Myocardial catecholamine responsiveness of spontaneously hypertensive rats as influenced by swimming training

Summary Alterations of myocardial mechanical catecholamine responsiveness by swimming training (2×90 min/day, 4 weeks) were examined in 13-week-old spontaneously hypertensive male rats (SHR). The relationships between myocardial mechanical catecholamine responsiveness and ventricular -adrenoceptors as well as myosin isoenzyme pattern were also examined. Compared with sedentary controls, trained rats showed a greater responsiveness to isoproterenol (10^–6 mol/l) on isometric tension (T) and its first derivative (dT/dt) (T:0.45±0.55 vs. –0.15±0.11 10^–2 N/mm², p<0.01, dT/dt: 17.1±10.1 vs. 8.3±3.6 10^–2 N/mm²·s, p<0.05). In sedentary SHR, dT/dt_max increased significantly, whereas developed tension decreased slightly, coupled with a decrease of time to peak tension by high dose (10^–6 mol/l) isoproterenol. Therefore, it can be stated that dT/dt is a better indicator for catecholamine sensitivity than isometric tension. -adrenoceptor density ([³H]-dihydroalprenolol binding) decreased significantly in trained rats (68.7±7.62 vs. 102.4±4.37 fmol/mg protein, p<0.01) with no significant difference in K_D values (4.61±2.26 vs. 6.11±1.94 nM, ns). In addition, myosin isoenzyme pattern revealed by pyrophosphate gel electrophoresis shifted towards VM-1 after swimming training. The increased catecholamine sensitivity of fast contracting myocardium is, in principle, compatible with the assumption of cAMP-dependent regulation of myofibrillar ATPase activity (21) or cross bridge kinetics (9), although other postreceptor processes should also be taken into consideration for the increased catecholamine sensitivity.Supported by the Deutsche Forschungsgemeinschaft     

Mechanical catecholamine responsiveness and myosin isoenzyme pattern of pressure-overloaded rat ventricular myocardium

Summary Pressure-overloaded cardiac hypertrophy was induced by abdominal aortic constriction in 10-week-old male Wistar rats. 24–26 weeks after aortic constriction, the hearts were excised and a myocardial mechanical study was performed using isolated left ventricular papillary muscles. There was no significant difference in isometric developed tension (T) between sham-operated control and aortic constriction (AC) rats (control vs AC rats=2.9±0.6 vs 2.7±0.7 g/mm²). dT/dt_max of AC rats, on the other hand, was significantly lower than that of controls (controls vs AC rats=32.8±7.5 vs 26.3±6.1 g/mm²sp<0.05). Myocardial mechanical responses to isoproterenol (10^–7 mol/l) were depressed in the group with aortic constriction compared with the control group (T:18.5±6.7 vs 12.1±4.9%,p<0.05, dT/dt: 25.2±6.2 vs 17.5±5.8%,p<0.02). Responses of the parameters to dibutyryl cyclic AMP (10^–5 mol/l) were also smaller in the AC group than in the control group (T: 18.0±5.6 vs 13.3±4.0%,p<0.05, dT/dt: 20.4±6.9 vs 14.7±4.1%,p<0.05). Left ventricular myosin isoenzyme pattern, revealed by pyrophosphate gel electrophoresis, shifted towards VM-3 under pressure overload. The present study demonstrates that post-membrane processes may be mainly responsible for the decreased myocardial mechanical catecholamine responsiveness in pressure-overloaded cardiac hypertrophy.Supported by Tanaka Memorial Medical Research Fund     

Myocardial contractility and ventricular myosin isoenzymes as influenced by cardiac hypertrophy and its regression

Summary Changes in myocardial contractility and ventricular myosin isoenzymes were examined during pressure-overloaded cardiac hypertrophy in rats. Effects of regression of cardiac hypertrophy were also examined. Cardiac hypertrophy was induced by abdominal aortic constriction in 7-week-old male Wistar rats. Regression of cardiac hypertrophy was obtained by opening the aortic band. Myocardial contractility was estimated by measuring isometrically developed tension and maximum rate of tension rise (+dT/dtmax) in isolated left-ventricular papillary muscles perfused with Tyrode solution (32°C, pH 7.4, bubbled with 95% O₂·5% CO₂, stimulation frequency: 0.2 Hz). Left-ventricular myosin isoenzymes were separated by pyrophosphate gel electrophoresis and the isoenzyme pattern was determined by densitometry. Isometrically developed tension (T) in hypertrophic myocardium remained unchanged, but ±dT/dtmax decreased as compared with hearts of normal rats. Decreased ±dT/dtmax recovered near to the level in normal rats by regression of cardiac hypertrophy. Leftventricular myosin isoenzyme pattern shifted towards VM-3 in hypertrophied myocardium and shifted again toward VM-1 by regression of cardiac hypertrophy. In conclusion, myocardial contractility and ventricular myosin isoenzymes were changed in pressure-overloaded hypertrophy in rats and these changes were reversible to a normal level by regression of cardiac hypertrophy.     

