Prevention of exercise-induced cardiac hypertrophy in rats by chemical sympathectomy (guanethidine treatment)

Effects of sympathectomy on ovarian follicular development and steroid secretion

Recently, the influence of adrenergic activity over ovarian function, and thus fertility, has begun to gain importance. Previous studies have shown that adrenergic activity through norepinephrine (NE) participates in the control of follicular development and steroidal secretion from the ovary, among other functions. To examine this phenomenon, the denervation of the gonad has been widely used to observe changes in the ovary’s performance. Nevertheless, the effect of the absence of adrenergic nerves in the ovary has only been studied in short times periods. In the present work, we used guanethidine (a drug that produces an irreversible sympathectomy) during the infantile period of rats, and we observed its effects in the adult rat (6 months old). Our results indicate that ovarian NE content is recovered at 6 months old, alongside with an increase of the adrenal content of NE and a dysfunctional celiac ganglion. Together, these results suggest that the recovery of ovarian NE does not come from a neural origin. In addition, ovarian performance was impaired because the changes in follicular development and steroidal secretion are not recovered despite the recovery of ovarian NE content. In conclusion, these results suggest that the nerve–ovarian connections, which are established during infantile development, are necessary for the accurate response of the ovary to sympathetic stimulation.

A study of the physiological consequences of sympathetic denervation of the heart caused by the arterial switch procedure

BACKGROUND

The arterial switch operation is the corrective operation for transposition of the great arteries, defined as the combination of concordant atrioventricular and discordant ventriculo-arterial connections, but there have been concerns about silent subendocardial ischaemia on exercise and coronary artery growth. The arterial switch divides the majority of the sympathetic nerves entering the heart; we have studied the effects of coronary flow and sensitivity to catecholamine stimulation in an animal model.

METHODS

A total of 10 piglets were operated on cardiopulmonary bypass with section and resuturing of aortic trunk, pulmonary artery and both coronary arteries, with 13 sham-operated controls. After 5-7 weeks of recovery, seven simulated switch survivors and 13 controls were studied.

RESULTS

Basal heart rate was significantly higher in switch piglets: in vivo mean (standard deviation) 112 (12) versus sham 100 (10) beats per minute, (p = 0.042); in vitro (Langendorff preparation): 89 (9) versus sham 73 (8) beats per minute (p = 0.0056). In vivo maximal heart rate in response to epinephrine was increased in switch piglets, 209 (13) versus 190 (17) beats per minute (p = 0.044). In vitro dose-response curves to norepinephrine were shifted leftward and upwards (p = 0.0014), with an 80% increase in heart rate induced by 0.095 (0.053) norepinephrine micromole per litre perfusate in switch hearts versus 0.180 (0.035) norepinephrine micromole per litre (p = 0.023). Increase in coronary flow on norepinephrine stimulation and maximal coronary flow were significantly reduced in switch hearts: 0.3 (0.2) versus 0.8 (0.4) millilitre per gram heart weight (p = 0.045) and 2.5 (0.4) versus 3.1 (0.4) millilitre per gram heart (p = 0.030), respectively.

CONCLUSIONS

A combination of increased intrinsic heart rate, increased sensitivity to chronotropic actions of norepinephrine, and a decreased maximal coronary flow creates potential for a mismatch between perfusion and energy demands.

beta-Adrenoceptor blockade and exercise: effects on endurance and physical training

beta-adrenoceptor antagonists influence almost all haemodynamic and metabolic actions in the body. High levels of sympathetic stimulation accompany aerobic exercise and it is known that beta-blockade results in a decreased working capacity. Furthermore it has also been questioned whether beta-blockade inhibits the normal response to physical training. Although adrenergic mechanisms are involved in muscle and liver glycogen breakdown, beta-blockade does not seem to reduce glycogen utilisation during exercise. Both selective and non-selective beta-blockade inhibit lipolysis and result in less free fatty acids being available for muscle utilisation. Surgical and chemical sympathectomy in animals has been shown to inhibit the responses to physical training but results are now available showing that beta-adrenergic blockade does not prevent the effect of physical conditioning in patients treated with propranolol. It is concluded that beta-blockade during prolonged exercise a) does not reduce oxygen uptake by the working muscles b) decreases fat metabolism, which secondarily increases the use of carbohydrates, resulting in earlier hypoglycaemia and/or depletion of muscle glycogen with reduction in working capacity c) does not inhibit central and peripheral adaptation to physical conditioning.

