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

Investigation of L-type Ca(2+) current in the aganglionic bowel segment in Hirschsprung's disease

BACKGROUND

Studies on animal models of Hirschsprung's disease (HD) suggest that L-type Ca(2+) channels are down-regulated in the aganglionic bowel segment, however, this has yet to be confirmed in HD patients. The objective of this study was to test the hypothesis that L-type Ca(2+) current density is decreased in smooth muscle cells (SMC) obtained from the aganglionic bowel segment of patients with HD in comparison with those from the ganglionic segment.

METHODS

Smooth muscle cells were freshly isolated from colon samples obtained from HD patients undergoing pull-through surgery. L-type Ca(2+) currents were recorded using the perforated patch configuration of the whole cell voltage clamp technique and the expression levels of CACNA1C transcripts (which encode L-type Ca(2+) channels) in the ganglionic and aganglionic bowel segments were compared using real-time quantitative PCR.

KEY RESULTS

All SMC displayed robust currents that had activation/inactivation kinetics typical of L-type Ca(2+) current, were inhibited by nifedipine and enhanced by the L-type Ca(2+) channel agonists FPL 64176 and Bay K 8644. Moreover, FPL 64176 activated currents were also inhibited by nifedipine. However, there was no significant difference in L-type Ca(2+) current density, CACNA1C subunit expression or sensitivity to the pharmacological agents noted above, between SMC isolated from the ganglionic and aganglionic regions of the HD colon.

CONCLUSIONS & INFERENCES

In contrast to studies on genetic animal models of HD, L-type Ca(2+) currents are not down-regulated in the aganglionic bowel segment of HD patients and are therefore unlikely to account for the impaired colonic peristalsis observed in these patients.

Voltage-gated delayed rectifier K v 1-subunits may serve as distinctive markers for enteroglial cells with different phenotypes in the murine ileum

Due to entangled results concerning K(v)1 subunit distribution in the gastrointestinal wall, we aimed to unravel the expression of the delayed rectifier potassium subunits K(v)1.1 and K(v)1.2 in the murine ileum. Presence and distribution of both subunits were determined in cryosections and whole-mount preparations of the ileum of three different murine strains by indirect immunofluorescence, and analysed by conventional fluorescence and confocal microscopy. Distribution of both subunits was similar in the ileum of the three strains. K(v)1.1 immunoreactivity (IR) was found in some S100-expressing enteroglial cells (EGC) located at the periphery of myenteric ganglia, in S100-positive EGC along interganglionic, intramuscular and vascular nerve fibres, and in S100-positive EGC of the submucous plexus. K(v)1.1 IR was also observed in some GFAP-expressing EGC at the periphery of myenteric ganglia, and in GFAP-positive EGC of submucous ganglia. K(v)1.2 IR was detected in some intramuscular S100-positive EGC, in almost all submucous S100-expressing EGC, and in a few GFAP-expressing EGC. K(v)1.2 IR was also expressed in a majority of enteric neurons. Coding of these neurons showed that all cholinergic and most nitrergic neurons express K(v)1.2. In conclusion, the results showed that K(v)1.1 and K(v)1.2 were predominantly expressed in distinct EGC phenotypes. K(v)1.2 was also observed in distinct neuron subpopulations. Our results support the active role of EGC with distinct phenotypes in intestinal functions, which is relevant in view of their modulating role on intestinal barrier and inflammatory responses.

Simultaneous monitoring of cellular depolarization and contraction: a new method to investigate excitability and contractility in isolated smooth muscle cells

The present experiments were performed to establish a method for simultaneous monitoring of excitation and contraction in isolated smooth muscle cells. The smooth muscle cells were dissociated from the colons of Wistar rats by enzymatic digestion. All the experiments were performed on mixtures of circular and longitudinal cells. In a first set of experiments, focal extracellular potentials (FEPs) and transmembrane action potentials (APs) were simultaneously recorded from the cells by use of extracellular and intracellular pipettes, respectively. In a second set of experiments, cellular contraction induced by chemical stimulation was monitored simultaneous with the FEP recordings. The FEPs had spike and plateau amplitudes of 44.5 +/- 2.3 and 8.9 +/- 0.7 mV, respectively, and reproduced the general morphology of gastrointestinal APs. The parallel mechanical measurements from the rat colonic cells showed that they shortened with an average peak contraction of 8.8 +/- 1.4 microm and an average contraction velocity of 8.2 +/- 0.9 microm/s, to develop an average peak force of 1.2 +/- 0.2 microN, and generated an average peak power of 36 +/- 15 pW. Simultaneous monitoring of FEPs and cellular contraction demonstrates correlations between the electrical and mechanical events taking place in the investigated cells.

