Impaired inotropic response to alpha 1- but not to beta-adrenoceptor stimulation in isolated hearts from spontaneously hypertensive rats.

OBJECTIVES: Hypertension in humans and experimental animals is known to be associated with an increase in left ventricular myocardial mass. The development of cardiac hypertrophy is not caused by increased blood pressure alone; the autonomic nervous system may also play an important role. DESIGN: The functional responses to the beta-adrenoceptor agonists isoprenaline, dobutamine, salbutamol and terbutaline, and the alpha 1-adrenoceptor agonists methoxamine, cirazoline and phenylephrine were studied in isolated (Langendorff) hearts from spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) controls. The results were compared with data from radioligand binding experiments. RESULTS: There was no significant difference in the increase of left ventricular pressure induced by all beta-adrenoceptor agonists studied in SHR and WKY rat hearts. Although there was no significant difference in the response to phenylephrine, the inotropic responses to cirazoline and methoxamine proved to be significantly weaker in hearts from SHR than in those from WKY rats. Binding experiments with 3H-prazosin revealed no differences in density or affinity for cardiac tissues from SHR and WKY rats. CONCLUSIONS: Long-standing hypertension leads to an impaired response of the isolated heart to alpha 1-adrenoceptor stimulation, without changes in alpha 1-receptor density or affinity. It seems likely that changes in postreceptor events are responsible for the impaired inotropic response to alpha 1-adrenoceptor agonists in hearts from SHR.     

1,2-Diacylglycerol content in myocardium from spontaneously hypertensive rats during the development of hypertension

Summary 1,2-Diacylglycerol (DAG) has been considered to play an important role as an activator of protein kinase C in the signal transduction of inositol phospholipid metabolism. To examine the relation of 1,2-DAG in heart tissues to cardiac hypertrophy associated with hypertension, we measured the amount of 1,2-DAG in spontaneously hypertensive rat (SHR) hearts at 4,10 and 20 weeks of age, and in age-matched normotensive Wistar-Kyoto (WKY) rat hearts using thin-layer chromatography with flame ionization detection (TLC-FID). Significant cardiac hypertrophy was found in 4-week-old SHR, while SHR did not yet have significant hypertension. Major phospholipids such as phosphatidylcholine and phosphatidylethanolamine increased from 4 to 20 weeks in the myocardium, but there was no difference between the two strains. The cholesterol levels of 4- and 20-week-old SHR were significantly higher than WKY rats. The 1,2-DAG contents of SHR hearts were significantly higher than WKY rats at 4 weeks. An increase in the RNA contents of SHR hearts were significantly higher than WKY rats at 4 weeks. An increase in the RNA content was also observed in 4-week-old SHR hearts. However, analysis of the fatty acid composition of 1,2-DAG revealed no difference between the two strains. However, there was no significant difference in the 1,2-DAG content or in its fatty acid composition between SHR and WKY rat hearts at 10 and 20 weeks of age. It is suggested that an increase in the 1,2-DAG content of SHR hearts during the early stages appears related to the initiation of cardiac hypertrophy in SHR hearts before developed hypertension.     

Impaired vasodilator and chronotropic responses to 5-hydroxytryptamine in two models of hypertension-associated cardiac hypertrophy.

OBJECTIVE: In connection with hypertension, research concerning 5-hydroxytryptamine (5-HT) receptors and subtypes in the cardiovascular system has so far been predominantly focused on various vascular tissues. In this study, the effects of 5-HT were investigated in isolated hearts with experimental cardiac hypertrophy. DESIGN AND METHODS: Cardiac hypertrophy was induced by stenosing the abdominal aorta (ASR) of 5-week-old Wistar rats. The functional response to serotonin was measured in unpaced, ASR hearts (18-20 weeks) and compared with those of "sham" operated SHR and WKY rats. RESULTS: The ASR, less hypertensive than SHR, showed more pronounced cardiac hypertrophy. The positive chronotropic and coronary vasodilator response to 5-HT was reduced in hypertrophied hearts from SHR and ASR when compared to "sham" operated and normotensive controls. The positive chronotropic effect of 5-HT could be antagonised with ketanserin, without affecting the coronary vasodilation. 5-HT did not induce any change in contractile force. CONCLUSIONS: Cardiac hypertrophy is associated with impaired coronary vasodilator and chronotropic responsiveness to serotonin. The chronotropic response to 5-HT is mediated by the 5-HT2-receptor subtype.     

