Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: I. Tight-seal whole-cell recordings

The C-terminal domain of the neutral amino acid transporter SNAT2 regulates transport activity through voltage-dependent processes

SNAT (sodium-coupled neutral amino acid transporter) 2 belongs to the SLC38 (solute carrier 38) family of solute transporters. Transport of one amino acid molecule into the cell is driven by the co-transport of one Na(+) ion. The functional significance of the C-terminus of SNAT2, which is predicted to be located in the extracellular space, is currently unknown. In the present paper, we removed 13 amino acid residues from the SNAT2 C-terminus and studied the effect of this deletion on transporter function. The truncation abolished amino acid transport currents at negative membrane potentials (<0 mV), as well as substrate uptake. However, transport currents were observed at positive membrane potentials demonstrating that transport was accelerated while the driving force decreased. Membrane expression levels were normal in the truncated transporter. SNAT2(Del C-ter) (13 residues deleted from the C-terminus) showed 3-fold higher apparent affinity for alanine, and 2-fold higher Na(+) affinity compared with wild-type SNAT2, suggesting that the C-terminus is not required for high-affinity substrate and Na(+) interaction with SNAT2. The pH sensitivity of amino acid transport was retained partially after the truncation. In contrast with the truncation after TM (transmembrane domain) 11, the deletion of TM11 resulted in an inactive transporter, most probably due to a defect in cell surface expression. Taken together, the results demonstrate that the C-terminal domain of SNAT2 is an important voltage regulator that is required for a normal amino acid translocation process at physiological membrane potentials. However, the C-terminus appears not to be involved in the regulation of membrane expression.

Highly conserved asparagine 82 controls the interaction of Na+ with the sodium-coupled neutral amino acid transporter SNAT2

The neutral amino acid transporter 2 (SNAT2), which belongs to the SLC38 family of solute transporters, couples the transport of amino acid to the cotransport of one Na(+) ion into the cell. Several polar amino acids are highly conserved within the SLC38 family. Here, we mutated three of these conserved amino acids, Asn(82) in the predicted transmembrane domain 1 (TMD1), Tyr(337) in TMD7, and Arg(374) in TMD8; and we studied the functional consequences of these modifications. The mutation of N82A virtually eliminated the alanine-induced transport current, as well as amino acid uptake by SNAT2. In contrast, the mutations Y337A and R374Q did not abolish amino acid transport. The K(m) of SNAT2 for its interaction with Na(+), K(Na(+)), was dramatically reduced by the N82A mutation, whereas the more conservative mutation N82S resulted in a K(Na(+)) that was in between SNAT2(N82A) and SNAT2(WT). These results were interpreted as a reduction of Na(+) affinity caused by the Asn(82) mutations, suggesting that these mutations interfere with the interaction of SNAT2 with the sodium ion. As a consequence of this dramatic reduction in Na(+) affinity, the apparent K(m) of SNAT2(N82A) for alanine was increased 27-fold compared with that of SNAT2(WT). Our results demonstrate a direct or indirect involvement of Asn(82) in Na(+) coordination by SNAT2. Therefore, we predict that TMD1 is crucial for the function of SLC38 transporters and that of related families.

The role of nucleoside transporters in cancer chemotherapy with nucleoside drugs

Nucleoside analogs are important components of treatment regimens for various malignancies. Nucleoside-specific membrane transporters mediate plasma membrane permeation of physiologic nucleosides and most nucleoside analogs, for which the initial event is cellular conversion of nucleosides to active agents. Understanding of the roles of nucleoside transporters in nucleoside drug toxicity and resistance will provide opportunities for potentiating anticancer efficacy and avoiding resistance. Because transportability is a possible determinant of toxicity and resistance of many nucleoside analogs, nucleoside transporter abundance might be a prognostic marker to assess drug resistance. Elucidation of the structural determinants of nucleoside analogs for interaction with transporter proteins as well as the structural features of transporter proteins required for permeant interaction and translocation will lead to "transportability guidelines" for the rational design and therapeutic application of nucleoside analogs as anticancer drugs. It should eventually be possible to develop clinical assays that predict sensitivity and/or resistance to nucleoside anti-cancer drugs and thus to identify those patient populations that will most likely benefit from optimal nucleoside analog treatments. This review discusses recent results from structure/function studies of human nucleoside transporters, the role of nucleoside transport processes in the cytotoxicity and resistance of several anticancer nucleoside analogs and strategies to improve the nucleoside transporter-related anticancer effects of nucleoside analogs.

Functional properties and cellular distribution of the system A glutamine transporter SNAT1 support specialized roles in central neurons

Glutamine, the preferred precursor for neurotransmitter glutamate and GABA, is likely to be the principal substrate for the neuronal System A transporter SNAT1 in vivo. We explored the functional properties of SNAT1 (the product of the rat Slc38a1 gene) by measuring radiotracer uptake and currents associated with SNAT1 expression in Xenopus oocytes and determined the neuronal-phenotypic and cellular distribution of SNAT1 by confocal laser-scanning microscopy alongside other markers. We found that SNAT1 mediates transport of small, neutral, aliphatic amino acids including glutamine (K0.5 approximately 0.3 mm), alanine, and the System A-specific analogue 2-(methylamino)isobutyrate. Amino acid transport is driven by the Na+ electrochemical gradient. The voltage-dependent binding of Na+ precedes that of the amino acid in a simultaneous transport mechanism. Li+ (but not H+) can substitute for Na+ but results in reduced Vmax. In the absence of amino acid, SNAT1 mediates Na+-dependent presteady-state currents (Qmax approximately 9 nC) and a nonsaturable cation leak with selectivity Na+, Li+ >> H+, K+. Simultaneous flux and current measurements indicate coupling stoichiometry of 1 Na+ per 1 amino acid. SNAT1 protein was detected in somata and proximal dendrites but not nerve terminals of glutamatergic and GABAergic neurons throughout the adult CNS. We did not detect SNAT1 expression in astrocytes but detected its expression on the luminal membranes of the ependyma. The functional properties and cellular distribution of SNAT1 support a primary role for SNAT1 in glutamine transport serving the glutamate/GABA-glutamine cycle in central neurons. Localization of SNAT1 to certain dopaminergic neurons of the substantia nigra and cholinergic motoneurons suggests that SNAT1 may play additional specialized roles, providing metabolic fuel (via alpha-ketoglutarate) or precursors (cysteine, glycine) for glutathione synthesis.

