Endogenous D-glucose transport in oocytes of Xenopus laevis

The β2-adrenergic receptor in the apical membrane of intestinal enterocytes senses sugars to stimulate glucose uptake from the gut

The presence of sugar in the gut causes induction of SGLT1, the sodium/glucose cotransporter in intestinal epithelial cells (enterocytes), and this is accompanied by stimulation of sugar absorption. Sugar sensing was suggested to involve a G-protein coupled receptor and cAMP - protein kinase A signalling, but the sugar receptor has remained unknown. We show strong expression and co-localization with SGLT1 of the β2-adrenergic receptor (β ₂-AR) at the enterocyte apical membrane and reveal its role in stimulating glucose uptake from the gut by the sodium/glucose-linked transporter, SGLT1. Upon heterologous expression in different reporter systems, the β ₂-AR responds to multiple sugars in the mM range, consistent with estimated gut sugar levels after a meal. Most adrenergic receptor antagonists inhibit sugar signaling, while some differentially inhibit epinephrine and sugar responses. However, sugars did not inhibit binding of I¹²⁵-cyanopindolol, a β ₂-AR antagonist, to the ligand-binding site in cell-free membrane preparations. This suggests different but interdependent binding sites. Glucose uptake into everted sacs from rat intestine was stimulated by epinephrine and sugars in a β ₂-AR-dependent manner. STD-NMR confirmed direct physical binding of glucose to the β ₂-AR. Oral administration of glucose with a non-bioavailable β ₂-AR antagonist lowered the subsequent increase in blood glucose levels, confirming a role for enterocyte apical β ₂-ARs in stimulating gut glucose uptake, and suggesting enterocyte β ₂-AR as novel drug target in diabetic and obese patients. Future work will have to reveal how glucose sensing by enterocytes and neuroendocrine cells is connected, and whether β ₂-ARs mediate glucose sensing also in other tissues.

A hyperpolarization-activated ion current of amphibian oocytes

A comparative analysis of a hyperpolarization-activated ion current present in amphibian oocytes was performed using the two-electrode voltage-clamp technique in Xenopus laevis, Xenopus tropicalis, and Ambystoma mexicanum. This current appears to be driven mainly by Cl(-) ions, is independent of Ca(2+), and is made evident by applying extremely negative voltage pulses; it shows a slow activating phase and little or no desensitization. The pharmacological profile of the current is complex. The different channel blocker used for Cl(-), K(+), Na(+) and Ca(2+) conductances, exhibited various degrees of inhibition depending of the species. The profiles illustrate the intricacy of the components that give rise to this current. During X. laevis oogenesis, the hyperpolarization-activated current is present at all stages of oocytes tested (II-VI), and the amplitude of the current increases from about 50 nA in stage I to more than 1 μA in stage VI; nevertheless, there was no apparent modification of the kinetics. Our results suggest that the hyperpolarization-activated current is present both in order Anura and Urodela oocytes. However, the electrophysiological and pharmacological characteristics are quite perplexing and seem to suggest a mixture of ionic conductances that includes the activation of both anionic and cationic channels, most probably transiently opened due to the extreme hyperpolarizion of the plasma membrane. As a possible mechanism for the generation of the current, a kinetic model which fits the data suggests the opening of pores in the plasma membrane whose ion selectivity is dependent on the extracellular Cl(-) concentration. The extreme voltage conditions could induce the opening of otherwise latent pores in plasma membrane proteins (i.e., carriers), resembling the ´slippage´ events already described for some carriers. These observations should be valuable for other groups trying to express cloned, voltage-dependent ion channels in oocytes of amphibian in which hyperpolarizing voltage pulses are applied to activate the channels.

Human SLC2A9a and SLC2A9b isoforms mediate electrogenic transport of urate with different characteristics in the presence of hexoses

Human SLC2A9 (GLUT9) is a novel high-capacity urate transporter belonging to the facilitated glucose transporter family. In the present study, heterologous expression in Xenopus oocytes has allowed us to undertake an in-depth radiotracer flux and electrophysiological study of urate transport mediated by both isoforms of SLC2A9 (a and b). Addition of urate to SLC2A9-producing oocytes generated outward currents, indicating electrogenic transport. Urate transport by SLC2A9 was voltage dependent and independent of the Na(+) transmembrane gradient. Urate-induced outward currents were affected by the extracellular concentration of Cl(-), but there was no evidence for exchange of the two anions. [(14)C]urate flux studies under non-voltage-clamped conditions demonstrated symmetry of influx and efflux, suggesting that SLC2A9 functions in urate efflux driven primarily by the electrochemical gradient of the cell. Urate uptake in the presence of intracellular hexoses showed marked differences between the two isoforms, suggesting functional differences between the two splice variants. Finally, the permeant selectivity of SLC2A9 was examined by testing the ability to transport a panel of radiolabeled purine and pyrimidine nucleobases. SLC2A9 mediated the uptake of adenine in addition to urate, but did not function as a generalized nucleobase transporter. The differential expression pattern of the two isoforms of SLC2A9 in the human kidney's proximal convoluted tubule and its electrogenic transport of urate suggest that these transporters play key roles in the regulation of plasma urate levels and are therefore potentially important participants in hyperuricemia and hypouricemia.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

