Regulation of water and ion movement in intestine

Compared effects of calcium and sodium polystyrene sulfonate on mineral and bone metabolism and volume overload in pre-dialysis patients with hyperkalemia

BACKGROUND

Hyperkalemia is prevalent in end-stage renal disease patients, being involved in life-threatening arrhythmias. Although polystyrene sulfonate (PS) is commonly used for the treatment of hyperkalemia, direct comparison of effects between calcium and sodium PS (CPS and SPS) on mineral and bone metabolism has not yet been studied.

METHODS

In a randomized and crossover design, 20 pre-dialysis patients with hyperkalemia (>5 mmol/l) received either oral CPS or SPS therapy for 4 weeks.

RESULTS

After 4-week treatments, there was no significant difference of changes in serum potassium (K) from the baseline (ΔK) between the two groups. However, SPS significantly decreased serum calcium (Ca) and magnesium (Mg) and increased intact parathyroid hormone (iPTH) values, whereas CPS reduced iPTH. ΔiPTH was inversely correlated with ΔCa and ΔMg (r = -0.53 and r = -0.50, respectively). Furthermore, sodium (Na) and atrial natriuretic peptide (ANP) levels were significantly elevated in patients with SPS, but not with CPS, whereas ΔNa and ΔANP were significantly correlated with each other in all the patients. We also found that ΔNa and Δ(Na to chloride ratio) were positively correlated with ΔHCO₃^-. In artificial colon fluid, CPS increased Ca and decreased Na. Furthermore, SPS greatly reduced K, Mg, and NH₃.

CONCLUSION

Compared with SPS, CPS may be safer for the treatment of hyperkalemia in pre-dialysis patients, because it did not induce hyperparathyroidism or volume overload.

Mechanisms by which cyclic nucleotides and other intracellular mediators regulate secretion

The regulation of Na+ and Cl- transport across surface membranes and tight junctions of intestinal epithelium is mediated through at least three intracellular messengers: (i) 3',5'-cyclic AMP, activating two types of cyclic AMP-dependent protein kinase, (ii) 3',5'-cyclic GMP, binding to a unique isoenzyme of cyclic GMP-dependent protein kinase, enriched in the intestinal brush border, and (iii) Ca2+ ions, partially acting through calmodulin and a Ca2+/phospholipid-dependent protein kinase (C kinase). Recent data on the subcellular distribution and molecular properties of the high affinity cyclic nucleotide and Ca2+ receptors, their influence on the phosphorylation state of specific membrane proteins, and the possible role of these target proteins in ion transport regulation, are reviewed. The following aspects are accentuated: (1) the asymmetrical compartmentation of cyclic AMP-dependent protein kinase isoenzymes in the enterocyte and its functional implications; (2) the structure and function of microvillous cyclic GMP-dependent protein kinase; (3) the integration of cyclic AMP and cyclic GMP signals through co-phosphorylation of a 25 000 Mr protein in the intestinal-microvilli; (4) the identification of C kinase in villous and crypt cells; (5) various levels of interaction between cyclic nucleotide and Ca2+ signals; and (6) priority areas for future studies on stimulus-secretion coupling.

Toxins which activate guanylate cyclase: heat-stable enterotoxins

Certain enteropathogenic bacteria, including strains of Escherichia coli, Klebsiella pneumoniae and Yersinia enterocolitica, elicit their diarrhoeagenic effects by elaborating small molecular weight, heat-stable enterotoxins (STs). Structural and functional characteristics indicate that ST peptides are heterogeneous and two major subtypes, STa and STb, have been identified. Molecules of STa, unlike those of STb, are methanol-soluble and elicit their pathogenic effects by activating host cell guanylate cyclase activity and thereby increasing tissue cyclic GMP content: this increase in cyclic GMP causes fluid secretion. STa binds to specific proteinaceous receptors on intestinal cells but the nature of STa-receptor coupling to guanylate cyclase is poorly understood. The actions of STa, including binding to its receptor, activation of guanylate cyclase and stimulation of electrolyte transport, are rapid, reversible and tissue-specific. STa activates only particulate and not soluble guanylate cyclase. It alters the Vmax but not the apparent Km of this enzyme for Mg-GTP or Mn-GTP. At concentrations above 0.5-1 mM, calcium inhibits the STa activation of guanylate cyclase. The effects of calmodulin antagonists such as chlorpromazine on the activation of guanylate cyclase by STa are less clear. Inhibitors of phospholipid and arachidonic acid cascade pathways interfere with both basal and STa-stimulated guanylate cyclase. Membrane integrity is essential for STa activation of guanylate cyclase and the STa-receptor complex may activate the enzyme by intramembrane protein-protein interactions and/or perturbations. Interference with membrane phospholipid could alter such coupling.

