Voltage clamp of single freshly dissociated smooth muscle cells: current-voltage relationships for three currents

Spontaneous Electrical Activity and Rhythmicity in Gastrointestinal Smooth Muscles

The gastrointestinal (GI) tract has multifold tasks of ingesting, processing, and assimilating nutrients and disposing of wastes at appropriate times. These tasks are facilitated by several stereotypical motor patterns that build upon the intrinsic rhythmicity of the smooth muscles that generate phasic contractions in many regions of the gut. Phasic contractions result from a cyclical depolarization/repolarization cycle, known as electrical slow waves, which result from intrinsic pacemaker activity. Interstitial cells of Cajal (ICC) are electrically coupled to smooth muscle cells (SMCs) and generate and propagate pacemaker activity and slow waves. The mechanism of slow waves is dependent upon specialized conductances expressed by pacemaker ICC. The primary conductances responsible for slow waves in mice are Ano1, Ca2+-activated Cl- channels (CaCCs), and CaV3.2, T-type, voltage-dependent Ca2+ channels. Release of Ca2+ from intracellular stores in ICC appears to be the initiator of pacemaker depolarizations, activation of T-type current provides voltage-dependent Ca2+ entry into ICC, as slow waves propagate through ICC networks, and Ca2+-induced Ca2+ release and activation of Ano1 in ICC amplifies slow wave depolarizations. Slow waves conduct to coupled SMCs, and depolarization elicited by these events enhances the open-probability of L-type voltage-dependent Ca2+ channels, promotes Ca2+ entry, and initiates contraction. Phasic contractions timed by the occurrence of slow waves provide the basis for motility patterns such as gastric peristalsis and segmentation. This chapter discusses the properties of ICC and proposed mechanism of electrical rhythmicity in GI muscles.

Seeing is believing! Imaging Ca2+-signalling events in living cells

Ever since it was shown that maintenance of muscle contraction required the presence of extracellular Ca(2+), evidence has accumulated that Ca(2+) plays a crucial role in excitation-contraction coupling. This culminated in the use of the photoprotein aequorin to demonstrate that [Ca(2+)](i) increased after depolarization but before contraction in barnacle muscle. Green fluorescent protein was extracted from the same jellyfish as aequorin, so this work also has important historical links to the use of fluorescent proteins as markers in living cells. The subsequent development of cell-permeant Ca(2+) indicators resulted in a dramatic increase in related research, revealing Ca(2+) to be a ubiquitous cell signal. High-speed, confocal Ca(2+) imaging has now revealed subcellular detail not previously apparent, with the identification of Ca(2+) sparks. These act as building blocks for larger transients during excitation-contraction coupling in cardiac muscle, but their function in smooth muscle appears more diverse, with evidence suggesting both 'excitatory' and 'inhibitory' roles. Sparks can activate Ca(2+)-sensitive Cl() and K(+) currents, which exert positive and negative feedback, respectively, on global Ca(2+) signalling, through changes in membrane potential and activation of voltage-operated Ca(2+) channels. Calcium imaging has also demonstrated that agonists that appear to evoke relatively tonic increases in average [Ca(2+)](i) at the whole tissue level often stimulate much higher frequency phasic Ca(2+) oscillations at the cellular level. These findings may require re-evaluation of some of our models of Ca(2+) signalling to account for newly revealed cellular and subcellular detail. Future research in the field is likely to make increasing use of genetically coded Ca(2+) indicators expressed in an organelle- or tissue-specific manner.

Diversity of K+ channels in circular smooth muscle of opossum lower esophageal sphincter

We previously demonstrated that a balance of K+ and Ca2+-activated Cl– channel activity maintained the basal tone of circular smooth muscle of opossum lower esophageal sphincter (LES). In the current studies, the contribution of major K+ channels to the LES basal tone was investigated in circular smooth muscle of opossum LES in vitro. K+ channel activity was recorded in dispersed single cells at room temperature using patch-clamp recordings. Whole-cell patch-clamp recordings displayed an outward current beginning to activate at –60 mV by step test pulses lasting 400 ms (–120 mV to +100 mV) with increments of 20 mV from holding potential of –80 mV ([K+]I = 150 mM, [K+]o = 2.5 mM). However, no inward rectification was observed. The outward current peaked within 50 ms and showed little or no inactivation. It was significantly decreased by bath application of nifedipine, tetraethylammonium (TEA), 4-aminopyridine (4-AP), and iberiotoxin (IBTN). Further combination of TEA with 4-AP, nifedipine with 4-AP, and IBTN with TEA, or vice versa, blocked more than 90% of the outward current. Ca2+-sensitive single channels were recorded at asymetrical K+ gradients in cell-attached patch-clamp configurations (100.8 ± 3.2 pS, n = 8). Open probability of the single channels recorded in inside-out patch-clamp configurations were greatly decreased by bath application of IBTN (100 nM) (Vh = –14.4 ± 4.8 mV in control vs. 27.3 ± 0.1 mV, n = 3, P < 0.05). These data suggest that large conductance Ca2+-activated K+ and delayed rectifier K+ channels contribute to the membrane potential, and thereby regulate the basal tone of opossum LES circular smooth muscle.Key words: large conductance Ca2+-activated K+ channels, delayed rectifier K+ channels, patch-clamp recording, visceral smooth muscle.

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).

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)

The action potential and net membrane currents in isolated human detrusor smooth muscle cells

The basic electrophysiological properties of the human detrusor have been investigated in vitro using isolated single cells obtained by collagenase digestion of bladder biopsy specimens. Recordings were made using the 'whole-cell patch clamp' technique using either a physiological filling solution or one in which cesium was used to block any outward current. Spontaneous and stimulated action potentials have been recorded and we have performed the first voltage clamp analysis of the currents that underlie the action potential in human detrusor. The depolarising phase of the action potential occurs by an inward current of Ca2+ ions which can be shown to be of sufficient magnitude to support the rate of upstroke. Repolarisation occurs due to an outward K+ current that is partially Ca2+ dependent. The techniques described here permit the investigation of the ionic basis for the control of contractility in the human bladder and may permit the characterisation of any underlying abnormality in the overactive detrusor.

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.

