Increase In Beta-adrenoceptor Density Research Articles

The effect of streptozotocin-induced diabetes on cardiac beta-adrenoceptor subtypes in the rat.

1. The present study investigates the effect of short-term experimental diabetes of 14-days duration on the beta-adrenoceptor subtypes of the rat heart. 2 .beta-adrenoceptor-mediated functional responses to submaximal doses of isoprenaline were enhanced in Langendorff-perfused hearts from diabetic rats, manifested as greater changes in tension, heart rate and rates of tension development (+dT/dt) and decline (-dT/dt). 3. Radioligand binding data demonstrated that total cardiac beta-adrenoceptor density and affinity for [3H]-dihydroalprenolol was unchanged by diabetes, although a decrease in beta1-adrenoceptor density and increase in beta2-adrenoceptor density was observed. 4. In conclusion, hearts from 14-day streptozotocin-induced diabetic rats demonstrate a number of alterations within the beta-adrenoceptor system. However, the enhanced beta-adrenoceptor-mediated responses to isoprenaline were not caused by an overall increase in density of beta-adrenoceptors, but were accompanied by changes in the ratio of the beta-adrenoceptor subtypes.

Dynamic regulation of beta-adrenergic receptors by endothelin-1 in smooth-muscle cells.

Elevated endothelin-1 (ET-1) levels are found in atherosclerosis, myocardial ischemia, and heart failure, and are correlated with increased mortality rates. Contrary to expectations, elevations of endogenous ET-1 levels in transgenic mice are not associated with increases in arterial blood pressure or with vasospasm, although these effects can be observed after i.v. ET-1 administration. The aim of this study was to determine the regulatory effects of ET-1 on the expression of vasodilator beta-adrenergic receptors and their ability to activate adenylyl cyclase. Smooth-muscle cells were incubated with ET-1 (10(-7) mol/L) for 3 days. The density of ET-1 or beta-adrenergic receptor binding sites was determined using a radioligand binding procedure. Adenylyl cyclase activity was measured to assess any functional changes in the beta-adrenergic receptor density. ET-1 incubation reduced ET-1 binding sites by 70%. In contrast, the beta-adrenergic receptor density increased from 354 +/- 35 to 538 +/- 50 fmol/mg protein (p < 0.01; n = 7) after 3 days. ET-1 increased beta-adrenergic receptors dose-dependently. Incubation with ET-1 for different periods of time showed an initial decrease of 30% after 6 h of ET-1 incubation. However, after 24 h ET-1 induced an increase of beta-adrenergic receptors, reaching a maximal amount after 48 h. An increased stimulation of beta-adrenergic receptor-activated adenylyl cyclase was observed after 3-day ET-1 incubation compared to controls. These data demonstrate that chronic ET receptor activation by ET-1 results in a functionally significant increase in beta-adrenergic receptor density and adenylyl cyclase activity.

Elevated blood pressure and enhanced myocardial contractility in mice with severe IGF-1 deficiency.

To circumvent the embryonic lethality of a complete deficiency in insulin-like growth factor 1 (IGF-1), we generated mice homozygous for a site-specific insertional event that created a mutant IGF-1 allele (igf1m). These mice have IGF-1 levels 30% of wild type yet survive to adulthood, thereby allowing physiological analysis of the phenotype. Miniaturized catheterization technology revealed elevated conscious blood pressure in IGF-1(m/m) mice, and measurements of left ventricular contractility were increased. Adenylyl cyclase activity was enhanced in IGF-1(m/m) hearts, without an increase in beta-adrenergic receptor density, suggesting that crosstalk between IGF-1 and beta-adrenergic signaling pathways may mediate the increased contractility. The hypertrophic response of the left ventricular myocardium in response to aortic constriction, however, was preserved in IGF-1(m/m) mice. We conclude that chronic alterations in IGF-1 levels can selectively modulate blood pressure and left ventricular function, while not affecting adaptive myocardial hypertrophy in vivo.

Myocardial beta-adrenoceptors in pacing-induced heart failure: regulation by enalapril?

