Voltage Clamp Studies of the Slow Inward Current

A long lasting Ca2+-activated outward current in guinea-pig atrial myocytes

Among other characteristics, the steady-state current-voltage relationship of patch-clamped single atrial myocytes from guinea-pig hearts is defined by an outward current hump in the potential region -15 to +40 mV. This hump was reversibly suppressed by Co2+ (3 mM) or nitrendipine (5 microM) and enhanced by Bay K 8644 (5 microM). The maintained outward current component suppressed by Co2+ extended between -15.2 +/- 1.9 mV and +39.5 +/- 1.7 mV (mean +/- SEM of 14 cells) and has an amplitude of 95.7 +/- 9.4 pA at +10 mV. In isochronal I-V curves, the hump was already visible at 400 ms with essentially the same amplitude as at 1500 ms. The Co2+-sensitive outward current underlying the hump was poorly time-dependent during 1.5 s voltage pulses but slowly relaxed upon repolarization. Tail currents reversed near the K+ equilibrium potential under our experimental conditions. The current hump of the steady-state I-V curve was also abolished by caffeine (10 mM) or ryanodine (3 microM), both drugs that interfere with sarcoplasmic reticulum function. Apamin (1 microM) or quinine (100 microM) but not TEA (5-50 mM) markedly reduced its amplitude. However, at similar concentrations as required to inhibit the hump, both apamin and quinine appeared to be poorly specific for Ca2+-activated K+ currents in heart cells since they also inhibited the L-Type Ca2+ current. It is concluded that a long lasting Ca2+-activated outward current, probably mainly carried by K+ ions but not sensitive to TEA, exists in atrial myocytes which is responsible for the current hump of the background I-V curve.

Currents through ionic channels in multicellular cardiac tissue and single heart cells

Ionic channels are elementary excitable elements in the cell membranes of heart and other tissues. They produce and transduce electrical signals. After decades of trouble with quantitative interpretation of voltage-clamp data from multicellular heart tissue, due to its morphological complexness and methodological limitations, cardiac electrophysiologists have developed new techniques for better control of membrane potential and of the ionic and metabolic environment on both sides of the plasma membrane, by the use of single heart cells. Direct recordings of the behavior of single ionic channels have become possible by using the patch-clamp technique, which was developed simultaneously. Biochemists have made excellent progress in purifying and characterizing ionic channel proteins, and there has been initial success in reconstituting some partially purified channels into lipid bilayers, where their function can be studied.

On the relationship between V max of slow responses and Ca-current availability in whole-cell clamped guinea pig heart cells

The relationship between Ca current availability and maximum rate of rise (V max) of slow responses was determined in the same single guinea pig ventricular heart cell under voltage and current clamp conditions (whole-cell clamp technique). The results are as follows. (1) Cell capacitance measured in 32 cells from the current response to a fast ramp voltage-clamp pulse (119.6 +/- 4.6 pF, mean +/- SE) or from Vmax values at a holding potential of -50 or -40 mV (118.6 +/- 5.3 pF) are identical. (2) In control conditions ([Ca]o 1.8, [K]o 4 and [Cs]i 140 mM), voltage-dependence of steady-state inactivation of Ca current (ICa) or Vmax are similar up to -35 mV. However, Vmax significantly (P less than 0.005) underestimates ICa availability at more positive potentials. At -30 mV, ICa and Vmax amplitudes represent respectively 35.6 and 22.4% (n = 14) of their maximum value. (3) In the presence of 50 nM isoprenaline, Vmax and the underlying ICa are respectively increased by 79.2 +/- 13.8% and 71.2 +/- 13.8% (n = 15). No statistically significant deviation from linearity is then observed. (4) When Vmax amplitude is expressed as a function of ICa density, an almost linear relationship is observed for Vmax values between 0 and 25 V/s. Vmax is then best described by the equation: Vmax (V/s) = 1.043 ICa (pA/pF) -0.514 (46 cells). (5) We conclude that, under conditions that minimize outward currents, Vmax of slow responses accurately measures ICa amplitude, except when ICa is decreased to less than 40% of its maximum control amplitude (i.e., below 4 pA/pF). At that point, Vmax underestimates ICa.

The time course of action potential repolarization affects delayed afterdepolarization amplitude in atrial fibers of the canine coronary sinus

The effects of changing the time course of action potential repolarization on the amplitude and coupling interval of delayed afterdepolarizations were studied in small preparations of coronary sinus atrial fibers exposed to catecholamines. Repolarization was accelerated or retarded by current pulses passed through an intracellular microelectrode in the depolarizing or repolarizing direction. Acceleration of repolarization decreased the amplitude of delayed afterdepolarizations, prolonged their coupling interval to the action potential upstroke, and prevented triggered activity. Prolonging the time for repolarization increased afterdepolarization amplitude, shortened the coupling interval, and caused triggered activity. The afterdepolarization amplitude and coupling interval had a linear relationship to the duration of the action potential plateau. In some preparations, the action potential plateau increased spontaneously at stimulation rates that caused afterdepolarization amplitude to increase and triggering to occur, and this may have contributed to the occurrence of triggering. The effects of action potential repolarization on delayed afterdepolarizations suggest that pharmacological agents such as antiarrhythmic drugs which alter action potential duration should influence afterdepolarizations. Drugs which shorten action potential duration might prevent triggered activity from occurring, whereas drugs which prolong duration might cause triggering.

