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Effects of oxidant stress on steady-state background currents in isolated ventricular cells of rabbit heart
J Mol
pW55
Cell
Cardiol
22 (Supplement
III)
(1990)
THE EFFECT OF HYPOTHERMIA ON CARDIAC MUSCLE DURING THE CALCIUM PARADOX Mohammed-Saadeh Suleiman & R. A. Chapman. Department of Physiology, Bristol University, School of Veterinary Science, Park Row, Bristol BSl 5LS. Hypothermia is known to protect cardiac tissue from the Ca-paradox. This effect would seem to conflict with the notion that a rise in [Na]i predisposes the heart to the Ca paradox. Changes in [Na]i (measured using ion selective microelectrcde) membrane potential and contracture were monitered in the isolated ferret ventricular trabeculae during the Ca-paradox, at four temperatures ; 37. 30. 20 8 10°C. Cooling protects against the Caparadox by slowing down the rise in [Na]i during Ca-depletion. This effect is time dependent, where the steady state value for [Na]i at 20 & 30°C reached similar levels. However, at 10°C [Na]i rose to higher levels and subsequently the damaging effect of the paradox was stronger. The reason for this was found to be the complete inhibition of the Na-pump at 1 OoC. The inhibition of the pump with strophanthidin at 20, 30 & 37OC yield an estimate of the energy of activation at 74KJmole.t . This is higher than that of the Na-pump, thus explaining the initial protective effect of hypothermia. However the observation that a Pmin pulse at 30°C followed by cooling to 20°C during Ca-depletion which resulted in a greater rise in [Na]i cannot be simply explained by a temperature effect only on the influx of Na ions. An additional effect of temperature on the loss of ionic selectivity of the 1. type Ca-channels is suggested. This work provides further evidence for the hypothesis that the damaging effect of the paradox is determined by the degree of rise in [Na]i which is the outcome of the race between the Na ~nfiux through the L-type Ca-channels and the efflux via the Na-pump.
PW56ST;WT;;
THE PATHOGENESIS OF THE CARDIAC DYSFUNCTION IN DIABETES : ROLE OF MYOINOSITOL ON THE MEMBRANE TRANSPORT OF CATIONS. Hideaki Suzuki, Mahito Kawamura, Masahiro Kawanishi, Makoto Nagano. Department of Internal Mediciny,Avto Hosp.,Jikei University,TqkyoZ+Japan Depressed activity of Na -K ATPase, Ca2+ ATPase and Na -Ca exchange of myocardial membrane were thought to be the cause of cardiac dysfunction of experimental diabetes. It is claimed that the impaired transport of myoinositol( MI ) might be the patogenesis of diabetic angiopathy. Phosphatidylinositol( PI ) which is sensitive to the ex$ra+ cellulat MI was demonstrated to be involved in thy re ulation of Na -K 9+ ATPase. Involvement f PI in the regulation of Na -Ca exchange of sarS+ colemma( SL ) and Ca transport ATPase of sarcoplasmic reticulum( SR ) were also reported. For this reason effects of MI administration on the Na+-K+ ATPase activity and Na+-Ca2+exchange of SL and Ca2+ uptake of SR were studied in streptozotocin diabetic rats. Depressed Na+-K+ ATPase activity of diabetic SL was restored to the normal value by the administration of MI. Decreased Na+-Ca2+exchange of SL and Ca2+ uptake of SR were not recovered. Reported improvement of depressed cardiac function in diabetes by MI administration will be due to the restration of Na+-K+ ATPase activity by reprenishment of specific PI fraction by MT
t’w57
EFFECTSOF OXIDANI’STRESS ON STEADY-STATE BACKGROUND CURRENTSIN ISOLATED VENTRICULAR CELLS OF RABBIT HEART. Hkoshl Matswra and Ybhael J. Shaltock. Cardiovascular Research, St. Thomas’ Has I, London SEl. We have studied the effectsof oxidant stresson steady-statemembrane currents in isolated rJ% ’ ventricular myocytes using the whole-cell voltage clamp technique. Singlet ox en and su roxide were generated by the hotoactivationof rose al (1 O-l 00 nM added to the external solution). I?ose benga I+, in the absence of illumination, lad no detectable effects. 9 r?illumination, however, rose bengal gradually decreased resting membrane potential, induced repetitiveaction pote$ial $scharqes and prolonged action potential duration (to 2-I 0 s). In voltage clamp studies, the steady-state I-V relationship was N’shaped and was outward at all potentials positive to -8OmV. After exposure to rose bengal, the I-V relationshipshied inwardataroundplateau potentialsandshowed threezerocunentintercepts (at-70, -40and -lOmV), two of these (-70 and -10 mV) hati a posibve slope. This indicates that, in additon to -70 rnV! -10 mV may be a stable resting potential. This may explam 3 t e prolongation of the action potential and failure to repolanze seen in current clamp expenments. The slope conductance at the level of the resting potential (-75 mV) decreased from 403~7nS (control) to 25s nS rose hsn al), indiiting a reduction in the inward rectifier K+current (i ,). In the absence of iK,,the rose bengalsensitiveIra&n. of the background conductance showed an almost linear I-V rJationshipand a reversal potential around 0 mV. This linear I-V relationship may be due to the Ca%tivated non-selective cation channel current (i . The slope conductance of i increased with time to 15 nS, indicating that the intracellular CP concentration may g rardll ly increase during exposure% rose bengal. In conclusion, oxidant stress reduces k, and activates iw a.ti this depoladzes the cell membraneandpromotesabnormalautomaticity. Thesechangesinsteady-statebackgroundcurrentsmaythuscontribute to the arrhythmias seen in isolated hearts during the photoactivation of rose bengal. s.19
