Effect of mucosal potassium on the intestinal glucose transport

Summary The extracellular space (inulin or raffinose space) and the intracellular concentration of sodium and potassium were determined in the intestinal mucosal scrapings of rats following a perfusion of the intestine with isosmotic solutions of electrolytes or non-electrolytes. Following a perfusion with K₂SO₄ or mannitol, the intracellular concentration of potassium increased significantly. Perfusion with sorbitol or raffinose did not produce such change. Mannitol (but not sorbitol or raffinose) increased the rate of mediated diffusion (but not that of active transport) of glucose from the gut lumen. It is concluded that the increased glucose absorption is due to the high intracellular concentration of potassium which causes an enhanced metabolic disappearance of the free intracellular glucose. In light of modern concepts regarding the intestinal absorption of glucose and of the findings of present and related studies, Verzár's theory concerning the role of intracellular metabolism (phosphorylation) in the absorption is reassessed.Dedicated to Prof. F. Verzár on the occasion of his 80th birthday.Supported by grants from the U. S. Public Health Service and from the American Heart Association.     

Transcellular transport of calcium and inorganic phosphate in the small intestinal epithelium

Transport mechanisms involved in the small intestinal handling of inorganic phosphate and calcium have been studied by different in vitro methods during the last few years. In concordance with studies on intact epithelial preparations, studies with brush-border and basal-lateral membrane vesicles isolated from the small intestinal epithelial cell revealed that transcellular calcium and inorganic phosphate fluxes are coupled to transcellular sodium flux, i.e., secondary active via coupling to the primary active sodium flux. A sodium-coupled mechanism in the brush-border membrane leads to cellular accumulation of inorganic phosphate. A sodium-coupled mechanism leads to extrusion of calcium from the cell into the serosal interstitium. A primary active transport mediated by the Ca-ATPase and located in the basal-lateral membrane also exists for calcium. Regulation of transcellular phosphate and calcium flux proceeds via altered influx rates at the luminal cell pole.     

Vitamin D-dependent, particulate calcium-binding activity and intestinal calcium transport.

A vitamin D-dependent calcium-binding activity of relatively high molecular weight has been identified in the particulate fraction of rat small intestinal mucosa. The Ca-binding activity is sedimented at 140,000 X g after treatment of the mucosal particulate fraction with Triton X-114. Intestinal brush-border suspensions can also be used as starting material. The Ca-binding component is inactivated by heat and repeated freeze-thawing and consists of one or more protein complexes in the range of 0.5-1.0 million mol wt as indicated by gel filtration. The Ca-binding activity correlates positively with known features of the intestinal Ca transport mechanism, as demonstrated by studies of the distribution in the small intestine and the effects of vitamin D, dietary Ca, cycloheximide treatment, and rat age. It is suggested that the component might function in the transit of Ca across the brush-border surface to the cytosol of intestinal mucosal cells.     

Effects of trifluoperazine, ouabain, and ethacrynic acid on intestinal calcium transport

Bidirectional steady-state calcium fluxes were measured in vitro under short-circuited conditions in segments of rat duodenum and descending colon. The calcium-activated ATPase (Ca-ATPase) inhibitor trifluoperazine (TFP, 0.1 mM) reduced net calcium absorption in both tissues by decreasing the absorptive flux from mucosa to serosa (Jm leads to s) without consistently altering the secretory flux from serosa to mucosa. 1,25-Dihydroxyvitamin D3 administration (50 ng/day for 4 days) increased net calcium absorption by increasing Jm leads to s, and TFP reduced Jm leads to s to the same extent across tissues from vehicle- or 1,25-dihydroxyvitamin D3-treated animals. Na-K-ATPase inhibitors ouabain and ethacrynic acid both reduced short-circuit current without affecting calcium fluxes. These data suggest that Ca-ATPase, located in the basolateral membrane of intestinal epithelial cells, plays a role in the transepithelial transport of calcium. More general effects of TFP on intestinal epithelium may also contribute to the reduction in calcium fluxes. Duodenal and descending colon calcium transport appears independent of transcellular sodium transport mediated by Na-K-ATPase.     

Calcium ATPase and intestinal calcium transport in uremic rats

Calcium ATPase, an enzyme involved in intestinal calcium transport, was measured in homogenates of duodenal mucosal scrapings of normal and uremic rats. The effects of calcium deprivation and treatment with 1 alpha,25-dihydroxycholecalciferol [1,25-(OH)2D3] were investigated as well. Uremia decreased the enzyme activity and impaired the rise after calcium deprivation as observed in intact rats. The 1,25-(OH)2D3 treatment increased the enzyme activity in uremic animals and resulted in an identical response to calcium deprivation as observed in intact rats; parathyroidectomy abolished this effect. A striking correlation between everted duodenal gut sac calcium transport and calcium ATPase activity could be demonstrated for all groups of rats studied. It is concluded that the calcium ATPase activity is linked to the production of 1,25-(OH)2D3 as well as to an additional factor, probably parathyroid hormone. The close relationship between enzyme activity and in vitro calcium transport, even during constant physiological supplementation with 1,25-(OH)2D3, suggests an autonomous role of the calcium ATPase activity for mediation of calcium transport in the duodenum in addition to the well-known mechanisms related to vitamin D and its metabolites.     

