Calcium- and voltage-activated potassium channels in human macrophages.

Single calcium-activated potassium channel currents were recorded in intact and excised membrane patches from cultured human macrophages. Channel conductance was 240 pS in symmetrical 145 mM K+ and 130 pS in 5 mM external K+. Lower conductance current fluctuations (40% of the larger channels) with the same reversal potential as the higher conductance channels were noted in some patches. Ion substitution experiments indicated that the channel is permeable to potassium and relatively impermeable to sodium. The frequency of channel opening increased with depolarization and intracellular calcium concentration. At 10(-7) M (Ca++)i, channel activity was evident only at potentials of +40 mV or more depolarized, while at 10(-5) M, channels were open at all voltages tested (-40 to +60 mV). In intact patches, channels were seen at depolarized patch potentials of +50 mV or greater, indicating that the ionized calcium concentration in the macrophage is probably less than 10(-7) M.     

Nonselective cation channels in intact human T lymphocytes.

Nonselective cation channels were found in single channel recordings from cell-attached patches on human T lymphocytes. These channels were active under conditions that should lead to cell swelling (hypotonic bath solutions with NaCl or KCl); however, a definite dependence of activity on cell swelling has not been proven. Under these conditions similar channels were found in 20 of 23 patches from 11 different blood donors. The current-voltage relation was approximately linear for outward current (11-14 pS) and inwardly rectifying (to 23 pS) when the intact cells were depolarized with high KCl in the bath. The voltage dependence of channel activity is consistent with closing at hyperpolarized membrane potentials (Vm less than or equal to -50 mV) and block of open channels at strongly depolarized membrane potentials (Vm greater than 0 mV). Reversal potentials under all ionic gradients tested are consistent with a channel that is poorly selective between Na+ and K+ ions. Active channels in cell-attached patches were rapidly blocked by bath addition of the membrane-permeant inhibitor quinine. Channels that were active in cell-attached became quiescent after patch excision; however, two patches remained active long enough to obtain current-voltage relations. These were linear with a slope conductance for outward current of 8-11 pS. Because of the clustering of single-channel openings, detailed voltage dependence of kinetics and probability of opening were not studied.     

Regulation of the hyperpolarization-activated K+ channel in the lateral membrane of the cortical collecting duct [published erratum appears in J Gen Physiol 1995 Sep;106(3):579]

An intermediate-conductance K+ channel (I.K.), the activity of which is increased by hyperpolarization, was previously identified in the lateral membrane of the cortical collecting duct (CCD) of the rat kidney (Wang, W. H., C. M. McNicholas, A. S. Segal, and G. Giebisch. 1994. American Journal of Physiology. 266:F813-F822). The biophysical properties and regulatory mechanisms of this K+ channel have been further investigated with patch clamp techniques in the present study. The slope conductance of the channel in inside-out patches was 50 pS with 140 mM KCl in the pipette and 5 mM KCl, 140 mM NaCl (NaCl Ringer''s solution) in the bath. Replacement of the bath solution with symmetrical 140 mM KCl solution changed the slope conductance of the channel to 85 pS and shifted the reversal potential by 55 mV, indicating that the selectivity ratio of K+/Na+ was at least 10:1. Channel open probability (Po) in inside-out patches was 0.12 at 0 mV and was increased by hyperpolarization. The voltage-dependent Po was fitted with the Boltzmann''s equation: Po = 1/[1 + exp(V-V1/2)zF/RT], with z = 1.2 and V1/2 = -40 mV. Addition of 2 mM tetraethylammonium or 500 mM quinidine to the bath blocked the activity of the K+ channel in inside-out patches. In addition, decrease in the bath pH from 7.40 to 6.70 reduced Po by 30%. Addition of the catalytic subunit of protein kinase A (PKAc; 20 U/ml) and 100 microM [corrected] MgATP to the bath increased Po from 0.12 to 0.49 at 0 mV and shifted the voltage dependence curve of channel activity toward more positive potentials by 40 mV. Two exponentials were required to fit both the open-time and the closed-time histograms. Addition of PKAc increased the long open-time constant and shortened the long closed-time constant. In conclusion, PKA-mediated phosphorylation plays an important role in the regulation of the voltage dependence of the hyperpolarization-activated K+ channel in the basolateral membrane of CCD.     