The whole-cell Ca2+ channel current in single smooth muscle cells of the guinea-pig ureter

1. Calcium channel currents were recorded in single, enzymatically isolated smooth muscle cells of the guinea-pig ureter using a single-electrode whole-cell voltage clamp technique. Calcium and barium currents through voltage-activated Ca2+ channels were recorded in cells dialysed with Cs(+)- or Na(+)-containing saline which suppressed K+ currents. 2. Inward currents in Ca2+ (1.5-7.5 mM) or Ba2+ (1.5-7.5 mM) were recorded at potentials positive to -50 to -30 mV. Inward currents were maximal at 0 mV in 1.5 mM-Ca2+ and at +10 mV in 7.5 mM-Ba2+. Current flow through Ca2+ channels in Cs(+)-filled cells (in 1.5 mM-Ca2+ or 7.5 mM-Ba2+) changed from inward to outward at potentials positive to +70 mV. In Na(+)-filled cells this reversal potential was between +50 and +60 mV. 3. Replacing Ca2+ or Ba2+ with Co2+ (1.5 mM) blocked all inward current flow through these Ca2+ channels; outward currents at potentials positive to +40 mV, however, were increased. Cadmium (100 microM) and nifedipine (0.1-10 microM) reduced both inward and outward current flow. 4. Calcium channel activation showed a sigmoidal relationship with membrane potential; the potential of half-maximal activation was -8.4 mV in 1.5 mM-Ca2+ and -10.8 mV in 7.5 mM-Ba2+. The maximum membrane conductance to Ca2+ (in 1.5 mM-Ca2+) was 2.57 nS/cell or approximately 0.05 mS/cm2. 5. Evidence for a voltage-dependent inactivation mechanism included (a) the time-dependent relaxation of the outward currents at potentials positive to the reversal potential and (b) a steady-state inactivation (f infinity (V] vs. membrane potential relationship (in 7.5 mM-Ba2+) which ranged between -80 and 0 mV, with a half-maximal availability at -40.5 mV. 6. The voltage dependencies of the inward current elicited from -80 and -30 mV were similar, suggesting that depolarization activated only L-type Ca2+ channels. 7. It was concluded that the processes controlling the time course of the Ca2+ current in single ureteral cells bathed in physiological concentrations of Ca2+ were mostly voltage-dependent.

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)

Two components of potassium current activated by depolarization of single smooth muscle cells from the rabbit portal vein

1. Using the patch-clamp technique at 20-23 degrees C membrane currents were recorded from single smooth muscle cells enzymatically isolated from the rabbit portal vein. Single-channel currents were observed in outside-out patches excised from these. 2. Outward current elicited upon depolarization from -70 mV was not activated as a result of Ca2+ influx. It could be divided into two components: an inactivating, 4-aminopyridine- and phencyclidine-sensitive low-noise current (IdK), and a non-inactivating, tetraethylammonium (TEA)- and charybdotoxin-sensitive high-noise current (IcK). 3. IdK activated with a threshold around -40 mV and was carried by K+. It was substantially inhibited by 4-aminopyridine (5 mM) or phencyclidine (0.1 mM) but was insensitive to TEA+ (4 mM), charybdotoxin (0.1 microM) or apamin (0.1 microM). Upon stepping to 0 mV it reached a maximum within about 0.2 s. The time course of its activation could be described by a fourth-order single exponential; the time constants of these exponentials changed e-fold every 56 mV. It inactivated in a time- and voltage-dependent manner with a fast and slow component, and was about 50% available at -30 mV. From single-channel recordings in isolated patches single channels underlying this current have a small unitary conductance (around 5 pS). 4. IcK did not inactivate significantly over 6 s. It activated with a less negative threshold than IdK, usually near 0 mV when the pipette solution contained 0.8 mM-EGTA with no added calcium. It was blocked by TEA (4 mM) or charybdotoxin (0.1 microM), but not by 4-aminopyridine (5 mM), phencyclidine (0.1 mM) or apamin (0.1 microM). Estimates of the single-channel conductance from the noise variance of the whole-cell current IcK indicated a value at +80 mV of 115 pS, very similar to that of the large-conductance Ca2(+)-activated K+ channels studied in these cells using single-channel recording. 5. The results suggest that outward current evoked by depolarization from the resting potential can be carried by 100 pS Ca2(+)-activated K+ channels and by small-conductance delayed-rectifier K+ channels. It is likely that opening of both types of channel contributes to the repolarization phase of the action potential in this smooth muscle.

Macroscopic K+ currents in single smooth muscle cells of the rabbit portal vein

1. Single smooth muscle cells isolated from rabbit portal vein were voltage clamped at room temperature using the whole-cell configuration of the patch-clamp technique. These cells exhibited a mean resting potential of -47.9 mV and a mean input resistance of 376 M omega. 2. Using small tip diameter micropipettes (to avoid dialysis of the cells), depolarizing voltage-clamp pulses from a holding potential of -50 mV elicited two distinct outward currents: a quasi-instantaneous background current and a time-dependent current that did not appear to inactivate (delayed rectifier). Upon return to the holding potential, an outward tail current decaying back to the holding current was observed. 3. The time course of development of the tail current as estimated from envelopes of tail current protocols followed the kinetics of activation of the delayed rectifier elicited during the preceding test pulse. The tail current reversed close to the equilibrium potential for K+ ions indicating that it is mainly carried by potassium ions. 4. Using large tip diameter micropipettes to internally dialyse the cells (EGTA = 0.1 mM; ATP = 5 mM), two additional outward currents having transient kinetics were revealed: a smooth transient outward current (Ito) and spontaneous transient outward currents (STOCs). Ito was found to be mainly selective for K+ ions and exhibited voltage-dependent inactivation with half-maximal availability near -40 mV. 5. Removal of calcium from the bathing solution significantly reduced the background current and abolished both Ito and STOCs. The delayed rectifier current appeared to be insensitive to this procedure. The two types of transient outward currents were never recorded when EGTA was elevated to 5 mM inside the micropipette whereas the background and delayed rectifier currents were not affected. These results suggested that Ito and the spontaneous transient outward currents are activated by internal calcium. 6. External application of TEA (0.5-20 mM) blocked all four outward currents. Calcium replacement by barium significantly reduced the background current and Ito, and had small effects on the delayed rectifier current. When potassium was replaced with caesium (130 mM) and TEA (20 mM) inside the pipette, none of the outward currents described was ever observed. In about 60% of the cells dialysed with this solution a small inward Ca2+ current was revealed. 7. External application of caffeine (5 mM) abolished STOCs in cells in which this activity was present under control conditions. In cells lacking this type of activity under control conditions caffeine induced and later abolished this type of current.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of calcium channels in guinea-pig gastric myocytes

