Selectivity and gating of the type L potassium channel in mouse lymphocytes

Type l voltage-gated K+ channels in murine lymphocytes were studied under voltage clamp in cell-attached patches and in the whole-cell configuration. The kinetics of activation of whole-cell currents during depolarizing pulses could be fit by a single exponential after an initial delay. Deactivation upon repolarization of both macroscopic and microscopic currents was mono-exponential, except in Rb-Ringer or Cs-Ringer solution in which tail currents often displayed "hooks," wherein the current first increased or remained constant before decaying. In some cells type l currents were contaminated by a small component due to type n K+ channels, which deactivate approximately 10 times slower than type l channels. Both macroscopic and single channel currents could be dissected either kinetically or pharmacologically into these two K+ channel types. The ionic selectivity and conductance of type l channels were studied by varying the internal and external permeant ion. With 160 mM K+ in the cell, the relative permeability calculated from the reversal potential with the Goldman-Hodgkin-Katz equation was K+ (identical to 1.0) greater than Rb+ (0.76) greater than NH4+ = Cs+ (0.12) much greater than Na+ (less than 0.004). Measured 30 mV negative to the reversal potential, the relative conductance sequence was quite different: NH4+ (1.5) greater than K+ (identical to 1.0) greater than Rb+ (0.5) greater than Cs+ (0.06) much greater than Na+, Li+, TMA+ (unmeasurable). Single channel current rectification resembled that of the whole-cell instantaneous I-V relation. Anomalous mole-fraction dependence of the relative permeability PNH4/PK was observed in NH4(+)-K+ mixtures, indicating that the type l K+ channel is a multi-ion pore. Compared with other K+ channels, lymphocyte type l K+ channels are most similar to "g12" channels in myelinated nerve.     

Characterization of the inward-rectifying potassium current in cat ventricular myocytes

Whole-cell membrane currents were measured in isolated cat ventricular myocytes using a suction-electrode voltage-clamp technique. An inward-rectifying current was identified that exhibited a time-dependent activation. The peak current appeared to have a linear voltage dependence at membrane potentials negative to the reversal potential. Inward current was sensitive to K channel blockers. In addition, varying the extracellular K+ concentration caused changes in the reversal potential and slope conductance expected for a K+ current. The voltage dependence of the chord conductance exhibited a sigmoidal relationship, increasing at more negative membrane potentials. Increasing the extracellular K+ concentration increased the maximal level of conductance and caused a shift in the relationship that was directly proportional to the change in reversal potential. Activation of the current followed a monoexponential time course, and the time constant of activation exhibited a monoexponential dependence on membrane potential. Increasing the extracellular K+ concentration caused a shift of this relationship that was directly proportional to the change in reversal potential. Inactivation of inward current became evident at more negative potentials, resulting in a negative slope region of the steady state current-voltage relationship between -140 and -180 mV. Steady state inactivation exhibited a sigmoidal voltage dependence, and recovery from inactivation followed a monoexponential time course. Removing extracellular Na+ caused a decrease in the slope of the steady state current-voltage relationship at potentials negative to -140 mV, as well as a decrease of the conductance of inward current. It was concluded that this current was IK1, the inward-rectifying K+ current found in multicellular cardiac preparations. The K+ and voltage sensitivity of IK1 activation resembled that found for the inward-rectifying K+ currents in frog skeletal muscle and various egg cell preparations. Inactivation of IK1 in isolated ventricular myocytes was viewed as being the result of two processes: the first involves a voltage-dependent change in conductance; the second involves depletion of K+ from extracellular spaces. The voltage-dependent component of inactivation was associated with the presence of extracellular Na+.     

