Effect of fatty acids on plasma membrane lipid dynamics and cation permeability in neuroblastoma cells

In this study the effects of experimental modifications of plasma membrane lipid lateral mobility on the electrical membrane properties and cation transport of mouse neuroblastoma cells, clone Neuro-2A, have been studied. Short-term supplementation of a chemically defined growth medium with oleic acid or linoleic acid resulted in an increase in the lateral mobility of lipids as inferred from fluorescence recovery after photobleaching of the lipid probe 3,3′-dioctadecylindocarbocyanide iodide. These changes were accompanied by a marked depolarization of the membrane potential from ?51 mV to ?36 mV, 1.5 h after addition, followed by a slow repolarization. Tracer flux studies, using ⁸⁶Rb⁺ as a radioactive tracer for K⁺, demonstrated that the depolarization was not caused by changes in (Na⁺ + K⁺)-ATPase-mediated K⁺ influx or in the transmembrane K⁺ gradient. The permeability ratio ($\text{P}{}_{\mathrm{<mtext>Na</mtext>}}\text{P}{}_{\mathrm{<mtext>K</mtext>}}$), determined from electrophysiological measurements, however, increased from 0.10 to 0.27 upon supplementation with oleic acid or linoleic acid. This transient rise of $\text{P}{}_{\mathrm{<mtext>Na</mtext>}}\text{P}{}_{\mathrm{<mtext>K</mtext>}}$ was shown by ²⁴Na⁺ and ⁸⁶Rb⁺ flux measurements to be due to both an increase of the Na⁺ permeability and a decrease of the K⁺ permeability. None of these effects occurred upon supplementation of the growth medium with stearic acid.     

Effect of oxytocin on the electrical potential and ion permeability of the apical membrane of frog gall bladder epithelial cells

The influence of oxytocin on the intracellular Na+ and K+ concentrations, the level of transmembrane potential differences, and on the relative ionic permeability (PNa/PK) of the apical zones of the superficial epithelium membrane was studied in experiments on the isolated frog gallbladder (GB). Oxytocine introduced into the outer incubation solution in a dose of 20 mulliunits/ml caused a reduction of transmembrane potential difference, and an increase of PNa/pk coefficient and an insignificant shift of the Na+ and K+ concentrations in the intracellular medium. Thirty minutes after the oxytocine action of the organ the membrane potential (MP) of the cells decreased from 52.7 mV to 38.7 mV (the cell is negatively charged inside), and PNa/PK increased from 0,083 (control) to 0,175 (test) with a simultaneous increase in the intracellular Na+ concentration by 18.3 milliequiv./kg of (H2O)i. Such a shift in the intracellular Na+ and K+ concentrations may cause a decrease of the MP by only--0.7 mV, but actually the membrane potential decreased by--14.0 mV. Thus, the reduction of the transmembrane potential difference results from increase of PNa/PK under the influence of oxytocine. No electrogenic ionic transport through the apical membrane of frog gallbladder epithelial cells was revealed.     

Mechanism of depolarization of rat cortical synaptosomes at submicromolar external Ca2+ activity. The use of Ca2+ buffers to control the synaptosomal membrane potential.

Rat cortical synaptosomes responded to a reduction of external Ca2+ from pCa 3.5 to pCa 4.8 in the absence of MgCl2 with a slight decrease of internal K+ and an increase of Na+. The effects were prevented by tetrodotoxin or millimolar concentrations of MgCl2. Further lowering of external pCa to 7.7 with N-hydroxyethylethylenediaminetriacetate evoked a rapid fall of internal K+, which was specifically blocked by Ruthenium Red; tetrodotoxin and nifedipine were ineffective. A linear relationship was established between K+ and methyltriphenylphosphonium cation distribution ratios by varying external pCa between 4.8 and 7.7, indicating that K+ efflux resulted from a depolarization of the plasma membrane. An increase of Na+ permeability was suggested by the synaptosomes' gain of Na+ and the disappearance of the depolarization in an Na+-free sucrose medium. According to the constant field equation, the permeability ratio PNa/PK increased from 0.029 at pCa4.8 to 0.090 at pCa 7.7 with plasma membrane potentials of -74mV and -47mV, respectively. Since the plasma membrane responded to variation of external Ca2+ activities in the micromolar range with a graded and sustained depolarization, the use of Ca2+ buffers to control membrane potentials is suggested.     