Charge-to-substrate ratio during organic cation uptake by rat OCT2 is voltage dependent and altered by exchange of glutamate 448 with glutamine

Uptake of substrate and electric charge was measured simultaneously in voltage-clamped Xenopus laevis oocytes expressing rat organic cation transporter 2 (rOCT2). At 0 mV, saturating substrate concentrations induced uptake of more positive elementary charges than monovalent organic cations, with charge-to-substrate ratios of 1.5 for guanidinium+, 3.5 for tetraethylammonium+, and 4.0 for 1-methyl-4-phenylpyridinium+. At negative holding potentials, the charge-to-substrate ratios decreased toward unity. At 0 mV, charge-to-substrate ratios higher than unity were observed at different extracellular pH and after replacement of extracellular Na+, K+, Ca2+, Mg2+, and/or Cl−. Charge-to-substrate ratios were not influenced by intracellular succinate2− or glutarate2−. The effects of membrane potential and ion substitution strongly suggest that the surplus of transported positive charge is not generated by passive ion permeabilities. Rather, we hypothetize that small cations are taken up together with organic cation substrates whereas the outward reorientation of the empty transporter is electroneutral. Nonselective cotransport of small cations was supported by the three-dimensional structures of rOCT2 in its inward-facing and outward-facing conformations, which we determined by homology modeling based on known corresponding structures of H+-lactose permease of E. coli, and by functional analysis of OCT mutants. In our model, the innermost cavity of the outward-open binding cleft is negatively charged by Glu448 and Asp475, whereas the inward-open innermost cavity is electroneutral, containing Asp379, Asp475, Lys215, and Arg440. Substitution of Glu448 by glutamine reduced the charge-to-TEA+ ratio at 0 mV to unity. The observed charge excess associated with organic cation uptake into depolarized cells may contribute to tubular damage in renal failure.

Endogenous lactate transport in Xenopus laevis oocyte: dependence on cytoskeleton and regulation by protein kinases

Carbon flux in Xenopus laevis oocyte is glycogenic and an endogenous monocarboxylate transporter is responsible for intracellular lactate uptake. The aim of the present study was to determine if direct activation of protein kinases C and A modulates the activity of lactate transporter, as well as to investigate the possible role of cytoskeleton in these regulatory phenomena. The modulation was studied in isolated Xenopus oocytes of stage V-VI by measuring (14)C-lactate uptake, both in the absence and in the presence of cytoskeletal-perturbing toxins. We found that the basal lactate transporter activity depends on the integrity of the cytoskeleton since it is partially inhibited by cytoskeleton disorganisation. Both PKA and PKC activation caused a significant decrease in transport activity and this decrease could be blocked by specific protein kinase inhibitors. The evidenced effects were not additive. Transport inhibition was annulled by agents that destabilize actin filaments or microtubules. We conclude that both protein kinases A and C, whose effects are mediated by cytoskeleton, negatively regulate the endogenous lactate transporter of Xenopus oocyte, suggesting that these kinases may have a role in the control of cytosolic pyruvate/lactate pool in the oocyte.

Substrate specificity of a chimera made from Xenopus SGLT1-like protein and rabbit SGLT1

To characterize the sugar translocation pathway of Na(+)/glucose cotransporter type 1 (SGLT1), a chimera was made by substituting the extracellular loop between transmembrane domain (TM) 12 and TM13 of Xenopus SGLT1-like protein (xSGLT1L) with the homologous region of rabbit SGLT1. The chimera was expressed in Xenopus oocytes and its transport activity was measured by the two-microelectrode voltage-clamp method. The substrate specificity of the chimera was different from those of xSGLT1L and SGLT1. In addition the chimera's apparent Michaelis-Menten constant (K(m)) for myo-inositol, 0.06 mM, was about one fourth of that of xSGLT1L, 0.25 mM, while the chimera's apparent K(m) for d-glucose, 0.8 mM, was about one eighth of that of xSGLT1L, 6.3 mM. Our results suggest that the extracellular loop between TM12 and TM13 participates in the sugar transport of SGLT1.

Different activation mechanisms of cystic fibrosis transmembrane conductance regulator expressed in Xenopus laevis oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP sensitive Cl- channel that is defective in cystic fibrosis (CF). The most frequent mutation, namely deltaF508-CFTR, accounts for 66% of CF. Here we show that cAMP-activation of CFTR occurs via at least two distinct pathways: activation of CFTR molecules already present in the plasma membrane and protein kinase A (PKA)-mediated vesicular transport of new CFTR molecules to the plasma membrane and functional insertion into the membrane. We investigated the mechanisms that are responsible for these activation pathways using the Xenopus laevis oocytes expression system. We expressed CFTR and recorded continuously membrane current (Im), conductance (Gm) and capacitance (Cm), which is a direct measure of membrane surface area. Expression of CFTR alone did not change the plasma membrane surface area. However, activation of CFTR with cAMP increased Im, Gm and Cm while deltaF508-CFTR-expressing oocytes showed no response on cAMP. Inhibition of protein kinase A or buffering intracellular Ca2+ abolished the cAMP-induced increase in Cm while increases of Im and Gm were still present. ATP or the xanthine derivative 8-cyclopentyl-1,3-dipropylxanthine (CPX) did not further activate CFTR. Insertion of pre-formed CFTR into the plasma membrane could be prevented by compounds that interfere with intracellular transport mechanisms such as primaquine, brefeldin A, nocodazole. From these data we conclude that cAMP activates CFTR by at least two distinct pathways: activation of CFTR already present in the plasma membrane and exocytotic delivery of new CFTR molecules to the oocyte membrane and functional insertion into it.

