Heinz-Gerd Zimmer

Significance of myocardial alpha- and beta-adrenoceptors in catecholamine-induced cardiac hypertrophy.

The role of alpha- and beta-adrenoceptors in the development of catecholamine-induced cardiac hypertrophy in vivo was investigated. Rats received a constant intravenous infusion of norepinephrine or sodium chloride (control) for 3 days. The norepinephrine infusion was combined with the alpha-blocker prazosin, the beta-blocker metoprolol, or both blockers. For modulation of the work load of the heart, the calcium channel blocker verapamil was added to the norepinephrine infusion. A further group of animals was treated with the alpha-adrenergic stimulator norfenephrine, which also was combined with prazosin or verapamil. Norepinephrine induced significant increases in mean aortic pressure, left ventricular dP/dtmax, heart rate, and total peripheral resistance. The left ventricular weight/body weight ratio was significantly elevated and was accompanied by an increase in the RNA concentration and the RNA/DNA ratio. Prazosin as well as metoprolol partially antagonized the increase in left ventricular weight and RNA concentration, whereas simultaneous prazosin and metoprolol treatment prevented the norepinephrine-induced alterations. Although combination of norepinephrine with verapamil resulted in considerable reduction of all functional parameters, the development of cardiac hypertrophy and the elevated RNA/DNA ratio were not significantly influenced. Stimulation of alpha-receptors with norfenephrine elicited an increase in total peripheral resistance and in left ventricular weight, which was abolished by prazosin. Verapamil did not affect the norfenephrine-induced cardiac hypertrophy, although it normalized essentially all functional parameters. Thus, the rapid development of cardiac hypertrophy in the norepinephrine model seems to be directly mediated by stimulation of myocardial alpha- and beta-adrenoceptors rather than by hemodynamic changes.

Changes in heart function and cardiac cell size in rats with chronic myocardial infarction

Myocardial infarctions were induced in rats by ligation of the left anterior descending (LAD) coronary artery. After 4 weeks, parameters of left (LV) and right ventricular (RV) function and of peripheral circulation were measured in the intact, anesthetized animals. The morphology of the heart chambers was examined at the macroscopic and cell size level. In animals with slight reduction in LV function, LVEDP was elevated from 3.4 +/- 0.8 to 8.8 +/- 1.5 mmHg, LV stroke work was reduced by 14%, and dP/dtmax of both ventricles was depressed by 10% compared with sham-operated controls. Myocytes isolated from the LV and RV were elongated to some extent and had a greater cell volume, but there was no change in heart weight. These rats had infarctions that were small or medium in size. In rats with severe depression in left heart function, cardiac output, LVSP, LV stroke work, mean arterial pressure, and the LV weight/body weight ratio were markedly lower than in sham-operated controls. LVEDP was elevated up to 32 +/- 2 mmHg. These rats had large infarctions. RVSP, RV weight/body weight ratio, and the volume of myocytes isolated from the RV were doubled. RV stroke work was increased by 58%. Myocytes from the LV, RV and from the septum were elongated to about the same extent. The septum had developed a 23% hypertrophy. Histological examination of the lungs revealed marked thickening of the tunica medica of small pulmonary arteries with narrowing of the lumen. These changes are considered to represent the morphological basis for the increased pulmonary vascular resistance that was associated with RV pressure overload and hypertrophy.

Stimulation of myocardial adenine nucleotide biosynthesis by pentoses and pentitols.

In rats, pentoses and pentitols, intravenously injected in a single dose of 100 mg/kg, induced a considerable enhancement of the available pool of 5-phosphoribosyl-1-pyrophosphate and of the rate of adenine nucleotide biosynthesis in the heart, but not in liver and kidney. De novo synthesis of adenine nucleotides not detectable in skeletal muscle of normal rats became measurable after application of ribose. The stimulatory effect of isoproterenol on myocardial adenine nucleotide biosynthesis could be further potentiated by ribose and xylitol, but not by glucose. The isoproterenol-induced decrease of cardiac adenine nucleotide concentrations could be almost completely prevented by repeated administrations of ribose. Thus, pentoses and pentitols in combination with β-receptor stimulation markedly and quite specifically enhance adenine nucleotide biosynthesis in the rat heart. The results indicate that the increase in the available pool of 5-phosphoribosyl-1-pyrophosphate is an important factor for the enhancement of cardiac adenine nucleotide biosynthesis. Moreover, the availability of 5-phosphoribosyl-1-pyrophosphate and the rate of de novo synthesis of adenine nucleotides in the heart seem to be limited by the flow through the hexose monophosphate shunt.