Sodium channel selectivity. Dependence on internal permeant ion concentration

The selectivity of sodium channels in squid axon membranes was investigated with widely varying concentrations of internal ions. The selectivity ratio, PNa/PK, determined from reversal potentials decreases from 12.8 to 5.7 to 3.5 as the concentration of internal potassium is reduced from 530 to 180 to 50 mM, respectively. The internal KF perfusion medium can be diluted by tetramethylammonium (TMA), Tris, or sucrose solutions with the same decrease in PNa/PK. The changes in the selectivity ratio depend upon internal permeant ion concentration rather than ionic strength, membrane potential, or chloride permeability. Lowering the internal concentration of cesium, rubidium, guanidnium, or ammonium also reduces PNa/Pion. The selective sequence of the sodium channel is: Na greater than guanidinium greater than ammonium greater than K greater than Rb greater than Cs.     

Ionic permeability of K, Na, and Cl in potassium-depolarized nerve. Dependency on pH, cooperative effects, and action of tetrodotoxin.

The passive ionic membrane conductances (gj) and permeabilities (Pj) of K, Na, and Cl of crayfish (Procambarus clarkii) medial giant axons were determined in the potassium-depolarized axon and compared with that of the resting axon. Passive ionic conductances and permeabilities were found to be potassium dependent with a major conductance transition occurring around an external K concentration of 12-15 mM (Vm = -60 to -65 mV). The results showed that K, Na, and Cl conductances increased by 6.2, 6.9, and 27-fold, respectively, when external K was elevated from 5.4 to 40 mM. Permeability measurements indicated that K changed minimally with K depolarization while Na and Cl underwent an order increase in permeability. In the resting axon (K0 = 5.4 mM, pH = 7.0) PK = 1.33 X 10(-5), PCl = 1.99 X 10(-6), PNa = 1.92 X 10(-8) while in elevated potassium (K0 = 40 mM, pH 7.0), PK = 1.9 X 10(-5), PCl = 1.2 X 10(-5), and PNa = 2.7 X 10(-7) cm/s. When membrane potential is reduced to 40 mV by changes in internal ions, the conductance changes are initially small. This suggests that resting channel conductances depend also on ion environments seen by each membrane surface in addition to membrane potential. In elevated potassium, K, Na, and Cl conductances and permeabilities were measured from pH 3.8 to 11 in 0.2 pH increments. Here a cooperative transition in membrane conductance or permeability occurs when pH is altered through the imidazole pK (approximately pH 6.3) region. This cooperative conductance transition involves changes in Na and Cl but not K permeabilities. A Hill coefficient n of near 4 was found for the cooperative conductance transition of both the Na and Cl ionic channel which could be interpreted as resulting from 4 protein molecules forming each of the Na and Cl ionic channels. Tetrodotoxin reduces the Hill coefficient n to near 2 for the Na channel but does not affect the Cl channel. In the resting or depolarized axon, crosslinking membrane amino groups with DIDS reduces Cl and Na permeability. Following potassium depolarization, buried amino groups appear to be uncovered. The data here suggest that potassium depolarization produces a membrane conformation change in these ionic permeability regulatory components. A model is proposed where membrane protein, which forms the membrane ionic channels, is oriented with an accessible amino terminal group on the axon exterior. In this model the ionizable groups on protein and phospholipid have varied associations with the different ionic channel access sites for K, Na, and Cl, and these groups exert considerable control over ion permeation through their surface potentials.     

