Volume overload influence on hypertrophied myocardium function

The aim of this study was to demonstrate that hypertrophied cardiac muscle is more sensitive to volume-overload than normal cardiac muscle. We assessed the mechanical function of isolated left ventricular papillary muscle from male spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto rats (WKY) submitted to volume overload caused by aortocaval fistula (ACF) for 30 days. Muscles were perfused with Krebs-Henseleit solution at 28 degrees C and studied isometrically at a stimulation rate of 0.2 Hz. The ACF increased the right and left ventricular weight-to-body weight ratio in WKY rats; it also promoted right ventricular hypertrophy and further increased the basal hypertrophy in the left ventricle from SHR. The arterial systolic pressure was greater in SHR than in WKY rats, and decreased with ACF in both groups. Developed tension (DT) and maximum rate of DT (+dT/dt) were greater in the SHR-control than in the WKY-control (P < 0.05); the time from peak tension to 50% relaxation (RT 1/2) was similar in these animals. ACF did not change any parameters in the SHR group and increased the resting tension in the WKY group. However, the significant difference observed between myocardial contraction performance in WKY-controls and SHR-controls disappeared when the SHR-ACF and WKY-controls were compared. Furthermore, RT 1/2 increased significantly in the SHR-ACF in relation to the WKY-controls. In conclusion, the data lead us to infer that volume-overload for 30 days promotes more mechanical functional changes in hypertrophied muscle than in normal cardiac muscle.     

Mechanical properties of papillary muscle in cardiac failure: importance of pathogenesis and of ventricle of origin

In an attempt to better understand what causes impairment of failing myocardium, the mechanical characteristics of papillary muscles from three different models of congestive heart failure were compared at varying stages of hypertrophy and failure: adriamycin cardiotoxic rabbit, cardiomyopathic hamster and infra-renal aorto-caval shunted dog. Except for right ventricular muscles from the shunted dogs, in all groups there was a significant decrease in total twitch tension, +dT/dt, -dT/dt, Vmax, +dL/dt, and -dL/dt. However, there were major differences in twitch duration between the three models with time to attain peak tension and peak shortening and time to attain half tension decline decreasing in right ventricular papillary muscles from adriamycin rabbits but increasing in right ventricular papillary muscles from shunted dogs and no change occurring in left ventricular papillary muscles from cardiomyopathic hamsters or shunted dogs. Load dependence as assessed by time to relaxation index for 30% afterload contractions was decreased in all but right ventricular muscles from shunted dogs. These results indicate that despite some common characteristics, major differences exist between papillary muscles from different models of congestive heart failure depending on the pathophysiological process involved and the ventricle of origin.     

Effects of captopril on contractility after myocardial infarction: experimental observations.

After large myocardial infarction, compromised left ventricular (LV) function and changes in the peripheral circulation result in the syndrome of chronic congestive heart failure. Although treatment with angiotensin-converting enzyme inhibitors improve cardiovascular function, it is difficult to determine whether this benefit is due to changes in organ versus muscle function. The rat model of heart failure, created by ligating the left coronary artery, results in pathophysiology that is similar to that seen in patients, i.e., increased LV end-diastolic pressure and volume, hypertrophy of the noninfarcted myocardium, prolongation of the time constant of LV relaxation, decreased venous compliance, and increased total blood volume. In noninfarcted papillary muscles, isolated from rats with heart failure, maximal developed tension and peak rate of tension rise (+dT/dt) are decreased, time to peak tension is prolonged, and myocardial stiffness is increased. Morphologic changes include an increase in papillary muscle myocyte cross-sectional area and an increase in myocardial hydroxyproline content. Captopril (2 g/liter drinking water) alters LV loading by decreasing arterial pressure, increasing venous compliance, and decreasing blood volume. This results in a decrease in LV end-diastolic pressure and volume. In the noninfarcted myocardium, time to peak tension is shortened, whereas developed tension, +dT/dt, and muscle stiffness remain abnormal. Captopril decreases myocyte cross-sectional area, but collagen content remains elevated. Thus, in the rat infarct model of heart failure, treatment with captopril alters LV remodeling and hypertrophy but produces only modest improvement in muscle function.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of long term treatment with an alpha 1 adrenoceptor blocker on cardiac hypertrophy, contractility, and myosin isoenzymes in spontaneously hypertensive rats.

