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1. 1. The aim of the experiments was to test in vivo whether absorption of lipid varied with the concentration solubilized in the lumen. Such a dependence has been demonstrated for everted sacs in vitro. 2. 2. Two types of perfusates were prepared in which the total concentration of absorbable labelled lipid and of conjugated bile acid was the same. In one type, most of the lipid was in micellar solution; in the other type, the solubilized concentration was lowered by adding an unabsorbable oil, glyceryl triether, into which micellar lipid was partitioned. 3. 3. Absorption was tested in rats prepared in two ways: (A) unanaesthetized with common bile duct and thoracic lymph duct cannulated in which lipid was continuously perfused by duodenal cannula; (B) anaesthetized with the small intestine cannulated proximally below the entrance of the common bile duct and distally in the lower third of the ileum. In Preparation B, the whole perfusate was injected at zero time and an even exposure to the entire mucosa ensured by to and fro displacement. 4. 4. In Preparation A absorption of [ 14 C]oleic acid and glyceryl[ 3 H]monooleyl ether into the lymph was the same for both lipids and both perfusates. Lipid was almost completely absorbed before the perfusate reached the distal half of the small intestine. 5. 5. In Preparation B disappearance of lipid from the lumen and uptake by the mucosa was less for the perfusate in which the concentration solubilized was lower. Oleic acid was absorbed more rapidly than monoether, relative to the concentrations solubilized. These findings agreed with results for everted sacs in vitro. 6. 6. It is suggested that in Preparation A decreased absorption per unit area can be compensated by utilization of more of the total absorptive area. In Preparation B and in everted sacs, no such compensation is possible and variation in absorption rate with solubilized concentration can be readily demonstrated.

The Ca2+ and Mg2+ contents of strips of the circular smooth muscle from the stomach of the toad (Bufo marinus) were measured after varying lengths of exposure to NaCl-Ringer and K2SO4 depolarizing Ringer solutions in which the Ca2+ and Mg2+ concentration was varied. The Ca2+ content of the smooth muscle was correlated with the contractility of the muscle when the tissue was first depleted of Ca2+ to abolish responses and then controlled replacements made. In Ca2+-free depolarizing K2SO4-Ringer the Ca2+ content fell from 0.95 ± 0.02 (mean ± S.E. of mean) to 0.14 ± 0.01 μmole per g wet weight before contractility to acetylcholine was lost. When only 9% of the tissue Ca2+ was replaced by exposure to 0.08 mM Ca2+ in the Ringer solution, full contractility was restored, the muscle then containing 0.23 ± 0.01 μmole per g wet weight. However, contractions produced by alternating current applied in the longitudinal direction of the fibres were practically eliminated when the tissue still contained 0.31 ± 0.01 μmole of Ca2+ per g wet weight. 26% of the maximal contractility was restored when the tissue contained 0.43 ± 0.01 μmole per g wet weight and 42% response when the tissue contained 0.53 ± 0.02 μmole per g wet weight. When the tissue contained supranormal amounts of Ca2+, the contractile response to maximum electrical stimulus equalled that produced by a maximal dose of acetylcholine. The experiments suggest that acetylcholine response depends on a small fraction of Ca2+ located in the cortex of the cell while the effect of alternating current is to displace Ca2+ from myoplasmic sites.

1. 1. Uptake and efflux of labelled micellar cholesterol by everted sacs of rat jejunum was compared for two detergents, (a) pure sodium taurocholate-sodium taurodeoxycholate (4:1, on molar basis) and (b) a high-molecular-weight non ionic detergent, Pluronic F68. Oleic acid and monoolein were also incorporated in the micellar solutions. 2. 2. Uptake of cholesterol from bile salt micelles was 2–4 times faster than from non-ionic micelles, for incubation periods varying from 10 min to 2 h. 3. 3. Uptake from bile salt micelles increased with temperature (apparent activation energy about 6 kcal/mole cholesterol) but uptake from non ionic micelles did not. Metabolic inhibitors had little effect. 4. 4. When sacs were loaded with isotopic cholesterol from bile salt micellar media, subsequent efflux into buffer, or into unlabelled cholesterol in non-ionic micelles or in bile salt micelles was slower than was efflux into a given medium after loading with isotopic cholesterol from non-ionic micelles. On the other hand, efflux into bile salt micellar medium was faster for a given loading medium than efflux into non-ionic micelles. 5. 5. It is suggested that cholesterol uptake may involve two stages only one of which is temperature sensitive and is promoted by bile salts.