1. The inward membrane current in enzymatically dispersed guinea-pig gastric myocytes was studied using whole-cell voltage clamp technique. 2. Only one inward membrane current was found in gastric myocytes which was identified as the Ca2+ current based on its inhibition by Ni2+, Cd2+ and Co2+, its dependence on [Ca2+]o, and its insensitivity to variations of [Na+]o. 3. Ca2+ current activated at -20 mV, peaked around +10 mV and was markedly enhanced when the holding potential was increased from -40 to -90 mV. The enhancement of ICa at negative holding potentials did not alter the activation threshold of ICa. When Ba2+ was substituted for Ca2+, IBa was similarly enhanced at more negative potentials. 4. In cells where internal Ca2+ was buffered with 10 mM-EGTA, the time course of inactivation was fitted with two exponentials, with time constants: tau f = 53.4 +/- 18.1 ms and tau s = 175.2 +/- 46.1 ms. When Ba2+ was the charge carrier through the channel, the time course of inactivation could be fitted often by only one exponential which approximated tau s for inactivation of ICa. The voltage dependence of steady-state inactivation of Ca2+ channels was not significantly altered when Ba2+ was the charge carrier. 5. Using different buffering systems (EGTA, EDTA and citrate), we found that citrate maintained the ICa and slowed inactivation more effectively than the other buffers tested. Because the calculated change in [Ca2+]i did not differ significantly between buffer systems, we speculate that suppression of inactivation by citrate is related to increased accessability of the buffer to cytoplasmic Ca2+ near the Ca2+ channel. Changes in [Mg2+]i affected peak ICa but not the kinetics of inactivation indicating that [Mg2+]i may regulate the steady-state inactivation or the availability of the Ca2+ channels. 6. The divalent selectivity of the Ca2+ channel had the following sequence: Ba2+ greater than Ca2+ greater than or equal to Sr2+ much greater than Mg2+. In very low extracellular Ca2+ (less than 10(-7) M), the Ca2+ channel conducted Na+. 7. Increasing [H+]o appeared to differentially affect peak and maintained components of ICa. At pH less than 6.5, the maintained component of ICa was suppressed more than the peak component indicating possible time- and voltage-dependent inhibition of ICa by protons. 8. Nifedipine, D600 and diltiazem inhibited ICa in a voltage-dependent manner. The order of potency for inhibition of peak ICa was nifedipine approximately D600 much greater than diltiazem.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of the major membrane currents in freshly dispersed single smooth muscle cells of guinea-pig ureter

1. The passive and active electrical properties of freshly dispersed single cells of the guinea-pig ureter were investigated using standard patch-clamp techniques. 2. Action potentials, having a rapid rising phase and a prolonged plateau, were recorded on passing depolarizing currents through the patch pipette when 'near-normal' physiological gradients were established across the cell membrane (5.9 mM-K+, 1.5 mM-Ca2+ in the bath; 126 mM-K+ in the pipette). 3. Under voltage clamp, depolarization to potentials positive of -50 mV (from a holding potential of -70 or -80 mV) triggered a net inward current which reached a peak in 5-10 ms and then slowly inactivated. 4. The averaged membrane current to depolarization to potentials between -30 and 0 mV showed two distinct patterns after the peak of the inward current; the membrane current either moved slowly outward over 400 ms or there was one or more transient outward currents superimposed on the slowly decreasing inward current. Both outward currents were blocked when 5 mM-tetraethylammonium (TEA) was added to the bathing solution, resulting in an increased inward current at all potentials. 5. Replacing the extracellular Ca2+ with Co2+ (1.5-5 mM) blocked the inward current and the outward currents to reveal another transient outward current (voltage activated) which activated rapidly to reach a peak within 5 ms and which inactivated exponentially with a time constant of 10 ms. This voltage-activated outward current was inactivated if the membrane was held at -50 mV, but could be reactivated by short hyperpolarizing pre-pulses. The amplitude of this transient current in response to a fixed depolarization (to 0 mV) was half-maximum when the hyperpolarizing pre-pulse was to -66 mV. The voltage-activated outward current was reduced in amplitude when the extracellular potassium was raised to 46 mM or upon exposure to 1 mM-4-aminopyridine (4-AP), but was not affected by 5 mM-TEA. 6. Replacing K+ in the pipette and bathing solution with caesium (Cs) blocked all outward currents, revealing the time course and voltage dependence of the inward current, which could be carried by Ca2+ or Ba2+ with little effect on its rate of inactivation. 7. It was concluded that the inward current recorded in single ureter cells was due to the flow of current through voltage-activated Ca2+ channels. The TEA-sensitive outward currents, whether transient or slowly activating, are presumably K+ channels activated by Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Inward current in single smooth muscle cells of the guinea pig taenia coli

Using the tight-seal voltage-clamp method, the ionic currents in the enzymatically dispersed single smooth muscle cells of the guinea pig taenia coli have been studied. In a physiological medium containing 3 mM Ca2+, the cells are gently tapering spindles, averaging 201 (length) x 8 microns (largest diameter in center of cell), with a volume of 5 pl. The average cell capacitance is 50 pF, and the specific membrane capacitance 1.15 microF/cm2. The input impedance of the resting cell is 1-2 G omega. Spatially uniform voltage-control prevails after the first 400 microseconds. There is much overlap of the inward and outward currents, but the inward current can be isolated by applying Cs+ internally to block all potassium currents. The inward current is carried by Ca2+. Activation begins at approximately -30 mV, maximum ICa occurs at +10-+20 mV, and the reversal potential is approximately +75 mV. The Ca2+ channel is permeable to Sr2+ and Ba2+, and to Cs+ moving outwards, but not to Na+ moving inwards. Activation and deactivation are very rapid at approximately 33 degrees C, with time-constants of less than 1 ms. Inactivation has a complex time course, resolvable into three exponential components, with average time constants (at 0 mV) of 7, 45, and 400 ms, which are affected differently by voltage. Steady-state inactivation is half-maximal at -30 mV for all components combined, but -36 mV for the fast component and -26 and -23 mV for the other two components. The presence of multiple forms of Ca2+ channel is inferred from the inactivation characteristics, not from activation properties. Recovery of the fast channel occurs with a time-constant of 72 ms (at +10 mV). Ca2+ influx during an action potential can transfer approximately 9 pC of charge, which could elevate intracellular Ca2+ concentration adequately for various physiological functions.

Ionic currents of smooth muscle cells isolated from the ctenophore Mnemiopsis

The ionic currents of smooth muscle cells isolated from the ctenophore Mnemiopsis were examined by using conventional two-electrode voltage clamp and whole-cell patch clamping methods. Several separable currents were identified. These include: (1) a transient and (2) a steady-state voltage-activated inward current; both are tetrodotoxin (TTX) and saxitoxin (STX) insensitive, partly reduced by decreasing external Ca2+ or Na+ or by addition of 5 mM Co2+, D-600 or verapamil and are totally blocked with 5 mM Cd2+; (3) an early, transient, cation-dependent, outward K+ current (IKCa/Na); (4) a transient, voltage-activated, outward K+ current provisionally identified as IA; (5) a delayed, steady-state, voltage-activated outward K+ current (IK) and (6) a late, transient, outward K+ current which is blocked by Cd2+ and evident only during long voltage pulses. Despite their phylogenic origin, most of these currents are similar to currents identified in many vertebrate smooth and cardiac muscle preparations, and other excitable cells in higher animals.

