Role of facilitated diffusion of calcium by calbindin in intestinal calcium absorption

Abstract

Computer simulations of transcellular Ca2+ transport in enterocytes were carried out using the simulation program SPICE. The program incorporated a negative-feedback entry of Ca2+ at the brush-border membrane that was characterized by an inhibitor constant of 0.5 microM cytosolic Ca2+ concentration ([Ca2+]). The basolateral Ca(2+)-ATPase was simulated by a four-step mechanism that resulted in Michaelis-Menten kinetics with a Michaelis constant of 0.24 microM [Ca2+]. The cytosolic diffusion of Ca2+ was simulated by dividing the cytosol into 10 slabs of equal width. Ca2+ binding to calbindin-D9K was simulated in each slab, and diffusion of free Ca2+, free calbindin, and Ca(2+)-laden calbindin was simulated between each slab. The cytosolic [Ca2+] of the simulated cells was regulated within the physiological range. Calbindin-D9K reduced the cytosolic [Ca2+] gradient, increased Ca2+ entry into the cell by removing the negative-feedback inhibition of Ca2+ entry, increased cytosolic Ca2+ flow, and increased the efflux of Ca2+ across the basolateral membrane by increasing the free [Ca2+] immediately adjacent to the pump. The enhancement of transcellular Ca2+ transport was nearly linearly dependent on calbindin-D9K concentration. The values of the dissociation constant (Kd) for calbindin-D9K were previously obtained experimentally in the presence and absence of KCl. Calbindin with the Kd obtained in the presence of KCl enhanced the simulated Ca2+ transport more than with the Kd obtained in the absence of KCl. This result suggests that the physiological Kd of calbindin is optimal for the enhancement of transcellular Ca2+ transport. The simulated Ca2+ flow was less than that predicted from the "near-equilibrium" analytic solution of the reaction-diffusion problem.

Co-ordinated control of apical calcium influx and basolateral calcium efflux in rabbit cortical collecting system

“…[Ca2+]j could serve as this signal, analogous to its role in th e regulation of solute transport in various epithelia [18,19]. For instance, in intestine and MDCK-F cells, [Ca2+]j has been implicated in the regulation of Ca2+ entry [2,20]. In the present study, reduction of th e basolateral efflux capacity evoked by reversal of the operation of the Na+/Ca2+ exchanger was paralleled by a m arked increase in [Ca2+].. At first sight, this observation is in agreem ent w ith the idea th at an overall increase in [Ca2'^ is responsible for the inhibition of the apical Ca2+ influx.…”

“…Knowledge of the local nature of these events is, therefore, of importance and is now becoming feasible through the use of high spatial resolution confocal laser scanning microscopy [19]. Thirdly, cytosolic calbindin-D2gk levels could also control the rate of transcellular Ca2+ transport [2], In a previous study, we have implicated calbindin-D28k in the long-term control of Ca2+ reabsorption by l, 25[OH]2D3 [23], It is conceivable th at the level of calbindin-D28k determines the maximal achievable rate of transcellular Ca2+ transport, but it is rather unlikely that in short-term regulation, as investigated in th e present study, calbindinDjak represents the rate-limiting step.…”

“…Feher et al developed a m athem atical m odel to explain the role of calbindins in transcellular Ca2+ transport [2], In this model, calbindin enhances Ca2+ transport by stimulating the apical entry of this ion, increasing its rate of cytosolic diffusion and enhancing its efflux by feeding Ca2f to Ca2+-starved basolateral pumps. In this way, the cytosolic level of calbindin ultimately determines the rate of transcellular Ca2+ transport and represents the ratelimiting step in transcellular Ca2+ movement.…”

Regulated redistribution of calretinins in WiDr cells

“…The mRNA level is increased as early as 3 h after the application of a dosage of 1,25-(OH) 2 D3 and the increase persists for more than 24 h. In Madin ± Darbey bovine kidney cells, it was demonstrated that calbindin-D28k mRNA was induced by 1,25-(OH) 2 D3 (10 77 M) and significantly higher levels of calbindin-D28k were observed at 8 and 24 h after induction (Gagnon et al, 1994). It is thought that calbindin-D28k present at a higher concentration in induced cells, acts as a cytosolic Ca 2+ buffer and presumably facilitates the net diffusional flux of Ca 2+ through the cytosol from the luminal to the apical side of the intestine (Feher et al, 1992). CR, the protein most closely related to calbindin-D28k (60% identity on the amino acid level) is normally not expressed in the intestine.…”

Molecular Mechanisms of Primary Hypercalciuria

“…The active, transcellular pathway is downregulated by a diet replete in calcium (37). Luminal calcium enters the enterocyte at the microvillus border of the apical membrane via facilitated translocation through an epithelial calcium channel, either CaT1 (38) or ECaC (39); diffusion of calcium through the cell is facilitated by binding to calbindin D 9k (40), and extrusion of calcium through the basolateral membrane against an electrochemical gradient is achieved by plasma membrane Ca 2ϩ -ATPase (PMCA) (41,42).…”