Reduction by oral propranolol treatment of left ventricular hypertrophy secondary to pressure-overload in the rat

1. Studies on cardiac myocyte cell cultures have postulated a role for alpha 1-adrenoceptors and mechanical stretch in the induction of cellular changes thought to be important in compensatory cardiac hypertrophy. However, in vivo work suggests that beta-adrenoceptors are important and the present study was designed to analyse the effect of propranolol on the cardiac hypertrophy caused by a pressure-overload in a way that takes account of the effects of propranolol on the work load itself. 2. The compensatory cardiac hypertrophy that develops in response to experimental coarctation of the aorta was studied in the rat. Pressure gradients and total cardiac work load (expressed as rate x pressure product) were assessed, and the relationship between increasing cardiac work load and the resulting left ventricular hypertrophy was established in a control group and compared with left ventricular hypertrophy in a group treated with a high dose of oral propranolol (80 mg kg-1 body weight). 3. In the rats with mean pressure gradients over the coarctation in the range of 15-31 mmHg, the animals on control diet showed a 38% increase in left ventricular weight/body weight ratio (LV ratio) and a 30% increase in heart weight/body weight ratio (heart ratio), whereas rats given high dose oral propranolol-treatment showed increases of only 13% and 9%, respectively. 4. In a second series of rats with a wider range of pressure gradients, the regression lines of LV ratio versus mean pressure gradient, and of LV ratio versus cardiac work, were different in the two groups with a slope that was only half as steep in the propranolol-treated rats as in the controls. Thus, for the same increment in cardiac work load, the degree of compensatory cardiac hypertrophy in propranolol-treated rats was half that observed in controls. 5. The reduction in compensatory cardiac hypertrophy was not associated with an increase in incidence of congestive heart failure and the propranolol-treated rats were able to sustain equally high (or higher) degrees of pressure over-load as controls did. 6. It is concluded that propranolol treatment approximately halves the compensatory cardiac hypertrophy occurring in response to a left ventricular pressure over-load by a mechanism independent of its effect on cardiac work load. This finding provides further support for the view that noradrenaline released from sympathetic nerve terminals in the heart exerts a trophic effect on cardiac myocytes, and that the sympathetic nervous system may be the final common pathway in many forms of compensatory cardiac hypertrophy. In contrast to in vitro models, this effect appears to be largely mediated via beta-adrenoceptors in the intact animal.

Reduction by beta-adrenoceptor blockade of hypoxia-induced right heart hypertrophy in the rat

1. The study was undertaken to assess the role of beta-adrenoceptors in the induction of compensatory cardiac hypertrophy in an in vivo model. 2. In the rat, exposure to severe hypoxia (6% inspired oxygen for 8 h day) caused a 51% increase in right heart weight and a 75% increase in haematocrit. 3. The hypoxia-induced right ventricular hypertrophic response was reduced by 65% by oral treatment with a high dose of the non-selective beta-adrenoceptor antagonist, propranolol (80 mg kg-1 body weight); the drug treatment caused only a minor reduction (6%) in secondary polycythaemia. 4. With a less severe degree of hypoxia (7% inspired oxygen) there was only minimal secondary polycythaemia (+15%), and a lesser degree of compensatory right ventricular hypertrophy in untreated rats (+33%). 5. Treatment with the beta 1-adrenoceptor antagonist, atenolol, in a dose of 80 mg kg-1 body weight abolished right ventricular hypertrophy in response to 7% inspired oxygen, without affecting haematocrit and caused a small reduction in the ratio of heart weight to body weight in normoxic rats. 6. The results show that the effect of propranolol on hypoxic right ventricular hypertrophy is not secondary to any effect on secondary polycythaemia as has previously been suggested and that a marked reduction of compensatory cardiac hypertrophy can be obtained by a beta 1-selective adrenoceptor antagonist. Thus these findings support the view that noradrenaline released from cardiac sympathetic nerve terminals exerts a trophic effect on myocardial cells and demonstrates that in vivo, this trophic effect can be reduced by beta 1-adrenoceptor blockade.

