Effects of saline loading on jejunal absorption of calcium,sodium, and water,and on parathyroid hormone secretion in the rat

Summary To investigate whether intestinal calcium absorption parallels that of sodium following extracellular fluid volume expansion, the effects of saline loading on intestinal transport of calcium. sodium and water were studied in rats by perfusing jejunal loops in situ.After calcium-free saline infusion net calcium absorption was reversed similar to that of sodium and water and net secretion occurred. Concurrently, blood-to-lumen (b-l) calcium flux, measured using⁴⁵Ca, increased significantly (P<0.001). Following expansion with calcium-containing Ringer a similar reversal of net calcium, sodium and water flux was also observed. Again, the b-l calcium flux increased but to a significantly lesser extent (P<0.05). Plasma ionized calcium remained unchanged after calcium-rich Ringer loading, but decreased significantly (P<0.001) when calcium was omitted from the solution. Plasma immunoreactive parathyroid hormone was unchanged after expansion with the calcium containing solution but increased following calcium-free infusion.It is concluded that after extracellular fluid volume expansion: 1. net jejunal calcium absorption is decreased; 2. the decrease parallels that of sodium and water; 3. b-l calcium transport is enhanced to a greater degree by calcium-free Ringer infusion than by a calcium-rich solution. This difference could be the result of increased parathyroid hormone secretion.     

Effect of aldosterone and dexamethasone pretreatment on sodium transport in rat distal colon in vitro

Aldosterone and dexamethasone stimulate sodium absorption in the rat colon in vivo. In vitro, increased amiloride inhibitable short-circuit current (I_SC) has been demonstrated following aldosterone or dexamethasone treatment. Since I_SC bears no relationship to sodium flux (J_Na) in the untreated rat colon, we measured J_Na in partially stripped voltage clamped segments of rat distal colon. Our results demonstrate directly that continuous infusion of aldosterone or dexamethasone for 4–7 days stimulated amiloride inhibitable J_Na by stimulating J_NaM-S. The amiloride inhibitable portion of J_NaM-S was highly correlated with and approximately equal to the amiloride inhibitable I_SC. Amiloride had no effect in controls. We conclude that J_NaM-S in the rat distal colon is only sensitive to mucosal amiloride after treatment with aldosterone or dexamethasone. the amiloride sensitive I_SC in these treated tissues was a good measure of the amiloride sensitive J_Na. Small differences between aldosterone and dexamethasone treatment were noted in the effect on transepithelial resistance, potential difference, and the I_SC after amiloride.     

The action of inorganic phosphate on thiamin transport by rat everted jejunal sacs

Rat everted jejunal sacs were incubated at 37°C in Krebs-Henseleit buffer, pH 7.4, containing thiazole-2-¹⁴C-thiamin (0.2 M), in the absence or in the presence of inorganic phosphate at a final concentration of 6 mM. Control experiments under the same conditions, but without labeled thiamin were also carried out. In the intestinal sac wall, the content of labeled and endogenous (unlabeled) thiamin in the free and phosphorylated form was measured, while in the serosal fluid, total transported thiamin (labeled and endogenous) was evaluated. Accumulation in the enterocyte of inorganic phosphate, an inhibitor of thiamin pyrophosphate enzymatic hydrolysis, lowered the dephosphorylation of endogenous thiamin phosphates, both in the presence and in the absence of labeled thiamin. In the presence of labeled thiamin, the following results were observed: a) decreased labeled thiamin uptake and accumulation, both in the phosphorylated and free form; b) reduction of labeled thiamin transport to the serosal side; c) increase of endogenous thiamin exit from the intestinal cell. These findings seem to point out an important role of thiamin phosphorylation-dephosphorylation coupling in thiamin intestinal transport in vitro.Abbreviations used TMP and TPP thiamin mono- and pyrophosphate - TPPase thiamin-pyrophosphatase - TPKase thiaminpyrophosphokinase - P_i inorganic phosphate - EDTA ethylenediaminetetraacetic acid - ATPase adenosintriphosphatase - TCA trichloroacetic acid     

Unstirred water layer and age-dependent changes in rabbit jejunal D-glucose transport.