An endogenous monocarboxylate transport in Xenopus laevis oocytes

We investigated the existence of an endogenous system for lactate transport in Xenopus laevis oocytes. (36)Cl-uptake studies excluded the involvement of a DIDS-sensitive anion antiporter as a possible pathway for lactate movement. L-[(14)C]lactate uptake was unaffected by superimposed pH gradients, stimulated by the presence of Na(+) in the incubating solution, and severely reduced by the monocarboxylate transporter inhibitor p-chloromercuribenzenesulphonate (pCMBS). Transport exhibited a broad cation specificity and was cis inhibited by other monocarboxylates, mostly by pyruvate. These results suggest that lactate uptake is mediated mainly by a transporter and that the preferred anion is pyruvate. [(14)C]pyruvate uptake exhibited the same pattern of functional properties evidenced for L-lactate. Kinetic parameters were calculated for both monocarboxylates, and a higher affinity for pyruvate was revealed. Various inhibitors of monocarboxylate transporters reduced significantly pyruvate uptake. These studies demonstrate that Xenopus laevis oocytes possess a monocarboxylate transport system that shares some functional features with the members of the mammalian monocarboxylate cotransporters family, but, in the meanwhile, exhibits some particular properties, mainly concerning cation specificity.

Cloning and functional expression of an SGLT-1-like protein from the Xenopus laevis intestine

A cDNA encoding an Na+-glucose cotransporter type 1 (SGLT-1)-like protein was cloned from the Xenopus laevis intestine by the 5'- and 3'-rapid amplification of cDNA ends method. The deduced amino acid sequence was 673 residues long, with a predicted mass of 74.1 kDa and 52-53% identity to mammalian SGLT-1s. This gene was expressed in the small intestine and kidney, reflecting a tissue distribution similar to that of SGLT-1. The function of the protein was studied using the two-microelectrode voltage-clamp technique after injection of cRNA into Xenopus laevis oocytes. Perfusion with myo-inositol elicited about twofold larger inward currents than perfusion with D-glucose. The order of the substrate specificity was myo-inositol > D-glucose > D-galactose >/= alpha-methyl-D-glucoside. The current induced by myo-inositol increased with membrane hyperpolarization and depended on external myo-inositol and Na+: the apparent Michaelis-Menten constant was 0.25 +/- 0.07 (SD) mM with myo-inositol, whereas the apparent concentration for half-maximal activation was 12.5 +/- 1.0 mM and the Hill coefficient was 1.6 +/- 0.1 with Na+. In conclusion, the cloned protein shares features with both SGLT-1 and the Na+-myo-inositol cotransporter.

Stretch-independent activation of the mechanosensitive cation channel in oocytes of Xenopus laevis

Oocytes of the South African clawed toad Xenopus laevis possess in their plasma membrane a so-called stretch-activated cation channel (SAC) which is activated by gently applying positive or negative pressure (stretch) to the membrane patch containing the channels. We show here that this mechanosensitive channel acted as a spontaneously opening, stretch-independent non-selective cation channel (NSCC) in more than half of the oocytes that we investigated. In 55% of cell-attached patches (total number of patches, 58) on 30 oocytes from several different donors, we found NSCC opening events. These currents were increased by elevating the membrane voltage or raising the temperature. NSCC and SAC currents shared some properties regarding the relative conductances of Na+>Li+>Ca2+, gating behaviour and amiloride sensitivity. Stretch-independent currents could be clearly distinguished from stretch induced SAC currents by their voltage and temperature dependence. Open events of NSCC increased strongly when temperature was raised from 21 to 27 degrees C. NSCC currents could be partly inhibited by high concentrations of extracellular Gd3+ and amiloride (100 and 500 microM, respectively). We further show exemplarily that NSCC can seriously hamper investigations when oocytes are used for the expression of foreign ion channels. In particular, NSCC complicated investigations on cation channels with small conductance as we demonstrate for a 4 pS epithelial Na+ channel (ENaC) from guinea pig distal colon. Our studies on NSCCs suggest the involvement of these channels in oocyte temperature response and ion transport regulation. From our results we suggest that NSCC and SAC currents are carried by one protein operating in different modes.

