Voltage clamping of smooth muscle from Taenia coli

Patch and whole-cell voltage clamp of single mammalian visceral and vascular smooth muscle cells

Dispersal of the constituent cells of mammalian visceral and vascular smooth muscles has permitted recordings both of membrane currents under whole-cell voltage clamp, and of currents through single ionic channels using the patch-clamp technique. A rectangular depolarizing step applied to a single cell under voltage clamp yielded a net inward current followed by a net outward current in normal physiological solution. In isolated, 'inside-out' patches of cell membrane a calcium- and potential-sensitive K channel (100 pS conductance) and a calcium-insensitive, potential-sensitive K+ channel (50 pS conductance) with slow kinetics have so far been identified and characterized.

Single Ca channel currents in mammalian visceral smooth muscle cells

Recordings of single Ca channel currents in mammalian visceral smooth muscle cells were obtained using patch clamp techniques. Smooth muscle cells from guinea-pig taenia coli were prepared by enzymatic dispersion using 0.3% collagenase. The recordings were obtained from cell-attached membrane patches of isolated cells with a pipette filled with isotonic 50 mM Ba2+. When the membrane patch was depolarized, brief inward current pulses of unitary size and small amplitude were observed. The amplitude of these single channel currents decreased linearly with increasing depolarization in a voltage range from -20 mV to +50 mV about the resting potential. The slope conductance was estimated to be about 30 pS. The mean current reconstructed by averaging individual current responses showed kinetic behaviour with a rapid activation and a slower inactivation process similar to the macroscopic Ca2+ current observed in strips of guinea-pig taenia coli. The present study suggests that the inward current pulses of unitary size induced by voltage-clamp pulses were due to Ba ions passing through a single voltage dependent Ca channel.

Effects of conditioning polarization on the membrane ionic currents in rat myometrium

Membrane ionic currents were measured in pregnant rat uterine smooth muscle under voltage clamp conditions by utilizing the double sucrose gap method, and the effects of conditioning pre-pulses on these currents were investigated. With depolarizing pulses, the early inward current was followed by a late outward current. Cobalt (1 mM) abolished the inward current and did not affect the late outward current per se, but produced changes in the current pattern, suggesting that the inward current overlaps with the initial part of the late outward current. After correction for this overlap, the inward current reached is maximum at about +10 mV and its reversal potential was estimated to be +62 mV. Tetraethylammonium (TEA) suppressed the outward currents and increased the apparent inward current. The increase in the inward current by TEA thus could be due to a suppression of the outward current. The reversal potential for the outward current was estimated to be -87 mV. Conditioning depolarization and hyperpolarization both produced a decrease in the inward current. Complete depolarization block occurred at membrane potential of -20 mV. Conditioning hyperpolarization experiments in the presence of cobalt and/or TEA revealed that the decrease in the inward current caused by conditioning hyperpolarization was a result of an increase in the outward current overlaping with the inward current. It appears that a part of the potassium channel population is inactivated at the resting membrane potential and that this inactivation is removed by hyperpolarization.

Voltage-clamp analysis of transmembrane ionic currents in guinea-pig myometrium: evidence for an initial potassium activation triggered by calcium influx

Voltage-clamp analysis of ionic transmembrane currents in very small strands of guinea-pig myometrium was carried out with a double sucrose-gap technique. It was found that the electrical activity, consisting of a spike followed by a long plateau, is controlled by, at least, four ionic conductances. (1) A fast inward current is responsible for the spike generation. Its low equilibrium potential accounts, partly, for the low amplitude of the spike. (2) The fast inward current is antagonized by an early outward current which occurs almost simultaneously. This fast outward current is blocked by TEA. Its reversal potential is about -95 mV. A tenfold increase in the external K-concentration shifts the reversal potential by 50 mV. Thus, it is concluded that the initial outward current is carried by K+. (3) A slow current, whose reversal potential ranges from -40 to -10 mV, is responsible for the negative after-potential. Cl-depletion (to one-ninth) does not modify this current while Na-depletion (to one-ninth) decreases its reversal potential by about 20 mV. (4) A late current which shows delayed rectification is elicited by long pulses. Its analysis is made difficult by the change mainly of the K-equilibrium potential suggesting accumulation of K+ outside the cell membrane. (5) The availability of the inward current and of the slow current, determined in TEA solution, shows that both currents are half-inactivated by a 8 mV conditioning depolarization. Using a slope factor of -2-5 or -3 the availability curve fits the experimental values. In normal solution, the availability curve of the initial current appears complex in the hyperpolarization range. The fast outward current, which is partly inactivated at the resting potential, is restored by conditioning hyperpolarization and then antagonizes the Ca inward current more. (6) It is concluded that the fast K-current controls the spike generation and accounts for the fast repolarization of the spike. The fast and transient increase in K-conductance may be the result of a momentary local increase in Ca concentration at the internal surface of the membrane.

