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PubMed1988

Inhibition of taurocholate and ouabain transport in isolated rat hepatocytes by cyclosporin A.

The use of cyclosporin A in transplantation procedures has been reported to cause hepatotoxicity as evidenced by elevated serum bilirubin and bile salt levels. However, these biochemical abnormalities could also result from interference with hepatic transport processes. This possibility was investigated in the present study in which the effect of cyclosporin A on transport processes was examined in isolated rat liver cells. Taurocholate, ouabain, and alpha-aminoisobutyric acid were selected as compounds known to enter liver cells by distinct active transport systems and cadmium was selected as a substance taken up by a combination of simple and facilitated diffusion. Cyclosporin A was found to cause a dose-related inhibition of both taurocholate and ouabain uptake. On the other hand, the uptake of alpha-aminoisobutyric acid and of cadmium were unaffected by cyclosporin A. These findings indicate a substrate-specific effect of cyclosporin A rather than a general effect on cellular transport. Efflux of taurocholate from preloaded hepatocytes was also inhibited by cyclosporin A. Cyclosporin A caused a decrease in maximum velocity for ouabain uptake with no change in Km. Kinetic analysis for both uptake and efflux of taurocholate showed an unchanged maximum velocity and an increased Km. The data indicate that the ability of liver cells to take up and release bile acids is impaired in the presence of cyclosporin A. These findings provide a possible explanation for the finding of increased serum bile acids during cyclosporin A therapy and suggest that hepatic clearance of other compounds could also be impaired.

Aminoisobutyric Acids↗
PubMed1997

Transport of glucose across the blood-tissue barriers.

In specialized parts of the body, free exchange of substances between blood and tissue cells is hindered by the presence of a barrier cell layer(s). Specialized milieu of the compartments provided by these "blood-tissue barriers" seems to be important for specific functions of the tissue cells guarded by the barriers. In blood-tissue barriers, such as the blood-brain barrier, blood-cerebrospinal fluid barrier, blood-nerve barrier, blood-retinal barrier, blood-aqueous barrier, blood-perilymph barrier, and placental barrier, endothelial or epithelial cells sealed by tight junctions, or a syncytial cell layer(s), serve as a structural basis of the barrier. A selective transport system localized in the cells of the barrier provides substances needed by the cells inside the barrier. GLUT1, an isoform of facilitated-diffusion glucose transporters, is abundant in cells of the barrier. GLUT1 is concentrated at the critical plasma membranes of cells of the barriers and thereby constitutes the major machinery for the transport of glucose across these barriers where transport occurs by a transcellular mechanism. In the barrier composed of double-epithelial layers, such as the epithelium of the ciliary body in the case of the blood-aqueous barrier, gap junctions appear to play an important role in addition to GLUT1 for the transfer of glucose across the barrier.

Animals↗
PubMedDate not supplied

Nucleoside transport by perfused human placenta.

Nucleoside transport and metabolism by human placenta was studied using the dual perfusion technique. With [3H] thymidine added to the maternal perfusate and neither perfusate recirculated (steady-state studies) around 40 per cent of the thymidine in the maternal outflow and 50 per cent of the transferred thymidine was degraded. In similar studies with adenosine, over 95 per cent of the nucleoside was degraded. Even with the bolus technique which sharply limits the duration of contact with the placenta, degradation of adenosine was over 95 per cent. Uptake as calculated by the dual-tracer method ([3H] adenosine/[14C] L-glucose) was equally rapid from the maternal and fetal perfusates, was saturable and inhibited by nitrosobenzylthioinosine, consistent with the facilitated diffusion system for nucleosides. Thymidine was taken up at one-third the rate of adenosine. Thymidine in large excess (500 microM) reduced adenosine uptake suggesting a common transporter. Zidovudine, a thymidine analogue used for the treatment of AIDS in which the ribose is modified at the 2' 3' position, did not compete with adenosine for uptake consistent with previous reports that zidovudine is transferred across the placenta by simple diffusion.

Adenosine↗
PubMed2003

Using RNA interference to manipulate endogenous gene expression in Schistosoma mansoni sporocysts.

