Calcium-activated potassium channels in single smooth muscle cells of rabbit jejunum and guinea-pig mesenteric artery

Delayed rectifier potassium channels in canine and porcine airway smooth muscle cells

1. In order to define the ion channels underlying the inactivating, calcium-insensitive current in airway smooth muscle cells, unitary potassium currents were recorded from canine and porcine trachealis cells, and compared with macroscopic currents. On-cell and inside-out single-channel currents were compared with whole-cell recordings made in dialysed cells. 2. Depolarizing voltage steps evoked outward unitary currents. In addition to a large conductance, calcium-activated potassium channel (KCa), a lower conductance potassium channel was identified. This channel has a conductance of 12.7 pS (on-cell; 1 mM-K+ in the pipette). 3. The lower conductance channel (Kdr) was not sensitive to cytosolic Ca2+ concentration and unitary current openings occurred following a delay after the voltage step. The time course of activation of the current composed of averaged single-channel events was very similar to that of the whole-cell, delayed rectifier potassium current (IdK), recorded under conditions of low intracellular calcium (Kotlikoff, 1990). 4. Kdr channels also inactivated with kinetics similar to those of the macroscopic current. Averaged single-channel records revealed a current that inactivated with kinetics that could be described by two exponentials (tau 1 = 0.14 s, tau 2 = 1.1 s; at 5 mV). These values corresponded well with previously determined values for time-dependent inactivation of IdK. Inactivation of Kdr channels was markedly voltage dependent, and was well fitted by a Boltzmann equation with V50 = -53 mV; this was similar to measurements of the macroscopic current, although the V50 value was shifted to more positive potentials in whole-cell measurements. When only the inactivating component of the macroscopic current was considered, the voltage dependence of inactivation of the single-channel current and macroscopic current were quite similar. 5. Single-channel kinetics indicated that Kdr channels occupy one open and two closed states. The mean open time was 1.7 ms. Inactivation results in a prominent increase in the long closed time, with little effect on the mean open time or short closed time. 6. The Kdr channel was not blocked by tetraethylammonium (TEA; 1 mM), charybdotoxin (ChTX; 100 nM) or glibenclamide (20 microM), but was blocked by 4-aminopyridine (4-AP; 1 mM). Similarly, 4-AP blocked the inactivating component of the macroscopic current, but a non-inactivating current remained. KCa currents were blocked by TEA (0.5-1 mM) and charybdotoxin (40 nM), but were insensitive to to 4-AP (1 mM) and glibenclamide (20 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of membrane currents in single smooth muscle cells from the guinea-pig gastric antrum

1. Smooth muscle cells, enzymatically isolated from the antrum of the guinea-pig stomach, were voltage clamped at room temperature using the whole-cell patch clamp technique. In physiological salt solution (PSS), step depolarization from a holding potential of -90 mV elicited inward calcium current (ICa) followed and superimposed by outward potassium current. 2. Outward current was divided into components depending on the presence of extracellular Ca2+ and others which were not activated as a result of Ca2+ influx. Ca(2+)-dependent components were (1) a fast transient component most likely representing Ca(2+)-activated K+ current (IK(Ca)) immediately triggered by the initial peak of ICa and (2) spontaneous transient outward currents (STOCs) apparently reflecting synchronized opening of IK(Ca) channels by cyclic release of Ca2+ from intracellular stores. Ca2+ influx-independent outward current could be divided into two main components: (1) a transient component (I(to)) showing voltage-dependent activation and inactivation and (2) a non-inactivating component (Ini). 3. I(to) activated with a threshold around -30 mV, was fully available at -90 mV and completely inactivated at -10 mV. The time course of both activation and inactivation of I(to) at different potentials could be described by single exponential functions. Time constants of activation decreased from 35 ms at -10 mV to 10 ms at +40 mV. The time constant of inactivation was about 2 s and only weakly voltage dependent. Time constants for exponentially developing recovery from inactivation of I(to) ranged from 0.1 s at -100 mV to 10 s at -30 mV. I(to) was insensitive to 4-aminopyridine (4-AP, 5 mmol/l), slightly sensitive to tetraethylammonium (TEA, 10 mmol/l), but substantially inhibited by caffeine (10 mmol/l) and Cd2+ (5 mmol/l). Estimates of the single-channel conductance by current fluctuation analysis indicated a small value of about 2.5 pS. 4. The action of TEA on current elicited from a holding potential of -10 mV indicated a major contribution to Ini of a distinct component (Ini,K) that was completely blocked by this substance at a concentration of 10 mmol/l. Ini was almost unaffected by 4-AP (5 mmol/l) and caffeine (10 mmol/l), but strongly suppressed by Cd2+ (5 mmol/l). Current fluctuation analysis of Ini,K gave a value for the single-channel conductance of about 60 pS. 5. Ca2+ inward current was studied in PSS ([Ca2+]o = 2.5 mmol/l) using pipette solution in which K+ was replaced by Cs+ to suppress outward K+ currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous sarcoplasmic reticulum calcium release and extrusion from bovine, not porcine, coronary artery smooth muscle

1. We tested the hypothesis that the Ca(2+)-loaded sarcoplasmic reticulum (SR) of coronary artery smooth muscle spontaneously releases Ca2+ preferentially toward the sarcolemma to be extruded from the cell without increasing the average free myoplasmic [Ca2+] (Ca(im)) concentration. 2. The SR of bovine cells was Ca(2+)-loaded by depolarization-induced Ca2+ influx. Release (unloading) of Ca2+ from the SR during recovery from depolarization was determined by Fura-2 microfluorometry of Ca(im). The SR Ca2+ unloading was maximal following a long (14 min) recovery from depolarization, as shown by the 66% decrease in the peak caffeine-induced Ca(im) transient compared to the Ca(im) transient after a short (2 min) recovery. No increase in Ca(im) occurred during the long recovery. No unloading of the SR Ca2+ store was noted in porcine cells. 3. Approximately 80% of the outward K+ current in bovine and porcine cells was sensitive to subsarcolemmal Ca2+ (Ca(is)) concentrations. Whole-cell voltage clamp using pipette solutions with Ca2+ concentrations clamped between 0 and 1000 nM with Ca(2+)-EGTA or Ca(2+)-BAPTA buffers showed increasing K+ currents (normalized for cell membrane surface area) as a function of both membrane potential and Ca(is). Clamping of Ca(im) and Ca(is) was verified by the lack of changes in K+ current and Fura-2 ratio in response to Ca2+ influx, Ca(2+)-free external solution, or caffeine-induced Ca2+ release. At +30 to +50 mV the K+ current amplitude showed a similar sensitivity to Ca2+ as Fura-2. These data indicate that in this experimental preparation Ca(2+)-activated K+ current is a valid estimate of Ca(is). 4. Simultaneous Ca(im) and Ca(is) measurements in bovine cells which were not Ca(2+)-clamped (2 x 10(-4) M-EGTA pipette solution) showed that during the long recovery period the K+ current (reflecting Ca(is)) increased 55%, while Ca(im) did not change. 5. In quiescent bovine cells the Ca(is) was higher than Ca(im), while the higher resting Ca(is) gradient was not apparent in porcine cells. 6. The Ca(is) concentration was directly related to the amount of Ca2+ in the SR in bovine, but not porcine cells. Depletion of the SR in bovine cells by caffeine resulted in a 58% decrease in K+ current compared to the resting K+ current. 7. Caffeine-induced Ca2+ release caused an increase in Ca(is) which preceded the increase in Ca(im) by approximately 2 s.(ABSTRACT TRUNCATED AT 400 WORDS)

Muscarinic inhibition of single KCa channels in smooth muscle cells by a pertussis-sensitive G protein

Application of a muscarinic agonist to the extracellular surface of membrane patches from airway smooth muscle cells resulted in an inhibition of calcium-activated potassium (KCa) channels in outside-out patches. Methacholine (50 microM) inhibited channel activity at physiological cytosolic calcium concentrations and resulted in a marked shift in channel open-time kinetics. In inside-out patches, KCa channels were inhibited upon addition of GTP (100 microM) when methacholine was present in the patch pipette. Muscarinic inhibition was blocked when guanosine 5'-O-(2-thiodiphosphate) was used to compete with endogenous GTP in outside-out or inside-out experiments. Pretreatment of dissociated cells with pertussis toxin (0.1 micrograms/ml) blocked muscarinic inhibition of the channel in a time-dependent fashion. These results demonstrate, at the single-channel level, a coupling between muscarinic receptor stimulation and inhibition of KCa in smooth muscle and demonstrate the guanine nucleotide dependence of this coupling.