STUDY OBJECTIVE--The aim was to investigate the effects of long-term treatment with an alpha 1 blocker, bunazosin hydrochloride, on blood pressure, heart weight, myocardial contractility, and ventricular myosin isoenzyme pattern in spontaneously hypertensive rats (SHR). DESIGN--Bunazosin hydrochloride was given orally in a dose of 10 mg.kg-1.d-1 for 8-9 weeks. Isometric tension development was measured in isolated left ventricular papillary muscles. The left ventricular myosin isoenzyme pattern was determined by pyrophosphate gel electrophoresis. SUBJECTS--14 male SHR (seven treated and seven untreated rats) aged 23 to 24 weeks were studied. MEASUREMENTS AND MAIN RESULTS--The blood pressure of the bunazosin treated rats was approximately 14% lower than that of the untreated rats at the end of the treatment period. The ventricular weight of treated SHR was significantly lower (around 8%) than that of untreated rats, but there were no significant differences between the two groups in the mechanical data obtained from isolated left ventricular papillary muscles. The myosin isoenzyme pattern of the left ventricle was significantly shifted toward VM-1 in the bunazosin treated SHR, the VM-1 concentration in the treated group being 38% greater than in the untreated group. CONCLUSIONS--These results indicate that long term treatment with bunazosin hydrochloride reduces blood pressure and leads to the regression of cardiac hypertrophy in SHR. Myocardial energetics (as represented by the myosin isoenzyme pattern) were affected, but there was no influence on myocardial tension development.     

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.     

Contractility and stiffness of noninfarcted myocardium after coronary ligation in rats. Effects of chronic angiotensin converting enzyme inhibition

BACKGROUND. Previous studies have shown that global left ventricular function is depressed after myocardial infarction. However, little is known about the effects of myocardial infarction on contractility and the passive-elastic properties of residual myocardium. METHODS AND RESULTS. We evaluated isometric function and passive myocardial stiffness in isolated, noninfarcted left ventricular papillary muscle from rats 6 weeks after sham operation or myocardial infarction. Maximal developed tension and peak rate of tension rise (+dT/dt) were significantly decreased in untreated rats with large myocardial infarction compared with controls (3.3 +/- 1.1 versus 4.3 +/- 0.6 g/mm2 and 49.5 +/- 17.5 versus 72.5 +/- 10.5 g/mm2/sec, respectively). Time to peak tension was prolonged (120 +/- 8 versus 102 +/- 4 msec) and myocardial stiffness was increased in untreated myocardial infarction rats compared with controls (35.2 +/- 4.9 versus 24.2 +/- 3.7). Rats with smaller myocardial infarctions differed from controls only with respect to a prolongation of time to peak tension. Papillary muscle myocyte cross-sectional area was increased by 44% (p less than 0.05), and myocardial hydroxyproline content was increased by 160% (p less than 0.05) in rats with large myocardial infarctions compared with controls. To determine whether treatment that improves left ventricular function after myocardial infarction also improves myocardial function, rats were treated with captopril beginning 3 weeks after myocardial infarction and continuing for 3 weeks. Treatment with captopril attenuated the prolongation in time to peak tension in the myocardial infarction rats; however, developed tension, +dT/dt, and muscle stiffness remained abnormal. Compared with untreated myocardial infarction rats, captopril-treated myocardial infarction rats had a 9% decrease in myocyte cross-sectional area (p = 0.1) but a persistent increase in myocardial collagen content. In summary, large myocardial infarction in rats causes contractile dysfunction, increased stiffness, myocyte hypertrophy, and increased collagen content in the residual noninfarcted myocardium. Treatment with captopril alters the process of cardiac remodeling and hypertrophy and improves one parameter of contractility in noninfarcted myocardium; however, myocardial collagen content and myocardial stiffness remain abnormal. CONCLUSIONS. These findings suggest that angiotensin converting enzyme inhibition in the rat infarct model of heart failure improves global cardiac performance via combined effects on myocardial function and the peripheral circulation.     

