Influence of permeating ions on Kv1.5 channel block by nifedipine

Nifedipine can block K(+) currents through Kv1.5 channels in an open-channel manner (32). Replacement of internal and external K(+) with equimolar Rb(+) or Cs(+) reduced the potency of nifedipine block of Kv1.5 from an IC(50) of 7.3 microM (K(+)) to 16.0 microM (Rb(+)) and 26.9 microM (Cs(+)). The voltage dependence of block was unaffected, and a single binding site block model was used to describe block for all three ions. By varying ion species at the intra- and extracellular mouth of the channel and by using a nonconducting W472F-Kv1.5 mutant, we demonstrated that block was conditioned by the ion permeating the pore and, to a lesser extent, by the extracellular ion species alone. In Kv1.5, the outer pore mutations R487V and R487Y reduced nifedipine potency close to that of Kv4.2 and other Kv channels with an equivalent valine. Although changing this residue can affect C-type inactivation of Kv channels, the normalized reduction and time course of currents blocked by nifedipine in 5, 135, and 300 mM extracellular K(+) concentration was the same. Similarly, a mean recovery time constant from nifedipine block of 316 ms was unchanged (332 ms) after 5-s prepulses to allow C-type inactivation. This is consistent with the conclusion that nifedipine block and C-type inactivation in the Kv1.5 channel can coexist but are mediated by distinct mechanisms coordinated by outer pore conformation.

The characterisation and uptake of paraquat in cultured baboon kidney proximal tubule cells (bPTC)

A primary culture of baboon proximal tubule cells (bPTC) was prepared and characterised using LLC-PK1 cells of proximal tubule origin and MDCK cells of distal tubule origin, as positive and negative references, respectively. The proximal tubular origin of the bPTC was determined by morphological studies, immunoperoxidase staining and the expression of proximal tubule markers alkaline phosphatase and gammaglutamyltransferase. The hypothesis that paraquat (PQ) is transported by the bPTC was investigated. The cytotoxic threshold for PQ in these cells was determined and compared to the LLC-PK1 and MDCK cells. Furthermore, this study investigated the transport of the monovalent cation tetraethyl ammonium (TEA) and the polyvalent cation cimetidine in the bPTC and demonstrated their effect on the cellular uptake of PQ. The cytotoxic threshold of PQ in the bPTC, determined by cellular viability studies using the method of Trypan blue exclusion, is 0.05 mM at 2 h incubation. The LC50 after 24 h is 76, 61 and 455 microM for the bPTC, LLC-PK1 and MDCK cells, respectively. This indicates that proximal tubule cells are more susceptible to PQ toxicity compared to distal tubule cells, which is consistent with clinical PQ toxicity where renal damage is found predominantly in the proximal renal tubules. The cations PQ and cimetidine were actively transported by the bPTC. The uptake of PQ (0.05 mM) commenced after 15 min whereas cimetidine (0.5 mM) uptake was evident after 2 min. Furthermore, cimetidine was shown to compete with PQ for uptake in the bPTC. Coincubating PQ (0.05 mM) and cimetidine (0.5 mM) for 60 min resulted in an approximate 50% decrease in PQ uptake. The cation TEA was not transported by the bPTC suggesting either a genetic mutation or complete absence of the transporter for TEA in the cells. The results suggest that PQ may be transported by the same cation transporter as cimetidine and not TEA, indicating PQ uptake in the bPTC to be via a polyvalent organic cation transporter.

Effect of organic cations on the renal tubular secretion of pseudoephedrine in the rat