In heart failure, both the sympathetic nervous system and the renin angiotensin system play important pathophysiological roles, and the two systems may interact with each other, e.g., angiotensin II facilitating noradrenaline release. An abnormality in beta-adrenoceptor density (i.e., a decrease) occurs in clinical and pacing-induced heart failure. This observation together with the therapeutic effectiveness of converting-enzyme inhibitors in the management of patients with heart failure led to the current investigation. The aim was to explore the impact of chronic enalapril treatment on the status of myocardial beta-adrenoceptors in dogs paced (250 beats.min-1) to end-stage heart failure. Placebo or enalapril treatment (5 mg b.i.d.) commenced 1 week after the onset of ventricular pacing and continued until end-stage heart failure was reached. Myocardial beta-adrenoceptor density and affinity were assessed by radioligand binding with [125I]iodocyanopindolol. Left ventricular angiotensin II formation and noradrenaline concentration were measured. In addition, plasma renin activity and plasma noradrenaline levels were determined. The results showed that there was a significant increase in beta-adrenoceptor density following enalapril treatment compared with placebo in the heart-failure group. Enalapril did not change the beta-adrenoceptor density in the control animals. However, in both heart failure and control animals, enalapril caused an unexpected increase in Kd. Furthermore, in heart failure, enalapril caused a significant increase in myocardial angiotensin II formation. We conclude that enalapril prevents or reverses the myocardial beta-adrenoceptor abnormality seen in heart failure and promotes angiotensin II formation.

Expression of beta-adrenoceptors on circulating mononuclear cells in hypertensives and normotensives before and after reduction of central sympathetic outflow by clonidine.

We have studied beta-adrenoceptor number and affinity on peripheral blood mononuclear cells (PBMCs) in normotensives (NT) and hypertensives (HT), before and after intravenous administration of clonidine, an alpha 2-adrenoceptor agonist which lowers blood pressure predominantly by reducing central nervous system sympathetic outflow. After clonidine, there was a decrease in blood pressure and plasma noradrenaline (NA) and adrenaline (Ad) levels, with an increase in growth hormone (GH) levels, in both NT and HT. There was no difference in basal beta-adrenoceptor densities on PBMCs between NT and HT. After clonidine at 30 and 60 min, there was an increase in beta-adrenoceptor density associated with a low affinity in NT. In HT, no changes were observed. The increased beta-adrenoceptor densities on PBMCs in NT after clonidine, returned to baseline values after 2 h. Short term up-regulation of beta-adrenoceptors on PBMCs in NT after clonidine is accompanied by a fall in blood pressure (BP) and plasma levels of catecholamines. The changes may represent a compensatory mechanism reflecting a rapid externalization-activation of adrenoceptors residing on the internal surface of the membranes with a change of the coupling ability between the receptor and the catalytic component. In HT, although the haemodynamic and neurohormonal response to clonidine was similar to NT, short term upregulation of receptors did not occur. The lack of such response may mirror a form of regulatory dysfunction of beta-adrenoceptors in HT.

Effect of thyroxine on urinary bladder autonomic receptor densities and contractility.

The effect of thyroxine on the response of urinary bladder strips to autonomic drugs and electrical stimulation was studied. Bladder strips from euthyroid, hyperthyroid and thyroidectomized rats were used. Rats were made hyperthyroid by intramuscular injection of thyroxine (250 micrograms/rat for 5 days). Surgical thyroidectomy was performed 12-15 weeks before the experiment. The contractile response of bladder strips from each of the 3 groups of rats to electrical field stimulation and the following drugs was measured: isoproterenol; bethanechol; methoxamine; adenosine triphosphate, and potassium chloride. Isoproterenol produced a significantly greater relaxation in strips from hyperthyroid than from euthyroid rats. The enhanced relaxation response to isoproterenol was accompanied by an increase in beta-adrenergic receptor density. The contractile response to methoxamine, adenosine triphosphate and potassium chloride was unchanged by pretreating the rats with thyroxine. The increase in contraction produced by bethanechol was the same in strips from hyperthyroid and euthyroid rats and the muscarinic receptor densities were also unaltered by thyroxine. Bladder strips from thyroidectomized rats showed the same response as strips from control animals following electrical field stimulation or exposure to isoproterenol, bethanechol, methoxamine, adenosine triphosphate and potassium chloride. These results demonstrate that thyroxine pretreatment enhances the relaxation of urinary bladder strips produced by isoproterenol and that this response is correlated with a hormone-induced increase in beta-adrenergic receptors in this tissue.