Modulation of Ca2+-mediated K+-gating of erythrocyte ghosts by external Ca-EGTA

Using 86Rb+ as a marker for K+ permeability, we find that extracellular Ca-EGTA influences the rate of 86Rb+ efflux from erythrocyte ghosts preloaded with 86Rb+ and "buffered" Ca2+. At an internal free Ca2+, where the rate of 86Rb+ efflux is minimal and uninfluenced by either external EGTA or external Ca2+, external Ca-EGTA at 0.2-0.5 mM can raise the flux rate to as high as can be attained by raising internal Ca2+, in the presence of an excess externally either of Ca2+ or of EGTA. Higher concentrations of Ca-EGTA (up to 1-2 mM) diminish the flux rate. External Ca-EDTA or Mg-EDTA can substitute for Ca-EGTA in enhancing and suppressing flux rate. The peak rate is insensitive to external free Ca2+ but depends on internal Ca2+; internal Mg-EDTA does not substitute for internal Ca-EGTA. Thus, the erythrocyte membrane is asymmetric with respect to its interaction with Ca2+ and Ca-EGTA. Also, 22Na+ does not substitute for 86Rb+. The peak rate of 86Rb+ flux produced by external Ca-EGTA is diminished by chlorpromazine (0.1 mM) and augmented by 1-propranolol (25 microM), in the same way as the rate produced by increasing internal Ca2+. The results suggest that external Ca-EGTA enhances the affinity of internal Ca2+ for its receptor(s) which operate the K+-gate at the inner surface of the membrane. At external concentrations of Ca-EGTA above 1-2 mM, 86Rb+ flux rate again rises with increase of Ca-EGTA. This phenomenon does not depend upon internal Ca2+, is not affected by chlorpromazine or by 1-propranolol, and is associated with an enhanced permeability to 22Na+, inulin, and haemoglobin.     

Electrical properties of normal and transformed mammalian cells.

The transmembrane potential difference, Em, and DC membrane resistance were measured in 3T3 and polyoma virus-transformed 3T3 cells. Em was a function of cell density and was -12 and -25 mV for the normal and transformed cells, respectively. The external concentrations of K+, Na+, and Cl were varied in order to study the nature of the differences between the two cell types. The relative permeability of ions was calculated to be: PNa/PK, 1.0; PCl/PK, 1.88; PNa/PCl, 0.53 for 3T3 cells, and 0.27, 1.75, and 0.15 for the transformed cells. In contrast to the normal cells, PNa/PK varied as a function of the external K+ concentration for the transformed cells. It was emphasized that the manipulation of variables directly affecting the electrical properties of cells also involves the indirect manipulation of a network of interconnected physiological determinants.     

Permeation of Na+ through a delayed rectifier K+ channel in chick dorsal root ganglion neurons

In whole-cell patch clamp recordings from chick dorsal root ganglion neurons, removal of intracellular K+ resulted in the appearance of a large, voltage-dependent inward tail current (Icat). Icat was not Ca2+ dependent and was not blocked by Cd2+, but was blocked by Ba2+. The reversal potential for Icat shifted with the Nernst potential for [Na+]. The channel responsible for Icat had a cation permeability sequence of Na+ >> Li+ >> TMA+ > NMG+ (PX/PNa = 1:0.33:0.1:0) and was impermeable to Cl-. Addition of high intracellular concentrations of K+, Cs+, or Rb+ prevented the occurrence of Icat. Inhibition of Icat by intracellular K+ was voltage dependent, with an IC50 that ranged from 3.0-8.9 mM at membrane potentials between -50 and -110 mV. This voltage- dependent shift in IC50 (e-fold per 52 mV) is consistent with a single cation binding site approximately 50% of the distance into the membrane field. Icat displayed anomolous mole fraction behavior with respect to Na+ and K+; Icat was inhibited by 5 mM extracellular K+ in the presence of 160 mM Na+ and potentiated by equimolar substitution of 80 mM K+ for Na+. The percent inhibition produced by both extracellular and intracellular K+ at 5 mM was identical. Reversal potential measurements revealed that K+ was 65-105 times more permeant than Na+ through the Icat channel. Icat exhibited the same voltage and time dependence of inactivation, the same voltage dependence of activation, and the same macroscopic conductance as the delayed rectifier K+ current in these neurons. We conclude that Icat is a Na+ current that passes through a delayed rectifier K+ channel when intracellular K+ is reduced to below 30 mM. At intracellular K+ concentrations between 1 and 30 mM, PK/PNa remained constant while the conductance at -50 mV varied from 80 to 0% of maximum. These data suggest that the high selectivity of these channels for K+ over Na+ is due to the inability of Na+ to compete with K+ for an intracellular binding site, rather than a barrier that excludes Na+ from entry into the channel or a barrier such as a selectivity filter that prevents Na+ ions from passing through the channel.     

