Amino Acid Absorption in the Locust (Schistocerca Gregaria Forsk.)

Insects are known to possess an exceedingly high concentration of amino acids in the haemolymph, probably higher than in any other animal group (cf. Buck, 1953). On the other hand, recent work has shown that in the locust the mid-gut region is permeable to the relatively large monosaccharide molecules which appear to be absorbed largely by facilitated diffusion (Treherne, 1958a, c). With such an apparently permeable absorptive surface the back diffusion of the rather small rapidly diffusing amino-acid molecules might be expected to present certain difficulties in their absorption. Furthermore, most membranes possessing secretory or transport mechanisms for non-electrolytes appear to be relatively impermeable to passively diffusing molecules (Wilbrandt, 1954) and it is, therefore, of interest to know something of the mechanism of amino-acid absorption in the locust. In the present investigation an attempt has been made to throw some light on these processes by following the uptake from, and the concentration changes occurring within, the lumen of the alimentary canal of Schistocerca gregaria (Forsk.).

All experiments were carried out on adult female Schistocerca gregaria (Forsk.) which had been deprived of food for the preceding 24 hr. They were reared and maintained at 28 ± 1·0° C. and fed on bran and on fresh green wheat grown in pots.

The absorption of amino acids was studied using some techniques previously employed in investigations on sugar absorption in insects (Treherne, 1957, 1958a). The various experimental solutions used were injected into the alimentary canal by means of a fine nylon hypodermic needle which was thrust into the rectum of a CO₂-anaesthetized insect and sealed into position with wax. In each case 0·15 ml. was used, which was sufficient to fill the whole of the mid-gut and hind-gut. To determine the major sites of absorption in the alimentary canal the gut lumen was filled with a solution containing ¹⁴C-labelled amino acid together with the dye Amaranth (Azo-Rubin S) which has been shown not to be absorbed from the gut lumen in this insect. After an appropriate period the gut was excised, quickly washed in saline, ligatured and then cut into appropriate portions which were dropped into a suitable volume of a solution buffered at pH 10·0. The net percentage absorption was calculated from the ratio of radioactive amino acid to dye in the various parts of the alimentary canal. The concentration of Amaranth used in these experiments was 0·04 M/I. Dye concentration was determined in solution at pH 10·0 using a Unicam absorptiometer at an absorption maximum of 510μu. The radioactivity of the relabelled compounds was assayed in solution using a thin-windowed G.M. tube (G.E.C. CV 2139) as previously described (Treherne, 1957) or on a planchette after diluting the radioactive fluid to a standard volume (0·1 ml.) and evaporating to dryness.

In some further experiments the concentrations of glycine and serine, the radioactivity and the fluid volume in the mid-gut caeca were followed over a period of 45 min. To collect samples of the experimental solution from the lumen of the caeca the whole gut was removed from a CO₂-anaesthetized insect and the caeca were isolated by ligatures. The whole gut was then very quickly washed in distilled water and dried with filter-paper. The mid-gut caeca were punctured over a waxed slide and the fluid was collected for analysis in a 5·0 μl. waxed micropipette. The volume changes were followed using ¹³¹I labelled albumen as a volume indicator (Phillips, unpublished). The amount of albumen added to the solution was so small that it did not have a detectable effect on the osmotic pressure. Tests showed that the radioactivity of ¹⁸¹I in the haemolymph was only 1·6% of that in the gut lumen, after a period of 45 min., showing that only a very small amount of the labelled material was lost from the lumen during this period.