Effect of cross-linkers on the structure and function of pig-renal sodium-glucose cotransporters after papain treatment

Kidney brush-border membranes contain two sodium-dependent glucose transporters, one with low and one with high affinity for phlorizin, the specific inhibitor of these transporters. Using Scatchard analysis of phlorizin binding and Western blotting with specific antibodies against these transporters, we demonstrate in this study that although both transporters were proteolysed by papain treatment, only the high-affinity phlorizin-binding sites were decreased. Papain treatment followed by cross-linking with homobifunctional disuccinimidyl tartarate restored only the structure of the low-affinity phlorizin-binding protein (approx. molecular mass 70 kDa) without modifying the phlorizin-binding sites. When disuccinimidyl tartarate was replaced with dithiobis(succinimidyl acetate), another homobifunctional cross-linker with a higher spacer arm, the low- and high-affinity sites were both restored, with reappearance of two phlorizin-binding proteins with approx. molecular masses of 70 and 120 kDa. We conclude that high-affinity phlorizin-binding sites depend on the presence of the heterodimeric 120 kDa protein.

Electrophysiological analysis of the function of the mammalian renal peptide transporter expressed in Xenopus laevis oocytes

1. To gain information on the mode of operation of the renal proton-coupled peptide transporter PepT2, voltage clamp studies were performed in Xenopus laevis oocytes expressing the rabbit renal PepT2. 2. Using differently charged glycyl-dipeptides we show that PepT2 translocates these dipeptides by an electrogenic pH-dependent process that is essentially independent of the substrate net charge. The apparent substrate affinities are in the micromolar range (2-50 microM) between pH 5.5 and 7.4 and membrane potentials of +/- 0 to -50 mV. 3. Maximal substrate-evoked inward currents (Imax) are affected by membrane voltage (Vm) and extracellular pH (pHo). Potential-dependent interactions of H+/H3O+ with PepT2 seem to be mediated by a single low affinity binding site and PepT2 remains pH dependent at all voltages. 4. The effects of voltage on apparent Imax and substrate affinity display an inverse relationship. As Vm is altered from -50 to -150 mV substrate affinities decrease 10- to 50-fold whereas apparent Imax increases almost 10-fold. 5. Even at saturating H+/H3O+ and dipeptide concentrations the I-V curves did not show saturation at negative membrane potentials, suggesting that other steps in the reaction cycle and not the ligand affinity changes are rate limiting. These are possibly the conformational changes of the empty and/or loaded transporters. 6. These findings demonstrate that not only substrate affinities but also other kinetic characteristics of PepT2 differ markedly from those of the intestinal peptide transporter isoform PepT1.

cAMP-activation of amiloride-sensitive Na+ channels from guinea-pig colon expressed in Xenopus oocytes

Guinea-pig distal colonic mRNA injection into Xenopus laevis oocytes resulted in expression of functional active epithelial Na+ channels in the oocyte plasma membrane. Poly(A)+ RNA was extracted from distal colonic mucosa of animals fed either a high-salt (HS) or a low-salt (LS) diet. The electrophysiological properties of the expressed amiloride-sensitive Na+ conductances were investigated by conventional two-electrode voltage-clamp and patch-clamp measurements. Injection of poly(A)+ RNA from HS-fed animals [from hereon referred to as HS-poly(A)+ RNA] into oocytes induced the expression of amiloride-sensitive Na+ conductances. On the other hand, oocytes injected with poly(A)+ RNA from LS-fed animals [LS-poly(A)+ RNA] expressed a markedly larger amount of amiloride-blockable Na+ conductances. LS-poly(A)+ RNA-induced conductances were completely inhibitable by amiloride with a Ki of 77 nM, and were also blocked by benzamil with a Ki of 1.8 nM. 5-(N-Ethyl-N-isopropyl)-amiloride (EIPA), even in high doses (25 "mu"M), had no detectable effect on the Na+ conductances. Expressed amiloride-sensitive Na+ channels could be further activated by cAMP leading to nearly doubled clamp currents. When Na+ was replaced by K+, amiloride (1 "mu"M) showed no effect on the clamp current. Single-channel analysis revealed slow gating behaviour, open probabilities (Po) between 0.4 and 0.9, and slope conductances of 3. 8 pS for Na+ and 5.6 pS for Li+. The expressed channels showed to be highly selective for Na+ over K+ with a permeability ratio PNa/PK > 20. Amiloride (500 nM) reduced channel Po to values < 0.05. All these features make the guinea-pig distal colon of LS-fed animals an interesting mRNA source for the expression of highly amiloride-sensitive Na+ channels in Xenopus oocytes, which could provide new insights in the regulatory mechanism of these channels.

The Ca(2+)-induced leak current in Xenopus oocytes is indeed mediated through a Cl- channel

Defolliculated oocytes of Xenopus laevis responded to removal of external divalent cations with large depolarizations and, when voltage clamped, with huge currents. Single channel analysis revealed a Cl- channel with a slope conductance of about 90 pS at positive membrane potentials with at least four substates. Single channel amplitudes and mean channel currents had a reversal potential of approximately -15 mV as predicted by the Nernst equation for a channel perfectly selective for Cl-. Readdition of Ca2+ immediately inactivated the channel and restored the former membrane potential or clamp current. The inward currents were mediated by a Ca2+ inactivated Cl- channel (CaIC). The inhibitory potency of Ca2+ was a function of the external Ca2+ concentration with a half maximal blocker concentration of about 20 microM. These channels were inhibited by the Cl- channel blockers flufenamic acid, niflumic acid and diphenylamine-2-carboxylate (DPC). In contrast, 4,4'-acetamido-4'-isothiocyanatostilbene-2, 2'-disulfonicacid (SITS), another Cl- channel blocker, led to activation of this Cl- channel. Like other Cl- channels, the CaIC was activated by cytosolic cAMP. Extracellular ATP inhibited the channel while ADP was without any effect. Injection of phorbol 12-myristate 13-acetate (PMA), a protein kinase C activating phorbol ester, stimulated the Cl- current. Cytochalasin D, an actin filament disrupting compound, reversibly decreased the clamp current demonstrating an influence of the cytoskeleton. The results indicate that removal of divalent cations activates Cl- channels in Xenopus oocytes which share several features with Cl- channels of the CLC family. The former so-called leak current of oocytes under divalent cation-free conditions is nothing else than an activation of Cl- channels.

