Rabbits (Oryctolagus cuniculus) increase caecal calcium absorption at increasing dietary calcium levels

Hindgut fermenting herbivores from different vertebrate taxa, including tortoises, and among mammals some afrotheria, perissodactyla incl. equids, several rodents as well as lagomorphs absorb more calcium (Ca) from the digesta than they require, and excrete the surplus via urine. Both proximate and ultimate causes are elusive. It was suggested that this mechanism might ensure phosphorus availability for the hindgut microbiome by removing potentially complex-building Ca from the digesta. Here we use Ussing chamber experiments to show that rabbits (Oryctolagus cuniculus) maintained on four different diets (six animals/diet) increase active Ca absorption at increasing Ca levels. This contradicts the common assumption that at higher dietary levels, where passive uptake should be more prevalent, active transport can relax and hence supports the deliberate removal hypothesis. In the rabbits, this absorption was distinctively higher in the caecum than in the duodenum, which is unexpected in mammals. Additional quantification of the presence of two proteins involved in active Ca absorption (calbindin-D9K CB; vitamin D receptor, VDR) showed higher presence with higher dietary Ca. However, their detailed distribution across the intestinal tract and the diet groups suggests that other factors not investigated in this study must play major roles in Ca absorption in rabbits. Investigating strategies of herbivores to mitigate potential negative effects of Ca in the digesta on microbial activity and growth might represent a promising area of future research.

Calcium Transport in the Kidney and Disease Processes

Calcium is a key ion involved in cardiac and skeletal muscle contractility, nerve function, and skeletal structure. Global calcium balance is affected by parathyroid hormone and vitamin D, and calcium is shuttled between the extracellular space and the bone matrix compartment dynamically. The kidney plays an important role in whole-body calcium balance. Abnormalities in the kidney transport proteins alter the renal excretion of calcium. Various hormonal and regulatory pathways have evolved that regulate the renal handling of calcium to maintain the serum calcium within defined limits despite dynamic changes in dietary calcium intake. Dysregulation of renal calcium transport can occur pharmacologically, hormonally, and via genetic mutations in key proteins in various nephron segments resulting in several disease processes. This review focuses on the regulation transport of calcium in the nephron. Genetic diseases affecting the renal handling of calcium that can potentially lead to changes in the serum calcium concentration are reviewed.

Akt Substrate of 160 kD Regulates Na+,K+-ATPase Trafficking in Response to Energy Depletion and Renal Ischemia

Renal ischemia and reperfusion injury causes loss of renal epithelial cell polarity and perturbations in tubular solute and fluid transport. Na(+),K(+)-ATPase, which is normally found at the basolateral plasma membrane of renal epithelial cells, is internalized and accumulates in intracellular compartments after renal ischemic injury. We previously reported that the subcellular distribution of Na(+),K(+)-ATPase is modulated by direct binding to Akt substrate of 160 kD (AS160), a Rab GTPase-activating protein that regulates the trafficking of glucose transporter 4 in response to insulin and muscle contraction. Here, we investigated the effect of AS160 on Na(+),K(+)-ATPase trafficking in response to energy depletion. We found that AS160 is required for the intracellular accumulation of Na(+),K(+)-ATPase that occurs in response to energy depletion in cultured epithelial cells. Energy depletion led to dephosphorylation of AS160 at S588, which was required for the energy depletion-induced accumulation of Na,K-ATPase in intracellular compartments. In AS160-knockout mice, the effects of renal ischemia on the distribution of Na(+),K(+)-ATPase were substantially reduced in the epithelial cells of distal segments of the renal tubules. These data demonstrate that AS160 has a direct role in linking the trafficking of Na(+),K(+)-ATPase to the energy state of renal epithelial cells.