Connective tissue content and myocardial stiffness in pressure overload hypertrophy A combined study of morphologic,morphometric, biochemical,and mechanical parameters

We investigated samples of left ventricular myocardium from Goldblatt II (4 and 8 weeks after operation) and spontaneously hypertensive rats (SHR; 40 and 80 weeks old) by histological and morphometric methods. From the same hearts, the distensibility of the left ventricular papillary muscle was analyzed by means of resting tension curves, and the collagen content of the whole left ventricular wall was determined by means of hydroxyproline concentration. In all groups, myocardial fibrosis was observed to accompany myocardial hypertrophy. The severity of fibrotic lesions increased with the duration of hypertension, and, in late stages, degenerative changes of cardiac myocytes were found. Morphometric determinations and chemical analysis of the hydroxyproline concentration revealed a decrease in myocardial muscle content, which was paralleled by an increase in collagen content when compared to the respective controls. In general, morphometric and chemical findings correlate with increased myocardial stiffness observed during mechanical measurements in isolated papillary muscle preparations from the same hearts. Differences were found, however, between chemical analysis and mechanical measurements in the 40-week-old SHR group, which may result from different patterns of collagen distribution between interstitium, perivascular spaces, and the walls of blood vessels. The comparison between histological, morphometric, chemical, and physiological data shows that (1) cardiac hypertrophy of Goldblatt and SH-rats is accompanied by myocardial fibrosis, and (2) changes in passive elastic properties of myocardium is better reflected in morphometric than in chemical analysis.     

Augmented right ventricular function in systemic hypertension-induced hypertrophy.

The contractile properties of right ventricular papillary muscles from the hearts of 15 rats which had developed hypertension 6 weeks following renal artery ligation were compared with those from 14 normal litter-mates. In the experimental group, the heart weight-body weight ratio was increased by 39%, while the right ventricular weight-body weight ratio increased 20%. Right ventricular papillary muscles from the hypertensive rats demonstrated increased tension development at the apex of the length-active tension curve (P less than 0 X 0001), elevated maximal rate of tension development (P less than 0 X 001), and increased maximal velocity of contraction at muscle lengths corresponding to both a light preload and at Lmax (P less than 0 X 05). Resting tension and time-to-peak tension in the muscles from the hypertensive group were not significantly different from the normal group. Thus, improved right ventricular performance, in the presence of increased left ventricular afterload, may indicate the existence of a stimulus to increased function in hypertrophied muscle not yet negated by the adverse effects of direct exposure to stress.     

Effects of beta-adrenergic blockade on papillary muscle function and the beta-adrenergic receptor system in noninfarcted myocardium in compensated ischemic left ventricular dysfunction.

BACKGROUND. beta-Adrenergic receptor blockade has been reported to improve hemodynamics and beta-adrenergic receptor-adenylate cyclase function in idiopathic dilated cardiomyopathy. The purpose of this study was to determine the effects of beta-adrenergic receptor blockade on the beta-adrenergic receptor system and myocardial function in a model of compensated ischemic heart failure. METHODS AND RESULTS. We examined the effects of propranolol treatment on the beta-adrenergic receptor-adenylate cyclase system and isolated papillary muscle isometric function in noninfarcted left ventricular myocardium in rats after coronary artery ligation. In untreated rats with large myocardial infarction (MI), developed tension (DT) (3.0 +/- 0.7 versus 5.1 +/- 1.1 g/mm2), peak rate of tension rise (+dT/dt) (40.3 +/- 9.5 versus 71.2 +/- 12.0 g/mm2/sec), and peak rate of tension fall (-dT/dt) (24.4 +/- 5.0 versus 38.2 +/- 6.0 g/mm2/sec) were decreased (p < 0.05). In addition, DT, +dT/dt, and -dT/dt of untreated MI rats demonstrated an impaired response to isoproterenol stimulation compared with controls. beta-Adrenergic receptor density (Bmax) measured by [125I]iodocyanopindolol (ICYP) binding was decreased 23% after infarction (9.3 +/- 0.6 versus 12.0 +/- 1.8 fmol/mg protein [prot]) (p < 0.05); however, the dissociation constant (Kd) for ICYP was not changed (24.1 +/- 5.7 versus 33.2 +/- 12.1 pM). Adenylate cyclase activity in the presence of 10(-2) M MgCl2 was reduced (p < 0.05) in MI rats (30.3 +/- 10.8 versus 45.9 +/- 12.5 pmol cAMP/min/mg prot). Maximal isoproterenol (52.5 +/- 7.3 versus 79.9 +/- 10.0 pmol cAMP/min/mg prot), guanyl-5'-imidodiphosphate (GppNHp) (95 +/- 8 versus 141 +/- 25 pmol cAMP/min/mg prot) and forskolin (503 +/- 76 versus 753 +/- 157 pmol cAMP/min/mg prot) stimulation of adenylate cyclase was also decreased (p < 0.05). In addition, manganese-stimulated adenylate cyclase activity was depressed (p < 0.05) in MI rats compared with controls (23.5 +/- 2.8 versus 52.1 +/- 9.0 pmol cAMP/min/mg prot). Chronic propranolol treatment in MI rats improved DT (4.1 +/- 0.9 versus 3.0 +/- 0.7 g/mm2) and +dT/dt (54.4 +/- 11.3 versus 40.5 +/- 9.5 g/mm2/sec) (p < 0.05); however, isoproterenol-stimulated isometric function remained impaired. Propranolol treatment normalized Bmax (11.9 +/- 1.7 versus 9.3 +/- 0.6 fmol/mg prot) (p < 0.05), whereas adenylate cyclase activity remained depressed. CONCLUSIONS. After large MI in rats, there is impaired papillary muscle function with decreased beta-adrenergic receptors and adenylate cyclase activity in the noninfarcted myocardium. Propranolol treatment improved basal isometric muscle function and beta-adrenergic receptor density in rats after myocardial infarction but did not improve adenylate cyclase activity or isoproterenol-stimulated muscle function. These data suggest that there is a primary defect in adenylate cyclase function that persists despite upregulation of receptors with propranolol treatment.     

