostasis will have applied relevance for aquaculture.

Use of Urine Electrolytes and Urine Osmolality in the Clinical Diagnosis of Fluid, Electrolytes, and Acid-Base Disorders

We discuss the use of urine electrolytes and urine osmolality in the clinical diagnosis of patients with fluid, electrolytes, and acid-base disorders, emphasizing their physiological basis, their utility, and the caveats and limitations in their use. While our focus is on information obtained from measurements in the urine, clinical diagnosis in these patients must integrate information obtained from the history, the physical examination, and other laboratory data.

The Mineralocorticoid Receptor in Salt-Sensitive Hypertension and Renal Injury

Hypertension and its comorbidities pose a major public health problem associated with disease-associated factors related to a modern lifestyle, such high salt intake or obesity. Accumulating evidence has demonstrated that aldosterone and its receptor, the mineralocorticoid receptor (MR), have crucial roles in the development of salt-sensitive hypertension and coexisting cardiovascular and renal injuries. Accordingly, clinical trials have repetitively shown the promising effects of MR blockers in these diseases. We and other researchers have identified novel mechanisms of MR activation involved in salt-sensitive hypertension and renal injury, including the obesity-derived overproduction of aldosterone and ligand-independent signaling. Moreover, recent advances in the analysis of cell-specific and context-dependent mechanisms of MR activation in various tissues-including a classic target of aldosterone, aldosterone-sensitive distal nephrons-are now providing new insights. In this review, we summarize recent updates to our understanding of aldosterone-MR signaling, focusing on its role in salt-sensitive hypertension and renal injury.

Electrolyte handling in the isolated perfused rat kidney: demonstration of vasopressin V2-receptor-dependent calcium reabsorption

BACKGROUND

The most profound effect of vasopressin on the kidney is to increase water reabsorption through V₂-receptor (V₂R) stimulation, but there are also data suggesting effects on calcium transport. To address this issue, we have established an isolated perfused kidney model with accurate pressure control, to directly study the effects of V₂R stimulation on kidney function, isolated from systemic effects.

METHODS

The role of V₂R in renal calcium handling was studied in isolated rat kidneys using a new pressure control system that uses a calibration curve to compensate for the internal pressure drop up to the tip of the perfusion cannula.

RESULTS

Kidneys subjected to V₂R stimulation using desmopressin (DDAVP) displayed stable osmolality and calcium reabsorption throughout the experiment, whereas kidneys not administered DDAVP exhibited a simultaneous fall in urine osmolality and calcium reabsorption. Epithelial sodium channel (ENaC) inhibition using amiloride resulted in a marked increase in potassium reabsorption along with decreased sodium reabsorption.

CONCLUSIONS

A stable isolated perfused kidney model with computer-controlled pressure regulation was developed, which retained key physiological functions. The preparation responds to pharmacological inhibition of ENaC channels and activation of V₂R. Using the model, the dynamic effects of V₂R stimulation on calcium handling and urine osmolality could be visualised. The study thereby provides evidence for a stimulatory role of V₂R in renal calcium reabsorption.

Dietary potassium and the kidney: lifesaving physiology

Potassium often has a negative connotation in Nephrology as patients with chronic kidney disease (CKD) are prone to develop hyperkalaemia. Approaches to the management of chronic hyperkalaemia include a low potassium diet or potassium binders. Yet, emerging data indicate that dietary potassium may be beneficial for patients with CKD. Epidemiological studies have shown that a higher urinary potassium excretion (as proxy for higher dietary potassium intake) is associated with lower blood pressure (BP) and lower cardiovascular risk, as well as better kidney outcomes. Considering that the composition of our current diet is characterized by a high sodium and low potassium content, increasing dietary potassium may be equally important as reducing sodium. Recent studies have revealed that dietary potassium modulates the activity of the thiazide-sensitive sodium-chloride cotransporter in the distal convoluted tubule (DCT). The DCT acts as a potassium sensor to control the delivery of sodium to the collecting duct, the potassium-secreting portion of the kidney. Physiologically, this allows immediate kaliuresis after a potassium load, and conservation of potassium during potassium deficiency. Clinically, it provides a novel explanation for the inverse relationship between dietary potassium and BP. Moreover, increasing dietary potassium intake can exert BP-independent effects on the kidney by relieving the deleterious effects of a low potassium diet (inflammation, oxidative stress and fibrosis). The aim of this comprehensive review is to link physiology with clinical medicine by proposing that the same mechanisms that allow us to excrete an acute potassium load also protect us from hypertension, cardiovascular disease and CKD.

