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

Effect of a novel BKCa opener on BKCa currents and contractility of the rabbit corpus cavernosum

Large-conductance Ca2+-activated K+ (BKCa) channels are thought to play a key role in the regulation of corpus cavernosum smooth muscle (CCSM) excitability. Few BKCa channel openers have been accepted for clinical development. The effect of the novel BKCa channel opener GoSlo-SR5-130 on electrical activity in isolated rabbit CCSM cells and mechanical activity in strips of rabbit CCSM was examined. Single-channel currents were observed in inside-out patches. These channels were sensitive to Ca2+, blocked by penitrem A, and had a conductance of 291 ± 20 pS ( n = 7). In the presence of GoSlo-SR5-130, the number of open BKCa channels increased. Using voltage-ramp protocols, GoSlo-SR5-130 caused currents to activate at more negative potentials in a concentration-dependent manner, shifting the half-maximal activation voltage potential to the left on the voltage axis. Therefore, BKCa channels were open within the physiological range of membrane potentials in the presence of GoSlo-SR5-130. GoSlo-SR5-130 also resulted in an increase in the activity of spontaneous transient outward currents in myocytes isolated from CCSM, and this effect was reversed by iberiotoxin. In current-clamp mode, GoSlo-SR5-130 hyperpolarized the cell membrane. Isometric tension recording of strips of rabbit corpus cavernosum showed that GoSlo-SR5-130 inhibited spontaneous contractions in a concentration-dependent manner. This effect was reversed in the presence of iberiotoxin, suggesting that GoSlo-SR5-130 exerts its effect through BKCa channels. These findings suggest that GoSlo-SR5-130 is an effective tool for the study of BKCa channels and that these channels can modulate CCSM activity and are possible targets for the treatment of erectile dysfunction.

Gastric emptying and Ca(2+) and K(+) channels of circular smooth muscle cells in diet-induced obese prone and resistant rats

OBJECTIVE

Accelerated gastric emptying that precipitates hunger and frequent eating could be a potential factor in the development of obesity. The aim of this study was to study gastric emptying in diet-induced obese-prone (DIO-P) and DIO-resistant (DIO-R) rats and explore possible differences in electrical properties of calcium (Ca(2+) ) and potassium (K(+) ) channels of antral circular smooth muscle cells (SMCs).

DESIGN AND METHODS

Whole-cell patch-clamp technique was used to measure Ca(2+) and K(+) currents in single SMCs. Gastric emptying was evaluated 90 min after the ingestion of a solid meal.

RESULTS

Solid gastric emptying in the DIO-P rats was significantly faster compared with that in the DIO-R rats. The peak amplitude of L-type Ca(2+) current (IBa,L ) at 10 mV in DIO-P rats was greater than that in DIO-R rats without alternation of the current-voltage curve and voltage-dependent activation and inactivation. The half-maximal inactivation voltage of transient outward K(+) current (IKto ) was more depolarized (∼4 mV) in DIO-P rats compared with that in DIO-R rats. No difference was found in the current density or recovery kinetics of IKto between two groups. The current density of delayed rectifier K(+) current (IKdr ), which was sensitive to tetraethylammonium chloride but not 4-aminopyridine, was lower in DIO-P rats than that in DIO-R rats.

CONCLUSION

The accelerated gastric emptying in DIO-P rats might be attributed to a higher density of IBa,L , depolarizing shift of inactivation curve of IKto and lower density of IKdr observed in the antral SMCs of DIO-P rats.

