Recycling of D-glucose in collagenous cuticle: A means of nutrient conservation?

Effect of heavy metals on the uptake of [3H]-L-histidine by the polychaete Nereis succinea

Integumentary uptake of [3H]-L-histidine by Nereis succinea was measured in the presence and absence of selected heavy metals and the amino acid L-leucine in 60% artificial seawater (ASW). The time course of 10 microM [3H]-L-histidine uptake into worms over a 60 min incubation was approximately doubled in the presence of 0.5 microM zinc and when calcium in the incubation medium was reduced from 6 mM to 5 microM the stimulatory effect of zinc on amino acid accumulation was reduced and uptake under the latter conditions was approximately half that of the control. Zinc stimulation of [3H]-L-histidine influx was a hyperbolic function of zinc concentration over the range 0 to 50 microM metal and displayed an apparent activation or affinity constant of 385+/-127 nM Zn(2+). The hyperbolic stimulatory effect of 1 microM Zn(2+) on the time course of 10 microM [3H]-L-histidine uptake was abolished in the presence of 25 microM L-leucine, suggesting that this amino acid shared the same transport system as [3H]-L-histidine and acted as a potential competitive inhibitor. Influx of [3H]-L-histidine was a hyperbolic function of external amino acid concentration and displayed an apparent affinity constant (Km) of 23.71+/-5.02 microM and an apparent aximal velocity (J(max)) of 4701+/-449 pmol/g dry wt.x15 min. Addition of 0.5 microM zinc resulted in a four-fold increase in J(max) and a doubling of K(m), suggesting the effect of the metal was mostly on the rate of amino acid transport. [3H]-L-histidine influx was mildly stimulated by Fe(2+) (0.5 microM), but was unaffected by either Ag(+) or Al(3+) (both at 0.5 microM). These results suggest that [3H]-L-histidine uptake into worm integument may take place by the classical Na(+)-independent L-transport system shared by L-leucine and regulated by exogenous calcium and other divalent metal concentrations.

Epithelial transport and osmoregulation in annelids

Epithelial transport related to osmoregulation has so far not been extensively investigated in annelids. Compared with the large body of information about ion transport across crustacean or insect epithelia, only a few studies have been done with isolated preparations of annelids, using the body wall of marine polychaetes or Hirudinea. Nephridial function and general body homeostasis have received more attention, and have probably been best investigated in Hirudinea. With recent advances in the molecular physiology of epithelial transport systems in vertebrates, the cloning of various transporters and ion channels, and the considerable number of osmoregulatory peptides that have now been found and analyzed from annelids, it should now be possible, and is timely, to conduct functional studies on individual selected epithelial preparations or isolated cells from annelids. Such studies may be important for establishing useful models with somewhat less complexity than mammalian systems. For example, annelids lack aldosterone, an important osmoregulatory hormone, which is a key factor in the regulation of sodium reabsorption in vertebrates. Therefore, not only would such studies contribute to annelid physiology, but they would be important in a broader sense for understanding osmoregulation and its evolution. They should also facilitate the discovery and investigation of new specific regulatory pathways.

Integumentary L-histidine transport in a euryhaline polychaete worm: regulatory roles of calcium and cadmium in the transport event

Integumentary uptake of L-[(3)H]histidine by polychaete worms (Nereis succinea) from estuarine waters of Oahu, Hawaii was measured in the presence and absence of calcium and cadmium using a physiological saline that approximated the ion composition of 60 % sea water. In this medium 1 micromol L(−1) cadmium significantly increased (P<0.01) the uptake of 10 micromol L(−1)L-[(3)H]histidine, while 1 micromol L(−1) cadmium plus 25 micromol L(−1)L-leucine significantly decreased (P<0.01) amino acid uptake. L-[(3)H]histidine influx was a sigmoidal function (n=2. 21+/−0.16, mean +/− s.e.m.) of [L-histidine] (1?50 micromol L(−1)) in the absence of cadmium, but became a hyperbolic function with the addition of 1 micromol L(−1) cadmium. A decrease of calcium concentration from 6 to 0 mmol L(−1) (lithium substitution) significantly increased (P<0.01) amino acid influx in the presence and absence of cadmium. Calcium significantly reduced (P<0.01), and cadmium significantly increased (P<0.01), L-[(3)H]histidine influx J(max), without either divalent cation affecting amino acid influx K(t). Variation in external sodium concentration (0?250 mmol L(−1)) had no effect on 10 micromol L(−1)L-[(3)H]histidine influx, but amino acid entry was a sigmoidal function of both [cadmium] (n=2.34+/−0.44) and [lithium] (n=1.91+/−0.39) in the absence of calcium. A model is proposed for transapical L-[(3)H]histidine influx by a transporter that resembles the classical sodium-independent L-system carrier protein that is regulated by the external divalent cations calcium and cadmium.