GAT1 (GABA:Na+:Cl-) cotransport function. Steady state studies in giant Xenopus oocyte membrane patches

Neurotransmitter transporters are reported to mediate transmembrane ion movements that are poorly coupled to neurotransmitter transport and to exhibit complex "channel-like" behaviors that challenge the classical "alternating access" transport model. To test alternative models, and to develop an improved model for the Na+- and Cl--dependent gamma-aminobutyric acid (GABA) transporter, GAT1, we expressed GAT1 in Xenopus oocytes and analyzed its function in detail in giant membrane patches. We detected no Na+- or Cl--dependent currents in the absence of GABA, nor did we detect activating effects of substrates added to the trans side. Outward GAT1 current ("reverse" transport mode) requires the presence of all three substrates on the cytoplasmic side. Inward GAT1 current ("forward" transport mode) can be partially activated by GABA and Na+ on the extracellular (pipette) side in the nominal absence of Cl-. With all three substrates on both membrane sides, reversal potentials defined with specific GAT1 inhibitors are consistent with the proposed stoichiometry of 1GABA:2Na+:1Cl-. As predicted for the "alternating access" model, addition of a substrate to the trans side (120 mM extracellular Na+) decreases the half-maximal concentration for activation of current by a substrate on the cis side (cytoplasmic GABA). In the presence of extracellular Na+, the half-maximal cytoplasmic GABA concentration is increased by decreasing cytoplasmic Cl-. In the absence of extracellular Na+, half-maximal cytoplasmic substrate concentrations (8 mM Cl-, 2 mM GABA, 60 mM Na+) do not change when cosubstrate concentrations are reduced, with the exception that reducing cytoplasmic Cl- increases the half-maximal cytoplasmic Na+ concentration. The forward GAT1 current (i.e., inward current with all extracellular substrates present) is inhibited monotonically by cytoplasmic Cl- (Ki, 8 mM); cytoplasmic Na+ and cytoplasmic GABA are without effect in the absence of cytoplasmic Cl-. In the absence of extracellular Na+, current-voltage relations for reverse transport current (i.e., outward current with all cytoplasmic substrates present) can be approximated by shallow exponential functions whose slopes are consistent with rate-limiting steps moving 0.15-0.3 equivalent charges. The slopes of current-voltage relations change only little when current is reduced four- to eightfold by lowering each cosubstrate concentration; they increase twofold upon addition of 100 mM Na+ to the extracellular (pipette) side.

Bile acid uptake via the human apical sodium-bile acid cotransporter is electrogenic

Intestinal absorption of bile acids depends on a sodium-bile acid cotransport protein in the apical membrane of the ileal epithelial cell. Transport is Na+-dependent, but the Na+-bile acid stoichiometry and electrogenicity of transport are not known. Studies in whole intestine, isolated cells, and ileal membrane vesicles have been unable to resolve this issue because transport currents are small and can be obscured by other ionic conductances and transport proteins present in these membranes. In this study, the human apical sodium-bile acid transporter was expressed in stably transfected Chinese hamster ovary cells that lack other bile acid transporters. The Na+-dependent transport of a fluorescent bile acid analog, chenodeoxycholyl-Nepsilon-nitrobenzoxadiazol-lysine, was monitored by fluorescence microscopy in single, voltage-clamped cells. Bile acid movement was bidirectional and voltage-dependent with negative intracellular voltage-stimulating influx. A 3-fold reduction in extracellular Na+ produced a negative 52 mV shift of the flux-voltage relationship, consistent with a 2:1 Na+:bile acid coupling stoichiometry. No Na+- or voltage-dependent uptake was observed in nontransfected Chinese hamster ovary cells. These results indicate that the cotransport of bile acids and Na+ by human apical sodium-bile acid transporter is electrogenic and bidirectional and is best explained by a 2:1 Na+:bile acid coupling stoichiometry. These results suggest that membrane potential may regulate bile acid transport rates under physiological and pathophysiological conditions.

Outward K+ current in epithelial cells isolated from intermediate portion of endolymphatic sac of guinea pigs

Ion currents in epithelial cells isolated from the intermediate portion of endolymphatic sac (ES) in guinea pigs were investigated with the use of the whole cell patch-clamp technique. Depolarizing voltage steps from a holding potential of -60 mV induced a time- and voltage-dependent outward current, which is comparable to that of delayed rectifying K+ currents. The average resting membrane potential in the current-clamp mode was -54.8 +/- 11 mV (n = 45), which was similar to the value of zero current potential (-55.6 +/- 0.8 mV, n = 32) obtained from current-voltage (I-V) relationships of outward currents in voltage-clamp mode. The I-V relationship of the tail current exhibited a reversal potential (Erev) of -78.1 +/- 0.9 mV (n = 19) in standard external solution. The Erev of the outward current was linearly related to the logarithm of extracellular K+ concentrations. The slope was 48 mV per 10-fold change in extracellular K+ concentrations. The time constants of K+ current activation, inactivation, and K+ tail current deactivation were voltage dependent. The steady-state activation and inactivation of K+ current exhibited a sigmoidal relationship to voltage. The 50% maximal activation voltage and slope factor were -21 and 11 mV (n = 8), respectively. The 50% maximal inactivation voltage and slope factor were -45 and 13 mV (n = 7), respectively. The K+ current was blocked by externally applied 1 mM 4-aminopyridine (4-AP), 5 mM Ba2+ and 20 mM tetraethylammonium chloride (TEA). The sensitivity of the current to 4-AP and Ba2+ was higher than that to TEA. Elimination of external Ca2+ and increase of internal Ca2+ failed to significantly change the current, suggesting that the K+ current may be Ca2+ independent. The results show that epithelial cells in the intermediate portion of the ES possess a delayed-rectifier K+ current, which may be involved in membrane stability or in the ion balance between the cytosol and the extracellular environment.

MEMBRANE TRANSPORT CARRIERS

▪ Abstract  Plant and fungal membrane proteins catalyzing the transmembrane translocation of small molecules without directly using ATP or acting as channels are discussed in this review. Facilitators, ion-cotransporters, and exchange translocators mainly for sugars, amino acids, and ions that have been cloned and characterized from Saccharomyces cerevisiae and from various plant sources have been tabulated. The membrane topology and structure of the most extensively studied carriers (lac permease of Escherichia coli, Glut1 of man, HUP1 of Chlorella) are discussed in detail as well as the kinetic analysis of specific Na+ and H+ cotransporters. Finally, the knowledge concerning regulatory phenomena of carriers—mainly of S. cerevisiae—is summarized.

Mechanisms of the human intestinal H+-coupled oligopeptide transporter hPEPT1

The hPEPT1 cDNA cloned from human intestine (Liang, R., Fei, Y.-J., Prasad, P. D., Ramamoorthy, S., Han, H., Yang-Feng, T. L., Hediger, M. A., Ganapathy, V., and Leibach, F. H. (1995) J. Biol. Chem. 270, 6456-6463) encodes a H+/oligopeptide cotransporter. Using two-microelectrode voltage-clamp in Xenopus oocytes expressing hPEPT1, we have investigated the transport mechanisms of hPEPT1 with regard to voltage dependence, steady-state kinetics, and transient charge movements. The currents evoked by 20 mM glycyl-sarcosine (Gly-Sar) at pH 5.0 were dependent upon membrane potential (Vm) between -150 mV and +50 mV. Gly-Sar-evoked currents increased hyperbolically with increasing extracellular [H+], with Hill coefficient approximately 1, and the apparent affinity constant (K0.5H) for H+ was in the range of 0.05 1 microM. K0.5 for Gly-Sar (K0.5GS) was dependent upon Vm and pH; at -50 mV, K0.5H was minimal (approximately 0.7 mM) at pH 6.0. Following step-changes in Vm, in the absence of Gly-Sar, hPEPT1 exhibited H+-dependent transient currents with characteristics similar to those of Na+-coupled transporters. These charge movements (which relaxed with time constants of 2-10 ms) were fitted to Boltzmann relations with maximal charge (Qmax) of up to 12 nC; the apparent valence was determined to be approximately 1. Qmax is an index of the level of transporter expression which for hPEPT1 was in the order of 1011/oocyte. In general our data are consistent with an ordered, simultaneous transport model for hPEPT1 in which H+ binds first.