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.

Effect of treatment time on calcium antagonism by cadmium ions in a guinea-pig taenia coli

1. When pretreated for 1 min, with Cd2+ at low concentrations (0.001-0.01 mM), there was a parallel rightward shift of Ca2+ concentration-curves in guinea-pig taenia coli in K+-depolarized Ca2+-free medium. However, when pretreated for 30 min, Cd2+ reduced the maximal Ca2+ response size. 2. The application of 0.01 mM Cd2+ for 1, 5 and 30 min in Ca2+-free, K+-medium reduced to the same degree the Ca uptake after addition of 3 mM Ca2+. The inhibitory action on the tension by Cd2+ however, became greater as the pretreatment time with Cd2+ increased. 3. Within 5 min of Cd2+ (0.01 mM) treatment, Cd2+ chiefly bound to the cell membrane, however, with a longer duration (30 min), Cd2+ entered the cytoplasm where EDTA could not reach. 4. Cd2+ above 0.0005 mM reduced dose-dependently the respiration of isolated mitochondria. 5. These results suggest that with short duration exposure (1-5 min) of taenia coli cells to Cd2+, the interference with Ca2+ entry through voltage-dependent Ca2+ channels is predominant but for longer exposure times, intracellular actions of Cd2+ contribute to its inhibitory effects.

Postjunctional alpha 1- and beta-adrenoceptor effects of noradrenaline on electrical slow waves and phasic contractions of cat colon circular muscle

1. The postjunctional excitatory and inhibitory effects of noradrenaline and selective alpha 1- and beta-adrenoceptor agonists on electrical and mechanical activity of cat colon muscle strips were studied by microelectrode recordings and isometric force measurements. Experiments were performed in the presence of tetrodotoxin (0.5 microM) or atropine (0.5 microM). 2. Circular muscle cells near the submucosal border had a mean resting membrane potential of -76.1 +/- 1.2 mV and exhibited electrical slow waves at frequencies of 4-6 cycles min-1. The mean values of electrical slow wave components were: upstroke potential, -40.7 +/- 1.2 mV; plateau potential, -43.7 +/- 0.8 mV; and duration, 4.9 +/- 0.4 s. Electrical slow waves were in phase with rhythmic contractions of the circular muscle layer. Muscle cells near the myenteric border had a mean testing membrane potential of -51.1 +/- 5.5 mV and did not exhibit electrical slow waves. 3. Noradrenaline (1 microM) increased the duration of electrical slow waves. This effect was inhibited by prazosin (1 microM) and potentiated by propranolol (5 microM), indicating activation of alpha 1- and beta-adrenoceptors. Also, when alpha 1-adrenoceptors were irreversibly blocked by phenoxybenzamine (1 microM), noradrenaline decreased the duration of electrical slow waves. Phenylephrine (1 microM), a selective alpha 1-adrenoceptor agonist, and isoprenaline (1 microM), a beta-adrenoceptor agonist, increased or decreased the duration of electrical slow waves, respectively. 4. Phenylephrine (0.01-5 microM) caused a linear increase in the area of electrical slow waves and phasic contractions but did not affect resting membrane potential or resting muscle tension. Higher concentrations of phenylephrine (5-50 microM) depolarized the resting membrane potential (2-6 mV) and increased muscle tone. 5. Nitrendipine or verapamil (each at 5 microM) reduced the amplitude of the upstroke potential and nearly abolished the plateau phase of the electrical slow waves. In the presence of L-type Ca2+ antagonists, noradrenaline (1-10 microM) or phenylephrine (1-100 microM) had no effect on electrical slow waves and phasic contractions. This indicates that the effects of noradrenaline and phenylephrine involve the influx of extracellular Ca2+ through voltage-dependent L-type Ca2+ channels. 6. Ryanodine, an alkaloid that depletes intracellular Ca2+ stores nearly abolished phasic contractions. In muscle strips, pretreated with ryanodine (10 microM for 30 min), phenylephrine (1 microM) increased and isoprenaline (1 microM) decreased the duration of electrical slow waves but neither was able to reverse the ryanodine-suppressed phasic contractions. This suggests that adrenoceptor effects on electrical slow waves are coupled to contractions via Ca2+ release from ryanodine-sensitive intracellular stores. 7. In summary, noradrenaline activates postjunctional alpha 1- and beta-adrenoceptors. Activation of alpha 1-adrenoceptors increases the magnitude of electrical slow waves and phasic contractions, whereas activation of beta-adrenoceptors decreases them. The alpha 1-adrenoceptor mediated effects on electrical slow waves and phasic contractions require the influx of Ca2+ through voltage-gated L-type Ca2+ channels. Phasic contractions also involve Ca2+ release from ryanodine-sensitive intracellular stores.

