The influence of phosphorylable molecules on calcium transfer in the rat ileum

Stimulation of ileal calcium absorption by sorbitol, L-xylose, or creatine via a decrease in luminal sodium concentration: relation with concomitant changes in enterocyte energy metabolism

Ligated ileal loops, 30 cm in length, of 4-month-old male Wistar rats were instilled with 3 ml of a 10 mM CaCl2 solution (added with 0.25 muCi 45Ca) in the absence (control) or presence of 100mM sorbitol, L-xylose, or creatine. Ileal calcium (Ca) transport, measured by plasma 45Ca appearance, was found to be similar 30 minutes after fluid instillation in all four instances. However, thereafter, 45Ca appearance in plasma did not increase further in control animals whereas it increased twice as much during the subsequent 30 minutes in the presence of sorbitol, L-xylose, or creatine. However, when loops of similar length were instilled with only 1.0 ml of such solutions, the sorbitol effect was already observed during the first 30 minutes. The stimulation of ileal Ca absorption induced by the presence of sorbitol appeared to be due to a cellular effect, associated with a decreased flux across the paracellular pathway, as indicated by 3H-mannitol absorption. The presence of sorbitol in instilled ileal solution induced a significant decrease in luminal Na, K, bicarbonate, and Cl concentrations at each time point studied (30, 60, 120, or 240 minutes after instillation). Thirty minutes after instillation, no difference in soluble Ca concentration was observed between control and experimental rats. After 60 minutes, Ca concentration was dramatically decreased in control rats but it remained nearly constant in experimental animals. Thus, the presence of substances enhancing ileal Ca transport favored the maintenance of soluble Ca in ileal solution during longer time periods than their absence. In the ileal enterocyte, these substances induced a twofold increase of ATP content compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of L-lysine on cytosolic calcium homeostasis in cultured human normal fibroblasts

L-lysine, a cationic essential amino acid, has been reported to affect calcium transport in both intestine and kidney. In order to investigate whether this effect is associated with changes in cytosolic calcium homeostasis, we studied the effect of L-lysine deprivation on intracellular calcium concentration ([Ca2+]i), as well as 45Ca efflux and accumulation in normal human fibroblasts. Steady state [Ca2+]i, measured using fura-2 fluorescence in cells cultured for 18 hours in a L-lysine-free medium, was significantly higher than in cells grown in the presence of as little as 4 microM L-lysine. L-lysine deprivation also led to a significant decrease of 45Ca fractional efflux compared with cells grown in complete medium. This effect was paralleled by a significant decrease in 45Ca accumulation. Lack of L-arginine from the growth medium for the same time period had no effect on either [Ca2+]i, 45Ca efflux, or 45Ca accumulation rate. Presumably, the lack of L-lysine for a significant amount of time impairs the active mechanisms of calcium extrusion, which is only partially compensated by a reduction of calcium accumulation rate. This leads to an increased steady state [Ca2+]i. It is concluded that L-lysine is an important modulator of cytosolic calcium homeostasis.

Phosphorylated proteins from anuran intestinal microvilli membranes--I. Relations with alkaline phosphatase

The degree of phosphorylation of intestinal microvilli membrane proteins in an adult amphibian, Rana esculenta, was investigated under various experimental conditions. The microvilli protein phosphorylation rate rapidly increases during the first 4 min of incubation in a medium containing [gamma-32P]ATP. This increase is slower afterwards. Cyclic nucleotides (cyclic AMP, cyclic GMP) and sorbitol do not modify the microvilli protein phosphorylation rate. On the contrary, this phosphorylation rate significantly decreases in the presence of L-lysine, when its concentration in the incubation medium is greater than 25 mM. The time course of phosphorylation confirms the inhibitory effects of L-lysine (100 mM). The microvilli membrane proteins were distinguished by polyacrylamide gel electrophoresis. In heated samples, electrophoresis followed by an radioautograph systematically reveals the existence of a very phosphorylated protein with a mol. wt of 86 kDa. The phosphorylation of this protein is partially inhibited by L-lysine (100 mM). The very phosphorylated protein could be the monomer of alkaline phosphatase. The dimer (170 kDa) is visualized on electrophoretograms by its catalytic activity. In mammals, several authors have established a correlation between phosphorylation of the microvilli membrane proteins and the intensity of intestinal calcium absorption. Such a control is presently being investigated in adult Rana esculenta.

Enterocyte microvillus can phosphorylate molecules which inhibit endogenous phosphorylation of its proteins

D- and L-amino acids (arginine, lysine) and hydroxylated molecules (sorbitol, ethyl-ethanolamine), all of which increase the intestinal transfer of calcium, are phosphorylated by jejunal and ileal microvilli. All these molecules inhibit the endogenous phosphorylation of microvilli proteins. D- and L-valine, which are not phosphorylated in the same conditions, have no effect on the phosphorylation of microvilli proteins. These observations are in good agreement with the scheme previously proposed for the increase of calcium intestinal transfer by L-lysine: phosphorylation of L-lysine by mucous membrane may interfere with that of membrane proteins, the phosphorylation of which would lower the permeability to that cation. Autoradiographies of the electrophoretogram show that the principal phosphorylable microvilli proteins have the same electrophoretic properties as those of the dimer and the monomer of alkaline phosphatase from the same intestinal loci. The ability of the enzyme to be phosphorylated and its well-known transphosphorylating activity upon hydroxylated molecules suggest that it could, in different ways, affect intestinal transfer of calcium.