The free amino acids in the haemolymph and in the gut fluids were separated and identified by paper chromatography. The freshly collected haemolymph was precipitated with two volumes of absolute ethanol, dried in vacuo over P₂O₆ and then restored to twice its original volume by the addition of 60% ethanol (Raper & Shaw, 1948). Successive 5-10μl. volumes of this fluid (or mid-gut fluid) were then transferred to Whatman no. 1 filter-paper for separation by two-dimensional descending chromatography. The solvent systems used were 70 % n-propanol followed by water-saturated phenol (Chen, 1958) and n-butanol/acetic acid/water (74/19/50) followed by phenol (Fowden & Steward, 1957). The detection spray used contained 0·4% ninhydrin and 0·2% cobalt chloride in isopropanol (Wiggins & Williams, 1952). For quantitative determination of amino acids the papers were treated with the modified ninhydrin spray of Connell, Dixon & Hanes (1955). The amino-acid spots were cut out, sprayed with 0·05 M borate buffer at pH 9·0 in methanol to remove ammonia and then estimated by the procedure of Cocking & Yemm (1954).

Osmotic pressure determinations on haemolymph and experimental solutions were made by the technique of Ramsay (1949) as modified by Ramsay & Brown (1955).

The chromatographic analysis of the haemolymph revealed the presence of ten amino acids: glycine, alanine, valine, leucine (and/or isoleucine), proline, tyrosine, serine, threonine, glutamic acid and histidine. In addition appreciable amounts of glutamine were also detected. Fig. 1 is a diagram of the distribution of these compounds on a chromatogram developed with 70% n-propanol followed by phenol.

The concentration of these amino acids and glutamine are tabulated in Table 1. These results showed that glycine and serine were present in relatively high concentration, 33·2 and 34·6 mM/1. respectively. The remaining eight amino acids were found to be at very much lower concentrations, varying between 1·05 mM/1. for histidine and 5·1 mM/1. for glutamic acid. The concentration of glutamine averaged 10·9 mM./l. Despite the fact that the insects used in these experiments were taken from the same batch there was a great deal of individual variation in these results. This is particularly evident with the results for leucine (and isoleucine) in which values varied between 0·5 and 127 mM./l. The results for alanine and valine were also characterized by a high degree of variability. The concentrations of glycine and serine, and also of glutamine, exhibited much less individual variation, to an extent which cannot be attributed to a greater degree of accuracy in their determination at higher concentration.

The freezing-point depression of the haemolymph of adult females was also measured and is summarized in Table 2.

For injection into the gut lumen an experimental solution was devised in which the concentrations of amino acids and other substances were as close as possible to those of the haemolymph. The substances incorporated in this solution are tabulated in column I of Table 3. The amino acids were present in concentrations as measured in the present investigation, while the sugar concentrations have been taken from the data of Treherne (1958c). The salt concentrations were devised by maintaining the concentration of cations given by Duchâteau, Florkin & Leclercq (1953), the anions being in the same proportion as in Hoyle’s balanced salt solution (Hoyle, 1953). This gave a total osmotic pressure of Δ = 0·790° C., which was slightly in excess of that of the haemolymph. The dye Amaranth was incorporated in the solution, without unduly raising the osmotic pressure, by a reduction of the total salt concentration as shown in column II of Table 3. This solution had an osmotic pressure of A = 0·760° C. which approximated to that of the haemolymph.

The concentrations of glycine and serine in the haemolymph showed least variation and because of their high and relatively constant concentrations these amino acids were selected for the study of absorption from the gut lumen. The first experiments were concerned with the absorption of ¹⁴C-labelled glycine and serine in order to determine the sites of absorption in the alimentary canal. The concentrations of glycine and serine in the injected solution were similar to their concentrations in the haemolymph (i.e. 33·2 and 34·6 mM./l. respectively), the remaining substances being as in column II of Table 3. The results of these experiments are summarized in Fig. 2 from which it will be seen that both glycine and serine were absorbed most rapidly from the mid-gut caeca. After 15 min. 18·1 % of the labelled glycine and 14·8 % of the labelled serine had been absorbed from the caeca, while in the remaining parts of the gut the absorption occurred much less rapidly. After 1·0 hr. 57·7% of the labelled glycine and 63-1% of the labelled serine had been absorbed from the caeca, with a less rapid absorption from the ventriculus and a slight absorption from the hind-gut.

