Myocardium Of Spontaneously Hypertensive Rats Research Articles

The transverse tubular system of the hypertrophie myocardium: morphology and morphometry in spontaneous hypertensive rats (SHR)

We reported previously on a modified Golgi stain that, in conjunction with high voltage electron microscope stereoscopy, gives striking views of the elaborate network of the transverse tubular system (T system) in rat myocardium. In this report we used the same techniques to study three-dimensional arrangements of the T system in the left ventricular myocardium of spontaneous hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). High voltage electron microscope stereoscopy revealed distinctive morphological characteristics of the T system, such as undulating running, short dead-end branches, and labyrinth-like tubular aggregates in the hypertrophic myocardium of SHR. Quantitative analysis of the SHR T system indicated a surface area greater than that of WKY. These findings may support the hypothesis that making an additional T system membrane will compensate for the smaller surface-to-volume ratio. However, the normal regulatory mechanism required to maintain the surface-to-volume ratio does not function properly in SHR, resulting in morphological abnormalities and functional disturbances of the myocardium.

Enhanced Susceptibility to Histamine-Induced Cardiac Arrhythmias in Spontaneously Hypertensive Rats

To clarify mechanisms underlying an enhanced susceptibility to cardiac rhythm disturbances in hypertension and myocardial hypertrophy, we evaluated the vulnerability to histamine-induced arrhythmias of isolated left ventricles from spontaneously hypertensive rats (SHR) and age-matched controls (Wistar-Kyoto rats, WKY). Before drug administration, left ventricle-to-body weight ratios, spontaneous firing rates, and the incidence of arrhythmias (including delayed afterdepolarizations) were significantly increased in SHR ventricles versus WKY. In addition, action potential duration (APD) was prolonged in SHR at all levels of repolarization. In WKY but not SHR hearts, histamine (10(-7)-10(-4) M) increased spontaneous firing rates; the incidence of arrhythmias was increased in all hearts, but the response to histamine was most pronounced and occurred at lower threshold drug levels in SHR. After potentials and triggered activity were observed only in SHR. The H2-receptor blocker cimetidine (10(-5) M) and the Ca2(+)-channel antagonist verapamil (10(-6) M) each attenuated the arrhythmogenic influence of histamine in SHR and WKY preparations. Neither chlorpheniramine nor propranolol had any effect. The enhanced vulnerability to arrhythmogenesis observed in SHR myocardium may reflect elevated intracellular calcium levels, which may in turn be potentiated by local ischemia and/or intracardiac histamine release.

Properties of cardiac alpha- and beta-adrenoceptors in spontaneously hypertensive rats

The effects of isoprenaline, Ca2+ and phenylephrine (in the presence of propranolol) on force of contraction were studied in isolated electrically driven papillary muscles of spontaneously hypertensive rats (SHR) and age-matched (14-18 weeks) Wistar Kyoto control rats (WK). Cardiac alpha- and beta-adrenoceptors were characterized by radioligand binding studies. The positive inotropic effect of isoprenaline in SHR was less effective than in control rats. The EC50 values did not differ in both groups. In SHR, isoprenaline was less effective than Ca2+ to increase force of contraction whereas in WK it had the same effectiveness as Ca2+. The positive inotropic effect of phenylephrine in the presence of propranolol was similar in SHR and WK. In SHR, both the densities of cardiac alpha- and beta-adrenoceptors were reduced. In beta-adrenoceptor binding experiments, the nonhydrolysable GTP analog Gpp(NH)p caused a rightward shift of agonist competition curves of isoprenaline. Biphasic competition curves revealed a similar percentage of low and high affinity sites in SHR and WK, respectively. In alpha-adrenoceptor binding experiments, Gpp(NH)p caused no detectable shift of agonist competition curves with norepinephrine. It is suggested that cardiac beta-adrenoceptor down-regulation is involved in the reduced positive inotropic effect of isoprenaline in SHR. Functional uncoupling of beta-adrenoceptors does not appear to be involved in the reduced beta-adrenoceptor-mediated positive inotropism in SHR. Binding studies do not show evidence for a large number of alpha-adrenoceptors coupling to a guanine-nucleotide binding protein in the rat heart. Finally, in ventricular myocardium of SHR, cardiac alpha-adrenoceptors do not serve as a reserve mechanism during impaired beta-adrenergic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in the plasma membrane properties of the myocardium of spontaneously hypertensive rats.

Spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were used to investigate the adaptive biochemical changes in the myocardium in response to chronic afterload. Ouabain-inhibited Na+,K+-adenosine triphosphatase (ATPase) activity was decreased by 40% in myocardium of SHR compared with that from WKY, which may lead to increased intracellular Ca2+ through Na+-Ca2+ exchange. Similarly, alpha 1-adrenergic receptor density, estimated by [3H]prazosin binding, was decreased by 42% in myocardial membranes of SHR, while the affinity for the agonist and the antagonist was not altered. In contrast, the number of Ca2+ channels estimated by [3H]nitrendipine binding was increased by 45% in myocardial membranes of SHR, while the affinity was comparable between SHR and WKY. These differences between WKY and SHR in the membrane properties were not due to differential contamination of plasma membranes because the activities of other putative plasma membrane marker enzymes were comparable between WKY and SHR. There were no differences between WKY and SHR in the myosin ATPase activity estimated using myofibrils, actomyosin, and myosin. These results suggest that specific alterations have occurred in the plasma membrane properties of myocardium of SHR that result in altered intracellular Ca2+ metabolism. These alterations may have an important bearing on excitation-contraction coupling in myocardium of SHR.