Comparative effects of candesartan and enalapril on augmented vasoconstrictive responses to endothelin-1 in coronary vessels of spontaneously hypertensive rats

BACKGROUND: The aim of this study was to compare the effect of angiotensin type-1 receptor blockade (ARB) on augmented vasoconstrictive response to endothelin-1 (ET-1) in coronary vessels of hypertensive hearts with angiotensin converting enzyme (ACE) inhibitor, candesartan cilexetil (CAN) or enalapril was administered for 3 weeks in spontaneously hypertensive rats (SHR). METHODS: We used SHR (9 to 12 weeks old, n = 18) and Wistar-Kyoto (WKY) rats (n = 6). Systolic blood pressure was measured once a week. Spontaneously hypertensive rats were divided into three groups. Enalapril malate (10 mg/day) or CAN (10 mg/day) was administered orally in each of six SHR in each group receiving treatment for 3 weeks. The control group (n = 6) received no treatment. At the end of this experiment, the hearts were isolated. Isolated hearts mounted on a Langendorff apparatus after weighing were then perfused with modified Krebs-Henseleit buffer at constant pressure (75 mm Hg). The coronary perfusion pressure and coronary flow were measured during perfusion of isolated hearts. Coronary vascular resistance (CVR; mm Hg/mL/min/100 g) was calculated. RESULTS: The ET-1 elicited increases in CVR dose-dependently in both normotensive and hypertensive rat hearts. However, the responses were significantly greater in SHR than in WKY rat. Chronic treatment with enalapril or candesartan inhibited the development of hypertension and cardiac hypertrophy equally in SHR. Augmented vasoconstrictive responses to ET-1 were significantly reduced in treated SHR. There was no difference in these effects between enalapril and candesartan. CONCLUSION: These findings suggest that both ACE inhibitors and ARB can equally inhibit augmented coronary vascular response to ET-1 in hypertensive hearts.     

Spontaneously occurring hypertrophic cardiomyopathy in the rat. I. Pathologic features

The gross anatomic and microscopic appearance of the hearts of young and adult WKY/NCrj rats was examined in comparison with that of normotensive Wistar and SHR/NCrj rats. In a substantial number of the WKY rats, the heart weight and thickness of ventricular septum were much greater than those of the Wistar and SHR rats. The ventricular septum to left ventricular free wall thickness ratio was greater than 1.3 in about one sixth of the WKY rats. In most of the hypertrophied WKY hearts, the transverse area of the left ventricular cavity was smaller in relation to the wall area than in the Wistar and SHR rat hearts, although in a few it was greater. Abnormal fiber arrangement, myocyte hypertrophy, and myocardial fibrosis were far more prominent in the hypertrophied myocardium of the WKY rats compared with the Wistar or SHR rats. Intramural arteries with marked wall thickening existed frequently in the hypertrophied and dilated hearts. Electron microscopic examination revealed marked disarrangement of bundles of myofilaments and widened Z-bands in the hypertrophied myocardium. Blood pressure was not elevated in the rats with cardiac hypertrophy. These findings show that a disease of the myocardium with the pathologic features similar to those of hypertrophic cardiomyopathy in man occurs spontaneously in rats.     

Effect of ubiquinone on contractile function and antioxidant status of the myocardium in spontaneously hypertensive rats