Direct assessments of gene function in parasitic flatworms have been hampered by the lack of effective tools to alter gene expression. The aim of the present study was to use RNA-interference (RNAi) to achieve targeted gene knockdown in larval stages of the human blood fluke, Schistosoma mansoni. We selected two S. mansoni genes for RNAi experiments: SGTP1, a facilitated diffusion glucose transporter and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). When S. mansoni larvae were treated in vitro for 6 days with dsRNA specific to one of these two genes, targeted transcript levels were reduced by 70-80% as determined by quantitative PCR (qPCR), while non-targeted transcripts were unaffected. Parasite exposure to SGTP1 dsRNA, but not GAPDH dsRNA, reduced larval glucose-uptake capacity by 40%, demonstrating that SGTP1 transcript knockdown results in the functional phenotype of reduced glucose transport activity. The effect of dsRNA treatment on transcript level was evident for up to 28 days after an initial dsRNA treatment. Interestingly, dsRNA treatment was effective only when miracidia were allowed to undergo the transition to sporocysts in its presence, while treatment of fully transformed sporocysts was ineffective. Fluorescence patterns in larvae exposed to rhodamine-labeled dsRNA as miracidia and sporocysts were similar, suggesting that the difference in susceptibility to dsRNA treatment between the two life stages may not be due to differences in dsRNA entry. Overall, this technology will enable direct assessment of the roles of individual genes in physiological processes of larval stages of S. mansoni, a crucial step in the identification of novel intervention targets for this important human pathogen.

Animals↗
PubMed2000

Cloning and characterization of glucose transporter in teleost fish rainbow trout (Oncorhynchus mykiss).

The facilitated diffusion of monosaccharides across the plasma membrane is mediated by glucose transporters (GLUTs). In contrast to mammals, the glucose transport system of lower vertebrates remains unexplored. We detected glucose transport activity in rainbow trout embryos. Two GLUTs sharing 83% amino acid identity were cloned from juvenile fish, these have been denoted OnmyGLUT1A and OnmyGLUT1B. In adult trout OnmyGLUT1A is predominantly expressed in the heart with low expression in other tissues. An inverse terminal repeat of a Tc1-like transposable element was found in the 3'-untranslated region of OnmyGLUT1B. Phylogenetic analysis suggested that rainbow trout genes share a common ancestor with higher vertebrate GLUT1. We also found GLUT genes in several salmonid species.

Amino Acid Sequence↗
PubMed1998

Binding of terbium and cisplatin to C13* human ovarian cancer cells using time-resolved terbium luminescence.

Terbium (Tb3+) has been shown to increase the cellular accumulation and cytotoxicity of cisplatin in cisplatin-resistant human breast and ovarian cancer cells. Time-resolved Tb3+ luminescence was used to describe the binding of cisplatin to cisplatin-resistant C13* cells. A high-affinity Tb3+ binding site was identified in the plasma membrane of the C13* cells (n=105+/-2 fmol/cell and Kd=36. 3+/-5.2 microM). The binding of Tb3+ is suggested to occur through a cation-pi interaction with tryptophan residues in the plasma membrane, resulting in an enhancement of the intensity and lifetime of Tb3+. Stern-Volmer quenching analysis revealed that the Tb3+ binding site is not readily accessible to the aqueous environment. The quenching of the Tb3+-C13* intensity by cisplatin occurred by static quenching processes, involving both a direct electron-exchange interaction as well as an indirect dipole-dipole resonant energy transfer mechanism. Formation of the Tb3+-C13*-cisplatin complex does not interfere with the high-affinity binding of Tb3+; cisplatin and Tb3+ bind within 5 to 10 A of each other. A specific terbium/cisplatin binding protein is suggested to play a role in the cellular accumulation and cytotoxicity of cisplatin. Therefore, the transport of cisplatin across the plasma membrane must also involve a facilitated diffusion process. Our results indicate that the binding of Tb3+ to the plasma membrane may be potentially useful in the reversal of cisplatin resistance.

Antineoplastic Agents↗
PubMed2003

Efflux-mediated heavy metal resistance in prokaryotes.