Cellular effects of gastric electrical stimulation on antral smooth muscle cells in rats

The cellular effects of gastric electrical stimulation (GES), which has recently been introduced as a potential therapy for the treatment of gastroparesis and obesity, were investigated in rat antrum smooth muscle cells (SMCs). Effects on cell membrane potentials of single electrical current pulses (pulse width from 0.1 ms to 200 ms) and 2-s pulse train stimuli with different pulse widths (0.1–4 ms), different frequencies (20–200 Hz), and different intensities were studied: 1) the stimulus amplitude had an exponential relationship to the pulse width from 2 ms to 200 ms, along with a rapidly rising strength-duration curve at pulse widths less than 5 ms, and a relatively flat curve at pulse widths greater than 50 ms; 2) when the pulse frequency was at 80 Hz or above, pulse train electrical stimulation, with a pulse width of 2 ms or above but not ≤1 ms, was able to depolarize cell membrane potentials to above −30 mV and/or generate action potentials. Electrical stimulation with a single long pulse and a width of 50 ms or greater is effective in depolarizing cell membrane potentials of SMCs with low amplitude. Pulse train electrical stimulation with a pulse width of ≤1 ms fails to generate action potentials in SMCs, whereas pulse train electrical stimulation with a pulse width of 2–4 ms and a sufficiently high pulse frequency is able to generate action potentials. These cellular findings may be useful in optimizing stimulation parameters of GES.

K+ channels and their effects on membrane potential in rat bronchial smooth muscle cells

In order to investigate the K+ channels and their effects on resting membrane potential (Em) and excitability in rat bronchial smooth muscle cells (BSMCs), the components of outward K+ channel currents and the effects of K+ channels on Em and tension in rat bronchial smooth muscle were observed by using standard whole-cell recording of patch clamp and isometric tension recording techniques. The results showed that under resting conditions, total outward K+ channel currents in freshly isolated BSMCs were unaffected by ATP-sensitive K+ channel blocker. There were two types of K+ currents: voltage-dependent delayed rectifier K+ channel (Kv) and large conductance calcium-activated K+ channel (BKCa) currents. 1 mmol/L 4-aminopyridine (4-AP, an inhibitor of Kv) caused a significant depolarization (from -8.7 +/- 5.9 mV to -25.4 +/- 3.1 mV, n = 18, P < 0.001). In contrast, 1 mmol/L tetraethylammonium (TEA, an inhibitor of BKca) had no significant effect on Em (from -37.6 +/- 4.8 mV to -36.8 +/- 4.1 mV, n = 12, P > 0.05). 4-AP caused a concentration-dependent contraction in resting bronchial strips. TEA had no effect on resting tension, but application of 5 mmol/L TEA resulted in a left shift with bigger pD2 (the negative logarithm of the drug concentration causing 50% of maximal effect) (from 6.27 +/- 0.38 to 6.89 +/- 0.54, n = 10, P < 0.05) in the concentration-effect curve of endothine-1, and a right shift with smaller pD2 (from 8.10 +/- 0.23 to 7.69 +/- 0.08, n = 10, P < 0.05) in the concentration-effect curve of isoprenaline. It was suggested that in rat BSMCs there may be two types of K+ channels, Kv and BKca, which serve distinct roles. Kv participates in the control of resting Em and tension. BKca is involved in the regulation of relaxation or contraction associated with excitation.

Heterogeneous expression of transient outward currents in smooth muscle cells of the mouse small intestine

The objective for this paper was to characterize the transient outward current (I(to)) present in smooth muscle cells of the intestinal external muscularis. Two populations of cells were identified, one with a fast rate of I(to) inactivation (tau < 100 ms) and another with a slow rate of I(to) inactivating (tau > 150 ms). The chord conductance for the fast I(to) was smaller than the chord conductance of the slow I(to) (0.5 +/- 0.1 vs. 1.3 +/- 0.1 nS pF(-1), respectively). The inactivation was fitted by mono-exponentials to give a tau for the fast and slow I(to) of 44 and 229 ms, respectively. Combined plots of voltage dependent activation and inactivation processes revealed voltage ranges where window currents were possible; a 20 mV range for the fast I(to) from -56 to -36 mV and a 47 mV range for the slow I(to) from -42 to +5 mV. The fast I(to) recovered more quickly from inactivation than the slow I(to); tau(fast I(to)) = 11 +/- 4 ms compared to tau(slow I(to)) = 42 +/- 16 ms. The effect of different rates of depolarization on I(to) activation was examined. The plots of peak currents evoked by different rates of depolarization were well fitted by inverse exponential functions. The fast I(to) had a larger response to fast rates of depolarization by having a tau of 2 +/- 1 mV ms(-1) with maximal activation (95 % complete) at 5 mV ms(-1). The slow I(to) had a tau of 14 +/- 9 mV ms(-1) with maximal activation (95 % complete) at 42 mV ms(-1). The properties of these currents suggest that the two transient outward currents may contribute differently to slow waves and action potentials generated by the smooth muscle cells.