Phospholipid profiles of human colon cancer using 31P magnetic resonance spectroscopy

Phospholipids of 16 malignant and 11 non-malignant human colon specimens were analyzed using a chloroform-methanol analytical reagent in conjunction with 31P magnetic resonance spectroscopy (MRS) at 202.4 MHz. Sixteen individual generic phospholipids were identified and quantified for statistical intergroup comparisons. Statistically significant elevations in the relative concentrations of lysophosphatidylcholine and phosphatidylcholine plasmalogen were seen in malignant tissues along with significantly depressed levels of sphingomyelin and phosphatidylethanolamine plasmalogen. The malignant and non-malignant tissue groups were further differentiated by the detection of the minor phospholipids, lysophosphatidylcholine plasmalogen, lysophosphatidylethanolamine plasmalogen, lysophosphatidic acid and phosphatidylglycerol exclusively present in the malignant tissues and by significant changes in computed phospholipid metabolic indices that were dominated by choline containing lipids. The 31P MRS methods used represent an advancement over previous protocols for identifying and quantifying major and minor tissue phospholipids making this the first direct study of membrane phospholipids in human colon tissues using 31P MRS. The phospholipid profiles obtained may provide important information regarding the nature of the malignant cell's membrane system and identify markers which may be used to estimate malignant propensity, aggressiveness of disease and provide prognostic information.

Secretagogue-induced changes in membrane calcium permeability in chicken and chinchilla ileal mucosa. Selective inhibition by loperamide

Substance P (SP), neurotensin (NT), bombesin (BB), serotonin (5HT), and carbamylcholine (CCH) transiently increase electrogenic anion secretion in chinchilla and chicken ileum. SP and CCH also transiently inhibit amiloride-sensitive Na/H exchange in isolated chicken enterocytes. Loperamide (LP) inhibits the short-circuit current responses caused by SP, NT, and BB, but not those caused by CCH, 5HT, Ca ionophore, or cyclic nucleotides. Similarly, LP inhibits the effects of SP, but not those of CCH, on Na/H exchange. LP inhibition of the SP effects was further studied in isolated chicken enterocytes. CCH and SP transiently increased cytosolic Ca activity by 20-50 nmol/liter, but only the response to SP was inhibited by LP (10(-5) M) and by the absence of extracellular Ca. We conclude SP and CCH effects on intestinal electrolyte transport are mediated by increasing enterocyte Ca activity and LP specifically inhibits peptide hormone-activated Ca entry by an opiate receptor-independent mechanism.

Stimulation of porcine jejunal ion secretion in vivo by protein kinase-C activators

Microbial toxins act through cyclic nucleotide dependent (cAMP or cGMP) or cyclic nucleotide independent pathways to cause intestinal ion secretion. To explore the calcium dependent, cyclic nucleotide independent pathway that is postulated to involve protein kinase C activation, we measured protein kinase C activity and phorbol ester binding in isolated intestinal epithelial cells and examined the effects of the C-kinase activators, phorbol myristate acetate, phorbol dibutyrate, and 4-beta-phorbol-12,13-didecanoate, in weaned pig jejunum in vivo. We demonstrated both protein kinase C activity and specific phorbol ester binding in porcine jejunal epithelial cells. Phorbol myristate acetate, phorbol dibutyrate, and 4-beta-phorbol-12,13-didecanoate (10(-5) M) each caused striking secretory responses at 5 h with accumulation of Na+, K+, Cl-, and HCO3- intraluminally. In contrast, 4-alpha-phorbol and 4-alpha-phorbol-12,13-didecanoate, which do not affect protein kinase C, allowed normal net absorption of all electrolytes from the intestinal lumen equivalent to controls with only Ringer's lactate. Time course studies revealed significant secretion within 30 min after exposure to the C-kinase activators. These data suggest an important role for protein kinase C activation in intestinal ion secretion.

Pathophysiology of neonatal calf diarrhea

Neonatal calf diarrhea caused by bacterial enterotoxins, bacterial or parasitic-induced inflammation, or virus-induced villous atrophy leads to intestinal hypersecretion, malabsorption, or both. Mechanisms of secretion and malabsorption differ depending on the agent, suggesting that different modes of treatment must be employed to be effective. Currently, oral rehydration solutions and the pharmacologic blockade of secretory processes are being evaluated in these various diseases.