A model for the kinetic mechanism of sodium-coupled L-alanine transport in LLC-PK1 cells

The kinetics of sodium-dependent L-alanine transport were characterized in ATP-depleted LLC-PK1 cells, which allows experimental imposition of an interior negative diffusion potential across the plasma membrane. Under these conditions a wide range of sodium concentrations can be studied without altering the membrane potential. When Na+ is the variable substrate, the apparent maximal velocity (V max) for transport changes nearly fourfold for the five different alanine concentrations studied (0.05-2.0 mM). In contrast, at five different sodium concentrations, ranging from 10 to 135 mM, the apparent V max with variable alanine remains nearly constant at 5.3 +/- 1.2 nmol.min-1.mg cell protein-1. The ratio of the two primary kinetic parameters, Michaelis constant (Km)/V max, varies markedly no matter which solute is treated as the variable illustrate. These data are consistent with a simultaneous ordered transport mechanism in which sodium binds before alanine to the transport protein at the extracellular surface of the membrane. Alanine-dependent 22Na+ influx is more than five times faster if unlabeled intracellular sodium is present than in its absence. Sodium-dependent influx of [14C]alanine is more rapid than net alanine flux only if unlabeled Na+ and alanine are both present intracellularly. These results indicate that the cotransporter can function more rapidly in an exchange mode than when it catalyzes net solute uptake and that Na+ is the first solute to be released at the intracellular side of the membrane. A model is presented that can be used for further quantitative analysis of the kinetic and functional properties of the cotransport system.

Na(+)-coupled alanine transport in LLC-PK1 cells

Transport of alanine (Ala) was characterized in LLC-PK1 renal epithelial cells. Transport capability for Ala falls by 75% in postconfluent cultures, while Na(+)-coupled alpha-methylglucoside (AMG) transport rises more than fourfold during the same interval. The kinetics of Ala transport were characterized in ATP-depleted cells to allow experimental imposition of changes in Na+ gradient and control of membrane potential across the plasma membrane. At 100 microM Ala and 135 mM Na+, > 98% of the unidirectional Ala influx is dependent on the presence of Na+ in cells from postconfluent cultures. Li+ is only 1% as effective as Na+, and other monovalent cations are ineffective in supporting Ala uptake. alpha-(Methylamino)isobutyric acid (MeAIB; 5 mM) causes only a small inhibition (approximately 10%) of 100 microM Ala influx. The low selectivity for Li+; low sensitivity to competition by MeAIB or aminoisobutyric acid; pronounced inhibition by serine, homoserine, cysteine, homocysteine and threonine; moderate inhibition by valine, isoleucine, proline and histidine; and lack of inhibition by lysine, arginine, and aspartate are more consistent with those characteristics reported for entry via the ASC amino acid transport system rather than those associated with the A system. Alanine influx exhibits a hyperbolic relationship with increasing Ala or Na+ concentration. Kinetic analysis suggests a single transport pathway with a Michaelis constant (Km) for alanine of 380 microM (when Na+ is 135 mM), apparent Km for Na+ of 32 mM (with 100 microM Ala), and a maximum velocity of 7 nmol.min-1.mg cell protein-1. An interior-negative diffusion potential induces a similar enhancement of [14C]alanine or [14C]tetraphenylphosphonium influx (approximately 40%). In contrast, AMG influx is enhanced by a factor of 2.2 under the same conditions. AMG uptake also shows a sigmoidal relationship with Na+ concentration. Hill coefficients are 1.56 for AMG and 1.0 for alanine. Direct measurement of Na(+)-Ala coupling stoichiometry yields a value of 1.01 +/- 0.07. Under the same conditions, Na(+)-AMG coupling stoichiometry is 2.1 +/- 0.25. The difference in coupling stoichiometries provides an explanation for differences in intensity of interaction between Na(+)-coupled transport systems for sugars and amino acids.

Basolateral Na(+)-H+ antiporter. Mechanisms of electroneutral and conductive ion transport

The basolateral Na-H antiporter of the turtle colon exhibits both conductive and electroneutral Na+ transport (Post and Dawson. 1992. American Journal of Physiology. 262:C1089-C1094). To explore the mechanism of antiporter-mediated current flow, we compared the conditions necessary to evoke conduction and exchange, and determined the kinetics of activation for both processes. Outward (cell to extracellular fluid) but not inward (extracellular fluid to cell) Na+ or Li+ gradients promoted antiporter-mediated Na+ or Li+ currents, whereas an outwardly directed proton gradient drove inward Na+ or Li+ currents. Proton gradient-driven, "counterflow" current is strong evidence for an exchange stoichiometry of > 1 Na+ or Li+ per proton. Consistent with this notion, outward Na+ and Li+ currents generated by outward Na+ or Li+ gradients displayed sigmoidal activation kinetics. Antiporter-mediated proton currents were never observed, suggesting that only a single proton was transported per turnover of the antiporter. In contrast to Na+ conduction, Na+ exchange was driven by either outwardly or inwardly directed Na+, Li+, or H+ gradients, and the activation of Na+/Na+ exchange was consistent with Michaelis-Menten kinetics (K1/2 = 5 mM). Raising the extracellular fluid Na+ or Li+ concentration, but not extracellular fluid proton concentration, inhibited antiporter-mediated conduction and activated Na+ exchange. These results are consistent with a model for the Na-H antiporter in which the binding of Na+ or Li+ to a high-affinity site gives rise to one-for-one cation exchange, but the binding of Na+ or Li+ ions to other, lower-affinity sites can give rise to a nonunity, cation exchange stoichiometry and, hence, the net translocation of charge. The relative proportion of conductive and nonconductive events is determined by the magnitude and orientation of the substrate gradient and by the serosal concentration of Na+ or Li+.

