Electrogenic Sodium Pump Research Articles

Two components of inhibition induced by [formula omitted] in a snail neurone

1. 1. The mechanism of the effects on a snail neurone of f-β- phenylpropionyl- l-tyrosine (BPLT), a derivative of l-Phe- l-Tyr, which has been reported to produce a marked inhibition, was analyzed under voltage clamping. 2. 2. Slow and long-lasting outward current, together with a membrane conductance increase, was induced by BPLT (1mM) at the holding voltage (−55mV). However, the I–V curve affected by BPLT ( I– V BPLT) never intersected that of the control ( I– V cont) in the physiological state. 3. 3. The current induced by BPLT ( I BPLT), which is the difference between I– V BPLT and I– V cont, was shifted according to the extracellular K + concentration, but the reversal potential of i BPLT was different from the K + equilibrium potential ( E K). I BPLT is partly the K + current ( i BPLT-k), but includes another current ( i BPLT-nk). i BPLT has no component of the Cl − current. The change in the extracellular Ca 2+, Mg 2+ and Na + concentrations, from normal to free, hardly i BPLT. 4. 4. Under ouabain (0.1 mM), I– V BPLT intersected I– V cont near e k; I BPLT-NK is due to electrogenic sodium pump activation ( I BPLT-pump). Under TEA (10 mM), the I BPLT-pump was isolated, an effect that was hardly dependent, very slightly inverse parabolic, and somewhat greater at a more negative membrane potential. This is accelerated in the higher extracellular K + concentration.

Passive electrical properties and electrogenic sodium transport of cultured guinea-pig coronary endothelial cells.

1. Coronary endothelial cells were isolated from adult guinea-pig hearts (Nees, Gerbes & Gerlach, 1981) and the electrical properties of primary cultures were studied using the tight-seal whole-cell recording mode of the patch clamp technique. 2. On the third or fourth day in culture whole-cell clamp records from single coronary endothelial cells were obtained at 37 degrees C. The resting potential was -33 +/- 6 mV (n = 10). The membrane time constant determined with rectangular current pulses was 68 +/- 22 ms (n = 10). 3. In voltage clamp experiments, no time-dependent membrane conductance changes were found in the range -80 to +40 mV. The current-voltage relation was linear in normal physiological salt solution containing 5.4 mM-K+. The input resistance was 1.7 +/- 0.4 G omega. When the external K+ concentration was increased to 116 mM the cells depolarized to about -3 mV and the clamp currents showed marked inward rectification. 4. Between days four and seven in culture the endothelial cells formed confluent monolayers which showed the characteristic 'cobblestone' morphology. The input resistance of cells in a monolayer was 8 +/- 3 M omega, i.e. a factor of 200 lower than that found in single cells. It was concluded that the cells in the confluent monolayer are coupled electrically by gap junctions. 5. Exposure of coronary endothelial cells to K+-free solution for 5 min produced a depolarization of about 8 mV. Upon readmission of normal external K+ the cells transiently hyperpolarized by up to 20 mV. This transient hyperpolarization decayed with a time constant of 1.9 +/- 0.3 min. 6. The transient hyperpolarization could be abolished by application of 2 x 10(-4) M-dihydro-ouabain (DHO). Application of DHO in the steady state produced a depolarization of 8 +/- 1 mV. From these findings it was concluded that coronary endothelial cells possess an electrogenic sodium pump which contributes about -8 mV to the resting potential. 7. From the passive electrical properties of single cells and the morphological data available it was calculated that endothelium in situ may have a large electrical space constant, probably between 250 and 550 microns. 8. The functional implications of the large space constant of the endothelial monolayer are discussed. It is suggested that intra-endothelial conduction of electrical signals from capillaries to the resistance vessels may be involved in the local regulation of blood flow in the intact heart.

