K. C. Huang

INTESTINAL TRANSPORT OF AMINO ACIDS AND GLUCOSE IN FLOUNDER FISH.

Summary Everted intestinal sacs of winter flounders were used in the summer to study the transmucosal movement of glucose and certain aromatic amino acids and derivatives. It was found that glucose was not transported against a concentration gradient. L-tyrosine, D-tyrosine, m-tyrosine, o-tyro-sine, 3-amino-tyrosine, 3,5-dichloro tyrosine and 3,4-dihydroxyphenylalanine were transported from mucosal to serosal side against a concentration gradient. 3,5-dibromo- and 3,5-dinitro-tyrosine were not transported. Among the tryptophan compounds which were studied, only L-tryptophan was transported; D-tryptophan and tryptamine were not transported against a concentration gradient.

Glucose Transport by the Intestinal Mucosa of the Dogfish.

Summary A simple technic using the mucosal membrane of dogfish gut to study the transport of glucose was presented. It was found that the mucosal membrane can transport glucose from the mucosal to the submucosal fluid against concentration gradients. This transport was greater at 36° than at 26°, and was inhibited by phlorizin, sodium azide, iodoacetic acid and uranyl acetate. Michaelis-Menten analysis of data has shown that inhibition of glucose transport by phlorizin was competitive at 26° and non-competitive at 36°. It is postulated that a carrier system for glucose transport operates at the low temperature, while a more complicated mechanism handles glucose transport at 36°.

Effect of Staphylococcal Enterotoxins A, B, and C, on Ion Transport and Permeability Across the Flounder Intestine

Summary Purified staphylococcal enterotoxins A, B, and C added to the mucosal and serosal bathing solution of winter flounder intestine in vitro caused changes in ion fluxes and electrical properties. An increase in Na and Cl fluxes was observed after the addition of these enterotoxins, with the greatest increase being in the serosal to mucosal flux for each ion resulting in an upward movement of both PD and Isc. In the staphylococcal enterotoxin treated intestine, the membrane permeability to small solute such as urea and thiourea did not increase significantly. The tissue resistance and the residual flux (J R net) did not change significantly after adding the enterotoxins, suggesting that staphylococcal enterotoxins increase PD and Isc by stimulating active Na and Cl secretion via an electrogenic mechanism.

Effect of Glucose and Diuretics on Intracellular Potentials of Mouse Intestinal Mucosa

Summary To substantiate our previous hypothesis in explanation of the inhibitory effect of EA, furosemide, and amiloride on mouse intestine, the intracellular potential, V mc, and transmural potential, V sm, of the intestinal mucosal epithelium were measured simultaneously with a 0.5-μm diameter microelectrode. It was found when glucose was present in the Ringer bathing solution, an average V sm of 22.5 ± 0.87 mV was recorded and V sm was 2.68 ± 0.09 mV. EA, furosemide, and amiloride at 0.5 mM produced an hyperpolarization of V mc and a reduction of V sm. The V mc increased to 34.8 ± 0.68, 31.9 ± 2.6, and 31.2 ± 0.92 mV, respectively. When the bathing fluid contained no glucose, the average V mc was 29.5 ± 0.53 mV and the average V sm was 1.28 ± 0.10 mV. Addition of EA in the glucose-free bathing media caused no significant change in V mc, but a decrease in V sm. However, addition of furosemide or amiloride did cause further hyperpolarizations of V mc, to 40.7 ± 0.64 and 39.8 ±1.2 mV, respectively. These results support our conclusion that EA mainly acts on the Na-glucose co-transport system and the other two diuretics act on the membrane conductance. The Ar-rhenius plots of the variations of V sm and V mc with temperature provide the calculation of the activation energies for the trans-mural and transmucosal potentials as 19.3 and 14.02 kcal/mole, respectively.

Renal Excretion of Some Isomeric Hexoses in the Dog.

Summary Studies with close arterial injection technique have demonstrated that the renal excretion of 3MG, L-glucose, L-galac-tose, L-fucose and L-mannose was greater than that of the simultaneously injected inu-lin, suggesting a renal tubular secretion of these compounds. Steady-state experiments also showed that a positive T value was obtained with L-glucose and 3MG. Phlorid-zin abolished the tubular secretion of these compounds. It is therefore postulated that a carrier system for sugar transport is located in the brush border of renal tubules. This carrier has a complementary structure resembling that of hexoses. Its luminal site reacts complementarily with D-hexoses and the cyto-plasmic facing site of the carrier reacts with the 3MG and L-hexose.

