Gum Arabic enhances paracellular transport of water in amphibian everted small intestinal segments.

Gum Arabic (GA) is known for its proabsorbent activity in normal intestine as well as in animal models of diarrhea. The aim of the study was to find the effect of GA on intestinal transport of water and possible route of absorption in frog everted gut sacs. D-Mannitol was used as a marker of paracellular transport to find the route of absorption. Everted gut sacs (n = 4,5) were placed in Ringer containing GA (2.5 g/L) with or without D-Mannitol (0.5 g/L), incubated for 1 hour and analysed for change in weights of the sacs and D-Mannitol uptake. There was significant increase in uptake of water and D-Mannitol in the presence of GA compared to controls (P < 0.05). Gum Arabic improves water uptake by the intestinal mucosa, possibly by opening the paracellular pathways.

Evidence for functional interaction of plasma membrane electron transport, voltage-dependent anion channel and volume-regulated anion channel in frog aorta.

Frog aortic tissue exhibits plasma membrane electron transport (PMET) owing to its ability to reduce ferricyanide even in the presence of mitochondrial poisons, such as cyanide and azide. Exposure to hypotonic solution (108 mOsmol/kg H2O) enhanced the reduction of ferricyanide in excised aortic tissue of frog. Increment in ferricyanide reductase activity was also brought about by the presence of homocysteine (100 microM dissolved in isotonic frog Ringer solution), a redox active compound and a potent modulator of PMET. Two plasma-membrane-bound channels, the volume-regulated anion channel (VRAC) and the voltage-dependent anion channel (VDAC), are involved in the response to hypotonic stress. The presence of VRAC and VDAC antagonists-tamoxifen, glibenclamide, fluoxetine and verapamil, and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), respectively-inhibited this enhanced activity brought about by either hypotonic stress or homocysteine. The blockers do not affect the ferricyanide reductase activity under isotonic conditions. Taken together, these findings indicate a functional interaction of the three plasma membrane proteins, namely, ferricyanide reductase (PMET), VDAC and VRAC.

Influence of dietary calcium content on intestinal permeability in rat.

BACKGROUND & OBJECTIVE: Agents that increase the permeability of intestinal epithelium promote the absorption of nutrients by the gut. High calcium concentration in the gut has been shown to enhance passive transport of glucose in the rat intestine. An increase in the permeability of the intestinal epithelium may account for this observation. The present study was aimed at monitoring the permeability of intestine of rats fed high or low calcium diets. METHODS: Everted intestinal sacs were used to study transports of substances across the gut. While radioactive and non radioactive calcium isotopes were employed to study the active transport and passive transport of calcium, transport of labelled mannitol was taken as a measure of passive permeability. RESULTS: High calcium diet increased the passive transport of mannitol and calcium while decreasing the active transport of calcium by the everted gut sacs. INTERPRETATION & CONCLUSION: Passive mechanisms are enhanced by high calcium diet, while low calcium diet favours active transport. Calcium in the diet may be affecting intestinal transport.

Actions of antidiuretic hormone analogues on intact and nystatin-permeabilized frog skins.

The roles of two antidiuretic hormone analogues, namely arginine vasotocin (AVT) and lysine vasopressin (LVP), in solute transport across the ventral abdominal skin of frogs (Rana hexadactyla) were studied using voltage-clamp methods on intact and nystatin-permeabilized preparations. Arginine vasotocin (40 nm), the amphibian analogue of antidiuretic hormone, did not have any effect on the skin of Rana hexadactyla. However, LVP, the porcine antidiuretic hormone, increased the transepithelial potential difference (TEPD) and short-circuit current (SCC) significantly, without affecting the slope conductance. Lysine vasopressin had no action subsequent to addition of amiloride (100 microm) on the apical side or ouabain (10 microm) on the basolateral side. Lysine vasopressin increased slope conductance in the nystatin-permeablized skin while decreasing TEPD. Such a change was not seen in chloride-free solutions. To elucidate the mechanism of action of LVP on intact skin, experiments were done with forskolin and a V(2) receptor blocker. The effects of forskolin (10 microm) were different from those of LVP in that forskolin significantly increased SCC and conductance of the intact skin, while decreasing TEPD. The forskolin-induced increase in conductance was not abolished by amiloride. Use of the V(2) receptor blocker inhibited the effects of LVP. We conclude that AVT does not have an action on the skin of Rana hexadactyla. Lysine vasopressin enhances transepithelial sodium transport by increasing sodium-potassium pump activity, while not affecting the epithelial sodium channel conductance. Lysine vasopressin also enhances an inward-directed conductance on the basolateral membrane, probably a chloride conductance. The action of LVP on the intact frog skin is through the V(2) receptors; however, downstream signalling does not seem to be mediated by cAMP. Analysis of the electrophysiological model of frog skin with LVP allows us additionally to conclude that modulation of channel activity and not carrier-mediated transport affects slope conductance.

Plasma membrane electron transport in frog blood vessels.

In an attempt to see if frog blood vessels possess a plasma membrane electron transport system, the postcaval vein and aorta isolated from Rana tigrina were tested for their ability to reduce ferricyanide, methylene blue, and 2,6-dichloroindophenol. While the dyes remained unchanged, ferricyanide was reduced to ferrocyanide. This reduction was resistant to inhibition by cyanide and azide. Heptane extraction or formalin fixation of the tissues markedly reduced the capability to reduce ferricyanide. Denuded aortas retained only 30% of the activity of intact tissue. Our results indicate that the amphibian postcaval vein and aorta exhibit plasma membrane electron transport.

Inherent rhythmicity and interstitial cells of Cajal in a frog vein.

Interstitial cells of Cajal are responsible for rhythmic contractions of the musculature of the gastrointestinal tract and blood vessels.The existence of these cells and spontaneous rhythmicity were noticed in amphibian vein and the findings are reported in this paper.The postcaval vein was identified in the frog, Rana tigrina and was perfused with amphibian Ringer solution after isolation.Contractile activity was recorded through a tension transducer connected to a polygraph.The isolated postcaval vein showed spontaneous rhythmic activity. Addition of cold Ringer solution decreased, while warm Ringer increased, the rate of contraction. Adrenaline caused inhibition of rhythmic activity at a dosage that increased the rate of isolated sinus venosus.Sections of the postcaval vein,when stained supravitally with methylene blue, showed the presence of interstitial cells of Cajal. Photic stimulation of the vein in the presence of methylene blue led to a significant decrease in the rate of spontaneous beating of the vein.These findings indicate that the postcaval vein of frog is capable of inherent rhythmcity, which is dependent on the interstitial cells of Cajal but is independent of the sinus venosus.

Phenol red inhibits uptake of phosphate by the everted gut sacs of mice.

Uptake of phosphate by the everted intestinal sacs of mice was measured at various concentrations of this anion in the incubation medium. Phenol red when added to the serosal side was able to inhibit the uptake of phosphate significantly, while uptake of glucose or proline was unaffected. When para amino hippuric acid was present along with phenol red in the serosal compartment, the inhibition exerted by the dye on phosphate uptake was significantly less. These findings suggest that entry of phenol red into gut tissue from the serosal side interferes with the mucosal uptake of phosphate by the everted intestinal sacs of mice.