Electrical Properties and Anion Permeability of Doubly Rectifying Junctions in the Leech Central Nervous System

ABSTRACT A study has been made of the electrical connections between touch sensory (T) neurones in the leech central nervous system (CNS) which display remarkable double rectification: depolarization spreads in both directions although hyperpolarization spreads poorly. Tests were made to determine whether this double rectification was a property of the junctions themselves or whether it resulted from changes in the length constants of processes intervening between the cell body and the junctions. Following trains of action potentials, T cells and their fine processes within the neuropile became hyperpolarized through the activity of an electrogenic sodium pump. When any T cell was hyperpolarized by 25 mV by repetitive stimulation, hyperpolarization failed to spread to the T cells to which it was electrically coupled. Further evidence for double rectification of junctions linking T cells was provided by experiments in which Cl− was injected electrophoretically. Cl− injection into one T cell caused inhibitory potentials recorded in it to become reversed. After a delay, Cl− spread to reverse IPSPs in the coupled T cell. Movement of Cl−, like current flow, was dependent on membrane potential. When the T cell into which Cl− was injected was kept hyperpolarized, Cl− failed to move into the adjacent T cell. Upon release of the hyperpolarization in the injected T cell, Cl− moved and reversed IPSPs in the coupled T cell. Together these results indicate that the gating properties of channels linking T cells are voltage-dependent, such that depolarization of either cell allows channels to open whereas hyperpolarization causes them to close.

Indirect effects of acetylcholine on the electrogenic sodium pump in bull‐frog atrial muscle fibres.

1. Effects of acetylcholine (ACh) on the activity of electrogenic Na+ pump in bullfrog atrial muscle fibres were examined using the single sucrose-gap voltage clamp technique. 2. In the K+-free solution, 10 microM-ACh induced a large outward current (ACh-induced current) with an increase in the membrane conductance. 3. The amplitude of the ACh-induced current decreased to 15% of the control 10 min after application of 1 microM-ouabain, suggesting the contribution of electrogenic Na+ pump to the ACh-induced current. The remaining ACh-induced current was not affected even if the concentration of ouabain was increased ten times. 4. The K+-activated current induced by an activation of the electrogenic Na+ pump was suppressed or reversed its direction during the course of the ACh-induced current. 5. The ACh-induced current was completely inhibited by applications of either atropine or barium ions while the K+-activated current was not affected. 6. Both ouabain-sensitive and -insensitive ACh-induced currents were decreased when the membrane was hyperpolarized and eliminated around -95 mV. 7. The ouabain-sensitive component was decreased by increasing the external K+ concentration [K+]o; the proportions of this current to ACh-induced current in 0.5, 0.75, 1 and 2 mM [K+]o were 54, 42, 34 and 14%, respectively. 8. The current-voltage (i-v) relation obtained in 2 or 4 mM [K+]o, where the currents carried by Na+ and Ca2+ were blocked by application of 1 microM-TTX and 1 mM-Cd2+, exhibits marked inward-going rectification but does not show a clear N-shaped feature. Ba2+ (1 mM) induced an inward current at the holding potential (-80 mV) and eliminated the inward-going rectification of the membrane. 9. These results suggest that the increase in the K+ permeability by ACh increases the concentration of K+ immediately outside of the membrane, which in turn stimulates the electrogenic Na+ pump mechanism. The physiological significance of the action of ACh on the electrogenic Na+ pump in bull-frog atrium is discussed in relation to the background K+ current (IK,1).

Role of sodium pump in membrane potential gradient of canine proximal colon.

A large gradient in membrane potential exists through the thickness of the circular layer in canine colonic muscles. This study tested the effects of several experimental manipulations known to block electrogenic sodium pumping on the resting potentials of colonic muscles. Membrane potentials were recorded with microelectrodes from cells through the circular muscle layer. In cells adjacent to the submucosal surface of the circular layer, application of ouabain (10(-6) to 10(-5) M) caused an average membrane depolarization of 36 mV. Removal of the external K+ resulted in depolarizations similar to the effect of ouabain. Readmission of K+ (5.9 mM) produced repolarization and an additional hyperpolarization that averaged 13 mV beyond the resting potential. When exposed to 15 mM K+, cells hyperpolarized well beyond the estimated potassium equilibrium potential (EK). Ouabain blocked the repolarization in response to reintroduction of external K+. Lowering the bath temperature to 20 degrees C rapidly depolarized membrane potential; rewarming repolarized cells. Ouabain and K+-free solutions blocked the repolarization response to rewarming. Cells also depolarized when exposed to solutions in which the NaCl was replaced with LiCl. Membrane potentials of cells within the bulk of the circular layer decreased as a function of distance from the submucosal border. Cells at the myenteric border of the circular muscle were not significantly affected by ouabain and K+-free solution, but these treatments abolished the gradient in membrane potential across the circular layer.(ABSTRACT TRUNCATED AT 250 WORDS)

A slow potassium conductance after action potential bursts in rabbit vagal C fibers.

