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

Secretagogue-induced changes in system A amino acid transport in the rat exocrine pancreas: stimulation of 2-methylaminoisobutyric acid efflux by carbachol

Secretagogue-induced changes in exocrine pancreatic amino acid transport are poorly understood. In this study uptake of the specific non-metabolized System A amino acid analogue 2-methylaminoisobutyric acid (2-MeAIB) was measured in the isolated perfused rat pancreas during 60 min loading with D-[3H]mannitol (extracellular tracer) and 2-[14C]MeAIB. Tracer 2-MeAIB reached a maximal uptake of 37 +/- 4% (n = 4) after 3 min of loading and gradually decreased to a steady-state uptake of 13 +/- 1%. Infusion of carbachol (3.10(-7) M) during the tracer loading period abolished net tracer 2-MeAIB uptake, and reperfusion in the absence of carbachol restored net uptake to the prestimulus value. Less than 41% of the arterial 2-[14C]MeAIB or D-[3H]mannitol activity appeared in the basal pancreatic secretion. Carbachol evoked a 4.8-fold increase in pancreatic juice flow and appeared to reduce the activity of both tracers in the exocrine secretion. During washout of the pancreas with an isotope-free medium 2-[14C]MeAIB cleared from a rapidly exchanging pool with a time constant (tau 1) of 1.4 +/- 0.3 min (n = 4) and a more slowly exchanging pool with a time constant (tau 2) of 20.7 +/- 1.1 min. Carbachol accelerated efflux of 2-[14C]MeAIB from the epithelium but had no effect on the slow phase of D-[3H]mannitol washout. Our findings suggest that activation of cholinergic receptors modifies Na+-dependent System A amino acid transport in the basolateral membrane of the exocrine pancreatic epithelium.

Cell-to-cell channel conductance during loss of gap junctional coupling in pairs of pancreatic acinar and Chinese hamster ovary cells

Electrical coupling, mediated by gap junctional channels connecting pairs of murine pancreatic acinar cells and Chinese hamster ovary (CHO) cells, was studied using the double whole cell patch clamp technique. Two approaches were used to reduce the junctional conductance (gj) in order to study gj at the single channel level. During spontaneous uncoupling, single channel conductances of 130 pS and 27 pS could be characterized using freshly isolated acinar cells. In most experiments, stepwise conductances could not be discriminated while gj decreased gradually below 10 pS. In CHO cell pairs, discrete junctional channel conductances of 120 pS, 70 pS, 50 pS, 37 pS and 22 pS were identified. Exposure of pancreatic acinar cell pairs to 0.4 mM octanol resulted in rapid and reversible uncoupling. Discrete junctional conductance steps could not clearly be identified down to a gj of about 3 pS. The influence of the composition of the pipette solution on spontaneous uncoupling was investigated. Addition of 5 mM ATP and 0.1 mM cAMP to the pipette electrolyte was sufficient to stabilize coupling in the experimental time range of up to 1 h. Different mechanisms of uncoupling, including an increase of flickering in the channel open state, and modulation of the number of channels exhibiting different conductance or subconductance states are discussed.

Kinetics of the Na+/alanine cotransporter in pancreatic acinar cells

Electric currents associated with Na+-coupled alanine transport in pancreatic acinar cells were investigated by the technique of tight-seal whole-cell recordings. In a previous study the observed concentration dependence of alanine-dependent currents was found to be consistent with a 'simultaneous' transport mechanism with 1:1 stoichiometry. In the present work the sidedness of the cotransporter was investigated by comparing inward (I") and outward currents (I') measured under mirror-symmetrical conditions. I' and I" were found to be nearly equal (within a factor of approx. 2) in a wide range of Na+ and alanine concentrations. The transport model was further tested by 'infinite-cis' experiments with fixed, saturating concentrations of Na+ and L-alanine on one side of the membrane and variable concentrations on the other. By measuring transmembrane currents as a function of Na+ and alanine concentrations, numerical values of the equilibrium dissociation constants of both substrates could be estimated.

The control of ion channels and pumps in exocrine acinar cells

The historical development of ideas concerning mechanisms of exocrine fluid secretion will be traced from the original finding of stimulant-evoked K+ release in 1956 to current models involving Ca2+-activated K+ and Cl- channels.