南湖霉素选择地运载Li^＋,Na^＋通过磷脂双层

南湖霉素具有抑制枯草杆菌生长和抗鸡球虫病效应,是个新的多醚类抗生素,先前利用神经肌肉标本进行的研究提示,它对生物膜的作用都可用“Ｎａ＾＋载体”来解释。本文观察了南湖霉素对人工脂双层离子通透性的影响,获得的主要结果为：南湖霉素引起脂双层依剂量提高而增大的膜电导升高;通过测定在不同溶液系统中的平衡电位,确定膜电导的变化来源于脂双层对阳离子,特别是对Ｌｉ＾＋,Ｎａ＾＋通透性的升高,ＰＬｉ：ＰＮａ：ＰＫ：     

The electrical potential profile of gallbladder epithelium.

In this study the relative ionic permeabilities of the cell membranes of Necturus gallbladder epithelium have been determined by means of simultaneous measurement of transmural and transmucosal membrane potential differences (PD) and by ionic substitution experiments with sodium, potassium and chloride ions. It is shown that the mucosal membrane is permeable to sodium and to potassium ions. The baso-lateral membrane PD is only sensitive to potassium ions. In both membranes chloride conductance is negligible or absent. The ratio of the resistances of the mucosal and baso-lateral membranes, RM/RS, increases upon reducing the sodium concentration in the mucosal solution. The same ratio decreases when sodium is replaced by potassium which implies a greater potassium than sodium conductance in the mucosal membrane. The relative permeability of the shunt for potassium, sodium and chloride ions is: PK/PNa/PCl=1.81:1.00:0.32. From the results obtained in this study a value for the PK/PNa ratio of the mucosal membrane could be evaluated. This ratio is 2.7. From the same data the magnitude of the electromotive forces generated across the cell membranes could be calculated. The EMF's are -15mV across the mucosal membrane and -81mV across the baso-lateral one. Due to the presence of the low resistance shunt the transmucosal membrane PD is -53.2mV (cell inside negative) and the transmural PD is +2.6mV (serosal side positive). The change in potential profile brought about by the low resistance shunt favors passive entry of Na ions into the cell across the mucosal membrane. Calculations show that this passive Na influx is maximally 64% of the net Na flux estimated from fluid transport measurements. The C-1 conductive of the baso-lateral membrane is too small to allow electrogenic coupling of C1 with Na transport across this membrane. Experiments with rabbit gallbladder epithelium indicate that the membrane properties in this tissue are qualitatively similar to those of Necturus gallbladder epithelium.     

Properties of a Sodium Channel (Nax) Activated by Strong Depolarization of Xenopus Oocytes

Short (<1 sec) duration depolarization of Xenopus laevis oocytes to voltages greater than +40 mV activates a sodium-selective channel (Na(x)) with sodium permeability five to six times greater than the permeability of other monovalent cations examined, including K+, Rb+, Cs+, TMA+, and Choline+. The permeability to Li+ is about equal to that of Na+. This channel was present in all oocytes examined. The kinetics, voltage dependence and pharmacology of Na(x)distinguish it from TTX-sensitive or epithelial sodium channels. It is also different from the sodium channel of Xenopus oocytes activated by prolonged depolarization, which is more highly selective for Na+, requires prolonged depolarization to be activated, and is blocked by Li+. Intracellular Mg2+ reversibly inhibits Na(x), whereas extracellular Mg2+ does not have an inhibitory effect. Intracellular Mg2+ inhibition of Na(x), is voltage dependent, suggesting that Mg2+ binding occurs within the membrane field. Eosin is also a reversible voltage-dependent intracellular inhibitor of Na(x), suggesting that a P-type ATPase may mediate the current. An additional cytoplasmic factor is involved in maintaining Na(x) since the current runs down in internally perfused oocytes and excised membrane patches. The rundown is reversible by reintroduction of the membrane patch into oocyte cytoplasm. The cytoplasmic factor is not ATP, because ATP has no effect on Na(x) current magnitude in either cut-open or inside-out patch preparations. Extracellular Gd3+ is also an inhibitor of Na(x). Na(x) activation follows a sigmoid time course. Its half-maximal activation potential is +100 mV and the effective valence estimated from the steepness of conductance activation is 1.0. Na(x) deactivates monoexponentially upon return to the holding potential (-40 mV). The deactivation rate is voltage dependent, increasing at more negative membrane potentials.     