Quinine inhibits chloride and nonselective cation channels in isolated rat distal colon cells.

Isolated cells from rat distal colon were investigated with the patch-clamp technique. In cell-attached and cell-excised patches (inside-out) single chloride channels with outward-rectifying properties were observed. In excised patches the single-channel conductance g was 47 +/- 5 pS at positive and 22 +/- 2 pS at negative clamp potentials (n = 6). The Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 10 microM) induced fast closing events, whereas 10 microM of 3',5-dichlorodiphenylamine-2-carboxylic acid (DCDPC) had no effect when applied to the cytosolic side. Quinine in the bath inhibited the Cl- channel by reducing its single-channel amplitude and increased open channel noise. With 0.1 mM the current amplitude decreased by 54% and with 1 mM quinine by 67%. Ca2(+)-dependent nonselective cation channels where observed after excision of the membrane patch. This channel was completely and reversibly inhibited by 100 microM DCDPC. Application of 1 mM quinine to the bath induced flickering and reduced the open-state probability from 0.94 to 0.44. In summary, besides its well established effects on K+ channels, quinine also inhibits nonselective cation channels and chloride channels by inducing fast closing events.     

The low conductance K(+) channel in human colonic crypt cells has a voltage-dependent permeability not affected by Mg(++)

The low conductance K(+) channel found in human colonocytes was investigated using the patch-clamp technique. The channel is Ca(++)-dependent and is blocked by Ba(++) (5 mM) with a decrease in open probability from 0.42 to 0.19. At -40 mV the slope conductance was 29 pS (using intracellular solution in the pipette). In inside-out patches, inward rectification was seen both with KCl (pipette)/NaCl (bath) solutions as well as KCl/KCl solutions. The rectification could not be affected by omitting Mg(++) from the pipette or the bath solution, neither by exposing the patches to the polyamine spermine (1 mM). Using the Goldman-Hodgkin-Katz equation we show that the permeability decreased in a linear fashion from approximately 5.2 x 10(-14) cm(3)/s to 1.8 x 10(-14) cm(3)/s (-100 to +100 mV), both with and without Mg(++) in the solutions. There was no significant difference in the nominal values of permeability. This property of the K(+) channel may facilitate the hyperpolarization needed to sustain a chloride secretion.     

Voltage-dependent block of the cystic fibrosis transmembrane conductance regulator Cl- channel by two closely related arylaminobenzoates

The gene defective in cystic fibrosis encodes a Cl- channel, the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR is blocked by diphenylamine-2-carboxylate (DPC) when applied extracellularly at millimolar concentrations. We studied the block of CFTR expressed in Xenopus oocytes by DPC or by a closely related molecule, flufenamic acid (FFA). Block of whole-cell CFTR currents by bath-applied DPC or by FFA, both at 200 microM, requires several minutes to reach full effect. Blockade is voltage dependent, suggesting open-channel block: currents at positive potentials are not affected but currents at negative potentials are reduced. The binding site for both drugs senses approximately 40% of the electric field across the membrane, measured from the inside. In single-channel recordings from excised patches without blockers, the conductance was 8.0 +/- 0.4 pS in symmetric 150 mM Cl-. A subconductance state, measuring approximately 60% of the main conductance, was often observed. Bursts to the full open state lasting up to tens of seconds were uninterrupted at depolarizing membrane voltages. At hyperpolarizing voltages, bursts were interrupted by brief closures. Either DPC or FFA (50 microM) applied to the cytoplasmic or extracellular face of the channel led to an increase in flicker at Vm = -100 mV and not at Vm = +100 mV, in agreement with whole-cell experiments. DPC induced a higher frequency of flickers from the cytoplasmic side than the extracellular side. FFA produced longer closures than DPC; the FFA closed time was roughly equal (approximately 1.2 ms) at -100 mV with application from either side. In cell-attached patch recordings with DPC or FFA applied to the bath, there was flickery block at Vm = -100 mV, confirming that the drugs permeate through the membrane to reach the binding site. The data are consistent with the presence of a single binding site for both drugs, reached from either end of the channel. Open-channel block by DPC or FFA may offer tools for use with site-directed mutagenesis to describe the permeation pathway.     