High sensitivity to internal tetraethylammonium in K(Ca) channels of cerebrovascular smooth muscle cells

Patch clamp methods were used to study the properties of calcium-dependent K(Ca) channels in enzyme dissociated smooth muscle cells from the cerebral arteries of adult rats. Dissociated muscle cells were maintained at 4 degrees C for up to 48 h prior to use. Inside-out membrane patches excised from these cells contained a K(Ca) channel with a conductance of 92 +/- 2.6 pS in symmetrical 140 mM potassium solutions. This channel was activated by membrane depolarization and by cytoplasmic calcium, and showed negligible permeability to sodium or cesium ions. Single channel currents were reduced by internal application of tetraethylammonium ions, with a Kd = 0.31 mM.

Properties of the late transient outward current in isolated intestinal smooth muscle cells of the guinea-pig

1. Whole-cell membrane currents in voltage-clamped single isolated cells of longitudinal smooth muscle of guinea-pig ileum were studied at room temperature using patch pipettes filled with either high-K+ solution or high-Cs+ solution, to suppress K+ outward current, and containing 0.3 mM-EGTA. 2. In the presence of high-K+ solution in the pipette, membrane depolarization from the holding potential of -50 mV evoked an initial inward calcium current (ICa) followed by a large initial transient outward current and a sustained outward current with spontaneous oscillations superimposed. Prolonged depolarization above -20 mV produced a late transient outward current which reached a maximum (up to several nanoamps at +10 mV) within approximately 1 s and lasted several seconds. 3. The late outward current (ILTO) was voltage dependent and reversed at the EK (potassium equilibrium potential) in cells exposed to high-K+ external solution. It was blocked by TEA+ (tetraethylammonium) or Ba2+ applied externally (calculated Kd (dissociation constant) values were 0.67 and 4.43 mM, respectively) or by high-Cs+ solution perfusing the cell. The removal of extracellular Ca2+, application of Ca2+ channel blockers (3 mM-Co2+, 0.2 mM-Cd2+ or 1 microM-nifedipine) or perfusion of 5 mM-EGTA inside the cell also abolished the current. Thus, the current seems to be a Ca(2+)-activated K+ current. 4. There is a great discrepancy between the time course of the ICa and that of the late ILTO, which suggests that Ca2+ release from intracellular storage sites may contribute to the generation of the ILTO. 5. Bath application of caffeine (10 mM) during the development of ILTO enhanced the current. However, in the presence of caffeine ILTO was inhibited. Moderate inhibition of ICa by caffeine was also observed. 6. Ryanodine (5 microM) applied to the bathing solution completely inhibited ILTO within 3.5 min; however, it had no or little effect on the ICa. 7. Ruthenium Red (10 microM) completely blocked the ILTO and slightly and more slowly inhibited the ICa. 8. Increasing Mg2+ concentration in the pipette solution from 1 to 6 mM abolished the ILTO. 9. It was concluded that the ILTO was activated mainly by Ca2+ released from the intracellular storage sites following Ca2+ entry, presumably by a Ca(2+)-induced Ca2+ release mechanism.

Relation between muscarinic receptor cationic current and internal calcium in guinea-pig jejunal smooth muscle cells

1. The action of carbachol, which activates muscarinic receptors, was studied in single patch-clamped cells where free internal calcium concentration in the cell (Cai2+) was estimated using the emission from the dye Indo-1. Cells were dialysed with potassium-free caesium solution to block any Ca(2+)-activated K(+)-current. 2. Carbachol applied to the cell evoked an initial peak in Cai2+ followed by a smaller sustained rise (plateau) upon which several oscillations in Cai2+ were often superimposed; the changes in inward, cationic current (icarb) followed changes in Cai2+ closely. Calcium entry blocker did not affect these responses. 3. The initial peak in Cai2+ produced by carbachol was due to calcium store release: it was essentially unchanged at +50 mV, and abolished by prior application of caffeine (10 mM) to the cell or by inclusion of heparin (which blocks D-myoinositol 1,4,5-trisphosphate receptors) in the pipette. In contrast, the rise in Cai2+ produced by ATP in rabbit ear artery smooth muscle cells was unaffected by caffeine or heparin as it was due to calcium entry into the cell. 4. The later sustained rise (plateau) in Cai2+ produced by carbachol was due to the entry of calcium into the cell down its electrochemical gradient as it was affected by changing the cell membrane potential or the calcium concentration of the bathing solution. As the sustained rise in Cai2+ produced by caffeine had similar properties, it was suggested that depletion of calcium stores can evoke an increased calcium entry into the cell through some pathway. 5. The cationic current evoked by carbachol was strongly dependent on Cai2+. It was small if any rise in Cai2+ due to calcium store release was prevented by the inclusion of heparin in the pipette solution and increased greatly if calcium entry was provoked through voltage-dependent channels by applying a depolarizing pulse or if calcium was released from stores by caffeine. 6. In the longitudinal muscle of guinea-pig small intestine, activation of muscarinic receptors by carbachol results in the opening of cationic channels; the resulting depolarization increases the frequency of action potential discharge and this determines the degree of contraction. Muscarinic receptor activation opens cationic channels by two mechanisms: release of stored calcium increases Cai2+ and this strongly potentiates a primary mechanism which may involve a G-protein.

Calcium-activated potassium channels: regulation by calcium

A wide variety of calcium-activated K channels has been described and can be conveniently separated into three classes based on differences in single-channel conductance, voltage dependence of channel opening, and sensitivity to blockers. Large-conductance calcium-activated K channels typically require micromolar concentrations of calcium to open, and their sensitivity to calcium increases with membrane depolarization, suggesting that they may be involved in repolarization events. Small-conductance calcium-activated K channels are generally more sensitive to calcium at negative membrane potentials, but their sensitivity to calcium is independent of membrane potential, suggesting that they may be involved in regulating membrane properties near the resting potential. Intermediate-conductance calcium-activated K channels are a loosely defined group, where membership is determined because a channel does not fit in either of the other two groups. Within each broad group, variations in calcium sensitivity and single-channel conductance have been observed, suggesting that there may be families of closely related calcium-activated K channels. Kinetic studies of the gating of calcium-activated potassium channels have revealed some basic features of the mechanisms involved in activation of these channels by calcium, including the number of calcium ions participating in channel opening, the number of major conformations of the channels involved in the gating process, and the number of transition pathways between open and closed states. Methods of analysis have been developed that may allow identification of models that give accurate descriptions of the gating of these channels. Although such kinetic models are likely to be oversimplifications of the behavior of a large macromolecule, these models may provide some insight into the mechanisms that control the gating of the channel, and are subject to falsification by new data.

Potassium conductance of smooth muscle cells from rabbit aorta in primary culture

Vascular smooth muscle cells were obtained from rabbit aorta and were studied in primary culture on days 1-7 after seeding with electrophysiological techniques. In impalement experiments a mean membrane potential difference (PD) of -50 +/- 0.3 mV (n = 387) was obtained with Ringer-type solution in the bath. PD was depolarized by 6 +/- 0.3 mV (n = 45) and 16 +/- 2 mV (n = 5) when the bath K+ concentration was increased from the control value of 3.6 mmol/l to 13.6 and 23.6 mmol/l, respectively. Ba2+ (0.1-1 mmol/l) depolarized PD. Tetraethylammonium (TEA, 10 mmol/l) depolarized PD only slightly but significantly. Verapamil (0.1 mmol/l) and charybdotoxin (10 nmol/l) had no effect on PD. The conductance properties of these cells were further examined with the patch-clamp technique. K+ channels were spontaneously present in cell-attached patches. When the pipette was filled with 145 mmol/l KCl, a mean conductance (gK) of 209.6 +/- 4.6 mV (n = 17) was read from the current/voltage curves at a clamp voltage (Vc) of 0 mV. After excision K+ channels were found in 129 patches with inside-out and in 50 with outside-out configuration. With KCl on one and NaCl on the other side the mean gK at a Vc of 0 mV was 134.6 +/- 3.9 pS (n = 179). The mean permeability was 0.89 +/- 0.03 x 10(-12) cm3/s. With symmetrical KCl solution the mean gK was 227 +/- 6 pS (n = 17). The conductance sequence was gK much greater than gRb = gCs = gNa = 0. TEA blocked dose-dependently only from the outside (1-10 mmol/l). Lidocaine (5 mmol/l) quinidine (0.01-1 mmol/l) and quinine (0.01-1 mmol/l) blocked from both sides. Charybdotoxin (0.5-5 nmol/l) blocked only from the extracellular side. Ba2+ blocked from the cytosolic side and the inhibition was increased by depolarization and reduced by hyperpolarization. At a Vc of 0 mV a half-maximal inhibition (IC50) of 2 mumol/l was obtained. Verapamil and diltiazem blocked from both sides, verapamil with an IC50 of 2 mumol/l and diltiazem with an IC50 of 10 mumol/l. The open probability of this channel was increased by CA2+ on the cytosolic side at activities greater than 0.1 mumol/l. Half-maximal activation occurred at Ca2+ activities exceeding 1 mumol/l. The present data indicate that the vascular smooth muscle cells of rabbit aorta in primary culture possess a K+ conductance. In excised patches only a maxi K+ channel was detected. This channel has properties different from the macroscopic K+ conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Subconductance states in calcium-activated potassium channels from canine airway smooth muscle