Sucrose gap recordings were made from vagus nerve in rabbit to examine the mechanisms underlying the generation of the hyperpolarization that follows a burst of evoked action potentials in unmyelinated C fibers. Analysis of the posttetanic hyperpolarization was made by fitting the membrane potential changes with the sum of two exponential components. The posttetanic hyperpolarization consisted of two separable components with time constants of approximately 0.5 and 30 s. The slower exponentially decaying component was dependent on an increase in electrogenic sodium pumping as shown by the effect of ouabain and changes in extracellular chloride. The faster-decaying exponential component was caused by a potassium conductance as shown by the effect of varied extracellular potassium. This potassium conductance appears to be novel as its dynamics vary with the frequency and duration of the burst yet increases in reduced calcium. It is suggested that this slow decaying and modifiable potassium conductance can play a role in modulation of preganglionic and presynaptic action potential conduction.

Orthovanadate inhibits adrenaline-induced hyperpolarization in amphibian sympathetic ganglia

The adrenaline-induced hyperpolarization of neurones in sympathetic ganglia of Rana pipiens was recorded by means of the sucrose gap technique. This hyperpolarization was inhibited in a concentration-dependent manner by sodium orthovanadate (0.1–l.0mM). Although orthovanadate inhibited the hydrolysis of ATP by preparations of microsomal Na +/K + ATPase from frog nerve (IC 50 = 3.0 μ M), extracellulariy-applied orthovanadate did not inhibit electrogenic sodium pumping in intact sympathetic ganglia of Rana pipiens. This and other observations indicated that extracellularly-applied orthovanadate did not enter sympathetic neurones and did not have access to intracellular enzyme systems. It is suggested that orthovanadate acts at an extracellular site to inhibit the adrenaline-induced hyperpolarization.

Membrane properties of bovine airway smooth muscle cells: Effects of maturation

Airway smooth muscle cells of bovine trachealis muscle from immature (2 weeks old), developing (5 months old) and mature animals (>5 years old) have resting membrane potentials of −63.5±0.4 mV, −62.2±0.8 mV and −60.3±0.7 mV, respectively. These resting potentials were not significantly different. The contribution of the electrogenic sodium pump to the resting membrane potential of airway smooth muscle cells decreases significantly with age being 38.3%±0.9, 30.6%±0.8 and 21.3%±0.9 in tissues from immature, developing and mature cows, respectively. The contribution of the electrogenic sodium pump to resting membrane potential was not influenced by the degree of stretch (applied load) of the airway smooth muscle cells. However, resting membrane potential was reduced with increasing tone especially when loads between 5 to 10 g were applied. The densities of Na + K + pump sites, as measured by [ 3H] ouabain binding, decreased significantly as a consequence of maturation and were 4.12±5.23, 3.08±0.26 and 1.94±0.38 pmoles/mg protein ouabain binding in tissues from immature, developing and mature animals respectively. The affinity of the pump sites for the cardiac glycoside, ouabain, did not change during maturation. We conclude that maturation alters both the number of Na + K + pump sites and the contribution of the electrogenic sodium pump to the membrane potential of airway smooth muscle cells. These age related changes may contribute to the reduced airway reactivity to both bronchoconstrictor and bronchodilator agents previously observed both in vitro and in vivo.

Neurohumoral control of ionic pumping in molluscan muscle—II. evidence for an electrogenic sodium pump in radular protractor muscle of Busycon canaliculatum

Abstract 1. A sucrose gap technique was used to study the effects of brief periods of superfusion with solutions in which the potassium content of artificial sea water was reduced or omitted. 2. Stepwise reduction in bath potassium had a complex effect, culminating in the response to potassium-free solution. This was composed of a rapid initial hyperpolarizing phase, overtaken by a slower depolarizing phase, which was accompanied by force. 3. Readmission of bath potassium induced a transient after-hyperpolarization. 4. There was a high degree of individual variability in RPM preparations from different animals. This was particularly evident in cases in which either the hyperpolarizing phase or the depolarizing phase predominated, in the response to zero-potassium, but the muscles from any one animal showed reproducible responses. 5. The RPM behaved as predicted on Nernst equation grounds, to the extent that initial hyperpolarization showed stepwise increases with stepwise reduction in [K + ] 0 , but as the steps approached zero-potassium there was a stepwise increase in the slower depolarizing response, suggesting reduction in electrogenic Na-K exchange. 6. In Na-free solution the depolarizing phase of the response to zero-K was abolished, leaving only an enhanced hyperpolarizing phase. 7. Abrupt chilling had a depolarizing effect. 8. There was only a slight increase in resistance during the action of zero K.