Potassium conductance of the squid giant axon. Single-channel studies

The patch-clamp technique was implemented in the cut-open squid giant axon and used to record single K channels. We present evidence for the existence of three distinct types of channel activities. In patches that contained three to eight channels, ensemble fluctuation analysis was performed to obtain an estimate of 17.4 pS for the single-channel conductance. Averaged currents obtained from these multichannel patches had a time course of activation similar to that of macroscopic K currents recorded from perfused squid giant axons. In patches where single events could be recorded, it was possible to find channels with conductances of 10, 20, and 40 pS. The channel most frequently encountered was the 20-pS channel; for a pulse to 50 mV, this channel had a probability of being open of 0.9. In other single-channel patches, a channel with a conductance of 40 pS was present. The activity of this channel varied from patch to patch. In some patches, it showed a very low probability of being open (0.16 for a pulse to 50 mV) and had a pronounced lag in its activation time course. In other patches, the 40-pS channel had a much higher probability of being open (0.75 at a holding potential of 50 mV). The 40-pS channel was found to be quite selective for K over Na. In some experiments, the cut-open axon was exposed to a solution containing no K for several minutes. A channel with a conductance of 10 pS was more frequently observed after this treatment. Our study shows that the macroscopic K conductance is a composite of several K channel types, but the relative contribution of each type is not yet clear. The time course of activation of the 20-pS channel and the ability to render it refractory to activation only by holding the membrane potential at a positive potential for several seconds makes it likely that it is the predominant channel contributing to the delayed rectifier conductance.     

Ion channels in human endothelial cells.

Ion channels were studied in human endothelial cells from umbilical cord by the patch clamp technique in the cell attached mode. Four different types of ion channels were recorded: i) potassium channel current that rectifies at positive potentials in symmetrical potassium solutions (inward rectifier); ii) low-conductance non-selective cation channel with a permeability ratio K:Na:Ca = 1:0.9:0.2; iii) high-conductance cation-selective channel that is about 100 times more permeable for calcium than for sodium or potassium; iv) high-conductance potassium channel with a permeability ratio K:Na = 1:0.05. The extrapolated reversal potential of the inwardly rectifying current was near to the potassium equilibrium potential. The slope conductance decreased from 27 pS in isotonic KCl solution to 7 pS with 5.4 mmol/l KCl and 140 mmol/l NaCl in the pipette but 140 mmol/l KCl in the bath. The low-conductance non-selective cation channel showed a single-channel conductance of 26 pS with 140 mmol/l Na outside, 28 pS with 140 mmol/l K outside, and rectified in inward direction in the presence of Ca (60 mmol/l Ca, 70 mmol/l Na, 2.7 mmol/l K in the pipette) at negative potentials. The current could be observed with either chloride or aspartate as anion. The high-conductance non-selective channel did not discriminate between Na and K. The single-channel conductance was about 50 pS. The extrapolated reversal potential was more positive than +40 mV (140 K or 140 Na with 5 Ca outside). Both the 26 and 50 pS channel showed a run-down, and they rapidly disappeared in excised patches. The high-conductance potassium channel with a single-channel conductance of 170 pS was observed only rarely. It reversed near the expected potassium equilibrium potential. The 26 pS channel could be stimulated with histamine and thrombin from outside in the cell-attached mode. Both the 26 pS as well as the 50 pS channel can mediate calcium flux into the endothelial cell.     