During the period of aging of spontaneously hypertensive rats (SHR) between 6 and 13 weeks the systolic arterial pressure increased from 131+/-2 up to 176+/-3 mm Hg while in the control group of WKY rats it reached 122+/-2 mmHg. The hypertension was combined with myocardial hypertrophy -- the relative weight of SHR heart was 24% higher. The contractile myocardial function of the isolated isovolumic heart of SHR group did not differ from WKY group in a wide range of coronary perfusion rates. During oxidative stress induced by 40-min intracoronary introduction of H(2)O(2) function of hypertrophied SHR hearts fell significantly deeper. This coincided with decreased myocardial activity of superoxide dismutase and glutathione peroxidase by 29-30%, and increased catalase activity by 18%. The rate of generation of active forms of oxygen (hydroxyl radicals HO(.-)) in mitochondria from SHR hearts was higher as compared with WKY. Thus, the development of hypertension was combined with decreased antioxidant protection of the myocardium. The addition of ubiquinone to drinking water (approximately 10 mg/kg/day) for 6 weeks did not affect arterial pressure level, but was associated with two times lesser degree of myocardial hypertrophy. The hearts of SHR that received ubiquinone differed from those not treated with ubiquinone by increased maximal level of myocardial contractile function, and by improved myocardial relaxability and distensibility. After administration of H(2)O(2), myocardial function of SHR was kept on higher level. That was combined with less myocardial oedema, better preservation of antioxidant enzymes and reduced rate of succinate-dependent generation of superoxide radicals in mitochondria from hearts of ubiquinone treated SHR. The results have shown, that administration of ubiquinone to rats with hereditary hypertension reduces degree of myocardial hypertrophy, improves functional properties of the myocardium, promotes effective protection of antioxidant enzymes and increases the resistance of the cardiac muscle to oxidative stress.     

Reversal of changes in myocardial beta-receptors and inotropic responsiveness with regression of cardiac hypertrophy in renal hypertensive rats (RHR)

Our previous studies, in vivo and in vitro, have shown reduced inotropic responsiveness to isoproterenol of hypertrophied hearts in renovascular hypertensive rats. In the present study, we have investigated, in the same model, the effects of treatment either by nephrectomy or captopril on the inotropic responsiveness to isoproterenol and on the number and affinity of ventricular beta-receptors. Isoproterenol infusion of isolated hearts from renovascular hypertensive rats 12-18 weeks post-clipping produced lower inotropic responses (delta peak dP/dt) than age-matched sham-operated normotensive rats (P less than 0.001). Quantitative assessment of beta-adrenergic receptors in the same hearts showed a significant decrease in renovascular hypertensive rats ventricular receptor numbers, whether calculated per milligram membrane protein (22.3 +/- 2.66 fmol/mg vs. 37.9 +/- 4.34, P less than 0.005) or per gram wet ventricular weight (1.43 +/- 0.14 pmol/g vs. 2.2 +/- 0.21, P less than 0.005), with no significant change in Kd. Control of hypertension by either nephrectomy or captopril led to regression of hypertrophy 6 weeks after stabilization of blood pressure (12-18 weeks post-clipping) and returned both the ventricular receptor density and inotropic responsiveness toward normal. The improvement in inotropic responsiveness to isoproterenol in regressed hearts correlated with both the reduction in ventricular weight and the decrease of blood pressure. Regression of hypertrophy did not alter the relationship between inotropic response, receptor density, and ventricular weight. These results indicate that the increase in cardiac mass associated with renovascular hypertension may interfere with adrenergic support to the heart, and that proper control of hypertension and regression of hypertrophy could reverse that impairment and restore its responsiveness to adrenergic stimulation.     

Prolonged QT interval is associated with blood pressure rather than left ventricular mass in spontaneously hypertensive rats

QT interval is prolonged in hypertensive individuals, although the factors responsible for this increase are not completely understood. We questioned whether enhanced left ventricular mass (LVM) or increased systemic blood pressure represents the principal factor determining QT prolongation in the period of development of hypertension and left ventricular hypertrophy (LVH) in spontaneously hypertensive rats (SHR). In 12-and 20-week-old SHR (SHR12 and SHR20) and age-matched normotensive Wistar-Kyoto rats (WKY12 and WKY20), arterial systolic blood pressure (sBP) was measured using tail-cuff technique. Orthogonal Frank ECG was registered in anaesthetized animals in vivo, and bipolar ECG was measured in spontaneously beating isolated hearts in vitro. Progressive increase of sBP and LVM resulted in significant QT prolongation in SHR20 as compared to WKY12, WKY20, and also to SHR12 in vivo (WKY12: 82 +/- 9 ms, WKY20: 81 +/- 9 ms, SHR12: 88 +/- 15 and SHR20: 100 +/- 10, respectively; p < 0.05) but not in isolated hearts (WKY20: 196 +/- 39 ms and SHR20: 220 +/- 55, respectively; NS). In whole animals, QT duration was positively related to sBP (r = 0.6842; p < 0.001) but not to LVM (r = 0.1632, NS) in SHR20. The results suggest that QT prolongation in SHR developing hypertension and LVH depends on blood pressure rather than increase in LVM. In this period, myocardial hypertrophy is probably the predisposition for QT prolongation, but the significant change manifests only in the presence of elevated systemic factors.     