Potassium channels in gastrointestinal smooth muscle

1. Electromechanical coupling in smooth muscle serves to coordinate the contractile activity of the syncytium. Electrical activity of smooth muscle of the gut is generated by ionic conductances that regulate and in turn are regulated by the membrane potential of smooth muscle cells. This activity determines the extent of Ca2+ entry into smooth muscle cells, and thus, the timing and intensity of contractions. 2. Potassium channels play an important role in regulating the excitability of the syncytium. The different types of K+ channel are characterized by different sensitivities to membrane potential, to intracellular Ca2+ levels and to modulation by agonists. 3. This review highlights the different types of K+ channels found in gut smooth muscle and describes their possible roles in regulating the electrical activity of the muscle.

Characterization of outward K(+) currents in isolated smooth muscle cells from sheep urethra

The perforated-patch technique was used to measure membrane currents in smooth muscle cells from sheep urethra. Depolarizing pulses evoked large transient outward currents and several components of sustained current. The transient current and a component of sustained current were blocked by iberiotoxin, penitrem A, and nifedipine but were unaffected by apamin or 4-aminopyridine, suggesting that they were mediated by large-conductance Ca(2+)-activated K(+) (BK) channels. When the BK current was blocked by exposure to penitrem A (100 nM) and Ca(2+)-free bath solution, there remained a voltage-sensitive K(+) current that was moderately sensitive to blockade with tetraethylammonium (TEA; half-maximal effective dose = 3.0 +/- 0.8 mM) but not 4-aminopyridine. Penitrem A (100 nM) increased the spike amplitude and plateau potential in slow waves evoked in single cells, whereas addition of TEA (10 mM) further increased the plateau potential and duration. In conclusion, both Ca(2+)-activated and voltage-dependent K(+) currents were found in urethral myocytes. Both of these currents are capable of contributing to the slow wave in these cells, suggesting that they are likely to influence urethral tone under certain conditions.

Hyperpolarisation-activated inward current in isolated sheep mesenteric lymphatic smooth muscle

1. Freshly isolated sheep lymphatic smooth muscle cells were studied using the perforated patch-clamp technique. Hyperpolarisation with constant-current pulses caused a time-dependent rectification evident as a depolarising 'sag' followed by an anode-break overshoot at the end of the pulse. Both sag and overshoot were blocked with 1 mM Cs+. 2. Cells were voltage clamped at -30 mV and stepped to -120 mV in 10 mV steps of 2 s duration. Steps negative to -60 mV evoked a slowly activating, non-inactivating inward current which increased in size and rate of activation with increasing hyperpolarisation. 3. The slowly activating current was reduced in Na+-free bathing solution but enhanced when the extracellular K+ concentration was increased to 60 mM. The current was significantly reduced by 1 mM Cs+ and 1 microM ZD7288 but not by 1.8 mM Ba2+. 4. The steady-state activation curve of the underlying conductance showed a threshold at -50 mV and half-maximal activation at -81 mV. Neither threshold nor half-maximal activation was significantly affected by increasing the external K+ concentration to 60 mM. 5. The frequency of spontaneous contractions and fluid propulsion in isolated cannulated segments of sheep mesenteric lymphatics were decreased by 1 mM Cs+ and by 1 microM ZD7288. 6. We conclude that sheep lymphatics have a hyperpolarisation-activated inward current similar to the If seen in sinoatrial node cells of the heart. Blockade of this current slows spontaneous pumping in intact lymphatic vessels suggesting that it is important in normal pacemaking.