Potassium currents in rat colonic smooth muscle cells and changes during development and aging

In a previous study on freshly isolated single smooth muscle cells from the circular layer of the rat distal colon, we reported that the L-type Ca2+ current density increased during development and gradually declined with further aging [ZI Xiong, N Sperelakis, N Noffsinger, C Fenoglio-Preiser (1993) Am J Physiol 265: C617-C625]. Since K+ current plays a key role in controlling excitability of the cells and hence the motility of the colon, in the present study the voltage-gated K+ channel currents, (IK) were investigated using the whole-cell voltage-clamp technique in colonic myocytes from rats of different ages. A Ca(2+)-sensitive K+ current [IK(Ca)] and two kinds of Ca(2+)-insensitive outward K+ currents were identified and characterized. IK(Ca) was recorded at potentials more positive than -40 mV in Ca(2+)-containing bath solution, and was blocked by Ca2+ channel antagonists and tetraethylammonium ion (TEA+). After removing Ca2+ from the bath solution and using a high ethylenebis(oxonitrilo)tetraacetate (EGTA, 4 mM) concentration in the pipette, two types of Ca(2+)-insensitive IK were recorded. The first and faster component was usually activated at potentials more positive than -50 mV, and was more sensitive to 4-aminopyridine (4-AP). In contrast, the second and slower (delayed) component was activated at potentials more positive than -30 mV, and was more sensitive to TEA. The total density of the Ca(2+)-insensitive IK component decreased dramatically during the neonatal period: from 32.2 +/- 3.2 pA/pF in 3-day-old rats to 17.8 +/- 2.6 pA/pF in 40-day-old rats; there was no further decline during aging (up to 480 days).(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple components of delayed rectifier K+ current in canine colonic smooth muscle

1. We investigated the pharmacology and voltage-dependent activation and inactivation kinetics of the 'delayed rectifier' K+ current, IdK, in canine colonic myocytes and developed protocols which separate this current into three distinct components that differ in their kinetics and pharmacology. 2. Block of IdK by TEA or 4-aminopyridine (4-AP) alone was incomplete. Maximal concentrations of TEA or 4-AP blocked 76% (EC50 = 2.6 mM) and 51% (EC50 = 69 mM) of current, respectively. In the presence of 10 mM 4-AP, IdK could be blocked completely by TEA. 3. TEA and 4-AP had distinct effects on current activation: time constants for activation of IdK at +10 mV were 25.6 +/- 4.4 ms under control conditions, 40.3 +/- 7.6 ms in the presence of 10 mM 4-AP and 16.7 +/- 2.3 ms with 10 mM TEA in the bath solution. 4-AP block and removal of block were use dependent, but no frequency dependence or voltage dependence of steady-state block could be detected. These data are consistent with the presence of a rapidly activating 4-AP-sensitive current, IdK(f), and a more slowly activating TEA-sensitive current component, IdK(s). 4. A third component of the delayed rectifier current, IdK(n), was revealed when 10 mM TEA was included in the pipette solution. IdK(n) was rapidly activating, had a membrane potential at half-maximal inactivation (V1/2) for steady-state inactivation 13 mV negative of that for the mixed IdK, was completely insensitive to 4-AP (10 mM) and was blocked by external TEA with an EC50 of 7.7 mM. 5. These data demonstrate that the delayed rectifier current in canine colonic smooth muscle is composed of three currents, IdK(f), IdK(s) and IdK(n). All three currents are insensitive to charybdotoxin (100 nM).