The pacemaker current in cardiac Purkinje myocytes

It is generally assumed that in cardiac Purkinje fibers the hyperpolarization activated inward current i(f) underlies the pacemaker potential. Because some findings are at odds with this interpretation, we used the whole cell patch clamp method to study the currents in the voltage range of diastolic depolarization in single canine Purkinje myocytes, a preparation where many confounding limitations can be avoided. In Tyrode solution ([K+]o = 5.4 mM), hyperpolarizing steps from Vh = -50 mV resulted in a time-dependent inwardly increasing current in the voltage range of diastolic depolarization. This time- dependent current (iKdd) appeared around -60 mV and reversed near EK. Small superimposed hyperpolarizing steps (5 mV) applied during the voltage clamp step showed that the slope conductance decreases during the development of this time-dependent current. Decreasing [K+]o from 5.4 to 2.7 mM shifted the reversal potential to a more negative value, near the corresponding EK. Increasing [K+]o to 10.8 mM almost abolished iKdd. Cs+ (2 mM) markedly reduced or blocked the time-dependent current at potentials positive and negative to EK. Ba2+ (4 mM) abolished the time-dependent current in its usual range of potentials and unmasked another time-dependent current (presumably i(f)) with a threshold of approximately -90 mV (> 20 mV negative to that of the time-dependent current in Tyrode solution). During more negative steps, i(f) increased in size and did not reverse. During i(f) the slope conductance measured with small (8-10 mV) superimposed clamp steps increased. High [K+]o (10.8 mM) markedly increased and Cs+ (2 mM) blocked i(f). We conclude that: (a) in the absence of Ba2+, a time-dependent current does reverse near EK and its reversal is unrelated to K+ depletion; (b) the slope conductance of that time-dependent current decreases in the absence of K+ depletion at potentials positive to EK where inactivation of iK1 is unlikely to occur. (c) Ba2+ blocks this time-dependent current and unmasks another time-dependent current (i(f)) with a more negative (> 20 mV) threshold and no reversal at more negative values; (d) Cs+ blocks both time-dependent currents recorded in the absence and presence of Ba2+. The data suggest that in the diastolic range of potentials in Purkinje myocytes there is a voltage- and time-dependent K+ current (iKdd) that can be separated from the hyperpolarization- activated inward current i(f).     

Whole-cell and single channel K+ and Cl- currents in epithelial cells of frog skin.

Whole-cell and single channel currents were studied in cells from frog (R. pipiens and R. catesbiana) skin epithelium, isolated by collagenase and trypsin treatment, and kept in primary cultures up to three days. Whole-cell currents did not exhibit any significant time-dependent kinetics under any ionic conditions used. With an external K gluconate Ringer solution the currents showed slight inward rectification with a reversal potential near zero and an average conductance of 5 nS at reversal. Ionic substitution of the external medium showed that most of the cell conductance was due to K and that very little, if any, Na conductance was present. This confirmed that most cells originate from inner epithelial layers and contain membranes with basolateral properties. At voltages more positive than 20 mV outward currents were larger with K in the medium than with Na or N-methyl-D-glucamine. Such behavior is indicative of a multi-ion transport mechanism. Whole-cell K current was inhibited by external Ba and quinidine. Blockade by Ba was strongly voltage dependent, while that by quinidine was not. In the presence of high external Cl, a component of outward current that was inhibited by the anion channel blocker diphenylamine-2-carboxylate (DPC) appeared in 70% of the cells. This component was strongly outwardly rectifying and reversed at a potential expected for a Cl current. At the single channel level the event most frequently observed in the cell-attached configuration was a K channel with the following characteristics: inward-rectifying I-V relation with a conductance (with 112.5 mM K in the pipette) of 44 pS at the reversal potential, one open and at least two closed states, and open probability that increased with depolarization. Quinidine blocked by binding in the open state and decreasing mean open time. Several observations suggest that this channel is responsible for most of the whole-cell current observed in high external K, and for the K conductance of the basolateral membrane of the intact epithelium. On a few occasions a Cl channel was observed that activated upon excision and brief strong depolarization. The I-V relation exhibited strong outward rectification with a single channel conductance of 48 pS at 0 mV in symmetrical 112 mM Cl solutions. Kinetic analysis showed the presence of two open and at least two closed states. Open time constants and open probability increased markedly with depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)     

Monovalent ion current through single calcium channels of skeletal muscle transverse tubules.

Akali monovalents, Li, Na, K, Cs, and organic monovalents of molecular cross section less than 20 A2, ammonium, methylammonium, hydrazinium, guanidinium, are shown to have a measurable conductance through Ca channels of muscle transverse tubules reconstituted into planar bilayers. For the alkali series, single channel conductances follow the sequence Cs approximately equal to K greater than Na greater than Li with a conductance ratio [g(Cs)/g(Li)] = 1.7. For permeability ratios, the sequence is Li greater than Na greater than K approximately equal to Cs with [P(Li)/P(Cs)] = 1.5. Monovalent current is only unmasked when Ba ions are not present. In mixtures of Cs and Ba, single channel current reverses close to the Ba equilibrium potential and more than 100 mV away from the Cs equilibrium potential. A cutoff in conduction is found for organic cations larger than trimethylammonium; this suggests an apparent pore aperture of about 5 X 5 A. Even in such a large pore, the fact that the alkali cation permeability sequence and conductance sequence are inverted rules out molecular sieving as the mechanism of selection among monovalents.     