In Fig. 3 the absorption of labelled glycine is compared with that of labelled serine by plotting the percentage of the radioactive amino acids remaining in the caeca on a logarithmic scale against time. It will be seen that for both substances the rate of absorption from the caeca is approximately the same and falls off exponentially with time.

The changes in concentrations of glycine and serine in the gut following injection of the experimental solution (column I, Table 3) into the gut lumen were studied by separating the contents of the caeca on two-dimensional chromatograms and determining the concentrations of these two amino acids. The concentration of glutamine in the lumen of the caeca was also followed in this manner. It seemed possible that fluid already present in the gut lumen might significantly affect the concentration of the substances in the experimental solution. To test this possibility the fluid in the caeca was removed immediately after the experimental solution had been injected via the rectum and the concentrations of glycine, serine and glutamine determined. The results are summarized in Table 4, from which it will be seen that their concentrations in the fluid recovered from the caeca did not differ significantly from their concentrations in the experimental solution.

Experiments were also carried out in which the concentrations and radioactivity of ¹⁴C-labelled glycine and serine were followed after injection into the gut lumen. These results are illustrated in Figs. 4 and 5. With glycine (Fig. 4) the concentration in the lumen tended to rise above that of the haemolymph. After 15 min. the concentration in the lumen averaged 40·5 mM./l. as against 33·2 mM./l., in the haemolymph, a difference which was not significant (P < 0·1). After 30 min. the concentration averaged 45·5 mM./l. and after 45 min. was 43·4 mM./l., both being significantly different from that of the haemolymph (P<0·01 and <0·02 respectively). During this time the concentration of the labelled glycine in the caeca lumen fell relatively rapidly reaching a mean value of 16·5 mM./l. after 45 min.

The concentration of serine (Fig. 5) in the caecal fluid also showed a slow increase after injection of the experimental solution into the gut lumen. The concentrations after 15-0 and 30-0 min. were not statistically different from that of the haemolymph (P< 0·3 and < 0·10 respectively). After 45 min., however, the concentration in the lumen averaged 45·3 mM./l. which was significantly different from that of the haemolymph (P< 0·02). During this time there was a progressive rapid fall in the concentration of the ¹⁴C-labelled serine in the lumen.

With glutamine the mean concentration in the lumen of the caeca also showed a steady rise, to a value of 15·3 mM./l. which was significantly higher than the value of 10·9 mM./l. for the haemolymph (P <0·001).

The change in volume of the caecal contents is illustrated in Fig. 7. The volume showed a progressive fall to 57% of the original volume after 45 min.

Using the data illustrated in Fig. 7 it is possible to calculate the rate of disappearance of glycine and serine from the lumen of the caeca. In Fig. 8 the percentage of each amino acid remaining in the lumen has been plotted on a logarithmic scale against time. These results illustrate the very slow net absorption of the amino acids as compared with the fast absorption of the ¹⁴C-labelled molecules.

The amino acids in the haemolymph of Schistocerca gregaria are in general similar to those found in many other insects. All of the amino acids in the present investigation were found to occur in the haemolymph of a wide variety of insects by Du-château & Florkin (1958), with the exception of serine which these authors do not mention. There is also qualitative agreement with the amino-acid picture in the haemolymph of Aeschna (Raper & Shaw, 1948), Bombyx mori (Wyatt, Lougheed & Wyatt, 1956) and Culex pipiens (Chen, 1958).

Quantitatively the haemolymph of Schistocerca is characterized by high concentrations of glycine and serine, and also of glutamine, and is in this respect similar to that of the larva of Bombyx mori (Wyatt et al. 1956). The high concentration of these substances in the haemolymph is not apparently characteristic for all insects however—in Hydrophilus, Gasterophilus and Apis, for example, they occur in relatively low concentrations (Duchâteau & Florkin, 1958). The strikingly large degree of individual variation obtained in the concentrations of certain amino acids in Schistocerca, especially leucine, valine and alanine, has also been recorded in other insects. Wyatt et al. (1956), for example, quote values for the concentrations of proline, tyrosine and histidine in which the variation exceeded an order of magnitude.