The unidirectional flux of solutes into the intestinal mucosal cell is determined by passage through the microvillus membrane and the overlying unstirred water layer, UWL. An in vitro technique was employed to determine the effect of the UWL on glucose transport into the jejunum of suckling, mature, and old rabbits. When the resistance of UWL was high, the uptake from high concentrations of glucose increased as the animals grew older; this change with aging was not seen from low concentrations of glucose. When UWL resistance was minimized by stirring the bulk phase, similar amounts of glucose were absorbed from high doses, but uptake from low doses was greater in young than in old animals. Studies undertaken to determine the kinetic basis of these age-related changes showed that with increasing age i) the apparent passive permeability coefficient P of glucose fell, ii) the maximum transport rate Jdmax rose, and iii) the apparent affinity constant Km increased. These differences were not observed when the resistance of the UWL was high: P and Km were similar in suckling, mature, and old animals, and the increase in glucose uptake with age was due to the greater Jdmax. Thus the potential benefit of the high affinity, high permeability transport system of young animals may be obscured by high resistance of the UWL.     

Neuropeptide Y antagonises secretagogue evoked chloride transport in rat jejunal epithelium

Neuropeptide Y (NPY) inhibits electrogenic Cl secretion in rat jejunal epithelium under voltage clamp conditions. This effect is dependent upon endogenous eicosanoid formation since it is blocked by the cyclooxygenase inhibitor, piroxicam, which itself has an inhibitory action upon chloride secretion. A number of chloride secretagogues have been examined for their ability to restore the antisecretory effects of NPY. Data presented here shows that NPY responsiveness is restored, in piroxicam pretreated tissues, by vasoactive intestinal polypeptide (VIP), forskolin, prostaglandin E₂ (PGE₂) isobutyl-1-methyl-xanthine (IBMX) and dibutyryl cAMP added prior to the neuropeptide. While all these agents cause chloride secretion by elevating intracellular cAMP, NPY is also effective in inhibiting the secretory effects of carbachol (CCh) and substance P (SP), agents believed to act by raising intracellular calcium (Ca_i). Although there is evidence that NPY can inhibit adenylate cyclase, its ability to attenuate chloride secretion brought about by secretagogues acting through both adenylate cyclase and calcium mechanisms, implies that NPY has either a more general fundamental mechanism or has multiple interactions with different second messenger systems.This work was supported by the Medical Research Council     

Effect of modified sham feeding on jejunal transport and pancreatic and biliary secretion in man

Because previous evidence suggested that intestinal secretion was under parasympathetic control, we investigated the effect of modified sham feeding on the intestinal transport of water and electrolytes by jejunal perfusion using a triple lumen tube. By measuring the concentration of amylase, protein, and bile acids entering the intestinal test segment, we were also able to obtain data on the influence of sham feeding on pancreatic and biliary secretion. Gastric contents were aspirated throughout the experiment. Although we observed a clear-cut cephalic phase of gastric acid secretion, sham feeding had no effect on jejunal transport of water and electrolytes or on the rate at which bile acids, protein, or amylase were secreted into the duodenum.     

Effect of amiloride on sodium transport of frog skin

Summary The effect of Amiloride on intracellular sodium content of the isolated frog skin was investigated. The maximal effect on intracellular exchangeable sodium concentration (–10 meq/kg cell water) was found at concentrations of Amiloride in the incubation solution between 0.3 and 1.0×10^–4 M/l, and at an incubation time of 6–15 min. Since total intracellular sodium concentration is also reduced by approximately 10 meq/kg cell water, it follows that the intracellular non-exchangeable sodium concentration was not affected by Amiloride. Water content, extracellular volume, and intracellular potassium concentration remained constant. The short circuit current reached a new steady state within a few seconds after addition of Amiloride. It is concluded that Amiloride affects a sodium transport pool, which contains about 10% of the exchangeable intracellular sodium.Supported by the Deutsche Forschungsgemeinschaft.     

Effect of prolonged saline-exposure on sodium transport across frog-skin

1. Differences in Na transport between skins from Rana temporaria and R. esculenta maintained for up to several weeks in water or 0·7% saline (0·7 g NaCl in 100 ml. H₂O), with and without daily injections of 4% saline (4 g NaCl in 100 ml. H₂O), were measured, in vitro.