The single-channel patch clamp technique was used to analyze subconductance states in the 260 pS calcium-activated potassium channel from canine airway smooth muscle. More than sixty minutes of single channel data (greater than 87,000 events) from five excised patches were analyzed. Six subconductance amplitudes were clearly established to be 17, 33, 41, 52, 63 and 72% of the full conductance. Subconductance openings were usually brief (milliseconds) and represented less than 5% of the total channel open time, but they also persisted for several seconds on rare occasions. They appeared to be unaffected by voltage or time after seal formation, but may have increased in occurrence with decreasing calcium concentration. Irregular amplitude intervals, and the presence of ramp-like, analog transitions between conductance states, suggest a model for maxi-K subconductance states in which the channel protein undergoes random conformational changes causing a variable pore size.

Characterization of ionic currents of circular smooth muscle cells of the canine pyloric sphincter

1. The ionic currents of circular muscle cells from canine pyloric sphincter were characterized using the whole-cell patch clamp technique. 2. Subpopulations of circular muscle cells from the myenteric and submucosal halves of the circular layer were isolated and studied separately to determine whether differences in the currents expressed by these cells could explain differences in electrical behaviour observed in situ. 3. Resting potentials of isolated cells were about 20 mV positive to cells in intact muscles. Polarization under current clamp to the level of tissue resting potentials caused spontaneous discharge of action potentials in many cells. 4. Outward current measured under voltage clamp could be divided into a voltage-dependent component and a voltage- and Ca(2+)-dependent component. The latter was affected by manipulations of external [Ca2+], nifedipine and dialysis of cells with EGTA. 5. A few cells exhibited a channel that was activated with hyperpolarization. These channels produced inward current at potentials positive to the potassium reversal potential, EK, and reversed at -13 mV. 6. Inward currents, recorded from Cs(+)-loaded cells, were characterized by a transient phase and a sustained phase that persisted throughout the test depolarization. The inward current was reduced by nifedipine but in some cells a nifedipine-resistant component was observed. 7. There were no fundamental differences in the ionic currents recorded from circular muscle cells from the myenteric and submucosal regions, suggesting that the electrical activity of the tissue must be dependent upon structural characteristics (i.e. electrical coupling, fibre bundle dimensions, etc.) of the tissue. 8. The ionic conductance characterized can be related to many of the excitable events recorded from pyloric muscles.

Kinetic differences between Ca(2+)-dependent K+ channels in smooth muscle cells isolated from normal and atherosclerotic human aorta

The properties of large conductance Ca(2+)-dependent K+ channels in smooth muscle cells (SMC) isolated from normal and atherosclerotic human aorta were studied using the patch-clamp technique. It was shown that SMC from normal human aorta possess a homogeneous population of normal Ca(2+)-dependent K+ channels. In atherosclerotic aorta two kinetically different types of these channels could be distinguished: along with normal 'long' (L)-type channels there appeared channels of 'short' (s)-type. Under similar conditions s-type channels had about a four times shorter mean open time. About five times higher [Ca2+]in was necessary for s-type channels to reach the probability of the channels being open equal to L-type channels. No differences in conductance and voltage dependency were found between the two channel types. Channels of the s-type resembled those previously described in SMC isolated from foetal human aorta. Thus, it can be suggested that during the development of atherosclerosis a population of SMC with s-type Ca(2+)-dependent K+ channels appears in human aorta.

Characterization of an outward K+ current in freshly dispersed cerebral arterial muscle cells

This study was carried out to define some of the cellular ionic mechanisms controlling cerebral arterial muscle. Muscle cells were enzymatically dispersed from cat cerebral arteries. Cells were dialyzed and voltage-clamped using patch pipettes and whole-cell currents measured. Using pipette solutions allowing us to record K+ currents we identified an outward current elicited by depolarizing voltage steps beyond -20 mV. This outward current exhibited properties of delayed outward rectification having a peak macroscopic current at +90 mV of 504 +/- 236 pA. The current was sensitive to 4-aminopyridine, but was sensitive to tetraethylammonium only at very high doses. When CsCl was in the recording pipette, macroscopic outward currents could not be recorded. Variations in the extracellular Ca2+ concentration from 0.5 to 5.0 mM had no effect on current amplitude or voltage dependence; similarly the Ca2+ channel blockers nifedipine and Mn2+ were without effect on this outward current. The current inactivated slowly with no decay seen even with 3-s command pulses. Repetitive voltage pulses from -60 to +90 mV at a frequency of 1 Hz resulted in "cumulative reduction", depressing peak current by 60% after ten pulses. Upon reduction of pH from 7.43 to 7.20 we observed a 350% increase in peak outward current in 7 of 12 cells studied in this regard. Thus, the cellular mechanism responsible for cerebral vascular dilation to acidosis and/or hypercapnia may involve an increase in outward K+ current.

A patch-clamp study of K(+)-channel activity in bovine isolated tracheal smooth muscle cells

1. Single smooth muscle cells were isolated from bovine trachealis by enzymic digestion. The properties of large conductance plasmalemmal K(+)-channels in these cells were studied by the patch-clamp recording technique. 2. Recordings were made from inside-out plasmalemmal patches when [K+] was symmetrically high (140 mM) and when [Ca2+] on the cytosolic side of the patch was varied from nominally zero to 10 microM. Large unitary currents of both Ca(2+)-dependent and -independent types were observed. Measured between + 20 and + 40 mV, the slope conductances of the channels carrying these currents were 249 +/- 18 pS and 268 +/- 14 pS respectively. 3. Lowering [K+] on the cytosolic side of the patches from 140 to 6 mM, shifted the reversal potentials of the two types of unitary current from approximately zero to much greater than + 40 mV, suggesting that both currents were carried by K(+)-channels. 4. The Ca(2+)-dependent and -independent K(+)-channels detected in inside-out plasmalemmal patches could also be distinguished on the basis of their sensitivity to inhibitors (tetraethylammonium (TEA), 1-10 mM; Cs+, 10 mM; Ba2+, 1-10 mM; quinidine, 100 microM) applied to the cytosolic surface of the patches. 5. Recordings were made from outside-out plasmalemmal patches when [K+] was symmetrically high (140 mM) and when [Ca2+] on the cytosolic side of the patch was varied from nominally zero to 1 microM. Ca(2+)-dependent unitary currents were observed and the slope conductance of the channel carrying these currents was 229 +/- 5 pS. 6. Activity of the Ca2+-dependent K+-channel detected in outside-out patches could be inhibited by application of TEA (1 mM), Cs+ (10mM), Ba2(+210mM) or quinidine (100 microM) to the external surface of the patch. 4-Aminopyridine (4-AP; 1 mM) was ineffective as an inhibitor. 7. The activity of the Ca2+-dependent K+-channel recorded from outside-out patches was reversibly inhibited by charybdotoxin (100 nM). 8. When whole-cell recording was performed, the application of a depolarizing voltage ramp evoked outward current which was dependent on the [Ca2 +] in the recording pipette and which could be reversibly inhibited by charybdotoxin (50 nM-I microM) applied to the external surface of the cell.9. We conclude that bovine trachealis cells are richly endowed with charybdotoxin-sensitive, large conductance, Ca2 +-dependent K+-channels. These channels carry most of the outward current evoked by a depolarizing ramp and could play a major role in determining the outward rectifying properties of the trachealis cells. The role of the large Ca2 + -independent K+ -channels remains unclear.