pW55
Cell
Cardiol
22 (Supplement
III)
(1990)
THE EFFECT OF HYPOTHERMIA ON CARDIAC MUSCLE DURING THE CALCIUM PARADOX Mohammed-Saadeh Suleiman & R. A. Chapman. Department of Physiology, Bristol University, School of Veterinary Science, Park Row, Bristol BSl 5LS. Hypothermia is known to protect cardiac tissue from the Ca-paradox. This effect would seem to conflict with the notion that a rise in [Na]i predisposes the heart to the Ca paradox. Changes in [Na]i (measured using ion selective microelectrcde) membrane potential and contracture were monitered in the isolated ferret ventricular trabeculae during the Ca-paradox, at four temperatures ; 37. 30. 20 8 10°C. Cooling protects against the Caparadox by slowing down the rise in [Na]i during Ca-depletion. This effect is time dependent, where the steady state value for [Na]i at 20 & 30°C reached similar levels. However, at 10°C [Na]i rose to higher levels and subsequently the damaging effect of the paradox was stronger. The reason for this was found to be the complete inhibition of the Na-pump at 1 OoC. The inhibition of the pump with strophanthidin at 20, 30 & 37OC yield an estimate of the energy of activation at 74KJmole.t . This is higher than that of the Na-pump, thus explaining the initial protective effect of hypothermia. However the observation that a Pmin pulse at 30°C followed by cooling to 20°C during Ca-depletion which resulted in a greater rise in [Na]i cannot be simply explained by a temperature effect only on the influx of Na ions. An additional effect of temperature on the loss of ionic selectivity of the 1. type Ca-channels is suggested. This work provides further evidence for the hypothesis that the damaging effect of the paradox is determined by the degree of rise in [Na]i which is the outcome of the race between the Na ~nfiux through the L-type Ca-channels and the efflux via the Na-pump.
PW56ST;WT;;
THE PATHOGENESIS OF THE CARDIAC DYSFUNCTION IN DIABETES : ROLE OF MYOINOSITOL ON THE MEMBRANE TRANSPORT OF CATIONS. Hideaki Suzuki, Mahito Kawamura, Masahiro Kawanishi, Makoto Nagano. Department of Internal Mediciny,Avto Hosp.,Jikei University,TqkyoZ+Japan Depressed activity of Na -K ATPase, Ca2+ ATPase and Na -Ca exchange of myocardial membrane were thought to be the cause of cardiac dysfunction of experimental diabetes. It is claimed that the impaired transport of myoinositol( MI ) might be the patogenesis of diabetic angiopathy. Phosphatidylinositol( PI ) which is sensitive to the ex$ra+ cellulat MI was demonstrated to be involved in thy re ulation of Na -K 9+ ATPase. Involvement f PI in the regulation of Na -Ca exchange of sarS+ colemma( SL ) and Ca transport ATPase of sarcoplasmic reticulum( SR ) were also reported. For this reason effects of MI administration on the Na+-K+ ATPase activity and Na+-Ca2+exchange of SL and Ca2+ uptake of SR were studied in streptozotocin diabetic rats. Depressed Na+-K+ ATPase activity of diabetic SL was restored to the normal value by the administration of MI. Decreased Na+-Ca2+exchange of SL and Ca2+ uptake of SR were not recovered. Reported improvement of depressed cardiac function in diabetes by MI administration will be due to the restration of Na+-K+ ATPase activity by reprenishment of specific PI fraction by MT
t’w57
EFFECTSOF OXIDANI’STRESS ON STEADY-STATE BACKGROUND CURRENTSIN ISOLATED VENTRICULAR CELLS OF RABBIT HEART. Hkoshl Matswra and Ybhael J. Shaltock. Cardiovascular Research, St. Thomas’ Has I, London SEl. We have studied the effectsof oxidant stresson steady-statemembrane currents in isolated rJ% ’ ventricular myocytes using the whole-cell voltage clamp technique. Singlet ox en and su roxide were generated by the hotoactivationof rose al (1 O-l 00 nM added to the external solution). I?ose benga I+, in the absence of illumination, lad no detectable effects. 9 r?illumination, however, rose bengal gradually decreased resting membrane potential, induced repetitiveaction pote$ial $scharqes and prolonged action potential duration (to 2-I 0 s). In voltage clamp studies, the steady-state I-V relationship was N’shaped and was outward at all potentials positive to -8OmV. After exposure to rose bengal, the I-V relationshipshied inwardataroundplateau potentialsandshowed threezerocunentintercepts (at-70, -40and -lOmV), two of these (-70 and -10 mV) hati a posibve slope. This indicates that, in additon to -70 rnV! -10 mV may be a stable resting potential. This may explam 3 t e prolongation of the action potential and failure to repolanze seen in current clamp expenments. The slope conductance at the level of the resting potential (-75 mV) decreased from 403~7nS (control) to 25s nS rose hsn al), indiiting a reduction in the inward rectifier K+current (i ,). In the absence of iK,,the rose bengalsensitiveIra&n. of the background conductance showed an almost linear I-V rJationshipand a reversal potential around 0 mV. This linear I-V relationship may be due to the Ca%tivated non-selective cation channel current (i . The slope conductance of i increased with time to 15 nS, indicating that the intracellular CP concentration may g rardll ly increase during exposure% rose bengal. In conclusion, oxidant stress reduces k, and activates iw a.ti this depoladzes the cell membraneandpromotesabnormalautomaticity. Thesechangesinsteady-statebackgroundcurrentsmaythuscontribute to the arrhythmias seen in isolated hearts during the photoactivation of rose bengal. s.19