Pathophysiology of cardiovascular structural changes in hypertension

Hypertrophic adaptation of the left ventricle and blood vessels is a prominent feature of established essential hypertension. Presence of left ventricular hypertrophy in hypertension increases, independently of other risk factors, the risk for a number of the most important cardiovascular hypertensive complications. Available evidence indicates that left ventricular hypertrophy develops in close parallel with the peripheral vascular changes. Structural involvement can be detected already in early phases of borderline hypertension. The pathophysiology of structural changes in hypertension appears to be dependent on a complex interplay between genetic, hemodynamic, and humoral-metabolic factors.

Cardiac hypertrophy due to pressure and volume overload: distinctly different biological phenomena?

Myocardial hypertrophy is a morphological adaptive response to chronic work overload imposed on the heart. It has been categorized into two distinct basic types: concentric hypertrophy, occurring in response to a sustained pressure overload in which wall thickness increases without chamber enlargement, and eccentric hypertrophy, in response to a chronic volume overload in which chamber volume enlarges without a relative increase in its wall thickness. It should be emphasized, in this context, that these adjectives are somewhat confusing, since the hypertrophy observed is not eccentric in the fashion often seen in the left ventricle of patients with hypertrophic cardiomyopathy. In fact, the hypertrophy is concentric in both instances, but is associated with an increase in chamber volume when described as eccentric, yet occurring with a maintained volume when said to be concentric. In rats made anemic by iron deficiency, the volume overloaded heart achieves an adaptive increase in mass characterized as hypertrophy occurring in the setting of dilated ventricle. This so-called eccentric hypertrophy depends on catecholamines as possible signals for myocardial growth, and progresses with preserved ultrastructure and contractile performance of the cardiac muscle. A gradually imposed volume overload results in a harmonious growth of the heart (it retains a relative normal shape, becoming a magnified normal heart), probably mediated by release of catecholamines into the myocardium. This process resembles the normal cardiac growth in response to the obligatory volume load imposed by an increasing cardiac output (greater metabolic demands) and blood volume.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of adrenergic system in regulation of cardiac myosin heavy chain gene expression

The cardiac phenotype exhibits considerable plasticity, being under the regulation of numerous factors, such as developmental stage, functional load, as well as nutritional and hormonal states of the animal. Several lines of evidence indicate that the adrenergic nervous system plays an important role in the redistribution of myosin isoforms in the heart. For example, chemical sympathectomy favors the expression of V3 isomyosin at the expense of V1. In this study, we have examined the effect of adrenergic pathways on the expression of cardiac myosin heavy chain (MHC) genes. The level of cAMP was modulated by either adding forskolin or 8-bromo-cAMP to primary cultures of embryonic (18 d) cardiac myocytes. We have found that the level of mRNA coding for MHC-alpha was increased two- to three-fold. The effect was dose- and time-dependent and was potentiated further when the 8-Br-cAMP was given together with a phosphodiesterase inhibitor. The same changes were found in KCl arrested cells, indicating independence of contractile activity. Treatment of cells known to activate the protein kinase C (TPA) and inositol triphosphate pathways has increased the level of beta-MHC mRNA while that of alpha-MHC remained unchanged. These data lend strong support to direct effect of the adrenergic system on activity of cardiac genes.

Dietary salt intake. A determinant of cardiac involvement in essential hypertension

Because a given increase in afterload does not consistently produce the same degree of left ventricular hypertrophy, we evaluated several clinical, hemodynamic, and endocrine factors that are prone to modify the adaptation of left ventricular structure in patients with mild essential hypertension (World Health Organization stages I or II). Dietary salt intake assessed by sodium excretion over 24 hours was a powerful determinant of posterior wall thickness (r = 0.64, p less than 0.001), relative wall thickness (r = 0.67, p less than 0.001), and left ventricular mass (r = 0.37, p less than 0.05). In contrast, diastolic pressure, body mass index, hematocrit, and epinephrine were found to be weaker determinants of left ventricular structure (r = 0.31-0.40, p less than 0.05). A stepwise multiple regression analysis revealed that sodium excretion was the strongest predictor for posterior wall thickness (p less than 0.02) and relative wall thickness (p less than 0.05) independent of the other examined variables. These results identify dietary salt intake as a strong determinant of cardiac structural adaptation to a persistent increase in arterial pressure. Consequently, a high salt intake might aggravate and, conversely, dietary salt restriction might prevent (or at least mitigate) the development of left ventricular hypertrophy in patients with essential hypertension.

