ACTIVE TRANSPORT BY THE CECROPIA MIDGUT. I. INHIBITORS, STIMULANTS, AND POTASSIUM-TRANSPORT

Physiological screening for target site insensitivity and localization of Na(+)/K(+)-ATPase in cardenolide-adapted Lepidoptera

Cardenolides are toxic plant compounds which specifically inhibit Na(+)/K(+)-ATPase, an animal enzyme which is essential for many physiological processes, such as the generation of action potentials. Several adapted insects feeding on cardenolide-containing plants sequester these toxins for their own defence. Some of these insects were shown to possess Na(+)/K(+)-ATPases with a reduced sensitivity towards cardenolides (target site insensitivity). In the present study we screened five species of arctiid moths feeding on cardenolide-containing plants for target site insensitivity towards cardenolides using an in vitro enzyme assay. The derived dose response curves of the respective Na(+)/K(+)-ATPases were compared to the insensitive Na(+)/K(+)-ATPase of the monarch butterfly (Danaus plexippus). Na(+)/K(+)-ATPases of all arctiid species tested were highly sensitive to ouabain, a water-soluble cardenolide which is most widely used in laboratory studies. Nevertheless, we detected substantial amounts of cardenolides in the haemolymph of two of the arctiid species. In caterpillars of the sequestering arctiid Empyreuma pugione and of D. plexippus we localized Na(+)/K(+)-ATPase by immunohistochemistry and western blot (in D. plexippus). Both techniques revealed strong expression of the enzyme in the nervous tissue and indicated weak expression or even absence in other tissues tested. We conclude that instead of target site insensitivity the investigated arctiid species use a different strategy to tolerate cardenolides. Most plausibly, the perineurium surrounding the nervous tissue functions as a barrier which prevents cardenolides from reaching Na(+)/K(+)-ATPase in the ventral nerve cord.

Electrical potentials and ion concentrations across the gut of Cryptochiton stelleri

1. The electrical potential and concentration differences of Na, K, and Cl in the mucosal and serosal solutions of everted sacs of gut from Cryptochiton stelleri were determined.2. The parameters were used to determine if active ion transport was present in segments from the anterior and posterior intestine.3. The electrical potentials were of the order of 0.5 mV, serosa negative, in the anterior intestine, and 1 mV, serosa positive, in the posterior intestine. The potentials, although small, were significantly different from zero.4. The potentials of all segments responded to anoxia and incubation at different temperatures in a manner indicating that they were of active origin.5. Ions were accumulated against electrical potentials in some segments but did not respond to anoxia or incubation at different temperatures in a consistent fashion.6. It was concluded that active ion transport was present but no evidence for active transport of Na, K or Cl across the gut could be found.7. K leaked from the gut during incubation at non-physiological temperatures or during hypoxia and was removed upon restoration of physiological conditions.

K+ transport in the caterpillar intestine epithelium: role of osmolytes for the K+-secretory capacity of the tobacco hornworm midgut

The midgut of the tobacco hornworm, Manduca sexta, actively secretes potassium ions. This can be measured as short-circuit current (I(sc)) with the midgut mounted in an Ussing chamber and superfused with a high-K(+) saline containing as its major osmolyte 166 mM sucrose. Iso-osmotic substitution of sucrose by non-metabolisable compounds (mannitol, urea, NaCl and the polyethylene glycols 200, 400 and 600) led to a dramatic, though reversible, drop in the current. Acarbose, a specific inhibitor of invertase (sucrase) in vertebrates and insects, had no detectable influence on I(sc). Unexpectedly, after replacing sucrose iso-osmotically with the saccharides glucose, fructose, trehalose or raffinose, the K(+) current could no longer be supported. However, all osmolytes smaller than sucrose (except for NaCl), metabolisable or not, initiated an immediate, quite uniform but transient, increase in I(sc) by about 20%, before its eventual decline far below the control value. Hypo-osmotic treatment by omission of sucrose also transiently increased the K(+) current. Small osmolytes substituted for sucrose caused no transient I(sc) stimulation when the epithelium had been challenged before with hypo-osmolarity; however, the eventual decline in I(sc) could not be prevented. Our data seem inconsistent with a role of sucrose as energiser or simple osmolyte. Rather, we discuss here its possible role as analogous to that of sucrose in lower eukaryotes or plants, as an extra- and/or intracellular "compatible osmolyte" that stabilises structure and/or function of the proteins implicated in K(+) transport.