The permeability of the sodium channel in Myxicola to the alkali cations

Relative permeabilities to the alkali cations were determined, from the reversal potential (VRev), for the Na channel of internally perfused voltage-clamped Myxicola giant axons. PLi/PNa and PK/PNa are 0.94 and 0.076, respectively. Rb and Cs are not measurably permeant. VRev vs. the internal Na activity was well described by the constant field equation over a 300-fold range of internal Na concentrations. In agreement with findings on squid axons, the PK/PNa was found to increase when the K content of the internal perfusate was reduced (equivalent per equivalent substitution with TMA). Internal Rb and Cs also decreased the PK/PNa. The order of effectiveness of internal K, Rb, and Cs in increasing the Na selectivity of the Na channel was Cs greater than Rb greater than or equal to K. External Li increases the PK/PNa but this may be due to the formation of LiF internally. It may be that substances do not have to traverse the channel in order to affect the selectivity filter. Evidence is presented which suggests that the selectivity of the Na channel may be higher for Na in intact as compared to perfused giant axons. It was concluded that the channel selectivity properities do not reflect only some fixed structural features of the channel, but the selectivity filter has a labile organization.     

Explanation for constancy of intracellular chloride concentration with respect to membrane potential in guinea pig mammary gland.

1. The relative permeabilities for sodium, potassium and chloride in guinea pig mammary gland slices are determined by means of ion flux studies with radioisotopes. 2. Assuming that there are no significant electrogenic potential components, we calculate permeability ratios PNa/PK = 0.97 and Pc1/PK = 1.25. 3. Substitution of these values in the Goldman equation yields membrane potentials of--15 mV before and--13 mV after ouabain treatment. 4. This small change in membrane potential explains the absence of a significant change in chloride content upon ouabain application, which leads to large changes in intracellular sodium and potassium concentrations.     

Cation flux in the ehrlich ascites tumor cell. Evidence for Na+-for-Na+ and K+-for-K+ exchange diffusion.

In a previous study, evidence was presented for an external Na+-dependent, ouabain-insensitive component of Na+ efflux and an external K+-dependent component of K+ efflux in the Ehrlich ascites tumor cell. Evidence is now presented that these components are inhibited by the diuretic furosemide and that under conditions of normal extracellular Na+ and K+ they represent Na+-for-Na+ and K-+for-K+ exchange mechanisms. Using 86Rb to monitor K+ movements, furosemide is shown to inhibit an ouabain-insensitive component of Rb+ influx and a component of Rb+ efflux, both representing approx. 30 percent of the total flux. Inhibition of Rb+ efflux is greatly reduced by removal of extracellular K+. Furosemide does not alter steady-state levels of intracellular K+ and it does not prevent cells depleted of K+ by incubation in the cold from regaining K+ upon warming. Using 22Na to monitor Na+ movements, furosemide is shown to inhibit an ouabain-insensitive component of unidirectional Na+ efflux which represents approx. 22 percent of total Na+ efflux. Furosemide does not alter steady-state levels of intracellular Na+ and does not prevent removal of intracellular Na+ upon warming from cells loaded with Na+ by preincubation in the cold. The ability of furosemide to affect unidirectional Na+ and K+ fluxes but not net fluxes is consistent with the conclusion that these components of cation movement across the cell membrane represent one-for-one exchange mechanisms. Data are also presented which demonstrate that the uptake of alpha-aminoisobutyrate is not affected by furosemide. This indicates that these components of cation flux are not directly involved in the Na+-dependent amino acid transport system A.     

Identification of a stretch-activated monovalent cation channel from teleost urinary bladder cells.