Cyclic AMP in myocytes isolated from hypertrophied rat hearts.

Impaired inotropic responsiveness to isoproterenol stimulation has been reported in the hypertrophied hearts of spontaneously hypertensive rats and renal hypertensive rats. This study was carried out in order to investigate the possibility that a defect in cyclic AMP production by cardiac myocytes is responsible for the impaired inotropic responsiveness of these hearts. Basal and isoproterenol stimulated cyclic AMP levels were measured in ventricular myocytes isolated from hypertrophied rat hearts. Cyclic AMP accumulation was also measured in the presence of isobutyl-methyl-xanthine, a phosphodiesterase inhibitor, and the results were compared to the appropriate controls. In the spontaneously hypertensive rat, no changes were detected in the basal or isoproterenol stimulated cyclic AMP formation. This suggests that the biochemical alterations leading to a diminished inotropic response in this model of cardiac hypertrophy involve abnormalities in mechanisms other than cyclic AMP production. In the renal hypertensive rat, basal and isoproterenol stimulated cyclic AMP levels were significantly depressed as compared to controls. This suggests that abnormalities in the signal transduction mechanism and formation of cyclic AMP are, at least in part, responsible for the impaired inotropic responsiveness seen in this model. These results confirm that cardiac hypertrophy is a heterogeneous process. Reduced inotropic responsiveness to isoproterenol stimulation in the hypertrophied hearts of the SHR and the RHR, both models of pressure overload hypertrophy, involve different biochemical alterations. Results of this study suggest that the physiologic response of cardiac hypertrophy may not be as important as the underlying cause of hypertrophic stimuli in determining the pathophysiological consequences.     

Increase in G(i alpha) protein accompanies progression of post-infarction remodeling in hypertensive cardiomyopathy

Hypertensive cardiac hypertrophy and myocardial infarction (MI) are clinically relevant risk factors for heart failure. There is no specific information addressing signaling alterations in the sequence of hypertrophy and post-MI remodeling. To investigate alterations in beta-adrenergic receptor G-protein signaling in ventricular remodeling with pre-existing hypertrophy, MI was induced by coronary artery ligation in Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Ten weeks after the induction of MI, the progression of left ventricular dysfunction and increases in plasma atrial natriuretic peptide (ANP) and cardiac ANP mRNA were more pronounced in SHR than WKY. In addition, the impaired contractile response to beta-adrenergic stimulation was observed in the noninfarcted papillary muscle isolated from SHR. Immunochemical G(s alpha) protein and beta-adrenoceptor density were not significantly altered by MI in both strains. However, immunochemical G(i alpha) was increased (1.5-fold) in the noninfarcted left ventricle of the SHR in which infarction had been induced when compared with that in SHR that underwent sham operation. This increase was observed especially in rats with a high plasma ANP level. Furthermore, there was a positive correlation between G(i alpha) and the extent of post-MI remodeling in WKY. A similar correlation between G(i alpha) and the extent of hypertensive hypertrophy was observed in SHR. In conclusion, the vulnerability of hypertrophied hearts to ischemic damage is greater than that of normotensive hearts. An increase in G(i alpha) could be one mechanism involved in the transition from cardiac hypertrophy to cardiac failure when chronic pressure overload and loss of contractile mass from ischemic heart disease coexist.     

