A preparation of isolated small intestine of the rat or golden hamster (Mesocricetus auratus) is described which permits convenient measurement ofrespiration and glycolysis during periods of active transference of substances across the wall. The difficulty of adequate oxygenation is overcome by everting a piece of intestine, tying it at both ends and filling it with sufficient fluid to distend the wall. The eversion exposes the highly active mucosa to the well-oxygenated suspending medium, while the distension increases the surface area of the sac and reduces the thickness of the sac wall. The oxygenation of the inner layer of the serosal surface is facilitated by an oxygen bubble which is introduced into the sac along with the inner fluid.The relatively small volume of fluid contained in the sac (serosal side) allows a rapid rise in concentration of transferred substances. A number of adjacent segments of intestine from the same animal may be studied simultaneously.The method was tested with glucose and methionine, both substances being known to be transferred against a concentration gradient (Fisher & Parsons, 1949b;Wiseman, 1953). Aerobically both were transferred against a concentration gradient in this preparation, but no active transference took place anaerobically. EXPERIMENTAL Preparation of tissue. The animal (rat or hamster) was killed by a blow on the head, the abdomen opened by a midline incision, and the entire small intestine washed out with a solution of 0.9% (w/v) NaCl containing 03 % (w/v) glucose. The whole of the small intestine was then removed by cutting across the upper end of the duodenum and the lower end of the ileum and manually stripping the mesentery from the intestine. In the case of the hamster all the fat and mesentery were removed by this procedure, but in the rat a small amount of fat remained on the * Exchange Fellow of the American Cancer Society.

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The ability of the small intestine to transfer amino-acids against a concentration gradient has been demonstrated by a number of investigators in the past few years (Wiseman, 1953;Agar, Hird & Sidhu, 1953;Smyth & Whaler, 1953;Wilson & Wiseman, 1954). The results of those investigations have shown that an active mechanism exists for the absorption of certain L-amino-acids but not for their D-forms. The L-amino-acids which those workers showed to be actively transferred by the small intestine of the rat are alanine, phenylalanine, methionine, histidine and isoleucine, but the L-forms of glutamic acid and aspartic acid are not (Wiseman, 1953). In the present investigation the method described by Wilson & Wiseman (1954) has been used to study the transference of glycine and the L-forms of proline, histidine, methionine, lysine and ornithine by sacs of everted small intestine of the hamster when these amino-acids were present singly and in pairs. The results show that when only one amino-acid was present L-proline, L-histidine, L-methionine, and glycine were readily transferred against a concentration gradient but L-lysine and L-ornithine were not. The results of the experiments in which pairs of amino-acids were used show that amino-acids for which a mechanism for active transfer exists compete with each other for that mechanism. On the basis of their effectiveness as inhibitors the order of affinity for the 'carrier' part of such a mechanism is methionine, histidine, proline and glycine (methionine appearing to have an affinity of roughly 20 times that of proline). Those amino-acids which when present alone were transferred at the lower rates (methionine and histidine) inhibited the transfer of the amino-acids transferred at the higher rates

During a period of semistarvation there is a fall in the mass of body cells and a lowering of the basal metabolic rate (Wishart, 1934;Taylor & Keys, 1950;Grande, Anderson & Keys, 1958), the degree to which each change occurs depending upon the duration of the experiment and the severity of the dietary restriction. The loss of cellular mass and the fall in the metabolic rate depend upon the number of calories available, and provided that the caloric restriction is not too severe an equilibrium situation can be obtained. It was of interest therefore to investigate the absorptive ability of the small intestine during semistarvation, as it seemed a vulnerable organ in view of its great demands on the metabolic pool for the frequent replacement ofits epithelial lining (Leblond & Stevens, 1948;McMinn, 1954).We have found that after a period of semistarvation sufficient to cause a loss of about 20 % of the initial body weight, there is an enhanced rate of disappearance of both glucose and L-histidine from the small intestine of the rat in vivo, and that in vitro the small intestine from semistarved animals can transport both these substances against a concentration gradient to a greater extent than can the small intestine of rats fed on an 'ad libitum' diet. A preliminary report has been given by Neame & Wiseman (1959). METHODS Animal8 and diet. Male albino rats of an inbred strain were used, and were kept individually in separating cageswith free access to water. The food used throughout was Diet 86, purchased from The North-Eastern Agricultural Cooperative Society, Ltd., Bannermill Place, Aberdeen, its composition being: soluble carbohydrate 53.4%; protein, 20-0 %; fat, 3.8%; fibre, 3-3%; ash, 5-2%; moisture, 14-3%.All animals were inspected every day and the dietary regimen was as follows: Group A was fed ad libitum. Group B was fed 5 g food per rat per day for 5 days. Group C was fed 5 g food per rat per day for 9 days. Group D was fed 5 g food per rat per day for 9 days, and then fed ad libitum for 24 hr. Group E was fed 5 g food per rat per day for 9 days, and then fed ad libitum for 3 days. Group F was fed 5 g food per rat per day for 9 days, and then fed ad libitum for 7 days.

Amino-acids may be absorbed from the small intestine in two ways, either by diffusion alone, or by a method involving an active process in one or more steps. Previous work has supported both possibilities. Hober & Hober (1937) found that the rate of entry of glycine, alanine and valine was not proportional to the concentration in the gut. They also found that these amino-acids were absorbed faster than polyhydric alcohols of comparable molecular volume and concluded that an active process was involved. On the other hand, Kratzer (1944) reported that the rate of disappearance of amino-acids from the gut was inversely proportional to their molecular volume, while Chase & Lewis (1934), comparing the rates of disappearance of several Land DL-amino-acids, found no difference between the Lisomers and the racemic mixtures. Simple diffusion would account for these results. This problem has been reinvestigated using the stereochemically specific methods which are now available for the analysis of amino-acids. It has been found with thirteen amino-acids that the L-isomer disappears more rapidly than the D-isomer from a racemic mixture introduced into the small intestine and this is regarded as evidence for an active process in the absorption of amino-acids. EXPERIMENTAL Operative technique Adult rats were anaesthetized by intraperitoneal injection of nembutal (6 mg./100 g. rat). The ileocaecal junction was