Relationship between ejection phase indices of performance and myocardial functions during the development of pressure overload hypertrophy

The present study examines the temporal relationship between performance of the hypertrophied nonfalling rabbit heart and the contractility of muscles isolated from these same hearts. Ejection phase indices of ventricular function were determined cineradiographically during the development of hypertrophy. Isolated papillary muscle function was examined an average of 8.6 (early), 40.1 (middle), and 97.5 (late) days after banding of the pulmonary artery. Active tension development at Lmax (F) was depressed by 65% in the muscles examined early and by 43% in the middle group. By the late group, F was comparable to control levels. Early depression and a return to normal function were also observed for peak dF/dt and velocity of shortening at Lmax. Time to peak tension was unchanged at 8.6 days and increased at the middle and late time points. Both percentage of shortening and mean normalized velocity of shortening, determined cineradiographically in the intact heart, were depressed immediately following surgery, gradually returned toward "normal function" by the third week, and plateaued at a stable level of performance, which was maintained thereafter. The similarity of in vivo function of the hypertrophied and control hearts, despite the profound differences observed in the myograph, indicate that "ejection phase indices," such as fiber shortening rate, are poor indicators of intrinsic function during the development of hypertrophy. Moreover, these results demonstrate the extent to which intrinsic functional deficits may be overcome in the whole heart and suggest the presence of mechanisms such as enhanced sympathetic nervous activity, which contribute to the maintenance of this normal basal ventricular function.     

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.     

Myocardial function during chronic food restriction in isolated hypertrophied cardiac muscle

BACKGROUND: The effect of food restriction (FR) on myocardial performance has been studied in normal hearts. Few experiments analyzed the effects of undernutrition on hearts subjected to cardiac overload. The aim of this study was to determine whether chronic FR promotes more significant changes in hypertrophied hearts than in normal hearts. METHODS: Myocardial performance was studied in isolated left ventricular papillary muscle from young male spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto rats (WKY) submitted to FR or to control diet. The animals subjected to FR were fed 50% of the amount of food consumed by control groups for 60 days. Isolated muscles were studied while contracting isometrically and isotonically. RESULTS: FR decreased the body weight and the left ventricular weight in both groups. FR increased the left ventricular weight-to-body weight ratio in the WKY rats and tended to decrease this ratio in SHR (P = 0.055). The arterial systolic pressure was greater in SHR than in WKY groups and did not change with FR. In the animals with normal diet, myocardial performance was better in SHR than in WKY. FR increased time to tension to fall from peak to 50% of peak tension and time to peak tension in the WKY rats and time to peak tension in the SHR. CONCLUSIONS: FR for 60 days has a trend to attenuate the development of cardiac hypertrophy and does not promote more mechanical functional changes in the hypertrophied myocardium than in the normal cardiac muscle.     

Sarcomere length-tension relations in living rat papillary muscle.