Changes in cardiac beta-adrenoceptor concentrations in spontaneously hypertensive and experimental renal hypertensive rats

Cardiac beta-adrenoceptors were studied in membrane fractions from spontaneously hypertensive rats (SHR) and rats with two-kidney, one clip hypertension (2K, 1C HT), using radioligand binding method. beta-Adrenoceptor concentration measured by [3H]-dihydroalprenolol (DHA) binding was significantly lower in cardiac membranes from two months old SHR than those from Wistar-Kyoto rats (WKY) (38.2 +/- 2.6 vs 45.1 +/- 1.8 fmol/mg protein, means +/- SEM, p less than 0.05). Cardiac membranes from 2K, 1C HT rats had also a lower concentration of beta-adrenoceptors than those from the sham-operated control rats at a week after operation (30.9 +/- 2.2 vs 47.8 +/- 1.6 fmol/mg protein, p less than 0.01). But receptor affinity remained unchanged. These reduced concentrations of beta-adrenoceptors were restored to control levels at 12 months old in SHR and at 6 weeks after operation in 2K, 1C HT rats, although age-dependent decrease in beta-adrenoceptor was observed. The decrease in beta-adrenoceptor was associated with increase in plasma noradrenaline levels during the earlier stages of hypertension. But there is no correlation between beta-adrenoceptor concentrations and plasma noradrenaline levels in the chronic stages of hypertension. No significant difference was found in activities of 5'-nucleotidase, which is a marker enzyme of cell membrane, in membrane fractions between the hypertensive hearts and the controls, suggesting that the cardiac hypertrophy is not a determinant factor for change in beta-adrenoceptor. The observed decrease in beta-adrenoceptor concentration may reflect an increase in sympathetic nerve activity during development of hypertension. In the chronic stages of hypertension, additional factors may be involved in the restoration of beta-adrenoceptors.     

Functional and metabolic properties in hearts from aged spontaneously hypertensive rats

The effect of long-term pressure overload on myocardial functional and metabolic alterations was investigated in hearts from spontaneously hypertensive rats of 16 weeks (young SHR) and 44 weeks (aged SHR) and age matched normotensive Wistar Kyoto strain rats (young WKY, aged WKY). The hearts were perfused by working heart mode and whole heart ischemia was induced by one-way valve. Following 20 min of ischemia, the hearts were reperfused for 30 min. The heart-body weight ratio in both SHR groups was significantly higher than in the respective age-matched WKY groups. Coronary flow relative to heart weight in both SHR groups was significantly lower than that of the respective age-matched WKY during both preischemic and reperfused periods. There was no significant difference in the recovery rate of cardiac output between young and aged WKY, whereas the young and aged SHR revealed significantly less recovery than their respective age-matched WKY. Tissue creatine phosphate and energy charge in both aged groups were significantly lower than in the young groups. These results indicate that long-term pressure overload increases susceptibility to ischemia and decreases the myocardial reserve presumably resulting from relative ischemia, whereas deterioration was minimal in the normotensive aged rat heart.     

A treatment with rosuvastatin induced a reduction of arterial pressure and a decrease of oxidative stress in spontaneously hypertensive rats

The aim of this study was to appreciate consequences of rosuvastatin administration on hemodynamic function, vascular oxidative stress and ischemia/reperfusion disorders in normotensive and hypertensive rats. At 10 weeks of age, spontaneously hypertensive rats (SHR, n=20) and normotensive Wistar Kyoto male rats (WKY, n=20) were divided into four groups and given, either vehicle or 10 mg/kg/day of rosuvastatin by gavage for 3 weeks. Systolic blood pressure was assessed every week. At the end of these treatments, vascular NADPH oxidase activity was evaluated by chemiluminescence (lucigenin 0.5 microM). Hearts were isolated and perfused according to the Langendorff method and were subjected to 30 min of global ischemia. Reactive oxygen species (ROS) produced during reperfusion were quantified by electron spin resonance (ESR) spectroscopy using a spin probe (CP-H, 1 mM). After one week of treatment, rosuvastatin reduced the arterial pressure in SHR rats (180.3 +/- 2.1, SHR vs 169.7 +/- 2.3 mmHg, SHR+rosuvastatin; p < 0.01), without lowering plasma cholesterol levels; these effects were not observed in WKY. NADPH activity was 25% higher in control SHR rat aortas compared to control WKY, and was reduced by rosuvastatin in SHR rats. In isolated rat hearts subjected to ischemia/reperfusion sequences, there was a deterioration in functional parameters in control SHR compared to control WKY hearts. Rosuvastatin decreased post-ischemic contracture in WKY hearts by 50% (41.5 +/- 7.5, WKY control vs 18.4 +/- 4.6 mmHg, WKY+rosuvastatin; p < 0.01) and increased left ventricular developed pressure. This beneficial effect was accompanied by a decrease in ROS detected by ESR during reperfusion (312.5 +/- 45.3, WKY control; vs 219.3 +/- 22.9 AUC/mL, WKY+rosuvastatin; p < 0.05). In conclusion, these results are in accordance with the hypothesis that oxidative stress plays a crucial role in the pathogenesis of cardiovascular diseases including hypertension, and demonstrate the beneficial effects of rosuvastatin.     