Behavioral vs beta-blocker therapy in patients with primary hypertension: effects on blood pressure, left ventricular function and mass, and the pressor surge of social stress anger

We compared the relative effects of relaxation therapy, conventional hygienic techniques, and a beta-receptor blocker, atenolol, on control of arterial pressure, left ventricular mass, and diastolic function in patients with mild primary hypertension. Furthermore, we related these effects to baseline neural tone and its changes and assessed the efficacy of relaxation or the pressor surge of "social stress" anger. In group I left ventricular mass index was related to both systolic and diastolic blood pressure (r = 0.46; p less than 0.05). Plasma norepinephrine was related to age (r = 0.33; p less than 0.01). Slope was inversely related to both plasma norepinephrine (r = -0.29; p less than 0.05) and age (r = -0.31; p less than 0.05). Relaxation therapy reduced both supine systolic and diastolic blood pressures, 4.5% and 7.6%, respectively, but did not affect plasma norepinephrine. Hygienic informational therapy reduced plasma norepinephrine by 18%, (p less than 0.05) and did not change blood pressure. Blood pressure responders to nondrug therapy (i.e., diastolic blood pressure reduction of 7% or more) had a 7% reduction of left ventricular mass index (p less than 0.02). On the other hand, atenolol reduced systolic and diastolic blood pressure by 10% and 15%, respectively, (p less than 0.01) and improved left ventricular function by 17% (p less than 0.05) without a significant change in left ventricular mass index. Finally, relaxation therapy but not hygienic therapy reduced systolic blood pressures 4% and 6%, respectively (p less than 0.01), both before and during social stress anger.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of beta 1- and beta 1 + beta 2-antagonists on training-induced myocardial hypertrophy and enzyme adaptation

The effects of beta 1- and beta 1 + beta 2-antagonists on the myocardial adaptation to exercise training were investigated in male Sprague-Dawley rats randomly divided into trained (treadmill, 1 hr/day, 5 days/week for 10 weeks at 27 m/min, 15% grade) without drug (TC), sedentary without drug (SC), trained treated with atenolol (TA) (10 mg/kg body wt, i.p.), trained treated with propranolol (TP, 30 mg/kg body wt, i.p.), and sedentary propranolol. Doses of both beta-antagonists were titrated to decrease the exercise heart rate by 25% compared to the controls. The heart weight and heart/body weight ratio were significantly greater in TC (1.28 +/- 0.07 g (P less than 0.01); 296 +/- 12 mg/100 g body wt (P less than 0.05) respectively) than in SC (1.09 +/- 0.04 g and 268 +/- 11 mg/100 g body wt), or in TP and TA. Myocardial mitochondrial protein was unchanged by training or beta-blockade. Citrate synthase and beta-hydroxyacyl CoA dehydrogenase activities were not altered. Carnitine palmitoyltransferase activity was increased in SP compared to SC. Training increased hexokinase activity only in TC (5.22 +/- 0.12 vs 4.26 +/- 0.23 mumol/min/g wet wt, P less than 0.01). Lactate dehydrogenase activity increased significantly (P less than 0.01) in both TC (383 +/- 14 mumol/min/g wet wt) and TA (372 +/- 14 mumol/min/g wet wt) compared to SC (276 +/- 14 mumol/min/g wet wt), but not in TP versus SP. These data indicate that (1) beta-adrenergic blockade prevents training-induced cardiac hypertrophy; (2) beta-antagonists have little effect on the myocardial oxidative capacity; and (3) while the training induction of myocardial hexokinase is inhibited by both beta 1- and beta 1 + beta 2-antagonists, myocardium may increase its ability to utilize lactate during exercise with training despite beta 1-blockade.