Arachidonate activates muscle electrogenic sodium pump and brain microsome Na +,K +-ATPase under suboptimal conditions

Arachidonate 5 × 10 −5 mol·l −1 increased the rate of hyperpolarization induced in Na +-loaded mouse diaphragm fibers by 5 mmol·l −1 K +. When applied to Na +-loaded muscles without potassium, arachidonate 1 × 10 −6 and 5 × 10 −5 mol·l −1 induced a ouabain-sensitive hyperpolarization of the muscle fibers. The activity of rat brain microsomal Na +,K +-ATPase was stimulated by 1 × 10 −7−5 × 10 −6 mol·l −1 arachidonate in reaction media with reduced amounts of ATP or K + and after short-lasting sonication of the samples. It was concluded that, under particular conditions, arachidonate might serve as a Na +,K +-ATPase activator or inhibitor regulating its ion transport and electrogenicity.

The electrophysiological properties of the resting thyroid follicular cell

Glass microelectrodes have been useful in the study of the electrical properties of the resting thyroid follicular cell membrane. The resting transmembrane potential (RMP) has probably been underestimated in earlier work, possible as a result of leak artefacts, and it is clear that in most species the RMP is certainly greater than -60 mV. The ratio of membrane Na+ permeability to K+ permeability (PNa/PK) is of the order of 0.07 to 0.08, and Cl- is possibly (although not definitely) distributed in a passive fashion across the cell membrane, indicating that the transmembrane K+ gradient is the most important factor in the generation of the RMP. The existence of an electrogenic sodium pump in the follicular cell membrane has been demonstrated: the pump contributes about -2 mV to the RMP under control conditions. Follicular cells are completely electrically coupled, the basic coupled cellular unit probably being equivalent to the individual thyroid follicle, and the specific membrane resistance and specific membrane capacitance have been calculated to be 5 k omega. cm2 and 3.6 microF/cm2 respectively.

Mechanism of impaired myogenic response in cerebral blood vessels during posthypoxic recovery

Cerebral blood flow autoregulation is impaired in newborn animals when a brief period of hypoxia is followed by normoxia. Because myogenic mechanisms are widely thought to be responsible for autoregulation, this study examined the effect of hypoxia and reoxygenation on the isometric mechanical function of isolated cerebral (basilar) arterial smooth muscle made to contract rhythmically and thus simulate the behaviour of arterioles. Although at rest a small amount of active tension was present in these preparations, this tension was further increased and the muscle was rendered spontaneously rhythmic by treatment with 4-aminopyridine. Reduction of bath [Formula: see text] from 120 to 25 mmHg (1 mmHg = 133.32 Pa) caused an increase in tension followed by a return towards normal tension. At this time rhythmic oscillations gradually decreased in amplitude. Restoration of normal oxygen tension caused a rapid reduction and an undershoot in tension followed by recovery to normal level. Two possible mechanisms for this undershoot, which may be due to a decrease in myogenic responsiveness and therefore of autoregulatory function, were tested. 8-Phenyltheophylline, an antagonist of adenosine receptors, caused a decrease in the inhibitory mechanical effects of adenosine (10−6–10− M). However, in the presence of 8-phenyltheophylline, there was no change in the undershoot in tension caused by reoxygenation, thus ruling out involvement of adenosine released from the vessel in smooth muscle inhibition due to hypoxia. In a separate series of experiments, the role of an electrogenic sodium pump in causing the undershoot was tested. Treatment with 5-hydroxytryptamine in a K+ -free bathing medium caused an increase in basilar artery tension. When normal potassium concentration was restored the muscle relaxed with a transient undershoot below the basal tension level. In muscle preparations pretreated with ouabain (3 × 10−6 M), the K+-induced undershoot was abolished and replaced by a transient contraction. The same concentration of ouabain decreased by 92% the tension undershoot caused by reoxygenation. These experiments rule out participation of adenosine released from the blood vessel in the loss of myogenic tone during reoxygenation. The data also indicate that reactivation of the sodium pump may be responsible to some extent for the undershoot in tone during reoxygenation of isolated canine basilar artery.