Synthetic mammalian C-type natriuretic peptide forms large cation channels

We report the first evidence that synthetic human C-type natriuretic peptide-22 and the OaC-type natriuretic peptide-39(18-39), a 22 amino acid fragment of the OaC-type natriuretic peptide-39 from platypus venom, can function directly by forming a novel voltage-gated weakly cation-selective channel in negatively charged artificial lipid bilayer membranes. The channel activity is characterized by a tendency for inactivation at negative voltages, e.g. -60 and -70 mV, whereas at positive voltages the channel is fully open. The channel has a maximal cord conductance of 546+/-23 pS (n = 16) and shows weak outward rectification. The sequence and the permeability ratios were P(K)+: P(Cs)+: P(Na)+: P(choline)+ 1:0.88:0.76:0.13, respectively. The addition of 50 mM TEA+ cis (a blocker of outwardly rectifying K+ channels), 20 mM Cs+ cis (a blocker of inwardly rectifying K+ channels) or 0.5 mM glibenclamide cis (a blocker of ATP-sensitive K+ channels) to the cis chamber did not affect the conductance or the kinetics of the OaC-type natriuretic peptide-39(18-39)-formed channels (n = 2-5). It is concluded that the weak cation selectivity, large conductance and high open probability as well as their voltage dependency are consistent with the ability of these peptides to cause that loss of compartmentation of the membrane, which is a characteristic feature of adverse conditions that cause C-type natriuretic peptide-related pathologies.     

Voltage-dependent block of cardiac inward-rectifying potassium current by monovalent cations

The inward-rectifying K+ current (IK1) in cat ventricular myocytes, like inward-rectifying K+ currents in many other preparations, exhibited a negative slope conductance region at hyperpolarized membrane potentials that was time-dependent. This was evident as an inactivation of inward current elicited by hyperpolarizing voltage-clamp pulses resulting in a negative slope region of the steady-state current-voltage relationship at potentials negative to -140 mV. Removing extracellular Na+ prevented the development of the negative slope in this voltage region, suggesting that Na+ can block IK1 channels in a time- and voltage-dependent manner. The time and voltage dependence of Cs+-induced block of IK1 was also examined. Cs+ blocked inward current in a manner similar to that of Na+, but the former was much more potent. The fraction of current blocked by Cs+ in the presence of Na+ was reduced in a time- and voltage-dependent manner, which suggested that these blocking ions compete for a common or at least similar site of action. In the absence of Na+, inactivation of IK1 could also be induced by both Cs+ and Li+. However, Li+ was less potent than Na+ in this respect. Calculation of the voltage sensitivity of current block by each of these ions suggests that the mechanism of block by each is similar.     

A whole-cell and single-channel study of the voltage-dependent outward potassium current in avian hepatocytes

Voltage-dependent membrane currents were studied in dissociated hepatocytes from chick, using the patch-clamp technique. All cells had voltage-dependent outward K+ currents; in 10% of the cells, a fast, transient, tetrodotoxin-sensitive Na+ current was identified. None of the cells had voltage-dependent inward Ca2+ currents. The K+ current activated at a membrane potential of about -10 mV, had a sigmoidal time course, and did not inactivate in 500 ms. The maximum outward conductance was 6.6 +/- 2.4 nS in 18 cells. The reversal potential, estimated from tail current measurements, shifted by 50 mV per 10-fold increase in the external K+ concentration. The current traces were fitted by n2 kinetics with voltage-dependent time constants. Omitting Ca2+ from the external bath or buffering the internal Ca2+ with EGTA did not alter the outward current, which shows that Ca2+-activated K+ currents were not present. 1-5 mM 4-aminopyridine, 0.5-2 mM BaCl2, and 0.1-1 mM CdCl2 reversibly inhibited the current. The block caused by Ba was voltage dependent. Single-channel currents were recorded in cell-attached and outside-out patches. The mean unitary conductance was 7 pS, and the channels displayed bursting kinetics. Thus, avian hepatocytes have a single type of K+ channel belonging to the delayed rectifier class of K+ channels.     