Enhanced myocardial contractility but not tachycardia persists in isolated working hyperthyroid rat hearts

Summary It is generally believed that the increased contractility and tachycardia of the hyperthyroid heart are a result of thyroid hormone-induced alterations of the mechanical and electrical properties of the heart, respectively. We compared the contractility (dP/dt_max) and the spontaneous beating rate of hyperthyroid and euthyroid hearts perfused in vitro in either a non-working or a working mode. The dP/dt_max (4196±74 mm Hg s^–1) and beating rate (322±8 beats/min) of the non-working hyperthyroid hearts were significantly higher (p<0.001) than those of the euthyroid hearts (3267±115 mm Hg s^–1 and 260±6 beats/min at an external Ca²⁺ of 2.5 mM). At 2.5 mM Ca²⁺, the working hyperthyroid hearts again displayed enhanced contractility (5636±179 mm Hg s^–1 vs 4508±172 mm Hg s^–1; p<0.001) but the spontaneous beating rate (275±7 beats/min) was not significantly different from euthyroid (261±8 beats/min). When hearts were subjected to periods of alternate non-working and working perfusion, the beating rate of the hyperthyroid hearts was significantly higher than euthyroid during non-working (p<0.02) but not during working perfusion. Increasing the afterload on the non-working preparations in a stepwise fashion from 75 cm H₂O to 120 cm H₂O caused significant changes in left ventricular pressure and dP/dt_max in both heart types but the tachycardia in the hyperthyroid hearts persisted (at 120 cm H₂O; hyperthyroid, 294±9 beats/min; euthyroid, 224±10 beats/min; p<0.001). Alteration of the preload (10 to 25 cm H₂O) and afterload (75 to 105 cm H₂O) on working hyperthyroid and euthyroid hearts caused changes in both left ventricular pressure and dP/dt_max but the beating rates of both heart types were never significantly different. We conclude from our results that (i) the increased contractility of the hyperthyroid rat heart is due to thyroid hormone-induced alteration of the mechanical properties of the heart; (ii) the tachycardia of hyperthyroidism is not due to thyroid hormone-induced changes in the electrical properties of the heart, but probably involves some as yet unidentified chronotropic agent.     

Altered myocardial contractility and energetics in hypertrophied myocardium

Alterations of myocardial contractility and energetics were examined in cardiac hypertrophy induced by different types of cardiac overload. Myocardial contractility was estimated by isometric contraction of isolated left ventricular papillary muscles. Myocardial energetics were assessed from ventricular myosin isoenzyme patterns obtained by pyrophosphate gel electrophoresis. Cardiac hypertrophy was induced by endurance swim-training and sustained pressure-overload by abdominal aortic constriction or volume-overload created by an arteriovenous shunt. In swim-trained hypertrophied myocardium, isometric developed tension (T) and dT/dtmax showed a tendency to increase and response of dT/dtmax to isoproterenol increased significantly as compared with sedentary rats. Training shifted the left ventricular myosin isoenzyme pattern toward VM-1, which has the highest ATPase activity. In pressure- or volume-overloaded myocardium, dT/dtmax decreased significantly and mechanical response to isoproterenol also decreased (or tended to decrease in volume-overloaded hearts) as compared with the respective sham-operated controls. In pressure- or volume-overloaded hearts, left ventricular myosin isoenzyme pattern shifted toward VM-3, which has the lowest ATPase activity. These results indicate that alterations in myocardial contractility, mechanical catecholamine responsiveness and myocardial energetics in hypertrophied myocardium do not always display the same trend, but are greatly influenced by the causes or duration of cardiac overload.     