What makes a heavy metal resistant bacterium heavy metal resistant? The mechanisms of action, physiological functions, and distribution of metal-exporting proteins are outlined, namely: CBA efflux pumps driven by proteins of the resistance-nodulation-cell division superfamily, P-type ATPases, cation diffusion facilitator and chromate proteins, NreB- and CnrT-like resistance factors. The complement of efflux systems of 63 sequenced prokaryotes was compared with that of the heavy metal resistant bacterium Ralstonia metallidurans. This comparison shows that heavy metal resistance is the result of multiple layers of resistance systems with overlapping substrate specificities, but unique functions. Some of these systems are widespread and serve in the basic defense of the cell against superfluous heavy metals, but some are highly specialized and occur only in a few bacteria. Possession of the latter systems makes a bacterium heavy metal resistant.

Adenosine Triphosphatases↗
PubMed1998

Effect of atrial natriuretic peptide on alpha-methyl-D-glucoside intestinal active uptake in rats.

In vivo, atrial natriuretic peptide (ANP) inhibits water and sodium absorption by the intestine. In addition, ANP inhibits glucose (re)absorption at the level of both the intestine and kidney. ANP also decreases sodium absorption in the rat small intestine in vitro, but only if glucose is present on the luminal side of the tissue. These findings suggest that ANP inhibits the sodium-glucose cotransporter (SGLT) of enterocytes. In the present study the inhibitory effect of 1 microM ANP on SGLT1 in rat small intestine and colon was tested. For this purpose, the apparent kinetic constants of SGLT1 were determined using radioactive alpha-methyl-D-glucoside (alpha-MG), a non-metabolizable glucose analogue that selectively serves the luminal Na+-dependent intestinal uptake, but not the serosal-facilitated diffusion sugar carrier. In both tissues, incubation with ANP increased Km without modifying the Vmax. In addition, Vmax in the small intestine was found to be higher than in the colon. The evidence presented here suggests that ANP, through its second messenger, may be a competitive inhibitor of SGLT1. Since SGLT1 is also expressed in the brush-border membrane of the renal proximal tubule, we suggest that this peptide might regulate the hydro-saline balance at intestinal and proximal tubular nephron levels.

Animals↗
PubMed1998

[Hexose transporters in humans: their role in insulin sensitivity of peripheral tissues].

UNLABELLED: OBJECTIVES AND JUSTIFICATION: To describe facilitated diffusion glucose transporters (GLUT) in humans, and particularly the regulation of GLUT4 expression since it is predominantly responsible for insulin-mediated glucose transport in muscle and adipose tissue, and plays a crucial role in whole-body glucose homeostasis. MAIN POINTS: Numerous studies have shown abnormal expression of GLUT4 in insulin responsive tissues in diabetes mellitus and other insulin resistant states. The recent development of transgenic mice that overexpress GLUT4 in muscle, adipose tissue, or both has also emphasized the importance of GLUT4 in glucose homeostasis. PERSPECTIVES AND PROJECTS: The studies performed in transgenic mice suggest that strategies to increase GLUT4 transporters in insulin responsive tissues may lead to new therapeutic possibilities to improve insulin sensitivity in insulin resistant states such as non-insulin-dependent diabetes mellitus. CONCLUSIONS: Glucose transport is a crucial process in mammals, which needs specific transmembrane hexose transporters. Each transporter is important in the regulation of glucose homeostasis. Advances in the knowledge of both the action and the regulation of the glucose transport system may provide new insights for the development of therapeutic interventions in diseases such as obesity or diabetes mellitus.

Adipose Tissue↗
PubMed1998

Glucose uptake in Oesophagostomum dentatum and the effect of oxfendazole.

The uptake of 14C-glucose by adult Oesophagostomum dentatum was characterised. The uptake was a non-linear function of external glucose concentration. The maximum velocity of uptake (Vmax) was 0.964 nmol/100 mg dry weight (dw)/5 min, and the transport constant (Kt) was 10.02 microM. When phlorizin, phloretin and 3-O-methylglucose were tested for their effects on the uptake of 14C-glucose, phloretin and 3-O-methylglucose produced significant inhibitions, indicating that the uptake was mediated and occurred by facilitated diffusion. Exposure of the worms to oxfendazole prior to incubation with 14C-glucose did not affect the uptake of glucose. In another experiment worms were incubated with unlabelled glucose and the external glucose concentration was measured enzymatically. During a 7 h incubation period, the quantity of glucose remaining in the incubation medium of oxfendazole exposed worms was significantly greater than in the control group. It was concluded that oxfendazole did not influence the process of 14C-glucose uptake, but might induce changes in the parasite leading to a reduced ability to deplete the incubation medium of glucose.

