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M Field

Publications and source records attributed to M Field.

At least 217 records · Page 12Linked to original sources
PubMed1975

Catecholamine effects on cyclic AMP levels and ion secretion in rabbit ileal mucosa.

Effects of catecholamines on cyclic AMP (cAMP) levels and ion fluxes were examined in isolated rabbit ileal mucosa. The base-line cAMP level was unaffected by epinephrine (Epi), norepinephrine (Norepi), and isoproterenol. The theophylline-augmented cAMP level was decreased slightly be Epi in one series of experiments but not in another. Propranolol did not enhance this effect. The increase in cAMP level produced by cholera toxin was almost completely reversed by addition of Epi or Norepi. This reversal was prevented by phenoxybenzamine. Epi also partially reversed the increase in cAMP level produced by prostaglandin E1. Effects of Epi on ion fluxes were determined following addition of secretagogues. Epi significantly decreased theophylline-induced but not cAMP or cholera toxin-induced Cl secretion. A decrease in short-circuit current was nonetheless observed in the latter two instances. The observed discrepancies between alpha-adrenergic effects on cAMP levels and ion fluxes suggest the following possibilities: 1) ion transport-related cAMP is only a small fraction of total mucosal cAMP; 2) cAMP-induced active ion secretion is only slowly reversible, or 3) effects of alpha-adrenergic stimuli on ion transport are not due to inhibition of cAMP accumulation.

Adrenergic alpha-Agonists↗
PubMed1975

Effects of theophylline on Na and alanine transport across isolated rabbit ileum.

Addition of theophylline to isolated rabbit ileal mucosa resulted in a persistent increase in short-circuit current (SCC) and a persistent decrease in DC conductance. Under short-circuit condition, net Na flux (JNanet) was reversed by theophylline from absorption to secretion (-0.6 mueq/h cm2) and was due to a decrease in unidirectional mucosal- (M) toserosal (S) flux (JNams). Addition of L-alanine to theophylline-treated m-ucosa increased SCC, JNanet, and JNams. Theophylline reduced JAlanet by 25%. Na flux ratios, determined in the presence of theophylline at +26 and -11 mV (mucosal reference), differed from flux ratios expected for a diffusional process by less than 8%. In contrast, Na flux ratios, determined in the absence of theophylline and presence of glucose at +26 and O mV, differed from diffusional ratios by 35%. The results are consistent with the hypothesis that theophylline inhibits active Na absorption, but the data do not conclusively demonstrate the diffusional nature of unidirectional Na fluxes in the presence of theophylline.

Alanine↗
PubMed1975

A new assay for cystic fibrosis factor: effects of sera from patients with cystic fibrosis in the in vitro electrical properaties of rat jejunum.

The in vitro electrical properties of rat jejunum were utilized to assay a factor or factors in serum from patients with cystic fibrosis (CF). Sera from patients with CF were found to decrease short circuit current (SCC) and the SCC response to glucose, and to increase electrical resistance. These effects were present in all 24 patients with CF and 10 of 14 parents of CF patients and absent from the serum of 21 control subjects. This quantitative technique provides an alternative to the ciliostatic assays for the detection of the CF factor or factors.

Animals↗
PubMed1974

Specific binding of cholera toxin to isolated intestinal microvillous membranes.

A sucrose density gradient assay was used to demonstrate the specificity and saturation of the binding of [(125)I]cholera toxin to isolated intestinal microvillous membranes from rat small intestine. When the toxin is first complexed to antitoxin and then exposed to intestinal membranes, the binding of cholera toxin is inhibited. To emphasize the physiologic importance of these observations, similar concentrations of [(125)I]cholera toxin were shown to stimulate the accumulation of cyclic AMP in mucosal homogenates and to increase the secretion of fluid into intestinal loops, whereas the same concentrations of toxin mixed with antitoxin had no effect on cyclic AMP accumulation. These studies suggest that cholera toxin attaches to brush border binding sites before exerting its biologic effect and that local intestinal antibody protection against cholera toxin may be due to inhibition of toxin attachment to these binding sites.