Effect of angiotensin converting enzyme inhibition on pressure-induced left ventricular hypertrophy in rats.

The influence of angiotensin converting enzyme inhibition on the development of left ventricular (LV) hypertrophy due to stenosis of the aortic arch was studied in female Sprague-Dawley rats. The aortic arch was banded to an outer diameter of 1.0 mm. After 14 days, LV and right ventricular functional parameters and transstenotic pressure gradient were measured in anesthetized rats. In addition, regional heart weights were determined, and myocytes of three different heart regions were isolated and subjected to morphometric analysis. To inhibit the angiotensin converting enzyme, ramipril was administered orally by gavage in a single daily dose of 1 mg/kg. Rats with aortic stenosis showed a marked increase in LV systolic pressure, mean prestenotic aortic pressure, and LV stroke work compared with sham-operated rats and demonstrated a systolic transstenotic pressure gradient of 82 mm Hg. This increase in LV hemodynamic load was paralleled by the development of LV hypertrophy as determined by a 37% increase in LV weight and by a 20% increase in cell volume of isolated LV myocytes. Concomitant ramipril treatment did not significantly affect LV functional parameters. The transstenotic pressure gradient was the same as in untreated rats with aortic stenosis. Likewise, the weight gain of the LV as well as the development of cellular hypertrophy of the LV were not influenced. Thus, in this model, angiotensin converting enzyme inhibition did not reduce the development of LV hypertrophy independent of the hemodynamic load.

The oxidative pentose phosphate pathway in the heart: Regulation, physiologicla significance, and clinical implications

SummaryThe capacity of the oxidative pentose phosphate pathway (PPP) in the heart is small, since the activity of glucose-6-phosphate dehydrogenase (G-6-PD), the first and rate-limiting enzyme, is very low. Basically, two mechanisms are involved in the regulation of this pathway. Under normal conditions, G-6-PD is inhibited by NADPH. This can immediately be overcome in the isolated perfused rat heart by increasing the oxidized glutathione and by elevating the NADP+/NADPH ratio.Apart from this rapid control mechanism, there exists a long-term regulation which involves the synthesis of G-6-PD. All catecholamines that were administered stimulated the activity of myocardial G-6-PD in a time- and dose-dependent manner. This stimulation was due to increased new synthesis of enzyme protein, since the G-6-PDmRNA was specifically enhanced. As a consequence of the stimulation of the oxidative PPP, the available pool of 5-phosphoribosyl-1-pyrophosphate (PRPP) was elevated which serves as an important precursor substrate for purine and pyrimidine nucleotide synthesis.The limiting step in the oxidative PPP can be bypassed by ribose which leads to an elevation of the cardiac PRPP pool. The decline in the ATP that is induced in many pathophysiological conditions can be attenuated or even entirely prevented by i.v. infusion of ribose. In some experimental in vivo rat models such as in the overloaded and catecholamine-stimulated heart and in the non-ischemic region of the infarcted heart, the normalization of the metabolic situation was accompanied by an improvement of global heart function.Ribose application has been shown to be beneficial in several clinical disease states such as myoadenylate deaminase deficiency and McArdles disease. Moreover, ribose facilitated thallium-201 redistribution and markedly improved the detection of reversible ischemic injury of the pig and human heart.

Nuclear DNA content and nucleation patterns in rat cardiac myocytes from different models of cardiac hypertrophy.