3-O-Methylglucose↗
PubMed1998

Expression of high-affinity trehalose-H+ symport in Saccharomyces cerevisiae.

The expression of the high-affinity trehalose-H+ symport was investigated in various Saccharomyces cerevisiae strains and culture conditions. Previous kinetic studies of trehalose transport in yeast have revealed the existence of at least two different uptake mechanisms: a high-affinity trehalose-H+ symport activity repressed by glucose, and a constitutive low-affinity transport activity, a putative facilitated diffusion process. Exogenously added trehalose was not an inducer of the high-affinity transport activity, and a correlation between trehalose and maltose uptake by yeast cells was found. Our results indicate that the maltose-H+ symporters encoded by MAL11, MAL21, and MAL41 are not responsible for the trehalose transport activity. The analysis of both trehalose and maltose transport activities in wild-type and in laboratory strains with defined MAL genes showed that the trehalose-H+ symporter was under control of MAL regulatory genes. Our results also suggest that the recently characterized AGT1 gene of S. cerevisiae may encode the high-affinity trehalose-H+ symporter. During diauxic growth on glucose the transport activity was low during the first exponential phase of growth, increased as glucose was exhausted from the medium, and decreased again as the cells reached the late stationary phase. This pattern was coincident with that of the intracellular levels of trehalose. The strong correlation between these two parameters may be of physiological significance during adaptation of yeast cells to stress conditions.

Biological Transport↗
PubMed1998

Characterisation of non-transferrin-bound iron (ferric citrate) uptake by rat hepatocytes in culture.

Under conditions of iron overload plasma transferrin can be fully saturated and the plasma can transport non-transferrin-bound Fe which is rapidly cleared by the liver. Much of this Fe is complexed by citrate. The aim of the present work was to characterise the mechanisms by which Fe-citrate is taken up by hepatocytes using a rat hepatocyte cell culture model. The cells, after one day in culture, were incubated with 59Fe-labelled Fe-citrate for varying time periods, then washed and Fe uptake to the membrane and intracellular compartments of the cell was determined by radioactivity measurements. Maximal rates of internalisation of Fe occurred at a Fe:citrate molar ratio of 1:100 or greater, a pH of approximately 7.4 and an extracellular Ca2+ concentration of 1.0 mM. Fe uptake showed Michaelis-Menten kinetics and was a temperature-dependent process. The K(m) and Vmax for Fe internalisation by the cells at 37 degrees C were approximately 7 microM and 2 nmol/mg DNA/min (25 x 10(6) atoms/cell/min), respectively; and the Arrhenius activation energy was 35 kJ/mol. The transition metals, Zn2+, Co2+ and Ni2+, inhibited Fe uptake when used at 10 and 100 times the concentration of Fe. The rate of Fe internalisation from Fe-citrate was found to be approximately 20 times as great as that from Fe-transferrin with Fe concentrations of 1 and 2.5 microM for both forms of Fe. The rate of Fe uptake by iron-loaded hepatocytes obtained from rats which had been fed carbonyl Fe was not significantly different from that by normal hepatocytes. These experiments show that rat hepatocytes in primary culture have a high capacity to take up non-transferrin-bound Fe in the form of Fe-citrate and that uptake occurs by facilitated diffusion. The iron transport process does not appear to be regulated by cellular Fe levels.

Animals↗
PubMed1997

Autoradiographic studies on the uptake of 3H-dopamine by neurons and astrocytes in explant and primary cultures of rat CNS: effects of uptake inhibitors.