Whole-cell K+ currents in fresh and cultured cells of the human and monkey retinal pigment epithelium

1. Whole-cell potassium currents of freshly isolated human (adult and fetal) and monkey (adult) retinal pigment epithelial (RPE) cells, as well as cultured human and monkey RPE cells were studied using the patch-clamp technique. 2. In freshly isolated adult cells of both species, two currents were observed in the voltage range from -150 to +50 mV: an outwardly rectifying current and an inwardly rectifying current. These currents were also found in cultured cells of both species. 3. The outwardly rectifying current in freshly isolated adult human and monkey cells and some cultured cells was evoked by depolarizing voltage pulses more positive that -30 mV. The current activated with a sigmoidal time course after a brief delay, and was virtually non-inactivating. The conductance associated with the current was half-maximal at -16.4 mV for fresh human cells and -13.5 mV for fresh monkey cells, but was shifted 16.0 and 17.7 mV in the positive direction in cultured human and monkey cells, respectively. The reversal potential of the current in both human and monkey cells matched the potassium equilibrium potential (EK) over a wide range of external potassium concentrations. This current was blocked by 20 mM tetraethylammonium. 4. A membrane current that exhibited inward rectification was observed with hyperpolarizing voltage pulses. The zero-current potential of this current was close to EK. This current was blocked by 2 mM Ba2+ and 2 mM Cs+. In cultured human and monkey cells, but not in fresh cells, this current exhibited an inactivation when voltage pulses were more negative than -120 mV. External Na+ was responsible for the inactivation, as the inactivation was removed in a Na(+)-free solution. 5. Membrane currents in freshly isolated fetal human RPE cells were remarkably different from those in adult cells. A transient outward current resembling the A-type potassium current was observed as the dominant membrane current in freshly isolated fetal human cells. This current activated when voltage pulses were more positive than -30 mV. It inactivated rapidly after reaching a maximal level. Application of 5 mM 4-aminopyridine (4-AP) completely blocked this current. Although this current was never observed in fresh adult cells, it was found in 33% of the cultured adult cells with similar kinetics, ion selectivity, and pharmacological properties. 6. In about 26% of the freshly isolated fetal human cells, a more slowly activating outward current, which resembled the delayed rectifier, was found to co-exist with the transient outward current.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential potentiation of GABAA receptor function by two stereoisomers of diimidazoquinazoline analogues

1. U-84935, diimidazo[1,5-a;1',2'-C]quinazoline,5-(5-cyclopropyl-1,2,4-oxid iazol-3yl)- 2,3-dihydro, is a ligand of high affinity for the benzodiazepine site of the GABAA receptor composed of alpha 1 beta 2 gamma 2 subunits. 2. The efficacy of its analogues was measured with their ability to potentiate GABA-mediated Cl- currents in the whole cell configuration of the patch clamp techniques in human kidney cells (A293 cells) expressing the subtype of the GABAA receptor. 3. The analogues displayed various levels of efficacy including agonists, partial agonists and antagonists without marked changes in their affinity for the receptors. 4. The major determinant of their efficacy was the spacial configuration of a methyl substituent of the C2 atom of the rigid and planar diimidazoquinazoline ring: U-90167, containing the methyl substituent projected below the plane of the ring, markedly enhanced the GABA current with a maximal potentiation of 220 +/- 25%, while its stereoisomer, U-90168, marginally increased the GABA response with a maximal potentiation of 45 +/- 10%, to which its methyl group appeared to contribute very little. 5. U-90167 potentiated the GABA response with an EC50 of 8.1 nM and a Hill coefficient of 1.1 and did not alter the reversal potential for the Cl- current. 6. From computational modelling, the sensitive methyl group of U-90167 could be assigned to the general region for the 5-phenyl group of diazepam. The diimidazoquinazoline, because of its rigid and plantar ring structure, may be useful to define further the out-of-plane region responsible for agonistic activity and to pinpoint other areas pivotal to the functionality of benzodiazepine ligands.

Voltage-dependent and odorant-regulated currents in isolated olfactory receptor neurons of the channel catfish

The electrical properties of olfactory receptor neurons, enzymatically dissociated from the channel catfish (Ictalurus punctatus), were studied using the whole-cell patch-clamp technique. Six voltage-dependent ionic currents were isolated. Transient inward currents (0.1-1.7 nA) were observed in response to depolarizing voltage steps from a holding potential of -80 mV in all neurons examined. They activated between -70 and -50 mV and were blocked by addition of 1 microM tetrodotoxin (TTX) to the bath or by replacing Na+ in the bath with N-methyl-D-glucamine and were classified as Na+ currents. Sustained inward currents, observed in most neurons examined when Na+ inward currents were blocked with TTX and outward currents were blocked by replacing K+ in the pipette solution with Cs+ and by addition of 10 mM Ba2+ to the bath, activated between -40 and -30 mV, reached a peak at 0 mV, and were blocked by 5 microM nimodipine. These currents were classified as L-type Ca2+ currents. Large, slowly activating outward currents that were blocked by simultaneous replacement of K+ in the pipette with Cs+ and addition of Ba2+ to the bath were observed in all olfactory neurons examined. The outward K+ currents activated over approximately the same range as the Na+ currents (-60 to -50 mV), but the Na+ currents were larger at the normal resting potential of the neurons (-45 +/- 11 mV, mean +/- SD, n = 52). Four different types of K+ currents could be differentiated: a Ca(2+)-activated K+ current, a transient K+ current, a delayed rectifier K+ current, and an inward rectifier K+ current. Spontaneous action potentials of varying amplitude were sometimes observed in the cell-attached recording configuration. Action potentials were not observed in whole-cell recordings with normal internal solution (K+ = 100 mM) in the pipette, but frequently appeared when K+ was reduced to 85 mM. These observations suggest that the membrane potential and action potential amplitude of catfish olfactory neurons are significantly affected by the activity of single channels due to the high input resistance (6.6 +/- 5.2 G omega, n = 20) and low membrane capacitance (2.1 +/- 1.1 pF, n = 46) of the cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulus-secretion coupling: cytoplasmic calcium signals and the control of ion channels in exocrine acinar cells

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Inositol triphosphate produces different patterns of cytoplasmic Ca2+ spiking depending on its concentration

In single mouse pancreatic acinar cells the effects of intracellular infusion of inositol 1,4,5-trisphosphate (InsP3) or the non-metabolizable InsP3 analogue inositol 1,4,5-triphosphorothioate (InsPS3) have been investigated using a wide range of concentrations. Different types of cytosolic Ca2+ fluctuation patterns (monitored as Ca(2+)-dependent Cl- current in patch-clamp whole-cell recording experiments) could be generated by InsP3 or InsPS3, dependent on concentration, resembling those previously shown to be evoked by varying degrees of receptor activation in these cells. Low InsPS3 concentrations evoked repetitive local Ca2+ spikes whereas at relatively high concentrations repetitive Ca2+ waves were produced. In the presence of intracellular citrate a much lower messenger level was sufficient to generate waves. The InsP3 concentration determines whether the cytosolic Ca2+ signals are local or global.