Animals↗
PubMed1974

Mode of action of cholera toxin: stabilization of catecholamine-sensitive adenylate cyclase in turkey erythrocytes.

Preincubating turkey erythrocytes with cholera toxin alters their adenylate cyclase (EC 4.6.1.1) system: basal activity, maximal epinephrine-stimulatable activity, and affinity of the enzyme reaction for epinephrine are all increased. Pretreatment of erythrocytes with choleragenoid prevents these changes. Cholera toxin does not alter [(3)H]epinephrine uptake by intact erythrocytes. The increase in epinephrine-stimulatable cyclase activity appears to occur at the expense of fluoride-stimulatable activity, which is decreased by the toxin. In lysates from both toxin-treated and control cells, maximally stimulating amounts of epinephrine and fluoride, when added in combination, have a nearly additive effect on cyclase activity. These observations suggest that the adenylate cyclase system of the turkey erythrocyte may exist in two interconvertible forms, one that is catecholamine-responsive but fluoride-insensitive, and another that is fluoride-sensitive but not coupled to catecholamine receptors. Cholera toxin appears to stabilize the enzyme in its hormone receptor-coupled form.

Adenylyl Cyclases↗
PubMed1974

Reversal of cyclic AMP-mediated intestinal secretion by ethacrynic acid.

Ethacrynic acid (EA) has been reported to reduce cholera toxin-induced intestinal fluid secretion in the intact animal. We explored the nature of this inhibition in vitro by measuring unidirectional, transmural fluxes of (22)Na and (36)Cl across isolated rabbit ileal mucosa. Under control conditions (short-circuited mucosa bathed in bicarbonate-Ringer), there was net absorption of Na and Cl. Theophylline (10 mM), cyclic AMP (5 mM), and cholera toxin (added in vivo) abolished net Na flux and produced net Cl secretion. In the presence of either theophylline or cAMP, addition of 0.1 mM EA to the serosal bathing solution abolished net Cl secretion and restored net Na absorption. Cholera toxin-treated mucosa was exposed to 0.05 and 1.0 mM EA. The lower concentration restored net Na absorption but did not significantly reduce Cl secretion. The higher concentration abolished net transport of both Na and Cl. Short-circuit current and Na flux measurements in the presence and absence of glucose indicated that 0.1 mM EA does not inhibit glucose-coupled Na transport. Short-circuit current measurements in the presence of 1.0 mM EA suggested that even this concentration of EA does not inhibit glucose-coupled Na transport. Thus EA appears to specifically inhibit Cl (or NaCl) secretion without inhibiting the absorptive Na "pump." The anti-secretory effect of 0.1 mM EA does not appear to result from inhibition of adenylate cyclase since secretion stimulated by addition of 5 mM cAMP was abolished. Furthermore, 0.1 mM EA did not significantly reduce theophylline-augmented and cholera toxin-augmented cAMP levels in ileal mucosa. We conclude that EA interacts specifically with the active Cl (or NaCl) secretory mechanism of the small intestine at a step beyond generation of cAMP.

Animals↗
PubMed1974

Effects of prostaglandins and cholera enterotoxin on intestinal mucosal cyclic AMP accumulation. Evidence against an essential role for prostaglandins in the action of toxin.