Changes in calcium channel current densities in rat colonic smooth muscle cells during development and aging

The age-related changes of Ca2+ channel currents were investigated in freshly isolated single smooth muscle cells from the circular layer of the distal colon from the rat using the whole cell voltage clamp technique. Under physiological conditions (Ca2+ concentration of 2.0 mM), the averaged total Ca2+ current density increased markedly from 1.25 pA/pF in the newborn rat to 6.46 pA/pF in the 60-day-old rat; it then gradually declined with aging. Two types of Ca2+ channel currents seemed to be present; one type possessed more negative threshold potentials (-70 to -60 mV) when the cells were held at -80 or -100 mV and inactivated quickly. The voltage for peak current was -20 to -10 mV, and the reversal potential was +60 to +70 mV. This current was highly sensitive to low concentrations of Ni2+ (30 microM) but was resistant to nifedipine, diltiazem, cadmium, and tetrodotoxin. In contrast, the other type of Ca2+ channel current possessed more positive threshold potential (-40 mV) and inactivated more slowly. The voltage for peak current was 0 mV, and the reversal potential was +60 to +70 mV. This current was insensitive to low concentrations of Ni2+ but highly sensitive to nifedipine, diltiazem, and cadmium. These results suggest that the fast inactivating (transient) current might be T-type Ca2+ current [ICa(T)], and such cells were ICa(T) positive cells; whereas the sustained Ca2+ current was L-type Ca2+ current [ICa(L)], and such cells were ICa(L) positive cells. Our results showed that the fraction of ICa(T) positive cells increased with development; the current densities of both ICa(L) and ICa(T) also increased with development.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca(2+)-activated and voltage-gated K+ currents in smooth muscle cells isolated from human mesenteric arteries

1. Smooth muscle cells were enzymatically isolated from arteries dissected from mesenteric fat removed from patients undergoing routine surgery. The whole-cell patch clamp technique was used to characterize the potassium (K+) currents and passive electrical properties of these cells, using high-K(+)-containing pipette solutions with either 0.2 mM EGTA or 10 mM EGTA and 10 mM BAPTA. 2. Cell capacitance, which is proportional to membrane surface area, was normally distributed around a value of 46 pF, and independent of artery size between 0.4 and 3.6 mm. The mean membrane potential measured under current clamp was -44.1 +/- 1.9 mV (n = 52). 3. Cells dialysed with 0.2 mM EGTA in order to weakly buffer intracellular Ca2+ demonstrated a noisy outward current with an apparent threshold near -30 mV, upon which were superimposed spontaneous transient outward currents (STOCs). In the presence, but not the absence, of extracellular Ca2+, this current was potentiated if the holding potential was depolarized into the voltage range between -40 and +50 mV. This potentiation had a bell-shaped potential dependency which reflected the activation of voltage-gated Ca2+ channels in these cells. 4. The noisy current was blocked by externally applied tetraethylammonium (the dissociation constant, Kd = 0.85 mM), as were STOCs. This current was also reduced by about 40% by 8 nM charybdotoxin, and was transiently potentiated by 10 mM caffeine. The characteristics of this current therefore suggested that it was carried by large-conductance Ca(2+)-activated K+ channels. 5. Dialysis of human mesenteric arterial cells with 10 mM EGTA and 10 mM BATPA was not able to completely suppress the Ca(2+)-activated current, and reduced by approximately 50% the amplitude of the outward current recorded at positive potentials. 6. Depolarization of strongly Ca(2+)-buffered cells in the presence of 30 mM TEA to block Ca(2+)-activated K+ channels revealed a residual outward current which had both transient and sustained components. These were blocked by 4-aminopyridine (4-AP) with a similar efficiency (Kd was 1.04 and 1.16 mM at +60 mV for transient and sustained current, respectively), but the voltage ranges over which they inactivated, and their rates of recovery from inactivation, were significantly different. 7. The transient and sustained currents had different sensitivities to external Ca2+ and Cd2+ ions. Ca2+ (5 mM) significantly reduced the amplitude and shifted the voltage dependency of inactivation of the transient but not the sustained component of the outward current. Cd2+ (0.2 mM) reduced the transient current by about 30% without affecting the sustained component amplitude.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclopiazonic acid, an inhibitor of the sarcoplasmic reticulum Ca(2+)-pump, reduces Ca(2+)-dependent K+ currents in guinea-pig smooth muscle cells

