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Cardiovascular Research, 1990, 24, 748-753
Changes in performance of the surviving myocardium after left ventricular infarction in rats JoseGerald0 Mill, Ivanita Stefanon, Claudia M Leite, Dalton V Vassallo
Abstract Study objective - The aim was to investigate left ventricular performance of infarcted hearts during scar formation and development of hypertrophy in the surviving myocardium. Design - Hearts were perfused according to the Langendorff technique and left ventricular function curves were obtained by inserting a distensible balloon into the ventricular cavity. The isovolumetric systolic pressure was measured as diastolic pressure was changed from 0 to 25 mm Hg and during inotropic interventions produced by Ca and isoprenaline. Experimental materials - Hearts were obtained from albino rats of either sex, 180-250 g, killed 1 , 3 , 7 or 14 d after left coronary artery ligation (n = 24) or sham operation (n = 26). Normal rats (n = 6 ) were used as additional controls. Measurements and main results - After infarction, there was a progressive and almost parallel displacement of the ventricular function curves toward higher diastolic pressures. The positive chronotropic response to isoprenaline was similar in infarcted and non-infarcted hearts. The inotropic response to Ca and isoprenaline, however, was significantly depressed in the infarcted hearts throughout the observation period. Conclusims - Hypertrophy in the surviving myocardium did not result in improvement of the left ventricular systolic function assessed under in vitro conditions during the f m t two weeks after infarction.The decreased inotropic response of the infarcted left ventricle to isoprenaline is likely to be dependent on the reduced Ca sensitivity of the surviving myocardium. Departamento de Ciincias Fisiologicas, Centro Biomedico da Universidade Federal do Espirito Santo, Caixa Postal 780, 29.001 Vitoria ES, B r a d J G .bid1 I Stefanon iM Leire I f V Vassallo iorrespondencr TO: Dr Mill Key words: myocardial infarction; left ventricular contraculitv: calcium; f3 adrenoceptors Suhmtrred 24 August I989 Accepted 26 April 1990
Development of hypertrophy in the remaining regions of both ventricle~l-~supposedly is an important adaptive process for the maintenance of mechanical left ventricular function in hearts with healed infarctions.' Geenen et a16 recently studied the contractile properties of left ventricular papillary muscles from chronically infarcted rat hearts and showed that the force produced during isometric contractions was significantly increased in relation to controls. The mechanical performance of the infarcted left ventricle is not only dependent on the contractile characteristics of the surviving muscle but also on the properties of the fibrotic tissue replacing the necrotic area.' * Both phenomena are time dependent processes.e12 We therefore investigated the changes in left ventricular performance (in terms of the pressure generated during isovolumetric systoles) of isolated hearts obtained from rats killed 1 to 14 d after left coronary artery ligation. Methods Male and female albino rats (10 to 14 weeks old, 180-250g) were divided into three groups (male/female ratio the same for each group): control (C, n = 6), infarcted (INF, n = 24) and sham operated (SO, n = 26) rats. A left thoracotomy was performed in the fourth or fifth intercostal space under light ether anaesthesia and the heart immediately exteriorised by gentle pressure on the right side of the thorax.I3 The anterior descending branches of the left coronary artery and vein were ligated with 6-0 polyvicryl thread (Ethicon) between the pulmonary artery and the tip of the left atrial appendage. The heart was rapidly repositioned into the chest, the thoracotomy was closed and spontaneous respiratory movements occurred in the greater number of animals. Positive ventilation was applied when necessary. All surgical procedures to produce infarction were also used in sham operated rats, except that the suture around the coronary vessels was not tied. After surgery, the animals were housed in their habitual cages and food and water provided ad libitum. No surgical procedures were performed in control rats. These animals were randomly chosen among those that would be submitted to infarction or sham operation. The hearts of infarcted and sham operated rats were studied 1, 3,7 or 14 d after surgery. The animals were pretreated with a single injection of heparin (25 U
Left ventricular performance after infarction
749