The urinary bladder of euryhaline teleost is an important osmoregulatory organ which absorbs Na+, Cl-, and water from urine. Using patch clamp technique, single stretch-activated channels, which were permeable to K+ and Na+ (PNa/PK approximately 0.75) and had conductances of 55 and 116 pS, were studied. In excised, inside-out patches which were voltage-clamped in the physiological range of membrane potential, the single-channel open probability (Po) was low (approximately 0.02), and increased to a maximum of 0.9 with applied pipette suction. Single-channel conductance also increased with suction. The channels showed adaptation to applied suction and relaxed to a steady-state activity about 20 seconds after application of suction. The Po increased up to 0.9 with strong membrane depolarization (Vm = 0 to +80 mV); however, there was little dependence of Po on membrane potential in the physiological range. The kinetic data suggest that there is one conducting state and at least two non-conducting states of the channel. The open-time constant increased with suction but remained unchanged with membrane potential (Vm = -70 to +60 mV). The mean closed-time of the channel decreased with suction and membrane depolarization. These results demonstrate the presence of a non-selective monovalent cation channel which may be involved in cell volume regulation in the goby urinary bladder. Additionally, this channel may function as an enhancer of Na+ influx and K+ efflux across the bladder cell as part of transepithelial ion transport if it is located in apical membrane.     

Contribution of Na/Ca transport to the resting membrane potential

Relations are derived that describe the combined effects of electrodiffusion, the Na/K pump, and Na/Ca transport by carrier on the resting membrane potential. Equations are derived that apply to both steady-state and non-steady-state conditions. Some example calculations from the equations are plotted at different permeability coefficient ratios, PK:PCa:PNa. The equations predict a depolarizing action of Na/Ca transport when more than two Na ions per Ca ion are transported by the carrier. For all permeability ratios examined, a steady state for Ca ions is achieved with at most a few millivolts of depolarization.     

Single potassium channels with delayed rectifier behavior from lobster axon membranes.

Single-channel potassium currents from lobster axon membranes were studied in planar bilayers made from monolayers. Channel-opening events are grouped by time, forming bursts with an average duration of 4.5 ms. The mean open time at 0 mV is 1.8 ms. The frequency of bursts is voltage dependent, increasing e-fold per 12-16 mV. At sufficiently high positive voltages, channels inactivate. Measured from reversal potentials, channels discriminate against Na+ by a permeability ratio PNa/PK of 1:30. The channel is blocked by tetraethylammonium and nonyltrimethylammonium in a voltage-dependent manner and at concentrations similar to those used in whole-axon experiments. Voltage-dependent block by Cs+ suggests that more than one ion may occupy the channel simultaneously. The kinetics and selectivity of this channel suggest that purified axolemma contains active K+ channels that are likely to participate in delayed rectification in the lobster axon membrane.     

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.     

Membrane potential measurements in isolated rat liver plasma membrane vesicles: effect of transmembrane ion concentration gradients

In isolated basolateral and canalicular rat liver plasma membrane vesicles the membrane potential (measured with DiS-C2 (5] varied with transmembrane concentration gradients of Na+, K+ and Cl- revealing the following ion permeabilities: basolateral vesicles: PNa/PK: 0.76, PCl/PK: 0.45 and canalicular vesicles: PNa/PK: 0.69, PCl/PK: 0.56. The data indicate a permselectivity of PK greater than PNa greater than PCl for both membranes.     

Atrioventricular block in low potassium and K dependence of the nodal resting potential

A progressive conduction block leading to atrioventricular dissociation develops in perfused rabbit hearts within 20-30 min of exposure to Krebs containing 0.5 mM potassium (low K). A decrease in potassium permeability resulting in membrane depolarization (as seen in Purkinje fibers) could be responsible for the loss of excitability in nodal cells. We investigated the K dependence of the resting potential and the long-term effects of low K perfusion on the resting and action potentials of nodal cells in rabbit hearts. The resting potential of atrial, atrionodal, and nodal cells varied by 52, 41, and 34 mV per decade of change in Ko within the range of 5-50 mM K. Hyperpolarization of the resting membrane, a progressive decline in action potential amplitude, and a decrease in maximum rate of rise were observed in nodal fibers when exposed to low K. Loss of propagated activity occurred in the middle node within 20-30 min while the cells remained hyperpolarized. There was no evidence of electrogenic Na extrusion and it seems that the low nodal resting potential results from a high resting PNa/PK permeability ratio. The early decrease in rate of rise in low K probably reflects an increase in K-dependent outward currents, whereas the progressive deterioration and final loss of conducted electrical activity may result from an accumulation of internal Na and Ca overload produced by low K inhibition of the Na pump.