The influence of endurance training on mechanical catecholamine responsiveness, β-adrenoceptor density and myosin isoenzyme pattern of rat ventricular myocardium

Summary An investigation was carried out on the effects of 4 weeks' swimming training (2×90 min/day) on myocardial isometric tension development and rate of tension rise, and also on the changes induced therein by in vitro application of isoproterenol. This was done in 9 isolated papillary muscles of 9-weekold male Wistar rats and the results were compared with the data of age-matched sedentary controls. Ventricular -adrenoceptors ([³H]-dihydroalprenolol binding) and the isoenzyme pattern of myosin (pyrophosphate gel electrophoresis) were examined in the same individuals. Isometric tension (T) and its first derivative (dT/dt) measured at the optimum of the length-tension diagram were moderately increased by long-term swimming training. Isoproterenol (10^–5 mol/l) induced a greater absolute and relative increase of both mechanical parameters in specimens of trained animals than in age-matched controls (T: 3.6±1.6 vs. 1.9±0.6×10^–2 N/mm², p<0.05. dT/dt: 43.4±14.0 vs. 30.4±9.5×10^–2 N/mm²·s, p<0.05). K _d decreased significantly (4.23±1.0 vs. 2.44±0.3 nM, p<0.02), indicating an increase in receptor affinity, whereas receptor density revealed a tendency to decrease (98.8±22.6 vs. 67.1±18.0 fmol/mg protein, p<0.1). In addition, there was a shift in the isoenzyme pattern of myosin towards VM-1 after swimming training.Thus, under the conditions of the present experiments, the mechanical response to isoproterenol does not correlate to -adrenoceptor density. It is probable that, apart from the altered sensitivity of the receptors, other membrane or post-membrane processes, are responsible for the increased mechanical responsiveness to catecholamines. Although a relationship between myosin isoenzyme pattern and mechanical responsiveness to catecholamines is apparent taking into account our results and the findings on hypertensive rats as reported in the literature, it cannot be accounted for simply by altered -adrenoceptor density.     

Tolerance of myocardium of aged animals to repeated oxygen deficiency

Summary Hemodynamic and metabolic effects of three times 4 min of oxygen deficiency were investigated in 18-month-old rats in comparison to 4-month-old Wistar rats. Left-ventricular isovolumicpressure-generating capacity and dp/dt_max during isovolumic conditions and hemodynamic indices during intact circulation were determined in open-chest rats. Additionally, high-energy phosphates were measured at the end of the experiments after 20 min of postasphyxial recovery.Older rats had a significantly reduced isovolumic left-ventricular pressure generating capacity (236±9 vs 269±5 mm Hg; p<0.05) and a low cardiac index (55±9 vs 117±8 ml×min^–1×kg^–1). The effects of the oxygen deficiency were comparable in both groups. The isovolumic pressure generating capacity was reduced for 11% vs 14%, and dp/dt_max for 13% vs 13%. The myocardial ATP-content was also decreased for the same extent in both groups (0.6 vs 1.0 mol/gww). Both hemodynamic and biochemical results indicate that aged myocardium does not have a reduced tolerance to repeated periods of oxygen deficiency.Supported in part by the Deutsche Forschungsgemeinschaft, Bonn, FRG (HO 1003/1-2)Parts of the results were presented at the 10th meeting of the Internat. Soc. for Heart Research (Europ. Section), Rotterdam, 1989     

The role of calcium in the enhanced myocardial contractility of the hyperthyroid rat heart

Summary The hyperthyroid rat myocardium exhibits enhanced contractility. There is evidence that altered calcium handling by the myocardium may be responsible for this enhanced state. To investigate this, isolated hyperthyroid and cuthyroid hearts were perfused in the working mode and exposed to alterations in external calcium concentration. Heart rate was not significantly different in either group of hearts, nor was it altered by the change in calcium. The concentration of calcium needed to elicit half-maximal contractility (dP/dt_max) was lower in the hyperthyroid (0.81±0.07 mM) than in the cuthyroid hearts (1.12±0.09 mM, p<0.05). This increase in calcium sensitivity was unlikely to be at the site of the sarcolemma as verapamil exerted equal negative inotropic effects on both groups of hearts. Dantrolene, which blocks calcium release from the sarcoplasmic reticulum, exerted a significantly greater (p<0.01) depression in dP/dt_max after 12 min in the hyperthyroid (50±7%) than in the cuthyroid heart (15±2%). We conclude from our results that the enhanced contractile state of the hyperthyroid rat heart is likely to involve an altered mechanical response to calcium which is possibly at the level of enhanced calcium release from the sarcoplasmic reticulum.     