Dual action of endothelin-1 on the Ca2(+)-activated K+ channel in smooth muscle cells of porcine coronary artery

The effects of endothelin-1 (ET-1) on the activity of the large Ca2(+)-activated K+ channel (BK channel) in enzymatically dissociated smooth muscle cells of porcine coronary artery were studied with the cell-attached patch-clamp technique. ET-1 at concentrations between 0.1 and 10 nM potentiated the BK channel activity. This effect was maximal at 1 nM ET-1, resulting in an average of 4.2-fold increase in channel open-state probability as compared with control. ET-1 at concentrations higher than 10 nM produced an irreversible inhibition of the BK channel activity, primarily due to a marked decrease in the channel mean open-time. The activation by lower doses of ET-1, but not the inhibition by higher doses of ET-1, of the BK channel was blocked by 0.1 microM PN 200-110, a Ca2+ channel blocker. The modulation of the BK channel activity in smooth muscle cell membrane may be a possible mechanism for ET-induced vasodilator and vasoconstrictor actions.

Role of K+ channels in spontaneous electrical and mechanical activity of smooth muscle in the guinea-pig mesotubarium

1. The spontaneous electrical and mechanical activity and the efflux rate of 86Rb+ in the guinea-pig mesotubarium were studied in the presence of agents interacting with K+ channels. 2. Tetraethylammonium (TEA, 10 mM) increased the amplitude of the action potentials while having no consistent effect on the frequency or amplitude of spontaneous contractions. 3. 4-Aminopyridine (4-AP, 1-5 mM) caused a graded increase in the duration of the contractions and of the electrical slow waves, and a decrease in the duration of the relaxed period between contractions. At 4 mM-4-AP or more the cell was unable to repolarize from the slow wave and the membrane depolarized to -26 mV from the normal resting potential of -63 mV. The rate of 86Rb+ efflux in the presence of 5 mM-4-AP was higher than that at 60 mM-K+, where the membrane potential is -24 mV. 4. 4-AP (5 mM) evoked a contracture in Ca(2+)-free solution, containing 1 mM-EGTA, both at the normal [K+]o of 5.9 mM and at 60 mM-K+, suggesting release of intracellular Ca2+. 5. Apamin (0.1-1 microM) and charybdotoxin (1-10 nM), blockers of Ca(2+)-dependent K+ channels, were without effects on the spontaneous electrical and mechanical activity. 6. The K+ channel opener pinacidil (10 microM) inhibited the spontaneous contractions and hyperpolarized the membrane by about 7 mV. The permeability to 86Rb+ was increased by a factor of 1.4. 7. It is concluded that different K+ channels are involved in the generation of spikes and slow waves: one sensitive to TEA and responsible for repolarization of the individual action potential, and another sensitive to 4-AP and responsible for repolarization of the slow wave. The duration of the relaxed period can be influenced by activation of K+ channels sensitive to pinacidil.

Calcium release induced by inositol 1,4,5-trisphosphate in single rabbit intestinal smooth muscle cells

1. Single smooth muscle cells were isolated by enzymic digestion from the longitudinal muscle layer of rabbit jejunum, and the response of the cells to calcium (Ca2+) release by InsP3 (D-myo-inositol 1,4,5-trisphosphate) was studied. Changes in internal Ca2+ concentration were monitored by measuring Ca(2+)-activated K+ currents (outward currents) using the whole-cell voltage-clamp technique. 2. At break-through from cell-attached patch to whole-cell recording mode using a 100 microM-InsP3-filled pipette, cells exhibited a brief outward current which reached its peak in 1.1 s and terminated within 10 s. Following this the generation of spontaneous transient outward currents (STOCs) was inhibited. (STOCs are considered to represent bursts of openings of Ca(2+)-activated K+ channels in response to spontaneous discharges of Ca2+ from the stores.) When a pipette filled with 20 microM-InsP3 was used, similar current responses were also evoked, but some cells failed to respond. 3. The InsP3-induced outward current at membrane break-through was similar in size and time course to the outward current response of normal cells to bath-applied carbachol (CCh, 100 microM) or caffeine (20 mM). 4. Dialysis with InsP3-containing solution inhibited the caffeine-induced outward current, depending on the pipette InsP3 concentration. Inclusion of heparin (5 mg/ml) in the pipette completely prevented inhibition by InsP3 of the caffeine response and of STOC discharge. However, the InsP3-induced current at break-through remained unchanged, probably because of the slower rate of diffusion of heparin. 5. In cells dialysed with pipette solution containing 30 or 100 microM-caged InsP3, flash photolysis (producing up to 1.5 microM-InsP3) induced an outward current response after a latency of 31.0 +/- 1.8 ms (n = 15), which was followed by inhibition of STOCs. The reversal potential of the current to flash-release of InsP3 followed closely the Nernst potential for K+ ions (EK), suggesting negligible contributions from channels other than Ca(2+)-activated K+ channels. 6. Photolysis of caged InsP3 (30 or 100 microM) still produced a current response after 3-6 min in Ca(2+)-free (3 mM-EGTA added) bathing solution, but no response occurred if the cell was exposed to either caffeine (20 mM) or CCh (100 microM) to deplete Ca stores.(ABSTRACT TRUNCATED AT 400 WORDS)

Carbon monoxide inhibits depolarization-induced Ca rise and increases cyclic GMP in visceral smooth muscle cells

Monocytes were isolated from the urinary bladder of the guinea-pig. By means of the voltage clamp technique, whole cells were depolarized from -65 to +10 mV in order to increase the intracellular calcium concentration [Ca2+]i and to monitor this increase by means of the calcium activated potassium current IK.Ca. Superfusion of the cells with carbon monoxide-containing solutions for 2 min inhibited the signal to about 50% of the control suggesting depression of the depolarization-induced increase in [Ca2+]i. The CO-mediated inhibition of IK.Ca was partially reversed by wash-off of CO; flashes of high light intensity accelerated the rate of recovery. Sodium nitroprusside (0.01-1 mM) depressed the depolarization-induced increase in [Ca2+]i similar to CO. In multicellular preparations of the urinary bladder, CO-containing media were shown to increase the cGMP concentration by a factor of 2 in the absence and by a factor of 3 in the presence of 1 mM of the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX). According to our previous work, CO binds to and activates soluble guanylate cyclase [Brüne B and Ullrich V, Mol Pharmacol 32: 497-504, 1987; Utz J and Ullrich V, Naunyn Schmiedebergs Arch Pharmacol 337 (Suppl): 299, 1988] and the rise in cGMP could thus effect [Ca2+]i by still unknown mechanisms.

Properties of sympathetic neuromuscular transmission and smooth muscle cell membranes in vascular beds

In vascular smooth muscle tissues, the cycle of contraction-relaxation is mainly regulated by the cytosolic Ca, and many other factors, such as substances released from endothelial cells and perivascular nerve terminals (mainly sympathetic nerves). In this article, we introduce regional differences in specific features of ionic channels in vascular smooth muscle membranes (mainly on features of Ca, Na and K channels) in relation to mobilization of the cytosolic Ca. In many vascular tissues, neurotransmitters released from sympathetic nerve terminals activate post-junctional receptors, and subsequently modify ion channels (receptor-activated cation channel and voltage-dependent Ca channel), whereas in some tissues, ionic channels are not modified by receptor activations (pharmaco-mechanical coupling). However, activation of receptors, with or without modulation of ionic channels, regulates the cytosolic Ca through synthesis of second messengers. In addition, receptors distributed on prejunctional nerve terminals positively or negatively regulate the release of transmitters. Roles of neurotransmitters (mainly ATP and noradrenaline) are also discussed in relation to the generation of excitatory junction potentials.