The effects of Ni2+ on ionic currents and tension generation in frog ventricular muscle

The effect of Ni2+ on E-C coupling events of the frog ventricular muscle were studied using a single sucrose gap voltage clamp technique. The results showed that Ni2+ increased the overshoot potential and depressed and prolonged the plateau of the action potential. Ni2+ also increased the dependence of the overshoot potential on [Na]0 from 18 to 58 mV per decade. In the presence of Ni2+, TTX blocked both the upstroke and the plateau of the action potential. The combination of TTX and Ni2+ suppressed the tension-voltage relation, the time-dependent outward currents and K+ efflux. While suppression of the tension-voltage relation by Ni2+ alone was reversed by increasing [Ca]0, the effects of Ni2+ plus TTX are not reversed by addition of Ca2+. The results suggest that Ni2+ may alter the action potential by slowing the inactivation of the Na+ current and blocking the inward Ca2+ current. Although the tension-suppressant effects of Ni2+ could be attributed to the inhibition of a slowly inactivating Ca2+ current, the effects of Ni2+ in the presence of TTX were less readily explained. Several possible mechanisms are considered which are all consistent with the hypothesis that development of tension in ventricular strips is mediated by both a Ca2+ current and a Ca2+ counter-transport system.

Excitation-contraction coupling in frog ventricle. Possible Ca2+ transport mechanisms

In frog ventricular muscle generation of tension was found to be under direct and continuous control of membrane potential. No phasic component of tension was found at any membrane potential. Developed tension depended only on the duration and amplitude of depolarization and was independent of previous contractile history. Developed tension, in part, depended on Ca2+ influx through a slowly inactivating component of Isi. Using long voltage clamp steps to achieve steady-state tension, no decline or reversal of developed tension was found at ECa. Increasing the [Ca]o shifted the tension-voltage relation to more negative potentials and increased the net outward current at potentials positive to -10 mV. The increase in tension seemed to be related to the increase in outward membrane current and K+ efflux, as estimated from post clamp K+ accumulations. Increasing [K]o, either by clamp-induced K+-accumulation or by increasing the [K] of the bathing solution, decreased the developed tension. These results suggest that in frog ventricular muscle Ca2+ for activation of tension is transported primarily from the extracellular space. There was no trigger-release of internal stores or recirculation of sequestered Ca2+. Activator Ca2+ was transported in part by a slowly inactivating Isi channel and a coupled transport mechanism. The exact mechanism by which Ca2+ transport and K+ efflux were related could not be identified.

Voltage- and frequency-dependent block of diltiazem on the slow inward current and generation of tension in frog ventricular muscle

The effects of a new "Ca2+ -antagonist", diltiazem, were studied in frog ventricular myocardium. The single sucrose gap voltage clamp technique was used to control membrane potential and measure membrane current and tension. Diltiazem had no effect on the resting potential, but reduced the slow inward current (Isi) and increased the net outward current. Membrane conductance measurements suggest that the increase in the total membrane current may be due to the suppression of a maintained inward current. The blocking action of diltiazem is frequency-dependent such that at higher frequencies of stimulation, the steady-state amplitude of twitch is reduced. The diltiazem-induced suppression of tension and Isi could be partially reversed by hyperpolarizing the surface membrane for brief intervals. These results suggest that diltiazem blocks Isi in a voltage- and time-dependent manner. These effects of diltiazem on Isi seem to be, in part, responsible for the tension-suppressant effect of the drug.

The discontinuous nature of electrical propagation in cardiac muscle. Consideration of a quantitative model incorporating the membrane ionic properties and structural complexities. The ALZA distinguished lecture

The propagation of excitation in cardiac muscle has generally been treated as though it occurred in a continuous structure. However, new evidence indicates that propagation in cardiac muscle often displays a discontinuous nature. In this paper, we consider the hypothesis that this previously unrecognized type of propagation is caused by recurrent discontinuities of effective axial resistivity which affect the membrane currents. The major implication is that the combination of discontinuities of axial resistivity at several size scales can produce most currently known cardiac conduction disturbances previously though to require spatial nonuniformities of the membrane properties. At present there is no appropriate model or simulation for propagation in anisotropic cardiac muscle. However, the recent quantitative description of the fast sodium current in voltage-clamped cardiac muscle membrane makes it possible, for the first time, to apply experimentally based quantitative membrane models to propagation in cardiac muscle. The major task now is to account for the functional role of the structural complexities of cardiac muscle. The importance of such a model is that it would establish how the membrane ionic currents and the complexities of cell and tissue structure interact to determine propagation in both normal and abnormal cardiac muscle.