Independent mechanisms for the chronotropic and inotropic responses in hyperthyroidism

Summary We established a hyperthyroid rat model and compared the hemodynamic responses of the hypertrophied rat heartin vivo andin vitro. Heart rate (557±26 beats/min), systolic blood pressure (162±5 mm Hg) and dry heart mass (230±11 mg) in hyperthyroid rats were significantly greater than in control animals (408±12 beats/min, 140±5 mm Hg and 193±4 mg respectively).In vitro studies were performed in order to eliminate neurohumoral and peripheral circulatory factors which are presentin vivo. In thein vitro working heart preparation, there was no significant difference between the heart rates of L-thyroxine-treated (263±9 beats/min) and control (258±10 beats/min) animals, implying that the tachycardia of hyperthyroidism is partly mediated byin vivo factors. Consistent with this hypothesis was the observation that the hyperthyroid heart was more sensitive to the chronotropic effects of physiological concentrations of the synthetic catecholamine, isoproterenol (10^–8M, 10^–7 M) than the control heart. The maximum rate of left ventricular pressure rise (dP/dt_max) was used as an index of myocardial contractility.In vitro values for dP/dt_max were significantly greater in hearts from hyperthyroid rats (5338±228 mm Hg/s) than in control hearts (4583±158 mm Hg/s), suggesting that the increased contractile response of hyperthyroidism is intrinsic to the heart itself. Although persistence of the inotropic response of the hyperthyroid heartin vitro was associated with an increase in heart mass, this factor alone did not account entirely for the enhanced contractility. It appears that intrinsicfunctional changes also contribute to the inotropic response of the hyperthyroid heart.     

Effects of in vivo manganese administration on calcium exchange and contractile force of rat ventricular myocardium

Summary The purpose of this study was to investigate the effect of prolonged (14 days) intragastric administration of Mn²⁺ (0.25 mmol/kg daily) on Ca²⁺ exchange and contractility of rat ventricular myocardium. Left-ventricular pressure and its first derivative (dP/dt) were recorded by means of a balloon catheter inserted via the left atrium into the left ventricle of the rat heart perfused by Langendorff method. Ca²⁺ exchange in the stimulated and rested ventricular myocardium was investigated with the aid of⁴⁵Ca under the conditions of complete equilibration of preparations with a solution containing⁴⁵Ca²⁺. The cellular⁴⁵Ca²⁺ content was calculated by subtraction of⁴⁵Ca²⁺ dissolved in the free water of extracellular space from the total tissue⁴⁵Ca²⁺ content. The cellular⁴⁵Ca²⁺ content in the stimulated (60/min) ventricles of control rats (without Mn²⁺) was 0.83 ± 0.09 mmol/kg wet weight (w.w.). Ten minutes of rest resulted in a gain of 0.06 mmol⁴⁵Ca/kg w.w. (not statistically significant). Fourteen days' exposure to Mn²⁺ resulted in an increase of the mean⁴⁵Ca content to 1.61 ± 0.09 mmol/kg w.w. in the stimulated preparations and to 1.35 ± 0.06 mmol/kg w.w. in the rested ones (p < 0.001). Thus, the control rest preparations did not change their Ca²⁺ content, while in the rats treated with Mn²⁺ the rest resulted in an increase at exchangeable Ca by 52 %. The maximal ventricular developed pressure (P_max) after 14 days of Mn²⁺ administration was increased by 35 % and dP/dt_max was 228 % of the value in the control group. The mean time from the maximal value of the first derivative to P_max (t-dP/dt_max) was reduced by one-half and mean time to peak developed pressure (TPT) was shortened to one-third of these in the control group. These results, although far from conclusive, do suggest that the long-lasting exposition to Mn²⁺ resulted in increased myocardial contractility caused, most probably, by the inhibition of Ca²⁺ efflux from the cell. Perhaps this is a compensation for the inhibitory effect of Mn²⁺ on the slow calcium channels.     

Attenuation of the systemic and coronary hemodynamic effects of cocaine in conscious dogs: propranolol versus labetalol

Summary The interaction of cocaine with myocardial and vascular adrenoceptors is incompletely understood. The systemic and coronary hemodynamic effects of intravenous cocaine (1.5 mg/kg) were examined in dogs with and without pretreatment with propranolol (2 mg/kg i.v.) or labelatol (5 mg/kg i.v.) on different days. A total of 24 experiments was completed (three sets of experiments) using eight dogs chronically instrumented for measurement of aortic and left-ventricular pressure, left-ventricular dP/dt, subendocardial segment length, coronary blood flow, and cardiac output. Myocardial oxygen consumption was estimated from the pressure work index (PWI). Cocaine significantly (p<0.05) increased heart rate (+51±17 bpm), mean arterial pressure (+72±10 mm Hg), left-ventricular systolic and end-diastolic pressures (+56±9 and +14±6 mm Hg, respectively), coronary blood flow (+32±10 ml/min) and the PWI (+10.0±2.3 ml O₂/min/100 g). Significant reductions in stroke volume (–9±5 ml) and percent segment shortening (–7.1±1.7) were observed. These changes returned to control after 30 min. After pretreatment with propranolol, the cocaine-mediated increases in mean arterial pressure, left-ventricular systolic pressure, rate-pressure product, and the pressure work index (4.4±0.7 ml O₂/min/100 g) were significantly (p<0.05) less than those observed with cocaine alone. Cocaine also reduced contractility [dP/dt₅₀ (–341±80 mm Hg/s)] and increased systemic vascular resistance (+2703±339 dyn·s·cm^–5) in the resence of propranolol. Labetalol abolished the cocainemediated increases in heart rate and coronary blood flow and significantly attenuated the increases in mean arterial pressure, left-ventricular systolic pressure, cardiac output, rate-pressure product, and calculated myocardial oxygen consumption when compared to results obtained with cocaine alone. The results demonstrate that a portion of the basic dynamic effects of cocaine is mediated by stimulation of alpha and beta adrenoceptors. Combined alpha and beta adrenergic blockade reduces the hemodynamic effects of cocaine more than beta blockade alone. During antagonism of the sympathomimetic response of cocaine, direct negative inotropic actions of this drug are unmasked.This work was supported by US PHS grants HL 36144 and HL 32911, Anesthesiology Research Training Grant GM 08377, and VA Medical Research Funds.     

