calcimycin and Arrhythmias--Cardiac

Reperfusion of globally ischemic rat hearts in vitro causes release of inositol(1,4,5) trisphosphate (Ins(1,4,5)P3) which is associated with the development of reperfusion arrhythmias. Both of these responses require the presence of a receptor agonist, either norepinephrine or thrombin, and both responses are inhibited by the aminoglycoside, gentamicin and the polyamine, spermine. In the current study, the role of Ins(1,4,5)P3 in the development of arrhythmias under ischemic conditions was addressed. Arrhythmias [ventricular premature beats, ventricular tachycardia and ventricular fibrillation (VF)] occurring over 25 min subsequent to coronary artery ligation were shown to be independent of endogenous norepinephrine or adrenergic receptor stimulation but were effectively inhibited by gentamicin (0.15-1.5 mM, 95% VF in controls compared with 0% VF, at 1.5 mM, P < 0.01) and spermine (5 mM, 40% VF, P < 0.01). Depletion of Ca2+ stores, including Ins(1,4,5)P3-sensitive Ca2+ stores, with thapsigargin (300 nM) reduced the incidence of ischemic arrhythmias (40% VF, P < 0.01). [3H]-Inositol-labeled right atria incubated under conditions of simulated ischemia retained the ability to respond to norepinephrine by releasing inositol phosphates. Under ischemic conditions, gentamicin (1.5 mM) caused a reduction in [3H]Ins(1,4,5)P3 without any effect on the other inositol phosphates. Similar effects of gentamicin were observed under ischemic conditions in the absence of norepinephrine (95 +/- 8 cpm/mg, mean +/- S.E.M., n = 4, v 29 +/- 4, P < 0.0] for 1.5 mM gentamicin). Agonist independent release of [3H]Ins(1,4,5)P3 under ischemic conditions required extracellular Ca2+ suggesting the operation of a Ca(2+)-activated phospholipase C. In agreement with this, release of [3H]Ins(1,4,5)P3 could be initiated by Ca2+ overload under normoxic conditions and this was inhibited by gentamicin. These findings show that Ca2+ overload can enhance release of Ins(1,4,5)P3 under ischemic conditions and provide evidence that this release is involved in the genesis of arrhythmias under these conditions.

Topics: Animals; Arrhythmias, Cardiac; Calcimycin; Calcium; Gentamicins; Inositol 1,4,5-Trisphosphate; Inositol Phosphates; Ionophores; Male; Myocardial Ischemia; Norepinephrine; Rats; Rats, Sprague-Dawley; Receptors, Adrenergic

1. Effects of calcium ionophores (A23187 and X-537A) on the spontaneously beating sino-atrial (SA) node cells of rabbit heart were examined using electron microscopic and an electrophysiological techniques. 2. During exposure to A23187 or X-537A (2 x 10(-5) M), the cycle length was significantly prolonged by 11% (n = 12) or 118% (n = 11), respectively. But neither ionophore affected other action potential parameters. 3. X-537A (2 x 10(-5) M) induced irregular rhythm (dysrhythmia), probably due to cellular calcium overload. Similarly, ouabain (3 x 10(-7) M) also elicited dysrhythmia. In the presence of isoproterenol (ISP, 10(-7) M), X-537A potentiated dysrhythmia, and A23187 newly induced it. 4. In ultrastructural analyses, X-537A caused swelling of the cisternae of Golgi apparatus within 10 min, whereas A23187 and ouabain did not produce any changes even after 30 min-application. 5. Addition of high Ca2+ (10 mM) and/or ISP (10(-7) M) to X-537A produced a further dilation and vacuolization. In A23187 or ouabain, however, the addition of Ca2+ and ISP did not cause any changes, even during dysrhythmia. 6. These results indicate that X-537A elicited a more potent calcium overload than A23187, and that a discrepancy between ultrastructural damages and electrical changes exists.

Topics: Animals; Arrhythmias, Cardiac; Calcimycin; Electrophysiology; In Vitro Techniques; Isoproterenol; Lasalocid; Microscopy, Electron; Ouabain; Rabbits; Sinoatrial Node

Electrophysiological effects of calcium ionophores, A23187 and X-537A, on spontaneously beating and voltage-clamped rabbit sino-atrial node preparations were examined, using the voltage-clamp technique with two microelectrodes. (1) A23187 (administered cumulatively) increased the cycle length significantly at 3 x 10(-6) and 10(-5) mol/l, and X-537 only at 10(-5) mol/l. Other action potential parameters were unaffected in the presence of these concentrations of either agent. At 2 x 10(-5) mol/l, either agent prolonged the cycle length significantly, but increased the amplitude and the duration of the action potentials and the maximum diastolic potential not to any significant extent. Both X-537A and A23187, at 2 x 10(-5)mol/l, induced a dysrhythmia, which in the former was probably due to delayed afterdepolarizations. (2) In voltage-clamped sino-atrial node cells, the holding current was shifted outwardly, to a greater extent in the presence of X-537A than A23187 at the same concentration (2 x 10(-5) mol/l). The ionophores initially increased the slow inward current and then decreased it. The steady outward current was inhibited, and its activation curve was shifted to a more negative voltage range. X-537A caused a transient inward current and an inward tail current on repolarization to the holding potential. (3) At concentrations of 10 and 18 mmol/l [Ca2+]o or in the presence of isoprenaline 10(-7) mol/l, these ionophores induced a more severe dysrhythmia. Conversely in the nominal absence of [Ca2+]o the regular rhythm was resumed.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Action Potentials; Animals; Arrhythmias, Cardiac; Calcimycin; Calcium; Electrophysiology; In Vitro Techniques; Isoproterenol; Lasalocid; Membrane Potentials; Rabbits; Sinoatrial Node

Intracellular recordings were obtained from spheroidal aggregates of 7-day embryonic chick heart cells after 3 days in gyratory culture. Three types of perturbations in the membrane potential were observed under experimental conditions expected to increase intracellular calcium: 1) multiple oscillations (of 5-20 mV peak-to-peak amplitude) during diastole in aggregates exposed to 10-15 mM Ca, 5 microM strophanthidin, or K-free solutions; 2) less periodic spontaneous voltage fluctuations (of less than 1.5 mV peak-to-peak amplitude) in aggregates exposed to solutions containing 22% of the normal [Na], and 3) depolarizing afterpotentials (DAPs), following repolarization of the action potential, in aggregates treated with 20-50 microM A23187, a Ca ionophore, or 5-10 mM caffeine. The oscillations were reduced markedly by 0.03-3.0 microM tetrodotoxin (TTX) and were blocked by 5-10 mM caffeine. Spontaneous voltage fluctuations were increased by raising external Ca, were unaffected by 30 microM TTX, and were blocked by 5 mM caffeine. DAPs were not blocked by 5 mM caffeine or by 0.1 microM TTX and 1 microgram/ml D 600, concentrations that greatly reduced the action potential upstroke velocity and plateau, respectively. Two intracellular electrodes were employed to test for electrotonic coupling between cells within an aggregate. An electrotonic response in one cell could be recorded when current was injected into another cell during recordings of each of the perturbations but was somewhat less during spontaneous voltage fluctuations. Possible ionic mechanisms for the perturbations and for concomitant changes in the configuration of the action potential are discussed.

Topics: Animals; Arrhythmias, Cardiac; Caffeine; Calcimycin; Calcium; Cells, Cultured; Chick Embryo; Diastole; Electrophysiology; Gallopamil; Heart; Ion Channels; Membrane Potentials; Myocardium; Sodium; Strophanthidin; Tetrodotoxin