Ca2(+)-activated K+ channels in airway smooth muscle are inhibited by cytoplasmic adenosine triphosphate

Large-conductance Ca2(+)-activated K+ channels were studied in membranes of cultured rabbit airway smooth muscle cells, using the patch-clamp technique. In cell-attached recordings, channel openings were rare and occurred only at very positive potentials. Bradykinin (10 microM), an agonist which releases Ca2+ from the sarcoplasmic reticulum, transiently increased channel activity. The metabolic blocker 2,4-dinitrophenol (20 microM), which lowers cellular adenosine triphosphate (ATP) levels, induced a sustained increase of channel activity in cell-attached patches. In excised patches, these channels had a slope conductance of 155 pS at 0 mV, were activated by depolarization and by increasing the Ca2+ concentration at the cytoplasmic side above 10(-7) mol/l. ATP, applied to the cytoplasmic side of the patches, dose-dependently decreased the channel's open-state probability. An inhibition constant (Ki) of 0.2 mmol/l was found for the ATP-induced inhibition. ATP reduced the Ca2+ sensitivity of the channel, shifting the Ca2+ activation curve to the right and additionally reducing its steepness. Our results demonstrate that cytoplasmic ATP inhibits a large-conductance Ca2(+)-activated K+ channel in airway smooth muscle. This ATP modulation of Ca2(+)-activated K+ channels might serve as an important mechanism linking energy status and the contractile state of the cells.

ATP-gated channels in vascular smooth muscle cells

ATP acting through P2x-purinoceptors activates cation channels with some similarities to the activation of channels gated by acetylcholine and glutamate (channels that can also act as fast excitatory transmitters). These experiments clearly demonstrate an ATP-mediated Ca2+ influx through agonist-gated channels and a consequent elevation of [Ca2+]i in these single vascular smooth muscle cells. The combination of the ability to hold these cells under voltage-clamp and to measure [Ca2+]i simultaneously has allowed us to exclude other possible explanations for the rise in [Ca2+]i under these conditions. Thus, although the major cation entering through the channels is Na+, ATP receptor activation will also generate subtle, localized increases in [Ca2+]. These increases might directly activate contractile proteins or, if insufficient to do this, might upregulate other Ca2(+)-dependent enzymes modulating the contractile process and provide an enhanced source of Ca2+ for uptake into internal Ca2+ stores. Further understanding of the physiological role of this conductance pathway may require the development of specific receptor antagonists or channel blockers.

An ATP, calcium and voltage sensitive potassium channel in porcine coronary artery smooth muscle cells

There is increasing interest in the roles played by potassium channels of smooth muscle in protecting against ischemic and anoxic insults. Hence, potassium-selective channels were studied in freshly dispersed porcine coronary artery smooth muscle cells using the inside-out variant of the patch-clamp technique. The most abundant potassium channel had a conductance of 148 pS in a 5.4/140 mM K+ gradient, at 0 mV, and was regulated by cytoplasmic ATP (0.05-3.0 mM), cytoplasmic Ca2+ (0.1-10 microM) and voltage. ATP and AMP-PNP (0.5 mM) reduced the probability of channel opening (Po) by 87 and 92%, respectively. This inhibition was partially reversed by the addition of 0.5 mM ADP. ADP on its own (2 mM) reduced Po by 46%. It appears, therefore, that this channel shares properties with both the ATP-sensitive and the calcium-regulated potassium channels, raising the possibility that it plays a central role in the regulation of coronary blood flow.

Participation of Ca2(+)-activated K+ channels in electrical activity of canine gastric smooth muscle

1. The hypothesis that Ca2(+)-activated K+ channels participate in the repolarization of electrical slow waves was tested in isolated cells and intact muscles of the canine gastric antrum. 2. Freshly dispersed cells from the gastric antrum liberally express large conductance channels that were characterized as Ca2(+)-activated K+ channels by several criteria. 3. Mean slope conductance of these channels in symmetrical 140 mM-KCl solutions was 265 +/- 25 pS and reversal potential was 1.3 +/- 3.3 mV. The reversal potential was shifted when K+ was partially replaced with Na+ in a manner consistent with the Nernst equation for the K+ gradient. 4. Open probability was studied in excised patches in solutions containing 10(-7)-10(-6) M-Ca2+ with holding potentials ranging from -100 to +100 mV. Resulting activation curves were fitted by Boltzmann functions. 5. Increasing [Ca2+] from 10(-7) to 10(-6) M shifted the half-maximal activation from +99 to 0 mV. These data suggest that Ca2(+)-activated K+ channels may be activated in the voltage range and [Ca2+]i occurring during the plateau phase of the slow wave. 6. In intact muscles loaded with the photolabile Ca2+ chelator, nitr-5, photo-activated release of Ca2+ during the slow wave cycle produced changes consistent with activation of Ca2(+)-dependent outward currents. 7. The data are consistent with the idea that Ca2+ build-up during electrical slow waves shifts the activation voltage of Ca2(+)-activated K+ channels into the range of the plateau potential. Activation of these channels yields outward current and repolarization. 8. Since the force of contractions depends on slow wave amplitude and duration, regulation of these channels may be important in controlling gastric motility.

A maxi calcium-activated potassium channel from chick lens epithelium

The apical membrane of embryonic chick lens epithelium contains at high density, a large conductance K+ channel whose open probability is increased by Ca++ at the inner surface of the membrane and by depolarization. The conductance of the channel when it is fully open in symmetrical 150 mM K+ solutions is 214 +/- 3 pS (mean +/- std. error). The current through the channel is a function of the K+ concentration. Gating (open probability) at positive transmembrane voltages increases as the internal [Ca++] is raised above 10(-7) M. The open probability decreases monotonically as the transmembrane voltage is made more negative. The channel is at least 87 times more permeable to K+ than to Na+ or Li+ and shows appreciable permeability to Rb+ and NH4+. It has at least three subconductance levels amounting to approximately 3/4, 1/2, and 1/4 the fully open unitary conductance. The occurrence of these subconductance levels is highly variable from one patch to another. The channel is blocked by physiological levels of internal Na+ but not over a physiological voltage range. This block is partially overcome by elevated external K+. This K+ channel from chick lens epithelium is blocked by a number of compounds known to block BK channels in other tissues. Here we show that decamethonium and Ba++ are effective blockers when added to the inner bathing solution at concentrations greater than .1 mM. Tetraethylammonium, Cs+, quinine, quinidine and Ba++ are all effective blockers when applied to the outer side of the channel in the .1 mM - 5 mM range. With the exception of internal Ba++, all of these compounds produce a fast flicker-type blockade. We use a one-site model to quantify the blockade caused by these flicker producing agents. The voltage dependence of the blockade by Cs+ suggests that this channel probably allows multiple occupancy.

Effects of phenylephrine in single isolated smooth muscle cells of rabbit and guinea pig taenia caeci

Spontaneous transient and evoked outward currents were studied using the whole cell patch-clamp technique with freshly dispersed smooth muscle cells isolated from rabbit and guinea pig taenia caeci. Phenylephrine induced a low amplitude sustained outward current in both tissues. Simultaneously, the frequency and amplitude of spontaneous transient outward currents were increased. However, there were differences between the effects of phenylephrine on the evoked outward currents recorded from smooth muscle cells of rabbit (reduction) and guinea pig (enhancement) taenia caeci.

Isolated guinea pig coronary smooth muscle cells. Acetylcholine induces hyperpolarization due to sarcoplasmic reticulum calcium release activating potassium channels

Smooth muscle cells, dispersed from the circumflex coronary artery of the guinea pig, were studied with the whole-cell configuration of the patch-clamp. The resting potential of about -40 mV was superimposed by spikelike hyperpolarizations (SLHs) up to -20 mV amplitude. The SLHs resulted from spontaneous transient outward currents (spontaneous TOCs) measured under voltage-clamp (-40 or -50 mV). Acetylcholine (ACh; 10 microM) increased SLHs and TOCs in amplitude and frequency. Atropine blocked the ACh effects. ACh-induced SLHs or TOCs were suppressed by bath application of tetraethylammonium (1 or 10 mM) or by cell dialysis with cesium, suggesting that they result from induction of potassium currents. In cell-attached patches, induction of currents through 130-pS potassium channels was recorded when ACh was bath-applied. An ACh-induced increase in intracellular [Ca2+] is suggested as a second messenger since SHLs and TOCs were suppressed by cell dialysis of 10 mM EGTA. ACh induced SHLs and TOCs in the absence of extracellular calcium. Intracellular application of 5 mg/ml heparin blocked ACh-induced TOCs. When the intracellular calcium stores were depleted by pretreatment with caffeine, the ACh effects were suppressed. Similarly, ACh pretreatment reduced the caffeine-induced outward currents. The results suggested that ACh augments calcium release from the sarcoplasmic reticulum, and the released calcium activates maxi potassium channels. In the single cell, calcium-activated potassium channels generate TOCs and SLHs that sum up to a hyperpolarization of the multicellular tissue.