The cellular localization of the uptake of 3H-dopamine was studies in explant and primary cultures from various regions of rat central nervous system by means of autoradiography. In explant cultures of substantia nigra, 3H-dopamine was taken up by cell bodies and processes of many neurons. In cultures from striatum, cerebellum and spinal cord, neuronal cell bodies were not labelled, whereas outgrowing nerve fibres revealed intense uptake of the monoamine. Uptake of 3H-dopamine by neurons was Na(+)- and temperature-dependent, suggesting an active uptake mechanism. In explant cultures, astrocytes did not accumulate 3H-dopamine, whereas in primary cultures, which were prepared from the same regions of rat central nervous system as the explant cultures, astrocytes also revealed uptake of this monoamine. The intensity of labelling was dependent on the incubation time. Little uptake of 3H-dopamine was observed after an incubation time of 5 min and only after 10-15 min did the astrocytes show moderate labelling. Uptake of 3H-dopamine by astrocytes was not Na(+)- and temperature-dependent, indicating that glial cells do not possess an active uptake mechanism for this monoamine. This is consistent with biochemical investigations by other laboratories, demonstrating that astrocytes accumulate 3H-dopamine by a facilitated diffusion system. Addition of the uptake inhibitors nomifensine or GBR 12909 to explant cultures markedly reduced or inhibited uptake of 3H-dopamine by neurons at a concentration of 10(-6) M. In contrast, accumulation of 3H-dopamine by astrocytes in primary cultures was only slightly affected by nomifensine at 10(-6) M. At the highest concentration used (10(-5) M), nomifensine also blocked the uptake of 3H-dopamine by astrocytes. Our finding that GBR 12909 almost completely inhibited the uptake of 3H-dopamine by astrocytes already at 10(-6) M suggests that this compound is a more potent inhibitor of the glial uptake of dopamine than nomifensine.

Animals↗
PubMed2002

Blood-brain barrier, brain metabolism and cerebral blood flow.

For optimal function of the brain with its meticulous operations, an adequate and constant micro environment seems to be a prerequisite. This is secured by the blood-brain barrier which is impermeable to hydrophilic substances, with notable exceptions such as glucose, which cross the barrier by a mechanism of facilitated diffusion. A constant micro environment is further secured by the blood flow which is balanced to the metabolic demand of the cerebral tissue and which also contributes to the maintenance of a constant pH. During activation, blood flow and glucose consumption increase more than oxygen consumption in activated areas of the brain. The flow increase forms the physiological basis for measurement and mapping of functional activation using positron emission tomography and the changes in the metabolic pattern which has been called uncoupling of flow and oxygen metabolism is the basis for such measurements using functional magnetic resonance imaging.

Animals↗
PubMed1996

The preclinical basis for broad-spectrum selective cytoprotection of normal tissues from cytotoxic therapies by amifostine (Ethyol).

Administered prior to cytotoxic chemotherapy or radiation, the aminothiol amifostine provides broad-spectrum cytoprotection of various normal tissues without attenuating antitumour response. The basis for the selectivity of action resides in the anabolism of amifostine at the normal tissue site by membrane-bound alkaline phosphatase. Dephosphorylation to the free thiol, WR-1065, is followed by rapid uptake into normal tissues by a carrier mediated, facilitated diffusion process; in contrast, uptake into tumour tissue is slow to negligible. Preclinical studies have shown that pretreatment with amifostine provides protection of normal tissues from the cytotoxic effects of alkylating agents, organoplatinums, anthracyclines, taxanes and radiation. Normal tissues protected include bone marrow, kidney, neural tissues, the heart, intestinal crypt cells and pulmonary tissues. Additionally, the mutagenic and carcinogenic effects of these modalities are also attenuated. With respect to bone marrow, preclinical studies have shown significant protection of progenitor cells that give rise to the red and white cells and platelets. Comparative in vitro and in vivo studies using murine and human tumour xenografts show no decrease of antitumour effects of these same therapies despite the protection of normal organs. The unique preclinical profile of amifostine serves as a model for the clinical development programme for this important new broad-spectrum cytoprotective agent.

Amifostine↗
PubMed1995

Target DNA capture by HIV-1 integration complexes.

BACKGROUND: The early steps of human immunodeficiency virus 1 (HIV-1) replication involve reverse transcription of the viral RNA and integration of the resulting cDNA into a host chromosome. The DNA integration step requires the integration machinery ('preintegration complex') to bind to the host DNA before connecting the viral and host DNAs. Here, we present experiments that distinguish among three possible pathways of target-DNA capture: repeated binding and release of target DNA prior to the chemical strand-transfer step; binding followed by facilitated diffusion along target DNA (sliding); and integration at the initial target-capture site. The mechanism of target-DNA capture has implications for the design of gene therapy methods, and influences the interpretation of results on the selection of integration target sites in vivo. RESULTS: We present new in vitro conditions that allow us to assemble HIV-1 integrase--the virus-encoded recombination enzyme--with a viral DNA and then to trap assembled complexes bound to target DNA. We find that complexes of integrase and viral DNA do not slide along target DNA substantially after binding. We confirm and extend these results by analyzing target capture by a hybrid protein composed of HIV-1 integrase linked to a sequence-specific DNA-binding domain. We find that the integrase domain binds quickly and tightly under the above conditions, thereby obstructing function of the fused sequence-specific DNA-binding domain. We also monitor target-DNA capture by HIV-1 preintegration complexes purified from freshly infected cells. Partially purified complexes commit quickly and stably to the first target DNA added, whereas preintegration complexes in crude cytoplasmic extracts do not. The addition of extracts from uninfected cells to partially purified complexes blocks quick commitment. CONCLUSIONS: Under new conditions favorable for the analysis of target-DNA capture in vitro, HIV-1 integrase complexes bind quickly and stably to target DNA without subsequent sliding. Parallel studies of preintegration complexes support a model in which target-site capture in vivo is reversible as a result of the action of cellular factors.