Amiloride-sensitive Na+ conductance in native Xenopus oocytes

Endogenous Na+ conductances in the plasma membrane of oocytes of the South African clawed toad Xenopus laevis were investigated by microelectrode techniques and influx measurements. Removal of Na+ from the bath solution under voltage clamp conditions led to a decrease in the clamp current indicating the existence of native Na+ conductances. The observed current was voltage dependent but showed no marked rectification. Amiloride (10 microM) blocked this Na+ current reversibly. However, amiloride analogues such as benzamil and phenamil had no effect on this Na+ conductance. The Na+/H(+)-exchanger blocker EIPA (5-(N-ethyl-N-isopropyl)amiloride), another amiloride analogue, also had no effect thereby excluding a possible involvement of the Na+/H+ exchanger. The Na+ mediated current had a reversal potential of about 50 mV suggesting high selectivity of these Na+ conductances for Na+ over other monovalent cations. When Na+ was replaced by K+ in the bath solution, amiloride had no effect on the clamp current over the whole potential range demonstrating that only Na+ but not K+ can enter the cell via the investigated conductances. In radio tracer experiments 22Na+ influx into oocytes was nearly halved in presence of amiloride (10 microM), whereas benzamil and phenamil again failed to influence 22Na+ influx. These results suggest that the endogenous amiloride-sensitive Na+ conductance belongs to a new class of channels which is quite different from amiloride-sensitive epithelial Na+ channels.

Two endogenous methyl-alpha-D-glucopyranoside uptake activities in Xenopus oocytes

Endogenous methyl-alpha-D-glucopyranoside (AMG) uptake in Xenopus oocytes was measured with 14C-labeled AMG. Two AMG uptake activities, one Na(+)-dependent and the other Na(+)-independent, were observed in oocytes incubated with 2 mM AMG. However, only Na(+)-dependent uptake was observed at 0.1 mM AMG. Na(+)-dependent AMG uptake was attributed to the endogenous Na+/glucose co-transporter since it was inhibited by phlorizin. On the other hand, Na(+)-independent AMG uptake was inhibited by cytochalasin B or 2-deoxy-D-glucose. This suggests that AMG can be transported by the endogenous facilitated glucose transporter as well as by the endogenous co-transporter. In addition, a substrate range of the endogenous co-transporter was examined by competition experiments.

Insulin but not progesterone promotes the biosynthesis of glycogen in Xenopus laevis oocytes: implications on the control of glycogen synthase by phosphorylation, dephosphorylation

Insulin, the well-known hypoglycemic hormone, mimics progesterone in promoting the resumption of meiosis within the oocyte of Xenopus laevis. Both hormones exert their action through the inhibition of protein kinases and the activation of protein phosphatases. Because glycogen synthase is an enzyme regulated by a kinases/phosphatases cascade, we investigated the effect of insulin and progesterone on the regulation of glycogen synthesis and glycogen synthase throughout the oogenesis of Xenopus laevis oocytes. In this framework the maximal activity of synthase "a" is concomitant with the vitellogenic period characterized by a drastic increase in the amount of glycogen. Oocyte glycogen synthase is inhibited by cAMP-dependent phosphorylation and stimulated by 20 mM Mg2+. The magnesium effect is inhibited by mu molar concentrations of okadaic acid and suggests that oocyte glycogen synthase is activated by dephosphorylation achieved by protein phosphatase-1. The okadaic acid effect is itself thwarted by the specific inhibitor of protein kinase A, confirming the role of this kinase in the regulation of glycogen synthase. Finally, working on intact ripe oocytes, we showed that insulin but not progesterone increases glycogen synthesis and glycogen synthase "a" activity and lowers the rates of phosphorylation, especially in the glycogen-bound proteins.