The origins, fate, and ecological significance of free amino compounds released by freshwater pulmonate snails

The mass-specific accumulation rates (MSAR) of both total (TFAC) and individual free amino compounds (FAC) in conditioned media were measured by HPLC, using the orthophthaldialdehyde (OPA) methods, in the following cases: (a) laboratory-reared freshwater snails (B. glabrata) with chemosterilized shells; (b) Biomphalaria glabrata with non-chemosterilized shells; (c) B. glabrata faeces; (d) isolated shells of B. glabrata; and (e) 10 other species of freshwater gastropods from the Lewes Brooks, East Sussex, U.K. The MSAR values for B. glabrata show that 95% of the TFAC's (predominantly ethanolamine, phosphoserine, and the amino acids leucine, isoleucine, valine, aspartic acid, and glycine/threonine) originated from the snails themselves as the faeces and shells contributed only 5.0 and 0%, respectively. In contrast, epizootic organisms on the shells of all 10 snail species from the Lewes Brooks released significant amounts of FAC with the two smallest species (Planorbis vortex and Planorbis contortus) having the highest MSAR values. The MSAR for isolated B. glabrata mucus was 42.45 micromol x g(-1)h(-1). As 500 mg snails can release 16.67 mg of mucus daily, this could potentially result in the daily loss of 707.5 micromol of FAC. The cost/benefits of mucus secretion and the various anatomical, physiological, biochemical, and ecological mechanisms which allow freshwater snails to recover FAC's lost as a result of a high rate of urine production in their hypotonic environment, are discussed.

Trophic exchanges between parent and young during development of Glossiphonia complanata (Annelida, Hirudinea)

Glossiphoniids are iteroparous hirudineans that brood their offspring. The young are attached by the posterior sucker to the parent's ventral side until development is complete. The weight loss of the brooding adults is commonly attributed to their reduced possibility of feeding and to the increased metabolic expense for movements needed to ventilate the offspring. The present study showed that there is a passage of nutritive substrates between the parental body wall and the young's sucker. A possible correlation between this passage and the reabsorption of some muscle fibers that occurs in the adult's body wall is also analyzed.

Sodium D-glucose cotransport in the gill of marine mussels: studies with intact tissue and brush-border membrane vesicles

Glucose transport was studied in marine mussels of the genus Mytilus. Initial observations, with intact animals and isolated gills, indicated that net uptake of glucose occurred in mussels by a carrier-mediated, Na+-sensitive process. Subsequent studies included use of brush-border membrane vesicles (BBMV) in order to characterize this transport in greater detail. The highest activity of Na+-dependent glucose transport was found in the brush-border membrane fractions used in this study, while basal-lateral membrane fractions contained the highest specific binding of ouabain. Glucose uptake into BBMV showed specificity for Na+, and concentrative glucose transport was observed in the presence of an inwardly directed Na+ gradient. There was a single saturable pathway for glucose uptake, with an apparent Kt of 3 microM in BBMV and 9 microM in intact gills. The kinetics of Na+ activation of glucose uptake were sigmoidal, with apparent Hill coefficients of 1.5 in BBMV and 1.2 in isolated gills, indicating that more than one Na+ may be involved in the transport of each glucose. Harmaline inhibited glucose transport in mussel BBMV with a Ki of 44 microM. The uptake of glucose was electrogenic and stimulated by an inside-negative membrane potential. The substrate specificity in intact gills and BBMV resembled that of Na+-glucose cotransporters in other systems; D-glucose and alpha-methyl glucopyranoside were the most effective inhibitors of Na+-glucose transport, D-galactose was intermediate in its inhibition, and there was little or no effect of L-glucose, D-fructose, 2-deoxy-glucose, or 3-O-methyl glucose. Phlorizin was an effective inhibitor of Na+-glucose uptake, with an apparent Ki of 154 nM in BBMV and 21 nM in intact gills. While the qualitative characteristics of glucose transport in the mussel gill were similar to those in other epithelia, the quantitative characteristics of this process reflect adaptation to the seawater environment of this animal.

Amino acid transport in the gill epithelium of a marine bivalve

1. Amino acid transport across the intestine of bivalve molluscs is reviewed. 2. Transport of alanine or taurine is dependent on the sodium gradient across the intestine. 3. The time course of the uptake of alanine into the brush border membrane vesicles is also sodium dependent.