Effects of stimulating the acetylcholine receptor on the current-voltage relationships of the smooth muscle membrane studied by voltage clamp of potential recorded by micro-electrode

1. A double sucrose-gap voltage-clamp technique is described for use on smooth muscle strips longer than about 2 mm. It involves intracellular recording by microelectrode of the membrane potential of a narrow region of the strip ("node") sandwiched between two streams of deionized sucrose solution. Current was passed into the node across one or both sucrose streams. 2. Preliminary experiments in which potential was recorded intracellularly at two points during polarization of a "short cable" preparation, formed by folding over a strip of smooth muscle, suggested that a node width of less than 0-15 mm was needed to achieve uniform potential during inward current flow. However, when node width between sucrose-gaps was reduced to 0-5 mm, spontaneous electrical activity was lost, and below 0-5 mm spike threshold was raised and the regenerative spike became graded. The currents flowing during the application of rectangular voltage-clamp command potentials were described. 3. Using taenia smooth muscle it was shown by recording with a second, independent micro-electrode that potential was not uniform for up to 200 ms or more following a step change in potential under voltage-clamp in nodes 0-4-0-5 mm wide where current was passed across both sucrose gaps. However, reasonably uniform nodal potentials were obtained using ramps with relatively slow rates of rise (25 mV/s). 4. Using such slow ramp commands under voltage clamp, the effects of carbachol on the current-voltage relationship of longitudinal muscle of ileum and taenia were studied in hypertonic solution. 5. In the presence of carbachol (10(-6) to 10(-5) g/ml.) additional inward current flowed across the membrane (in some experiments an equilibrium potential was observed at which this current reversed direction). The magnitude of this additional current was linearly related to potential at potentials negative to the resting potential. At potentials positive to the resting membrane potential, this additional current increased with depolarization over the range -40 to -10 mV; in ileum the effect of this additional inward current on the current-voltage relationship was to produce a region of net inward current where before, in the absence of carbachol, a net outward current existed. In taenia the additional inward current flowing in the presence of carbachol was too small to produce a region of net inward current; thus carbachol produced regenerative slow oscillations of potential (slow waves) in ileum but not in taenia. 6. These results support a previous suggestion that activation of the acetylcholine receptor of ileal smooth muscle produces an additional inward current in the membrane which increases with depolarization and is responsible for the regenerative slow waves seen when muscarinic stimulants are applied. A similar effect apparently operates in taenia but the additional inward current is too small to produce regenerative slow waves.

