Slow inward current and action potentials of papillary muscles under non-steady state conditions
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The relationship of the contractile response of cat papillary muscles and of the slow inward current, recorded under voltage clamp conditions (single sucrose gap), has been studied. The preparations were driven at a rate of 30 per min at 31° C. Both variables were recorded during a train of 7 identical clamp depolarizations (for 1 s from resting potential to −15 to +40 mV). The contractility increased severalfold and reached the steady state within 5–6 consecutive depolarizations.
The voltage-dependence of slow inward current was confirmed: maximum was found at depolarizations near 0 mV. On repetition of clamp pulses the slow current gradually diminished in amplitude and was more slowly activated and inactivated. The shift of the current-voltage curve indicated a decrease of the reversal potential.
Under non-steady state conditions the amplitude of the slow current was found to correlate closely with the magnitude of the contractile response at any given level of depolarization. The relation was linear with negative slope. The largest contractile response was not found at voltages which elicited maximum slow current.
The progressive decrease of the slow current during repetition of voltage clamp depolarizations is not significantly affected by inadequate time for recovery of slowly changing conductances, since it occurs also at stimulation frequency 15 per min and the slow current remains virtually unaltered after 20 s period of quiescence.
The course of total ionic current during phase 1 and 2 of action potential was reconstructed from a family of current curves obtained as a response to clamp depolarizations to various voltages, respecting the contractility-dependence of the current. The resulting course was correlated with the first derivative of action potential. A general conformity was ascertained.
The correlation of slow inward current with action potential configuration indicates that the rate of its activation determines the depth of the notch separating spike and plateau, its magnitude determines the voltage of the plateau phase and its rate of inactivation affects repolarization.
It is concluded that the described simultaneous changes of mechanical and electrical phenomena might be due to increased [Ca]i, which is responsible for more intense activation of the contractile proteins on the one hand, and decreased driving force of the slow inward current, carried by Ca ions, on the other.
Key wordsCat papillary muscle Action potential Slow inward current Contractility dependence
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- Bassingthwaighte, J. B., Reuter, H.: Calcium movements and excitation-contraction coupling in cardiac cells. In: Electrical phenomena in the heart (W. C. Mello, ed.). New York: Academic Press 1972Google Scholar
- Beeler, G. W., Reuter, H.: Voltage clamp experiments on ventricular myocardial fibres. J. Physiol. (Lond.)207, 169–190 (1970a)Google Scholar
- Beeler, G. W., Reuter, H.: Membrane calcium current in ventricular myocardial fibres. J. Physiol. (Lond.)207, 191–209 (1970b)Google Scholar
- Beeler, G. W., Reuter, H.: The relation between membrane potential, membrane currents and activation of contraction in ventricular myocardial fibres. J. Physiol. (Lond.)207, 211–229 (1970c)Google Scholar
- Bravený, P., Šumbera, J.: Electromechanical correlations in the mammalian heart muscle. Pflügers Arch.319, 36–48 (1970)Google Scholar
- Bravený, P., Šimurdová, M., Šumbera, J.: Effect of epinephrine on the duration of action potential of papillary muscle. Experientia (Basel)30, 166–167 (1974)Google Scholar
- Katz, A. M.: Regulation of cardiac muscle contractility. In: The contractile process. Boston: Little, Brown & Comp. 1967Google Scholar
- McGuigan, J. A. S.: Some limitations of the double sucrose gap and its use in a study of the slow outward current in mammalian ventricular muscle. J. Physiol. (Lond.)240, 775–806 (1974)Google Scholar
- Morad, M., Orkand, R. K.: Excitation-contraction coupling in frog ventricle: Evidence from voltage clamp studies. J. Physiol. (Lond.)219, 167–189 (1971)Google Scholar
- New, W., Trautwein, W.: Inward membrane currents in mammalian myocardium. Pflügers Arch.334, 1–23 (1972a)Google Scholar
- New, W., Trautwein, W.: The ionic nature of slow inward current and its relation to contraction. Pflügers Arch.334, 24–38 (1972b)Google Scholar
- Noble, D., Tsien, R. W.: Outward membrane currents activated in the plateau range of potentials in cardiac Purkinje fibres. J. Physiol. (Lond.)200, 205–231 (1969a)Google Scholar
- Noble, D., Tsien, R. W.: Reconstruction of the repolarization process in cardiac Purkinje fibres based on voltage clamp measurements of membrane current. J. Physiol. (Lond.)200, 233–254 (1969b)Google Scholar
- Reuter, H.: Time- and voltage-dependent contractile responses in mammalian cardiac muscle. Europ. J. Cardiol.1, 177–181 (1973)Google Scholar
- Reuter, H.: Exchange of calcium ions in the mammalian myocardium. Circulat. Res.34, 559–605 (1974)Google Scholar
- Ridgway, E. B., Ashley, C. C.: Calcium transients in single muscle fibres. Biochem. biophys. Res. Commun.29, 229–234 (1967)Google Scholar
- Solaro, R. J., Wise, R. M., Shiner, J. S., Briggs, F. N.: Calcium requirements for cardiac myofibrillar activation. Circulat. Res.34, 525–530 (1974)Google Scholar
- Trautwein, W.: The slow inward current in mammalian myocardium. Europ. J. Cardiol.1, 169–175 (1973)Google Scholar
- Trautwein, W., McDonald, T. F., Tripathi, O.: Calcium conductance and tension in mammalian ventricular muscle. Pflügers Arch.354, 55–74 (1975)Google Scholar
- Tritthart, H., Kaufmann, R., Volkmer, H. P., Bayer, R., Krause, H.: Ca-movement controlling myocardial contractility, I. Pflügers Arch.338, 207–231 (1973)Google Scholar