Three-dimensional analysis of regional mechanical function,blood flow and electrophysiological parameters during early myocardial ischemia in dogs

Summary Ventricular arrhythmias are primarily responsible for sudden cardiac death early after the onset of acute myocardial ischemia. We designed an experimental model to simultaneously characterize regional myocardial function, myocardial blood flow, and electrophysiological parameters, and to determine predisposing factors for the development of early ventricular arrhythmias (EVA).The left circumflex coronary artery was occluded in six anesthetized (n=2 piritramide/N₂O, n=4 chloralose/urethane) mongrel dogs. Systolic wall thickening (% WT) in a control zone and in the central ischemic zone was measured with sonomicrometry and regional myocardial blood flow (RMBF) with colored microspheres. Excitability and relative refractory period at the stimulus electrode and conduction times to all other electrodes were determined with a three-dimensional transmural multi(16)-electrode array using a computer algorithm.In three of six dogs spontaneous EVA occurred 4 to 6 min after coronary occlusion, degenerating to ventricular fibrillation in two of these dogs. The three dogs developing EVA were not distinguished from those not developing EVA, neither by the kind of anesthesia nor by ischemic % WT (–6.6±3.8 [SD] vs –7.8±1.6, ns). Also, dogs with and without EVA did not differ significantly in excitability and relative refractory period. In contrast, dogs with EVA were characterized by a greater mass of severely ischemic myocardium, i.e., exhibiting a RMBF reduction to less than 0.1 ml/(min · g) (18±3 g vs 7±4 g, p<0.05), and by an increase in subendocardial conduction times of greater than 100% above the respective pre-ischemic values (120±18% vs 66±9%, p<0.05). Dogs with and without EVA were not as clearly distinguished by the increases in subepicardial (81±22% vs 46±15%, ns) and transmural (98±31% vs 67±14%, ns) conduction times.The development of EVA is associated with a greater mass of severely ischemic myocardium and a greater increase in subendocardial conduction times.Dedicated to Prof. Dr. Werner Meesmann on the occasion of his 68th birthday     

Myocardial mechanical and myosin isoenzyme alterations in streptozotocin-diabetic rats

Fifteen week old male Wistar rats (n = 7) were made diabetic by intravenous injection of streptozotocin (50 mg/kg). Age-matched, untreated male Wistar rats (n = 9) served as controls. Hearts were removed after 5-6 weeks of diabetes, and the isometric developed tension (T) of isolated left ventricular papillary muscles and its first derivative (dT/dt) were measured at a frequency of 0.2 Hz. During testing, the muscles were perfused with Tyrode's solution (Ca2+ concentration was half of normal Tyrode's solution, pH 7.4, 32 degrees C, bubbled with 95% O2 and 5% CO2). In addition, the left ventricular isoenzyme pattern, which is related to myocardial energetics, was determined by pyrophosphate gel electrophoresis. There was no significant difference in isometric developed tension between diabetic and control rats (DM: 2.90 +/- 0.89 vs controls: 2.87 +/- 0.85 g/mm2, mean +/- SD), but in diabetic rats, dT/dtmax decreased significantly as compared with controls (DM: 23.5 +/- 4.2 vs controls: 31.9 +/- 7.9 g/mm2.s, p less than 0.05). Myocardial mechanical responses to isoproterenol (10(-7)M) and dibutyryl cyclic AMP (10(-5)M) also decreased in diabetic rats. The left ventricular myosin isoenzyme pattern shifted toward VM-3 in diabetic rats (VM-3: DM: 74.9 +/- 10.7 vs controls: 9.5 +/- 4.1%, p less than 0.001). These results indicate that diabetes influences myocardial contractility and changes cardiac energetics. Post-receptor processes may play a role in myocardial mechanical responses to catecholamines in streptozotocin-diabetic rats.     

