Calcium/calmodulin inhibition of coupled NaCl transport in membrane vesicles from rabbit ileal brush border

Calcium-calmodulin-dependent Activation of Adenylate Cyclase in Prostaglandin-induced Electrically-monitored Intestinal Secretion in the Rat

The calcium-calmodulin antagonist 5-iodo-C8-W7 inhibited the PGE2-induced stimulation of cAMP production by isolated enterocytes from rat small intestine. It also reduced the secretory response of intestinal sheets to PGE2, measured as a rise in short-circuit current. It did not however, inhibit the electrical responses to forskolin and dibutyryl cAMP, nor to acetylcholine, a secretagogue whose effect is not mediated by cAMP. It is concluded that the receptor-mediated activation of adenylate cyclase and the subsequent secretory response are dependent upon calcium-calmodulin.

Regulation of water and ion movement in intestine

The direction of net fluid transport in the gut is determined by the algebraic sum of Na+ absorption and Cl- secretion. Na+ absorption by small intestinal villous cells and colonic surface cells is controlled primarily by electrically neutral (NaCl) and electrogenic (Na+-glucose, Na+-amino acid, amiloride-insensitive, and amiloride-sensitive Na+ conductance) entry processes in the apical membrane. Neutral NaCl entry appears to be the result of parallel Na+:H- and Cl-:HCO3- exchangers operating at equal stoichiometry. Uncoupled exchangers operating at different stoichiometry may result in net HCO3- absorption (jejunum), net HCO3- secretion (ileum and proximal colon) or HCO3-:Cl- exchange (distal colon). Increases in intracellular cyclic nucleotides and/or ionized Ca2+ inhibit NaCl entry and, in vivo, promote HCO3- and Cl- secretion. Cl- secretion by crypt cells is the result of cyclic nucleotide-mediated or Ca2+-mediated Cl- conductance channels in the apical membrane which allow Cl- to exit down an electrochemical gradient created by a basolateral NaKCl2 entry process. Cyclic nucleotides may act via specific A and G protein kinases. They also release Ca2+ from intracellular stores and thus could alter transport via Ca2+ (and calmodulin)-activated kinases. Ca2+-dependent secretory agents initiate phospholipid hydrolysis and stimulate secretion via the resulting hydrolytic products: arachidonic acid metabolites when bradykinin is the stimulus or diacylglycerol and/or inositol trisphosphate when acetylcholine is the stimulus. The arachidonic acid metabolites may then stimulate cyclic nucleotide production, while diacylglycerol activates a specific Ca2+/phospholipid-dependent protein kinase (C kinase), and inositol trisphosphate releases Ca2+ from the endoplasmic reticulum. The interrelationships between these intracellular messengers and their exact modes of action remain to be clarified.

The effect of [Arg8]vasopressin on electrogenic chloride secretion in cultured rat epididymal epithelia

Primary cultured rat efferent ductal epithelia and cauda epididymal epithelial were mounted in Ussing chambers to study the effect of arginine vasopressin (AVP) on chloride secretion in the male excurrent duct. The regional differences in the signal transduction pathways involved were also investigated. In both the efferent duct and the cauda epididymidis, basolateral addition of AVP resulted in a dose-dependent increase in the short-circuit current (Isc), which was mediated via V1 receptor. Replacement of ambient Cl- with gluconate or pretreatment of a Cl- channel blocker, diphenylamine-2-carboxylate (apical, 1 mM), completely abolished the response, whereas addition of amiloride had no effect on the Isc. Pretreating the epithelia of the efferent duct with indomethacin (apical, 5 microM) or forskolin (basolateral, 1 microM), but not thapsigargin (apical, 1 microM) or trifluoperazine (apical, 20 microM), significantly inhibited the AVP response (P < 0.001). By comparison, pretreating the epithelia of the cauda epididymidis with any of the four agents significantly reduced the AVP-evoked response. These results suggested that the stimulation of chloride secretion by AVP in the efferent duct and the cauda epididymidis is mediated by prostaglandin synthesis and involves adenosine 3',5'-cyclic monophosphate (cAMP) as a second messenger. In the cauda epididymidis, calcium, in addition to cAMP, may play a role in mediating the AVP-induced response.