Small papillary muscles about 2 mm long and 0.2 mm thick were dissected from the right ventricles of 16-19 day-old rats. Resting (between twitches) and active (at twitch peaks) striation patterns were photographed in living muscles using a light microscope. External muscle length was varied from Lmax, the length at which peak twitch tension was maximum, to 0.75Lmax, the length at which peak twitch tension was about 10% of maximum. Resting and active tension versus muscle length curves were similar to those obtained from other papillary muscle preparations. Resting average sarcomere length at Lmax was about 2.23 mu; it decreased with decreasing muscle length in the range between Lmax and 0.75Lmax. Near 0.75Lmax, resting average sarcomere length was about 1.5-1.6mu. Considerable internal shortening occurred during contractions, and the active average sarcomere lengths measured at the twitch peaks were less than the resting values. At Lmax, the active average sarcomere length was 1.98mu. At 0.75Lmax, there was only about a 3-6% decrease in average sarcomere length at the twitch peaks. However, at external muscle lengths between Lmax and 0.75Lmax more internal shortening was present than there was at Lmax, since average sarcomere length decreases of about 15% were observed. The finding that peak active tension decreases as sarcomere length decreases below about 2.0mu suggests that some of the factors limiting force generation at short lengths in skeletal muscle may also limit it in mammalian cardiac muscle.     

Measurement of relaxation in isolated rat ventricular myocardium during hypoxia and reoxygenation

The effects of hypoxia and subsequent reoxygenation were examined in isolated left ventricular papillary muscles from the rat at 28 degrees C and 33 degrees C. Studies of relaxation were carried out in isometric and physiologically sequenced contractions. In studies of isometric contractions, the following variables were determined: active tension (AT), maximum rate of tension increase (+dT/dt), time to peak tension (TPT), time for tension to fall from peak to 50% of peak tension (RT1/2), maximum rate of tension decline (-dT/dt), isometric peak (-dT/dt/T), and isometric maximum (-dT/dt per T). Variables measured in physiologically sequenced contractions were the slopes of the relation between -dT/dtmax and end systolic length (SIM) and maximum rate of isotonic muscle lengthening (+dL/dtmax) and end systolic length (SIT). Tau, the exponential time constant for isometric relaxation, was also examined. Pronounced changes in active tension and +dT/dt were seen during hypoxia at both temperatures, whereas changes in measured relaxation variables were less prominent and inconsistent. At neither 28 degrees C nor 33 degrees C did TPT or RT1/2 indicate impaired relaxation during hypoxia. Isometric peak -dT/dt declined with hypoxia at both temperatures but the normalised indices, isometric peak -dT/dt/T, peak (-dT/dt per T), and tau showed consistent impairment of relaxation only at 33 degrees C. In physiologically sequenced contractions, SIM suggested impaired relaxation during hypoxia at 28 degrees C but not at 33 degrees C. SIT showed impaired relaxation at both 28 degrees C and 33 degrees C. These findings are consistent with data suggesting impairment of the cardiac relaxing system during hypoxia. Nevertheless, relaxation appears less affected than contraction, and impairment is best seen late in the cardiac cycle.     

Dependence of peak dP/dt and mean ejection rate on load and effect of inotropic agents on the relationship between peak dP/dt and left ventricular developed pressure--assessed in the isolated working rat heart and cardiac muscles.

To examine the effects of preload (mean left atrial pressure) and afterload on two so-called "contractility indices" and the effects of inotropic agents (isoproterenol and calcium) on the relationship between left ventricular developed pressure and peak dP/dt when afterload was increased, we used a modified working rat heart preparation (perfused with Krebs-Henseleit solution bubbled with a 95% O2 -5% CO2 gas mixture at 37 degrees C). The atrium was stimulated at a rate of 240/min. An increase in preload from 5 to 15 mmHg caused an increase in peak dP/dt from 1,711 +/- 293 mmHg/s to 1,971 +/- 387 mmHg/s (p less than 0.001, n = 15), and an increase in the mean systolic ejection rate from 1.28 +/- 0.31 ml/s to 2.74 +/- 0.80 ml/s (p less than 0.001). An increase in afterload (left ventricular developed pressure) from 66 to 97 mmHg produced by elevating aortic pressure caused an increase in peak dP/dt from 1,829 +/- 222 mmHg/s to 2,449 +/- 254 mmHg/s (p less than 0.001, n = 7), and a decrease in the mean systolic ejection rate from 2.12 +/- 0.36 ml/s to 1.95 +/- 0.33 ml/s (p less than 0.001). Studies using isolated rat papillary muscles ruled out the contribution of an increase in myocardial perfusion to the increase in peak dP/dt, since the maximum rate of rise in tension (dT/dt) increased with an increase in afterload during afterloaded isotonic contraction. Peak dP/dt showed a linear relationship to left ventricular developed pressure when the latter was increased by elevating the aortic reservoir and then clamping the aortic outflow tube (n = 8).(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Contractile effects of Ghrelin and expression of its receptor GHS-R1a in normal and hypertrophic myocardium.