Activation of osmotically-activated potassium transporters after injection of mRNA from A6 cells in Xenopus oocytes

The different potassium pathways, under iso-osmotic or hypo-osmotic conditions, were examined in Xenopus oocytes that were micro-injected with mRNA from A6 cells. Hypo-osmotically stimulated 86Rb (K+) effluxes could be measured from intact oocytes 1-4 days after injection of 25 ng of poly (A)+ RNA isolated from A6 cells. 86Rb (K+) effluxes were 2.2 times higher from oocytes micro-injected with 25 ng of poly(A)+ RNA, than from water injected control oocytes. Water-injected oocytes themselves, however, were 7-fold more responsive to a hypo-osmotic shock than non-injected Xenopus oocytes. There was no significant effect of the different K+ transport blockers tested (TEA, bumetanide, glybenclamide or quinidine) on the endogenous 86Rb (K+) effluxes from non-injected oocytes in either iso- or hypo-osmotic media. The 86Rb (K+) effluxes from water-and mRNA-injected oocytes in hypo-osmotic media were both inhibited by TEA. In mRNA-injected oocytes the increase in 86Rb (K+) transport following a medium dilution was also inhibited in the presence of glybenclamide or bumetanide. The present study reports that the activation of hypo-osmotically-activated potassium transporters in the oocytes of Xenopus laevis. after injection of mRNA from A6 cells differs quantitatively and in part qualitatively (glybenclamide-sensitivity) from the endogenous K+ pathways of non-injected and of water-injected Xenopus oocytes.

Function and presumed molecular structure of Na(+)-D-glucose cotransport systems

Functional characterization of Na(+)-D-glucose cotransport in intestine and kidney indicates the existence of heterogeneous Na(+)-D-glucose cotransport systems. Target size analysis of the transporting unit and model analysis of substrate binding have been performed and proteins have been cloned which mediate (SGLT1) and modulate (RS1) the expression of Na(+)-D-glucose cotransport. The experiments support the hypothesis that functional Na(+)-D-glucose cotransport systems in mammals are composed of two SGLT1-type subunits and may contain one or two RS1-type proteins. SGLT1 contains up to twelve membrane-spanning alpha-helices, whereas RS1 is a hydrophilic extracellular protein which is anchored in the brush-border membrane by a hydrophobic alpha-helix at the C-terminus. SGLT1 alone is able to translocate glucose together with sodium; however, RS1 increases the Vmax of transport expressed by SGLT1. In addition, the biphasic glucose dependence of transport, which is typical for kidney and has been often observed in intestine, was only obtained after coexpression of SGLT1 and RS1.

Characterization of the endogenous Na(+)-K(+)-2Cl- cotransporter in Xenopus oocytes

Over time, Xenopus laevis changed from producing stage V and VI oocytes with little native Na(+)-K(+)-2Cl- cotransport activity to those with substantial activity. In oocytes with high endogenous activity, K+ uptake, using the tracer 86Rb+ was approximately 20 pmol.min-1.oocyte-1 in the presence of blockers of Na(+)-K(+)-ATPase and conductive K+ transport. Bumetanide (10 microM) inhibited > 90% of this uptake, suggesting involvement of Na(+)-K(+)-2Cl- cotransport. This was confirmed by two observations that are found in this cotransporter in other tissues: 1) The related diuretics, thiobenzmetanide [50% inhibitory concentration (IC50), 2 x 10(-11) M] > bumetanide (IC50, 7 x 10(-8) M) > furosemide (IC50, 2.5 x 10(-6) M) inhibited the cotransporter in a dose-dependent manner. 2) There was little uptake of K+ in the absence of extracellular Na+ or Cl-. Halving medium osmolarity to 92 mosM decreased bumetanide-sensitive K+ uptake by approximately 75%, whereas a doubling of medium osmolarity increased it by approximately 50%. The cotransport activity was increased fourfold by the phosphatase inhibitor calyculin A (200 nM) but was unaffected by 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate, 8-bromoguanosine 3',5'-cyclic monophosphate, ATP, ionomycin, or okadaic acid. Both the photoaffinity bumetanide analogue, 4-[3H]benzoyl-5-sulfamoyl-3-(3-thenyloxy)benzoic acid, and an antiserum raised against Ehrlich ascites cell cotransporter specifically labeled an approximately 140-kDa oocyte membrane protein. These results demonstrated that, in addition to the Na+ pump and K+ channels, K+ uptake in Xenopus oocytes occurs via a loop-diuretic-sensitive Na(+)-K(+)-2Cl- cotransporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of amiloride-sensitive Na+ channels of hen lower intestine in Xenopus oocytes: electrophysiological studies on the dependence of varying NaCl intake

Epithelial Na+ channels were incorporated into the plasma membrane of Xenopus laevis oocytes after micro-injection of RNA from hen lower intestinal epithelium (colon and coprodeum). The animals were fed either a normal poultry food which contained NaCl (HS), or a similar food devoid of NaCl (LS). Oocytes were monitored for the expression of amiloride-sensitive sodium channels by measuring membrane potentials and currents. Oocytes injected with poly(A)+RNA prepared from HS animals or non-injected control oocytes showed no detectable sodium currents, whereas oocytes injected with LS-poly(A)+RNA had large amiloride-blockable sodium currents. These currents were almost completely saturated by sodium concentrations of 20 mM with a Km of about 2.6 mM sodium. Amiloride (10 microM) inhibits the expressed sodium channels entirely and examination of dose response relationships yielded a half-maximal inhibition concentration (Ki) of 120 nM amiloride. I-V difference curves in the presence or absence of sodium or amiloride (10 microM) indicate a potential dependence of the sodium transport which can be described by the Goldman equation. When Na+ is replaced by K+, no amiloride response was detected indicating a high selectivity for Na+ over K+. These results provide strong evidence that intestinal Na+ channels are regulated by dietary salt intake on the RNA level.