Nuclear DNA content and number of nuclei were examined in cardiac myocytes isolated from controls and rats with volume and pressure overload hypertrophy to determine if haemodynamic overload alters these nuclear parameters. The experimental groups were comprised of normotensive (WKY) and Spontaneously Hypertensive rats (SHR). Additionally, Sprague-Dawley rats with aortic constriction (AC), pulmonary stenosis (PS), myocardial infarction (MI), and 5 month arteriovenous fistulas (F) were studied along with appropriate shams for each of these groups. Nuclear DNA content was measured from DAPI-stained nuclei using an image analysis microdensitometry system. Myocyte volume was measured with a Coulter Channelyzer system. Approximately 83% of the left ventricular myocytes from the SHR and WKY groups contained a diploid DNA content with the remainder being tetraploid. The remaining experimental and sham groups, all female Sprague-Dawley rats (SD), were approximately 93% diploid. The nucleation patterns differed slightly between rat strains with the SHR/WKY expressing approximately 85% binucleation, 14% mononucleation and 5% tri- or tetranucleation. All SD groups, control and hypertrophied, showed approximately 89% binucleation, and 10% mononucleation with the remainder being tri- or tetranucleated. In summary: (1) cardiac myocytes from SHR/WKY strains are predominantly diploid but to a lesser degree than myocytes from SD; (2) nuclear number follows the same pattern with SHR/WKY showing a smaller percentage of binucleated myocytes than SD myocytes; (3) neither the duration, severity, or type of overload caused a significant change in the extent of polyploidy in overloaded hearts from SD rats; and (4) the extent of polyploidy in cardiac myocytes from both the right and left ventricles of SHR and WKY animals does not differ statistically.

Effect of Beta-Adrenergic Stimulation on Myocardial Adenine Nucleotide Metabolism

The effects of isoproterenol, propranolol, and compound D600 (α-isopropyl-α-[(N-methyl-N-homoveratryl)-γ-aminopropyl1–3, 4, 5-trimethoxyphenylacetonitrile) on myocardial adenine nucleotide metabolism were studied in rat hearts in situ. Isoproterenol in doses between 0.1 and 25 mg/kg induced an increase in heart rate concomitant with a significant acceleration in the de novo synthesis of adenine nucleotides (ATP, ADP, and AMP) and a diminution in their concentration. The effects of isoproterenol were antagonized by propranolol (1 and 50 mg/kg), which alone caused a reduction in the de novo synthesis of adenine nucleotides without inducing a change in their concentration. Compound D600 (10 mg/kg) brought about a slight elevation in the concentration of adenine nucleotides but did not influence the rate of de novo synthesis. The isoproterenol-induced diminution in adenine nucleotide concentration was prevented by D600; under these conditions, the acceleration of de novo synthesis was attenuated. These findings indicate that de novo synthesis of myocardial adenine nucleotides in the normal and the isoproterenol-stimulated heart is regulated not only by a feedback mechanism dependent on the concentration of adenine nucleotides but also by β-receptor-mediated alterations in carbohydrate metabolism which can cause changes in the size of the available pool of 5-phosphoribosyl-1-pyrophosphate.

Different response of the rat left and right heart to norepinephrine.

The in vivo hemodynamic and morphologic responses of the rat left (LV) and right (RV) ventricle to continuous long-term i.v. infusion of norepinephrine (NE) at different dosages and for different durations of infusion were studied. Female Sprague-Dawley rats received continuous intravenous infusion of norepinephrine from infors syringe pumps for 24, 48 and 72 h at a dose of 200 mu g center dot kg-1 x h-1. Furthermore, NE was infused for 72 h at dosages of 50, 100 and 200 mu g center dot kg-1 x h-1. The beta-adrenergic blocker and vasodilator with alpha1-blocking activity carvedilol (0.5 mg x kg-1 x h-1) was coinfused with NE for 72 h. The hemodynamic effects were measured on intact, anesthetized rats with special Millar ultraminiature pressure tip catheters, and the weights of the left and right ventricles were measured. NE increased heart rate at any time or dose, whereas cardiac output and total peripheral resistance remained unchanged. LV and RV dP/dtmax were nearly doubled as compared to control values and RVSP was elevated by more than 100%. The effect of NE on LVSP was much less pronounced (< 20%) and only significant at 50 mu g x kg-1 x h-1 for 72 h. Neither LV nor RV end-diastolic pressures were elevated, indicating that cardiac failure had not occurred. The LV developed hypertrophy with an increase of the ventricular weight/body weight ratio (LVW/BW) of 22% even after only 2 days of NE (200 mu g x kg-1 center dot h-1). The RV showed no hypertrophy at any time of the experiments. The NE-induced changes in HR, dP/dtmax, RVSP and LVW/BW were completely prevented by the coinfusion of carvedilol. These studies show that the hemodynamic responses to continuous infusion of NE are more pronounced in the RV than in the LV. Conversely, NE induced hypertrophy only in the LV, not in the RV. The hemodynamic effects of chronic NE infusion did not change significantly between 1 and 3 days of infusion. The in vivo responses to exogenous NE therefore were unaffected by adaptive effects such as downregulation of adrenergic receptors.

