65Zn2+ transport by lobster hepato-pancreatic baso-lateral membrane vesicles

Intracellular calcium phosphate deposits contribute to transcellular calcium transport within the hepatopancreas of Porcellio scaber

Like in most Crustacea, the cuticle of terrestrial isopods is hardened by a calcareous mineral phase. This rigid cuticle is frequently shed during a process called moulting. To reduce calcium loss, Porcellio scaber eats the shed cuticle, the exuviae, and absorb the calcium from it through large tubular diverticula of the intestine, called the mid gut glands or hepatopancreas. After moulting the absorbed calcium should be transported immediately into the hemolymph from which it is used to rapidly mineralize the new cuticle. This suggests that the hepatopancreas epithelium transports calcium from the lumen to the hemolymph. We used TEM, energy-filtered TEM and electron-probe X-ray microanalysis to analyse the distribution of elevated calcium within the hepatopancreas cells of P. scaber. We used animals in the postmoult stage that have eaten their exuviae and, as a control, those that have not ingested the exuviae. To minimize calcium loss within the samples, we used high pressure frozen and freeze substituted samples and propane-1-3-diol as floatation medium for thin-sectioning. The results reveal intracellular dense deposits containing calcium, phosphorus and oxygen at the apical microvillus membrane, within the cytoplasm, attached to vesicles and to the basolateral membrane, as well as extracellular between cells and the basal lamina. Control animals were devoid of these deposits. The results indicate that calcium from the exuviae is absorbed and transported across the epithelium into the hemolymph. We propose that during transport, intracellular calcium is bound to phosphate avoiding toxic effects of high concentrations of ionized calcium.

FMRF-amide is a glucose-lowering hormone in the snail Helix aspersa

Although glucose is metabolically the most important carbohydrate in almost all living organisms, still little is known about the evolution of the hormonal control of cellular glucose uptake. In this study, we identify Phe-Met-Arg-Phe-amide (FMRFa), also known as molluscan cardioexcitatory tetrapeptide, as a glucose-lowering hormone in the snail Helix aspersa. FMRFa belongs to an evolutionarily conserved neuropeptide family and is involved in the neuron-to-muscle signal transmission in the snail digestive system. This study shows that, beyond this function, FMRFa also has glucose-lowering activity. We found neuronal transcription of genes encoding FMRFa and its receptor and moreover the hemolymph FMRFa levels were peaking at metabolically active periods of the snails. In turn, hypometabolism of the dormant periods was associated with abolished FMRFa production. In the absence of FMRFa, the midintestinal gland ("hepatopancreas") cells were deficient in their glucose uptake, contributing to the development of glucose intolerance. Exogenous FMRFa restored the absorption of hemolymph glucose by the midintestinal gland cells and improved glucose tolerance in dormant snails. We show that FMRFa was released to the hemolymph in response to glucose challenge. FMRFa-containing nerve terminals reach the interstitial sinusoids between the chondroid cells in the artery walls. We propose that, in addition to the known sites of possible FMRFa secretion, these perivascular sinusoids serve as neurohemal organs and allow FMRFa release. This study suggests that in evolution, not only the insulin-like peptides have adopted the ability to increase cellular glucose uptake and can act as hypoglycemic hormones.

Transepithelial transport of zinc and L-histidine across perfused intestine of American lobster, Homarus americanus