1. Pseudoephedrine is a weak organic base that undergoes renal tubular secretion. The aim of the present study was to assess whether two other commonly used weak organic bases (cimetidine and morphine) inhibit the renal tubular secretion of pseudoephedrine in the rat isolated perfused kidney. 2. A total of 12 perfusions were performed with four perfusions in each of three treatment groups. In the control group, pseudoephedrine was administered as a bolus dose of [14C]-pseudoephedrine and unlabelled pseudoephedrine to achieve an initial perfusate concentration of 0.4 microg/mL. For the treatment groups, pseudoephedrine was administered as above and cimetidine or morphine was added to the perfusion medium in increasing concentrations of 0.5-12.5 and 0.2-5.0 microg/mL, respectively. 3. The mean (+/-SD) fraction unbound of pseudoephedrine alone in perfusate was 0.866+/-0.014 and was not different (P> 0.05) in the presence of cimetidine or morphine. 4. In control experiments, the renal excretory clearance (CLR) of pseudoephedrine was three-fold greater than glomerular filtration rate (GFR), yielding a ratio consistently greater than unity, which indicates extensive net tubular secretion of pseudoephedrine. The CLR and total clearance of pseudoephedrine were similar, suggesting an absence of renal metabolism of pseudoephedrine. 5. The CLR/GFR ratio for pseudoephedrine was not affected by morphine, but was significantly reduced (P < 0.05) in the presence of cimetidine. 6. The results indicate that cimetidine inhibits the renal tubular secretion of pseudoephedrine.

Apical uptake of organic cations by human intestinal Caco-2 cells: putative involvement of ASF transporters

The aim of this work was to characterise the intestinal absorption of organic cations, by testing the possibility of involvement of known members of the amphiphilic solute facilitator (ASF) family in this process. For that purpose, the characteristics of the uptake of 1-methyl-4-phenylpyridinium, a model organic cation, at the brush-border membrane of Caco-2 cells were compared with those of the extraneuronal monoamine transporter (EMT)-mediated transport. Uptake of [3H]MPP+ by Caco-2 and 293hEMT cells showed pH-dependence: it was significantly reduced (to 86% and 62% of control, respectively) when the pH of the extracellular medium was decreased to 6.2, and increased (to 116% and 136% of control, respectively) when the extracellular pH was increased to 8.2. Uptake of [3H]MPP+ by Caco-2 cells and 293hEMT cells showed potential-dependence: substitution of KCl for NaCl in the incubation medium resulted in a reduction in the inward transport of [3H]MPP+ (to 70% and 40% of control, respectively). Uptake of [3H]MPP+ by Caco-2 and 293hEMT cells showed only little dependence on Na+: substitution of NaCl of the incubation media with LiCl resulted in a small decrease (of 19% and 14%, respectively) in [3H]MPP+ uptake. However, when NaCl was substituted with choline chloride, a significant reduction in [3H]MPP+ uptake by Caco-2 and 2931hEMT cells (of 56% and 68%, respectively) was observed. The effect of various compounds on initial rates of [3H]MPP+ uptake into Caco-2 and 293hEMT cells was tested. All compounds tested interacted with the specific [3H]MPP+ uptake in both cell lines. There was no correlation between the IC50s in relation to inhibition of [3H]MPP+ uptake into Caco-2 cells and into 293hEMT cells. Reverse transcriptase-polymerase chain reaction indicates that mRNA of hEMT and of the human organic cation transporter 1 (hOCT1) are present in Caco-2 cells. In conclusion, our results suggest that uptake of organic cations at the brush-border membrane of Caco-2 cells may occur through two distinct Na+-independent transporters belonging to the ASF family: hEMT and hOCT1.

Effect of protamine on cation-selective permeability in hamster medullary thick ascending limb of Henle's loop

To estimate the contribution of the paracellular shunt pathway to cation-selective permeability in the hamster medullary thick ascending limb of Henle's loop, we observed the effect of protamine, a selective blocker of paracellular conductance, on salt-diffusion voltage (dVT) in the isolated nephron segment perfused in vitro. When 300 microg/ml protamine was added to the lumen, the lumen-positive dVT generated upon reduction of the lumen NaCl concentration was decreased from 5.1 +/- 0.9 to 0.8 +/- 0.8 mV and the calculated Na+/Cl- permeability ratio was decreased from 1.40 +/- 0.14 to 0.86 +/- 0.08. Although the effect of protamine persisted after removal of the agent from the lumen, addition of 30 U/ml heparin, which neutralizes protamine, returned the dVT toward the control level. This effect was almost the same when the orientation of the imposed NaCl gradient was reversed. Protamine exhibited a similar effect on dVT in the presence of ouabain added to the bath. Protamine was without effect from the bath. Protamine did not affect the basel VT perfused with the control solution. Increased VT by decreasing perfusion pressure was inhibited by adding protamine from the lumen. These observations suggest that the paracellular pathway contributes to the cation selectivity of the medullary thick ascending limb. The cation selectivity of the paracellular shunt pathway may mainly account for the changes in VT which are either dependent on the luminal flow rate or transmural NaCl concentration gradient, while it may not contribute to the basal level of VT.