Cytoplasmic Ca2+ signals evoked by activation of cholecystokinin receptors: Ca(2+)-dependent current recording in internally perfused pancreatic acinar cells

The effects on the cytosolic Ca2+ concentration of activating cholecystokinin receptors on single mouse pancreatic acinar cells have been investigated using patch-clamp whole-cell recording of Ca(2+)-dependent Cl- current. We used the nonsulphated octapeptide of cholecystokinin (CCK8-NS) since the effects of even high concentrations were rapidly reversible which was not the case for the sulphated octapeptide. A submaximal concentration of CCK8-NS (10 nM) evoked a current response consisting of short-lasting (a few seconds) spikes, and some of these spikes were seen to trigger larger and longer (about half a minute) current pulses. At a higher concentration (100 nM) CCK8-NS evoked smooth and sustained responses. The effect of CCK8-NS was almost abolished when the internal perfusion solution contained a high concentration of the Ca2+ chelator EGTA (5 mM). The responses evoked by CCK8-NS were independent of the presence of Ca2+ in the external solution at least for the first 5 min of stimulation. Internal perfusion with GTP-gamma-S markedly potentiated the effect of CCK8-NS or at a higher concentration itself induced responses very similar to those normally evoked by CCK8-NS. Caffeine added to the external solution at a low concentration (0.2-1 mM) enhanced weak CCK8-NS responses, whereas high caffeine concentrations always inhibited the CCK8-NS-evoked responses. These inhibitory caffeine effects were quickly reversible. Forskolin evoked a similar inhibitory effect. Intracellular heparin (200 micrograms/ml) infusion markedly inhibited the response to CCK8-NS stimulation. We conclude that the primary effect of activating CCK receptors is to induce inositoltrisphosphate (IP3) production.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor-activated cytoplasmic Ca2+ spikes in communicating clusters of pancreatic acinar cells

The transmission of receptor-generated cytoplasmic Ca2+ signals between communicating pancreatic acinar cells has been investigated by comparing patch-clamp recordings of Ca(2+)-dependent Cl- current in internally perfused single cells and small multi-cell clusters. Acetylcholine (50 nM) generates shortlasting repetitive spikes of Ca(2+)-dependent current and these spikes are not transmitted to neighbouring cells. Cholecystokinin octapeptide (5 pM) also generates repetitive spikes, but a significant proportion of these trigger longer and larger pulses of Ca(2+)-dependent current and these waves can easily spread from cell to cell. In pancreatic acinar units it is therefore possible to observe both local Ca2+ signals confined to the cell of its origin as well as Ca2+ signals that spread through communicating junctions to all cells in the unit.

Sodium-alanine cotransport in renal proximal tubule cells investigated by whole-cell current recording

Sodium-alanine cotransport was investigated in single isolated proximal tubule cells from rabbit kidney with the whole-cell current recording technique. Addition of L-alanine at the extracellular side induced an inward-directed sodium current and a cell depolarization. The sodium-alanine cotransport current was stereospecific and sodium dependent. Competition experiments suggested a common cotransport system for L-alanine and L-phenylalanine. Sodium-alanine cotransport current followed simple Michaelis-Menten kinetics, with an apparent Km of 6.6 mM alanine and 11.6 mM sodium and a maximal cotransport current of 0.98 pA/pF at -60 mV clamp potential. Hill plots of cotransport current suggested a potential-independent coupling ratio of one sodium and one alanine. The apparent Km for sodium and the maximal cotransport current were potential dependent, whereas the apparent Km for L-alanine was not affected by transmembrane potential. The increase in Km for alanine with decreasing inward-directed sodium gradients suggested a simultaneous transport mechanism. These results are consistent with a cotransport model with potential-dependent binding or unbinding of sodium (high-field access channel) and a potential-dependent translocation step.

Receptor-activated cytoplasmic Ca2+ oscillations in pancreatic acinar cells: generation and spreading of Ca2+ signals

Receptor-activated cytoplasmic Ca2+ oscillations have been investigated using both single cell microfluorometry and voltage-clamp recording of Ca(2+)-dependent Cl- current in single internally perfused acinar cells. In these cells there is direct experimental evidence showing that the ACh-evoked [Ca2+]i fluctuations are due to an inositol trisphosphate-induced small steady Ca2+ release which in turn evokes repetitive Ca2+ spikes via a caffeine-sensitive Ca(2+)-induced Ca2+ release process. There is indirect evidence suggesting that receptor-activation in addition to generating the Ca2+ releasing messenger, inositol trisphosphate, also produces another regulator involved in the control of Ca2+ signal spreading. Intracellular inositol trisphosphate or Ca2+ infusion produce short duration repetitive spikes confined to the cytoplasmic area close to the plasma membrane, but these signals can be made to progress throughout the cell by addition of caffeine or by receptor activation.

Receptor-activated cytoplasmic Ca2+ spiking mediated by inositol trisphosphate is due to Ca2(+)-induced Ca2+ release

Receptor-mediated inositol 1,4,5-trisphosphate (Ins-(1,4,5)P3) generation evokes fluctuations in the cytoplasmic Ca2+ concentration ([Ca2+]i). Intracellular Ca2+ infusion into single mouse pancreatic acinar cells mimicks the effect of external acetylcholine (ACh) or internal Ins(1,4,5)P3 application by evoking repetitive Ca2+ release monitored by Ca2(+)-activated Cl- current. Intracellular infusion of the Ins(1,4,5)P3 receptor antagonist heparin fails to inhibit Ca2+ spiking caused by Ca2+ infusion, but blocks ACh- and Ins(1,4,5)P3-evoked Ca2+ oscillations. Caffeine (1 mM), a potentiator of Ca2(+)-induced Ca2+ release, evokes Ca2+ spiking during subthreshold intracellular Ca2+ infusion. These results indicate that ACh-evoked Ca2+ oscillations are due to pulses of Ca2+ release through a caffeine-sensitive channel triggered by a small steady Ins(1,4,5)P3-evoked Ca2+ flow.

Effects of alanine on insulin-secreting cells: patch-clamp and single cell intracellular Ca2+ measurements

The effects of alanine, glucose and tolbutamide on insulin-secreting cells (RINm5F) have been investigated using patch-clamp and single cell intracellular Ca2+ measurements. When directly challenged with the amino acid L-alanine (2-10 mM) the cells underwent a sharp depolarization, which led to the generation of Ca2+ spike potentials and an increase in [Ca2+]i. The L-alanine-induced depolarization was associated with a net inward membrane current but no measurable change in the resistance of the cell. The latter effect was found to be in contrast to the actions of glucose (5-10 mM) and tolbutamide (100 microM), both of which depolarized cells and raised [Ca2+]i by an increase in the input resistance of the cell membrane, due to the closure of ATP-sensitive potassium channels. In the complete absence of external Na+ (by replacement with 140 mM NMDG+), L-alanine had no effects on either the membrane potential or [Ca2+]i. Similarly, replacing Na+ with NMDG+ in the continued presence of the amino acid resulted in a repolarization of the membrane and an attenuation of the L-alanine-induced rise in [Ca2+]i. The Na+ channel blocker TTX (1-2 microM) had no effects on the alanine-evoked electrical activity. Exchange of the L-form of the amino acid with the D-stereoisomer had similar actions to those of removing external Na+, since D-alanine had no effects on the membrane potential or [Ca2+]i. The actions of L-alanine were also found to be mimicked by the N-methylated amino acid analogue methylamino isobutyric acid (MeAIB) (2-10 mM), suggesting that the A-type electrogenic amino acid cotransport system operates in the RINm5F insulin-secreting cell line.