Binding Sites↗
PubMed2001

Do circulating plasma AVT and/or cortisol levels control pulsatile urea excretion in the gulf toadfish (Opsanus beta)?

Previous work has shown that pulsatile urea excretion at the gills of the gulf toadfish is due to periodic activation of a facilitated diffusion transport system with molecular and pharmacological similarity to the UT-A transport system of the mammalian kidney. In mammals, AVP and glucocorticoids are two important endocrine regulators of this system. The present study focused on the potential role of circulating AVT (the teleost homologue of AVP) and cortisol levels as possible triggers for urea pulses. Long-term (34-84 h) monitoring of plasma levels by repetitive sampling at 2-h intervals from chronic cannulae in individual toadfish demonstrated that circulating AVT concentrations are low (10(-12)-10(-11) M), and show no relationship to the occurrence of natural urea pulses. In contrast, plasma cortisol levels decline greatly prior to natural pulses and rise rapidly thereafter. AVT injections into the caudal artery or ventral aorta elicited pulse events, but these were extremely small (1-10%) relative to natural pulses, and occurred only at unphysiological dose levels (10(-9) M in the plasma). AVP was a partial agonist, but isotocin, insulin-like growth factor-1, and atrial natriuretic peptide were without effect at the same concentration. Artificially raising plasma cortisol levels by cortisol injection tended to reduce responsiveness to AVT. Pharmacological reduction of plasma cortisol levels by metyrapone injection elicited small pulses similar to those caused by AVT. Following such pulse events, AVT was ineffective in inducing pulses. We conclude that decreases in circulating cortisol play an important permissive role in urea pulsing, but that circulating AVT levels are not involved.

Animals↗
PubMed2003

Branchial and renal handling of urea in the gulf toadfish, Opsanus beta: the effect of exogenous urea loading.

The objective of this study was to determine whether the pulsatile facilitated diffusion transport mechanism (tUT) found in the gills of the gulf toadfish (Opsanus beta) and the active secretion transporter thought to be present in its kidney could be saturated when faced with elevated plasma urea concentrations. Toadfish were infused with four consecutive exogenous urea loads at a rate of 0, 150, 300 and 600 micromol kg(-1) h(-1). Initial plasma and urine urea concentrations were 8.1+/-0.9 and 12.4+/-1.5 mmol l(-1), respectively, and steadily increased with increasing infused loads of urea to a maximum of 36.8+/-2.8 mmol l(-1) in the plasma and 39.8+/-6.5 mmol l(-1) in the urine. There was only a very weak relationship (r=0.17) between pulse size (measured as branchial excretion during pulsatile excretion of urea) and plasma urea concentration (slope=9.79 micromol-N kg(-1) per mmol-N l(-1); P<0.05) suggesting that the branchial excretion mechanism was already saturated at normal plasma urea concentrations. Urine flow rate (0.15+/-0.03 ml kg(-1) h(-1)) and glomerular filtration rate (0.025+/-0.004 ml kg(-1) h(-1)) remained constant throughout the experiment despite the increased volume load. Renal urea secretion rate maintained a strong linear relationship (r=0.84) to plasma urea levels (slope=0.391 micromol-N kg(-1) h(-1) per mmol-N l(-1); P<0.001) with no observable transport maximum, suggesting that the renal secretory transport mechanism was not saturated even at plasma urea levels well above normal, in contrast to the branchial excretion mechanism.

Animals↗