2. In saline-treated skins, the following changes were found:

(a) An increased Na content.

(b) A consistent decrease in short-circuit current (I_sc).

(c) An increased d.c. resistance, R, the consistency of which varied with the anion content of the Ringer solution.

(d) A highly significant fall in Na influx, accounting for the reduced I_sc; a small reduction in Na efflux was not significant, statistically.

(e) The Pitressin-induced increment in I_sc was usually considerably lower compared with that in water-exposed skins; considered relative to the pre-Pitressin values, however, there were no clear differences.

(f) By calculation from the changes in resistance (R) caused by replacement of outer Na₂SO₄ Ringer by K₂SO₄ Ringer solution,

I. E₀, the electromotive force of the active sodium transport system, was moderately, but significantly, reduced,

II. R _shunt, the shunt path resistance, was moderately, but significantly, increased, and

III. R_ser, the series path resistance, was considerably, and highly significantly, increased.

(g) K influx from outer K₂SO₄ Ringer solution was reduced.

3. Differences between skins from water-exposed and saline-treated frogs persisted, in vitro, despite the occurrence of anionic-dependent acute changes after mounting in Ringer solution.

4. There were seasonal changes in I_sc, and in the effects of saline treatment.

5. The findings are discussed in terms of decreased permeability of outer barriers to ion-diffusion, and reduced activity of a Na pump.

     

Effect of Amiloride on sodium transport in frog skin

Summary The effect of Amiloride on several parameters of sodium transport was investigated on the isolated frog skin. Amiloride at a concentration of 10^–4 M/l decreased the sodium concentration in the sodium transport pool from 8.9 meq/kg cell water to 3.7 meq/kg cell water. No effect was observed on the intracellular sodium which is exchangeable from the corium side. Short circuit current and unidirectional sodium influx were diminished to the same extent whilst the unidirectional sodium efflux was not affected. In contrast to the short circuit current, which reaches a new steady state value within seconds, the unidirectional sodium influx reaches its new steady state with a half-time of 3.3 min. From the difference in the time courses of the decreases of short circuit current and unidirectional sodium influx, an amount of sodium could be calculated which agreed well with the directly measured fall in the sodium transport pool.The results provide further evidence for the concept that Amiloride inhibits the passive entrance of sodium into the sodium transport compartment, without influencing the transport capacity of the sodium pump.Supported by the Deutsche Forschungsgemeinschaft.     

Increased sodium-dependent D-glucose transport in the jejunal brush-border membrane of spontaneously hypertensive rat

The current studies explore the effect of hypertension on D-glucose transport into jejunal brush-border membrane vesicles (BBMV). Spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats, as a control group, were used. The purity of the BBMV from both groups of animals was validated by the finding that the specific activity of brush-border enzyme marker, sucrase, was severalfold greater in membrane vesicles compared with corresponding values in mucosal homogenate. D-glucose uptake was Na⁺ dependent in both groups of animals, with a transient increase in the intravesicular concentration of D-glucose. However, the initial rate and the magnitude of the accumulation of Na⁺-dependent D-glucose was significantly higher in SHR compared with WKY rats. In order to investigate the mechanism(s) for the increase in Na⁺-dependent D-glucose transport in SHR, several experiments were performed: (1) an experiment that indicated ²²Na uptake, as an indicator for Na⁺ permeability, was similar between SHR and WKY rats, (2) kinetic studies that indicated that V _max values of SHR were significantly greater that those of WKY rats. In contrast, similar K _m values for glucose were found between SHR and WKY rats, (3) Na⁺-dependent phlorizin binding measurements that were not altered by hypertension and (4) a study of the brush-border membrane lipid composition that showed a significant increase in the free cholesterol/phospholipid ratio in SHR. We conclude that altered membrane cholesterol content and consequently altered lipid fluidity could be, at least in part, responsible for the observed increase in Na⁺-dependent D-glucose transport in SHR. Received: 27 October 1995/Received after revision and accepted: 23 January 1996