PI3-Kinase-dependent electrogenic intestinal transport of glucose and amino acids

Intestinal glucose and amino acid transport is stimulated by the serum- and glucocorticoid-inducible kinase isoforms SGK1, SGK2, and SGK3 and protein kinase B which are, in turn, stimulated following activation of the phosphoinositol-3 kinase (PI3 kinase). The present study has been performed to explore whether pharmacological inhibition of the PI3 kinase affects electrogenic jejunal transport of glucose and amino acids. In Ussing chamber experiments, glucose (20 mM), phenylalanine (20 mM), glutamine (20 mM), cysteine (20 mM), and proline (20 mM) generated lumen negative currents (I _glc, I _phe, I _gln, I _cys, and I _pro), respectively, which gradually declined following application of the PI3 kinase inhibitor Wortmannin (1 μM). Within 40 min, Wortmannin treatment significantly decreased I _glc by 39 ± 10% (n = 5), I _phe by 70 ± 7% (n = 4), I _gln by 69 ± 8% (n = 4), I _cys by 67 ± 8% (n = 6), and I _prol by 79 ± 12% (n = 3). A similar decline of I _glc was observed following application of the PI3 kinase inhibitor LY294002 (50 μM). Exposure to the inhibitors did not significantly alter transepithelial potential difference and resistance in the absence of substrates for electrogenic transport. The observations suggest that the electrogenic transport of glucose and several amino acids requires the continued activity of PI3 kinase. R. Rexhepaj and F. Artunc shared first authorship.     

Effects of somatostatin on intestinal calcium transport in the rat

The addition of somatostatin (SRIF) to rat descending colon in vitro increased the calcium secretory flux from serosa to mucosa (Js leads to m) and reduced tissue short-circuit current (Isc) but did not alter the absorptive flux from mucosa to serosa (Js leads to m). Js leads to m increased by 37% at 10(-9) M SRIF and by 48% at 10(-6) M. The response to SRIF was not altered by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], and SRIF did not interfere with stimulation of calcium Jm leads to s by 1,25(OH)2D3. Removal of sodium from the buffer abolished the stimulation of Js leads to m by SRIF without reducing basal Js leads to m. Secretory fluxes of mannitol and calcium were strongly correlated in the presence and absence of SRIF, suggesting that SRIF stimulates a paracellular transepithelial pathway for calcium. In the duodenum, SRIF altered neither calcium Js leads to m nor Isc. In the ileum, calcium Js leads to m increased and Isc decreased, as in the colon, but only by 28 and 12%, respectively. The maximal change in calcium Js leads to m caused by SRIF in these three intestinal segments was negatively correlated with the tissue concentration of immunoreactive SRIF. These results suggest that intestinal calcium secretion could, in part, be regulated by intestinal SRIF.     

Sodium ion transport in the intestinal wall: A mathematical model

A theoretical approach to study the uptake of sodium ions acrossthe gastrointestinal mucosa and the concentrations at whichthey are taken up into the underlying blood capillaries hasbeen attempted. A two-phase model of the mucosa is considered,consisting of an extracellular phase and a cellular phase. Themodel takes into account two important modes of transport: diffusionunder concentration gradient and potential gradient (electrodiffusion)and active transport which is ATPase enzyme mediated. Appropriatepartial differential equations for the two mechanisms of transportare derived and are solved by iterative methods. An approximatedistance from the lumen where the capillaries lie is somewherebetween 10 and 15% of the total wall length. The mean concentrationof sodium ions made available for absorption at the capillariesis studied with time. A functional form for the variation ofpotential with respect to distance is proposed, and by comparingmodel solutions with experimental data it is calculated explicitly.     

Decreased intestinal glucose transport in the sgk3-knockout mouse

Xenopus oocyte coexpression experiments revealed the capacity of the serum- and glucocorticoid-inducible kinase isoform 3 (SGK3) to up-regulate a variety of transport systems including the sodium-dependent glucose transporter SGLT1. The present study explored the functional significance of SGK3-dependent regulation of intestinal transport. To this end, experiments were performed in gene targeted mice lacking functional sgk3 (sgk3^–/–) and their wild type littermates (sgk3^+/+). Oral food intake and fecal dry weight were significantly larger in sgk3^–/– than in sgk3^+/+ mice. Glucose-induced current (I_g) in Ussing chamber as a measure of Na⁺ coupled glucose transport was significantly smaller in sgk3^–/– than in sgk3^+/+ mouse jejunal segments. Fasting plasma glucose concentrations were significantly lower in sgk3^–/– than in sgk3^+/+ mice. Intestinal electrogenic transport of phenylalanine, cysteine, glutamine and proline were not significantly different between sgk3^–/– and sgk3^+/+ mice. In conclusion, SGK3 is required for adequate intestinal Na⁺ coupled glucose transport and impaired glucose absorption may contribute to delayed growth and decreased plasma glucose concentrations of SGK3 deficient mice. The hypoglycemia might lead to enhanced food intake to compensate for impaired intestinal absorption.Ciprian Sandu, Rexhep Rexhepaj Shared first authorship.     

A mechanistic explanation of the effect of potassium on goldfish intestinal transport

Partial replacement of sodium by potassium or rubidium in the solution used to perfuse isolated intestinal segments of goldfish causes an increase in transmural electrical resistance. Serosal replacements have a stronger effect than mucosal replacements.A 70% inhibition of the glucose-evoked transmural electrical current is brought about by serosal replacement of 40 mM sodium by potassium. Transmural mucosal to serosal flux of 3-O-methyl-D-glucose is also strongly inhibited by serosal potassium. These inhibitory effects of potassium do not occur when the intestinal mucosa is stripped free from the intestinal muscular layers. It is concluded that potassium-induced muscular contractures cause a decrease in transport area by pressing the mucosal folds closer against each other.Certain effects of high potassium concentrations that have been reported in mammalian intestinal preparations may involve a similar mechanism.