Age dependent changes in myosin ATPase activity in the myocardium of spontaneously hypertensive rats.

Male spontaneously hypertensive rats (SHR) and age matched Wistar Kyoto normotensive (WKY) rats of 5 weeks, 16 weeks, and 52 weeks of age were used to determine whether duration of hypertension has any effect on contractile protein ATPase and myosin isoenzyme distribution. Myofibrils, actomyosin, and myosin were isolated from the left ventricles of WKY rats and SHR and assayed for myosin ATPase activity and myosin isoenzyme distribution. Myofibrillar ATPase activity was assayed at various free [Ca++] ranging from 10(-7) to 10(-5) mol X litre-1. Ca++ stimulated actomyosin ATPase activity was determined at several Ca++ concentrations both at low ionic strength, which favours actin-myosin interaction, and at high ionic strength, which diminishes actin interaction with myosin. Purified myosin ATPase activity was assayed in the presence of K+-EDTA and in the presence of several concentrations of Ca++. Actin activated myosin ATPase activity was assayed using 26 mumol X litre-1 skeletal muscle actin. Under all these assay conditions no differences were observed in the contractile protein ATPase activity between SHR and WKY rats in any age group. On the other hand, in both SHR and WKY rats the contractile protein ATPase activity under all assay conditions was significantly decreased in 52 week old rats compared with 5 week old rats. The predominant myosin isoenzyme was Vi in 5 week and 16 week old WKY rats and SHR. In 52 week old WKY rats and SHR, however, significant amounts of isoenzymes V2 and V3 were present along with V1. Percentage distribution of V1, V2, V3 isoenzymes calculated from densitometric scans of gels did not show any differences between WKY rats and SHR in any age group. These results suggest that neither myosin ATPase activity nor myosin isoenzyme distribution is altered in the moderately hypertrophied left ventricles of SHR. Moreover, the data indicate that the myocardium of SHR, despite the persistence of pressure overload, undergoes a similar decrease in myosin ATPase activity and an increase in myosin isoenzyme V3 to age matched normotensive WKY rats.

長期ストレスによる高血圧と動脈病変

The purpose of this study is to report the effects of long-term stress to the rats on the blood pressure or the spontaneous hypertension, and on pathological changes especially in cardiovascular system. Methods of stress.Experiment 1: Male Wistar rats were stressed with thrushing unceasingly and harassingly with a bamboo stick for 20 min./day for three and twelve months. Experiment 2: Male Wistar rats were fed by high fat diet and stressed with the same manner as Experiment 1. for two months. Experiment 3: Spontaneously hypertensive rats (SHRs) and normotensive rats (WKYs) were subjected to combination of immobilization and electrical stress for three months.Results. 1. The blood pressure reached a high value (191±3mmHg) in twelve months in Experiment 1. The development of hypertension was accelerated in stressed SHRs when compared with unstressed SHRs. 2. Organ-body weight ratios of heart, kidney and adrenal gland were increased by stress. 3. Mean values of relative thickness of media to radius of small arteries were increased in heart of stressed group of twelve months in Experiment 1. and in kidney and mesentery of stressed SHRs in Experiment 2. 4. Myocardium of stressed SHRs in Experiment 3. disclosed severe degree of fibrosis.

Distribution and properties of cAMP-dependent protein kinase isozymes in the myocardium of spontaneously hypertensive rat

cAMP-dependent protein kinase isozymes were isolated from the soluble fraction of cardiac tissue of 12 to 16-week spontaneously hypertensive rat (SHR) and Kyoto Wistar Normotensive rat (WKY). No differences were observed in the relative distribution and elution profile of isozyme I and II between SHR and WKY. In both types of rat isozyme I was the major enzyme comprising approximately 65% of total enzyme activity. Specific activity of cAMP-dependent protein kinase was significantly decreased in the soluble fraction as well as isozymes I and II of SHR as compared to WKY. V max of cAMP stimulation was decreased for Types I and II isozymes of SHR as compared to WKY, with no differences in the K a value (concentration of cAMP required for half maximal activation). K m values for ATP and Mg 2+ were not altered for isozymes I and II of SHR, but V max was significantly decreased in SHR as compared to WKY. Thermostability and salt dissociation experiments did not reveal any differences between SHR and WKY isozymes I and II. These data would suggest that cAMP-dependent protein kinase activity could be decreased in SHR myocardium due to a reduction in the number of enzyme molecules.

Possible role of phosphorylation-dephosphorylation in the regulation of calcium metabolism in cardiovascular tissues of SHR.