Characteristics of transient outward currents in single smooth muscle cells from the ureter of the guinea-pig

1. Two kinds of transient outward currents were observed upon depolarization of single smooth muscle cells isolated from guinea-pig ureter. The major transient outward current was through Ca2(+)-activated K+ channels (IK(Ca) which had a large conductance (130 pS; 126 mM [K+]i/5.9 mM [K+]o). 2. The smaller transient outward current (ITO) was pharmacologically separated from other membrane currents in the presence of 1 mM-Cd2+ and 2 mM-tetraethylammonium(TEA+) and was selectively blocked by 3 mM-4-aminopyridine. It peaked (approximately 200 pA) within 10 ms upon depolarization from -80 to +20 mV and its half-inactivation time was approximately 50 ms at +20 mV. Half-maximum voltages (V 1/2) for activation and inactivation were about -8 and -50 mV, respectively, in the presence of 1 mM-Cd2+ and 2 mM-TEA+. The time course of recovery from inactivation of ITO was fitted with a single-exponential function (tau = 100 ms at -80 mV). A tenfold change of [K+]o resulted in a 53 mV change in the reversal potential of the tail of ITO. 3. Cadmium reduced peak ITO and shifted the voltage dependence of activation and inactivation in the positive direction in a concentration-dependent manner. The V 1/2 for inactivation in the absence of Cd2+ was estimated to be approximately -64 mV. 4. Single-channel outward currents which appeared only in the initial part of a depolarizing pulse from about -100 mV were recorded using the cell-attached patch clamp. The decay of the ensemble average of the current was similar to the macroscopic ITO under whole-cell clamp. When the holding potential was less negative, the opening probability of the channel greatly decreased. The channel conductance in normal extracellular medium was 14 pS. 5. In ureter cells ITO resembles A-type current. ITO does not contribute significantly to the repolarization of the action potential but it may regulate membrane excitability by opposing Ca2+ current activated around the threshold of the action potential.

GTP-dependent regulation of myometrial KCa channels incorporated into lipid bilayers

The regulation of calcium-activated K (KCa) channels by a G protein-mediated mechanism was studied. KCa channels were reconstituted in planar lipid bilayers by fusion of membrane vesicles from rat or pig myometrium. The regulatory process was studied by exploring the actions of GTP and GTP gamma S on single channel activity. KCa channels had a conductance of 260 +/- 6 pS (n = 25, +/- SE, 250/50 mM KCl gradient) and were voltage dependent. The open probability (Po) vs. voltage relationships were well fit by a Boltzmann distribution. The slope factor (11 mV) was insensitive to internal Ca2+. The half activation potential (V1/2) was shifted -70 mV by raising internal Ca2+ from pCa 6.2 to pCa 4. Addition of GTP or GTP gamma S activated channel activity only in the presence of Mg2+, a characteristic typical of G protein-mediated mechanisms. The Po increased from 0.18 +/- 0.08 to 0.49 +/- 0.07 (n = 7, 0 mV, pCa 6 to 6.8). The channel was also activated (Po increased from 0.03 to 0.37) in the presence of AMP-PNP, a nonphosphorylating ATP analogue, suggesting a direct G protein gating of KCa channels. Upon nucleotide activation, mean open time increased by a factor of 2.7 +/- 0.7 and mean closed time decreased by 0.2 +/- 0.07 of their initial values (n = 6). Norepinephrine (NE) or isoproterenol potentiated the GTP-mediated activation of KCa channels (Po increased from 0.17 +/- 0.06 to 0.35 +/- 0.07, n = 10). These results suggest that myometrium possesses beta-adrenergic receptors coupled to a GTP-dependent protein that can directly gate KCa channels. Furthermore, KCa channels, beta-adrenergic receptors, and G proteins can be reconstituted in lipid bilayers as a stable, functionally coupled, molecular complex.

Intracellular regulation of ion channels in cell membranes

Cells communicate with their environment through receptor proteins on the cell membrane. Some ion channels are receptors, whereas others are linked to receptors through guanine nucleotide-binding proteins (G proteins). Ion channels control intracellular concentrations of ions such as calcium, and these concentrations control cell functions such as secretion and cell division. This review summarizes the current state of knowledge about the control of ion channels.

Calcium channels, potassium channels, and voltage dependence of arterial smooth muscle tone

Resistance arteries exist in a maintained contracted state from which they can dilate or constrict depending on need. In many cases, these arteries constrict to membrane depolarization and dilate to membrane hyperpolarization and Ca-channel blockers. We discuss recent information on the regulation of arterial smooth muscle voltage-dependent Ca channels by membrane potential and vasoconstrictors and on the regulation of membrane potential and K channels by vasodilators. We show that voltage-dependent Ca channels in the steady state can be open and very sensitive to membrane potential changes in a range that occurs in resistance arteries with tone. Many synthetic and endogenous vasodilators act, at least in part, through membrane hyperpolarization caused by opening K channels. We discuss evidence that these vasodilators act on a common target, the ATP-sensitive K (KATP) channel that is inhibited by sulfonylurea drugs. We propose the following hypotheses that presently explain these findings: 1) arterial smooth muscle tone is regulated by membrane potential primarily through the voltage dependence of Ca channels; 2) many vasoconstrictors act, in part, by opening voltage-dependent Ca channels through membrane depolarization and activation by second messengers; and 3) many vasodilators work, in part, through membrane hyperpolarization caused by KATP channel activation.

Effects of tetraethylammonium and 4-aminopyridine on outward currents and excitability in canine tracheal smooth muscle cells

1. The effects of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on membrane currents and on single channel K currents in smooth muscle cells isolated from canine trachea were examined by use of tight seal whole cell- and patch-clamp techniques. 2. Depolarizing current applied through a recording pipette did not elicit an action potential under current clamp. A strong outward rectification was observed. 3. In most cells under voltage-clamp, only an outward current was observed upon depolarization from -60 mV when a pipette solution contained mainly KCl. The outward current consisted of three components; a large initial transient, a following sustained component and an additional component of irregular small transients on the sustained one. The two transient components were almost abolished when extracellular and pipette solutions contained 2.2 mM Cd2+ (0 mM Ca2+) and 10 mM EGTA, respectively. The sustained component was well maintained under these conditions. 4. TEA at low concentrations (less than 1 mM) effectively decreased the transient components and made the outward current smooth; it also suppressed the sustained component at higher concentrations. In outside-out patches, external 1 mM TEA reduced the single channel conductance of Ca-activated K channels by about 87% whereas 3 mM 4-AP did not. 4-AP at low concentrations (less than 3 mM) selectively reduced the sustained component of the outward current. 5. A Ca current recorded after the suppression of outward current by internal Cs+ had a peak of approximately 200 pA at +10 mV (holding potential: -60 mV). The half inactivation voltage in the steady-state was approximately -30 mV. 6. Simultaneous application of 1 mM TEA and 4-AP reduced the outward current and unmasked a Ca current. Under these conditions, an action potential with overshoot was easily elicited under current clamp. 7. It is concluded that the low excitability of canine tracheal smooth muscle cell upon depolarization is due to a large outward K current which consists of Ca-dependent and Ca-independent components. The peak amplitude of the Ca current is similar to that in highly excitable smooth muscle cells such as those of the ureter.

Effects of calmodulin antagonists on calcium-activated potassium channels in pregnant rat myometrium

1. The effects of W-7, trifluoperazine, and W-5 on Ca2(+)-activated K(+)-channels were investigated with the inside-out patch-clamp method in smooth muscle cells freshly dispersed from pregnant rat myometrium. These drugs are known to have different potencies as calmodulin antagonists. 2. In the presence of 1 microM Ca2+ on the cytoplasmic side ([Ca2+]i), the fraction of time the channel was open (open probability, Po) was about 0.9 and the calmodulin antagonists (1-30 microM) applied to the cytoplasmic face reduced Po to 0.65-0.55 dose-dependently. In the presence of 0.1-0.16 microM Ca2+, when Po was very low (0.02), calmodulin antagonists increased Po. All antagonists used produced almost identical effects at the same concentration. 3. The probability density function of the open time distribution could be described by the sum of two exponentials. W-7 decreased the time constant of slow component of distribution and at 30 microM the slow component disappeared both at 1 and 0.25 microM [Ca2+]i, reflecting the appearance of flickering channel activity. The probability density function of the closed time distribution could be fitted with three exponentials. The time constants of these components were not significantly altered by W-7. 4. Internally applied calmodulin (1-5 microM) did not produce any significant effect on channel activity. 5. The effects of calmodulin antagonists are considered to be due to a direct action of these compounds on the channel, and suggest that channel activation by Ca2+ is not mediated by calmodulin.