Fast and slowly inactivating components of Ca-channel current and their sensitivities to nicardipine in isolated smooth muscle cells from rat vas deferens

(1) Fast and slowly inactivating components of Ca-channel current were compared to clarify whether more than one type of Ca-channel exists in smooth muscle cells from rat vas deferens using the whole cell variant of the patch clamp technique. The pipette was filled with 150 mM Cs solution to eliminate outward current and Ba was used as the charge carrier for Ca-channel current. (2) When activated by a 5 s test pulse to O mV from a holding potential of -60 mV, the inactivation process of Ba-current was well fitted by the sum of two exponentials. The time constant of the faster inactivating component was 100-300 s and that of the slower inactivating component was 1.5-3 s. Steady-state inactivation curves of the fast- and slow-components were very similar. (3) The inward current activated at O mV from -80 mV was inactivated faster than that from -30 mV. The voltage-dependencies of the peak current from holding potentials of -30 mV and -80 mV were similar. Both had voltage threshold at -30 mV and were maximal at +10 mV. (4) Low concentrations of nicardipine (10(-9) to 10(-7) M) preferentially inhibited the slow component while higher concentration (10(-6) to 10(-5) M) were required to block the fast component. The current activated from a holding potential of -30 mV was almost fully suppressed by 10(-7) M nicardipine whereas that from -80 mV was blocked only slightly.(ABSTRACT TRUNCATED AT 250 WORDS)

Whole-cell voltage clamp and intracellular perfusion technique on single smooth muscle cells

Using freshly isolated single smooth muscle cells prepared by collagenase treatment, membrane currents were recorded by whole-cell voltage clamp. Intracellular constituents were modified by using an intracellular perfusion technique, i.e., pipette solutions were continuously exchanged from control to test solutions during current recording. In smooth muscle cells, intracellular application of ATP, but not cyclic AMP, enhanced the amplitude of Ca2+ currents and prevented current run-down. In addition, with this stabilization of Ca2+ current recording by ATP, introduction of various chemicals into the cell using the intracellular perfusion technique is useful for investigations of regulation of ion channels in smooth muscle cells.

Membrane currents that govern smooth muscle contraction in a ctenophore

Ctenophores are transparent marine organisms that swim by means of beating cilia; they are the simplest animals with individual muscle fibres. Predatory species, such as Beroe ovata, have particularly well-developed muscles and are capable of an elaborate feeding response. When Beroe contacts its prey, the mouth opens, the body shortens, the pharynx expands, the prey is engulfed and the lips then close tightly. How this sequence, which lasts 1 s, is accomplished is unclear. The muscles concerned are structurally uniform and are innervated at each end by a neuronal nerve net with no centre for coordination. Isolated muscle cells studied under voltage-clamp provide a solution to this puzzle. We find that different groups of muscle cells have different time-dependent membrane currents. Because muscle contraction depends upon calcium entry during each action potential, these different currents produce different patterns of contraction. We conclude that in a simple animal such as a ctenophore, a sophisticated set of membrane conductances can compensate for the absence of an elaborate system of effectors.

Influence of prostaglandins on electrical and mechanical activities of gastric muscles of Bufo marinus

1. Study was performed to compare the role of prostaglandins in regulating gastric contractile activity in an amphibian model, Bufo marinus, with mammalian models. 2. The prostaglandin synthesis inhibitor, indomethacin, had little effect on spontaneous mechanical activity, but increased the force and frequency of contractions stimulated by acetylcholine. 3. PGE2 reversed the effects of indomethacin and reduced the force and frequency of contractions. These effects were concentration-dependent. 4. Intracellular measurement of membrane potential demonstrated that the effects of PGE2 could be explained by basic effects on membrane potential and slow wave activity. 5. The data shown that many similarities exist between amphibian and mammalian gastric muscles in terms of the regulatory role played by endogenous PGs. It also appears that the mechanisms of PGE2 action are similar.

Isolated muscle cells as a physiological model

Summary of a symposium presented by the American Physiological Society (Cell and General Physiology Section and Muscle Group) at the 70th Annual Meeting of the Federation of American Societies for Experimental Biology, St. Louis, Missouri, April 15, 1986, chaired by M. Lieberman and F. Fay. This symposium reflects a growing interest in seeking new technologies to study the basic physiological and biophysical properties of cardiac, smooth, and skeletal muscle cells. Recognizing that technical and analytical problems associated with multicellular preparations limit the physiological significance of many experiments, investigators have increasingly focused on efforts to isolate single, functional embryonic, and adult muscle cells. Progress in obtaining physiologically relevant preparations has been both rapid and significant even though problems regarding cell purification and viability are not fully resolved. The symposium draws attention to a broad, though incomplete, range of studies using isolated or cultured muscle cells. Based on the following reports, investigators should be convinced that a variety of experiments can be designed with preparations of isolated cells and those in tissue culture to resolve questions about fundamental physiological properties of muscle cells.

Blocking actions of Ca2+ antagonists on the Ca2+ channels in the smooth muscle cell membrane of rabbit small intestine

Actions of Ca2+ antagonists, verapamil, nicardipine and diltiazem, were investigated on the Ca2+ inward current in the fragmented smooth muscle cell membrane (smooth muscle ball; SMB) obtained from the longitudinal muscle layer of the rabbit ileum, by enzymatic dispersion. All Ca2+ antagonists inhibited the inward current, in a dose-dependent manner. The ID50 value on the maximum amplitude of the inward current of nicardipine was 24 nM, and this value was roughly 50 times lower than values obtained with verapamil and diltiazem, when the inward current was provoked by 0 mV command pulse from the holding potential of -60 mV. Lowering the holding potential to -80 mV shifted the dose-response curve to the right. When depolarizing pulses (100 ms, stepped up to 0 mV from -60 mV or -80 mV) were applied every 20 s, the peak amplitude of the inward current remained unchanged, but nicardipine immediately, and diltiazem and verapamil slowly reduced the peak amplitude. These slow inhibitions by the latter two drugs depended on the frequency or number of stimulations. Nicardipine but not diltiazem and verapamil shifted the voltage-dependent inactivation curve to the left (3 s duration of the conditioning pulse). However, with a longer conditioning pulse (10 s) verapamil and diltiazem shifted the voltage-dependent inactivation curves to the left. Therefore, the inhibitory actions of these Ca2+ antagonists differ. Namely, diltiazem and verapamil inhibit the Ca2+ channels, mainly in a frequency- or use-dependent manner while nicardipine does so in a voltage-dependent manner.

Cellular calcium regulates outward currents in rabbit intestinal smooth muscle cell

The nature of transient and oscillatory outward currents (ITO and IOO) in fragmented smooth muscle cells (smooth muscle ball, SMB) from the longitudinal muscle layer of the rabbit ileum, was studied using a single electrode voltage clamp technique. With a high K+ solution containing 0.3 mM ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) in the pipette and physiological salt solution (PSS) in the bath, the Ca inward current was followed by a large transient outward current (ITO) and spontaneous oscillations of the outward current (IOO) on the sustained outward current (ISO) were elicited by a depolarizing pulse, positive to -30 mV (holding potential of -60 mV). When the internal fluid of the SMB was replaced with Cs+-tetraethylammonium+ (TEA+) solution, or when the concentration of EGTA in the pipette was increased to 4 mM, using the intracellular perfusion technique, both ITO and IOO were abolished. In Mn2+ solution both currents were also inhibited. Bath application of TEA+, procaine or A23187 completely blocked both ITO and IOO. Caffeine (0.3-1 mM) enhanced the amplitude of ITO and generations of IOO, and concentrations of caffeine over 3 mM transiently enhanced, but finally suppressed both these currents. These results suggest that the generation of ITO is closely related to the Ca2+ influx, whereas the generation of IOO may be initiated by an increment in the intracellular concentration of Ca2+, possibly released from store sites.