Liposomes targeted to deliver antisecretory agents to jejunal mucosa

The B subunit of cholera toxin has been covalently attached to the surface of liposomes made from a mixture of phosphatidylethanolamine, phosphatidylcholine and cholesterol. Adenylate cyclase inhibitors and chloride conductance inhibitors were encapsulated within the liposomes. These "targeted" liposomes were used to study the combined effects of this novel delivery system, and a limited number of possible antisecretory agents, on net fluid flux into the pig jejunum. A state of net secretory fluid flux was induced in isolated jejunal loops in weanling pigs by adding theophylline or cholera toxin to the lumen of the isolated loops. There was no reduction in net fluid secretion when liposome suspensions without encapsulated secretory inhibitors were added to fluid in the lumen of loops treated with theophylline. There was also no reduction in net fluid secretion when miconazole, alpha-phenylcinnamate or 5 nitro-2-(3-phenethylamino)benzoate were encapsulated within targeted liposomes added to isolated jejunal loops. The net fluid flux induced by exposure of jejunal loops to theophylline was significantly reduced by adding targeted liposomes containing 2'-deoxy-3'-AMP. The reduction involved a reversal of net secretory fluid flux to an absorptive value. The net fluid secretory response to treatment of loops with cholera toxin was also inhibited by treating loops with targeted liposomes containing 2'-deoxy-3'-AMP. However, the reversal of secretion was less complete for secretion induced by cholera toxin than for secretion induced by theophylline. The reduced antisecretory efficacy versus cholera toxin was not improved by encapsulating higher concentrations of 2'-deoxy-3'-AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Calmodulin content and activity in normal and coeliac duodenum

Calmodulin is an important modulator of intracellular calcium processes and may be implicated in the calcium malabsorption of coeliac disease. The calmodulin content in extracts of duodenal biopsy specimens from 48 normal control subjects and 28 patients with coeliac disease was determined. Radioimmunoassay was used to measure immunoreactive calmodulin while a cyclic adenosine 3',5'-monophosphate phosphodiesterase activity assay was used to measure biologically active calmodulin. Calmodulin values measured by both assays were similar for control and disease groups. Mean (SEM) immunoreactive calmodulin values were 1.68 (0.09) micrograms/mg protein for controls and 1.67 (0.15) and 1.45 (0.15) micrograms/mg protein for partial and total villous atrophy respectively. These values were not significantly different. Biologically active calmodulin values were 2.77 (0.21), 1.82 (0.34), and 3.24 (0.33) micrograms/mg protein for control, partial, and total villous atrophy subjects respectively. The biologically active calmodulin values in the partial villous atrophy group were significantly lower than in controls and total villous atrophy subjects. In the phosphodiesterase assay, the calmodulin antagonist trifluoperazine inhibited the activity stimulated by purified calmodulin and by the extracts to the same extent. These results show that calmodulin values are normal in coeliac disease and provide no evidence that changes in calmodulin account for the abnormal calcium absorption in these patients.

Effect of antipsychotic drugs on the molecular action of cholera toxin in rabbit intestinal epithelial cells

Antipsychotic drugs of known antidiarrhoeal and anticalmodulin activity inhibited the cholera-toxin-catalysed ADP-ribosylation of proteins of Mr 37,000, 40,000 and 45,000 (thought to be regulatory components of the adenylate cyclase complex) that was previously shown to occur in plasma membranes from rabbit intestinal epithelial cells [(1989) Biochim. Biophys. Acta 1014, 289-297]. There was no obvious correlation between the different activities of the drugs. The drugs also inhibited adenylate cyclase activity, but in this case the inhibition correlated well with the known IC50 values of the drugs for anticalmodulin activity and with their antidiarrhoeal activities.

Role of calcium and calmodulin in the regulation of the rabbit ileal brush-border membrane Na+/H+ antiporter

In rabbit ileum, Ca2+/calmodulin (CaM) appears to be involved in physiologically inhibiting the linked NaCl absorptive process, since inhibitors of Ca2+/CaM stimulate linked Na+ and Cl- absorption. The role of Ca2+/CaM-dependent phosphorylation in regulation of the brush-border Na+/H+ antiporter, which is believed to be part of the neutral linked NaCl absorptive process, was studied using purified brush-border membrane vesicles, which contain both the Na+/H+ antiporter and Ca2+/CaM-dependent protein kinase(s) and its phosphorprotein substrates. Rabbit ileal villus cell brush-border membrane vesicles were prepared by Mg precipitation and depleted of ATP. Using a freezethaw technique, the ATP-depleted vesicles were loaded with Ca2+, CaM, ATP and an ATP-regenerating system consisting of creatine kinase and creatine phosphate. The combination of Ca2+/CaM and ATP inhibited Na+/H+ exchange by 45 +/- 13%. This effect was specific since Ca2+/CaM and ATP did not alter diffusive Na+ uptake, Na+-dependent glucose entry, or Na+ or glucose equilibrium volumes. The inhibition of the Na+/H+ exchanger by Ca2+/CaM/ATP was due to an effect on the Vmax and not on the Km for Na+. In the presence of CaM and ATP, Ca2+ caused a concentration-dependent inhibition of Na+ uptake, with an effect 50% of maximum occurring at 120 nM. This Ca2+ concentration dependence was similar to the Ca2+ concentration dependence of Ca2+/CaM-dependent phosphorylation of specific proteins in the vesicles. The Ca2+/CaM/ATP-inhibition of Na+/H+ exchange was reversed by W13, a Ca2+/CaM antagonist, but not by a hydrophobic control, W12, or by H-7, a protein kinase C antagonist. We conclude that Ca2+, acting through CaM, regulates ileal brush-border Na+/H+ exchange, and that this may be involved in the regulation of neutral linked NaCl absorption.