The effect of orthotopic transplantation on total, beta 1- and beta 2-adrenoceptors in the human heart.

1. [125I]-(-)pindolol binding was used to determine beta-adrenoceptor density in homogenate preparations of right ventricular endomyocardial biopsies from 43 non-rejecting patients over the first 13 months following cardiac transplantation. The selective beta 1 subtype antagonist ligand CGP 20712A was used to determine the subtype density in 32 of the patients. Biopsy specimens from 15 donor hearts were used as controls. 2. beta-adrenoceptor density (expressed in terms of fmol mg-1 protein) was increased in the group of transplanted hearts as a whole compared with the donor hearts with respect to total (35 +/- 2 vs 23 +/- 2) and the beta 1 subtype (25 +/- 2 vs 16 +/- 2) whereas the beta 2 subtype and radioligand dissociation constant did not differ. 3. Non-parametric analysis of variance of total receptor density over time revealed significant heterogeneity which appears to be due to a discrete increase in beta-adrenoceptor density during the 4th post operative month. 4. These results indicate that beta-adrenoceptor density is not constant following transplantation. Furthermore, the increase in receptor density following transplantation is due mainly to an increase in the beta 1 subtype without a significant change in the beta 2 subtype.

Radiation-Induced Heart Disease: Morphology, Changes in Catecholamine Synthesis and Content, β-Adrenoceptor Density, and Hemodynamic Function in an Experimental Model

To study further the pathophysiology of radiation-induced cardiomyopathy, we investigated resting hemodynamics, myocardial catecholamine synthesis and storage, and beta-adrenoceptor density after local heart irradiation. In Wistar rats, a radiation dose of 20 Gy eventually leads to compromised myocardial function which is characterized by a reduction in cardiac output to 43 +/- 11% and in the left ventricular ejection fraction to 66 +/- 7.5%, and an increase in the left ventricular end-diastolic volume to 187 +/- 17% of control values. This reduction in function is correlated with focal degeneration of 23 +/- 4% of the myocardium. Measurement of tyrosine hydroxylase activity and catecholamine content revealed that catecholamine biosynthesis is unchanged in the adrenals but is significantly reduced in the hearts of irradiated animals, while cardiac beta-adrenoceptor density is increased to about 140% of that in age-matched controls. This is in contrast to findings in dilated or ischemic cardiomyopathy. Time-course studies showed that the development of myocardial degeneration starts simultaneously with the decrease in cardiac output and ejection fraction and the increase in beta-adrenoceptors at 50-80 days postirradiation. Myocardial degeneration is maximal in extent and severity at 100 days and does not progress thereafter. Cardiac output decreases at 80-100 days postirradiation to 60 +/- 7% of control values. A significant further decrease is seen only when congestive heart failure becomes manifest at 249 +/- 21 days after 20 Gy. Thus there is a delay between structural myocardial injury and hemodynamic deterioration which could be due to a compensatory increase in beta-adrenoceptor density during the initial stages of the cardiomyopathy.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of beta-adrenergic regulation of adenylate cyclase in intrapulmonary arteries from fetal and postnatal lambs.