Whole-cell and single-channel calcium currents of isolated smooth muscle cells from saphenous vein

Whole-cell and single-channel calcium currents in enzymatically isolated dog saphenous vein cells were recorded by the patch-clamp method. Test pulses to negative potentials from holding potentials of -90 to -40 mV elicited currents that inactivated quickly and in a voltage-dependent manner (called I low for low threshold). A second calcium current persisted even at relatively positive holding potentials of -30 to -10 mV, required stronger depolarizations for maximum current, and inactivated slowly (I high for high threshold). I high transported barium more than calcium, whereas I low transported the two ions equally. Single-channel current records (90 mM barium) showed a larger conductance that activated at relatively positive potentials and a smaller (about one-third) conductance that activated at weak depolarizations. Nitrendipine suppressed I high, and the effect was voltage dependent as observed in cardiac cells, although block of resting channels was much greater in vein cells (KR approximately 10(-8) M). Exposure to the stereoisomer (-)Bay K 8644 increased I high but not I low. The (-)Bay K 8644 also increased the channel activity and prolonged the open time of the larger conductance current. Thus, two types of calcium channels, differing in potential-dependence of activation and inactivation, calcium/barium selectivity, single-channel conductance, and sensitivities to dihydropyridines were identified in smooth muscle cells isolated from a large cutaneous vein.

Acetylcholine increases voltage-activated Ca2+ current in freshly dissociated smooth muscle cells

The regulation of voltage-activated Ca2+ current by acetylcholine was studied in single freshly dissociated smooth muscle cells from the stomach of the toad Bufo marinus by using the tight-seal whole-cell recording technique. Ca2+ currents were elicited by positive-going command pulses from a holding level near -80 mV in the presence of internal Cs+ to block outward K+ currents. Ca2+ current was greatest in magnitude at command potentials near 10 mV. At such command potentials, acetylcholine increased the magnitude of the inward current and slowed its decay. The effects of acetylcholine were seen in the absence of external Na+ or with low Cl- (aspartate replacement) in the bathing solution and could be mimicked by muscarine. The peak of the current-voltage relationship for the Ca2+ current was not discernibly shifted along the voltage axis by acetylcholine. These results demonstrate that activation of muscarinic receptors not only suppresses a K+ current (M-current), as we have previously demonstrated [Sims, S. M., Singer, J. J. & Walsh, J. V., Jr. (1985) J. Physiol. (London) 367, 503-529], but also increases the magnitude and slows the decay of Ca2+ current.

Characterization of calcium-activated potassium channels in single smooth muscle cells using the patch-clamp technique

Single-channel currents were recorded with the patch-clamp technique from freshly dissociated vertebrate smooth muscle cells from the stomach of Bufo marinus. Of the variety of channels observed, one displayed a large linear conductance of 250 pS (in symmetric 130 mM KCl) which in excised patches was shown to be highly K+ selective. The probability of the channel being open (Po) increased when [Ca2+]i was elevated and/or when the membrane potential was made more positive. Thus, the features of this channel resemble the large-conductance Ca2+-activated K+ channel found in a wide variety of cell types. The voltage sensitivity of the channel was studied in detail. For patches containing a single large-conductance channel a plot of Po versus membrane potential followed the Boltzmann relationship. Increasing [Ca2+]i shifted this plot to the left along the voltage axis to more negative potentials. Both the mean closed time and mean open time varied with potential as a single exponential with almost all of the voltage sensitivity of Po residing in the mean closed time. These results were verified with a series of experiments carried out at low Po (less than 0.1) in patches containing multiple (N) large-conductance channels. Here the ln (NPo) was a linear function of potential with an inverse slope of 9 mV. Almost all of the potential sensitivity lay in the mean closed time the natural log of which was also a linear function of potential with an inverse slope 11 mV in magnitude. The characteristics of this channel as well as the appearance of several of them in almost every patch suggest that they underlie the large peak outward macroscopic current found with whole-cell voltage-clamp studies.

Contractile response and electrophysiological properties in enzymatically dispersed smooth muscle cells of rat vas deferens

Electrophysiological studies were performed on single smooth muscle cells isolated from the vas deferens of the rat. The tissue was preincubated in Ca-free modified Tyrode's solution for 1 h and then transferred to a high-K solution for 1 h. It was next minced and treated with the enzyme solution composed of 600-800 unit/ml collagenase and 40 unit/ml elastase. The procedure yielded about 50% spindle shaped Ca-tolerant cells (100-250 microns in length and about 10 microns in diameter). These cells could contract during the superfusion with the solutions containing 10(-8) to 10(-3) M norepinephrine (NE) or adenosine triphosphate (ATP). The cells isolated from the epididymal portion were more sensitive to norepinephrine than were those from the prostatic part. Their basic electrical properties were studied using tight-seal suction electrode technique. The cells had resting potentials around -40 mV and their input resistance was about 0.8 G omega. Action potentials could be evoked by application of depolarizing current. During whole cell voltage clamp, an inward current followed by an outward current was recorded when 800 ms pulses from a holding potential of -60 mV to test potentials positive than -40 mV were applied. The transient outward current generally recorded in other smooth muscle cells was not seen in these cells. The amplitude of the inward current was Ca dependent and sensitive to a Ca antagonist, nicardipine, indicating that Ca ion is the main carrier of this component of the current. When the pipette was filled with Cs-containing solution, the outward current was abolished.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of major ionic currents in isolated smooth muscle cells using the voltage-clamp technique

Voltage-clamp experiments were carried out on freshly dissociated single vertebrate smooth muscle cells from the stomach muscularis of Bufo marinus. Conventional two-microelectrode methodology was used, thus avoiding rapid dialysis of the cytosol. Four major phases of current were identified upon voltage jumps from negative holding levels to more positive levels. The first phase of current was an initial, inward current. This current was blocked by external Mn2+ and was of the correct magnitude to account for the rising phase of the Ca2+-dependent, TTX-independent action potentials found in these cells. Following this initial, inward Ca2+ current, a large outward current was observed which reached its peak over a period of hundreds of milliseconds and then decayed over a period of seconds to a steady-state level. The peak outward current and the steady-state outward current constitute the second and third major currents. The peak outward current was the largest current observed, with a magnitude as large as tens of nanoamps whereas the inward current was at most about one nanoamp. The peak outward current was reduced more than tenfold in the presence of external TEA. It was also decreased or abolished when the preceding inward current was diminished or eliminated by using external Mn2+ or less negative holding potentials. In this way the peak outward current was identified as a Ca2+-activated K+ current whose slow decay was hypothesized to result from removal of internal Ca ions by cellular mechanisms following the initial rise in [Ca2+]i resulting from the inward current. A fourth major current was an early transient outward current observed most clearly upon voltage jumps to more positive potentials when the inward current was eliminated by using less negative holding potentials or external Mn2+. A classical steady-state inactivation relationship as a function of membrane potential was constructed for the inward current. A substantial portion of this inactivation curve lies at potentials negative to the apparent threshold for activation of inward current, suggesting a true voltage-dependent inactivation. Although additional Ca2+-dependent inactivation could not be ruled out, neither could evidence for it be found.