Interaction of cationic colloids at the surface of J774 cells: a kinetic analysis

We have characterized the binding of multilamellar colloids to J774 cells. Cationic colloids were shown to bind much more efficiently than neutral ones. Particle uptake by cells was followed by flow cytometry and fluorescence microscopy. Analysis of the kinetics of uptake of cationic particles indicated that binding on the cell surface occurred with two characteristic times. Analysis of the dissociation properties allowed discriminating between several alternative models for adsorption and led us to propose a mechanism that involved two independent classes of binding sites on the cell surface. One class of sites appeared to be governed by a classic mass action law describing a binding equilibrium. The other sites were populated irreversibly by particles made of 10% cationic lipids. This was observed in the absence of endocytosis, under conditions where both the equilibrium and the irreversible binding occurred at the cell surface. We determined the rate constants for the different steps. We found that the reversible association occurred with a characteristic time of the order of tens of seconds, whereas the irreversible binding took a hundred times longer. The presence of serum proteins in the incubation medium did not drastically affect the final uptake of the particles. In contrast, the capture of the particles by cells significantly dropped when the fraction of positively charged lipids contained in the colloids was decreased from 10% to 5%. Finally, the results will be discussed within a comprehensive model where cationic particles find labile binding sites in the volume of the pericellular network (glycocalyx and extracellular matrix) and less-accessible irreversible binding sites at the cell membrane itself.

Significance of lysine/glycine cluster structure in gastric H+,K+-ATPase

Gastric H+,K+-ATPase consists of alpha- and beta-subunits. The catalytic alpha-subunit contains a very unique structure consisting of lysine and glycine clusters, KKK(or KKKK)AG(G/R)GGGK-(K/R)K, in the amino-terminal cytoplasmic region. This structure is well conserved in all gastric H+,K+-ATPases from different animal species, and was postulated to be the site controlling the access of cations (or proton) to its binding site. In this report, we studied the role of this unique structure by expressing several H+,K+-ATPase mutants of the alpha-subunit together with the wild-type beta-subunit in HEK-293 cells. Even after replacing all the positively-charged amino acid residues (six lysines and one arginine) in the cluster with alanine or removing all the glycine residues in the cluster, the mutants preserved the H+,K+-ATPase activity, and showed similar affinity for ATP and K+ as well as similar pH profiles as those of wild-type H+,K+-ATPase, indicating that the cluster is not indispensable for H+,K+-ATPase activity and not directly involved in determination of the affinity for cation (proton).

Functional impairment of sinusoidal membrane transport of organic cations in rats with CCl4-induced hepatic failure

PURPOSE

The effect of CCl4-induced experimental hepatic failure (EHF) on the sequential hepatobiliary transport of model organic cations (OCs), triethylmethylammonium (TEMA), and tributylmethylammonium (TBuMA), was investigated in rats.

METHODS

EHF was induced by an i.p. injection of CCl4 at a dose of 1 ml/kg 24 hr prior to the transport study. The cumulative in vivo biliary excretion, in vitro hepatic uptake by isolated hepatocytes, in vitro efflux (i.e., release) from hepatocytes, and in vivo hepatobiliary excretion clearance were measured for normal and CCl4-EHF rats.

RESULTS

The CCl4-EHF decreased the apparent in vivo biliary clearance (CL(b)) and the in vitro maximum uptake rate (Vmax, uptake) of TBuMA by 66 and 48%, respectively. The CCl4-EHF had no effect on the CL(b) of TEMA. but decreased both the Vmax, uptake (59%) and the in vitro maximum hepatic efflux rate (Vmax, efflux) of TEMA (80%). On the contrary, the CCl4-EHF had no influence on the in vivo hepatobiliary excretion clearance (CL(exc)) of both OCs.

CONCLUSIONS

Transport systems for the OCs on the sinusoidal membrane (uptake and/or efflux), rather than those on the bile canalicular membrane (excretion) appear to be prone to damage by the CC14-EHF.