Enzymatic adaptation to physical training under beta-blockade in the rat. Evidence of a beta 2-adrenergic mechanism in skeletal muscle

Nonselective and beta 1-selective adrenergic antagonists were tested for their effects on enzymatic adaptation to exercise training in rats as follows: trained + placebo (TC); trained + propranolol (TP); trained + atenolol (TA); and corresponding sedentary groups, SC and SP. Trained rats ran 1 h/d at 26.8 m/min, 15% grade, 5 d/wk, 10 wk. Both beta-antagonists were given at doses that decreased exercise heart rates by 25%. Training increased skeletal muscle citrate synthase, cytochrome c oxidase (Cyt-Ox), carnitine palmitoyltransferase (CPT), beta-hydroxyacyl coenzyme A dehydrogenase, mitochondrial malate dehydrogenase (MDH), and alanine aminotransferase (ALT) activities significantly in the TC group, but not in TP. These enzyme activities, except Cyt-Ox and CPT, were also significantly increased in TA. Hepatic phosphoenolpyruvate carboxykinase activity did not alter with training or beta-blockade. Fructose 1,6-bisphosphatase activity was lower in TC than in SC, but unchanged in TP or TA. Hepatic mitochondrial MDH and ALT activities increased with training only in TC. It is concluded that beta 2-adrenergic mechanisms play an essential role in the training-induced enzymatic adaptation in skeletal muscle.

Pathogenesis of ventricular hypertrophy

Growth of the vertebrate heart during embryonic and fetal life is characterized by hyperplasia of myocardial cells. Shortly after birth, myocardial cells lose the capability of dividing, and further growth of the heart is due to myocardial cell hypertrophy and nonmuscle cell hyperplasia. This process results in a 30- to 40-fold increase in volume of individual myocardial cells during normal postnatal growth and maturation. The transition from hyperplastic to hypertrophic growth is related to formation of binucleated myocardial cells as a result of karyokinesis without cytokinesis. The molecular mechanism of this transition is uncertain. The response of the heart to increased metabolic demands or an increased work load depends on the age of the animal at the time when the stress is imposed. Increased myocardial work loads in fetal or early neonatal life lead to cardiac enlargement by causing an increased rate of hyperplasia of myocardial cells or continuation of hyperplasia beyond the normal period of hyperplastic growth. In contrast, imposition of increased loads on the hearts of older animals results in cardiac hypertrophy due to enlargement of myocardial cells and hyperplasia of nonmuscular components. In addition to cellular enlargement, structural remodeling of the myocardial cells and of the chambers of the heart occurs during the development of hypertrophy. Important stimuli of cardiac hypertrophy include increased systolic force or tension generated by the myocardial fibers (pressure overload), increased end-diastolic wall stress (volume overload) and neurohumoral factors such as increased circulating catecholamines or discharge of cardiac sympathetic nerves, or both, activation of the renin-angiotensin system and increased levels of thyroxine and growth hormone.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in left ventricular mass during a double-blind study with chlorthalidone and slow-release nifedipine

The presence of a possible correlation between changes in left ventricular mass of hypertensive patients and the degree of blood pressure reduction with different antihypertensive drugs has been investigated in 40 outpatients by M-mode echocardiography. Ten of these, with blood pressure in normal limits with different antihypertensive treatment had their therapy changed in chlorthalidone 25 mg/day without any run-in (Group A); other 30 patients, with a previously uncontrolled blood pressure, after a 14 day run-in, were randomly allocated to chlorthalidone 25 mg/day (Group B), slow release nifedipine 20 mg/day (Group C) or placebo (Group D). At the end of the eight week treatment period a further decrease in systolic blood pressure was observed in Group A without changes in ventricular mass; an highly significant decrease in both systolic and diastolic blood pressure was observed in B and C but only patients on chlorthalidone changed their ventricular mass; no change in both blood pressure and ventricular mass was observed on placebo. As changes in ventricular mass are not correlated with blood pressure reduction, we conclude that other, not well defined factors, apart from the decrease in duration and degree of left ventricular systolic wall tension, may be responsible for reversal of left ventricular hypertrophy.