The enzyme adenylate cyclase is critically involved in the regulation of vasodilatation in the developing pulmonary circulation in that it mediates the vascular smooth muscle effects of beta-adrenergic agonists and vasodilatory prostaglandins. These agonists activate receptors coupled to the catalytic subunit of the enzyme by stimulatory guanine nucleotide-dependent regulatory proteins. We examined the ontogeny of the function of the adenylate cyclase system in intrapulmonary arterial segments from fetal lambs at 110-115 (F1) and 125-135 d gestation (F2) and from postnatal lambs at 7-14 (P1) and 35-56 d of age (P2). The function of the intact enzyme system and its components was assessed in incubations measuring cAMP accumulation during phosphodiesterase inhibition. beta-Adrenergic mediation of adenylate cyclase was examined because the binding characteristics of the smooth muscle receptors can be readily quantified. Nonstimulated (basal) cAMP accumulation was similar in F1 and F2, it increased 8-fold from F2 to P1, and it was equivalent in P1 and P2. cAMP accumulation with isoproterenol increased 5.9-fold from F1 to F2 and was similar in F2, P1, and P2. Radioligand binding studies revealed that the greater response to isoproterenol in F2 versus F1 is not related to an increase in beta-adrenergic receptor density or affinity. cAMP accumulation with forskolin, which activates adenylate cyclase by actions that involve both the stimulatory guanine nucleotide-dependent regulatory protein and the catalytic subunit, was similar in the four age groups, indicating that there are no maturational changes in the funciton of these enzyme system components.(ABSTRACT TRUNCATED AT 250 WORDS)

Cigarette Smoking Alters Sympathoadrenal Regulation by Decreasing the Density of β2-Adrenoceptors. A Study of Monitored Smoking Cessation

We report the effects of monitored smoking cessation on adrenergic regulation in chronic smokers. The beta 2 adrenoceptor density of mononuclear leukocytes (MNLs) and plasma catecholamines was analyzed before cessation and 2, 3, and 8 weeks after cessation. We found a progressive increase in beta-adrenoceptor density after smoking cessation. During smoking the beta-adrenoceptor density was 1.456 +/- 83 (mean +/- SEM) binding sites per cell (n = 10), whereas 3 weeks after cessation the density was 1,774 +/- 157 sites per cell (n = 10; p less than 0.05), and at 8 weeks, 1,900 +/- 227 sites per cell (n = 8; p less than 0.05), representing an overall increase of 23%. Smoking cessation had no effect on binding affinity nor on lymphocyte subgroup distribution. The density of MNL cell beta-adrenoceptors in age-matched nonsmoking men was higher, at 1,896 +/- 271 sites per cell, than that of the chronic smokers before cessation, 1,419 +/- 117 sites per cell (n = 14; p less than 0.01). Plasma epinephrine decreased as a result of cessation from 0.36 pmol/ml (0.26-0.44, 95% confidence interval; baseline) to 0.26 pmol/ml (0.20-0.32) at 8 weeks (p less than 0.05), and norepinephrine decreased from 2.09 pmol/ml (1.38-2.80) to 1.69 pmol/ml (1.14-2.24; p = 0.06). We conclude that stopping smoking progressively increases beta 2-adrenoceptor density on MNL cells. Eight weeks after cessation the adrenoceptor density reaches the corresponding level of nonsmokers. These reversible changes in adrenergic regulation after smoking cessation may be associated with the relatively rapid reduction in cardiovascular disease risk among ex-smokers.

Inotropic response to norepinephrine is augmented early and maintained late in conscious dogs with perinephritic hypertension.