Effect of ionization on the variable uptake of valacyclovir via the human intestinal peptide transporter (hPepT1) in CHO cells

Carrier-mediated transport of valacyclovir (vacv), the L-valyl ester prodrug of acyclovir (acv), via the human peptide transporter (hPepT1) has been shown in Xenopus laevis oocytes and in cell lines such as Chinese hamster ovary (CHO) and Caco-2 transfected with the hPepT1 gene. However, significant differences in vacv uptake were observed in those models as extracellular pH varied. The purpose of this work was to characterize the interactions of various ionic species of vacv with the peptide transporter by overexpressing the transporter gene, hPepT1, in CHO cells. Based on the pK(a) values of vacv, it was determined that vacv exists as four different ionic species (di-cationic, cationic, neutral and anionic) with a predominance of cationic and neutral species at physiologically relevant pH conditions. Vacv uptake was shown to increase with increasing pH of the extracellular medium from 5.5 to 7.2. The uptake value was maximal at around pH 7.2 and did not vary for studies done at higher pH. Vacv uptake was concentration dependent and saturable at all pH conditions (5.5, 6.2, 6.8, 7.5 and 7.9) with apparent Michaelis-Menten constants, mean (S.D.), of 7.42(0.32), 6.64(1.20), 5.38(0.88), 2.69(0.23) and 2.23(0.33) mM, respectively. The current results demonstrate that the estimated affinities of the cationic and the neutral species of vacv with hPepT1 are significantly different (7.4 versus 1.2 mM, respectively). Given the axial and radial (microclimate) pH gradients known to exist in the intestine, the greater than six-fold difference in affinity constants suggests that intestinal pH fluctuations may significantly impact upon the variability of vacv uptake.

Characterization of L-arginine transporters in rat renal inner medullary collecting duct

Previous work from our laboratory has demonstrated that the inner medullary collecting duct (IMCD) expresses a large amount of nitric oxide synthase (NOS) activity. The present study was designed to characterize the transport of NOS substrate, L-arginine, in a suspension of bulk-isolated IMCD cells from the Sprague-Dawley rat kidney. Biochemical transport studies demonstrated an L-arginine transport system in IMCD cells that was saturable and Na(+) independent (n = 6). L-Arginine uptake by IMCD cells was inhibited by the cationic amino acids L-lysine, L-homoarginine, and L-ornithine (10 mmol/l each) and unaffected by the neutral amino acids L-leucine, L-serine, and L-glutamine. Both L-ornithine (n = 6) and L-lysine (n = 6) inhibited NOS enzymatic activity in a dose-dependent manner in IMCD cells, supporting the important role of L-arginine transport for NO production by this tubular segment. Furthermore, RT-PCR of microdissected IMCD confirmed the presence of cationic amino acid transporter CAT1 mRNA, whereas CAT2A, CAT2B, and CAT3 were not detected. These results indicate that L-arginine uptake by IMCD cells occurs via system y(+), is encoded by CAT1, and may participate in the regulation of NO production in this renal segment.

Effect of lamivudine on uptake of organic cations by rat renal brush-border and basolateral membrane vesicles

The effect of lamivudine on uptake of a representative organic cation, tetraethylammonium (TEA), by rat renal brush-border membrane vesicles (BBMV) and basolateral membrane vesicles (BLMV) has been investigated. The pH-driven uptake of TEA by BBMV (pHin = 6.0, pHout = 7.5) was inhibited by lamivudine. The IC50 value (concentration resulting in 50% inhibition) for the concentration-dependent effect of lamivudine on TEA uptake by BBMV after 30 s was 2668 microM whereas IC50 values for cimetidine and trimethoprim were < 2.5 microM and < 25 microM, respectively. The early uptake of TEA by BLMV was also reduced significantly by lamivudine. The IC50 value for the concentration-dependent effect of lamivudine on uptake of TEA by BLMV at 30 s was > 25 mM, whereas the IC50 values for cimetidine and trimethoprim were 2116 microM and 445 microM, respectively. These findings suggest that compared with other cationic drugs, such as trimethoprim and cimetidine, lamivudine is a weak inhibitor of organic cation transport into the tubules by the brush-border and basolateral membranes of renal epithelial cells. It is unlikely lamivudine will have any significant effect on the excretion of co-administered cationic drugs by the renal tubules.