1. Effects of cyclopiazonic acid (CPA), a specific inhibitor of the Ca(2+)-ATPase in sarcoplasmic reticulum (SR), on membrane ionic currents were examined in single smooth muscle cells freshly isolated from ileal longitudinal strips and urinary bladder of the guinea-pig. 2. Under whole-cell clamp, CPA (1-10 microM) reduced peak outward current elicited by depolarization in a concentration-dependent manner. The concentration of CPA required for 50% decrease in the peak outward current was approximately 3 microM in ileal cells under these conditions. The current reduced by CPA recovered by more than 70% after washout. 3. The transient outward current elicited by application of 5 mM caffeine at a holding potential of -50 mV in Ca2+ free solution was almost abolished, when the preceding Ca(2+)-loading of the cell in a solution containing 2.2 mM Ca2+ was performed in the presence of 3 microM CPA. 4. When the Ca(2+)-dependent K+ current (IK-Ca) and Ca2+ current (ICa) were inhibited by addition of Ca2+, the remaining delayed rectifier type K+ current was not affected by 10 microM CPA. When outward currents were blocked by replacement of K+ by Cs+ in the pipette solution, the remaining ICa was not affected by 10 microM CPA. 5. CPA (10 microM) did not affect the conductance of single maxi Ca(2+)-dependent K+ channels or the Cd(2+)-dependence of their open probability in both inside- and outside-out configurations. 6. These results indicate that IK-Ca is selectively and strongly suppressed by CPA.Its effects may be attributed to a decrease in Ca2"-uptake into SR, resulting in a decrease in Ca2"-induced Ca24 release which is triggered by Ca24 entering through voltage-dependent Ca24 channels and therefore less activation of these K channels.7. CPA may be extremely valuable pharmacological tool for investigating intracellular Ca24 mobilization and ionic currents regulated by intracellular Ca24.

A potential-dependent fast outward current in single smooth muscle cells isolated from the newborn rat ileum

1. Whole-cell outward currents have been studied in single smooth muscle cells isolated from newborn and adult rat ileum, using fire-polished glass micropipettes. 2. Two major outward currents, delayed (I(do)) and fast inactivating potential-dependent (I(fo)), have been observed in the newborn rat ileal cells. I(fo) is activated between -50 and -40 mV from the holding potential of -80 mV, whereas I(do) usually starts to activate at membrane potentials positive to -20 mV. Activation of I(do) was fast, its time-to-peak decreased from 10.8 +/- 0.9 ms (n = 5) at -30 mV to 4.5 +/- 0.7 ms (n = 4) at 20 mV. 3. I(fo) decay was monoexponential and its time constant did not depend on the membrane potential. Dependence of I(fo) inactivation on membrane voltage in normal physiological salt solutions (PSS) could be described by the Boltzmann equation with the following parameters: a half-inactivation potential, V0.5 = -70.8 mV and slope factor, k = 7.7 mV. 4. Recovery of I(fo) from inactivation was fitted by a single exponential and was potential dependent. The average time constant was 28.4 +/- 2.4 ms (n = 11) at -120 mV, 47.7 +/- 3.0 ms (n = 6) at -100 mV and 89.6 +/- 5.3 ms (n = 13) at -80 mV. 5. Removal of Ca2+ ions from the PSS (in the presence of 5 mM-Mg2+) increased I(fo) amplitude by about two times, and shifted its voltage dependence of inactivation towards negative membrane potentials by about 16 mV (V0.5 = -87.2 mV). Removal of Mg2+ from the PSS (in the presence of 2.5 mM-Ca2+) exerted no effects upon either inactivation dependence (V0.5 = -74.2 mV) or I(fo) amplitude. 6. I(do) and I(fo) had different sensitivities to K+ channel blockers. With 10 mM-external TEA+ I(do), was preferentially suppressed, while 5 mM-4-aminopyridine (4-AP) completely blocked I(fo). I(fo) was also partially blocked by a higher TEA+ concentration (30 mM), which suppressed I(fo) to 0.55 +/- 0.02 (n = 9). The blocking effect of 4-AP on I(fo) was potential, use and time dependent. 7. Ileal cells isolated from the adult rat demonstrated the presence of two populations of smooth muscle cells. One has an outward current which seems to be similar to that described in the newborn rat. However, in other cells spontaneous transient outward currents, well described in other single smooth muscle cells, but not found in newborn rat ileal cells, have been observed.