We studied the inotropic responses to intravenous infusions of norepinephrine in nine conscious chronically instrumented dogs before and early (2-4 weeks) in the development of perinephritic hypertension; seven conscious dogs were studied later (approximately 14 weeks), during a more stable phase of hypertension. perinephritic hypertension was associated with a 24% increase in left ventricular (LV) mass during developing hypertension; no further increase was seen during the stable hypertension phase. LV end-systolic stress was increased early (p less than 0.01) but was normalized later. The LV end-systolic stress-volume relation demonstrated an enhanced contractile response to norepinephrine during developing hypertension, which returned toward control later in the course of stable hypertension. The LV dP/dt responses to norepinephrine (0.4 microgram/kg/min) were significantly greater during developing hypertension (7,509 +/- 337 mm Hg/sec, p less than 0.05) compared with the control period (4,737 +/- 286 mm Hg/sec) and returned toward the control value during stable hypertension (5,168 +/- 465 mm Hg/sec). The enhanced inotropic responses to norepinephrine in developing hypertension were preserved in the presence of ganglionic blockade, suggesting that the augmentation was not mediated via reflex mechanisms. These physiological responses were associated with an increase in beta-adrenergic receptor density, but no significant change in basal or maximal adenylate cyclase stimulation occurred during developing hypertension. Thus, in contrast to prior studies in anesthetized animals, the inotropic response to beta-adrenergic stimulation is not depressed in conscious dogs but is enhanced selectively during the development of hypertension and maintained during stable hypertension.

Studies on the Pathogenesis of Hypertension in Cushing's Disease and Acromegaly

The pathogenesis of the hypertension associated with Cushing's syndrome and with acromegaly is poorly understood. We have investigated the possible roles of sodium retention, activation of the renin-angiotensin system and increased sympathetic nervous system activity in untreated patients. In 11 patients with Cushing's disease, seven of whom were hypertensive, total exchangeable sodium was normal despite increased levels of the mineralocorticoid hormones, 11-deoxy-corticosterone and corticosterone. The renin-angiotensin system was also normal. Cardiac sensitivity to the beta-receptor agonist isoprenaline was increased, but this was not due to an increase in beta-adrenoceptor density. Hypertension in Cushing's disease is neither sodium-dependent nor angiotensin II-mediated, but increased cardiac sensitivity to catecholamines, by increasing cardiac output, may contribute to the pathogenesis of hypertension. In nine patients with acromegaly (three of whom were hypertensive) total exchangeable sodium was elevated. Although no correlation between blood pressure and exchangeable sodium was found, hypertension in acromegaly is probably sodium dependent. No evidence was found for a pathogenetic role for either the renin-angiotensin-aldosterone or the sympathetic nervous system.

Beta-adrenergic supersensitivity of the transplanted human heart is presynaptic in origin.

An increase in cardiac beta-adrenergic sensitivity or beta-receptor density or both has been described in several animal species after denervating the heart. The transplanted human heart is also denervated and, therefore, may exhibit supersensitivity to beta-adrenergic agonists and an increase in beta-adrenergic receptor density. In 16 patients examined 1-3 months after orthotopic cardiac transplantation, beta-adrenergic receptor density measured by [125I]iodocyanopindolol binding in endomyocardial biopsy specimens was not significantly different in transplant recipients compared with normal controls (transplant = 1,429 +/- 199, control = 1,728 +/- 263 fmol/g wet wt; p = NS). However, when normalized to Lowry protein, the [125I]iodocyanopindolol in beta-adrenergic receptor density in biopsy tissue from transplant recipients was significantly lower than in tissue from controls (transplant = 58.1 +/- 6.2, control = 93.5 +/- 13.4 fmol/g Lowry protein; p = 0.011). Atrial sinus node activity of the denervated donor heart and the innervated atrial cuff of the native recipient heart could be detected on the surface electrocardiogram in six patients. In these six patients, the heart rate response to graded infusions of epinephrine (taken up by the adrenergic nerve terminals) and isoproterenol (not taken up by the adrenergic nerve terminals) was measured. The epinephrine dose-response curve in transplanted donor atria was significantly to the left of the native recipient atrial dose-response curve (p less than 0.0001). The isoproterenol dose-response curves for native and transplanted atria were not different. We conclude that myocardial beta-adrenergic receptors are not increased in human orthotopic cardiac allografts and that there is no evidence for beta-receptor-mediated supersensitivity of postsynaptic origin.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms and time course of beta 1 adrenoceptor desensitisation in mammalian cardiac myocytes.