A cationic lipid emulsion/DNA complex as a physically stable and serum-resistant gene delivery system

PURPOSE

To develop a non-viral gene delivery system in the form of an oil-in-water (o/w) lipid emulsion.

METHOD

Cationic lipid emulsions were formulated with soybean oil, 1,2-dioleoyl-sn-glycero-3-trimethylammonium-propane (DOTAP) as a cationic emulsifier and other co-emulsifiers. The physical characteristics of the lipid emulsion and the emulsion/DNA complex were determined. The in vitro transfection efficiency of the emulsion/DNA complex was determined in the presence of up to 90% serum.

RESULTS

The average droplet size and zeta potential of emulsions were ca. 180 nm and ca. +50 mV, respectively. Among the emulsions, a stable formulation was selected to form a complex with a plasmid DNA encoding chloramphenicol acetyltransferase. By increasing the ratio of emulsion to DNA. zeta-potential of the emulsion/DNA complex increased monotonously from negative to positive without any changes in the complex size. The complex was stable against DNase I digestion and an anionic poly-L-aspartic acid (PLAA). The complex delivered DNA into the cells successfully, and the transfection efficiency was not affected by complex formation time from 20 min to 2 h. More importantly, the cationic lipid emulsion facilitated the transfer of DNA in the presence of up to 90% serum.

CONCLUSIONS

The cationic lipid emulsion/DNA complex has physical stability and serum resistant properties for gene transfer.

Cationic amino acid transport through system y+L in erythrocytes of patients with lysinuric protein intolerance

We test the hypothesis that lysinuric protein intolerance (LPI), a rare autosomal recessive defect of cationic amino acid transport, results from the absence of the recently described y+L amino acid transporter. We compare fluxes of lysine (1 microM) into erythrocytes of normal subjects with those of patients homozygous for the LPI mutation. No significant differences in fluxes through system y+L in normal or LPI cells were found, excluding the possibility that system y+L cannot be expressed in patients with LPI. Reasons for supposing that there may be tissue-specific processing of two recently described genes encoding the y+L transporter are discussed. Polymerase chain reaction measurement of expression of these two genes in an erythroleukemic cell line suggests that alternatively there may be an as-yet-unidentified additional member of this gene family.

Reduction in water and metabolite apparent diffusion coefficients during energy failure involves cation-dependent mechanisms. A proton nuclear magnetic resonance study of rat cortical brain slices

Proton (1H) nuclear magnetic resonance (NMR) diffusion spectroscopy was used to assess apparent diffusion coefficients (ADCs) in rat brain slices. Aglycemic hypoxia caused reductions in the ADC of N-acetylaspartate (NAA) (0.15 to 0.09 x 10(-3) mm2/s) and "slow" diffusion coefficient (D2) of tissue water (0.51 to 0.37 x 10(-3) mm2/s), together with a 32+/-11% increase in tissue water volume, attributable to tissue swelling. The ADC and D2 reductions were diminished, however, by removing external Ca2+, and under 10 mmol/L Mg2+, normoxic diffusion coefficients persisted until 40 minutes of hypoxia. The data suggest that the shift of water into the intracellular space alone cannot satisfactorily explain the reduced cerebral diffusion upon energy failure and that external Mg2+ and Ca2+ play crucial modulatory roles.

Low potassium enhances sodium uptake in red-beet under moderate saline conditions