A study of pace-maker activity in intestinal smooth muscle

1. Electrical activity of longitudinal muscle from cat intestine was recorded in the double sucrose gap.2. Approximately 20% of the preparations demonstrated slow, spontaneous fluctuations of membrane voltage, slow waves. This activity, although quite uniform in a given preparation, showed considerable inter-preparation variation with respect to amplitude, frequency and wave form.3. Application of steady hyperpolarizing current decreased slow-wave frequency and increased slow-wave amplitude while depolarizing currents increased frequency and decreased amplitude.4. Some preparations with no spontaneous slow-wave activity developed slow waves when the membrane was hyperpolarized into a given range which, depending on the preparation, varied in size from 10 to 40 mV. Step or ramp depolarization of the membrane from hyperpolarized levels triggered slow waves in some preparations.5. When the membrane potential of a slow-wave generating preparation was clamped at the resting potential, spontaneous inward-directed current transients were observed.6. No changes in membrane conductance were observed during the course of a slow wave.7. The slow-wave pattern was simulated for individual preparations by applying the membrane current measured under voltage clamp to the passive membrane resistance and capacitance measured independently under current clamp.8. In addition to the defined slow-wave activity, voltage-dependent oscillations in membrane potential were sometimes observed.9. Application of 10(-5)M ouabain irreversibly blocked slow waves and produced a membrane depolarization equal to or slightly greater than the slow wave crest. Repolarization of the membrane to the resting potential, or hyperpolarization, failed to restore slow-wave activity.10. Removal of external potassium produced a reversible sequence of events almost identical to those following ouabain application.11. Replacement of 50% of the external sodium chloride with sucrose produced no changes in slow-wave activity with respect to rates of rise or fall, maximum amplitude or frequency. Sucrose replacement of all external sodium chloride eliminated slow waves after 5 min; however, activity could be restored by a slight hyperpolarization. Longer exposures to the modified bath abolished activity.12. Following a conditioning exposure to potassium-free Krebs solution, readmission of potassium at normal concentration produced a mean hyperpolarization of 20.5 mV and in spontaneous preparations an arrest of activity.13. Pump current in sodium-loaded, non-spontaneously active preparations was measured by voltage clamp and was observed to be voltage-dependent.14. The results of this study indicate that an electrogenic pump is present in longitudinal muscle of cat duodenum, and that oscillations in the level of pump current produce slow waves.

Studies on calcium and sodium in uterine smooth muscle excitation under current-clamp and voltage-clamp conditions

The objective of these studies was to define the roles of calcium and sodium in uterine smooth muscle excitation. The double sucrose-gap technique was used for current-clamp and voltage-clamp experiments. It was shown that neither sodium nor calcium alone is capable of supporting excitation in estrogen-dominated uterine smooth muscle. Calcium dependence was explained in part by increased membrane "leakage" current in calcium-free solution and calcium control of the voltage dependence of the early transient conductance. High concentrations of TTX did not affect the magnitude of the peak transient current while La(+++), Mn(++), and Co(++) greatly reduced or abolished it and decreased the steady-state current. From these and other data it was concluded that the regenerative mechanism in uterine smooth muscle has the functional characteristics of a single transient conductance channel whose activation requires the presence of both sodium and calcium. Insensitivity to TTX indicates that the molecular structure of the channel is unlike that in certain sodium-dependent systems, while the effects of La(+++), Mn(++), Co(++), and Ca(++) reveal a similar dependence of conductances on extracellular polyvalent cations.

Electrogenic sodium pump in smooth muscle cells of the guinea-pig's taenia coli

1. The changes of the membrane potential, of the K equilibrium potential, and of the membrane conductance during K accumulation by K-depleted tissues have been studied. Three subsequent characteristic periods can be described.2. Readmission of 5.9 mM-K after complete depletion results in a rapid extrusion of Na and uptake of K, and in a rapid hyperpolarization of the cells. Initially the time course of the K equilibrium potential and the membrane potential are similar except in propionate solution. This initial period is characterized by a high membrane conductance. No change of membrane potential occurs if 10(-5)M ouabain is present.3. After 5-7 min the membrane potential becomes more negative than the K equilibrium potential. The difference between both values is larger in solutions containing propionate or in hypertonic solutions. This second phase of the recovery period is characterized by a progressive decrease of the membrane conductance.4. In a third phase both the membrane potential and the membrane resistance return to their steady-state value.5. If the external K concentration in the recovery solution is increased, the maximal hyperpolarization is less and has a shorter duration. A decrease of the temperature of the recovery solution results in a slower initial rate of repolarization and in a decrease of the maximal value of the hyperpolarization.6. These observations demonstrate the existence of an electrogenic sodium pump in smooth muscle cells during stimulation of the Na pump. An analysis of the experimental data obtained under steady-state conditions in normal Krebs solution suggests that also under these conditions an electrogenic Na pump might take part in the maintenance of the resting potential.