Inward rectification in freshly isolated single smooth muscle cells of the rabbit jejunum

1. Single smooth muscle cells were obtained by collagenase digestion of longitudinal muscle of rabbit jejunum. Membrane potential or current under voltage clamp was recorded by patch-clamp technique in the whole-cell recording mode at 22-25 degrees C. 2. At a membrane potential of -50 mV small hyperpolarizing current pulses produced electrotonic potentials which asymptotically approached a steady-state size and did not 'sag'. Stronger hyperpolarization resulted in a 'sag' of the electrotonic potential. Under voltage clamp an inward current, i, was activated in the range -60 to -110 mV. 3. This current had an equilibrium potential of -24.5 +/- 3.5 mV which was shifted negatively by reducing the sodium concentration, and positively by raising the potassium concentration of the bathing solution. 4. This current was blocked by caesium (1 mM) but less affected by barium ions up to 10 mM. 5. The time course of i upon stepping into its activation range was accelerated by increasing negativity and following an initial short delay could be described by a single exponential with a time constant in the range 60 s(-60 mV) to 1 s(-130 mV). 6. It is concluded that these jejunal cells possess a current to which sodium and potassium ions make a contribution which is responsible for the inward rectification they show upon hyperpolarization. This current is activated in a range which would allow it to contribute to the slow potential changes shown by longitudinal jejunal muscle.

Potential-dependent calcium inward current in a single isolated smooth muscle cell of the guinea-pig taenia caeci

A single glass micropipette voltage-clamp technique was used to study a potential-dependent calcium inward current in isolated smooth muscle cells of the guinea-pig taenia caeci. Experiments were performed at 22-24 degrees C. With potassium as the main cation in the pipette solution, a transient inward current appeared in response to a depolarizing pulse, followed by an outward current. The replacement of potassium ions by caesium ions and TEA (tetraethyl-ammonium) in the pipette solution resulted in an effective suppression of potassium outward current permitting a study of the calcium current solely. The calcium inward current was blocked by 5 mM-cobalt and 5 X 10(-6) M-verapamil. Activation of the calcium current occurred at a membrane potential of between -35 and -25 mV. The calcium current was maximal in the potential range +10 to +20 mV and did not reverse even at +60 mV. Inactivation of the calcium current had a complex nature. It did not inactivate completely even during depolarizations lasting many seconds. During the first 400 ms the decay of the calcium current followed a time course described by two exponentials. The fast time constant of decay was in the range of 40 to 53 ms (n = 3) and the slow time constant was approximately 10-fold greater (at 0 mV). The fast time constant did not depend on the membrane potential while the slow time constant decreased with depolarization. Availability of the calcium current was estimated in double-pulse experiments. It had a U-shaped dependence on the conditioning potential; maximal inactivation was observed at potentials corresponding to the maximal calcium current. It was suggested that a component of inactivation was dependent on the calcium current which flowed. Calculations of calcium entry at various depolarizations showed that large amounts of calcium ions enter the cell. Also, it was suggested that calcium ions are effectively bound within the smooth muscle cell.

Substance P and acetylcholine both suppress the same K+ current in dissociated smooth muscle cells

The effect of substance P on freshly dissociated gastric smooth muscle cells was examined electrophysiologically. Substance P caused depolarization, associated with a membrane conductance decrease, which led to the generation of action potentials and contraction. When the membrane potential was held constant under voltage clamp, substance P induced a net inward current, also associated with a conductance decrease. The net inward current resulted from suppression of an outward K+ current, one which resembled the acetylcholine-sensitive M-current in these cells. When substance P maximally suppressed this outward K+ current, acetylcholine (ACh) had no additional effect. Conversely, when ACh fully suppressed the M-current, substance P was without additional effect. These results indicate that substance P suppresses the same outward K+ current affected by ACh. Suppression of M-current by substance P was observed in approximately half (44 of 85) of the cells studied in these experiments. In those cells that did not respond to substance P, ACh was nevertheless capable of suppressing the M-current. Thus both substance P and cholinergic agonists appear to exert their excitatory effects on smooth muscle cells by inhibiting a common K+ current.

Isolation and contractile responses of single pregnant rat myometrial cells in short-term primary culture and the effects of pharmacological and electrical stimuli

A modified method for enzymatically isolating myometrial cells from the pregnant rat has been developed and the mechanical properties of single cells in short-term primary culture have been studied in response to various stimuli. The dissociation method produced a high proportion of fully relaxed cells and these cells shortened and subsequently relaxed completely in response to successive applications of acetylcholine, angiotensin II, high K+ solution or depolarizing current. In single cells, the contractions induced by acetylcholine and high K+ solution were concentration-dependent. Maximal contractions were obtained with 135.6 mM K+ and 5 X 10(-4)M acetylcholine. In single myometrial cells, the time course of contractions induced by acetylcholine, high K+ solution or depolarizing current was similar, suggesting that the rate of shortening was determined by limits of the contractile mechanism. Scanning electron microscopy revealed a smooth surface to the relaxed cells which contrasted with the numerous evaginations present on fully contracted cells. These results demonstrate the retention of structural integrity, acetylcholine and angiotensin II receptors, and potential-dependent Ca channels in myometrial single cells in short-term primary culture. Cells produced by this technique may provide a useful model for detailed electrophysiological studies.

Calcium channels of amphibian stomach and mammalian aorta smooth muscle cells

Whole-cell and single-channel calcium currents were studied using single smooth muscle cells enzymatically-isolated from stomach of Amphiuma tridactylum and from guinea-pig aorta. These cells have a high specific resistance and can sustain calcium action potentials after suppression of potassium currents. Dialyzed Amphiuma smooth muscle cells had calcium currents which were stable for several hours whereas the calcium currents of aortic cells ran down quickly. Single channel calcium currents in cell-attached patches behaved similarly for the two cell types. Calcium channel conductance in 110 mM barium was 12 pS and the mean open time was 1.4 ms at a nominal membrane potential of +10 mV. Exposure of both cell types to BAY K8644 resulted in a dramatic prolongation of the calcium channel open times and a shift in the probability of opening to more negative potentials. Low-threshold calcium channels were not identified in the extensively studied amphibian cells. High-threshold calcium channels therefore appear to be the primary pathway for the calcium influx that produces contraction in these smooth muscle cells.