Discovery of Novel SPAK Inhibitors That Block WNK Kinase Signaling to Cation Chloride Transporters

Upon activation by with-no-lysine kinases, STE20/SPS1-related proline-alanine-rich protein kinase (SPAK) phosphorylates and activates SLC12A transporters such as the Na(+)-Cl(-) cotransporter (NCC) and Na(+)-K(+)-2Cl(-) cotransporter type 1 (NKCC1) and type 2 (NKCC2); these transporters have important roles in regulating BP through NaCl reabsorption and vasoconstriction. SPAK knockout mice are viable and display hypotension with decreased activity (phosphorylation) of NCC and NKCC1 in the kidneys and aorta, respectively. Therefore, agents that inhibit SPAK activity could be a new class of antihypertensive drugs with dual actions (i.e., NaCl diuresis and vasodilation). In this study, we developed a new ELISA-based screening system to find novel SPAK inhibitors and screened >20,000 small-molecule compounds. Furthermore, we used a drug repositioning strategy to identify existing drugs that inhibit SPAK activity. As a result, we discovered one small-molecule compound (Stock 1S-14279) and an antiparasitic agent (Closantel) that inhibited SPAK-regulated phosphorylation and activation of NCC and NKCC1 in vitro and in mice. Notably, these compounds had structural similarity and inhibited SPAK in an ATP-insensitive manner. We propose that the two compounds found in this study may have great potential as novel antihypertensive drugs.

Evaluation of endoscopically obtained duodenal biopsy samples from cats and dogs in an adapter-modified Ussing chamber

This study was conducted to evaluate an adapter-modified Ussing chamber for assessment of transport physiology in endoscopically obtained duodenal biopsies from healthy cats and dogs, as well as dogs with chronic enteropathies. 17 duodenal biopsies from five cats and 51 duodenal biopsies from 13 dogs were obtained. Samples were transferred into an adapter-modified Ussing chamber and sequentially exposed to various absorbagogues and secretagogues. Overall, 78.6% of duodenal samples obtained from cats responded to at least one compound. In duodenal biopsies obtained from dogs, the rate of overall response ranged from 87.5% (healthy individuals; n = 8), to 63.6% (animals exhibiting clinical signs of gastrointestinal disease and histopathological unremarkable duodenum; n = 15), and 32.1% (animals exhibiting clinical signs of gastrointestinal diseases and moderate to severe histopathological lesions; n = 28). Detailed information regarding the magnitude and duration of the response are provided. The adapter-modified Ussing chamber enables investigation of the absorptive and secretory capacity of endoscopically obtained duodenal biopsies from cats and dogs and has the potential to become a valuable research tool. The response of samples was correlated with histopathological findings.

Effects of age and controlled oral dosing of Enterococcus faecium on epithelial properties in the piglet small intestine

Enterococcus faecium NCIMB 10415 is a licensed probiotic for piglets that has been shown to positively affect diarrhoea incidence and to act on transport properties and immunological parameters in the porcine intestine. The aim of the present study was to examine its effects on jejunal absorptive and secretory capacities around weaning. Furthermore, the possible involvement of heat shock proteins in the effects of probiotics on epithelial functions was investigated. A significant part of the probiotic was dosed orally to reduce the variability of intake of the probiotic. The piglets were randomly assigned to a control and a probiotic feeding group, the latter receiving 4.5×109 cfu/day of E. faecium directly into the mouth for 34 days starting after birth. Additionally, their feed was supplemented with the probiotic strain. Piglets were weaned at day 29 after birth. Ussing chamber studies were conducted with the mid-jejunum of piglets aged 14, 28, 31, 35 and 56 days. Changes in short-circuit current (ΔIsc) were measured after stimulation of Na+-coupled absorption with L-glutamine or glucose or with the secretagogue prostaglandin E2 (PGE2). The mRNA expression for SGLT1, CFTR and various heat shock proteins was determined. The transport properties changed significantly with age. The glucose-, L-glutamine- and PGE2-induced changes in Isc were highest at day 31 after birth. No significant differences between the feeding groups were observed. The mRNA of HSP60, HSC70, HSP70 and HSP90 was expressed in the jejunal tissues. The mRNA expression of HSC70 was higher and that of HSP60 was lower in the probiotic group. HSC70 expression increased with age. In conclusion, whereas age effects were observed on absorptive and secretory functions, controlled E. faecium dosing had no measurable effects on these functional parameters in this experimental setup. The possible role of heat shock proteins should be further evaluated.