Due to the discrepancy in metabolic sodium (Na) requirements between plants and animals, cycling of Na between humans and plants is limited and critical to the proper functioning of bio-regenerative life support systems, being considered for long-term human habitats in space (e.g., Martian bases). This study was conducted to determine the effects of limited potassium (K) on growth, Na uptake, photosynthesis, ionic partitioning, and water relations of red-beet (Beta vulgaris L. ssp. vulgaris) under moderate Na-saline conditions. Two cultivars, Klein Bol, and Ruby Queen were grown for 42 days in a growth chamber using a re-circulating nutrient film technique where the supplied K levels were 5.0, 1.25, 0.25, and 0.10 mM in a modified half-strength Hoagland solution salinized with 50 mM NaCl. Reducing K levels from 5.0 to 0.10 mM quadrupled the Na uptake, and lamina Na levels reached -20 g kg-1 dwt. Lamina K levels decreased from -60 g kg-1 dwt at 5.0 mM K to -4.0 g kg-1 dwt at 0.10 mM K. Ruby Queen and Klein Bol responded differently to these changes in Na and K status. Klein Bol showed a linear decline in dry matter production with a decrease in available K, whereas for cv. Ruby Queen, growth was stimulated at 1.25 mM K and relatively insensitive to a further decreases of K down to 0.10 mM. Leaf glycinebetaine levels showed no significant response to the changing K treatments. Leaf relative water content and osmotic potential were significantly higher for both cultivars at low-K treatments. Leaf chlorophyll levels were significantly decreased at low-K treatments, but leaf photosynthetic rates showed no significant difference. No substantial changes were observed in the total cation concentration of plant tissues despite major shifts in the relative Na and K uptake at various K levels. Sodium accounted for 90% of the total cation uptake at the low K levels, and thus Na was likely replacing K in osmotic functions without negatively affecting the plant water status, or growth. Our results also suggest that cv. Ruby Queen can tolerate a much higher Na tissue concentration than cv. Klein Bol before there is any growth reduction. Grant numbers: 12180.

Drug release from cation exchange membrane in rabbit eye

Cations were adsorbed onto a poly(acrylic acid) (PAA) grafted poly(vinylidene fluoride) (PVDF) membrane that served as a cation exchange membrane. The aim of this study was to evaluate the effect of ionic strength of the adsorption medium on cation release from the PAA-PVDF membrane in the eye. Model cations, propranolol and timolol, were adsorbed onto the membranes in solutions with different ionic strengths (micron = 0.018 - 0.40) at pH 7.0. The circular drug-containing membranes were applied to both eyes of pigmented rabbits in the lower conjunctival sac. The membranes were well tolerated and well retained in the rabbit eye. Membranes containing either propranolol or timolol were removed from the eyes at preset time intervals, and the remaining drug content in the membranes was determined. The release rates of both propranolol and timolol decreased with increasing ionic strength of the adsorption medium. This was probably due to cationic exchange properties, as well as swelling properties of the membrane.

Adsorption of drugs onto a poly(acrylic acid) grafted cation-exchange membrane

The influence of pH, ionic strength and the concentration of albumin in the adsorption medium as well as the charge and lipophilicity of a model drug on their adsorption onto poly(acrylic acid) grafted poly(vinylidene fluoride) (PAA-PVDF) membranes was evaluated. The PAA-PVDF membrane is a responsive porous polymer membrane that we have studied for controlled drug delivery. Sodium salicylate (anionic), flunitrazepam (neutral), primidone (neutral), desipramine (cationic) and thioridazine (cationic) were used as model drugs. The extent of drug adsorption was dependent on pH. Drug adsorption was enhanced by the dissociation of the grafted PAA chains and by a positive charge and a high lipophilicity of the drug. Increasing the ionic strength of the medium retarded the adsorption of the cationic drugs. Interestingly, the present results showing that drugs are adsorbed onto the membrane while albumin is not adsorbed onto the membrane suggest that the PAA-PVDF membrane may be suitable for separating drugs from proteinaceous substances for subsequent monitoring and evaluation.

Effect of molecular charge on intestinal epithelial drug transport: pH-dependent transport of cationic drugs

The aim of this study was to investigate the effect of ionization on drug transport across the intestinal epithelium in order to include this effect in structure-absorption relationships. The pH-dependent permeation of one rapidly (alfentanil) and one slowly (cimetidine) transported basic model drug across Caco-2 cell monolayers was investigated. Both drugs had pK(a)values in the physiological pH range. The permeability coefficients (P(c)) of the model drugs were obtained at varying apical buffer pHs, thus varying the degree of drug ionization (from 5 to 95%). The relationship between P(c) and the fraction of the drug in un-ionized form (f(u)) was analyzed to delineate the permeability coefficients of the un-ionized (P(c,u)) and ionized (P(c,i)) forms of the drugs. Theoretical estimates of the pK(a) values were also calculated from ionization energies for each model compound. For both drugs, a linear increase in P(c) was observed with increasing f(u). Transport of the un-ionized form was 150- and 30-fold more rapid than transport of the ionized form for alfentanil and cimetidine, respectively. However, when f(u) <0.1, the contribution of the ionized form was significant. Because f(u) is <0.1 over the entire physiological pH range for a large number of drugs, these results will have implications on predictions of in vivo intestinal drug absorption both from in vitro studies in cell cultures and from computed structural properties of drug molecules.