Voltage-dependent large conductance channels in visceral smooth muscle

The ionic conductances that underlie the resting membrane potential of visceral smooth muscle are not fully understood. Using the patch-clamp technique in the whole-cell configuration, single large conductance channels (LCCs) with unitary conductances of up to 400 pS were recorded in isolated smooth muscle cells of the opossum esophagus. These channels were active at physiological potentials (-100 to -40 mV) and opened with increasing frequency as the membrane potential was hyperpolarized. This voltage dependence gave rise to an inwardly rectifying macroscopic current which was half-maximally activated at -65 mV. The current through LCCs was carried by cations because reduction of external [NaCl] shifted the reversal potential of the LCC current towards the predicted Nernst potential for a nonselective cation current. These results suggest that LCCs may contribute to resting membrane potential in the circular muscle of the opossum esophagus.

Delayed rectifier and Ca(2+)-dependent K(+) currents in human esophagus: roles in regulating muscle contraction

We have examined K(+) channels and their function in human esophageal smooth muscle using perforated patch recording, RT-PCR to identify channel mRNA, and muscle contraction to study the effects of channel blockers. Depolarization revealed at least two types of currents: a 4-aminopyridine (4-AP)-sensitive transient delayed rectifier K(+) (K(V)) and a Ca(2+)-dependent K(+) (K(Ca)) current. K(Ca) current was active at positive potentials and was blocked by tetraethylammonium (TEA), iberiotoxin, and charybdotoxin but was insensitive to 4-AP. The mRNA encoding the gene products of Kv1.2 and Kv1.5 was identified in muscle and dissociated cells, consistent with these channel types contributing to K(V) current. 4-AP increased resting tension of muscle strips, suggesting a role for K(V) in setting the membrane potential. TEA, but not 4-AP, augmented the amplitude and duration of electrically evoked contraction, effects that were abolished by nifedipine. Here we provide the first description of macroscopic K(+) currents in human esophagus. K(V) channels participate in regulation of resting tension, whereas the K(Ca) channel limits depolarization and contraction during excitation.

Effect of nitric oxide on calcium-activated potassium channels in colonic smooth muscle of rabbits

Nitric oxide (NO) hyperpolarizes intestinal smooth muscle cells. This study was designed to determine the mechanism whereby NO activates KCa channels of circular smooth muscle of the rabbit colon. Transmural biopsies of the rabbit colon were stained for NADPH-diaphorase. Freshly dispersed circular smooth muscle cells were studied in the whole cell configuration, as well as in on-cell and excised inside-out patch recording configurations, while KCa current and the activity of KCa channels, respectively, were monitored. NADPH-diaphorase-positive nerve fibers were found in both muscle layers. NO (1%) increased whole cell net outward current by 79% and hyperpolarized resting membrane voltage from -59 to -73 mV (n = 8 cells, P < 0.01). In the on-cell patch recording configuration. NO (0.5% or 1%) in the bath increased NPo of KCa channels; charybdotoxin (125 nM) in the pipette solution blocked this effect. In the excised inside-out patch recording configuration, NO (1%) had no effect on NPo of KCa channels. In the on-cell patch recording configuration, methylene blue (1 microM) or cystamine (5 mM) in the bath solution decreased the effect of NO (1%) on NPo of KCa channels. NPo was increased by 8-bromo-cGMP (8-BrcGMP; 1 mM), a cGMP analog, and zaprinast (100 microM), an inhibitor of cGMP phosphodiesterase. These data suggest that NO increased whole cell outward K+ current by activating KCa channels through a cGMP pathway.