Functional expression of a plant plasma membrane transporter in Xenopus oocytes

A full-length cDNA clone for the H+/hexose co-transporter (STP1) from Arabidopsis thaliana has been transcribed in vitro and the mRNA injected into Xenopus oocytes. Under optimized conditions, oocytes injected with the STP1 mRNA accumulated 3-O-[methyl-14C]glucose at rates of more than a 1000-fold greater than water-injected control oocytes. A hexose-elicited depolarization of the oocyte membrane potential was demonstrated, and uptake was shown to be stimulated by low external pH, confirming the activity of a H+/hexose co-transport system. This is the first example of the functional expression of a plant membrane transporter in oocytes.

An endogenous carrier-mediated uptake system for folate in oocytes of Xenopus laevis

We investigated the existence of an endogenous uptake system for folate in Xenopus laevis oocytes. This was done by performing uptake measurements using [3H]folic acid. Uptake of folic acid was linear with time for 4 h of incubation, and was similar in collagenase-treated and non-treated oocytes. The uptake process was carrier-mediated, as suggested by the saturation of folic acid uptake with concentration, and by the ability of unlabelled folic acid and its related compounds to significantly inhibit the uptake of [3H]folic acid. The apparent Km and Vmax of the uptake process were 42 +/- 7 nM and 10.56 +/- 0.46 fmol per oocyte per 2 h, respectively. The uptake of folic acid was independent of the presence of Na+ in the incubation medium, but was highly pH dependent with severe inhibition occurring at pH lower than 6.5. Folic acid uptake was energy- and temperature-dependent, and was significantly inhibited by the anion transport inhibitors DIDS and SITS. These results demonstrate the existence of an endogenous carrier-mediated system for folic acid uptake in Xenopus oocytes. Further characterization of the molecular mechanism of folic acid uptake and its regulation in this non mammalian in vitro unicellular system may prove useful in furthering our understanding of folate movement across biological membranes.

Endogenous L-glutamate transport in oocytes of Xenopus laevis

The existence of an endogenous Na(+)-glutamate cotransporter in the oocytes of Xenopus laevis is demonstrated. The transporter does not accept D-glutamate as substrate. The dependence on substrate displays two saturating components with low (K1/2 = 9 mM) and high (K1/2 = 0.35 microM) affinities for L-glutamate. The dependence on external Na+ exhibits a saturating component with a K1/2 value of about 5 mM and a component that has not saturated up to 110 mM Na+. In voltage-clamped oocytes, it is possible to demonstrate that Na(+)-dependent L-glutamate transport is directly coupled to countertransport of Rb+. The analysis of the voltage dependence of the Na+,K(+)-dependent L-glutamate uptake suggests that positive charges are moved inwardly during the transport cycle.

A possible Na/Ca exchange in the follicle cells of Xenopus oocyte

In manually dissected Xenopus oocytes, we found that the replacement of external sodium by Tris, choline, or lithium induced a large membrane depolarization and, in voltage clamp, a large inward current. This current appears to be due to activation of a calcium-dependent chloride conductance since it is reversed near ECl, increased by the removal of external chloride, and can be abolished by an injection of BAPTA or by the removal of external Ca2+. Using the Ca-dependent Cl current as a monitor of Ca concentration at the inner surface of the oocyte membrane, we are led to propose that the removal of external Na+ induces an increase in internal Ca2+ via the activation of a Na/Ca exchanger operating in the reverse mode. This interpretation is supported by the finding that the chloride current is diminished in either 3',4'-dichlorobenzamyl (DCB) or high external [Mg2+]o, both of which are known to block the Na/Ca exchanger, whereas it is increased when Li+, rather than Tris or choline, is used as the substitute for Na. The effect of zero [Na+]o was not obtained in oocytes from which follicular cells were removed by enzymatic treatment. This observation led us to test the possibility that the Na/Ca exchanger was present in the follicle cells and not in the oocyte membrane, assuming that entering Ca2+ could pass into the oocyte through gap junctions. Octanol, which blocks gap junctions, or a high [Ca2+]o both considerably reduced the inward current. While octanol probably blocked the gap junctions directly, we propose that the block by high [Ca2+] was due to an excessive rise of [Ca2+]i in the follicular cells. These results, taken together, indirectly suggest the presence of a Na/Ca exchanger in the follicular cells. These results, taken together, indirectly suggest the presence of a Na/Ca exchanger in the follicle cells of Xenopus oocyte which could contribute to the regulation of the internal Ca concentration of the oocyte before fertilization.

Comparison of a Na+/D-glucose cotransporter from rat intestine expressed in oocytes of Xenopus laevis with the endogenous cotransporter

Epithelial Na+/D-glucose cotransport was incorporated into the plasma membrane of Xenopus oocytes after microinjection of poly(A)(+)-mRNA from rat intestine tissue and was detected by measurements of uptake of [14C]AMG (methyl alpha-D-glucopyranoside). In mRNA-injected oocytes, the rate of AMG uptake exceeds the rate of endogenous Na+/AMG cotransport by a factor of up to 30. It is demonstrated that the additionally expressed transport differs qualitatively from the endogenous transport with respect to several parameters which is a prerequisite for the demonstration of expression of a foreign transporter: (1) The expressed system is more sensitive to external glucose or AMG and to the specific inhibitor phlorizin, (2) it is less sensitive to external Na+ and to changes in membrane potential, and (3) it is susceptible to inhibition by monoclonal antibodies, known to bind specifically to Na+/glucose cotransporters and to modulate the cotransport in kidney and intestine. The use of the antibodies allows one to distinguish between endogenous Na+/AMG cotransport and foreign cotransport expressed by injection of foreign mRNA. The expression of the foreign transport leads to transport rates that are high enough to detect the electrical current generated by the Na+/glucose cotransport. This allows future characterization of the cotransport system under voltage-clamp conditions by analyzing membrane current.