Spontaneously hypertensive rats (SHR) and Wistar-Kyoto normotensive rats (WKY) were compared for phosphorylation-dephosphorylation mechanism(s) in aorta, caudal artery, inferior vena cava, and right and left ventricles. Reduction of cAMP-induced phosphorylation of microsomes and cAMP-dependent protein kinase activity was significant in the aorta and caudal artery of SHR compared with WKY. These changes were not observed in the vena cava of SHR. Phosphoprotein phosphatase activity was significantly increased (p less than 0.05) in the soluble fraction of arterial smooth muscle. No changes were observed, however, in the myocardium or vein. Furthermore, the extent of phosphorylation, and Ca2+ uptake ability and the protein kinase activity in the soluble and the microsomal fractions were not reduced in the myocardium of SHR compared with WKY. These data suggest that phosphorylation-dephosphorylation mechanisms are altered in the microsomal fraction of the aorta and caudal artery of SHR, which may result in reduced Ca2+ uptake by the intracellular organelle. The changes observed could have a significant effect on vasodilatation of arteries in the hypertensive state. The lesion appears specific to the arterial smooth muscle in the cardiovascular tissues.

ULTRASTRUCTURAL ASPECT OF LEFT VENTRICULAR HYPERTROPHY IN SPONTANEOUSLY HYPERTENSIVE RATS : A QUALITATIVE AND QUANTITATIVE STUDY

The morphology of the left ventricular myocardium of spontaneously hypertensive rats (SHR) (Okamoto and Aoki) was studied with light and electron microscopes, with special reference to a quantitative analysis of subcellular organelles and a description of the characteristics of the capillaries in myocardium. Paired groups of SHR and control Wistar-Kyoto rats were sacrificed at 3, 5, 9, 11, 15 and 21 weeks and one year of age. In the left ventricular lateral wall of SHR, starting at the age of 11 weeks, the cardiocytes became progressively thicker as the blood pressure rose and exhibited various ultrastructural alterations. In these cardiocytes, the proliferation of Z-band materials was occasionally present in the myofibrils, and was closely associated with the subsarcolemmal area and intercalated discs. Convolutions and extensive foldings of the intercalated discs were prominent. Mitochondrial abnormalities consisted of fragmentation and stacks of cristae, aggregates of small mitochondria and intramitochondorial α-glycogen rosettes. In one-year-old SHR, malaligned cardiocytes as well as myofibrils were seen in certain areas of the myocardium. In some cardiocytes, streaming and clumping of Z-band material, myofibrillolysis and proliferation of sarcoplasmic reticulum appeared. In addition, there were clusters of spherical microparticles, tubular aggregates and intramitochondrial inclusions, suggestive of degenerative changes. Stereological analysis of subcellular organelles in the hypertrophied cardiocytes of SHR revealed an increase of myofibrillar volume fractions and a decrease of mitochondrial volume fractions, resulting in an increase in the ratio of myofibrils to mitochondria. These findings differed from those in the posterior papillary muscle of the left ventricle in the same animal hearts. Thus, it is apparent that myocardial hypertrophy varies in each area of the myocardium due to different stresses during the course of its development. In the hypertrophied myocardium of SHR, the capillaries were seen to proliferate with the appearance of a "tunnel capillary", while interstitial fibrosis and hypertensive vascular lesions progressed with aging. The capillary proliferation maintained a constant diffusion distance from the capillary to the cardiocyte, probably as one of adaptation in cardiac hypertrophy in SHR.

Hemodynamics of the spontaneously hypertensive rat: effects of isoproterenol.

Increased sympathetic neural activity has been implicated in the pathogenesis and maintenance of elevated arterial pressure in several forms of hypertension. Although increased adrenergic participation may alter cardiovascular performance by modifying both vascular smooth and cardiac muscle function, the effect upon the latter has received little attention. Fujiwara, Kuchii, and Shibata have recently reported that atria isolated from spontaneously hypertensive rats (SHR) exhibited lesser increments of cardiac responses from isoproterenol stimulation than normotensive rat atria (NR) (1). In our previous studies, the intact SHR myocardium failed to demonstrate the increases in heart rate and cardiac output which were observed in sex- and age-matched NR during infusions of small doses of exogenously administered norepinephrine (5). These differences of cardiac responses to norepinephrine could indicate possible quantitative or qualitative alterations in the cardiac administered sympathetic neurotransmitter, intrinsic changes in the myocardial contractile machinery, or possible differences in tonic autonomic influences on myocardial function. The present study was undertaken to examine further the responses of intact NR and SHR myocardia to betaadrenergic stimulation with the specific agonist isoproterenol. Methods. Studies were performed on the 21st and 22nd filial generation of spontaneously hypertensive rats (3) and on normotensive Wistar rats (West Jersey Biological Supply) ranging from 22 to 27 weeks of age. Commercially obtained normotensive Wistar rats (from the original Wistar strain) were used since Wistar-Kyoto (WKY) rats were not available at the time this study was initiated. In previous studies we have compared the hemodynamic and myocardial functions of these two groups of rats over a wide range (6) and following acute elevation of arterial pressure (5). It was therefore necessary to follow up the mechanisms for these observed differences, and present studies