Effect of motilin and erythromycin on calcium-activated potassium channels in rabbit colonic myocytes

BACKGROUND & AIMS

Motilin and erythromycin are prokinetic agents that act on the same receptor in gastrointestinal smooth muscle to cause contraction. Both agonists may also cause an increase in outward current. The aim of this study was to determine whether motilin and erythromycin activate calcium-activated potassium (KCa) channels.

METHODS

Freshly dispersed longitudinal smooth muscle cells of the rabbit colon were used to measure whole-cell outward current and single-channel activity using patch clamp recording methods.

RESULTS

Erythromycin and motilin increased a calcium-dependent outward potassium current and increased the open probability of KCa channels of cell- attached patches.

CONCLUSIONS

Erythromycin and motilin activate KCa channels via an intracellular second messenger system. This effect may modulate the increase in contractility caused by these agonists.

Outward currents in smooth muscle cells isolated from sheep mesenteric lymphatics

1. The patch-clamp technique was used to measure membrane currents in isolated smooth muscle cells dispersed from sheep mesenteric lymphatics. Depolarizing steps positive to -30 mV evoked rapid inward currents followed by noisy outward currents. 2. Nifedipine (1 microM) markedly reduced the outward current, while Bay K 8644 (1 microM) enhanced it. Up to 90% of the outward current was also blocked by iberiotoxin (Kd = 36 nM). 3. Large conductance (304 +/- 15 pS, 7 cells), Ca(2+)- and voltage-sensitive channels were observed during single-channel recordings on inside-out patches using symmetrical 140 mM K+ solutions (at 37 degrees C). The voltage required for half-maximal activation of the channels (V1/2) shifted in the hyperpolarizing direction by 146 mV per 10-fold increase in [Ca2+]i. 4. In whole-cell experiments a voltage-dependent outward current remained when the Ca(2+)-activated current was blocked with penitrem A (100 nM). This current activated at potentials positive to -20 mV and demonstrated the phenomenon of voltage-dependent inactivation (V1/2 = -41 +/- 2 mV, slope factor = 18 +/- 2 mV, 5 cells). 6. Tetraethylammonium (TEA; 30 mM) reduced the voltage-dependent current by 75% (Kd = 3.3 mM, 5 cells) while a maximal concentration of 4-aminopyridine (4-AP; 10 mM) blocked only 40% of the current. TEA alone had as much effect as TEA and 4-AP together, suggesting that there are at least two components to the voltage-sensitive K+ current. 7. These results suggest that lymphatic smooth muscle cells generate a Ca(2+)-activated current, largely mediated by large conductance Ca(2+)-activated K+ channels, and several components of voltage-dependent outward current which resemble 'delayed rectifier' currents in other smooth muscle preparations.

Ouabain-induced excitation of colonic smooth muscle due to block of K+ conductance by intracellular Na+ ions

The mechanism by which ouabain causes excitation of canine colonic circular smooth muscle was investigated. Ouabain-induced depolarization and increase in contractility were related to the concentration of extracellular sodium and prevented by complete substitution of sodium ions with N-methyl-D-glucamine or lithium ions. Absence of external sodium ions did not prevent the depolarization and increase in contractility induced by tetraethylammonium. Exposure of the muscle strips to sodium-free solutions produced a transient hyperpolarization and decrease in the input membrane resistance consistent with the hypothesis that intracellular sodium blocks potassium conductance. The relationship between the membrane potential and the extracellular potassium concentration indicated that the resting membrane potential is mainly determined by the membrane potassium conductance. Our data suggest the following mechanism of action for ouabain: (a) ouabain blocks Na+/K+ pump thereby increasing the intracellular sodium concentration; (b) increase in intracellular sodium inhibits membrane potassium conductance, which depolarizes the membrane and prolongs the slow wave plateau, resulting in an increase of the force of contraction. The direct contribution of the sodium pump to the resting membrane potential, if any, can only be minor (< 6 mV).