Association of ventricular myosin heavy chains in functional states which lead to isoenzyme populations encompassing the whole range of possible distribution

Summary A statistical model is presented which describes quantitatively the distribution of ventricular myosin (VM) isoenzymes VM-1, VM-2 and VM-3. In order to account for the actual distribution of the isoenzymes, it was assumed that the probability for formation of the heterodimer VM-2 (alpha- and beta-heavy chain) is lower than that of the homodimers VM-1 (2 alpha-heavy chains) or VM-3 (2 beta-heavy chains). The relation VM-2=0.85 (VM-1·VM-3)^0.5 describes quantitatively the proportion of the 3 isoenzymes in a given population. The model was established for 252 sedentary normotensive Wistar rats and spontaneously hypertensive rats (SHR) fed ad libitum. It is demonstrated that the isoenzyme populations of rats subjected to 8 weeks experimental routines involving intermittent feeding (1 day feeding ad libitum, followed by 1 or 2 days fasting) or swimming also obey this theoretical distribution. Intermittent feeding led to an increased proportion of VM-3 in Wistar rats and SHR, whereby the latter approached the limits of the possible distribution. Intermittent swimming resulted in an increased proportion of VM-1 which was independent of the feeding schedule in SHR. In Wistar rats, however, the swimming rats fed intermittently exhibited a significantly smaller proportion of VM-1. By combining certain experimental routines, it is thus possible to induce within 8 weeks an isoenzyme population of a predefined composition, nearly within the whole range of possible distribution. The data further demonstrate that the formal correlation between the proportion of beta-heavy chains in a myosin population and ventricular mass holds only when the rats are kept under standard conditions. Following the routines involving an altered pattern of food intake or intense physical activity, the isoenzyme population can be altered in a manner unrelated to ventricular mass. Thus, swimming induced a redistribution in favour of VM-1 irrespective of the feeding pattern, although ventricular mass differed greatly in both the intermittently or ad libitum fed swimming rats. It is concluded that, besides ventricular mass, neuro-endocrine status is decisive for the isoenzyme population of myosin.Dedicated to Professor Dr. R. Jacob on the occasion of his 60th birthday.Presented at the Symposium Aktuelle Probleme der Herzdynamik (1984) supported by Fritz-Thyssen-Stiftung.     

Regional oxygen supply and consumption balance in experimental left ventricular hypertrophy

Summary The aim of the present study was to determine if the relationship between myocardial O₂ supply and O₂ consumption was preserved after prolonged pressure overload due to aortic valve stenosis. This was examined in anesthetized open-chest dogs in which the aortic valve was plicated 6 months previously. We measured coronary blood flow with radioactive microspheres and regional small vessel O₂ saturation with microspectrophotometry, to obtain O₂ supply, and O₂ consumption. Regional O₂ consumption was calculated as the product of flow and O₂ extraction. The left ventricular weight/body weight ratio was 81% greater in the dogs with aortic valve stenosis. There were no hemodynamic differences between the groups except that left ventricular systolic pressure was 38±22 mm Hg greater than aortic in the hypertrophied group. Coronary blood flow did not differ between the control and hypertrophied groups nor were there subepicardial vs subendocardial differences. When maximal coronary flow was determined with chromonar (10 mg/kg), the flow increase was significantly attenuated in the hypertrophied subendocardium (242.1±82.3 (hypertrophy) vs 512.4±204.1 ml·min^–1·100 g^–1 (control)). There were no significant differences in O₂ extraction or O₂ consumption/g between control and hypertrophied animals. There was a significantly lower O₂ supply/consumption ratio in the subendocardium compared to the subepicardium of both groups. However, the O₂ supply/consumption ratio was not decreased by hypertrophy. Thus, despite significant hypertrophy, a loss of flow reserve and a high left ventricular pressure, O₂ supply/consumption balance is preserved in valvular aortic stenosis at rest.     