Effect of cooling on vascular smooth muscle from the thirteen-lined ground squirrel

Peripheral vascular resistance in the ground squirrel ( Spermophilus tridecemlineatus) increases when the animal enters hibernation. The goals of this study were (1) to determine if a change in vascular reactivity contributes to this hemodynamic response, and (2) to compare the effects of temperature on vascular responsiveness in a hibernator (ground squirrel) and a nonhibernating mammal (rat). Helically cut strips of aortae and femoral arteries were mounted in organ chambers (37 °C) and isometric contractions were recorded. The arteries were made to contract in response to exogenous norepinephrine (5.9 × 10 − 7 M). Cooling the organ chamber (11 °C) potentiated contractions to norepinephrine (5–15% increase) in ground squirrel femoral arteries but depressed those (80–100% decrease) in ground squirrel aortae and rat aortae and femoral arteries. Contractions in response to depolarizing concentrations of potassium in ground squirrel femoral arteries were depressed by cooling (11 °C), suggesting that the augmented response to norepinephrine at low temperature is specific. Treatment with indomethacin, propanolol, and ouabain did not alter the potentiating effect of temperature on contractions to norepinephrine in ground squirrel femoral arteries. Apparently, the potentiation is not related to prostaglandins generated in the vascular wall, to blockade of β-adrenergic receptors, nor to inhibition of the electrogenic sodium pump. The observations are consistent with the hypothesis that a change in vascular responsiveness contributes to the regional control of blood flow in hibernation. This adaptive response is specific in that it does not occur in the aorta of the ground squirrel and the response is not present in the vasculature of the rat, a nonhibernating mammal.

Relative contribution of two mechanisms to post-stimulus hyperpolarization in leech retzius cells

1. 1. In Retzius cells of the horse leech, Haemopis sanguisuga, intensive firing is followed by prolonged post-stimulus hyperpolarization (PSH) mediated by an increase in K-conductance and activation of the electrogenic sodium pump. 2. 2. The recovery of membrane potential during PSH can be described by a biexponential model from which the relative contribution of the early and the late component to the peak amplitude of PSH may be calculated. 3. 3. The time course of the calculated early component coincides with the time course of changes that occur in input membrane resistance during PSH. 4. 4. The relative contribution of the late component increases linearly with an increase in stimulus duration. The rate constants of the two phases did not change with changes in stimulus duration. 5. 5. It was concluded that the proposed model is useful for the estimation of the relative contribution and kinetics of two PSH mediating mechanisms (K-conductance increase and activation of an electrogenic sodium pump) in different experimental conditions.

Opposite effects of adrenaline and ouabain on the resting potential of rat atrial cells

In the left rat atrium changes in diastolic potential (E max) evoked by sudden stimulation train were modified by adrenaline and ouabain. The early stimulation depolarization phase (SDP) of E max occuring on stimulation was shortened by adrenaline, but lengthened and strongly enchanced by ouabain. The stimulation repolarization phase (SRP) following SDP was markedly inhibited by ouabain, while accelerated and increased by adrenaline. In continuously stimulated atria E max was decreased by ouabain and augmented by adrenaline. The adrenaline-induced hyperpolarization was reduced or suppressed in the presence of 10-4 M or 10-3 M ouabain, respectively. The present data suggest that adrenaline could stimulate the electrogenic sodium pump in the rat atrium.

Activation of electrogenic sodium pump in hippocampal CA1 neurons following glutamate-induced depolarization

Intracellular recordings were obtained from guinea pig hippocampal CA1 pyramidal neurons maintained in vitro. Focal applications of glutamate produced depolarizations followed by prolonged hyperpolarizations. The mechanisms underlying this postglutamate hyperpolarization (PGH) were investigated. PGH did not reverse polarity with hyperpolarization to potentials at or near the presumed K+ equilibrium potential. A transient increase in conductance was associated with the PGH; control values returned well before the termination of PGH. Application of Mn2+, an antagonist of voltage-dependent calcium conductance, blocked synaptic transmission and the afterhyperpolarization (AHP) that follows a directly evoked train of action potentials but did not diminish the PGH or the transient conductance increase. Intracellular application of the calcium chelator ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid blocked AHP but did not affect PGH. Reductions in temperature from 37 to 27-32 degrees C reduced the amplitude of PGH and prolonged its duration but increased the amplitude and duration of AHP. The transient conductance increase associated with PGH was unaffected. Application of strophanthidin, a specific antagonist of Na+-K+-ATPase, reversibly blocked PGH and led to large increases in the amplitude and duration of the AHP. It is concluded that PGH is produced by activation of the electrogenic sodium pump by glutamate-induced excitation. As such, PGH is a useful physiological assay of electrogenic sodium transport. In addition, maintenance of the Na+ gradient by the sodium pump is important for the buffering of Ca2+ influx.

