Abstract
The inhibition of the pacemaker current (i f) in sheep cardiac Purkinje fibres by ZD 7288 [4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)pyrimidinium chloride] is lost use-dependently. This disinhibition of i f was investigated by using the two-microelectrode voltage-clamp technique. The pulse protocol consisted of a rest period (holding potential of about -50 mV, 1–10 μmol/l ZD 7288) followed by a train of test pulses (potential negative to -100 mV, stimulation frequency 0.05 Hz). At the beginning of the first test pulse there was an immediate reduction of i f but inhibition was lost during continued stimulation. Activation of i f is sigmoidal and the early delay in current activation was prolonged from 33 ms (no ZD 7288) to 424 ms (10 μmol/l ZD 7288). Therefore hardly any disinhibition occurred during short test pulses (0.5 s). During longer test pulses (5 s, -120 mV, 10 μmol/l) disinhibition developed with a time constant of about 2 s. The inhibition of i f by ZD 7288 was lost voltage-dependently. With 10 μmol/l ZD 7288 the half-maximal disinhibition occurred at -92 mV and the slope factor of the disinhibition/voltage curve (Boltzmann relation) was 4.8 mV. The voltage-dependent disinhibition could be abolished largely by extracellular application of protease (0.5 mg/ml, 7 min). After prior disinhibition, reinhibition at the holding potential (about -50 mV) followed a bi-exponential time course indicating that inhibition may be produced by a fast (τ=0.7 min) and a slow component (τ=20–30 min). Increasing ZD 7288 concentration from 1 to 10 μmol/l accelerated reinhibition, mainly by an increase of the amplitude (A) of the fast component. The ratio A fast/A sIow was 0.399 at 1 μmol/l and 2.65 at 10 μmol/1 ZD 7288. The reinhibition of i f was unchanged by shifting the holding potential from -50 mV to -20 mV Trials to wash out the effects of 10 μmol/l ZD 7288 gave two results. The inhibition of i f was slightly reversed after a wash-out of 1.5 h with drug-free solution. A second effect of the drug, the fast reinhibition, could be completely removed by washout. In summary i f is inhibited by ZD 7288 at membrane potentials at which the virtual i f gate is closed. Disinhibition occurs during long-lasting hyperpolarization but will hardly be operative in unclamped fibres under physiological conditions.
Similar content being viewed by others
References
Aronson RS, Gelles JM, Hoffman BF (1973) A new method for producing short cardiac Purkinje fibers suitable for voltage clamp. J Appl Physiol 34:527–530
Berger F, Borchard U, Hafner D, Kammer T, Weis T (1991) Inhibition of potassium outward currents and pacemaker current in sheep cardiac Purkinje fibres by the verapamil derivative YS 035. Naunyn-Schrniedeberg's Arch Pharmacol 344:653–661
Berger F, Borchard U, Gelhaar R, Hafner D, Weis T (1994) Effects of bradycardiac agent ZD 7288 on membrane voltage and pacemaker current in sheep cardiac Purkinje fibres. Naunyn-Schmiedeberg's Arch Pharmacol 350:677–684
BoSmith RE, Briggs I, Sturgess NC (1993) Inhibitory actions of ZENECA ZD 7288 on whole-cell hyperpolarization activated inward current (if) in guinea-pig dissociated sinoatrial node cells. Br J Pharmacol 110:343–349
Briggs I, BoSmith RE, Heapy CG (1994) Effect of Zeneca ZD 7288 in comparison with alinidine and UL-FS 49 on guinea-pig sinoatrial node and ventricular action potentials. J Cardiovasc Pharmacol 24:380–387
Callewaert G, Carmeliet E, Vereecke J (1984) Single cardiac Purkinje cells: general electrophysiology and voltage-clamp analysis of the pace-maker current. J Physiol (Lond) 349:643–661
Deck KA, Kern R, Trautwein W (1964) Voltage clamp technique in mammalian cardiac fibres. Pflügers Arch 280:50–62
Denyer JC, Brown HF (1990) Pacemaking in rabbit isolated sino-atrial node cells during Cs+ block of the hyperpolarization-activated current if. J Physiol (Lond) 429:401–409
DiFrancesco D (1981) A new interpretation of the pace-maker current in calf Purkinje fibres. J Physiol (Lond) 314:359–376
DiFrancesco D (1986) Characterization of single pacemaker channels in cardiac sino-atrial node cells. Nature 324:470–473
DiFrancesco D (1993) Pacemaker mechanisms in cardiac tissue. Annu Rev Physiol 55:455–472
DiFrancesco D, Ferroni A, Mazzanti M, Tromba C (1986) Properties of the hyperpolarizing-activated current (if) in cells isolated from the rabbit sino-atrial node. J Physiol (Lond) 377:61–88
Frace AM, Maruoka F, Noma A (1992) Control of the hyperpolarization-activated cation current by external anions in rabbit sino-atrial node cells. J Physiol (Lond) 453:307–318
Glitsch HG, Pusch H, Verdonck F (1986) The contribution of Na and K ions to the pacemaker current in sheep cardiac Purkinje fibres. Pflügers Arch 406:464–471
Goethals M, Raes A, Van Bogaert PP (1993) Use-dependent block of the pacemaker current if in rabbit sinoatrial node cells by zatebradine (UL-FS 49). Circulation 88:2389–2401
Hagiwara N, Irisawa H (1989) Modulation by intracellular Ca2+ of the hyperpolarization-activated inward current in rabbit single sino-atrial node cells. J Physiol (Lond) 409:121–141
Hondghem LM, Katzung BG (1984) Antiarrhythmic agents: the modulated receptor mechanism of action of sodium and calcium channel-blocking drugs. Annu Rev Pharmacol Toxicol 24:387–423
Marshall PW, Rouse W, Briggs J, Hargreaves RB, Mills SD, McLoughlin BJ (1993) ICI D 7288, a novel sinoatrial node modulator. J Cardiovasc Pharmacol 21:902–906
Noble D, Tsien RW (1968) The kinetics and rectifier properties of the slow potassium current in cardiac Purkinje fibres. J Physiol (Lond) 195:185–214
Snyders DJ, Van Bogaert P-P (1987) Alinidine modifies the pacemaker current in sheep Purkinje fibers. Pflügers Arch 410:83–91
Thuringer D, Lauribe P, Escande D (1992) A hyperpolarization-activated inward current in human myocardial cells. J Mol Cell Cardiol 24:451–455
Van Bogaert PP, Goethals M (1992) Blockade of the pacemaker current by intracellular application of UL-FS 49 and UL-AH 99 in sheep cardiac Purkinje fibers. Eur J Pharmacol 229:55–62
Van Bogaert PP, Goethals M, Simoens C (1990) Use- and frequency-dependent blockade by UL-FS 49 of the if pacemaker current in sheep cardiac Purkinje fibres. Eur J Pharmacol 187:241–256
Van Ginneken ACG, Giles W (1991) Voltage clamp measurements of the hyperpolarization-activated inward current if in single cells from rabbit sino-atrial node. J Physiol (Lond) 434:57–83
Yanagihara K, Irisawa H (1980) Inward current activated during hyperpolarization in the rabbit sino-atrial node cell. Pflügers Arch 385:11–19
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Berger, F., Borchard, U., Gelhaar, R. et al. Inhibition of pacemaker current by the bradycardic agent ZD 7288 is lost use-dependently in sheep cardiac Purkinje fibres. Naunyn-Schmiedeberg's Arch Pharmacol 353, 64–72 (1995). https://doi.org/10.1007/BF00168917
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF00168917