To study the mechanisms and time course of beta1 adrenoceptor desensitisation in mammalian heart tissue neonatal rat cardiac myocytes (greater than 90% pure) were cultured in serum free medium. Cells were exposed to 1 mumol X litre-1 (-)-isoprenaline for 30 min, 4 h, and 16 h. In myocyte membranes mean(SEM)) 125I-iodocyanopindolol binding was 167(46) pmol X litre-1 (n = 5) and did not differ at 30 min, 4 h, or 16 h in control compared with (-)-isoprenaline treated cells. The maximum number of binding sites was 84(32) fmol X mg protein-1 and was unchanged at 30 min, but (-)-isoprenaline stimulated adenylate cyclase activity significantly decreased from 221(62) to 103(37) pmol X mg protein-1 30 min-1. (-)-Isoprenaline competition curves at 30 min showed a significant increase in the proportion of low affinity binding sites from 46% to 62% (n = 5). By 4 h the maximum number of binding sites was significantly decreased by 54%, adenylate cyclase activity remained depressed, and agonist affinity decreased threefold in the (-)-isoprenaline treated cells. At 16 h (-)-isoprenaline treated cells showed alterations similar to the 4 h values in the maximum number of binding sites, adenylate cyclase activity, and affinity for (-)-isoprenaline. (-)-Isoprenaline stimulated adenylate cyclase activity took 72 h to recover after desensitisation. Overnight ultracentrifugation of the cytosol showed a significant 40% increase in beta adrenoceptor density in cells exposed to (-)-isoprenaline for 4 h (n = 5), suggesting receptor internalisation.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a potentially reversible increase in beta-adrenergic receptors in isolated, neonatal rat cardiac myocytes with impaired energy metabolism.

Previous studies have reported that the numbers of beta- and alpha-adrenergic receptors increase in ischemic myocardium. In vivo studies have raised questions regarding the mechanisms involved in the adrenergic receptor alterations and the consequences of these alterations. The purpose of this study was to evaluate potential relationships among beta-adrenergic receptor changes, high energy phosphate reduction, and severity of cell injury in cultured neonatal rat myocytes treated with metabolic inhibitors. The potential for reversal of the receptor changes also was addressed. Binding parameters were measured using [125I]iodocyanopindolol. After 4 hours incubation in potassium cyanide and 2-deoxyglucose, there was a 43% increase in beta-adrenergic receptor number, 41% decrease in adenosine triphosphate, and minimal morphological change in myocytes. Twenty-four hours after removal of the inhibitors, myocytes exhibited a return to normal of the receptor number and adenosine triphosphate level. Iodoacetate treatment for up to 3 hours resulted in marked reduction in adenosine triphosphate and increasing severity of cell injury. The number of beta-adrenergic receptors was unchanged at 1.2 hours, increased at 1.5-2 hours, and decreased at 3 hours. Thus: beta-adrenergic receptor density increases during relatively early stages of injury in metabolically impaired myocytes with reduced adenosine triphosphate levels and decreases subsequently, after the myocytes become irreversibly injured; the increased beta-adrenergic receptor density in moderately injured myocytes can be reversed upon removal of the injurious agent and restoration of the cellular adenosine triphosphate level; and changes in catecholamines mediated by an intact nervous system are not required for an increase in beta-adrenergic receptor density in the setting of impaired energy metabolism.

Long-term beta 1-selective adrenergic blockade and adrenergic receptors in human subcutaneous adipocytes.

The influence of beta-adrenergic blockade with metoprolol, a beta 1-selective agent, on the adrenergic regulation of lipid mobilization was explored in subcutaneous adipocytes removed from 13 patients with essential hypertension. Treatment with metoprolol, which was associated with adequate beta-adrenergic blockade and an antihypertensive effect, resulted in a significant increase (p less than 0.05) in the binding of the beta-adrenergic antagonist (-)-(3H)-dihydroalprenolol and a 50% increase (p less than 0.01) in the maximum lipolytic response to the beta-adrenergic agonist isopropylnoradrenaline. In 7 patients with normotriglyceridaemia the total plasma triglyceride level increased significantly (p less than 0.025) during metoprolol treatment, a change that was due to an increase in the very low density lipoprotein triglycerides. The findings suggest that chronic treatment with the beta 1-selective adrenergic blocker metoprolol leads to a significant increase in beta-adrenoceptor density and an increase in the lipolytic response to beta-adrenergic agonists. This latter finding may, in some measure, account for the increased plasma triglyceride level observed.