Case Report: Renal potassium wasting in SARS-CoV-2 infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is associated with many potentially fatal complications. Renal involvement in various forms is common in addition to serum electrolyte disturbances. Early reports suggest that hypokalaemia may frequent those with SARS-CoV-2 infection and various aetiological factors may cause this electrolyte disturbance. A Chinese retrospective study has demonstrated renal potassium wasting in patients infected with SARS-CoV-2, however, it is not known if these patients were receiving diuretic therapy which may be a contributing factor. This case report illustrates an example of renal potassium wasting in SARS-CoV-2 infection in the absence of diuretics and extra-renal mechanisms with important lessons learned.

Case Report: Renal potassium wasting in SARS-CoV-2 infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is associated with many potentially fatal complications. Renal involvement in various forms is common in addition to serum electrolyte disturbances. Early reports suggest that hypokalaemia may frequent those with SARS-CoV-2 infection and various aetiological factors may cause this electrolyte disturbance. A Chinese retrospective study has demonstrated renal potassium wasting in patients infected with SARS-CoV-2, however, it is not known if these patients were receiving diuretic therapy which may be a contributing factor. This case report illustrates an example of renal potassium wasting in SARS-CoV-2 infection in the absence of diuretics and extra-renal mechanisms with important lessons learned.

SARS-CoV-2 and hypokalaemia: evidence and implications

The global pandemic secondary to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is leading to unprecedented global morbidity and mortality. With a bewildering array of complications, renal involvement in various forms is common, including serum electrolyte derangements. Hypokalaemia secondary to SARS-CoV-2 was common in a reported Chinese cohort. Here we review the emerging evidence on hypokalaemia and SARS-CoV-2 infection, the potential pathophysiological mechanisms based on early clinical and histopathological data and important clinical implications. Mechanisms of hypokalaemia are multifactorial and so the electrolyte disturbance can be difficult to avoid. We provide further support to the theory of renin-angiotensin-aldosterone (RAS) activation, discuss the strengths and weaknesses of implicating RAS involvement and highlight the importance of calculating the transtubular potassium gradient to identify those at risk of hypokalaemia and its complications.

Electrolyte Disturbances in SARS-CoV-2 Infection

The global pandemic secondary to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is leading to unprecedented global morbidity and mortality. With a bewildering array of complications, renal involvement in various forms is common, including serum electrolyte derangements. Hypokalaemia secondary to SARS-CoV-2 was common in a reported Chinese cohort. Here we review the emerging evidence on hypokalaemia and SARS-CoV-2 infection, the potential pathophysiological mechanisms based on early clinical and histopathological data and important clinical implications. Mechanisms of hypokalaemia are multifactorial and so the electrolyte disturbance can be difficult to avoid. We provide further support to the theory of renin-angiotensin-aldosterone (RAS) activation, discuss the strengths and weaknesses of implicating RAS involvement and highlight the importance of calculating the transtubular potassium gradient to identify those at risk of hypokalaemia and its complications.

Overview of Monogenic Forms of Hypertension Combined With Hypokalemia

Hypertension is an important risk factor in many conditions and creates a heavy burden of disease and mortality globally. Polygenic hypertension is the most common form; however, it is increasingly recognized that monogenic hypertension is not rare, especially in patients with electrolyte disorders. Single genetic alterations are associated with plasma volume expansion and catecholamines/sympathetic excess with simultaneously increased potassium excretion in the urine and potassium intracellular shift. Early-onset refractory hypertension and profound hypokalemia are characteristics of monogenic hypertension. However, accumulated evidence shows the existence of phenotypic heterogeneity in monogenic hypertension meaning that, even for mild symptoms, clinicians cannot easily exclude the possibility of monogenic hypertension. Genetic, epigenetic and non-genetic factors are all possible mechanisms influencing phenotypic diversity. Genetic sequencing is a precise and efficient method that can broaden the mutant gene spectrum of the disease and is very helpful for understanding the pathophysiology of monogenic hypertension. Genetic sequencing, along with biochemical tests and imaging modalities, is essential for the early diagnosis and targeted management of monogenic hypertension to avoid long-term catastrophic complications.

Interactions of the DASH Diet with the Renin-Angiotensin-Aldosterone System

BACKGROUND

The Dietary Approaches to Stop Hypertension (DASH) diet is widely recommended to lower blood pressure, but its mechanisms of action are unclear. Lines of evidence suggest an interaction with the renin-angiotensin-aldosterone system (RAAS).

OBJECTIVE

We conducted a randomized, controlled, cross-over feeding trial to test RAAS-related mechanisms underlying the DASH diet in patients with isolated systolic hypertension.

METHODS

Participants entered a 1-wk run-in period on a control (CON) diet and then consumed the DASH or CON diets for 4 wk each in randomized sequence. Calorie content was controlled to maintain weight, and sodium intake was set at 3 g daily. After each diet, participants had hormonal and hemodynamic assessments obtained at baseline, in response to RAAS inhibition with captopril (CAP) 25 mg, and to graded angiotensin II (AngII) infusions (1 ng/kg and 3 ng/kg × 45 min). Primary outcomes were mean arterial pressure (MAP) and renal blood flow (RBF), and secondary outcomes were diastolic function, pulse wave velocity (PWV), plasma renin activity (PRA), and aldosterone (ALDO) responses by diet.