Alterations in the myocardial capillary vasculature accompany tachycardia-induced cardiomyopathy

Summary Changes in capillary structure and distribution within the left ventricle (LV) occur with pressure and volume overload hypertrophy. These changes may cause an impairment in myocardial blood flow (MBF) and oxygen delivery resulting in myocyte injury and LV dysfunction. However, it is unknown whether changes in the capillary vasculature accompany the development of LV dysfunction in dilated cardiomyopathies. Accordingly, this study examined the relation between LV function, MBF and capillary architecture after the development of dilated cardiomyopathy in pigs produced by chronic supraventricular tachycardia (SVT). LV function was examined by echo-catheterization, and capillary morphometrics were computed using lectin histochemistry in two groups of pigs: sham controls (n=8); and after 3 weeks of pacing-induced SVT (n=8). Chronic SVT resulted in significantly incresed LV end-diastolic dimension and pressure with a 50% reduction in LV fractional shortening compared to CON (p<0.05). Although no significant change in capillary density (2180±164 vs 2402±147/mm², p=0.25) occurred in the SVT group compared to CON, a significant reduction in the volume fraction of capillaries (12.2±0.5 vs 9.9±0.7%, p<0.05) and increased capillary diffusion distance (8.4±0.5 vs 7.5±0.3 m, p<0.05) was observed. Frequency distribution analysis revealed a higher percentage of smaller diameter capillaries with chronic SVT vs CON (p<0.05). Ultrastructural examination revealed an increased capillary-myocyte distance with chronic SVT (0.95±0.08 vs 1.95±0.12 m, p<0.05). These changes were accompanied by ultrastructural evidence of significant subendocardial injury (p<0.05). MBF was measured using microspheres in five additional conscious pigs in each group. MBF was significantly reduced and coronary vascular resistance increased in the SVT group compared to CON (p<0.05). Chronic SVT caused significant remodeling of the capillary vasculature; these changes were associated with reduced MBF, myocyte injury, and LV dysfunction.     

Fluoro-fatty acids and the impairment of cardiac function in the rat in vivo and in vitro

Summary Dichapetalum toxicarium seeds contain long chain fluoro-fatty acids, particularly fluorooleic acid, which in doses as low as 10 mg/kg can cause death. We have used the rat heart both in vivo and in vitro to assess the cardiovascular effects of various doses of the fluoro-oleic acid extract of the seeds ofDichapetalum toxicarium. Intraperitoneal administration of 0.25 ml of seed extract solution/kg body weight (estimated to be equivalent to 10 mg fluoro-oleic acid/kg body weight) or 0.5 ml/kg body weight (equivalent to 20 mg fluoro-oleic acid/kg body weight) resulted in death in all animals (n–6 in each group). The mean time from administration to death was 36.4±4 h and 21.0±2 h, respectively. Death was attributable to severe bradycardia which developed progressively throughout the experiment. Thus, during the first 6 h, heart rate fell by 32.2% from 450±7 beats/min to 305±36 beats/min (p<0.01) in the 0.25 ml/kg group and by 66±10% to 150±20 beats/min (p<0.001) in the 0.5 ml/kg group. Administration of the extract solution alone or oleic acid alone (equivalent to 0.5 ml/kg seed extract) to control rats had no effect. Investigating the effects of the seed extract in vitro, hearts (n=6 in each group) were perfused with buffer containing 0.5 ml/l seed extract (equivalent to 20 mg fluoro-oleic acid/l) or with buffer containing extract solution alone. In control hearts, there were no changes in heart rate or LVEDP over 150 min of aerobic perfusion; there were small declines in coronary flow, LVDP and LVdP/dt_max. Perfusion with seed extract resulted in a progressive decline in heart rate which, over the first 60 min of perfusion, fell by 53±7% from its control value of 323±11 beats/min to 153±20 beats/min (p<0.001). Similarly, significant declines in coronaryflow (fell by 53±12% over 60 min), LVDP (fell by 61±12% over 60 min) and LVdP/dt_max (fell by 63±7% over 60 min) were observed. LVEDP began to increase after 30 min and by 60 min had increased to 2.8±0.4 kPa (control value=0 kPa). Perfusion with oleic acid (20 mg/l) alone had no adverse effect on cardiac contractile function. Metabolic studies showed that the administration of the seed extract resulted in a loss of myocardial high energy phosphates. Thus, ATP and creatine phosphate fell from their control values of 20.6±0.4 and 27.8±0.6 mol/g dry weight to 3.9±0.4 and 2.4±0.4 mol/g dry weight (p<0.001 in each instance), respectively. Lactic acid content increased from 1.4±0.2 to 21.7±1.4 mol/g dry weight (p<0.001). In conclusion, our results indicate that the fluoro-oleic acid-containing extract ofDichapetalum toxicarium exerts its toxic effects by severely reducing cardiac function. In vivo this can lead to death of the animal.     