Regulation of the rabbit ileal brush-border Na+/H+ exchanger by an ATP-requiring Ca++/calmodulin-mediated process

Brush-border vesicles purified from rabbit ileal villus cells were used to evaluate how Ca++/calmodulin (CaM) regulates the neutral linked NaCl absorptive process, part of which is a Na+/H+ exchanger. After freezing and thawing to allow incorporation of macromolecules into the vesicles, the effect of Ca++/CaM on brush-border Na+ uptake with an acid inside pH gradient, and on Na+/H+ exchange was determined. Freezing and thawing vesicles with 0.85 microM free Ca++ plus 5 microM exogenous CaM failed to alter Na+/H+ exchange as did the addition of exogenous ATP plus an ATP regenerating system, which was sufficient to elevate intravesicular ATP to 47 microM from a basal level of 0.4 microM. However, the combination of Ca++/CaM plus ATP inhibited Na+ uptake in the presence of an acid inside pH gradient and inhibited Na+/H+ exchange, while Na+ uptake in the absence of a pH gradient was not altered. This effect required a hydrolyzable form of ATP, and did not occur when the nonhydrolyzable ATP analogue, AMP-PNP, replaced ATP. Under the identical intravesicular conditions used for the transport studies, Ca++ (0.85 microM) plus exogenous CaM (5 microM), in the presence of magnesium plus ATP, increased phosphorylation of five brush-border peptides. These data are consistent with Ca++/CaM acting via phosphorylation to regulate the ileal brush-border Na+/H+ exchanger.

Effect of two potent calmodulin antagonists on calcium transport of brush border and basolateral vesicles from human duodenum

In the present in-vitro study we investigated the possible role of the calmodulin-antagonistic drugs loperamide and calmidazolium in the regulation of transepithelial Ca2+ transport of human duodenum. Brush border membrane vesicles and basolateral membrane vesicles were simultaneously prepared from surgically resected pieces of morphologically intact human duodenum with a modified Percoll-gradient centrifugation method. Brush border and basolateral membrane vesicles were characterized using enzyme marker analysis and electron microscopy: alkaline phosphatase was enriched 20-fold in brush border membrane vesicles, whereas [Na+ + K+]-stimulated adenosine triphosphatase was enriched 15-fold in basolateral membrane vesicles. Calmodulin activity was determined by a specific radioimmunoassay after solubilizing brush border and basolateral membrane vesicles in 1% Triton X-100. In basolateral membrane vesicles, we found no calmodulin activity. In brush border membrane vesicles calmodulin activity was impaired by 50% after pre-incubation with loperamide or calmidazolium. We measured calcium, sodium, D-glucose and D-mannitol uptake with a rapid filtration technique. Before the transport experiments, brush border and basolateral membrane vesicles were pre-incubated with 5 microM loperamide or 5 microM calmidazolium for 60 min at 5 degrees C. In drug-pretreated, brush border membrane vesicles calcium uptake was significantly reduced after 1 min incubation (-25% +/- 5%, P less than 0.05); this effect was completely reversed in the presence of 5 microM calmodulin. In basolateral membrane vesicles, we found two Ca2+ transport systems: (1) Na+/Ca2+ exchange and (2) ATP-dependent Ca2+ transport. In basolateral membrane vesicles loperamide had no effect. Calmidazolium had no effect on Na+/Ca2+ exchange, but significantly inhibited ATP-dependent Ca2+ transport. This effect could not be reversed by calmodulin.

Inhibition by calmodulin of calcium/phospholipid-dependent protein phosphorylation

Calmodulin was previously found to inhibit the Ca2+/phospholipid-dependent phosphorylation of an endogenous substrate, called the 87-kilodalton protein, in a crude extract prepared from rat brain synaptosomal cytosol. We investigated the mechanism of this inhibition, using Ca2+/phospholipid-dependent protein kinase and the 87-kilodalton protein, both of which had been purified to homogeneity from bovine brain. Rabbit brain calmodulin and some other Ca2+-binding proteins inhibited the phosphorylation of the 87-kilodalton protein by this kinase in the purified system. Calmodulin also inhibited the Ca2+/phospholipid-dependent phosphorylation of H1 histone, synapsin I, and the delta subunit of the acetylcholine receptor, with use of purified components. These results suggest that calmodulin may be a physiological regulator of Ca2+/phospholipid-dependent protein kinase.