Possible reduction of surface charge by a mutation inParamecium tetraurelia

Summary Under voltage clamp, a mutant ofParamecium tetraurelia (teaB) shows a shift in the positive direction of the voltage sensitivity of the Ca conductance and the depolarization inactivation curve by 10 mV with no change in the total conductance. This effect can be mimicked in the wild type by the addition of external Ca²⁺ or Mg²⁺. The mutation also shifts the resting potential and the voltage sensitivities of the delayed rectification (depolarization-sensitive) K conductance and the anomalous rectification (hyperpolarization-sensitive) K conductance in the positive direction to a similar extent. This systematic shift of channel voltage sensitivities is best explained by the reduction of the surface negative charges of the membrane due to the mutation.     

Role of hyperpolarization-activated currents for the intrinsic dynamics of isolated retinal neurons

The intrinsic dynamics of bipolar cells and rod photoreceptors isolated from tiger salamanders were studied by a patch-clamp technique combined with estimation of effective impulse responses across a range of mean membrane voltages. An increase in external K(+) reduces the gain and speeds the response in bipolar cells near and below resting potential. High external K(+) enhances the inward rectification of membrane potential, an effect mediated by a fast, hyperpolarization-activated, inwardly rectifying potassium current (K(IR)). External Cs(+) suppresses the inward-rectifying effect of external K(+). The reversal potential of the current, estimated by a novel method from a family of impulse responses below resting potential, indicates a channel that is permeable predominantly to K(+). Its permeability to Na(+), estimated from Goldman-Hodgkin-Katz voltage equation, was negligible. Whereas the activation of the delayed-rectifier K(+) current causes bandpass behavior (i.e., undershoots in the impulse responses) in bipolar cells, activation of the K(IR) current does not. In contrast, a slow hyperpolarization-activated current (I(h)) in rod photoreceptors leads to pronounced, slow undershoots near resting potential. Differences in the kinetics and ion selectivity of hyperpolarization-activated currents in bipolar cells (K(IR)) and in rod photoreceptors (I(h)) confer different dynamical behavior onto the two types of neurons.     

The mechanism of inward rectification of potassium channels: "long-pore plugging" by cytoplasmic polyamines

The mechanism of inward rectification was examined in cell-attached and inside-out membrane patches from Xenopus oocytes expressing the cloned strong inward rectifier HRK1. Little or no outward current was measured in cell-attached patches. Inward currents reach their maximal value in two steps: an instantaneous phase followed by a time-dependent "activation" phase, requiring at least two exponentials to fit the time- dependent phase. After an activating pulse, the quasi-steady state current-voltage (I-V) relationship could be fit with a single Boltzmann equation (apparent gating charge, Z = 2.0 +/- 0.1, n = 3). Strong rectification and time-dependent activation were initially maintained after patch excision into high [K+] (K-INT) solution containing 1 mM EDTA, but disappeared gradually, until only a partial, slow inactivation of outward current remained. Biochemical characterization (Lopatin, A. N., E. N. Makhina, and C. G. Nichols, 1994. Nature. 372:366-396.) suggests that the active factors are naturally occurring polyamines (putrescine, spermidine, and spermine). Each polyamine causes reversible, steeply voltage-dependent rectification of HRK1 channels. Both the blocking affinity and the voltage sensitivity increased as the charge on the polyamine increased. The sum two Boltzmann functions is required to fit the spermine and spermidine steady state block. Putrescine unblock, like Mg2+ unblock, is almost instantaneous, whereas the spermine and spermidine unblocks are time dependent. Spermine and spermidine unblocks (current activation) can each be fit with single exponential functions. Time constants of unblock change e-fold every 15.0 +/- 0.7 mV (n = 3) and 33.3 +/- 6.4 mV (n = 5) for spermine and spermidine, respectively, matching the voltage sensitivity of the two time constants required to fit the activation phase in cell-attached patches. It is concluded that inward rectification in intact cells can be entirely accounted for by channel block. Putrescine and Mg2+ ions can account for instantaneous rectification; spermine and spermidine provide a slower rectification corresponding to so-called intrinsic gating of inward rectifier K channels. The structure of spermine and spermidine leads us to suggest a specific model in which the pore of the inward rectifier channel is plugged by polyamines that enter deeply into the pore and bind at sites within the membrane field. We propose a model that takes into account the linear structure of the natural polyamines and electrostatic repulsion between two molecules inside the pore. Experimentally observed instantaneous and steady state rectification of HRK1 channels as well as the time-dependent behavior of HRK1 currents are then well fit with the same set of parameters for all tested voltages and concentrations of spermine and spermidine.     