Participation of fast-activating, voltage-dependent K currents in electrical slow waves of colonic circular muscle

The plateau phase of electrical slow waves in phasic gastrointestinal muscles is critical for excitation-contraction coupling. The plateau appears to depend upon a balance between inward Ca2+ current and outward K+ currents that is sustained for several seconds. Voltage-dependent, non-Ca(2+)-dependent K currents were studied in canine colonic circular muscle cells using the whole cell patch-clamp technique. At room temperature, depolarization activated a slow outward current that showed little inactivation during 500 ms. Increasing the temperature to 37 degrees C significantly increased the rate of activation of voltage-dependent outward current. The onset of the outward current overlapped the transient inward Ca2+ current, suggesting that this K current may act as a brake on the upstroke depolarization of electrical slow waves in intact muscles. Voltage-dependent outward current was sustained for the duration of test pulses. This current balanced the sustained inward current that was also activated at physiological test potentials. The outward current evoked by test pulses positive to -20 mV inactivated by at least 50% within 500 ms. Half inactivation occurred at -36 mV. Voltage-dependent K current was reduced by 4-aminopyridine (4-AP; 1-5 mM), but difference currents obtained by subtracting currents elicited from holding potentials of -45 mV from currents obtained from holding potentials of -100 mV were not affected by 4-AP (1 mM). Studies were also performed on intact muscles to test the effects of 4-AP on electrical slow waves. 4-AP increased the amplitude and rate of rise of the upstroke potential and increased the amplitude and prolonged the plateau phase of slow waves. These data suggest that a rapidly activating, inactivating, voltage-dependent K current participates in electrical slow waves of colonic circular smooth muscles.

Outward currents in longitudinal colonic muscle cells contribute to spiking electrical behavior

Electrical events in longitudinal and circular muscles of the colon are different. Longitudinal muscles generate action potentials superimposed upon small depolarizations termed myenteric potential oscillations and circular muscles generate slow wave events that persist for several seconds. Differences between circular and longitudinal muscles may be related to the potassium channels these cells express. We have studied Ca(2+)-dependent and voltage-dependent K currents of isolated longitudinal cells with the whole cell patch-clamp technique. Test depolarizations positive to -40 mV yielded a transient inward current followed by a large sustained outward current. Blockade of the inward Ca2+ current reduced the amplitude of the outward current. Outward current was also reduced by tetraethylammonium (TEA; 1 mM), suggesting that a component of the outward current is Ca2+ dependent. After blockade of the Ca(2+)-dependent outward current, a voltage- and time-dependent component of outward current remained. The activation and inactivation properties and sensitivity to TEA and 4-aminopyridine (4-AP) were characterized. The voltage-dependent outward current in longitudinal cells had different properties than the voltage-dependent K currents in circular muscle cells (i.e., more negative inactivation, less sensitivity to 4-AP). TEA (1-5 mM) increased the amplitude and frequency of action potentials in intact longitudinal muscles; 4-AP (1 mM) had little effect on electrical activity of longitudinal muscles. The data suggest that differences in electrical behavior of the 2 muscle layers may be related to the expression of different species of K channels.

Effect of cromakalim and lemakalim on slow waves and membrane currents in colonic smooth muscle

The effects of cromakalim (BRL 34915) and its optical isomer lemakalim (BRL 38227) were investigated in intact tissue and freshly dispersed circular muscle cells from canine proximal colon. Cromakalim and lemakalim hyperpolarized resting membrane potential, shortened the duration of slow waves by abolishing the plateau phase, and decreased the frequency of slow waves. Glyburide, a K channel blocker, prevented the effect of cromakalim on slow-wave activity. The mechanisms of these alterations in slow-wave activity were studied in isolated myocytes under voltage-clamp conditions. Cromakalim and lemakalim increased the magnitude of a time-independent outward K current, but cromakalim also reduced the peak outward K current. Glyburide inhibited lemakalim stimulation of the time-independent background current. Nisoldipine also reduced the peak outward current, and in the presence of nisoldipine, cromakalim did not affect the peak outward component of current. This suggested that cromakalim may block a Ca-dependent component of the outward current. Lemakalim did not affect the peak outward current. We tested whether the effects of cromakalim on outward current might be indirect due to an effect on inward Ca current. Cromakalim, but not lemakalim, was found to inhibit L-type Ca channels; however, glyburide did not alter cromakalim inhibition of inward Ca current. We conclude that the effects of cromakalim and lemakalim on membrane potential and slow waves in colonic smooth muscle appear to result primarily from stimulation of a time-independent background K conductance. The effects of these compounds on channel activity may explain the inhibitory effect of these compounds on contractile activity.