1. 1. Everted sacs of rat jejunum were used to measure the effect on uptake of labelled oleic acid, 1 mM, when an unabsorbable oil (glyceryl-trioleyl ether) was present. The incubation medium also contained monoolein 1 mM, and pure bile salts (sodium taurocholate-sodium taurodeoxycholate, 4:1 on a molar basis). 2. 2. Increasing concentrations of triether had no effect on the low uptake of oleic acid in the absence of a micellar phase, bile salts 1 mM, when virtually all the lipid was in emulsified particles. 3. 3. When most of the oleic acid (and monoolein) was present in a micellar phase, bile salts 4 mM or 5 mM, uptake was greatly accelerated. Addition of increasing amounts of triether now led to partition of an increasing proportion of oleic acid into the emulsified oil particles with a corresponding reduction in concentration of oleic acid in the micellar or isotropic phase. Uptake of oleic acid was reduced in linear proportion to reduction in micellar concentration. 4. 4. The regression of oleic acid uptake on isotropic aqueous concentration was the same in these experiments, in which bile salt concentration remained constant, as in previous experiments in which the isotropic concentration was varied by altering the bile salt concentration. 5. 5. These experiments provide further evidence that uptake in vitro depends on the concentration of fatty acid in the isotropic aqueous phase, and not on the concentration of bile salts per se .

Summary The effect of aminopterin on fat absorption into the lymph of unanesthetized rats has been studied at various times after the administration of the drug. A consistent effect on fat absorption has been shown, and this has been related to the age of the mucosal cells. After inhibition of mitosis with aminopterin, fat absorption remains normal for at least 32 hr, and then there follows a period of severe malabsorption which lasts for 2 days. This malabsorption has been shown to be mucosal in origin. Mitosis in the crypts recommences on the fourth day after aminopterin, but fat absorption does not begin to recover until about 24 hr later. The results indicate that a preliminary correlation of mucosal cell age to fat absorption is possible. Newly arisen cells appear to be unable to absorb fat until they are about 24 hr old, and then they retain this ability until they are due to be shed from the tips of the villi.

The rate of uptake and esterification of fatty acid by everted intestinal sacs is reduced when micellar media of a non-ionic detergent are compared with bile salt micellar media 1 ( Hoffman , Biochim. Biophys. Acta , 196 (1970) 193). In this paper it has been shown that: 1. (1) The reduced rate of uptake correlates with a reduced diffusion coefficient of fatty acid in the non-ionic detergent micellar solution. 2. (2) The sac preparation has a maximal esterifying capacity. 3. (3) This is the same for non-ionic detergent and bile salt micellar media.

1. 1. Individuals of the Australian marsupial species, the possum ( Trichosurus vulpecula ), differ in erythrocyte cation concentration. ATPase activity was measured in the fragmented membranes of erythrocytes in which potassium predominates (HK cells) and in which sodium predominates (LK cells). A comparison was made with erythrocyte membrane ATPase activity in the marsupial quokka ( Setonyx brachyurus ) in which sodium and potassium are present in approximately equal concentration. 2. 2. All erythrocyte preparations showed basal ATPase activity. Significant (Na + -K + )-activated ATPase was present only in HK possum and quokka cells, absolute values and values relative to basal ATPase being higher in HK possum than in quokka cells. 3. 3. These results suggest that the low erythrocyte potassium concentrations in LK possums may be due to the absence of (Na + -K + )-activated ATPase and pump activity in their cell membranes.

Absorption of two lipids from a mixture was compared in unanaesthetized bile diverted rats. The isotopically labelled lipids, oleic acid and glyceryl-1-monoether, were infused intraduodenally in bile salts either as a micellar solution or in an emulsion with no micellar phase present. The two isotopes were absorbed equally into the thoracic duct lymph from micellar solutions. The absorption of both was depressed in the absence of micelles with the monoether label being more affected than the fatty acid label. This suggested a rate-limiting step in the absorptive pathway prior to the appearance of lipid in the lymph. To further localize this step animals were sacrificed at 4 and 6 h and isotope recovered from intestinal lumen and wall as well as lymph. This showed that there was no accumulation of isotope in the intestinal wall but that there was accumulation in the intestinal lumen, suggesting the rate-limiting step to be in the luminal phase of absorption.