Beta-adrenergic inhibition of bovine mesenteric lymphatics

The beta-action of catecholamines on lymphatic smooth muscle was studied by observing the effect of isoprenaline on electrical and mechanical activity in the double sucrose-gap. Action potentials and phasic contractions evoked by depolarizing pulses were abolished within 2 min of drug addition. Isoprenaline hyperpolarized the membrane and increased membrane conductance. Tetraethylammonium (10 mM) did not itself affect membrane resistance but reduced the hyperpolarization and the increase in conductance caused by isoprenaline. Removal of K+ from the external solution reduced membrane conductance and increased the hyperpolarization due to isoprenaline. When the NaCl content of Krebs solution was replaced with LiCl or choline chloride, isoprenaline no longer blocked action potential firing and its effects on phasic contractions and membrane conductance were reduced. In contrast, ouabain (10(-5) M) did not block the effect of isoprenaline on membrane potential and membrane conductance. These results suggest that beta-adrenergic inhibition of lymphatic smooth muscle involves an increase in an outward K+ current, though an additional metabolic effect cannot be ruled out.

Ca2+-channel current and its modification by the dihydropyridine agonist BAY k 8644 in isolated smooth muscle cells

The electrophysiological properties of single smooth muscle cells isolated from the longitudinal layer of the guinea-pig ileum were studied with the whole-cell patch-clamp technique. The finding of resting potentials between -45 and -50 mV and the occurrence of spontaneous electrical activity when K+ was the predominant intracellular cation indicated that the cells were not leaky or hyperpermeable. The existence of an inward Ca2+ current overlapping in time with an outward rectifying K+ current was demonstrated. The latter could be selectively blocked by replacing internal K+ with Cs+ and external Ca2+ with Ba2+. Depolarizations to potentials between -40 and +50 mV evoked time-dependent inward currents, with a maximum peak value between -20 and 0 mV. For depolarizations beyond +50 mV time-dependent outward currents appeared. These currents were inhibited by 0.1 mM CdCl2. The activation of the inward current showed a sigmoidal time course, and the rate of onset of the current increased at more positive potentials. Inactivation could be described by two exponentials. The threshold for activation was about -40 mV, and full activation was reached at 0 mV. Inactivation was complete near 0 mV, whereas the channels were fully available at -80 mV. The fully-activated Ca2+-channel current was strongly voltage dependent. The conductance decreased for potentials close to the reversal potential, and showed rectification for hyperpolarizing potentials. The Ca2+ agonist BAY k 8644 enhanced the Ca2+-channel current without a significant effect on its kinetics. The fully-activated current and the steady-state activation were enhanced in a rather voltage-independent way.

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

Single-channel studies were made using the patch-clamp technique of K channels in dispersed single smooth muscle cells from rabbit longitudinal jejunal muscle and guinea-pig small (less than 0.2 mm o.d.) mesenteric arteries. In isolated inside-out patches from these two types of smooth muscle cell there was a population of K channels which had single-channel conductances of about 100 pS in near physiological K gradients and about 200 pS with symmetrical 126 mM-K solutions. Their conductance and other properties distinguish them from a K channel of smaller conductance which we have previously described in these cells. The relative permeability of the channel with respect to K was 1.4 Tl:1.0 K:0.7 Rb: less than 0.05 Na: less than 0.05 Cs. Cs (1 mM applied to the outside of the membrane) interfered with inward K movement when the membrane was hyperpolarized. Rb conductance of the channel when both sides of the membrane were exposed to 126 mM-Rb was 30 pS. When the Ca concentration on the inside of the membrane ([Ca]i) was about 10(-9) M, K channel opening was rarely observed and then only at strongly positive potentials. At [Ca]i between 10(-9) M and 10(-7) M mean channel open time increased and the probability of channel opening increased steeply; both were further increased by increasing membrane positivity. At [Ca]i between 10(-6) M and 2.5 mM the channel was mainly in the open state and the probability of channel conducting state often declined with increasing membrane positivity. The effects of varying [Ca]i from 10(-7) M to 2.5 mM on the kinetic activity of a single channel was studied largely in mesenteric artery patches containing one active channel. The distribution of open times could be fitted by a single exponential at low (less than 10(-6) M) [Ca]i but a component of fast openings (to less than 1.0 ms) was observed at all potentials at [Ca]i 2.5 mM. Closed time distribution required the sum of three exponentials to fit it all [Ca]i greater than 10(-7) M; at [Ca]i 10(-6) M or greater evidence of a fourth component, probably caused by Ca block of open channels, was obtained. Raising [Ca]i increased the mean duration of the (long) open state and decreased or had no effect on the duration of short, intermediate, and long mean closed states.

Characterization of a calcium-activated potassium channel from rabbit intestinal smooth muscle incorporated into planar bilayers

Interaction of vesicles from a microsomal fraction of rabbit intestinal smooth muscle with planar bilayers promotes the incorporation of a large conductance potassium-selective channel. The channel conductance fluctuates between two states: closed and open and the fraction of time the channel dwells in the open state is a function of the electric potential difference and the calcium concentrations. This channel seems to correspond to a Ca-activated K channel described by other authors in smooth muscle cells with the patch-clamp technique. Single-channel conductance is a saturating function of the potassium concentration. The relationship between conductance and concentration cannot be described by a hyperbolic function, suggesting multiple occupancy of the channel. The single-channel conductance is 230 pS in symmetrical 0.1 M KCl. Current is a linear function of the applied voltage in the range between -100 and +100 mV, at concentrations of 0.1 M KCl or higher. At lower concentrations, current-to-voltage curves bend symmetrically to the voltage axis. Sodium, lithium and cesium ions do not pass through the channel and the permeability for Rb is 66% that of potassium. All these alkali cations and Ca2+ block the channel in a voltage-dependent manner. A two-site three-barrier model on Eyring absolute reaction rate theory can account for the conduction and blocking characteristics.

Large-conductance Ca2+-activated K+ channels in freshly dissociated smooth muscle cells

Freshly dissociated cells from the stomach muscularis of the toad Bufo marinus have been employed to carry out a systematic set of electrophysiological studies on the membrane properties of smooth muscle. The existence of Ca2+-activated K+ channels became apparent during the first studies under current clamp. In subsequent studies under voltage clamp, a Ca2+-activated. TEA-sensitive outward current was evident, and it was more than an order of magnitude larger than any other current observed in the cells. The channel responsible, at least in part, for this large outward current has been identified on the basis of single-channel records, and some of its main characteristics have been studied. It is similar in many respects to the large-conductance, Ca2+-activated K+ channel seen in other preparations. This channel has now been found in a considerable diversity of smooth muscle types.

