Postprandial adjustments in renal phosphate excretion do not involve a gut-derived phosphaturic factor

Phosphate intake, hyperphosphatemia, and kidney function

Phosphate is essential in living organisms and its blood levels are regulated by a complex network involving the kidneys, intestine, parathyroid glands, and the skeleton. The crosstalk between these organs is executed primarily by three hormones, calcitriol, parathyroid hormone, and fibroblast growth factor 23. Largely due to a higher intake of ultraprocessed foods, dietary phosphate intake has increased in the last decades. The average intake is now about twice the recommended dietary allowance. Studies investigating the side effect of chronic high dietary phosphate intake suffer from incomplete dietary phosphate assessment and, therefore, often make data interpretation difficult. Renal excretion is quickly adapted to acute and chronic phosphate intake. However, at the high ends of dietary intake, renal adaptation, even in pre-existing normal kidney function, apparently is not perfect. Experimental intervention studies suggest that chronic excess of dietary phosphate can result in sustained higher blood phosphate leading to hyperphosphatemia. Evidence exists that the price of the homeostatic response (phosphaturia in response to phosphate loading/hyperphosphatemia) is an increased risk for declining kidney function, partly due by intraluminal/tubular calcium phosphate particles that provoke renal inflammation. High dietary phosphate intake and hyperphosphatemia are progression factors for declining kidney function and are associated with higher cardiovascular disease and mortality risk. This is best established for pre-existing chronic kidney disease, but epidemiological and experimental data strongly suggest that this holds true for subjects with normal renal function as well. Here, we review the latest advances in phosphate intake and kidney function decline.

The Complexities of Organ Crosstalk in Phosphate Homeostasis: Time to Put Phosphate Sensing Back in the Limelight

Phosphate homeostasis is essential for health and is achieved via interaction between the bone, kidney, small intestine, and parathyroid glands and via intricate processes involving phosphate transporters, phosphate sensors, and circulating hormones. Numerous genetic and acquired disorders are associated with disruption in these processes and can lead to significant morbidity and mortality. The role of the kidney in phosphate homeostasis is well known, although it is recognized that the cellular mechanisms in murine models and humans are different. Intestinal phosphate transport also appears to differ in humans and rodents, with recent studies demonstrating a dominant role for the paracellular pathway. The existence of phosphate sensing has been acknowledged for decades; however, the underlying molecular mechanisms are poorly understood. At least three phosphate sensors have emerged. PiT2 and FGFR1c both act as phosphate sensors controlling Fibroblast Growth Factor 23 secretion in bone, whereas the calcium-sensing receptor controls parathyroid hormone secretion in response to extracellular phosphate. All three of the proposed sensors are expressed in the kidney and intestine but their exact function in these organs is unknown. Understanding organ interactions and the mechanisms involved in phosphate sensing requires significant research to develop novel approaches for the treatment of phosphate homeostasis disorders.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Circadian rhythm of activin A and related parameters of mineral metabolism in normal and uremic rats

Activin A is a new fascinating player in chronic kidney disease-mineral and bone disorder (CKD-MBD), which is implicated in progressive renal disease, vascular calcification, and osteodystrophy. Plasma activin A rises early in the progression of renal disease. Disruption of circadian rhythms is related to increased risk of several diseases and circadian rhythms are observed in mineral homeostasis, bone parameters, and plasma levels of phosphate and PTH. Therefore, we examined the circadian rhythm of activin A and CKD-MBD-related parameters (phosphate, PTH, FGF23, and klotho) in healthy controls and CKD rats (5/6 nephrectomy) on high-, standard- and low-dietary phosphate contents as well as during fasting conditions. Plasma activin A exhibited circadian rhythmicity in healthy control rats with fourfold higher values at acrophase compared with nadir. The rhythm was obliterated in CKD. Activin A was higher in CKD rats compared with controls when measured at daytime but not significantly when measured at evening/nighttime, stressing the importance of time-specific reference intervals when interpreting plasma values. Plasma phosphate, PTH, and FGF23 all showed circadian rhythms in control rats, which were abolished or disrupted in CKD. Plasma klotho did not show circadian rhythm. Thus, the present investigation shows, for the first time, circadian rhythm of plasma activin A. The rhythmicity is severely disturbed by CKD and is associated with disturbed rhythms of phosphate and phosphate-regulating hormones PTH and FGF23, indicating that disturbed circadian rhythmicity is an important feature of CKD-MBD.

The role of SLC34A2 in intestinal phosphate absorption and phosphate homeostasis

There has recently been significant interest in the concept of directly targeting intestinal phosphate transport to control hyperphosphatemia in patients with chronic kidney disease. However, we do not have a complete understanding of the cellular mechanisms that govern dietary phosphate absorption. Studies in the 1970s documented both active and passive pathways for intestinal phosphate absorption. However, following the cloning of the intestinal SLC34 cotransporter, NaPi-IIb, much of the research focused on the role of this protein in active transcellular phosphate absorption and the factors involved in its regulation. Generation of a conditional NaPi-IIb knockout mouse has demonstrated that this protein is critical for the maintenance of skeletal integrity during periods of phosphate restriction and that under normal physiological conditions, the passive sodium-independent pathway is likely be the more dominant pathway for intestinal phosphate absorption. The review aims to summarise the most recent developments in our understanding of the role of the intestine in phosphate homeostasis, including the acute and chronic renal adaptations that occur in response to dietary phosphate intake. Evidence regarding the overall contribution of the transcellular and paracellular pathways for phosphate absorption will be discussed, together with the clinical benefit of inhibiting these pathways for the treatment of hyperphosphatemia in chronic kidney disease.

Mechanisms and Regulation of Intestinal Phosphate Absorption

States of hypo- and hyperphosphatemia have deleterious consequences including rickets/osteomalacia and renal/cardiovascular disease, respectively. Therefore, the maintenance of appropriate plasma levels of phosphate is an essential requirement for health. This control is executed by the collaborative action of intestine and kidney whose capacities to (re)absorb phosphate are regulated by a number of hormonal and metabolic factors, among them parathyroid hormone, fibroblast growth factor 23, 1,25(OH)₂ vitamin D₃ , and dietary phosphate. The molecular mechanisms responsible for the transepithelial transport of phosphate across enterocytes are only partially understood. Indeed, whereas renal reabsorption entirely relies on well-characterized active transport mechanisms of phosphate across the renal proximal epithelia, intestinal absorption proceeds via active and passive mechanisms, with the molecular identity of the passive component still unknown. The active absorption of phosphate depends mostly on the activity and expression of the sodium-dependent phosphate cotransporter NaPi-IIb (SLC34A2), which is highly regulated by many of the factors, mentioned earlier. Physiologically, the contribution of NaPi-IIb to the maintenance of phosphate balance appears to be mostly relevant during periods of low phosphate availability. Therefore, its role in individuals living in industrialized societies with high phosphate intake is probably less relevant. Importantly, small increases in plasma phosphate, even within normal range, associate with higher risk of cardiovascular disease. Therefore, therapeutic approaches to treat hyperphosphatemia, including dietary phosphate restriction and phosphate binders, aim at reducing intestinal absorption. Here we review the current state of research in the field. © 2017 American Physiological Society. Compr Physiol 8:1065-1090, 2018.