Regression of cardiac hypertrophy. Experimental animal model

Regression of cardiac hypertrophy has been proven to occur in experimental animals following some types of antihypertensive therapy. However, no direct and necessary correlation can be found between hypertension and cardiac hypertrophy. The effect of different antihypertensive drugs on regression of cardiac hypertrophy varies despite the same degree of reduction in arterial pressure in hypertensive control subjects. Therapy with alpha-methyldopa resulted in reversal of hypertrophy with effective blood pressure control, whereas treatment with vasodilators, namely hydralazine and minoxidil, either did not alter (hydralazine) degree of myocardial hypertrophy or increase it (minoxidil) despite normalization of blood pressure. The biochemical profile of the myocardium after regression of hypertrophy following antihypertensive therapy is not homogeneous; for example, reversal with alpha-methyldopa is associated with increased collagen content, whereas reversal with Captopril did not alter collagen content of the heart. Adrenergic factors seem to play an important role in the modulation of myocardial structure to variations in arterial pressure. Furthermore, in each type of hypertrophy in hypertension, a combination of different factors might be responsible, and it may not be correct to assume that the same factors must be involved in the regression of all types of myocardial hypertrophy.

Left ventricular hypertrophy and function in high, normal, and low-renin forms of essential hypertension

To assess the relative importance of the level of blood pressure (BP) and renin profile status as determinants of hypertensive left ventricular hypertrophy (LVH) and dysfunction, we studied, by quantitative echocardiography, 118 hypertensive patients off medication. The 19 high-renin patients were younger (31 +/- 13 years; p less than 0.01) but had hypertension of severity (152 +/- 13/95 +/- 11 mg Hg) similar to the 79 normal-renin patients (42 +/- 14 years; 152 +/- 17/98 +/- 12 mg Hg) and 20 low-renin patients (49 +/- 13 yrs; 157 +/- 17/95 +/- 11 mm Hg). Left ventricular (LV) mass index (normal = 70 +/- 25 g/m2) was similar in the high- (113 +/- 21 g/m2, p less than 0.001), normal = (114 +/- 31 g/m2, p less than 0.001), and low-renin patients (115 +/- 18 g/m2, p less than 0.01). End-diastolic relative wall thickness (nl = 0.32 +/- 0.05) was equally elevated in high- (0.41 +/- 0.09), normal (0.42 +/- 0.08) and low-renin groups (0.41 +/- 0.08) (all p less than 0.001). In the entire population, there was a closer correlation of relative wall thickness with total peripheral resistance (r = 0.54, p less than 0.001) than with mean blood pressure (r = 0.31, p less than 0.05). LV dysfunction (LV fractional systolic shortening less than 26%) occurred only in two high-renin patients, whereas LV fractional shortening was significantly increased in the low-renin subgroup (p less than 0.01). We conclude that the degree of LVH is similar in low-, normal-, and high-renin hypertensives and is proportional to the degree of hypertension, even though the high-renin patients were significantly younger; but that the low-renin patients with similar level of blood pressure, even though significantly older and with a longer duration of hypertension than the other patient groups, manifest increased LV function compared to normals.

Increased plasma catecholamines in patients with Friedreich's ataxia

We studied free plasma catecholamines in 23 patients with Friedreich's ataxia, having a mean age of 22 +/- 9.6 (SD) years. Conjugated catecholamines were also studied in 10 patients. Mean plasma norepinephrine and epinephrine were significantly higher than controls both in the supine and standing positions. In total 15 out of 23 patients (65%) had increase free and/or conjugated plasma catecholamines. The increased in plasma catecholamines was more marked in patients with severe neuromotor impairment. Among the patients with left ventricular concentric hypertrophy (wall thickness greater than 12 mm), only 3 had no demonstrable sympathetic hyperfunction. Since the high local concentrations of norepinephrine at the site of release from sympathetic nerve terminals may serve as a trigger for the hypertrophic response of the myocardial cell, it is suggested that early pharmacological intervention could prevent or limit the cardiomyopathic process or its clinical consequences.