Sodium-dependent suppression of gamma-aminobutyric-acid-gated chloride currents in internally perfused frog sensory neurones

1. The effects of the Na+ electrochemical potential gradient on gamma-aminobutyric acid (GABA)-induced Cl- currents (ICl) in frog sensory neurones were studied, using a suction pipette technique with which internal perfusion can be accomplished under current- and voltage-clamp conditions. 2. Under current clamp, the depolarizing response to GABA decreased in the presence of external Na+. A similar external Na+-dependent reduction in the GABA-induced inward ICl was observed under voltage clamp. The reversal potential of GABA-induced ICl (EGABA) was nearly equal to the Cl- equilibrium potential (ECl), irrespective of the presence or absence of external Na+. 3. Varying the Na+ influx by changing the holding membrane potential (VH) altered the GABA response: the GABA-induced ICl decreased progressively as VH became more negative. 4. The effects of changing the external and internal Na+ concentrations ([Na+]o and [Na+]i) on the GABA-induced ICl were also studied. Increasing [Na+]o at a constant [Na+]i reduced this current while increasing [Na+]i at a fixed [Na+]o facilitated it. 5. A high temperature coefficient of about 3 was estimated with respect to the percentage reduction in GABA-induced ICl due to [Na+]o. 6. These results indicate that the [Na+]o-dependent suppression of GABA-induced ICl was mediated chiefly by the uptake of GABA subserved by a Na-GABA co-transport mechanism. 7. GABA dose-response measurements were made with and without external Na+. The [Na+]o-induced suppression was more pronounced in relative amount at lower concentrations and in absolute amount at intermediate concentrations. Analysis of these data indicates, however, that the Na+-coupled GABA influx kept increasing at GABA concentrations high enough to nearly saturate GABA-induced ICl, and the same saturating level was observed as in the Na+-free case. This indicates that the electrogenic co-transport current was much smaller so that our measurements of GABA-induced ICl' were contaminated very little. Thus, the present method based on recording of GABA-induced ICl was legitimate for the analysis of the Na-GABA co-transport. 8. By analysing the [Na+]o-dependent suppression of GABA-induced ICl, the stoichiometric ratio of the underlying co-transport was estimated to be one: one Na+ ion per GABA molecule. 9. The ICl induced by GABA agonists such as beta-alanine, taurine, l-GABOB (l-gamma-amino-beta-hydroxybutyric acid) and muscimol was not affected by the amount of external Na+ present, suggesting difference in the affinity between receptor and transport carrier.

Sodium-dependent taurocholate uptake by isolated rat hepatocytes occurs through an electrogenic mechanism

The uptake mechanism for the bile salt, taurocholate, by the liver cell is coupled to sodium but the stoichiometry is controversial. A one-to-one coupling ratio would result in electroneutral transport, whereas cotransport of more than one sodium ion with each taurocholate molecule cause an electrogenic response. To better define the uptake of this bile salt, we measured the effect of taurocholate on the membrane potential and resistance of isolated rat hepatocytes using conventional microelectrode electrophysiology. The addition of 20 microM taurocholate caused transient but significant depolarization accompanied by a significant decrease in membrane resistance. The electrical effect induced by taurocholate mimicked that induced by L-alanine (10 mM), the uptake of which is known to occur through an electrogenic, sodium-coupled mechanism. The sodium dependence of taurocholate-induced depolarization was further confirmed by: (1) replacing Na+ with choline +, and (2) preincubating cells with ouabain (2 mM) or with the Na+-ionophore, gramicidin (25 micrograms/ml); both suppressed the electrogenic response. Further, cholic acid, which inhibits sodium-coupled taurocholate uptake in hepatocytes, inhibited taurocholate evoked depolarization. These results support the hypothesis that sodium-coupled taurocholate uptake by isolated hepatocytes occurs through an electrogenic process which transports more than one Na+ with each taurocholate molecule.

Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: II. Comparison with transport models

In this paper, the results of the preceding electrophysiological study of sodium-alanine cotransport in pancreatic acinar cells are compared with kinetic models. Two different types of transport mechanisms are considered. In the "simultaneous" mechanism the cotransporter C forms a ternary complex NCS with Na+ and the substrate S; coupled transport of Na+ and S involves a conformational transition between states NC'S and NC"S with inward- and outward-facing binding sites. In the "consecutive" (or "ping-pong") mechanism, formation of a ternary complex is not required; coupled transport occurs by an alternating sequence of association-dissociation steps and conformational transitions. It is shown that the experimentally observed alanine- and sodium-concentration dependence of transport rates is consistent with the predictions of the "simultaneous" model, but incompatible with the "consecutive" mechanism. Assuming that the association-dissociation reactions are not rate-limiting, a number of kinetic parameters of the "simultaneous" model can be estimated from the experimental results. The equilibrium dissociation constants of Na+ and alanine at the extracellular side are determined to be K"N less than or equal to 64 mM and K"S less than or equal to 18 mM. Furthermore, the ratio K"N/KS"N of the dissociation constants of Na+ from the binary (NC) and the ternary complex (NCS) at the extracellular side is estimated to be less than or equal to 6. This indicates that the binding sequence of Na+ and S to the transporter is not ordered. The current-voltage behavior of the transporter is analyzed in terms of charge translocations associated with the single-reaction steps. The observed voltage-dependence of the half-saturation concentration of sodium is consistent with the assumption that a Na+ ion that migrates from the extracellular medium to the binding site has to traverse part of the transmembrane voltage.