Glucose-induced excitation in molluscan central neurons producing insulin-related peptides

The light green cells (LGCs) are a group of identified central neurons in the pond snail, Lymnaea stagnalis, that produce a number of insulin-related peptides. Freshly dissociated LGCs are activated by physiological concentrations of extracellular glucose. The response to glucose consists of a slow depolarization, which, at concentrations of 1 mM or more, rapidly induces regular spiking activity. The response persists during prolonged application of glucose but is completely reversed upon washing. The threshold concentration is 0.2 mM; the maximal effect occurs at 5 mM. In LGCs in central nervous system preparations kept in organ culture for 16-24 h, glucose causes a similar depolarization, which may lead to spiking activity. In freshly isolated preparations, which have very inexcitable LGCs, no direct response to glucose was seen. The response is specific to the LGCs; no other central neurons in Lymnaea showed consistent responses. The glucose response is evoked by D-glucose and the non-metabolized analogue 2-deoxy-D-glucose, but not by related hexoses, including L-glucose, nor pentoses or disaccharides. The response is not affected by interfering with the glucose metabolism, nor is the response mimicked by the metabolite D-glyceraldehyde or by injection of glucose. This suggests that glucose metabolites are not involved in the response. The glucose response depends on the presence of extracellular Na+ and is blocked by phlorizin, which specifically inhibits Na(+)-coupled glucose transport. This suggests that the response is due to activation of an electrogenic Na(+)-coupled glucose transporter.

Characterization of a Na(+)-K(+)-2Cl- cotransport system in oocytes from Xenopus laevis

In order to characterize the transport systems mediating K+ uptake into oocytes, flux studies employing 86Rb were performed on Xenopus oocytes stripped of follicular cells by pretreatment with Ca2(+)-Mg2(+)-free Barth's medium. Total Rb+ uptake consisted of an ouabain-sensitive and an ouabain-insensitive flux. In the presence of 100 mmol/l NaCl and 0.1 mmol/l ouabain the ouabain-insensitive flux amounted to 754.7 +/- 59.9 pmol/oocyte per h (n = 30 cells, i.e., 10 cells each from three different animals). In the absence of Na+ (Na+ substituted by N-methylglucamine) or when Cl- was replaced by NO3- the ouabain-insensitive flux was reduced to 84.4 +/- 42.9 and 79.2 +/- 12.1 pmol/oocyte per h, respectively (n = 50 cells). Furthermore, this Na(+)- and Cl(-)-dependent flux was completely inhibited by 10(-4) mol/l bumetanide, a specific inhibitor of the Na(+)-K(+)-2Cl- cotransport system. These results suggest that K+ uptake via a bumetanide-sensitive Na(+)-K(+)-2Cl- cotransport system represents a major K+ pathway in oocytes.

Two substrate sites in the renal Na(+)-D-glucose cotransporter studied by model analysis of phlorizin binding and D-glucose transport measurements

Time courses of phlorizin binding to the outside of membrane vesicles from porcine renal outer cortex and outer medulla were measured and the obtained families of binding curves were fitted to different binding models. To fit the experimental data a model with two binding sites was required. Optimal fits were obtained if a ratio of low and high affinity phlorizin binding sites of 1:1 was assumed. Na+ increased the affinity of both binding sites. By an inside-negative membrane potential the affinity of the high affinity binding site (measured in the presence of 3 mM Na+) and of the low affinity binding site (measured in the presence of 3 or 90 mM Na+) was increased. Optimal fits were obtained when the rate constants of dissociation were not changed by the membrane potential. In the presence of 90 mM Na+ on both membrane sides and with a clamped membrane potential, KD values of 0.4 and 7.9 microM were calculated for the low and high affinity phlorizin binding sites which were observed in outer cortex and in outer medulla. Apparent low and high affinity transport sites were detected by measuring the substrate dependence of D-glucose uptake in membrane vesicles from outer cortex and outer medulla which is stimulated by an initial gradient of 90 mM Na+ (out greater than in). Low and high affinity transport could be fitted with identical Km values in outer cortex and outer medulla. An inside-negative membrane potential decreased the apparent Km of high affinity transport whereas the apparent Km of low affinity transport was not changed. The data show that in outer cortex and outer medulla of pig high and low affinity Na(+)-D-glucose cotransporters are present which contain low and high affinity phlorizin binding sites, respectively. It has to be elucidated from future experiments whether equal amounts of low and high affinity transporters are expressed in both kidney regions or whether the low and high affinity transporter are parts of the same glucose transport molecule.