Intracellular aluminium inhibits acetylcholine- and caffeine-evoked Ca2+ mobilization

The effect of intracellular aluminium on Ca2+ signalling in single internally perfused mouse pancreatic acinar cells was investigated by measurement of the Ca2(+)-dependent Cl- current using the patch-clamp whole-cell recording configuration. Acetylcholine (ACh) normally evoked a pulsatile Ca2(+)-dependent Cl- current, but when AlCl3 (1 mM) was present in the internal perfusion solution the ACh responses were virtually absent. When aluminium was acutely infused into the internal perfusion solution, the ACh-evoked Ca2+ signals and also the caffeine-evoked responses quickly disappeared, but the Ca2+ ionophore, ionomycin (100 nM), could still induce a large increase in the Cl- current. It is concluded that intracellular aluminium can abolish receptor-activated intracellular Ca2+ release probably by inhibition of Ca2(+)-induced Ca2+ release.

Intestinal Na+/glucose cotransporter expressed in Xenopus oocytes is electrogenic

The cloned rabbit intestinal Na+/glucose cotransporter was expressed in Xenopus oocytes, and transmembrane currents associated with this transporter were monitored using a two-electrode voltage clamp. Addition of D-glucose to a Na(+)-containing solution bathing these oocytes generated a current which was blocked by phlorizin. Water-injected control oocytes did not exhibit any currents under these conditions. The magnitude and shape of the currents were dependent on the extracellular glucose and Na+ concentrations and the membrane potential. At Vhold = -50 mV, the Km values for glucose and Na+ were 14 +/- 2 (N = 4) microM and 17 +/- 1 (N = 3) mM, respectively. These Km values and imax exhibited voltage dependence: increasing the membrane potential from -30 to -150 mV increased KGlcm and imax threefold and decreased KNam eightfold. The reversal potential (VR) of the phlorizin-sensitive, glucose-dependent current varied with log Nao+ (slope 46 +/- 6 [N = 9] mV). In the absence of sugar, a Na(+)-dependent, phlorizin-sensitive (Ki = 3 +/- 0.5 microM) current was detected only in RNA-injected oocytes. The amplitude of this current at -50 mV was 6 +/- 1% (N = 13) of the maximum current measured in the presence of D-glucose. The VR of this sugar-independent current varied with log Nao+ (slope 63 +/- 1 [N = 4] mV), indicating that the cotransporter may carry Na+ in the absence of sugar. We conclude that the Na+/glucose cotransporter is electrogenic and that investigations of currents associated with its operation can yield valuable insights into the mechanisms of solute translocation.

Cytoplasmic Ca2+ oscillations evoked by acetylcholine or intracellular infusion of inositol trisphosphate or Ca2+ can be inhibited by internal Ca2+

In single internally perfused mouse pancreatic acinar cells, changes in the free intracellular Ca2+ concentration ([Ca2+]i) were monitored by measuring the Ca2(+)-dependent transmembrane Cl- current under voltage-clamp conditions. Cytoplasmic Ca2+ oscillations were induced by external acetylcholine (ACh) application, internal infusion of inositol (1,4,5) trisphosphate or its non-metabolizable analogue inositol trisphosphorothioate or by intracellular Ca2+ infusion. Such [Ca2+]i oscillations could be rapidly inhibited by external application of the Ca2+ ionophore ionomycin (10-100 nM). Cytoplasmic Ca2+ oscillations could also be evoked by external caffeine (1 mM) application when the internal perfusion solution did not contain any Ca2+ chelator. In such cases intracellular Ca2+ infusion transiently abolished the [Ca2+]i oscillations. We conclude that although Ca2(+)-induced Ca2+ release is the cause of the ACh-evoked [Ca2+]i oscillations, there is also a negative feed-back since Ca2+ can inhibit Ca2+ release initiated by Ca2+.

Whole cell recording of sugar-induced currents in LLC-PK1 cells

Gigaohm-seal whole cell recording techniques were used to monitor function of the Na(+)-coupled sugar transport system in LLC-PK1 cells. The currents coupled to sugar transport were identified as those that are induced by the presence of 10 mM alpha-methylglucoside (AMG) in either the extracellular or intracellular compartment and were inhibited by addition of 320-800 microM phlorizin to the extracellular bathing medium. The sugar-induced currents are small, 15-20 pA, but of the expected magnitude as determined from the known kinetic parameters for Na(+)-coupled sugar transport in LLC-PK1 cells. The phlorizin-sensitive currents are Na+ dependent and can be studied under conditions in which the net Na+ and sugar flux (and consequently the Na+ electrical current) is in either the inward or outward direction. The reversal potential of the sugar-induced currents measured under conditions with high Na+ and AMG concentrations inside the cell is close to values predicted from thermodynamic principles, assuming a coupling stoichiometry of 2 Na+: 1 sugar for the transport system. The reversal potential of the sugar-induced currents with high extracellular Na+ and AMG is not equal to the predicted value, but it is of the polarity expected for inward-imposed solute gradients. Reasons for the observed discrepancy between observed and calculated values are discussed.

Electrophysiological investigation of the amino acid carrier selectivity in epithelial cells from Xenopus embryo

The electrical responses induced by external applications of neutral amino acids were used to determine whether different carriers are expressed in the membrane of embryonic epithelial cells of Xenopus laevis. Competition experiments were performed under voltage-clamp conditions at constant membrane potential. Gly, L-Ala, L-Pro, L-Ser, L-Asn and L-Gln generate electrical responses with similar apparent kinetic constants and compete for the same carrier.They are [Na]o and voltage-dependent, insensitive to variations in [Cl]o and [HCO3]o, inhibited by pHo changes, by amiloride and, for a large fraction of the current, by MeAIB. The increase in [K]o at constant and negative membrane potential reduces the response, whereas lowering [K]o augments it. L-Leu, L-Phe and L-Pro appear to compete for another carrier. They generate electrogenic responses insensitive to amiloride and MeAIB, as well as to alterations of membrane potential, [Na]o and [K]o. Lowering [Cl]o decreases their size, whereas increasing [HCO3]o at neutral pHo increases it. It is concluded that at least two and possibly three transport systems (A, ASC and L) are expressed in the membrane of the embryonic cells studied. An unexpected electrogenic character of the L system is revealed by the present study and seems to be indirectly linked to the transport function. L-Pro seems to be transported by system A or ASC in the presence of Na and by system L in the absence of Na. MeAIB induces an inward current.