The intestine of the American lobster, Homarus americanus, was isolated and perfused in vitro with a physiological saline, based on the ion composition of the blood, to characterize the mechanisms responsible for transmural transport of zinc and how the amino acid, L-histidine, affects the net movement of the metal across the tissue. Previous studies with this preparation, focusing on the characteristics of unidirectional mucosa to serosa (M to S) fluxes of (65)Zn(2+) and (3)H-L-histidine, indicated the presence of a brush border co-transport process responsible for simultaneously transferring the metal and amino acid across this tissue as an apparent bis-complex (Zn-[His](2)) using a PEPT-1-like dipeptide carrier mechanism. In addition, both zinc and L-histidine were also transferred toward the blood by separate transporters that were independent of the other substrate. The focus of the present study was to characterize the serosa to mucosa (S to M) flux of (65)Zn(2+) under a variety of conditions, and use these values in conjunction with those from the previous study, to assess the direction and magnitude of net metal movement across the tissue. Transmural S to M transport of (65)Zn(2+) was markedly reduced with the addition of the serosal inhibitors ouabain (32%), excess K(+) (25%), excess Ca(2+) (30%), Cu(2+) (38%), nifedipine (21%), and vanadate (53%). In contrast, this flux was markedly stimulated with the serosal addition of ATP (24%) and excess Na(+) (28%). These results suggest that S to M fluxes of zinc occurred by the combination of the basolateral Na/Ca exchanger (NCX), where zinc replaced calcium, and a basolateral nifedipine-sensitive calcium channel. Transmural M to S (65)Zn(2+) fluxes (5-100 microM) were threefold greater than S to M metal transport, and the addition of luminal L-histidine doubled the net M to S zinc flux over its rate in the absence of the amino acid. The results of this paper and those in its predecessor indicate that zinc transport by the lobster intestine is absorptive and significantly enhanced by luminal amino acids.

Physiological characterization of 45Ca2+ and 65Zn2+ transport by lobster hepatopancreatic endoplasmic reticulum

The crustacean hepatopancreas is an epithelial-lined, multifunctional organ that, among other activities, regulates the flow of calcium into and out of the animal's body throughout the life cycle. Transepithelial calcium flow across this epithelial cell layer occurs by the combination of calcium channels and cation exchangers at the apical pole of the cell and by an ATP-dependent, calcium ATPase in conjunction with a calcium channel and an Na+/Ca2+ antiporter in the basolateral cell region. The roles of intracellular organelles such as mitochondria, lysosomes, and endoplasmic reticulum (ER) in transepithelial calcium transport or in transient calcium sequestration are unclear, but may be involved in transferring cytosolic calcium from one cell pole to the other. The ER membrane has a complement of ATP-dependent calcium ATPases (SERCA) and calcium channels that regulate the uptake and possible transfer of calcium through this organelle during periods of intense calcium fluxes across the epithelium as a whole. This investigation characterized the mechanisms of calcium transport by lobster hepatopancreatic ER vesicles and the effects of drugs and heavy metals on them. Kinetic constants for 45Ca2+ influx under control conditions were K(n) (m)=10.38+/-1.01 microM, J(max)=14.75+/-1.27 pmol/mg protein x sec, and n=2.53+/-0.46. The Hill coefficient for 45Ca2+ influx under control conditions, approximating 2, suggests that approximately two calcium ions were transported for each transport cycle in the absence of ATP or the inhibitors. Addition of 1 mM ATP to the incubation medium significantly (P<0.01) elevated the rate of 45Ca2+ influx at all calcium activities used and retained the sigmoidal nature of the transport relationship. The kinetic constants for 45Ca2+ influx in the presence of 1 mM ATP were K(n) (m)=12.76+/-0.91 microM, J(max)=25.46+/-1.45 pmol/mg protein x sec, and n=1.95+/-0.15. Kinetic analyses of ER 65Zn2+ influx resulted in a sigmoidal relationship between transport rate and zinc activity under control conditions (K(n) (m)=38.63+/-0.52 microM, J(max)=19.35+/-0.17 pmol/mg protein x sec, n=1.81+/-0.03). The Addition of 1 mM ATP enhanced 65Zn2+ influx at each zinc activity, but maintained the overall sigmoidal nature of the kinetic relationship. The kinetic constants for zinc influx in the presence of 1 mM ATP were K(n) (m)=34.59+/-2.31 microM, J(max)=26.09+/-1.17 pmol/mg protein x sec, and n=1.96+/-0.17. Both sigmoidal and ATP-dependent calcium and zinc influxes by ER vesicles were reduced in the presence of thapsigargin and vanadate. This investigation found that lobster hepatopancreatic ER exhibited a thapsigargin- and vanadate-inhibited, SERCA-like, calcium ATPase. This transporter displayed cooperative calcium transport kinetics (Hill coefficient, n approximately 2.0) and was inhibited by the heavy metals zinc and copper, suggesting that the metals may reduce the binding and transport of calcium when they are present in the cytosol.