Single K+ channels in endocrine cells dispersed from the cricket (Gryllus bimaculatus) corpora allata

Single channel currents were recorded from cell-attached patches of endocrine cells of the adult male cricket corpora allata. Three distinct types of K+ channels were identified; a weak inward rectifier (Type 1), a strong inward rectifier (Type 2) and a weak outward rectifier (Type 3). The type 1 channel had a slope conductance of 191 +/- 9 pS (n = 4) at negative membrane potentials (Vm) and 101 +/- 6 pS (n = 6) at positive Vm. In addition, the channel showed fast open-closed kinetics at negative Vm and slow open-closed kinetics at positive Vm. The open probability (Po) of this channel was strongly voltage-dependent at positive Vm, but less voltage-dependent at negative Vm. The reversal potential was not modified significantly by the substitution of gluconate for external Cl- but was modified after N-methyl-D-glucamine (NMDG+) was substituted for external K+, according to the Nernst equation for a K+-selective channel. The type 2 channel had a slope conductance of 44 +/- 2 pS (n = 5) at negative Vm, but no detectable outward current was observed at positive Vm. This channel showed very slow open-closed kinetics at negative Vm and its Po was not voltage-dependent. The type 3 channel had a limit conductance of 55 +/- 12 pS (n = 3) at negative Vm and 88 +/- 10 pS (n = 3) at positive Vm. This channel showed slow open-closed kinetics at negative Vm and fast open-closed kinetics at positive Vm. The Po for the channel was voltage-dependent at positive Vm but was voltage-independent at negative Vm. These three types of K+ channels may be important for the control of the resting membrane potential, and may thus participate in the regulation of Ca2+ influx and juvenile hormone secretion in corpora allata cells.     

Mechanisms of Cs+ blockade in a Ca2+-activated K+ channel from smooth muscle.

Large unitary conductance Ca2+-activated K+ channels from smooth muscle membrane were incorporated into phospholipid planar bilayers, and the blockade induced by internally and externally applied Cs+ was characterized. Internal Cs+ blockade is voltage dependent and can be explained on the basis of a Cs+ binding to a site that senses 54% of the applied voltage, with an apparent dissociation constant, Kd(0), of 70 mM. On the other hand, external Cs+ blocks the channel in micromolar amounts, and the voltage dependence of blockade is a function of Cs+ concentration. The fractional electrical distance can be as large as 1.4 at 10 mM Cs+. This last result suggests that the channel behaves as a multi-ion pore. At large negative voltages the I-V relationships in the presence of external Cs+ show an upturn, indicating relief of Cs+ block. External Cs+ blockade is relieved by increasing the internal K+ concentration, but can be enhanced by increasing the external K+. All the characteristics of external Cs+ block can be explained by a model that incorporates a "knock-on" of Cs+ by K+.     

Steady state current rectification in potential clamped nodes of Ranvier (Xenopus laevis).

The membrane potential in single nodes of Ranvier was changed in rectangular pulse steps while the membrane currents, associated with the potential steps, were measured. Changes were made in the ionic composition of the external and the internal medium. The latter changes were obtained by free diffusion through a cut internode. The steady state currents, described on the basis of potassium and leak permeability, were affected by the solution composition in a characteristic way. Increased inside concentration of sodium and lithium caused a striking rectification of the outward steady state currents at large potential steps. Instantaneous potassium currents in high [K+]o at a second potential step to E approximately equal to minus 80 mV were not affected by [Na+]1. Neither [Na+]o nor [K+]i affected the potential at which this rectification appeared. Increased [K+]o shifted the region for rectification along the potential axis in positive direction. These findings form strict limitations for satisfactory models describing the mechanism for the steady state current in myelinated nerve.     