The calcium-activated potassium channels of turtle hair cells

A major factor determining the electrical resonant frequency of turtle cochlear hair cells is the time course of the Ca-activated K current (Art, J. J., and R. Fettiplace. 1987. Journal of Physiology. 385:207- 242). We have examined the notion that this time course is dictated by the K channel kinetics by recording single Ca-activated K channels in inside-out patches from isolated cells. A hair cell's resonant frequency was estimated from its known correlation with the dimensions of the hair bundle. All cells possess BK channels with a similar unit conductance of approximately 320 pS but with different mean open times of 0.25-12 ms. The time constant of relaxation of the average single- channel current at -50 mV in 4 microM Ca varied between cells from 0.4 to 13 ms and was correlated with the hair bundle height. The magnitude and voltage dependence of the time constant agree with the expected behavior of the macroscopic K(Ca) current, whose speed may thus be limited by the channel kinetics. All BK channels had similar sensitivities to Ca which produced half-maximal activation for a concentration of approximately 2 microM at +50 mV and 12 microM at -50 mV. We estimate from the voltage dependence of the whole-cell K(Ca) current that the BK channels may be fully activated at -35 mV by a rise in intracellular Ca to 50 microM. BK channels were occasionally observed to switch between slow and fast gating modes which raises the possibility that the range of kinetics of BK channels observed in different hair cells reflects a common channel protein whose kinetics are regulated by an unidentified intracellular factor. Membrane patches also contained 30 pS SK channels which were approximately 5 times more Ca-sensitive than BK channels at -50 mV. The SK channels may underlie the inhibitory synaptic potential produced in hair cells by efferent stimulation.     

Apical K+ channels in Necturus taste cells. Modulation by intracellular factors and taste stimuli

The apically restricted, voltage-dependent K+ conductance of Necturus taste receptor cells was studied using cell-attached, inside-out and outside-out configurations of the patch-clamp recording technique. Patches from the apical membrane typically contained many channels with unitary conductances ranging from 30 to 175 pS in symmetrical K+ solutions. Channel density was so high that unitary currents could be resolved only at negative voltages; at positive voltages patch recordings resembled whole-cell recordings. These multi-channel patches had a small but significant resting conductance that was strongly activated by depolarization. Patch current was highly K+ selective, with a PK/PNa ratio of 28. Patches containing single K+ channels were obtained by allowing the apical membrane to redistribute into the basolateral membrane with time. Two types of K+ channels were observed in isolation. Ca(2+)-dependent channels of large conductance (135-175 pS) were activated in cell-attached patches by strong depolarization, with a half-activation voltage of approximately -10 mV. An ATP-blocked K+ channel of 100 pS was activated in cell-attached patches by weak depolarization, with a half-activation voltage of approximately -47 mV. All apical K+ channels were blocked by the sour taste stimulus citric acid directly applied to outside-out and perfused cell-attached patches. The bitter stimulus quinine also blocked all channels when applied directly by altering channel gating to reduce the open probability. When quinine was applied extracellularly only to the membrane outside the patch pipette and also to inside-out patches, it produced a flickery block. Thus, sour and bitter taste stimuli appear to block the same apical K+ channels via different mechanisms to produce depolarizing receptor potentials.     

Basic properties and potential regulators of the apical K+ channel in macula densa cells

These studies examine the properties of an apical potassium (K+) channel in macula densa cells, a specialized group of cells involved in tubuloglomerular feedback signal transmission. To this end, individual glomeruli with thick ascending limbs (TAL) and macula densa cells were dissected from rabbit kidney and the TAL covering macula densa cells was removed. Using patch clamp techniques, we found a high density (up to 54 channels per patch) of K+ channels in the apical membrane of macula densa cells. An inward conductance of 41.1 +/- 4.8 pS was obtained in cell-attached patches (patch pipette, 140 mM K+). In inside- out patches (patch pipette, 140 mM; bath, 5 mM K+), inward currents of 1.1 +/- 0.1 pA (n = 11) were observed at 0 mV and single channel current reversed at a pipette potential of -84 mV giving a permeability ratio (PK/PNa) of over 100. In cell-attached patches, mean channel open probability (N,Po, where N is number of channels in the patch and Po is single channel open probability) was unaffected by bumetanide, but was reduced from 11.3 +/- 2.7 to 1.6 +/- 1.3 (n = 5, p < 0.02) by removal of bath sodium (Na+). Simultaneous removal of bath Na+ and calcium (Ca2+) prevented the Na(+)-induced decrease in N.Po indicating that the effect of Na+ removal on N.Po was probably mediated by stimulation of Ca2+ entry. This interpretation was supported by studies where ionomycin, which directly increases intracellular Ca2+, produced a fall in N.Po from 17.8 +/- 4.0 to 5.9 +/- 4.1 (n = 7, p < 0.02). In inside- out patches, the apical K+ channel was not sensitive to ATP but was directly blocked by 2 mM Ca2+ and by lowering bath pH from 7.4 to 6.8. These studies constitute the first single channel observations on macula densa cells and establish some of the characteristics and regulators of this apical K+ channel. This channel is likely to be involved in macula densa transepithelial Cl- transport and perhaps in the tubuloglomerular feedback signaling process.     