RESULTS

In total, 44 (19 female) participants completed the study. DASH + CAP significantly lowered MAP compared with CON + CAP (83 ± 11 mmHg compared with 88 ± 14 mmHg, P <0.01). RBF was increased with DASH + CAP compared with CON + CAP (486 ± 149 cc/min compared with 451 ± 171 cc/min, P <0.001). Study diet did not change PWV but CAP reduced diastolic function on the DASH diet (P <0.05). DASH + CAP significantly increased PRA compared with CON + CAP (1.52 ± 1.78 ng/mL/min compared with 0.89 ± 1.17 ng/mL/min; P <0.001). ALDO sensitivity to AngII infusion was greater with DASH when compared to CON (17.4 ± 7.7 ng/mL compared with 13.8 ± 6.2 ng/dL, P <0.05) as was DASH + CAP compared with CON + CAP (15.1 ± 5.3 ng/dL compared with 13.1 ± 5.9 ng/mL, P <0.05).

CONCLUSIONS

The DASH diet interacts with the RAAS resulting in vascular and hormonal responses similar to a natriuretic effect, which appears to augment the hypotensive effect of angiotensin-converting enzyme (ACE) inhibition in individuals with isolated systolic hypertension. This trial was registered at clinicaltrials.gov as NCT00123006.

In vitro antioxidant, anti-glycation and immunomodulation activities of fermented blue-green algae Aphanizomenon flos-aquae

To clarify the antioxidant, anti-glycation and immunomodulatory capacities of fermented blue-green algae Aphanizomenon flos-aquae (AFA), hot aqueous extract suspensions made from 10% AFA were fermented by Lactobacillus plantarum AN7 and Lactococcus lactis subsp. lactis Kushiro-L2 strains isolated from a coastal region of Japan. The DPPH and O₂^- radical scavenging capacities and Fe-reducing power were increased in the fermented AFA. The increased DPPH radical scavenging capacity of the fermented AFA was fractionated to mainly < 3 kDa and 30-100 kDa. The increased O₂^- radical scavenging capacities were fractionated to mainly < 3 kDa. Anti-glycation activity in BSA-fructose model rather than BSA-methylglyoxal model was increased by the fermentation. The increased anti-glycation activity was fractionated to mainly 30-100 kDa. The NO concentration in the murine macrophage RAW264.7 culture media was high with the fermented AFA. The increased immunomodulation capacity was also fractionated to mainly 30-100 kDa. These results suggest that the fermented AFA is a more useful material for health foods and supplements.

Hypokalemia: a clinical update

Hypokalemia is a common electrolyte disturbance, especially in hospitalized patients. It can have various causes, including endocrine ones. Sometimes, hypokalemia requires urgent medical attention. The aim of this review is to present updated information regarding: (1) the definition and prevalence of hypokalemia, (2) the physiology of potassium homeostasis, (3) the various causes leading to hypokalemia, (4) the diagnostic steps for the assessment of hypokalemia and (5) the appropriate treatment of hypokalemia depending on the cause. Practical algorithms for the optimal diagnostic, treatment and follow-up strategy are presented, while an individualized approach is emphasized.

Organotin Compounds Toxicity: Focus on Kidney

Organotin compounds (OTs) are synthetic persistent organometallic xenobiotics widely used in several commercial applications. They exert well-described harmful effects in brain, liver, adipose tissue, and reproductive organs, as they are endocrine-disrupting chemicals (EDCs), but the effects in the kidneys are less known. The kidneys are especially vulnerable to environmental contaminants because they are a metabolizing site of xenobiotics, therefore, pollutants can accumulate in renal tissue, leading to impaired renal function and to several renal abnormalities. Individuals chronically exposed to OTs present a threefold increase in the prevalence of kidney stones. These compounds can directly inhibit H⁺/K⁺-ATPase in renal intercalated cells, resulting in hypokalemia, renal tubular acidity, and increased urinary pH, which is a known risk factor for kidney stones formation. OTs effects are not only limited to induce nephrolithiasis, its nephrotoxicity is also due to increased reactive oxygen species (ROS). This increase leads to lipid peroxidation, abnormal cellular function, and cell death. Combined, the enzymatic and non-enzymatic antioxidant defense systems become deficient and there is a consequent uncontrolled generation of ROS that culminates in renal tissue damage. Still, few epidemiological and experimental studies have reported renal impact correlated to OTs exposure. This lack of investigation of the complete effect of OTs in renal function and structure led us to perform this review reporting the main researches about this subject.