Thiocyanate inhibition of active chloride absortion in Aplysia intestine

This investigation was principally undertaken to examine the mechanism of active chloride absorption across the Aplysia californica intestine by using various inhibitors of ion transport. Isolated intestine, mounted between identical oxygenated sodium-free seawater solutions, maintained stable transmural potential differences (serosa negative) and short-circuit currents for several hours at 25°C. The metabolic inhibitors, 2,4-dinitrophenol and flouride, reduced both transmural potential difference and short-circuit current; however, the electrical characteristics were predominantly dependent upon glycolytic energy. The addition of thiocyanate to the mucosal solution inhibited both electrical characteristics in parallel, and this inhibition could be titrated according to the thiocyanate concentration. The short-circuit current was carried wholly by a net active chloride transfer from mucosa to serosa as determined by flux measurements. These results suggest that active chloride absorption may be mediated by a primary active transport process.     

Phosphate absorption in Aplysia californica gut: glucocorticoid inhibition

Apical membranes of Aplysia californica foregut epithelia contain a sodium-phosphate symporter. Dexamethasone inhibited the absorptive activity of the sodium-phosphate symporter, whereas amiloride had no effect on the sodium-phosphate symporter. It appears that glucocorticoids or their molluscan equivalent play a role in the overall regulation of phosphate homeostasis by the Aplysia californica gut.     

The general fluid circuit theory of active chloride absorption

The original fluid circuit theory used to explain active intestinal absorption of chloride is modified to include diffusion and secretion of chloride and osmosis. The general differential equation developed is integrated in a particular case. The definition, “effective concentration of chloride in the fluid passing into the intestinal lumen,” leads to simplified general expressions.     

Intracellular chloride activities and active chloride absorption in the intestinal epithelium of the winter flounder

Summary Intracellular chloride activities, (Cl) _c , and the electrical potential difference across the mucosal membrane, _mc, were determined in the isolated small intestine of the winter flounder, using Cl-selective and conventional (KCl-filled) microelectrodes. In the presence of a Na-containing buffer _mc averages –69mV and (Cl) _c averages 24mM, a value that is 3.4 times that predicted for an equilibrium distribution across the mucosal membrane. On the other hand, when the tissue is then perfused with Na-free buffer, (Cl) _c slowly falls to a value that does not differ significantly from that predicted for an equilibrium distribution, and _mc depolarizes significantly. Finally, when the tissue is again bathed in the Na-containing buffer, (Cl) _c rapidly returns to a value well above equilibrium.These results, together with those of Frizzellet al. (J. Membrane Biol. 46:27, 1979), provide direct evidence that: (1) Cl is accumulated against its electrochemical potential difference (32mV) by this tissue, and (2) this accumulation is coupled to and energized by the entry of Na down its steep electrochemical potential difference.     

Thiocyanate transport across fish intestine (Pleuronectes platessa)

Summary When bathed on both sides with identical chloride-containing salines thein vitro preparation of the plaice intestine maintains a negative (serosa to mucosa) short-circuit current of 107±11 A/cm², a transepithelial potential difference of 5.5±0.6 mV (serosa negative), and a mean mucosal membrane potential of –45.4±0.6 mV. Under these conditions the intracellular chloride activity is 32mm.If chloride in the bathing media is partially, or completely substituted by thiocyanate the measured electrical parameters do not change but transepithelial flux determinations show a reduction in chloride fluxes and the presence of a significant thiocyanate flux. The addition of piretanide (10^–4 m) reduced the short-circuit current and the mucosa-to-serosa fluxes of chloride and thiocyanate; this inhibition is similar to the effect of piretanide on chloride transport in this tissue.The results indicate that thiocyanate is transported in this tissue via the piretanide-sensitive chloride pathway and are compared with the effects of thiocyanate on other tissues reported in the literature.     

The cellular mechanism of active chloride secretion in vertebrate epithelia: studies in intestine and trachea

The cellular mechanism of active chloride secretion, as it is manifested in the intestine and trachea, appears to possess the following elements: (1)NaCl cl-transport across the basolateral membrane; (2) Cl- accumulation in the cell above electrochemical equilibrium due to the Na+ gradient; (3) a basolateral Na+-K+ pump that maintains the Na+ gradient; (4) a hormone-regulated Cl- permeability in the apical membrane; (5) passive Na/ secretion through a paracellular route, driven by the transepithelial potential difference; and (6) an increase in basolateral membrane K+ permeability occurring in conjunction with an increase in Na+-K+ pump rate. Electrophysiological studies in canine trachea support this model. Adrenalin, a potent secretory stimulus in that tissue, increases apical membrane conductance through a selective increase in Cl- permeability. Adrenalin also appears to increase basolateral membrane K+ permeability. Whether or not adrenalin also increases paracellular Na+ permeability is unclear. Some of the testable implications of the above secretion model are discussed.     