Amino acid efflux in the isolated perfused rat pancreas: trans-stimulation by extracellular amino acids

1. Epithelial uptake and efflux of the non-metabolized system A analogue 2-methylaminoisobutyric acid (MeAIB) and L-serine were studied in the isolated perfused rat pancreas using a dual tracer loading and wash-out technique. Uptakes of 2-[14C]MeAIB and L-[3H]serine were measured relative to D-[3H or 14C]mannitol (extracellular tracer) during a 20 min cell loading period. Maximal uptake for MeAIB (34 +/- 2%, n = 6) occurred within 2-3 min and decreased to 14 +/- 2% after 20 min tracer loading. Uptake for L-serine reached a maximum (62 +/- 4%, n = 7) within 1 min and decreased to 19 +/- 2% after 20 min tracer loading. 2. When tracer wash-out was monitored during subsequent perfusion of the preloaded pancreas with an isotope-free solution, D-mannitol predominantly cleared from a fast exchanging compartment (0.54 +/- 0.05 ml g-1, n = 9) with a time constant (Tfast) of 0.68 +/- 0.04 min. Although MeAIB and L-serine exhibited similar fast phases of wash-out, a much larger efflux occurred from a slowly exchanging pool with respective time constants (Tslow) of 15.47 +/- 0.45 min (n = 6) and 5.98 +/- 0.46 min (n = 7). 3. A rapid vascular challenge of the pancreas with 100 mM-L-serine transiently accelerated cellular efflux of 2-[14C]MeAIB and L-[3H]serine without affecting wash-out of D-[14C]mannitol. Tracer efflux following cell loading with 2-[14C]MeAIB or L-[3H]serine was not stimulated by a challenge with 100 mM-MeAIB. 4. The time course of amino acid evoked 2-[14C]MeAIB and L-[3H]serine efflux paralleled the extracellular dilution profile of a vascular stimulus, suggesting that the acceleration of efflux was due to trans-stimulation. 5. Trans-stimulation of 2-[14C]MeAIB and L-[3H]serine efflux by a further twenty-two naturally occurring and three synthetic amino acids was then examined. L-Proline, N-methyl-DL-alanine, L-lysine and D-lysine selectively stimulated MeAIB efflux. Efflux of both tracer amino acids was accelerated by aminoisobutyric acid (AIB), L-serine, L-alanine, L-cysteine, L-threonine, glycine, branched-chain and large neutral amino acids, but affected negligibly by D-alanine, beta-alanine, MeAIB, L-arginine, L-aspartate, L-glutamate, taurine or D-mannitol. 6. In summary, we have characterized amino acid exchange mechanisms in the isolated pancreas and conclude that efflux of intracellular amino acids from pancreatic acinar cells may be mediated by at least two transporters with differing specificity for extracellular amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Inositol 1,3,4,5-tetrakisphosphate and inositol 1,4,5-trisphosphate act by different mechanisms when controlling Ca2+ in mouse lacrimal acinar cells

In internally perfused single lacrimal acinar cells the competitive inositol 1,4,5-trisphosphate (Ins 1,4,5-P3)-antagonist heparin inhibits the ACh-evoked K+ current response mediated by internal Ca2+ and also blocks both the Ins 1,4,5-P3-evoked transient as well as the sustained K+ current increase evoked by combined stimulation with internal Ins 1,4,5-P3 and inositol 1,3,4,5-tetrakisphosphate (Ins 1,3,4,5-P4). When, during sustained stimulation with both Ins 1,4,5-P3 and Ins 1,3,4,5-P4, one of the inositol polyphosphates is removed, the K+ current declines; whereas removal of Ins 1,4,5-P3 results in an immediate termination of the response, removal of Ins 1,3,4,5-P4 only causes a very gradual and slow reduction in the current. Ins 1,3,4,5-P4 is therefore not an acute controller of Ca2+ release from stores into the cytosol, but modulates the release of Ca2+ induced by Ins 1,4,5,P3 by an unknown mechanism, perhaps by linking Ins 1,4,5 P3-sensitive and insensitive Ca2+ stores.

Inositol 1,3,4,5-tetrakisphosphate is essential for sustained activation of the Ca2+-dependent K+ current in single internally perfused mouse lacrimal acinar cells

We have examined the effects of various inositol polyphosphates, alone and in combination, on the Ca2+-activated K+ current in internally perfused, single mouse lacrimal acinar cells. We used the patch-clamp technique for whole-cell current recording with a set-up allowing exchange of the pipette solution during individual experiments so that control and test periods could be directly compared in individual cells. Inositol 1,4,5-trisphosphate (Ins 1,4,5 P3) (10-100 microM) evoked a transient increase in the Ca2+-sensitive K+ current that was independent of the presence of Ca2+ in the external solution. The transient nature of the Ins 1,4,5 P3 effect was not due to rapid metabolic breakdown, as similar responses were obtained in the presence of 5 mM 2,3-diphosphoglyceric acid, that blocks the hydrolysis of Ins 1,4,5 P3, as well as with the stable analogue DL-inositol 1,4,5-trisphosphorothioate (Ins 1,4,5 P(S)3) (100 microM). Ins 1,3,4 P3 (50 microM) had no effect, whereas 50 microM Ins 2,4,5 P3 evoked responses similar to those obtained by 10 microM Ins 1,4,5 P3. A sustained increase in Ca2+-dependent K+ current was only observed when inositol 1,3,4,5-tetrakisphosphate (Ins 1,3,4,5 P4) (10 microM) was added to the Ins 1,4,5 P3 (10 microM)-containing solution and this effect could be terminated by removal of external Ca2+. The effect of Ins 1,3,4,5 P4 was specifically dependent on the presence of Ins 1,4,5 P3 as it was not found when 10 microM concentrations of Ins 1,3,4 P3 or Ins 2,4,5 P3 were used.(ABSTRACT TRUNCATED AT 250 WORDS)

Does inositol tetrakisphosphate play a role in the receptor-mediated control of calcium mobilization?

The evidence for and against an important role for inositol 1,3,4,5 tetrakisphosphate (Ins 1,3,4,5 P4) in receptor-mediated Ca2+ mobilization is reviewed. Data obtained from patch-clamp whole-cell current recording studies on internally perfused exocrine acinar cells show that the acetylcholine (ACh)-evoked sustained increase in Ca2+-dependent K+ current caused by an increase in [Ca2+]i cannot be mimicked by internal application of inositol 1,4,5-trisphosphate (Ins 1,4,5 P3), but only by a combination of Ins 1,4,5 P3 and Ins 1,3,4,5 P4. The sustained response evoked by Ins 1,4,5 P3 + Ins 1,3,4,5 P4 is dependent on the presence of external Ca2+ as is the effect of ACh. Only those inositol trisphosphates able to evoke Ca2+ release from internal stores can support the action of Ins 1,3,4,5 P4 in evoking responses that are acutely dependent on extracellular Ca2+ (Ca2+ influx). The various arguments presented against an involvement of Ins 1,3,4,5 P4 are discussed. The main point emerging is that most studies are inadequately controlled and it is concluded that there is a strong need for whole-cell current recording studies combined with pipette fluid exchange to be carried out in many more systems. The major problem in this field is that the precise site and mechanism of action of Ins 1,3,4,5 P4 are unknown and that the pathway for Ca2+ uptake during receptor activation is inadequately defined.