Myocardial performance and metabolism in non-ketotic, diabetic rat hearts: myocardial function and metabolism in vivo and in the isolated perfused heart under the influence of insulin and octanoate

SummaryThe influence of a non-ketonic, chronically diabetic state (60 mg/kg streptozotocin) on cardiac function and metabolism was studied under in vivo conditions by inserting a Millar-tip catheter into the left ventricle and in the model of the isolated perfused heart.In vivo heart rate and maximal left ventricular systolic pressure were reduced after a diabetes duration of 4 and 12 weeks. The maximal rise and fall in left ventricular pressure progressively declined with the duration of diabetes. The reduced myocardial function was associated with a loss in ATP and adenine nucleotides.In the perfused heart of chronically diabetic rats, heart function was also impaired and could not be restored in vitro by perfusion with glucose and insulin. In the presence of octanoate — a substrate which can be metabolized independently from insulin — heart function of diabetic rats was improved, but remained lowered as compared to controls. Since the content of myocardial creatine phosphate was reduced in diabetic hearts perfused with octanoate, these findings indicate that the suppression of cardiac performance is not only a result of an impaired glucose metabolism, but of a more general defect in energy provision and utilization.In contrast to hearts of acutely diabetic, ketotic rats most often used, the rate of lipolysis of endogenous triglycerides and the contribution of fatty acids to energy production was low in the chronically diabetic state. Inhibition of fatty acid oxidation by an inhibitor of carnitine palmitoyltransferase (CPTI) did not restore the reduced responsiveness of diabetic hearts to insulin.Analysis of intracardiac metabolites revealed that in the perfused heart of chronically diabetic rats glucose-6-phosphate and citrate do not accumulate as in hearts of ketotic, diabetic rats. Therefore, the impaired glucose metabolism presumably reflects a reduced uptake of glucose rather than in inhibition of glucolysis as in hearts of ketotic, diabetic rats.

Beta-adrenergic agonists stimulate the oxidative pentose phosphate pathway in the rat heart.

The oxidative pentose phosphate pathway is poorly developed in the rat heart compared with other organs, since the activity of glucose-6-phosphate dehydrogenase (G-6-PDH), the first and rate-limiting enzyme of the oxidative pentose phosphate pathway, is low. As a consequence, the available pool of 5-phosphoribosyl-1-pyrophosphate and the rate of adenine nucleotide biosynthesis are limited. Isoproterenol, 24 hours after subcutaneous administration at 0.1, 1, and 25 mg/kg, stimulated the activity of G-6-PDH in whole hearts dose-dependently from 4.3 +/- 0.16 (control) to 6.6 +/- 0.35, 10.3 +/- 0.82, and 11.5 +/- 0.56 units/g protein, respectively. The activity of 6-phosphogluconate dehydrogenase, another of the enzymes in the oxidative pentose phosphate pathway, remained unchanged. G-6-PDH activity started to increase 12 hours after isoproterenol application, when the glycogenolytic and functional response was over, and reached a peak value between 24 and 48 hours. This stimulating effect was also demonstrated in cardiac myocytes that were isolated 28 hours after isoproterenol application. beta-receptor blockade with atenolol reduced the isoproterenol-induced increase in cardiac G-6-PDH activity by 90%. Cycloheximide, which inhibits translation, and actinomycin D, which interferes with transcription, attenuated it by 83% and 78%, respectively. These results indicate that cardiac beta-adrenergic receptors and enzyme protein synthesis are involved in this effect. Other beta-sympathomimetic agents such as dopamine, dobutamine, fenoterol, salbutamol, and terbutaline also stimulated myocardial G-6-PDH activity in a time- and dose-related manner. The calcium antagonist D 600 (gallopamil) reduced the isoproterenol-elicited stimulation by 65%, and verapamil blunted the fenoterol-induced increase by 50%. This suggests that Ca2+ ions also contribute to the stimulation of the cardiac oxidative pentose phosphate pathway.