Macrophage-colony-stimulating factor (CSF-1) modulates a differentiation-specific inward-rectifying potassium current in human leukemic (HL-60) cells

A voltage-activated inward-rectifying K+ conductance (lKi) appears in human promyelocytic leukemia (HL-60) cells during phorbol ester-induced differentiation into macrophages. This conductance was detected in the cells 24 hours after exposure to phorbol-12-myristate-13-acetate (PMA), as the cells began to express the macrophage phenotype, and continued to increase for 4 days after PMA exposure. The magnitude of inward current was a function of external K+; current was blocked by extracellular or intracellular Cs+ and by extracellular Ba++. Hyperpolarization produced activation at membrane potentials more negative than -80 mV, and a slower, partial inactivation also occurred at potentials more negative than -100 mV. This conductance was not detected in proliferating cells nor in granulocytes derived from HL-60 cells which were induced to differentiate with retinoic acid (RA). Exposure of differentiated macrophages to recombinant human CSF-1 produced inhibition of the lKi beginning within 1 minute after exposure. CSF-1 inhibition of lKi channels in cell-attached patches indicated that channel modulation was via intracellular mediators. The rapid inhibition of the inward rectifier by the macrophage-specific CSF-1 appears to be one of the earliest cellular responses to this factor.     

Electrophysiologic study of the cholinergic receptor of the myocardial membrane]

A double sucrose gap voltage clamp technic has been used to study the extra-current induced by acetylcholine (Iach) on the myocardial membrane on frog atrial trabeculae. I) No desensitization of the Iach current is noted for repeated perfusions of Ach. II) The Iach current is suppressed by atropine. III) The reversal potential Each is more negative than the resting potential --20 mV less than or equal to Each less than or equal to OmV.IV) The relationship Iach/Holding potential for various [K+]o shows a) That Each behaved as a potassium electrode, b) an inward going rectification. These results indicate that the cholinergic receptor might be related with the gk1 channel.     

Inward rectification in rat olfactory receptor neurons.

Inwardly rectifying currents in enzymically dissociated olfactory receptor neurons of rat were studied by using patch-clamp techniques. Upon hyperpolarization to membrane potentials more negative than -100 mV, small inward-current relaxations were observed. Activation was described by a single exponential with a time constant that decreased e-fold for a 21 mV hyperpolarization. The current was not reduced by the external application of 5 mM Ba2+, but was abolished by the addition of 5 mM Cs+ to the bath solution. Increasing the external K+ concentration ([K+]o) to 25 mM dramatically enhanced the current without affecting the voltage range or the kinetics of activation. In 25 mM [K+]o, tail currents reversed at -26 mV, significantly more positive than the K+ equilibrium potential of -44 mV. These characteristics are consistent with those of a mixed Na+/K+ inward rectification that has been reported in several types of neuronal, cardiac and smooth muscle cells. The current may contribute to controlling cell excitability during the response to some odorants.     

Multiple conductances in the large K+ channel from Chara corallina shown by a transient analysis method.

The large conductance K+ channel in the tonoplast of Chara corallina has subconductance states (substates). We describe a method that detects substates by monitoring the time derivative of channel current. Substates near to the full conductance tend to have long durations and high probabilities, while those of smaller amplitude occur with less probability and short duration. The substate pattern is similar in cell-attached, inside-out and outside-out patches over a range of temperatures. The pattern changes at high Ca2+ concentration (10 mol m-3) on the cytoplasmic face of inside-out patches. One substate at approximately 50% of the full conductance is characterized by a high frequency of transitions from the full conductance level. This midstate conductance is not a constant proportion of the full conductance but changes as a function of membrane potential difference (p.d.) showing strong inward rectification. We suggest that the channel is a single pore that can change conformation and/or charge profile to give different conductances. The mean durations of the full conductance level and the midstate decrease as the membrane p.d. becomes more negative. Programs for analysis of channel kinetics based on an half-amplitude detection criterion are shown to be unsuitable for analysis of the K+ channel.     

Electrical activity in white adipose tissue of rat

Intracellular recording of white adipocytes was performed in an in vitro preparation. Resting potential, input resistance and membrane time constant averaged: -34 +/- 9 mV, 295 +/- 161 M omega, and 58 +/- 19 ms respectively (mean +/- SD, n = 32). Intracellular injection of positive and negative square current pulses elicited membrane voltage responses, characterized by a rectification of the voltage change evoked by positive pulses, and a slow return to baseline at the offset of hyperpolarizing pulses. The amplitude and duration of the slow return to resting potential was dependent on membrane potential, pulse duration, and extracellular K+ concentration. This response was depressed when external Ca2+ was replaced by Co2+, and by external application of 4-aminopyridine. These results indicate that white adipocytes can generate membrane voltage responses which may mostly be a consequence of the activity of ionic channels. The properties of the slow return to baseline suggest that it may be due to a transient K+ current.