Both cholera enterotoxin and certain prostaglandins have been shown to stimulate intestinal fluid secretion in vivo, to cause ion flux changes in vitro similar to those caused by addition of cyclic 3',5'-adenosine monophosphate (cyclic AMP), and to activate intestinal mucosal adenyl cyclase. It has been suggested that the effects of the enterotoxin on intestinal cyclic AMP metabolism may be indirect, and that locally synthesized prostaglandins may serve as required intermediates for the effects of the enterotoxin in activating intestinal mucosal adenyl cyclase. In order to clarify certain aspects of the mechanisms by which these two agents alter intestinal mucosal cyclic AMP metabolism and ion transport, their effects on cyclic AMP accumulation in rabbit ileal mucosa were examined in vitro. Addition of 5 mug per ml (75 mug per 150 mg mucosa) of purified cholera enterotoxin produced a peak increase in cyclic AMP level in 3 h but there was a time delay of at least 30 min before any effect was observed. Inhibition of cyclic nucleotide phosphodiesterase with theophylline failed to reduce this time delay. In contrast, addition of prostaglandin E(1) (PGE(1)) increased the cyclic AMP level rapidly, a peak effect being observed in 2 min. The time of the peak prostaglandin-induced changes in cyclic AMP level and short-circuit current correlated closely. A maximal increment in cyclic AMP level was achieved with 5 x 10(-5) M PGE(1). When 10(-4) M PGE(1) was added to mucosa already maximally stimulated with cholera toxin, the resulting cyclic AMP level was equal to the sum of the levels reached when each agent was added alone. Furthermore, the effects of the enterotoxin on mucosal cyclic AMP levels were not influenced by indomethacin under conditions where mucosal prostaglandins synthesis was inhibited. The results suggest that endogenous prostaglandins do not provide an essential link in the activation of intestinal mucosal adenyl cyclase by cholera enterotoxin. The present study also indicates that the effect of cholera enterotoxin on intestinal mucosal cyclic AMP metabolism involves a definite time delay which is not due to cyclic nucleotide phosphodiesterase activity.

Adenylyl Cyclases↗
PubMed1974

Vasoactive intestinal peptide stimulation of adenylate cyclase and active electrolyte secretion in intestinal mucosa.

Vasoactive intestinal peptide (VIP), originally isolated from hog small intestinal mucosa, has been shown to cause small intestinal secretion. More recently, this peptide has been identified in the plasma and tumors of patients with the so-called "pancreatic cholera" syndrome. In order to explore the possible role of VIP in the pathogenesis of this syndrome, we examined the effects of this peptide and other hormones on the cyclic AMP levels, adenylate cyclase activity, and ion transport in in vitro preparations of ileal mucosa. In rabbit ileal mucosa, VIP (20 mug/ml) caused a prompt fivefold increase in cyclic AMP level, whereas nine other hormones, which have been postulated to cause intestinal secretion, failed to exert such an effect. Pentagastrin and glucagon also failed to increase cyclic AMP levels in canine ileal mucosa. An increase in mucosal cyclic AMP levels was observed at a VIP concentration of 0.1 mug/ml and appeared to be nearly maximal at 2.0 mug/ml. VIP (100 mug/ml) stimulated adenylate cyclase activity in a membrane preparation from rabbit ileal mucosa. Secretin (6.0 x 10(-5) M) failed to do so. When added to the serosal side of isolated rabbit ileal mucosa clamped in an Ussing chamber, VIP (2 mug/ml) increased short-circuit current (SCC) and caused net secretion of both Cl and Na. Net Cl secretion exceeded net Na secretion. These effects of VIP on mucosal cyclic AMP metabolism and ion transport are similar to those observed with cholera enterotoxin and certain prostaglandins. VIP was also tested with normal human ileal mucosa. At a concentration of 2 mug/ml it caused a fivefold increase in cyclic AMP level and an increase in SCC of the same magnitude as that caused by 5 mM theophylline. Addition of a second 2-mug/ml dose of VIP and addition of theophylline after VIP produced no further change in SCC. We conclude the VIP stimulates adenylate cyclase and active ion secretion in both rabbit and human ileal mucosa. This may be related to the pathogenesis of diarrhea in patients with the pancreatic cholera syndrome.

Adenylyl Cyclases↗
PubMed1974

Intestinal secretion.

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Adrenergic beta-Antagonists↗
PubMed1973

Effects of cycloheximide on the response of intestinal mucosa to cholera enterotoxin.