Activity of cyclic GMP-dependent protein kinase in aortae from spontaneously hypertensive rats

It has been suggested that various agents induce relaxation of vascular smooth muscles through guanosine 3',5'-cyclic monophosphate (cGMP) and cGMP-dependent protein kinase (cGMP-PK). In this work, the activity of cGMP-PK was studied in the 30,000 g supernatant from aortae of 4, 6, 8 and 12-week-old spontaneously hypertensive (SHR) and age-matched normotensive Wistar-Kyoto (WKY) rats and also of 4 and 12-week-old normotensive Wistar (W) and Sprague Dawley (SD) rats. At 4 weeks of age, both basal and cGMP-stimulated activity were not different in SHR and WKY rats. Nevertheless, a greater basal activity was measured in W (+50%) and SD (+20%) rats than in SHR, while no difference was observed between stimulated activities. In contrast with observations in the three normotensive rat strains, cGMP-PK activity did not decrease in the aortae supernatant of SHR rats aged 4-12 weeks. This resulted in mean increases of 45 and 30% in the basal and the cGMP-stimulated activity, respectively, in the 12-week-old SHR rats. The abnormal evolution of cGMP-PK activity in the hypertensive strain was already detectable at 4-6 weeks of age. In apparent agreement with observations on protein kinase activity, cGMP binding activity attributable to cGMP-PK was 25% greater in 12-week-old hypertensive rats compared with age-matched WKY rats. These results indicate that in aortae of SHR rats, control of cGMP-PK activity is abnormal early in life.     

Increase of cardiac beta-adrenergic receptors in young spontaneously hypertensive rats

The authors studied the number of myocardial beta-adrenergic receptors and the cyclic nucleotide concentration in both male spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) at 4 to 5, 10 to 15, 20 to 25 and 35 to 55 weeks of age. A potent beta-adrenergic antagonist, (125I) iodohydroxybenzylpindolol was used to estimate the number and affinity of beta-adrenergic receptors. beta-adrenergic receptors in cardiac membranes from SHR of 4 to 5 weeks and 10 to 15 weeks numbered 63.1 +/- 4.6 and 51.6 +/- 4.6 f mol/mg protein, respectively. These were significantly (p less than 0.02) greater than the number in WKY at 4 to 5 weeks and 10 to 15 weeks (42.2 +/- 5.1 and 31.5 +/- 5.4 f mol/mg protein, respectively). The dissociation constant in the membranes was the same in WKY and SHR, and no significant differences were found in the number of receptors and affinity of SHR and WKY at 20 to 25 weeks or 35 to 55 weeks of age. Also, there was no difference in the concentration of myocardial cyclic nucleotides at the various ages. Since cardiac hypertrophy in SHR had appeared before the onset of hypertension at about 7 weeks, the present results suggest that the SHR heart is hypersensitive to catecholamines and hemodynamically hyperkinetic due to the increased numbers of beta-receptors in the pre- and early stages of hypertension.     