Carbonic anhydrase in the midgut of larvalAedes aegypti: cloning, localization and inhibition

The larval mosquito midgut exhibits one of the highest pH values known in a biological system. While the pH inside the posterior midgut and gastric caeca ranges between 7.0 and 8.0, the pH inside the anterior midgut is close to 11.0. Alkalization is likely to involve bicarbonate/carbonate ions. These ions are produced in vivo by the enzymatic action of carbonic anhydrase. The purpose of this study was to investigate the role of this enzyme in the alkalization mechanism, to establish its presence and localization in the midgut of larval Aedes aegypti and to clone and characterize its cDNA. Here, we report the physiological demonstration of the involvement of carbonic anhydrase in midgut alkalization. Histochemistry and in situ hybridization showed that the enzyme appears to be localized throughout the midgut, although preferentially in the gastric caeca and posterior regions with specific cellular heterogeneity. Furthermore, we report the cloning and localization of the first carbonic anhydrase from mosquito larval midgut. A cDNA clone from Aedes aegypti larval midgut revealed sequence homology to α-carbonic anhydrases from vertebrates. Bioinformatics indicates the presence of at least six carbonic anhydrases or closely related genes in the genome of another dipteran, the fruit fly Drosophila melanogaster. Molecular analyses suggest that the larval mosquito may also possess multiple forms.

Cation-stimulated ATPase activity in purified plasma membranes from tobacco hornworm midgut

Purified goblet cell apical membranes from Manduca sexta larval midgut exhibit a specific ATPase activity approx. 20-fold higher than that in the 100 000 X g pellet of a midgut homogenate. The already substantial ATPase activity in this plasma membrane segment is doubled in the presence of 20-50 mM KCl. At ATP concentrations ranging from 0.1 to 3.0 mM, the presence of 20 mM KCl leads to a 10-fold increase in the enzyme's affinity for ATP. ATPase activity is greatest at a pH of approx. 8. In addition to ATP, GTP serves as a substrate, but CTP, ADP, AMP and p-nitrophenyl phosphate do not. Either Mg2+ or Mn2+ is required for activity and cannot be replaced by Ca2+ or Zn2+. The ATPase activity of goblet cell apical membranes is inhibited by neither the typical (Na+ + K+)-ATPase inhibitors, ouabain and orthovanadate, nor by the typical mitochondrial F1F0-ATPase inhibitors, azide and oligomycin. Although 1.5 microM DCCD is ineffective, 150 microM DCCD leads to total inhibition of ATPase activity. The ATPase activity of goblet cell apical membranes is stimulated not only by K+, but also, in order of decreasing effectiveness, by Rb+, Li+, Na+ and even Mg2+. Replacement of Cl- by Br-, F- and HCO3- has less influence than variation of the cations. However, replacement of Cl- by NO3- inhibits strongly this ATPase activity. The ATPase activity described above is characteristic of the alkali metal ion pump containing apical membranes of goblet cells and is not enhanced to a similar degree in other purified midgut epithelial cell plasma membrane segments. Its localization, its broad cation specificity and its insensitivity to ouabain all mimic properties of active ion transport by the lepidopteran midgut and suggest this ATPase as a possible key component of the lepidopteran electrogenic alkali metal ion pump.

Intestinal electrophysiology and transmural ion transport in freshwater prawns

A steady-state transmural potential difference of 1.63 +/- 0.26 mV (mean +/- SE, lumen negative) is recorded across the isolated, perfused intestine of the freshwater prawn, Macrobrachium rosenbergii, when saline approximating hemolymph in composition is present on both surfaces of the preparation. The magnitude of this potential is a hyperbolic function of bilateral K concentration. It is abolished by metabolic poinsons (NaN3 and iodoacetate) or ouabain and appears to result largely from the combined net transmural fluxes of Na, Cl, and K. Net fluxes of Na (1,49 +/- 0;30 mu mol . cm-2 . h-1) and Cl (0.72 +/- 0.22) are absorptive, whereas that of K (0.47 +/- 0.11) IS SECRETORY. Bilateral absence of Na abolishes net K secretion, whereas bilateral addition of ouabain (0.5 mM) eliminates net Na absorption. One-third of net K secretion appears to be transcellular and coupled to the net cellular transfer of Na in the opposite direction. The remaining component of K secretion can be attributed to paracellular cation flow responsive to transmural PD, Transmural diffusion potentials generated in the presence and absence of metabolic poisons provide the following passive permeability properties of the tissue: PK:PNa:PCl = 5.1:1.1:0.5. The symmetrical nature of these diffusion potentials implies the occurrence of a single rate-limiting barrier to ion flow with the above characteristics--probably the cell junction (septate desmosome) and paracellular channel. A model for transepithelial ion transport is presented where transmural potential difference is largely a result of an apparent 9 Na/1 K basolateral cationic exchange.