Thiocyanate degradation by Acremonium strictum and inhibition by secondary toxicants

Acremonium strictum, capable of degrading 7.4 g thiocyanate l^–1, was isolated from wastewater condensate from coke-oven gas. Ammonia and sulfate were the final products from thiocyanate degradation with a stoichiometric ratio of near 1:1. The highest degradation activity was at pH 6. Although the degradation rate started to be inhibited above 4 g thiocyanate l^–1, thiocyanate was completely degraded up to 7.4 g l^–1 within 85 h in shake-flask cultures. The degradation of thiocyanate was inhibited by phenol above 625 mg l^–1, by cyanide above 16 mg l^–1, and by nitrite above 100 mg l^–1. However, ammonia and nitrate had negligible inhibition on thiocyanate degradation up to 3 g l^–1 and 1.5 g l^–1, respectively.     

The gill withdrawal reflex is suppressed in sexually active Aplysia

In Aplysia, the central nervous system and peripheral nervous system interact and form an integrated system that mediates adaptive gill withdrawal reflex behaviours evoked by tactile stimulation of the siphon. The central nervous system (CNS) exerts suppressive and facilitatory control over the peripheral nervous system (PNS) in the mediation of these behaviours. We found that the CNS's suppressive control over the PNS was increased significantly in animals engaged in sexual activity as either a male or female. In control animals, the evoked gill withdrawal reflex met a minimal response amplitude criterion, while in sexually active animals the reflex did not meet this criterion. At the neuronal level, the increased CNS suppressive control was manifested as a decrease in excitatory input to the central gill motor neurons.     

Differential effects of enflurane on Glu- and ACH-induced chloride currents in Aplysia neurons

The primary site of anesthetic action remains controversial. In addition to non-specific actions of hydrophobic substances on the membrane, specific effects of volatile anesthetics on neuronal activity have been reported. In the present study, effects of enflurane on the chloride currents (ICl) induced by L-glutamic acid (Glu) and acetylcholine (ACh) in isolated Aplysia neurons were examined, using the 'concentration clamp' technique. Enflurane increased the peak amplitude of the ICl induced by low concentrations of Glu but decreased those evoked by higher concentrations of the agonist. The anesthetic accelerated both activation and desensitization phases of the Glu-induced ICl. On the other hand, the ACh-induced ICl in the same neuron was depressed in an uncompetitive manner in the presence of enflurane. The desensitization phase was not affected, although the activation phase became more rapid and the mean open time obtained by noise analysis was shortened. These results suggest the existence of specific steps in the process of activation and desensitization of channels, at which the volatile anesthetic exerts differential effects on the postsynaptic currents.     

Sterol absorption by the small intestine

PURPOSE OF REVIEW: Cholesterol absorption is a selective process in that plant sterols and other non-cholesterol sterols are absorbed poorly or not at all. Recent research on the sterol efflux pumps adenosine triphosphate-binding cassette transporter G5 and adenosine triphosphate-binding cassette transporter G8 has not only provided an explanation for this selectivity, but also, together with the discovery of a new class of cholesterol absorption inhibitor, has yielded new insights into the mechanisms that potentially regulate the flux of cholesterol across the enterocyte. This review discusses these recent developments and their importance to the regulation of whole body cholesterol homeostasis. RECENT FINDINGS: Adenosine triphosphate-binding cassette transporters G5/8 regulate plant sterol absorption and also the secretion into bile of cholesterol and non-cholesterol sterols. Loss of adenosine triphosphate-binding cassette transporter G5/8 function results in sitosterolemia. Ezetimibe, a novel, potent and selective inhibitor of cholesterol absorption which is effective in milligram doses, lowers plasma plant sterol concentrations in sitosterolemic subjects, thus suggesting that this drug might be inhibiting the activity of a putative sterol permease in the brush border membrane of the enterocyte that actively facilitates the uptake of cholesterol as well as other non-cholesterol sterols. SUMMARY: Intestinal cholesterol absorption represents a major route for the entry of cholesterol into the body's miscible pools and therefore can potentially impact the plasma LDL-cholesterol concentration. The combined use of agents that inhibit the absorption and synthesis of cholesterol provides a powerful new approach to the prevention and treatment of atherosclerosis.     

Phosphate absorption by Aplysia californica gut: thyroid hormone stimulation

Mucosal membranes of foregut epithelia of Aplysia californica contain a sodium/phosphate symporter. Triiodothyronine stimulated the absorptive activity of the sodium/phosphate symporter, whereas reverse triiodothyronine had no effect on the sodium/phosphate symporter. It appears that thyroid hormone or its molluscan equivalent plays a role in the overall regulation of phosphate homeostasis by the Aplysia californica gut.     

Iron absorption by small intestine of chickens

Iron (Fe) absorption by three segments (duodenum, jejunum, and ileum) of the small intestine of chickens was studied by a perfusion technique in vivo in closed circuit using⁵⁹Fe Cl₃ and was related to the histological characteristics of each segment. The serosal transfers of Fe for the duodenum and jejunum were the same (14%/cm), but significantly different (p<0.05) from those of the ileum (9%/cm), which may be explained by the morphological and histological properties of the gut of chickens. However, the presence of Fe in blood and in liver was significantly lower after perfusion of the jejunum and ileum than after perfusion of the duodenum. It is concluded that chickens show an early adaptation of small intestine to Fe absorption in response to the considerable loss of Fe suffered during the laying process.