Effects of intracellular pH on calcium-activated potassium channels in rabbit tracheal smooth muscle

1. The effects of intracellular pH (pHi) on calcium-activated potassium channels (Ca2(+)-activated K+ channels) were studied in membrane patches of smooth muscle freshly dispersed from the rabbit trachea. Single-channel currents were recorded with an 'inside-out' patch clamp technique, mainly at 0 mV, with the external (electrode) medium containing 130 mM-K+ and the internal (bath) medium 6 mM-K+. 2. With an internal Ca2+ concentration ([Ca2+]i) of 1 microM, the fraction of time during which the channel was in an open state (the open probability, Po) was more than 0.8 at pHi 7.4. The channel activity nearly disappeared at pHi 7.0. The [Ca2+]i-Po relationship was shifted to higher [Ca2+]i by acidosis, the shift being approximately an 8-fold increase for a fall in pHi of 0.5 units. 3. The membrane potential and current intensity (V-I) relationship of single channels between +30 and -50 mV was shifted in a hyperpolarizing direction by intracellular acidosis. The shift was roughly 10 mV for 1 pH unit at 1 microM [Ca2+]i. At pHi 7.4 [Ca2+]i 1 microM, the V-Po relationship was shifted in a depolarizing direction by acidification. When [Ca2+]i was increased to 10 microM, V-Po relationship became less sensitive to V as well as pHi changes. 4. When Po was high, the probability density function of open and closed time distributions could be fitted by two exponentials. When Po was decreased to less than 0.3, either by reducing [Ca2+]i or by lowering pHi, another component having long closed times appeared. At similar Po values, the time constant of open time distribution was smaller with lower pHi. 5. It is concluded that the main effect of an increase in intracellular hydrogen ions is to decrease the open probability of the Ca2(+)-activated K+ channel, by reducing the sensitivity to Ca2+ and also shortening the open state.

Potassium, chloride and non-selective cation conductances opened by noradrenaline in rabbit ear artery cells

1. The action of noradrenaline on cells isolated from the rabbit ear artery was studied with the whole-cell configuration of the patch clamp technique. In normal potassium-containing solutions at a holding potential of -50 mV noradrenaline elicited either outward, inward or mixed outward and inward currents. These responses were blocked by the alpha-adrenoceptor antagonist, phentolamine (10(-6) M). 2. The outward current occurred as a consequence of an increase in membrane conductance and the reversal potential was close to the potassium equilibrium potential (EK). It was possible to record outward currents without external calcium but not when the concentration of EGTA in the pipette was increased to 10 mM or when potassium was absent from the pipette solution. It is concluded that the outward current evoked by alpha-adrenoceptor stimulation is produced by a calcium mediated increase in potassium conductance. 3. The ionic basis of the inward current was investigated in potassium-free external and pipette solutions. When sodium chloride was the major constituent of the external and pipette solutions the reversal potential (Er) of the noradrenaline-induced current was 0.63 mV, close to ENa and ECl. 4. When most of the external sodium chloride was replaced by sucrose Er was intermediate between ENa and ECl but had shifted significantly towards ENa. Further ion substitution experiments suggest that noradrenaline increased the membrane conductance to both anions and cations. 5. When the current was carried predominantly by anions, depolarizing steps (from -50 mV) produced outward current relaxations with a time constant of about 40 ms. Bath-applied caffeine also produced a membrane current which rectified in the outward direction. 6. When the response to noradrenaline was mediated mainly by cations, the relationship between the membrane current and clamp potential was non-linear and the amplitude of the currents at potentials positive to -50 mV became disproportionately smaller. In addition on repolarization to -50 mV the instantaneous current was followed by an inward relaxation. 7. In high external barium solution noradrenaline evoked a membrane current with a reversal potential much more positive than ENa or ECl. This current was recorded in the presence of 10(-5) M-nifedipine and diltiazem and in 10(-4) M-cadmium which suggests that the voltage-dependent calcium channel is not implicated in the generation of the non-selective cation current to noradrenaline.(ABSTRACT TRUNCATED AT 400 WORDS)

The calcium-dependent potassium conductance in rat sympathetic neurones

1. Adult and intact sympathetic neurones of isolated rat superior cervical ganglia were subjected to a two-electrode voltage-clamp analysis at 37 degrees C in order to investigate the Ca2(+)-dependent K+ conductance. 2. At each potential a Ca2(+)-dependent K+ current, IKCa, was determined as the difference between the current that could be attributed to the voltage-dependent K+ current, IKV, following Ca2+ channel blockade by Cd2+ and the total current generated. The final IKCa curves were obtained after correcting the experimental tracings for the underlying ICa current component. 3. IKCa became detectable during commands to -30 mV. About 3.6 x 10(5) Ca2+ ions are required to enter the cell before IKCa is initiated. The current was modelled on the basis of a 0.4-0.6 ms delay followed by an exponential activation of a fast component, IKCaf, simultaneously with a much slower exponential activation, IKCas. Experiments indicate a sigmoidal activation curve for the fast conductance, gKCf, with half-maximal activation at -13.0 mV and a slope factor of 4.7 mV (for 5 mM-Ca2+ in the bath). The associated time constant, tau kcf, ranged from 0.8 to 2.0 ms. The slow conductance exhibited a similar steady-state activation curve but an activation time constant in the 48-280 ms range. The maximum mean gKC was 0.32 microS per neurone for either the fast or slow component. 4. Excess K+ ions accumulate in the perineuronal space during K+ current flow giving rise to rapidly occurring, large K+ reversal potential (EK) modifications (up to -45 mV for the largest currents). The kinetics of K+ extracellular load can be described satisfactorily by a simple exponential function (tau = 0.9-2.8 ms). The characteristics of K+ wash-out appear similar to those of accumulation. 5. The immediate effect of such an extracellular K+ build-up is to make the apparent IKCa activation kinetics faster and to reduce (up to 50%) the true value of the K+ conductance. We simulated the predictions of a K+ diffusion model and generated new functions describing the IKCa steady-state activation, activation rate and maximum conductance values which satisfactorily reconstruct the IKCa current tracings together with the K+ accumulation process near the membrane. 6. A small component of the Ca2(+)-dependent K+ current, IAHP, was observed which survived at membrane potential levels negative to -40 mV. Increasing Ca2+ influx by applying longer pulses enhanced IAHP, which on the other hand was also activated by depolarizations of short duration.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of extracellular K+ and Ca2+ on membrane potential, contraction and 86Rb+ efflux in guinea-pig mesotubarium

The effects of varying extracellular concentrations of K+ and Ca2+ [K+]o and [Ca2+]o on force development and membrane potential were investigated in the guinea-pig mesotubarium. At [K+]o up to 40 mM, spontaneous action potentials were present, while higher [K+]o gave sustained contractures at a stable membrane potential (-24 to -12 mV for [K+]o from 60 to 120 mM). Tension decreased successively with increasing [K+]o from 30 to 120 mM. The relaxing potency of the dihydropyridine Ca2+ antagonist, felodipine, increased as the membrane was depolarized with increasing [K+]o and action potentials ceased. These results are compatible with the existence of Ca2+ channels showing voltage-dependent affinity with dihydrophyridines. Increasing [Ca2+]o from 2.5 to 10 mM caused membrane hyperpolarization by about 11 mV and was accompanied by a lower frequency of spontaneous contractions and a longer duration of the relaxation between contractions. 86Rb+ efflux measurements in 60 mM K+ in the absence and presence of felodipine revealed a Ca2(+)-dependent component of the voltage-activated efflux. In normal solution (5.9 mM K+), efflux in the presence of felodipine was similar to the minimal value during normal spontaneous activity. The results indicate regulation of the permeability of K+ channels by the intracellular Ca2+ concentration ([Ca2+]i) and suggest participation of such channels in the generation of the regularly occurring bursts of action potentials characteristic of spontaneous activity in the mesotubarium.