Amino acid specificity of the Na+/alanine cotransporter in pancreatic acinar cells

The method of tight-seal whole-cell recording was used to study the amino-acid specificity of the Na+/alanine cotransporter in pancreatic acinar cells. Single cells or small clusters of electrically coupled cells were obtained by enzymatic dissociation of mouse pancreas. Inward currents were measured under 'zero-trans' conditions, i.e., at finite concentrations of Na+ and amino acid at the extracellular side and vanishing concentrations at the cytoplasmic side. The cotransporter, which corresponds to 'system A', as previously defined in the literature, was found to exhibit a wide tolerance to neutral amino acids (L-cysteine, L-serine, L-alanine, glycine, L-phenylalanine). Competition experiments with 2-methylaminoisobutyric acid (MeAIB) indicate that for glycine a second electrogenic transport system exists in pancreatic acinar cells.

Paradoxical effects of endogenous and exogenous insulin on amino acid transport activity in the isolated rat pancreas: somatostatin-14 inhibits insulin action

Regulatory effects of insulin, somatostatin and cholecystokinin on amino acid transport in the isolated perfused rat pancreas have been studied using a rapid dual isotope dilution technique. Uni-directional L-serine transport (15 s) was quantified relative to an extracellular tracer D-mannitol over a wide range of substrate concentrations. In pancreata perfused with 2.5 mmol/l D-glucose, a weighted nonlinear regression analysis of overall transport indicated an apparent Km = 14.4 +/- 1.6 mmol/l and Vmax = 25.9 +/- 1.4 mumol.min-1.g-1 (n = 6). Although L-serine transport was stimulated during perfusion with 100 microU/ml bovine insulin, endogenous insulin (7-25 ng.min-1.g-1) released during continuous perfusion with either 8.8 mmol/l or 16.8 mmol/l D-glucose had no such effect. Exogenous somatostatin-14 (250 pg/ml) or cholecystokinin octapeptide (CCK-8, 3 x 10(-11) mol/l) appeared to increase only the Km for transport. Only CCK-8 evoked a notable protein output (2.9 +/- 0.3 mg.30 min-1.g-1) and juice flow (68 +/- 10 microliters.30 min-1.g-1, n = 3) from the exocrine pancreas. When pancreata were perfused with bovine insulin (100 microU/ml) and somatostatin-14 (250 pg/ml), the stimulatory action of exogenous insulin on L-serine transport was abolished. If endogenous insulin and somatostatin, released concurrently in response to 16.8 mmol/l D-glucose, were conveyed to the exocrine epithelium via an islet-acinar portal axis, it is conceivable that somatostatin modulates the stimulatory action of insulin on basolateral amino acid transport in the exocrine pancreas.

Patch clamp recording from anterior pituitary cells identified by reverse hemolytic plaque assay

The study of hormone secretion by anterior pituitary cells is complicated by the presence of multiple cell types. For unambiguous interpretation of data it is necessary to identify the cells from which measurements are made. We have described a reliable experimental approach involving the identification of cultured cells of a particular type with a reverse hemolytic plaque assay. The electrical characteristics of individual identified cells can then be studied using patch clamp recording. This electrophysiological approach is well suited to the study of complex systems in cultured cells. Although this combined approach requires some expertise in a variety of techniques, it is workable and should yield valuable information regarding the role of ion channels in the cellular control of hormone secretion by the anterior pituitary.

Agonist-induced changes in cell membrane capacitance and conductance in dialysed pancreatic acinar cells of rats

1. Single acinar cells enzymatically isolated from the rat pancreas were subjected to tight-seal whole-cell recordings. Changes in cell membrane capacitance and conductance were simultaneously recorded using a phase-sensitive detection method. 2. Acetylcholine (ACh, 0.05-0.5 microM) and cholecystokinin octapeptide (CCK-8, 10-50 pM) concomitantly induced transient increases in cell membrane current, capacitance and conductance only when cytosolic Ca2+ was weakly chelated by EGTA (70 microM). These responses were prolonged when the cells were dialysed with a solution containing GTP gamma S (a stable analogue of GTP, 50-100 microM), whereas they were inhibited by dialysing with that containing GDP beta S (a stable analogue of GDP). These results suggest that a type of guanine-nucleotide-binding protein (G-protein) could be involved in ACh- or CCK-receptor signalling. 3. The ACh- or CCK-induced responses (with or without GTP gamma S in the cytosol) were all abolished when a high dose of EGTA (1-2 mM) was injected into the acinar cells. In addition, A23187, a calcium ionophore, induced sustained responses when the cytosolic Ca2+ was weakly buffered by 70 microM-EGTA. These results suggest that the secretagogues regulate the changes in cell membrane capacitance and conductance via an increase and decrease of cytosolic Ca2+ concentration. 4. Oscillatory changes in cell membrane conductance and capacitance were consistently observed even without applying secretagogues when the cells were dialysed with a solution containing GTP gamma S (50-100 microM) and cytosolic free Ca2+ ions weakly buffered at about 10(-6) M with a low dose of EGTA and CaCl2. 5. The peak amplitude of changes in cell membrane capacitance induced by ACh or CCK-8, with or without GTP gamma S in the cytosol, varied between 200 and 1000 fF, thereby suggesting that 20-100 zymogen granules can fuse with the luminal cell membrane in response to these agonists in rat exocrine pancreatic acinar cells.

Na+-coupled sugar transport: membrane potential-dependent Km and Ki for Na+

Kinetic analysis of the characteristics of phlorizin binding and of the Na+, sugar, and potential dependence of alpha-methylglucoside (alpha-MG) influx into isolated avian intestinal cells has pointed toward two alternative models for the transport mechanism (D. Restrepo and G. A. Kimmich, J. Membr. Biol. 89: 269-280, 1986). One of these models envisions a potential-dependent Na+ binding event (Na+ well concept) as a part of the molecular mechanism. The data reported here show that the apparent Km for Na+ for sugar transport is sharply dependent on the magnitude of the membrane potential. When intracellular Na+ is absent, the maximal velocity (Vmax) achieved for sugar influx is the same with or without a potential, although Vmax is obtained at a lower Na+ concentration when a potential is imposed (interior negative). Intracellular Na+ severely inhibits the influx of sugar in the absence of a potential, but this effect is largely overcome when a potential is present. The Vmax obtained when intracellular Na+ is present is a function of the potential. These results are consistent with a transport model in which Na+ binding to the Na+-dependent sugar carrier at the extracellular surface of the membrane and debinding at the inner surface of the membrane are both potential-dependent events.