Cellular mechanism of the dependence of cardiotonic action of digoxin on the degree of ischemic damage to the myocardium Academic Article uri icon

abstract

  • The purpose of this investigation was a complex analysis of the influence of ischemia on changes in the activity of the sarcolemmal Na,K-ATPase and electromechanical coupling in the papillary muscle (PM) of the guinea pig under the action of digoxin. The selected model made it possible to dose out the degree of ischemic damage to the myocardium with minimal dispersions in experiments of the same series, to record the change in the Na,K-ATPase activity as disorders of the functions of the myocardium increase in stages under the influence of ischemia, and to quantitatively assess the change in the functional state precisely of the region of the heart in which the Na,K-ATPase activity is controlled. EXPERIMENTAL The experiments were conducted on guinea pigs weighing 200-250 g. The preparation used was PM with a length of 3-4 mm and a diameter of 1-1.5 mm from the left ventricle of the intact and the ischemically damaged heart. To damage the myocardium by total global ischemia, the isolated heart was placed in a dry hermetically closed box at a temperature of 37~ for various periods of time. The contractions of the PM were recorded in a system close to isometric, with the aid of a 6MXIC mechanotron, and the intracellular potentials were recorded with microelectrodes. The indicated standard procedures were described in detail previously [4]. A fragment of myocardium was continuously perfused with Tyrode's solution of the following composition (mM): NaCI, 121.5; KCI, 4.5; NaHC03, 20; NaH2P04, i.i; MgCI2, 0.25; CaCI 2, 2.16; glucose, ii; pH 7.2-7.4. Temperature 28  0.5~ In all the series of experiments, the dependence of the value of the established contractions on the rhythm of stimulation (0.i, 0.3, 0.5, 0.7, and 1 Hz) was recorded. In statistical treatment, the values of all the parameters of the contractile responses were usually normalized to the values of the corresponding parameters at a frequency of stimulation of 0.5 Hz. For a determination of the Na,K-ATPase activity, the fraction of the sarcolemma of the left ventricles was isolated at a temperature of 2-4~ in the following way: tissue of the left ventricles (2 g) was homogenized in eight volumes of the isolation medium (IM) containing 0.25 M sucrose, 1 mM EDTA, I mM ATP, 20 mM Tris-HCl (pH 7.4), with a blender of the Polytron type (three times for periods of 20-sec, 30-sec intervals). The homogenate was centrifuged at 2800g for 15 min. The precipitate was washed three times in the IM under the same conditions of centrifugation, suspended in double-distilled water, and treated with sodium iodide for 1 h according to the method of [10]. The NaI extract was centrifuged at 10,000g for I0 min, the supernatant precipitated at 45,000g for 30 min. The precipitate was washed three times with 1 mM EDTA (pH 7.4), suspended in a medium containing 0.25 M sucrose and 30 mM histidine (pH 7.2), and stored at a temperature of -150C. The total ATPase activity was determined according to the accumulation of P [ii] in a medium of the~.following composition (mM): NaCI, 10% KCI, 20; MgCI2, 5; Na2ATP, 3; TrisHCI, 50; pH 7.4 at a temperature of 37~ The Na,K-ATPase activity was calculated according

publication date

  • January 1, 1989