Action potentials and membrane currents of isolated single smooth muscle cells of cat and rabbit colon

Membrane potentials, action potentials and macroscopic currents in enzymatically dispersed, single smooth muscle cells of the circular layer of cat and rabbit colon were investigated. The cells did not exhibit spontaneous depolarizations and repolarizations (slow waves) or spontaneous action potentials. Single action potentials of smooth muscle cells were evoked by depolarizing current pulses of 5 ms to 3 s duration. A repetitive action potential discharge and an increase in the duration of the action potential was observed in cells during long depolarizing current pulses by superfusion with tetraethylammonium (TEA) or 4-aminopyridine (4-AP). Tetrodotoxin (TTX) did not alter the configuration of the action potential. Voltage-clamp experiments revealed two major outward macroscopic currents: a quasi-instantaneous (time-independent) and a time-dependent outward current. Both currents were identified as potassium (K) currents due to their pharmacological sensitivity to K antagonists [TEA, 4-AP and cesium (Cs)] and due to the reversal potential of outward tail currents. Barium selectively blocked the time-independent current. A time-dependent outward K current in colon cells was observed which appeared to be dependent upon entry of calcium ions (Ca2+) through voltage-dependent Ca-channels, since it was blocked by cadmium and low concentrations of nifedipine. The majority of cells did not exhibit transient outward currents. Inward currents were exposed in some of the cells when the K currents were blocked by external TEA and by replacement of K by Cs and TEA in the recording pipette. Inward currents were presumably carried by Ca2+, since they were not altered by TTX, were sensitive to external Ca concentrations and were abolished by the Ca channel antagonist, nifedipine. Carbachol augmented the amplitude of the inward Ca current.

Mutational and gene dosage analysis of calcium-activated potassium channels in Drosophila: correlation of micro- and macroscopic currents

In Drosophila, two Ca2(+)-activated K+ currents, ICF and ICS, have previously been distinguished in conventional voltage clamp experiments. The slowpoke (slo) mutation eliminates ICF specifically. We report that in patch clamp recordings a single-channel Ca2(+)-activated K+ current is readily distinguished from other channel activities in normal larval muscle membrane, whereas no such current is observed in slo muscles. This single-channel current thus correlates with the macroscopic ICF. No obvious differences in amplitude or properties were detected between normal (+/+) and heterozygous (slo/+) ICF channels in whole-cell voltage clamp recordings or single-channel patch clamp recordings. These results are consistent with the hypothesis that slo is a structural gene for the ICF channels only under certain conditions. The selective effect of the slo mutation may reflect a defect in a regulatory mechanism that is specific for the functioning of the ICF channel protein.

The activation of calcium and calcium-activated potassium channels in mammalian colonic smooth muscle by substance P

1. The regulation of Ca2(+)-activated K+ channels by the agonist substance P in freshly dissociated smooth muscle cells from the rabbit longitudinal colonic muscle was characterized using the patch clamp technique. 2. In the cell-attached recording mode, when pipette and bath solutions contained equal [K+] (126 mM), the Ca2(+)-activated K+ channels showed a linear current-voltage relationship (between -50 mV and 50 mV) with a slope conductance of 210 +/- 35 pS (n = 12). Reversal potential measurements indicated that the channel was highly selective for K+ over Na+ (PK/PNa = 110). 3. Channels were activated by depolarizing membrane voltages and cytosolic Ca2+, and in inside-out patches channel activation depended sigmoidally on voltage and [Ca2+]. The potential for half-activation at a cytosolic [Ca2+] of 5 x 10(-6) M was 0 mV. A tenfold increase in cytosolic Ca2+ resulted in a 60 mV shift of the sigmoidal voltage activation curve to more negative potentials. 4. Threshold concentrations of substance P (10(-12) M), which did not result in cell contraction, caused a prolonged activation of K+ channels. The K+ channels were observed to open in clusters: simultaneous opening of multiple channels was interrupted by complete, prolonged channel closure. 5. Lowering bath [Ca2+] to submicromolar concentrations abolished the effect of substance P. The activation of K+ channels by substance P (10(-12) M) was also inhibited by the dihydropyridine nifedipine (10(-6) M), a blocker of L-type Ca2+ channels. 6. In the whole-cell recording mode, with the pipette solution containing 126 mM-KCl, 0.77 mM-EGTA and 1 mM-ATP, depolarization from a holding potential of -70 mV elicited outward currents which increased to steady-state values. These were K+ currents as they were blocked by TEA (tetraethylammonium, 30 mM) and Ba2+ (1 mM) and were abolished when pipette K+ was replaced by Cs+. 7. The depolarization-activated outward current was not affected by lowering extracellular [Ca2+] or by the Ca2+ channel antagonists Cd2+ (200 microM), nifedipine (10(-6)-10(-5) M) or verapamil (10(-6) M). The current was greatly reduced when the EGTA concentration in the pipette solution was increased from 0.77 to 10 mM. 8. When the pipette solution contained CsCl, membrane depolarization activated inward currents. The peak inward current was identified as current through L-type Ca2+ channels based on its voltage- and time-dependent kinetics, and its modulation by dihydropyridines.(ABSTRACT TRUNCATED AT 400 WORDS)

Outward potassium currents in freshly isolated smooth muscle cell of dog coronary arteries

Outward membrane currents were characterized in single coronary smooth muscle cells of adult beagle dogs. The cells averaged 96.4 x 7.1 microns and had a resting potential of -50.7 mV, an input resistance of 307.9 M omega, a capacitance of 32.3 pF, and a calculated membrane surface area of 4,037 microns2. The cells contracted in response to external application of acetylcholine or high K+. In voltage clamp by use of the suction pipette method, outward current began to appear at -50 mV and reached 15.2 nA at 50 mV with a current density of 376.5 microA/cm2. The current was reduced by external tetraethylammonium, Ba2+, and internal Cs+, and its reversal potential had a Nernst relation to external K+ concentration. Elevation of external Ca2+ (Ca2+o) from 0 to 0.3 mM increased total K+ current by up to 300%; elevation of internal Ca2+ (Ca2+i) to 5 x 10(-7) M by internal perfusion increased total outward current to a similar extent, suggesting a large difference in Ca2+ transmembrane sensitivity. Total whole-cell K+ current consisted of two components: an initial time-independent current (Ii) followed by a time-dependent current (It). Ii and It were through separate K+ channels based on differences in a) sensitivity to Ca2+09b) modulation by an inward Ca2+ current, c) current amplitudes and activation kinetics, and d) responses to pharmacological agents. It was the largest component, measuring 4.5 nA in 0 mM Ca2+o but increasing to 11.9 nA in 0.3 mM Ca2+o with a steep 2.5 power function. It activated with a biexponential time course; in Ca2+o-free solution, its time course was relatively insensitive to voltage changes but became voltage sensitive in the presence of Ca2+o. Further, such sensitivity was abolished or enhanced by Co2+ or Bay K 8644, respectively. We concluded that there are two types of Ca2+-sensitive K+ currents, Ii and It, in coronary smooth muscle cells. Via an inward Ca2+ channel Ca2+o strongly modulates It, both in amplitude and kinetics.

ATP-activated channels gate calcium entry in single smooth muscle cells dissociated from rabbit ear artery

1. A combination of the techniques of microspectrofluorimetry and whole-cell patch clamp was used to investigate changes in cytoplasmic Ca2+ concentration (Cai2+) in single arterial smooth muscle cells on external application of ATP. 2. ATP applied to cells held under voltage clamp at --60 mV evoked an inward current and an associated rise in Cai2+. In the absence of extracellular Ca2+. ATP-activated inward currents were observed but there was no rise in Cai2+. 3. Pre-treatment of cells with noradrenaline or caffeine did not prevent the rise in Cai2+ on subsequent application of ATP. 4. The ATP-activated rise in Cai2+ was voltage dependent as outward currents evoked by ATP at positive membrane potentials were not associated with a change in Cai2+. 5. At --60 mV, the rise in Cai2+ due to ATP application was dependent on the magnitude of the ATP current response, such that Cai2+ increased by about 0.5 nM/pC charge transferred through ATP-gated channels. 6. The results suggest that ATP-gated channels in these cells admit sufficient Ca2+ in a physiological Ca2+ gradient to significantly elevate Cai2+. About 10% of the ATP-gated current may be carried by Ca2+ ions. Thus the ATP-activated channels have a dual excitatory function: depolarization due to Na+ entry promotes action potential discharge and voltage-gated Ca2+ entry, and also direct entry of Ca2+ through the ATP-activated channels.