Regulation of cation transport pathways and glycolytic enzyme activity by alterations in red cell volume

In the presence of NH4Cl and hypotonic solutions, Rana balcanica red cells respond by increasing their volume. The stimulation of cellular volume by hypotonicity is more rapid than that of NH4Cl, while the maximum value is less than that observed in the presence of NH4Cl. Depending on the cause of swelling, (nct uptake of NH4Cl or decrease in external osmolality) cells show specific responses. The NH4Cl treatment causes a significant increase in intracellular Na+, from 5.14 +/- 0.78 to 29.84 +/- 0.47 mmoles l-1 cell, while hypotonicity leads to a significant decrease of this cation, to 3.85 +/- 0.25 mmoles l-1 cell in relation to the control, after 30 min of incubation of Rana balcanica erythrocytes. In addition, amiloiride significantly reverses the NH4Cl effect with respect to intracellular Na+. Both treatments cause a significant K+ loss in comparison with controls. Two glycolytic enzymes glyceraldehyde phosphate dehydrogenase (GAPDH) and pyruvate kinase (PK) of Rana balcanica haemolysate were found to respond to the NH4Cl effect by significantly decreasing their activity.

ATP- and glutathione-dependent transport of chemotherapeutic drugs by the multidrug resistance protein MRP1

The present study was performed to investigate the ability of the multidrug resistance protein (MRPI) to transport different cationic substrates in comparison with MDR1-P-glycoprotein (MDR1). Transport studies were performed with isolated membrane vesicles from in vitro selected multidrug resistant cell lines overexpressing MDR1 (A2780AD) or MRP1 (GLC4/Adr) and a MRP1-transfected cell line (S1(MRP)). As substrates we used 3H-labelled derivatives of the hydrophilic monoquaternary cation N-(4',4'-azo-in-pentyl)-21-deoxy-ajmalinium (APDA), the basic drug vincristine and the more hydrophobic basic drug daunorubicin. All three are known MDR1-substrates. MRP1 did not mediate transport of these substrates per se. In the presence of reduced glutathione (GSH), there was an ATP-dependent uptake of vincristine and daunorubicin, but not of APDA, into GLC4/Adr and S1(MRP) membrane vesicles which could be inhibited by the MRP1-inhibitor MK571. ATP- and GSH-dependent transport of daunorubicin and vincristine into GLC4/Adr membrane vesicles was inhibited by the MRP1-specific monoclonal antibody QCRL-3. MRP1-mediated daunorubicin transport rates were dependent on the concentration of GSH and were maximal at concentrations > or = 10 mM. The apparent KM value for GSH was 2.7 mM. Transport of daunorubicin in the presence of 10 mM GSH was inhibited by MK571 with an IC50 of 0.4 microM. In conclusion, these results demonstrate that MRP1 transports vincristine and daunorubicin in an ATP- and GSH-dependent manner. APDA is not a substrate for MRP1.

Uptake and synthesis of compatible solutes as microbial stress responses to high-osmolality environments

All microorganisms possess a positive turgor, and maintenance of this outward-directed pressure is essential since it is generally considered as the driving force for cell expansion. Exposure of microorganisms to high-osmolality environments triggers rapid fluxes of cell water along the osmotic gradient out of the cell, thus causing a reduction in turgor and dehydration of the cytoplasm. To counteract the outflow of water, microorganisms increase their intracellular solute pool by amassing large amounts of organic osmolytes, the so-called compatible solutes. These osmoprotectants are highly congruous with the physiology of the cell and comprise a limited number of substances including the disaccharide trehalose, the amino acid proline, and the trimethylammonium compound glycine betaine. The intracellular amassing of compatible solutes as an adaptive strategy to high-osmolality environments is evolutionarily well-conserved in Bacteria, Archaea, and Eukarya. Furthermore, the nature of the osmolytes that are accumulated during water stress is maintained across the kingdoms, reflecting fundamental constraints on the kind of solutes that are compatible with macromolecular and cellular functions. Generally, compatible solutes can be amassed by microorganisms through uptake and synthesis. Here we summarise the molecular mechanisms of compatible solute accumulation in Escherichia coli and Bacillus subtilis, model organisms for the gram-negative and gram-positive branches of bacteria.