INTRODUCTION: Ghrelin, isolated in 1999, is an endogenous ligand for the growth hormone secretagogue receptor (GHS-R1a). Recent studies suggest that it may influence the function of normal and failing hearts. Nonetheless, it has been difficult to differentiate its effects on the intrinsic properties of the myocardium from the secondary effects resulting from growth hormone release and vasomotor action. This study investigated the contractile effects of ghrelin and expression of its receptor GHS-R1a in normal and hypertrophic myocardium. METHODS: Adult Wistar rats randomly received monocrotaline (MCT; n=9; 60 mg/kg, s.c.) or vehicle (n=7; 1 ml/kg). Three weeks later, after right ventricular (RV) hemodynamic evaluation, the effects of 10(-6) M of a pentapeptide active fragment of ghrelin (fG) were tested on contractile parameters of RV papillary muscles (Normal, n=7; MCT, n=9). GHS-R1a mRNA expression was estimated in RV transmural free-wall samples (Normal, n=7; MCT, n=9), using real-time RT-PCR. RESULTS: In the Normal group, fG reduced active tension (AT), maximum velocity of tension rise (dT/dt(max)) and maximum velocity of tension decline (dT/dt(min)), by 27.9 +/- 4.0%, 28.5 +/- 6.7% and 21.4 +/- 4.2% respectively. In the MCT group, fG reduced AT, dT/dt(max) and dT/dt(min) by 24.1 +/- 6.3%, 24.3 +/- 6.5% and 24.5 +/- 6.1% respectively. GHS-R1a mRNA expression was similar in the two groups (Normal: 2.3*10(5) +/- 5.4*10(4); MCT: 3.0*10(5) +/- 1.1*10(5): p > 0.05). CONCLUSION: This study shows that ghrelin has negative inotropic and lusitropic effects. These effects and expression of its receptor are preserved in RV hypertrophy, suggesting that ghrelin may be a new target in progression to heart failure.     

Regression of hypertensive myocardial fibrosis by Na(+)/H(+) exchange inhibition

We have recently reported that the inhibition of the Na(+)/H(+) exchanger (NHE) during 1 month in spontaneously hypertensive rats (SHR) is followed by regression of cardiomyocyte hypertrophy but not of myocardial fibrosis. The aim of this study was to evaluate whether a treatment of longer duration could reduce myocardial fibrosis and stiffness. SHR received 3.0 mg/kg per day of the specific NHE-1 inhibitor cariporide; the effect on cardiomyocyte cross-sectional area, myocardial collagen volume fraction, collagen synthesis, and myocardial stiffness (length-tension relation in left papillary muscles) was evaluated at several time points (after 1, 2, or 3 months). A slight decrease of approximately 5 mm Hg in systolic blood pressure was observed after 1 month of treatment with no further changes. After 2 and 3 months of treatment, the size of cardiomyocytes remained within normal values and myocardial fibrosis progressively decreased to normal level. Accordingly, myocardial stiffness and the serum levels of the carboxyterminal propeptide of procollagen type I, a marker of collagen type I synthesis, were normalized after 3 months. Left ventricular weight decreased from 910+/-43 (in untreated SHR) to 781+/-21 mg (treated SHR) after 3 months of treatment. No difference in body weight between treated and untreated SHR was observed after this period of treatment. The present data allow us to conclude that in the SHR the administration of an NHE-1 inhibitor for 2 or 3 months leads to the normalization of collagen type I synthesis, myocardial collagen volume fraction, and stiffness.     

Modulation of the myocardial effects of selective ETB receptor stimulation and its implications for heart failure.