Prior studies have indicated that effects of cholera enterotoxin (CT) on the small intestine are delayed in onset and involve an interaction with adenyl cyclase in the mucosa. It has also been shown that the administration of cycloheximide to rabbits in doses which inhibit crypt cell mitoses (20 mg/kg), diminishes CT-induced fluid production in jejunal loops. These latter studies have been interpreted as indications that CT-related intestinal secretion is a crypt cell function and that it is mediated by a CT-induced protein. The present study was undertaken to delineate more precisely the nature of the interaction in the intestine between cycloheximide and cholera toxin. Pretreatment of rabbits with cycloheximide reduced by 60% the secretory response to CT in isolated ileal loops with intact blood supply. Sodium and chloride flux measurements on mucosa isolated from these and control loops indicated that this antisecretory effect of cycloheximide persists in vitro. Measurements of radioactive leucine incorporation into mucosal protein indicated that the dose of cycloheximide employed inhibited protein synthesis by 90%. This inhibitory effect was shown to be independent of any effect of cycloheximide on amino acid uptake across the brush border. Measurements of adenyl cyclase activity and cyclic AMP levels in ileal mucosa of cycloheximide pretreated and control animals indicated that cycloheximide did not diminish the CT-induced increases in these parameters. These observations demonstrate that cycloheximide reduces CT-induced intestinal fluid production without interfering with the CT-induced augmentation of adenyl cyclase activity or the consequent rise in cyclic. AMP concentration. Since the antisecretory effect of cycloheximide persists in vitro, it probably involves a direct interaction of the antibiotic with mucosal cell ion transport mechanisms rather than an indirect effect mediated by other humoral or neurogenic factors. The present observations also suggest that the secretory response of the intestine to CT involves neither the synthesis of new adenyl cyclase nor that of a protein modifying its activity.

Adenylyl Cyclases↗
PubMed1972

Effect of cholera enterotoxin on ion transport across isolated ileal mucosa.

The effects of cholera enterotoxin on intestinal ion transport were examined in vitro. Addition of dialyzed filtrate of Vibrio cholerae (crude toxin) to the luminal side of isolated rabbit ileal mucosa caused a delayed and gradually progressive increase in transmural electric potential difference (PD) and shortcircuit current (SCC). A similar pattern was observed upon addition of a highly purified preparation of cholera toxin, although the changes in PD and SCC were smaller. Na and Cl fluxes across the short-circuited mucosa were determined with radioisotopes 3-4 hr after addition of crude toxin or at a comparable time in control tissues. The toxin caused a net secretory flux of Cl and reduced to zero the net absorptive flux of Na. Similar flux changes were observed when either crude or purified toxin was added in vivo and tissues were mounted in vitro 3-4 hr later. Additon of D-glucose to the luminal side of toxin-treated mucosa produced a large net absorptive flux of Na without altering the net Cl and residual ion fluxes. Adenosine 3',5'-cyclic phosphate (cyclic AMP) and theophylline had previously been shown to cause a rapid increase in SCC and ion flux changes similar to those induced by cholera toxin. Pretreatment of ileal mucosa with either crude or purified cholera toxin greatly reduced the SCC response to theophylline and dibutyryl cyclic AMP, which, together with the flux data, suggest that both cyclic AMP and cholera toxin stimulate active secretion by a common pathway. Inhibition of the SCC response to theophylline was observed after luminal but not after serosal addition of toxin. In vitro effects of cholera toxin correlated closely with in vivo effects: heating toxin destroyed both; two V. cholerae filtrates which were inactive in vivo proved also to be inactive in vitro; PD and volume flow measurements in isolated, in vivo ileal loops of rabbit revealed that the PD pattern after addition of toxin is similar to that seen in vitro and also correlates closely with changes in fluid movement. The results suggest that stimulation by cholera toxin of a cyclic AMP-dependent active secretory process of the intestinal epithelial cells is a major cause of fluid loss in cholera.

Animals↗