Age-related decline of PCr/ATP-ratio in progressively hypertrophied hearts of spontaneously hypertensive rats

Although the ultimate cause for the myocardial dysfunction of hypertensive heart disease is still unclear, a crucial role of the myocardial energy metabolism has been suggested. Therefore, the aim of the present study was to investigate whether age-related myocardial dysfunction in hearts of spontaneously hypertensive rats (SHR) is associated with an impaired myocardial energy metabolism. Isolated hearts of SHR and Wistar Kyoto rats (WKY) aged about 40, 60, and 80 weeks, respectively (each n = 4–5), were perfused according to the working heart technique. Cardiac work and coronary flow were monitored online. Myocardial energy metabolism was evaluated by calculating the ratio of phosphocreatine (PCr) and adenosine triphosphate (ATP) which were measured by nuclear magnetic resonance (³¹P-NMR) spectroscopy. All hearts were subjected to work for 30 min at baseline conditions (low afterload), followed by another 30 min under a moderate pressure load (high afterload). Each SHR group showed a higher heart weight/body weight ratio than the age-matched WKY controls. The SHR showed a progressive age-dependent reduction of cardiac work (40 weeks = 5.1 ± 0.3, 60 weeks = 4.0 ± 0.3, 80 weeks = 3.8 ± 0.2 (mW/g) at baseline conditions) and PCr/ATP-ratio (40 weeks = 1.82 ± 0.06, 60 weeks = 1.69 ± 0.05, 80 weeks = 1.59 ± 0.09 (PCr/ATP) at baseline conditions). Similar results were found for hearts of SHR at high afterload. In WKY no significant decline in cardiac work or PCr/ATP-ratio was found under either low or under high afterload. The cardiac work capacity of hearts of SHR progressively decreases with increasing age and left ventricular hypertrophy. This myocardial dysfunction is closely associated with an impaired PCr/ATP-ratio, suggesting a decreased energy reserve. Received: September 18, 2000 / Accepted January 5, 2001     

Lower Na(+)-H+ antiport activity in vascular smooth muscle cells of Wistar-Kyoto rats than spontaneously hypertensive and Wistar rats

To determine whether increased Na(+)-H+ antiport activity in vascular smooth muscle cells may relate to the pathogenesis of hypertension in the spontaneously hypertensive rat (SHR), we monitored Na(+)-dependent alkalinization of acidified cells from the hypertensive strain and two normotensive controls, the Wistar-Kyoto rat (WKY) and the Wistar rat. Changes in intracellular pH (pHi) of cultured aortic cells were measured using the fluorescent probe 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). The initial maximal reaction velocity of Na(+)-dependent alkalinization was significantly higher in SHR and Wistar than WKY cells. Similar results were obtained for the maximal velocity of the proton equivalent efflux: SHR, 7.51 +/- 0.71; Wistar, 9.14 +/- 0.85; WKY, 4.38 +/- 0.55 mmol H+/liter x 10 s. There were no differences in the basal pHi or cellular buffering power among the three rat strains. These findings indicate that the activity of the Na(+)-H+ antiport is higher in SHR vascular smooth muscle cells than in WKY cells. However, by itself, this difference cannot explain the hypertensive process in the SHR, since this transport system is also higher in vascular cells of the Wistar rat.     

Abnormal polyamine metabolism in hypertensive cardiac hypertrophy

In order to assess myocardial hypertrophic activity during the process of hypertensive cardiac hypertrophy in the presence and absence of treatment with anti-hypertensive agents, we analyzed myocardial polyamine concentrations in spontaneous hypertensive (SHR) rats and control rats of Wistar Kyoto (WKY) strain. The anti-hypertensive agents studied were diltiazem, hydralazine and captopril, each of which was administered for 5 weeks. In comparison with WKY rats, SHR rats showed elevated blood pressure and enlarged hearts with higher myocardial spermidine concentration. Although blood pressure was lowered in the diltiazem-treated SHR rats, heart weight and myocardial spermidine concentration increased as in untreated SHR rats. In the hydralazine-treated group increases in both blood pressure and myocardial spermidine concentration were suppressed, while an increase in heart weight was not. In the captopril-treated group, increases in blood pressure, heart weight and spermidine concentration were all suppressed. Since spermidine level appears to be a sensitive indicator of hypertrophic activity in the heart, this study suggests that captopril exerts an inhibitory effect on hypertensive cardiac hypertrophy whereas diltiazem does not. It also suggests that hypertrophy may reach a certain plateau level earlier in the hydralazine-treated animals than in others.     