Extracellular space values and intracellular ionic concentrations in the isolated midgut of Philosamia cynthia and Bombyx mori

Total extracellular space values have been determined in the midguts of 2 lepidopteran larvae, Philosamia cynthia and Bombyx mori. The values found are 42% and 45% tissue water respectively. Intracellular concentrations of Na+, Ca++ and Mg++ are very low, while K+ concentration is 197,2 mEq/l cell water Philosamia and 180.9 mEq/l cell water in Bombyx.

From frog skin to sheep rumen: a survey of transport of salts and water across multicellular structures

All higher animals, whether they live in water or on dry land, are faced with the necessity of regulating rather closely their intake and excretion of salts and water in order to maintain the constancy of their internal ionic environment. The kidney is in general the most important organ of the body as far as the excretion of sodium, potassium, chloride and water is concerned, but there are other tissues which also play a part in controlling the ionic balance between the internal and external environments, such as the intestinal mucosa, the skin and urinary bladder in amphibia, the gill epithelium in fishes, the salt gland in marine birds, and the epithelium of the rumen in ruminants. In addition to excretory and absorptive organs of this type, there are others which are secretory and whose function involves the production of fluids differing in ionic composition from the blood plasma. Examples include the glands which secrete saliva and sweat, the oxyntic acid-producing cells of the gastric mucosa, and the epithelium of the stria vascularis which generates the potassium-rich endolymph of the mammalian cochlea. The purpose of this article is to consider briefly what is known about the active transport of salts and water across some typical multicellular secretory tissues, and to attempt in the process to discern what properties they have in common and in what respects they are specialized.

Active transport by the cecropia midgut. II. Fine structure of the midgut epithelium

A morphological basis for transcellular potassium transport in the midgut of the mature fifth instar larvae of Hyalophora cecropia has been established through studies with the light and electron microscopes. The single-layered epithelium consists of two distinct cell types, the columnar cell and the goblet cell. No regenerative cells are present. Both columnar and goblet cells rest on a well developed basement lamina. The basal portion of the columnar cell is incompletely divided into compartments by deep infoldings of the plasma membrane, whereas the apical end consists of numerous cytoplasmic projections, each of which is covered with a fine fuzzy or filamentous material. The cytoplasm of this cell contains large amounts of rough endoplasmic reticulum, microtubules, and mitochondria. In the basal region of the cell the mitochondria are oriented parallel to the long axes of the folded plasma-lemma, but in the intermediate and apical portions they are randomly scattered within the cytoplasmic matrix. Compared to the columnar cell, the goblet cell has relatively little endoplasmic reticulum. On the other hand, the plications of the plasma membrane of the goblet cell greatly exceed those of the columnar cell. One can distinguish at least four characteristic types of folding: (a) basal podocytelike extensions, (b) lateral evaginations, (c) apical microvilli, and (d) specialized cytoplasmic projections which line the goblet chamber. Apically, the projections are large and branch to form villus-like units, whereas in the major portion of the cavity each projection appears to contain an elongate mitochondrion. Junctional complexes of similar kind and position appear between neighboring columnar cells and between adjacent columnar and goblet cells as follows: a zonula adherens is found near the luminal surface and is followed by one or more zonulae occludentes. The morphological data obtained in this study and the physiological information on ion transport through the midgut epithelium have encouraged us to suggest that the goblet cell may be the principal unit of active potassium transport from the hemolymph to the lumen of the midgut. We have postulated that ion accumulation by mitochondria in close association with plicated plasma membranes may play a role in the active movement of potassium across the midgut.