INTRODUCTION: Endothelin-1 (ET-1) acts on two types of receptors, ET(A) and ET(B). Recent functional studies suggest the existence of two ET(B) receptor subtypes in the heart: ET(B1), located on endocardial endothelial cells and responsible for negative inotropism, and ET(B2), located on myocardial cells and responsible for positive inotropism. The aim of the present study was to investigate the mechanisms underlying the myocardial effects of selective ET(B) receptor stimulation. METHODS: The study was performed on right papillary muscles from New Zealand white rabbits (n = 39; Krebs-Ringer; 1.8 mM CaCl2; 35 degrees C). The effects of sarafotoxin S6c (SRTXc, ET(B) agonist; 0.2 microM) were evaluated in muscles with: (i) intact endocardial endothelium (EE) (n = 6); (ii) damaged EE (Triton X100; 0.5%; n = 6); (iii) intact EE, in the presence of N(G)-nitro-L-arginine (L-NNA, nitric oxide synthase inhibitor; n = 6); (iv) intact EE, in the presence of indomethacin (INDO, cyclooxygenase inhibitor; n = 6); (v) intact EE, in the presence of BQ-123 (ET(A) antagonist; n = 7); and (vi) damaged EE, in the presence of BQ-123 (n = 8). Only significant results (mean +/- SEM, p < 0.05) are given, expressed as % change from baseline. RESULTS: In muscles with intact EE, SRTXc alone induced negative inotropic and lusitropic effects, decreasing active tension (AT) by 11.0 +/- 5.6%, maximum velocity of tension rise (dT/dt(max)) by 11.2 +/- 5.9% and maximum velocity of tension decline (dT/dt(min)) by 11.5 +/- 6.2%. However, after removal of EE, or in the presence of L-NNA or INDO, SRTXc increased AT by 35.2 +/- 11.7%, 22.8 +/- 2.9% and 15.2 +/- 3.4%, dT/dt(max) by 29.5 +/- 7.9%, 20.1 +/- 2.1% and 13.3 +/- 5.0%, and dT/dt(min) by 28.2 +/- 8.1%, 21.2 +/- 3.8% and 12.3 +/- 2.2%, respectively. In muscles with intact EE and in the presence of BQ-123, the negative inotropic and lusitropic effects of SRTXc were enhanced: AT decreased by 27.0 +/- 7.4%, dT/dt(max) by 13.3 +/- 4.9% and dT/dt(min) by 31.1 +/- 7.9%. On the other hand, the positive inotropic and lusitropic effects of SRTXc in the absence of intact EE were reversed in the presence of ET(A) blockade: AT decreased by 9.0 +/- 1.8%, dT/dt(max) by 4.1 +/- 3.5% and dT/dt(min) by 8.1 +/- 3.6%. CONCLUSIONS: The present study shows that the inotropic and lusitropic effects mediated by ET(B) receptors are modulated by endocardial endothelium and by ET(A) receptor activity. These results may have pathophysiological and therapeutic implications in heart failure, a condition in which ET-1 levels are increased and endothelial dysfunction may be present.     

Mechanism of additive effects of digoxin and quinidine on contractility in isolated cardiac muscle

To evaluate the mechanism of the effect of the interaction of digoxin and quinidine on myocardial contractility, ferret right ventricular papillary muscles were isolated and the effects of digoxin, 4 × 10^{?7 M}, quinidine, 1 × 10⁵M and atropine, 1.5 × 10^?6M, on peak developed force, peak rate of development of force (dF/dt) and time to peak tension were determined. The addition of quinidine to muscles treated with digoxin increased developed force 18 percent (p = 0.006) and dF/dt 37 percent (p = 0.001) without significantly changing time to peak tension. This effect was abolished by pretreatment with atropine. Quinidine alone increased developed force 35 percent (p < 0.001) and dF/dt 70 percent (p < 0.001) and decreased time to peak tension 22 percent (p < 0.001) from pretreatment control values. Atropine alone increased developed force 17 percent (p = 0.02) and dF/dt 32 percent (p = 0.001) and decreased time to peak tension 13 percent (p = 0.003) from pretreatment control values. The addition of quinidine to muscles treated with atropine or of atropine to muscles treated with quinidine did not significantly change developed force, dF/dt or time to peak tension from values with either drug alone.It is concluded that digoxin and quinidine in these doses have additive effects on myocardial contractility, and that this interaction is at least partially mediated through antagonism of cholinergic influences by quinidine.