The absorption of ¹⁴C-labelled glycine and serine took place largely in the mid-gut region and occurred most rapidly from the caeca. The disappearance of the amino acids from the ventriculus may not have been due entirely to an absorption by this organ, however, for it is possible that some may have diffused into the lumen of the caeca as the concentration there fell. The uptake of other nutrient substances has also been found to occur rapidly from the mid-gut caeca. In Periplaneta americana the uptake of glucose (Treherne, 1957) and triglyceride (Treherne, 1958^) occurred most rapidly in this region; in Schistocerca monosaccharides were also rapidly absorbed from the caeca (Treherne, 1958b, c).

The composition of the experimental solution was adjusted so that the concentrations of the amino acids and other substances, and also its osmotic pressure, were ‘similar to those in the haemolymph. The results showed that there was a definite rise in the concentrations of glycine and serine in the caeca, above their concentrations in the haemolymph, following injection of the experimental solution into the caeca. With glutamine also there was a slow increase in concentration. This effect resulted from the demonstrated removal of water, a process which occurred more rapidly than the net transfer of the amino acids across the gut wall (Fig. 8). Absorption of these amino acids could, therefore, be brought about as a result of a diffusion gradient thus established, the transfer of glycine and serine being dependent, in part at least, upon the net movement of water molecules into the haemolymph. These water movements could be produced either by an active transport of water or by a passive movement resulting from net ion movements into the haemolymph (cf. Robinson, 1954). In the absence of any demonstrated accumulation of amino acids against concentration gradients it seems likely that active uptake of glycine and serine would form only a relatively small part of the absorptive mechanism in the caeca of this insect. The absorption of these amino acids by the locust would thus seem to be fundamentally different from the processes of absorption in mammals. Wiseman (1955, 1956), for example, has demonstrated for several amino acids the development of steep concentration gradients from the serosal to the mucosal surface of the small intestine in hamsters.

The net absorption of glycine and serine in the locust appears to be a relatively slow process, at least under the experimental conditions employed in this investigation. This may be due perhaps to the relatively large volume of experimental solution introduced into the mid-gut region. It is of some interest in this respect to compare the absorption of amino acids with that of glucose. From the data available (Treherne, 1958c) the net absorption of glucose from the caeca at a concentration of 30·0 mM./l. would be expected to be about 54% after an experimental period of 15 min.; the net absorption of serine in a similar period represented only 8·4% of that originally present. The absorption of these amino acids and of glutamine would thus seem to be much less efficient than the facilitated diffusion mechanism postulated for the uptake of monosaccharides in the locust (Treherne, 1958a, b).

The rapid disappearance of ¹⁴C-labelled glycine and serine from the gut lumen was a particularly striking feature in these experiments, indicating the very rapid exchange occurring between the caeca and the haemolymph even though the net absorption was rather slow. These movements were exaggerated in this case as the loss of the ¹⁴C-labelled amino acids took place from a small compartment (the caeca) into a relatively large one (the haemolymph and tissues).

Glycine and serine are present at a higher order of concentration than the other amino acids in the haemolymph of the locust. They do not, however, occur in high concentration among the protein amino acids of plants. In the Gramineae, for example, the glycine content is only 2-3% that of arginine and about 4% that of leucine (Lugg, 1941). It seems possible that some of the amino acids released in the mid-gut as the result of the digestion of plant proteins might initially be at a higher concentration than those in the haemolymph, so that some absorption of other amino acids could be achieved as the result of diffusion down a concentration gradient. This process would also tend to be accelerated by the absorption of water from the gut lumen so that it might be expected that other amino acids would be absorbed more rapidly from the food than glycine and serine. The possibility that other amino acids might also be rapidly absorbed by some specific mechanisms has of course not been eliminated.