Two novel cardiac atrial K+ channels, IK.AA and IK.PC

Two K(+)-selective channels in neonatal rat atrial cells activated by lipophilic compounds have been characterized in detail. The arachidonic acid-stimulated channel (IK.AA) had a slope conductance of 124 +/- 17 pS at +30 mV in symmetrical 140 mM potassium and a mean open time of approximately 1 ms, and was relatively voltage independent. IK.AA activity was reversibly increased by lowering pH to 6.0. Arachidonic acid was most effective in activating this channel, although a number of lipophilic compounds resulted in activation. Surprisingly, choline, a polar molecule, also activated the channel. A second K+ channel was activated by 10 microM phosphatidylcholine applied to the intracellular surface of inside-out atrial patches. This channel (IK.PC) had a slope conductance of 60 +/- 6 pS at +40 mV and a mean open time of approximately 0.6 ms, and was also relatively voltage independent. Fatty acids are probably monomeric in the membrane under the conditions of our recording; thus detergent effects are unlikely. Since a number of compounds including fatty acids and prostaglandins activated these two channels, an indirect, channel-specific mechanism may account for activation of these two cardiac K+ channels.     

Single-channel recordings from two types of amiloride-sensitive epithelial Na+ channels

We report here the first evidence in intact epithelial cells of unit conductance events from amiloride-sensitive Na+ channels. The events were observed when patch-clamp recordings were made from the apical surface of cultured epithelial kidney cells (A6). Two types of channels were observed: one with a high selectivity to Na+ and one with relatively low selectivity. The characteristics of the low-selectivity channel are as follows: single-channel conductance ranged between 7 and 10 pS (mean = 8.4 +/- 1.3), the current-voltage (I-V) relationship displayed little if any nonlinearity over a range of +/- 80 mV (with respect to the patch pipette) and the channel Na+/K+ selectivity was approximately 3-4:1. Amiloride, a cationic blocker of the channel, reduced channel mean open time and increased channel mean closed times as the voltage of the cell interior was made more negative. Amiloride induced channel flickering at increased negative potentials (intracellular potential with respect to the patch) but did not alter the single-channel conductance or the I-V relationship from that observed in control patches. The characteristics of the high-selectivity channel are: a single-channel conductance of 1-3 pS (mean = 2.8 +/- 1.2), the current-voltage relationship is markedly nonlinear with a Na+/K+ selectivity greater than 20:1. The mean open and closed times for the two types of channels are quite different, the high-selectivity channel being open only about 10% of the time while the low-selectivity channel is open about 30% of the time.     

Ca2(+)-sensitive K+ channel in aortic smooth muscle of rats

We measured K+ channel activity in inside-out patches of cell membrane from aortic vascular smooth muscle cultured (Passages 1-3) from Wistar, Wistar-Kyoto, and spontaneously hypertensive rats (SHR). With [Ca2+]i between 25 and 100 nm and 150 mm K+ on both sides of the membrane, the conductance of this channel was 55 +/- 7 pS (slope of current-voltage curve through 0 mV) and the current was outwardly rectified. There was no difference in single-channel conductance among the three rat strains. Increasing negative holding voltages or increasing [Ca2+]i, increased the probability of this type channel being open (Npo; P less than 0.01); SHR had a larger NPo (P less than 0.01). Compared with cells from Wistar and Wistar-Kyoto, cells from SHR also had the longest mean open time. The increased NPo and mean open time we observed in this K+ channel of cells from SHR could contribute, at least in part, to the increased membrane K+ permeability, reported previously.     

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.     