Endogenous digitalislike circulating substances in spontaneously hypertensive rats

Circulating digitalislike compounds have been proposed to be involved in some Na+-dependent types of experimental hypertension and in human essential hypertension. The level of circulating Na+-K+ pump inhibitor(s) was investigated in the spontaneously hypertensive rat of the Okamoto strain (SHR), its normotensive control, Wistar-Kyoto rat (WKY), and the regular Wistar rat using the following criteria: the ability of whole plasma to inhibit the total active Na+ efflux from Wistar rat erythrocytes and to cross-react with digoxin antibodies and the ability of plasma extracts to inhibit Na+,K+-adenosine triphosphatase (ATPase) activity of membranes from rat kidney. SHR plasma inhibited the net Na+ efflux from Wistar erythrocytes by up to 27% compared with WKY or Wistar plasma. For a given number of cells, the inhibition increased with the amount of available plasma. Cross-reactivity with digoxin antibodies was twice as high in SHR as in WKY or Wistar plasma. It was already enhanced in 3- to 4-week-old rats. Plasma extracts from SHR significantly inhibited Na+,K+-ATPase activity when compared with WKY extracts (75.6 +/- 2.6 vs 89.3 +/- 2.4 mumol Pi/mg/hr; p less than 0.01) but did not differ from Wistar plasma extracts. These results strongly suggest that circulating digitalislike compound(s) are present in elevated amounts in SHR as early as 3 to 4 weeks of age, but their exact participation in blood pressure elevation or maintenance remains to be clarified.     

Cross-talk between cardiac kappa-opioid and beta-adrenergic receptors in developing hypertensive rats

The kappa-opioid receptor exerts a negative modulatory action on the beta-adrenoceptor and the action is blunted in adult spontaneously hypertensive rats (SHR). In order to determine whether the blunted negative modulation of the beta-adrenoceptor by the kappa-opioid receptor contributes to the development of hypertension, the electrically induced intracellular calcium ([Ca2+]i) transient was measured in single ventricular myocytes of SHR at 4, 6, 8 and 13-week-old and the age-matched Wistar Kyoto (WKY) rats. The electrically induced [Ca2+]i transients were augmented by norepinephrine (NE), a beta-adrenoceptor agonist, over four-fold in WKY rats of all ages studied and in SHR of 4 and 6 weeks of age. The enhancing effect of NE in 8- and 13-week-old SHR was, however, only approximately three-fold, significantly lower than the corresponding values in age-matched WKY rats. Similarly, the electrically induced [Ca2+]i transients were also augmented by forskolin, an activator of adenylate cyclase, by approximately two-fold in WKY rats of all ages and SHR aged 4 and 6 weeks. In SHR aged 8 and 13 weeks, the effect of forskolin was only 1.5-fold, significantly lower than the two-fold increase in the corresponding WKY rats. The enhancing effects of NE and forskolin were attenuated by U50,488H, a selective kappa-opioid agonist, by approximately 50 and 25%, respectively, in both types of rats of all ages studied, with the exception of 13-week-old rats. In rats of this age group, the attenuations by U50,488H on the enhancing effects of NE and forskolin were 17 and 9% in SHR, respectively, significantly less than the corresponding 54 and 29% in WKY. The fact that attenuation of U50,488H on the enhancing effects of NE and forskolin only occurs in 13-week-old SHR when hypertension has been fully developed indicates that the attenuated inhibitory modulation of kappa-opioid receptor stimulation does not contribute to the initiation of hypertension. Interestingly, the enhancing effects of NE and forskolin on the electrically induced [Ca2+]i transient was attenuated in SHR aged from 8 weeks when the blood pressure was rapidly increasing. The different time courses of altered responses to U50,488H, and NE and forskolin suggest that the attenuated negative modulation of kappa-receptor stimulation on the beta-adrenergic receptor is not due to the signal transduction pathway activated by beta-adrenergic stimulation. In 13-week-old SHR with the arterial blood pressure restored to normal by pharmacological manipulations, the blunted responses to NE, U50,488H and forskolin still occurred, indicating that the altered responses to activation of beta-adrenergic and kappa-opioid receptors and adenylate cyclase are not secondary to hypertension.