Vasodilator proteins: role in delayed cerebral vasospasm.

Blood proteins could play a critical role in the pathogenesis of cerebral vasospasm in subarachnoid hemorrhage (SAH) as agonists and as antagonists of vasoconstriction. The present study was designed primarily to quantify the inhibition produced by antithrombin III of the phasic responses elicited by cumulative doses of KCl, serotonin (5-HT), uridine triphosphate (UTP), and thrombin in isolated canine basilar arteries, and to ascertain whether other proteins might act similarly. Antithrombin III (1 unit/ml and 3 units/ml) given 2 min beforehand inhibited all agonists. The inhibition was not dependent on a functional endothelium nor due to stimulation of the electrogenic sodium pump. Alpha2-macroglobulin (0.1 mg/ml and 0.4 mg/ml) inhibited the contractile responses to high K+, 5-HT and thrombin. Kallikrein (1 and 4 units/ml) did not inhibit UTP but inhibited high K+ and 5-HT through an effect on the endothelium. Kallikrein (1 unit/ml) irreversibly blocked the responses to thrombin. Globulins (3 mg/ml) and fibrinogen (0.3 mg/ml) were not inhibitory. The results demonstrate that anticoagulant proteins are very effective nonspecific inhibitors of the vasoconstriction, whereas the serine protease kallikrein selectively blocks thrombin. The remarkable potency of antithrombin III suggests that it may protect cerebral arteries from exhibiting vasospasm in SAH.

Neurohumoral control of ionic pumping in molluscan muscle—I. Evidence for an electrogenic sodium pump in ventricular muscle of Aplysia californica

1. Abrupt warming induces a transitory hyperpolarization across a sucrose gap in ventricular myocardium of Aplysia californica. 2. Abrupt cooling induces a transitory depolarization. 3. The effects of a sudden change of temperature are reversed in bathing medium lacking potassium. 4. Ouabain counteracts the depolarization induced by cooling. 5. Stepwise reduction in bath potassium has a stepwise depolarizing excitatory effect. 6. Acetylcholine treatment or reduction in bath sodium counteracts the depolarizing effect of reduction in bath potassium. 7. The effects of warming and cooling, which are counteracted by ouabain or potassium deprivation, as well as the depolarizing effects of potassium deprivation, support the presence of an electrogenic sodium pump. The action of acetylcholine in counteracting depolarization is congruent with its hyperpolarizing effect on this muscle.

Evidence for the activation of an electrogenic sodium pump in leech retzius neurons following kainate depolarization

1. Intracellular recordings were made from Retzius cells in the segmental ganglia of the leech, Hirudo medicinalis. The mechanism associated with the afterhyperpolarization following kainate depolarization in these cells was investigated. 2. This kainate-induced afterhyperpolarization was dependent on external sodium and potassium and was blocked or reduced by strophanthidin. 3. It is concluded that it may be due to the activation of an electrogenic sodium pump.

The significance of adrenaline-induced potentiation of electrogenic sodium pumping in bullfrog sympathetic ganglia.

Two different electrophysiological responses in amphibian sympathetic ganglia were studied by means of the sucrose gap technique; the potassium-activated hyperpolarization (KH) which serves as an index of electrogenic Na+ pumping, and the hyperpolarization induced by adrenaline (AdH). Under appropriate experimental conditions, 0.1 microM adrenaline potentiated the KH to 121.5 +/- 7.5% of control (n = 7). This potentiation was blocked by both yohimbine (50 nM) and prazosin (1 microM) but not by propranolol (1 microM). Clonidine (10 nM) potentiated the KH to 113.5 +/- 3.4% of control (n = 5), whereas methoxamine (0.1 microM) was ineffective. Several lines of evidence argued against the hypothesis that the AdH may be generated, in whole or in part, by stimulation of the Na+ pump. For example, the AdH was sometimes completely unaffected when the KH was blocked by ouabain, and the AdH was eliminated by 2 mM Ba2+ even though this cation enhanced membrane hyperpolarization accompanying electrogenic Na+ pumping. These results imply that the electrogenic Na+ pump is not involved in the short-term electrophysiological effects of catecholamines. Despite this, it is possible that the homeostasis of Na+ and K+ in nerve may be regulated by alpha-adrenergic mechanisms.