Postnatal development of organic cation transport in the rat liver

Previous studies have demonstrated that the small permanently charged organic cation 1-methyl-4-phenylpyridinium (MPP+) is avidly taken up by rat hepatocytes. The aim of this study was to characterise the postnatal development of hepatic uptake of organic cations, using the model compound MPP+. Accumulation of [3H]MPP+ by liver slices obtained from rats ranging from 1 day to 7 weeks was measured, and the effect of a series of compounds on [3H]MPP+ uptake was examined. The accumulation of [3H]MPP+ by liver slices was similar in adult (87.5 +/- 19.9 pmol g-1; n = 7) and neonatal rats (110.6 +/- 11.5 pmol g-1; n = 15). Verapamil, quinidine (100 microM) and progesterone (200 microM) produced very marked reductions on [3H]MPP+ uptake at all ages, and the inhibitory effect of verapamil and quinidine was maximum in livers from 1-day-old rats. Bilirubin (200 microM) significantly reduced [3H]MPP+ uptake by liver slices from 1 day, 1 week and 7-week-old rats. However, [3H]MPP+ accumulation was reduced by cimetidine, vinblastine and daunomycin (100 microM) in 1-day-old rats, but the effect of these drugs disappeared as the animals age increased. These findings demonstrate that hepatic organic cation uptake capacity is remarkably high shortly after birth and suggest that at least two distinct uptake mechanisms are involved in this process. These uptake systems are the type I hepatic transporter of organic cations, active from birth to adulthood, and P-glycoprotein, active only in very young rats.

Membrane transporters in drug disposition

Many clinically used drugs and their metabolites as well as a variety of environmental toxins are organic cations at physiologic pH. Secretion in the renal proximal tubule constitutes a major pathway in the elimination of organic cations. In this report, the results of studies recently performed in this laboratory are presented. First, the molecular cloning of a novel splice variant of organic cation transporter from rat kidney (rOCT1A) is described. The functional characteristics of the transporter are discussed along with the implications of RNA splicing in enhancing transporter diversity. Second, the molecular cloning of the first human organic cation transporter (hOCT1) is described. Distinct interspecies differences in the tissue distribution and function of this transporter is presented. These studies have paved the way for elucidating molecular structure function relationships of organic cation transporters and for determining their physiologic role in drug absorption and elimination. The cloned transporters can be used in mammalian expression systems for screening candidate compounds identified during drug discovery and development and in the in vivo prediction of the pharmacokinetics of therapeutic agents.

Relationship between chemical structure and physicochemical properties of series of bulky organic cations and their hepatic uptake and biliary excretion rates

To obtain more insight in the relationship between physicochemical properties of cationic drugs and their hepatobiliary transport rate, a series of 12 aminosteroidal neuromuscular blocking agents (NMBAs), supplemented with data of four related NMBAs from the literature, were investigated in the isolated perfused rat liver. A significant correlation was found between plasma protein binding and the partition coefficient octanol/Krebs (log P), confirming results from the literature with other organic cations. Evidence was found for a saturable hepatic uptake of several NMBAs, indicating that carrier-mediated uptake processes are involved. Hepatic uptake rate was closely related to the lipophilicity of the compounds; the initial extraction ratio, the apparent clearance and the intrinsic clearance were significantly correlated to log P. We did not find a significant correlation between biliary clearance and lipophilicity in the current series of compounds. Pharmacokinetics analysis of perfusate disappearance and biliary excretion data revealed that a considerable fraction of the dose of these bulky organic cations is stored in the liver and seems to not be directly available for biliary excretion. This finding is in line with earlier observations showing a pronounced accumulation of this type of compounds in mitochondria and lysosomes.