Effect of hyperglycaemia on sugar transport in the isolated mucosa of guinea-pig small intestine

The effect of hyperglycaemia on sugar transport was studied by comparing transepithelial permeation and tissue content of 3-O-methyl-D-glucose (3-O-MG), beta-methyl-D-glucoside (beta-MDG) and D-glucose in isolated mucosae of guinea-pig jejunum mounted in a flux chamber. Sugars were administered either to the luminal or the blood side of mucosae prepared either from normal animals or those maintained in a hyperglycaemic state by I.V. glucose infusion for 12 h. In control animals, absorptive sugar fluxes increased in the order glucose greater than beta-MDG greater than 3-O-MG. Only beta-MDG was accumulated in the tissue beyond the medium concentration. Permeation of 3-O-MG and beta-MDG in the direction blood-to-lumen was mainly paracellular as indicated by the strict correlation with the simultaneous permeation of polyethylene glycol (mol. wt. 900). Luminal addition of 10(-3) M-phlorhizin increased permeation and decreased tissue content of beta-MDG and D-glucose when administered on the blood side, suggesting that these sugars are recaptured at the brush border even from vigorously mixed solutions. For flux coefficient calculation the preparation was regarded as a three-compartment system. With all three sugars, the influx coefficient was higher at the luminal, but lower at the basolateral membrane than the corresponding efflux coefficient. 3-O-MG displayed the highest basolateral influx coefficient of all three sugars, being even higher than its luminal influx coefficient. The luminal influx coefficient of beta-MDG was 22 times greater, and its basolateral efflux coefficient 2.5 times less than the corresponding values for 3-O-MG, resulting in cellular beta-MDG accumulation. D-Glucose was suited best for transepithelial transport, having a luminal influx coefficient only 1.6 times less, and a basolateral efflux coefficient almost 10 times greater than those for beta-MDG. Prolonged hyperglycaemia increased the lumen-to-blood permeation of all three sugars 1.3-2-fold. No significant differences in tissue content to control values were observed after 45 min (3-O-MG, D-glucose) or 90 min (beta-MDG) incubation. Therefore, flux coefficients increased by the same factors in luminal and basolateral membranes, i.e. 1.7, 1.3 and 1.7 for 3-O-MG, beta-MDG and D-glucose, respectively. These results indicate that changes in both the luminal and basolateral membranes play a role in the increase of sugar transport in hyperglycaemia and that a regulatory mechanism might exist between the transport systems located in both membranes.(ABSTRACT TRUNCATED AT 400 WORDS)

Epidermal nutrition of the alcyonarian Heteroxenia fuscescens (Ehrb.): absorption of dissolved organic material and lost endogenous photosynthates

The trophic strategies were studied of Heteroxenia fuscescens living in shallow tropical waters. Structural and physiological adaptations show that particulate food is of less nutritional importance than the uptake of organic material dissolved in the sea, the utilization of assimilates of cytosymbiotic algae (zooxanthellae) and even the symbionts themselves. The external and internal surfaces of the tentacles are enlarged by featherlike pinnules, on the one hand facilitating the epidermal uptake of dissolved organic compounds and on the other offering wellilluminated spaces in which large numbers of zooxanthellae can be 'cultivated'.Zooxanthellae expelled from gastrodermal cells may be taken up by the mesenteric filaments of the dorsal mesenteries, where they are often decomposed and utilized. The transport of photo-assimilates from the gastrodermis to the epidermis through the mesogloea takes place at a low rate. Most of the released assimilates of the symbionts appear in the coelenteron. One fraction of these assimilates is distributed within the gastric channel system and can be taken up by developing stages living there; another fraction reaches the epidermis extracorporally via the pharynx and the sea. Thus both the pharynx and the epidermis absorb these photo-assimilates. The epidermal uptake capacity serves two main purposes: (1) active uptake and incorporation of external organic material dissolved in the sea; (2) reabsorption of internal, self-produced organic material, i.e. reduction of the loss of endogenous compounds escaping from the gastric cavity necessarily due to the polyfunctionality of the coelenteron.

D-Glucose transport across the apical membrane of the surface epithelium in Nereis diversicolor

Epidermal D-glucose transport was investigated in vivo in the brackish-water polychaete worm Nereis diversicolor. Transfer across the apical membrane is rate-limiting to D-glucose uptake, but the cuticle and/or mucus presents some resistance to D-glucose diffusion between bulk solution and transporting membrane. Maximal D-glucose influx is about 10(-12) mol sec-1 per cm2 of apical plasmalemma. Under natural conditions (approximately 1 microM D-glucose in the medium), backflux from the epidermal transport pool is negligible, but a significant paracellular outflux may occur. D-glucose influx across the apical membrane is Na+-dependent and completely inhibitable by phlorizin and harmaline; phloretin is less effective, and cytochalasin B has no effect. Influx is moderately depressed by KCN and iodoacetate, alpha-methyl-D-glucopyranoside is an effective substitute of D-glucose in transport. Animals acclimated to a low salinity, in which epidermal salt transport takes place, show a marked decrease of D-glucose transport capacity. On transfer of animals from a high to a low salinity, or vice versa, the corresponding change of influx occurs after a time-lag of at least an hour. Permeability of the epidermis to simple diffusion of D-glucose is 8 X 10(-8) cm sec-1 (on basis of gross epidermal area).