Dose-Dependent Elimination of 5-Fluoro-2′-deoxyuridine in the Monkey

The kinetics of 5-fluoro-2′-deoxyuridine (FdUrd) and 5-fluorouracil (FUra) disposition after bolus intravenous injection were determined in anesthetized rhesus and cynomolgus monkeys. FdUrd disappearance from plasma was an apparent triexponential process with average half-lives of 0.5, 2, and 8 min; FUra disappearance was biphasic with average half-lives of 2 and 13 min. After FdUrd injection, FUra reached peak plasma concentrations of 15-30% of the initial FdUrd concentrations within 3 min, and then disappeared more slowly than FdUrd. Total FdUrd clearance fell from 105 to 73 to 56 ml/kg/min as the dose increased from 10 to 20 to 40 mg/kg. Metabolic clearance was about 85% of total clearance and fell similarly with increasing dosage. Total and metabolic FUra clearances were about 30% of FdUrd values at an equimolar dose. Renal FdUrd clearance exceeded glomerular filtration rate and was decreased by probenecid, indicating tubular secretion; renal FUra clearance was close to glomerular filtration rate. There was no apparent correlation between dose and renal clearance or volume of distribution. It was concluded that FdUrd, like FUra, is eliminated primarily by a dose-dependent process. The metabolic basis of the dose-dependent kinetics remains to be determined.

Effect of Diuretics on Intestinal Transport of Electrolytes, Glucose, and Amino Acid

Summary The jejunal mucosal membrane of albino mice was used to study the electrical properties and ion transport. The membrane was bathed in Krebs-Ringer solution with or without glucose. When ethac-rynic acid (EA), furosemide, or amiloride was added to the bathing fluid of both sides, a transient increase followed by a decrease of both potential difference (PD) and short circuit current (Isc) were observed. In glucose-containing bathing medium, EA inhibited both net Na and Cl flux and residual flux; however, EA had little effect on both Na and Cl flux in glucose-free bathing medium. Studies using everted intestinal sac technique showed that EA inhibited both glucose and L-tyrosine across the mucosal membrane against concentration gradients. Furosemide and amiloride were less potent than EA in inhibiting the Na and Cl flux when the bathing solution contained glucose. But these two compounds had no effect on glucose and L-tyrosine transport across the intestinal mucosa. Furthermore, they did inhibit Cl flux even in the condition of glucose-free bathing medium. It is postulated that all three diuretics act on the brush-border membrane of the intestine. EA probably inhibits the Na-glucose co-transporting system; furosemide and amiloride inhibit the simple diffusion process of Na entry or Cl exit by decreasing the conductance of the membrane.

Electrolyte Transport Across the Mouse Small Intestine

Summary The electrical and electrolyte transport properties of nonstripped mouse intestine were investigated using the Ussing chamber technique. In HCO3-NaCl 0Ringer solution a PD of 1.47 ± 0.17 mV and I se, of 15.8 ± 1.56 μA existed across the intestine, orientated such that the serosa was electropositive to the mucosa. Radioisotope flux measurements demonstrated net Na absorption and Cl secretion. The I se was less than the sum of net Na and Cl fluxes (J net Na + J net cl), yielding a negative net residual flux (J net R), indication of a simultaneous anion absorption or cation secretion. Glucose augmented J net Na + J net cl and increased the PD and I sc. In the absence of bicarbonate, the J net Na and J net R were reduced to practically zero, suggesting that either HCO3 - absorption or H+ secretion is responsible for the residual flux observed in HCO3 - Ringer. These findings also indicate that Na absorption is HCO3 - dependent.

Renal Tubular Secretion and Biosynthesis of Organic Acids

Summary With the in vitro kidney slice technique, the in vivo steady state and stopflow technique, we have studied the sites and the interrelationship of tubular secretion and biosynthesis of organic acids and found that both occur in the cortex, in the same proximal segment and probably in the same cells. Addition of transported substance, such as PAH or Diodrast, in the medium does not alter the rate of biosynthesis. Simultaneous administration of aminobenzoic acid does not augment, but exhibits either no effect or a slight decrease of tubular transport maximum of the aminohippurate. However, administration of Diodrast would markedly depress the urinary excretion of aminohippurate either by administration or by biosynthesis. Our results indicate that the second step of the transport process, from cell to lumen, is carrier-mediated, and that this carrier can be saturated at a Tm level.

Renal Excretion of 3-O-Methyl-D-glucose

Summary The renal tubular transport of 3-O-methyl-D-glucose (3MG) was studied in dogs and rats. The 3MG was found to be actively secreted by a phlorizin sensitive mechanism in dogs. The nature of the renal handling of 3MG in rats was found to be different from that found in dogs, since, 3MG was reabsorbed by the renal tubules of rats at a rate of about 10% of the 3MG filtration rate. This was interpreted as a result of a reabsorptive process having a larger magnitude than a simultaneously operating secretory process for the same compound. The secretion of 3MG in dogs was found to occur in the proximal tubules and was not inhibited by 2,4-DNP. It was significantly augmented by an intravenous infusion of D-glucose. The results of these experiments were taken as evidence for the existence of a bidirectional sugar transport system in the renal tubules of dogs. This system would be capable of reabsorbing D-glucose and also capable of secreting 3MG and L-glucose.