Electrophysiological characterization of single pregnant rat myometrial cells in short-term primary culture

Smooth muscle cells were isolated from the longitudinal layer of pregnant rat myometrium (18-19 days) and studied either freshly dissociated or during short-term primary culture (until 30 h) using intracellular microelectrode techniques and direct microscopic observation. The isolated myometrial cells excluded trypan blue vital stain and could repetitively contract in response to various stimuli. Electrophysiological studies at 37 degrees C showed normal resting potential (-54.5 +/- 7.5 mV, n = 71). Action potentials with overshoot (+7.8 +/- 4.6 mV, n = 71) could be elicited by intracellular stimulation. Moreover, the membrane potential was largely dependent on the external K+ concentration. The action potential was suppressed in a Ca2+-free solution [with 0.1 mM ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid], and the overshoot amplitude was clearly Ca2+ dependent. The action potential was inhibited by Mn2+ ions (1 mM), Co2+ ions (1 mM), and D 600 (1 microM) but was unaffected by tetrodotoxin (2 microM) and external Na+ removal. Tetraethylammonium chloride (TEA, 10 mM) and 4-aminopyridine (4-AP, 10 mM) increased both overshoot amplitude and duration of the electrical responses. When the cell surface area was measured with light microscopy, the mean specific membrane resistance was 14.8 +/- 4.6 k omega . cm2 (n = 14), and the mean specific membrane capacitance was 2.3 +/- 0.7 microF/cm2 (n = 14). Outward-going rectification was consistently observed in all cells examined. This was either inhibited by TEA and 4-AP (10 mM each) or reduced in the presence of 1 mM Mn2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Action potentials and net membrane currents of isolated smooth muscle cells (urinary bladder of the guinea-pig)

Cells were isolated by incubating chunks of tissue from the urinary bladder of the guinea-pig in a high potassium, low chloride medium containing 0.2 mM calcium plus the enzymes collagenase and pronase. After isolation, the cells were superfused with a physiological salt solution (PSS) containing 150 mM NaCl, 3.6 mM CaCl2 and 5.4 mM KCl (35 degrees C). Patch electrodes filled with an isotonic KCl-solution were used for whole cell recordings. With a single electrode voltage clamp we measured a capacitance of 50 +/- 5 pF per cell, an input resistance of 200 +/- 25 kOhm X cm2 and a series resistance of 44 +/- 4 Ohm X cm2. The cells had resting potentials of -52 +/- 2 mV. They did not beat spontaneously but responded to stimuli with single action potentials (APs) which rose from the threshold (-38 mV) with a maximal rate of 6.5 +/- 1.8 V/s to an overshoot of 22 +/- 3 mV. The AP lasted for 36 +/- 4 ms (measured between threshold and -40 mV). Continuous cathodal current produced repetitive activity, a pacemaker depolarization followed the AP and preceded the next upstroke. Net membrane currents evoked by clamp steps to positive potentials were composed of an inward and an outward component. The inward component generating the upstroke of the AP was carried by Ca ions (iCa, Klöckner and Isenberg 1985). The repolarization resulted from a potassium outward current iK. Ca-channel blockers (5 mM NiCl2) reduced iK suggesting that (part of) iK was Ca-activated. iK rose within about 100 ms to a peak of 40-200 muA/cm2 from which it inactivated slowly and incompletely. The inactivating iK followed a bell-shaped voltage-dependence, the noninactivating iK an outwardly rectifying one. Both parts had similar steady state inactivation curves with a half maximal inactivation potential at -36 mV and a slope of 9 mV. Repolarization to -50 mV induced outward tail currents which reversed polarity at -85 mV (the calculated potassium equilibrium potential). The amplitude and the time course of the envelope of the tail currents varied in proportion to iK during the prestep. Thus, the tail current is suggested to reflect the turning off of a potassium conductance which had been activated during the prepulse. iK was largely reduced but not blocked by 20 or 150 mM tetraethylammonium (TEA). TEA did not significantly change the resting potential, but it prolonged the AP and facilitated upstroke and overshoot.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium currents of cesium loaded isolated smooth muscle cells (urinary bladder of the guinea pig)

Single smooth muscle cells isolated from the urinary bladder of the guinea-pig were studied at 35 degrees C in a solution composed of 150 mM NaCl, 3.6 mM CaCl2, 1.2 mM MgCl2, 5.4 mM KCl, 20 mM TEA-Cl, 5 mM glucose, 10 mM HEPES/NaOH (pH 7.4). Whole cells were clamped with a single patch electrode. The clamp settled a step from -65 to -5 mV within 260 microseconds, and afterwards the voltage inhomogeneities were less than 2 mV (measured at the cell edge with a second electrode). The calcium inward current iCa was dissected from net currents by blocking potassium outward currents by means of patch electrodes filled with 130 mM CsCl (Klöckner and Isenberg 1985 a). Pyruvate, succinate and oxalacetate in the patch electrode stabilized iCa and prevented its "run down". 140 ms long clamp steps from -65 to -5 mV evoked a net inward current which could be reversibly blocked by 5 mM NiCl2. The "Ni-sensitive" difference current iCa peaked within 2-4 ms to about 1 nA per cell. Afterwards it completely inactivated; the inactivation could be fitted with three exponentials (time constants of 4, 30, and 250 ms, respectively). The half decay time of 16 ms suggests that most of the inactivation resulted from the fast exponential process. The reference current in the presence of Ni was nearly time independent and almost zero; therefore, iCa could be approximated from the net inward current using the zero current as a reference line.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic agonists suppress a potassium current in freshly dissociated smooth muscle cells of the toad

Single micro-electrode voltage-clamp and current-clamp techniques were used to study cholinergic responses in single freshly isolated gastric smooth muscle cells from the toad Bufo marinus. Acetylcholine (ACh) or muscarine caused membrane depolarization, which sometimes gave rise to action potentials and contractions. The agonist-induced depolarization is due to the suppression of a voltage-dependent K+ conductance, a conclusion based on the following observations. Depolarization was accompanied by an apparent membrane conductance decrease, seen as the increased size of voltage deflexions in response to constant current pulses. The conductance decrease was confirmed under voltage clamp, where current deflexions in response to constant voltage jumps were smaller in the presence of cholinergic agonists. Muscarine induced net inward currents at potentials positive to the K+ equilibrium potential (EK), and net outward currents at potentials negative to EK. In experiments where external K+ concentration ([K+]o) ranged from 20 to 90 mM the reversal potentials shifted 58 mV positive per tenfold elevation of [K+]o, as expected for a K+ current. The steady-state current-voltage relationship revealed that the K+ current inhibited by muscarine was larger at more positive potentials than expected from driving force considerations alone. Therefore, the underlying conductance suppressed by cholinergic agonists was voltage dependent, with almost complete deactivation at potentials more negative than approximately -70 mV and exhibiting a sigmoidal activation curve upon depolarization. The deactivation of this voltage-dependent K+ conductance caused slow current relaxations to occur in response to hyperpolarizing voltage commands from depolarized holding potentials. In experiments where [K+]o ranged from 3 to 30 mM, these current relaxations reversed direction at potentials near EK and the reversal potential shifted 52 mV positive per tenfold elevation of [K+]o, indicating that K ions carry most of the charge. The current relaxations that occurred in response to hyperpolarizing voltage commands were suppressed by ACh, muscarine and oxotremorine. The effects of muscarine persisted in nominally Ca2+-free solutions containing Mn2+. Ba2+ mimicked the effects of muscarinic agonists. Thus, isolated smooth muscle cells exhibit a K+ current resembling the M-current of sympathetic and other neurones, which is reversibly suppressed by cholinergic agonists. The existence of a cholinergic K+ conductance decrease is of interest because it has not previously been demonstrated in smooth muscle.