Single-channel properties of a volume-sensitive anion conductance. Current activation occurs by abrupt switching of closed channels to an open state

Swelling-induced loss of organic osmolytes from cells is mediated by an outwardly rectified, volume-sensitive anion channel termed VSOAC (Volume-Sensitive Organic osmolyte/Anion Channel). Similar swelling- activated anion channels have been described in numerous cell types. The unitary conductance and gating kinetics of VSOAC have been uncertain, however. Stationary noise analysis and single-channel measurements have produced estimates for the unitary conductance of swelling-activated, outwardly rectified anion channels that vary by > 15-fold. We used a combination of stationary and nonstationary noise analyses and single-channel measurements to estimate the unitary properties of VSOAC. Current noise was analyzed initially by assuming that graded changes in macroscopic current were due to graded changes in channel open probability. Stationary noise analysis predicts that the unitary conductance of VSOAC is approximately 1 pS at 0 mV. In sharp contrast, nonstationary noise analysis demonstrates that VSOAC is a 40-50 pS channel at +120 mV (approximately 15 pS at 0 mV). Measurement of single-channel events in whole-cell currents and outside- out membrane patches confirmed the nonstationary noise analysis results. The discrepancy between stationary and nonstationary noise analyses and single-channel measurements indicates that swelling- induced current activation is not mediated by a graded increase in channel open probability as assumed initially. Instead, activation of VSOAC appears to involve an abrupt switching of single channels from an OFF state, where channel open probability is zero, to an ON state, where open probability is near unity.     

Apical membrane ionic channels in the rabbit cortical thick ascending limb in primary culture.

Cortical thick ascending limbs of Henle's loop (cTAL) were microdissected from rabbit kidneys and cultured in a hormonally-defined medium. The cultured cells grew as a monolayer and retained the morphological and biochemical characteristics of the original tubule. Cyclic AMP production of the cultured cells was increased by human calcitonin (x13) and parathyroid hormone (x2). The cultured epithelial developed a transepithelial potential of 4.1 +/- 1.3 mV that was orientated positively towards the apical compartment. The basolateral membrane of the cells exhibited a chloride conductance sensitive to diphenylamine 2-carboxylate (DPC) and the apical membrane a barium-sensitive K+ permeability. Patch clamp analysis conducted on the apical membrane of the cells revealed the presence of three types of ionic channel. The first is a large conductance Ca(2+)-activated K+ channel (95 pS). The second K+ channel has a much smaller conductance (18.3 pS) and is insensitive to Ca2+. It may represent the conductive pathway for K+ recycling into the lumen in the original tubule. The last channel is cation selective, does not discriminate between Na+ and K+ and was found to have a conductance of 20.5 pS. Channel activity required a high cytoplasmic calcium concentration (1 mM), and was blocked by ATP (10 microM) applied on its cytoplasmic face.     

Single calcium channel behavior in native skeletal muscle

The purpose of this study was to use whole-cell and cell-attached patches of cultured skeletal muscle myotubes to study the macroscopic and unitary behavior of voltage-dependent calcium channels under similar conditions. With 110 mM BaCl2 as the charge carrier, two types of calcium channels with markedly different single-channel and macroscopic properties were found. One class was DHP-insensitive, had a single-channel conductance of approximately 9 pS, yielded ensembles that displayed an activation threshold near -40 mV, and activated and inactivated rapidly in a voltage-dependent manner (T current). The second class could only be well resolved in the presence of the DHP agonist Bay K 8644 (5 microM) and had a single-channel conductance of approximately 14 pS (L current). The 14-pS channel produced ensembles exhibiting a threshold of approximately -10 mV that activated slowly (tau act approximately 20 ms) and displayed little inactivation. Moreover, the DHP antagonist, (+)-PN 200-110 (10 microM), greatly increased the percentage of null sweeps seen with the 14-pS channel. The open probability versus voltage relationship of the 14-pS channel was fitted by a Boltzmann distribution with a VP0.5 = 6.2 mV and kp = 5.3 mV. L current recorded from whole-cell experiments in the presence of 110 mM BaCl2 + 5 microM Bay K 8644 displayed similar time- and voltage-dependent properties as ensembles of the 14-pS channel. Thus, these data are the first comparison under similar conditions of the single-channel and macroscopic properties of T current and L current in native skeletal muscle, and identify the 9- and 14-pS channels as the single-channel correlates of T current and L current, respectively.