Training-induced bradycardia and intrinsic heart rate in rats

After 10 weeks of treadmill training, female Sprague-Dawley rats had developed a bradycardia at exercise on submaximal work loads. This bradycardia was also present after autonomic denervation and in isolated perfused heart preparations. The heart weight/body weight ratio was increased in these trained animals compared to untrained littermates. Sympathectomized, trained rats developed the same degree of cardiac hypertrophy, but their heart rate after denervation and in the isolated heart was the same as in sympathectomized, untrained rats. It is concluded that the bradycardia of trained and thereafter denervated animals seen in this and a previous investigation represents an adaptation within the heart itself, since it was present in the isolated heart. These results thus provide further evidence for a non-neural component in training-induced bradycardia. Since the trained sympathectomized rats had a cardiac hypertrophy but no reduction of intrinsic heart rate, it seems likely that the myocardial mass is of minor importance for the level of intrinsic heart rate.

Hypoxic cardiac hypertrophy is not inhibited by cardioselective or non-selective beta-adrenoceptor antagonists

1. Young rabbits, eight at a time, were exposed to more than 170 hr of hypoxia (O2 at 70-80 torr) at atmospheric pressure during 9-11 days. 2. The animals were injected twice daily with doses of beta-blockers up to the highest used in human therapy, or with an equivalent volume of saline. These controls were litter mates of the treated rabbits. 3. In comparison with untreated normoxic rabbits of similar age, the hypoxia induced highly significant mean increases in right ventricular dry weight (+57%, controls; +55% treated). 4. In each group, the same degree of hypertrophy was induced in the treated and control animals, irrespective of whether the drug used was a non-selective beta-blocker (propranolol) or a cardioselective drug (atenolol). 5. The right atria were also hypertrophied, though not as much as the right ventricles. 6. In all the hypertrophied tissues the water content was increased. 7. No significant change was observed in the dry weight or water content of the left ventricles of treated or control animals.

Adaptational responses to prolonged beta-adrenoceptor blockade in adult rabbits

1 A method for measuring propranolol concentrations in plasma has been modified to permit estimations to be made on small volumes. The method has been used to correlate blockade of heart rate increases in response to intravenous isoprenaline, in both young and adult rabbits, with plasma concentrations of propranolol after subcutaneous injections of 2 or 4 mg/kg. It has been found that the relation between beta-adrenoceptor blockade and plasma concentrations is the same in rabbits as that determined by previous workers in man. 2 Adult Dutch dwarf rabbits were treated for 9 weeks twice daily subcutaneously with 2 mg/kg propranolol, or 10 mg/kg practolol, or with saline. 3 The treatment caused no change in heart weight in relation to body weight or in the water content of the hearts, in contrast to effects previously observed in young rabbits. 4 As in the young rabbits, treatment did cause a prolongation of action potential duration, as measured with intracellular electrodes in hearts of animals killed 24 h after the last dose of drug. 5 In another series of experiments, with a similar regime of treatment for 6 weeks, a significant reduction of diastolic blood pressure was observed in the propranolol group. 6 During the course of treatment and at the end, there was no change in the heart-rate increases observed in response to intravenous isoprenaline administered not less than 15 h after the previous dose of beta-blockers. Thus no functional alteration in sensitivity ot beta-adrenoceptor stimulation was apparent.

Hypertrophic smooth muscle. I. Size and shape of cells, occurrence of mitoses

An extensive hypertrophy of the muscle coat develops in the small intestine of the guinea pig oral to an experimental stenosis. The profiles of smooth muscle cells become larger and irregular in shape. From the analysis of serial sections the arrangement of the muscle cells is less orderly than in control muscles. Many muscle cells are split into two or more branches over part of their length. The average cell volume is 3--4 times that of control muscle cells; the cell surface increases less dramatically and, in spite of the appearance of deep invaginations of the cell membrane, the surface-to-volume ratio falls from 1.4 to 0.8. The average cell length is only slightly increased compared with controls. Smooth muscle cells in mitosis are observed in all the hypertrophic muscles examined, in both muscle layers; in the circular musculature they occur mainly found in the middle part of the layer.