intraperitoneally) and 10-15 min later they were killed with a blow to the neck and exsanguinated. The heart was rapidly excised and perfused through the aortic stump according to the Langendorff technique. A constant pressure of 65 mm Hg was maintained throughout the experiment. The nutrient solution had the following composition (in mM): NaCl 120, KCl 5.4, MgCl2 1.2, CaC12 1.25, NaH2P04 2, Na2S04 1.2, NaHC03 27, and glucose 11. This solution was continuously bubbled with 95% 0 2 + 5% C02 (pH = 7.3-7.4), and maintained at 31°C. The heart, spontaneously beating, was let to stabilise in the new condition for 10-15 min. The left atrium was then opened and a stab wound was made in the apex to drain the ventricle with a needle introduced via the atrium and mitral valve. A soft distensible balloon was inserted into the left ventricular cavity through the mitral orifice and held in place with a ligature at atrial level. The balloon was mounted at the tip of a polyethylene catheter (PE 50) and connected (via a Y piece) to a pressure transducer (Gold P23XL) and a syringe (1 ml) which allowed adjustment of the left ventricular diastolic pressure. Conventional silver electrodes were placed near the epicardial surface (at the atrioventricular ring, right side, and apex) for the electrocardiogram (ECG). ECG and isovolumetric left ventricular pressures were recorded. The following protocols were tested. Initially the steady state isovolumetric systolic pressure was recorded as the diastolic pressure was increased from 0 to 25 mm Hg, in steps of 5 mm Hg. After recording these curves the diastolic pressure was returned to 5 mm Hg and the chronotropic and inotropic effects of Ca2+ and isoprenaline (Sigma) were tested. CaC12 (5 and 10 pmol) and isoprenaline (4,12,36 and 108 nmol) were directly dissolved from concentrated stock solutions in the nutrient solution and injected as a bolus (0.1 ml) into the perfusion stream about 2 cm above the orifice of the coronary arteries. Heart rate was obtained from QRS under regular sinus rhythm. After changes of diastolic pressure, isolated ventricular extrasystoles were more frequently observed in infarcted hearts. After each experiment, a careful dissection of the free wall of the right and left ventricle (including the
septum) was performed. The muscles were blotted and weighed. Weights were normalised to body weight. In infarcted hearts, the damaged region can easily be identified because of its transparency under strong transillumination, especially in rats killed 3, 7 or 14 d after infarction. Using a dissection microscope the infarcted region was dissected free from the normal tissue. Normal and infarcted parts were drawn on millimeter paper and measured. The infarcted area was expressed as percentage of total left ventricular area, including the septum. The results are presented as mean (SEM). Student r test was used to compare two means. One way analysis of variance was used to detect significant differences between more than two means. Statistical significance was set at p
Cardiovascular Research, 1990, 24, 748-753
Changes in performance of the surviving myocardium after left ventricular infarction in rats JoseGerald0 Mill, Ivanita Stefanon, Claudia M Leite, Dalton V Vassallo
Abstract Study objective - The aim was to investigate left ventricular performance of infarcted hearts during scar formation and development of hypertrophy in the surviving myocardium. Design - Hearts were perfused according to the Langendorff technique and left ventricular function curves were obtained by inserting a distensible balloon into the ventricular cavity. The isovolumetric systolic pressure was measured as diastolic pressure was changed from 0 to 25 mm Hg and during inotropic interventions produced by Ca and isoprenaline. Experimental materials - Hearts were obtained from albino rats of either sex, 180-250 g, killed 1 , 3 , 7 or 14 d after left coronary artery ligation (n = 24) or sham operation (n = 26). Normal rats (n = 6 ) were used as additional controls. Measurements and main results - After infarction, there was a progressive and almost parallel displacement of the ventricular function curves toward higher diastolic pressures. The positive chronotropic response to isoprenaline was similar in infarcted and non-infarcted hearts. The inotropic response to Ca and isoprenaline, however, was significantly depressed in the infarcted hearts throughout the observation period. Conclusims - Hypertrophy in the surviving myocardium did not result in improvement of the left ventricular systolic function assessed under in vitro conditions during the f m t two weeks after infarction.The decreased inotropic response of the infarcted left ventricle to isoprenaline is likely to be dependent on the reduced Ca sensitivity of the surviving myocardium. Departamento de Ciincias Fisiologicas, Centro Biomedico da Universidade Federal do Espirito Santo, Caixa Postal 780, 29.001 Vitoria ES, B r a d J G .bid1 I Stefanon iM Leire I f V Vassallo iorrespondencr TO: Dr Mill Key words: myocardial infarction; left ventricular contraculitv: calcium; f3 adrenoceptors Suhmtrred 24 August I989 Accepted 26 April 1990