Open chest and open pericardium affect the distribution of myocardial blood flow in the right ventricle

Summary We have investigated the effects of open chest and open pericardium on the distribution of myocardial blood flow assessed with the radioactive microsphere technique (15 m). Five dogs with intact thorax served as controls (group I) and six dogs were studied after a right-sided thoracotomy and pericardiotomy (group II). Global myocardial blood flow (mean±S.D.) was 0.73±0.17 ml·min^–1·g^–1 in group I and 1.22±0.09 ml·min^–1·g^–1 in group II (p<0.05). Analysis of transmural blood flow distribution revealed that flow was 44% higher in the right and 60% higher in the left ventricular endocardial layers in the open-chest animals, whereas epicardial flow increased by 105% and 90%, respectively. As a result of the preferential blood flow to the epicardial layers of the right ventricle, the endo/epi ratio was reduced from 1.30±0.26 in group I to 0.86±0.11 in the open-chest group (p<0.05). Left ventricular endo/epi ratio was 1.27±0.16 and 1.06±0.11 (n.s.), respectively. External work and diastolic filling pressure of the right ventricle did not differ between the two groups and therefore cannot account for the redistribution of myocardial blood flow. It is concluded that the distribution of myocardial blood flow, particularly in the RV, is severely disturbed by thoracotomy and pericardiotomy. This is an important aspect for the planning and evaluation of studies under open-chest/open-pericardium conditions.Supported by Deutsche Forschungsgemeinschaft grant SFB 320     

Sex differences in the tolerance of immature rat myocardium to global ischemia

Summary Currently there is considerable interest in the metabolism of the immature myocardium and in particular the mechanisms underlying its greater tolerance to ischemia than that of the adult heart. In order to investigate whether this tolerance is sex-related, we compared the recovery of function in isolated hearts from male and female neonatal rats (three to five days old) following 60 min of normothermic global ischemia and 30 min of reperfusion (n=8 per group). The female hearts exhibited significantly better (p<0.05) recovery of rate (81±5% vs. 65±5%) and the rate-pressure product (73±9% vs. 37±8%), and a tendency towards better recovery of contractile function (left ventricular developed pressure, 89±9% vs. 59±12%; dP/dt, 84±12% vs. 54±13%). This evidence for greater resistance of female hearts to ischemic injury was supported by a delayed onset of contracture (mean time to onset, 29.4±2.7 min vs. 24.9±2.6 min). The loss in left ventricular compliance during ischemia and reperfusion was also smaller in the female hearts (increase in left ventricular end diastolic pressure, 6.5±1.2 mm Hg vs. 13.6±3.8 mm Hg). These results suggest that there may be sex-related differences in the tolerance of immature hearts to ischemia, a factor which should be taken into account in the design and interpretation of experimental studies.     

Exercise training,indomethacin, and isoproterenol-induced myocardial necrosis in the rat

Summary It was hypothesized that endurance exercise training would attenuate isoproterenol-induced myocardial necrosis in the rat by increasing the concentration of prostacyclin in the myocardial vasculature. Rats were randomly assigned to exercise and control groups. Exercisers ran on a motorized treadmill 1 h · d^–1, 5 d · week^–1 for 14 weeks. Immediately following the training program subgroups of rats were injected with 4 mg · kg^–1 indomethacin or saline. One day later, all rats were given a subcutaneous injection of isoproterenol (20 mg · kg^–1); after another 24h they were sacrificed. A decrease of myocardial creatine kinase (CK) activity was used as a marker for myocardial necrosis. Endurance exercise training attenuated the isoproterenol-induced decrease in myocardial CK relative to control by approximately 37% (exercise: 16.4 ± 0.6 U · mg^–1 protein; control: 10.5 ± 0.6 U · mg^–1 protein; p < 0.05). Pretreatment with indomethacin decreased myocardial CK in the exercise-trained rats (indomethacin: 15.4 ± 0.8 U · mg^–1 protein; saline: 17.7 ± 0.7 U · mg^–1 protein; p < 0.05), but not in the controls (indomethacin: 10.3 ± 1.0 U · mg^–1 protein; saline: 10.8 ± 0.6 U · mg^–1 protein; p > 0.05). The concentration of myocardial 6-keto-PGF₁ a marker for prostacyclin, was not altered by exercise but, as expected, was reduced by indomethacin pretreatment (p < 0.05). Thus, exercise training reduces myocardial damage caused by isoproterenol, but the evidence does not support the hypothesis that prostacyclin mediated this effect of training. Further research is needed to determine the extent to which exercise training-induced alterations in sensitivity to PGI₂ or TXA₂ affect myocardial damage from isoproterenol. The effect of any exercise-induced alteration in cardiac -adrenoceptor number or binding characteristics which result in myocardial resistance to isoproterenol also cannot be overlooked.