Development of hypertrophy in the remaining regions of both ventricle~l-~supposedly is an important adaptive process for the maintenance of mechanical left ventricular function in hearts with healed infarctions.' Geenen et a16 recently studied the contractile properties of left ventricular papillary muscles from chronically infarcted rat hearts and showed that the force produced during isometric contractions was significantly increased in relation to controls. The mechanical performance of the infarcted left ventricle is not only dependent on the contractile characteristics of the surviving muscle but also on the properties of the fibrotic tissue replacing the necrotic area.' * Both phenomena are time dependent processes.e12 We therefore investigated the changes in left ventricular performance (in terms of the pressure generated during isovolumetric systoles) of isolated hearts obtained from rats killed 1 to 14 d after left coronary artery ligation. Methods Male and female albino rats (10 to 14 weeks old, 180-250g) were divided into three groups (male/female ratio the same for each group): control (C, n = 6), infarcted (INF, n = 24) and sham operated (SO, n = 26) rats. A left thoracotomy was performed in the fourth or fifth intercostal space under light ether anaesthesia and the heart immediately exteriorised by gentle pressure on the right side of the thorax.I3 The anterior descending branches of the left coronary artery and vein were ligated with 6-0 polyvicryl thread (Ethicon) between the pulmonary artery and the tip of the left atrial appendage. The heart was rapidly repositioned into the chest, the thoracotomy was closed and spontaneous respiratory movements occurred in the greater number of animals. Positive ventilation was applied when necessary. All surgical procedures to produce infarction were also used in sham operated rats, except that the suture around the coronary vessels was not tied. After surgery, the animals were housed in their habitual cages and food and water provided ad libitum. No surgical procedures were performed in control rats. These animals were randomly chosen among those that would be submitted to infarction or sham operation. The hearts of infarcted and sham operated rats were studied 1, 3,7 or 14 d after surgery. The animals were pretreated with a single injection of heparin (25 U
Left ventricular performance after infarction
749
intraperitoneally) and 10-15 min later they were killed with a blow to the neck and exsanguinated. The heart was rapidly excised and perfused through the aortic stump according to the Langendorff technique. A constant pressure of 65 mm Hg was maintained throughout the experiment. The nutrient solution had the following composition (in mM): NaCl 120, KCl 5.4, MgCl2 1.2, CaC12 1.25, NaH2P04 2, Na2S04 1.2, NaHC03 27, and glucose 11. This solution was continuously bubbled with 95% 0 2 + 5% C02 (pH = 7.3-7.4), and maintained at 31°C. The heart, spontaneously beating, was let to stabilise in the new condition for 10-15 min. The left atrium was then opened and a stab wound was made in the apex to drain the ventricle with a needle introduced via the atrium and mitral valve. A soft distensible balloon was inserted into the left ventricular cavity through the mitral orifice and held in place with a ligature at atrial level. The balloon was mounted at the tip of a polyethylene catheter (PE 50) and connected (via a Y piece) to a pressure transducer (Gold P23XL) and a syringe (1 ml) which allowed adjustment of the left ventricular diastolic pressure. Conventional silver electrodes were placed near the epicardial surface (at the atrioventricular ring, right side, and apex) for the electrocardiogram (ECG). ECG and isovolumetric left ventricular pressures were recorded. The following protocols were tested. Initially the steady state isovolumetric systolic pressure was recorded as the diastolic pressure was increased from 0 to 25 mm Hg, in steps of 5 mm Hg. After recording these curves the diastolic pressure was returned to 5 mm Hg and the chronotropic and inotropic effects of Ca2+ and isoprenaline (Sigma) were tested. CaC12 (5 and 10 pmol) and isoprenaline (4,12,36 and 108 nmol) were directly dissolved from concentrated stock solutions in the nutrient solution and injected as a bolus (0.1 ml) into the perfusion stream about 2 cm above the orifice of the coronary arteries. Heart rate was obtained from QRS under regular sinus rhythm. After changes of diastolic pressure, isolated ventricular extrasystoles were more frequently observed in infarcted hearts. After each experiment, a careful dissection of the free wall of the right and left ventricle (including the
septum) was performed. The muscles were blotted and weighed. Weights were normalised to body weight. In infarcted hearts, the damaged region can easily be identified because of its transparency under strong transillumination, especially in rats killed 3, 7 or 14 d after infarction. Using a dissection microscope the infarcted region was dissected free from the normal tissue. Normal and infarcted parts were drawn on millimeter paper and measured. The infarcted area was expressed as percentage of total left ventricular area, including the septum. The results are presented as mean (SEM). Student r test was used to compare two means. One way analysis of variance was used to detect significant differences between more than two means. Statistical significance was set at p