Hamster and Rat Pineal Gland β‐Adrenoceptor Characterization With Iodocyanopindolol and the Effect of Decreased Catecholamine Synthesis on the Receptor

Rat and hamster pineal glands were used in binding studies to characterize their beta-adrenoceptors with a new specific antagonist ligand, iodocyanopindolol. The receptors were saturable, and the ligand was selective and demonstrated stereospecificity for both species. The rat pineal had a 20-fold greater density of beta-adrenoceptors, while the affinity was one-third that of the hamster pineal. utilizing this radioligand, we examined the effects of decreased sympathetic input to the pineal on beta-adrenergic receptors in both species. Decreased noradrenergic input to the pineal gland of the hamster was accomplished by superior cervical ganglionectomy, or by exposing the animals to continuous light for 36 hours. Parallel studies were conducted with hamster pineal gland in which catecholamine synthesis was measured. The results indicate that a selective decrease in catecholamine synthesis in the hamster pineal does not change the beta-adrenoceptor density or affinity. In contrast, a concomitant increase in beta-adrenoceptor density but not affinity occurs in the rat pineal gland after similar decreased sympathetic input.

Prepubertal ovarian function is finely regulated by direct adrenergic influences. Role of noradrenergic innervation.

In the rat, the main source of adrenergic fibers innervating nonvascular ovarian tissue is the superior ovarian nerve (SON). To determine the influence of the SON on prepubertal ovarian function and, hence, on the time of puberty, several experiments were conducted. Transection of the SON in early juvenile rats (day 24) led to more than 60% depletion in ovarian norepinephrine (NE) content, but affected neither the timing of vaginal opening nor that of first ovulation. SON section, however, produced a 2-fold increase in beta-adrenergic receptor density, with no change in affinity, suggesting that hypersensitivity by denervation had developed. Removal of all adrenergic influences from ovarian granulosa cells by culturing the isolated cells in vitro produced a striking increase in beta-adrenergic receptor number, which was maximal at 72 h. In spite of this elevated receptor content, the beta-adrenergic agonist Zinterol did not induce progesterone release unless the cells were previously exposed for 48 h to a physiological concentration of FSH. Both NE and epinephrine down-regulated the receptors in a dose-related manner, without affecting receptor affinity. To test the hypothesis that denervation of the ovary results in greater steroidal sensitivity to adrenergic stimulation, a model was constructed in which cells were simultaneously exposed to FSH and NE or epinephrine (innervated cells) or exposed only to FSH (denervated cells) for 48 h. The response to adrenergic stimulation was subsequently measured by incubating the cells with Zinterol for 24 h. Zinterol was highly effective in eliciting progesterone release from the denervated cells, but failed to do so in innervated cells. Additionally, the agonist increased cAMP in the former cells, but it was ineffective in the latter, indicating that prior exposure to catecholamines desensitized the adenyl cyclase enzyme coupled to beta-adrenergic receptor sites. The results suggest that during prepubertal maturation, noradrenergic fibers reaching the ovary via the SON may modify ovarian steridogenic sensitivity to catecholamines, by regulating the number of functional beta-adrenergic receptors in the gland. They also indicate that the apparent failure of SON denervation to alter the maturation of ovarian steroidogenesis and the time of puberty may be due to development of compensatory hypersensitivity after the denervation.

Short- and long-term influence of beta-adrenergic antagonists after acute myocardial infarction

After coronary arterial occlusion, catecholamines are released from storage depots in the left ventricle and injured myocardial cells are exposed to relatively high concentrations of catecholamines during the evolutionary period in which cell injury is becoming progressively more severe. In addition, in experimental animal models, there is a substantial increase in beta-adrenergic receptor density without any alteration in affinity within 1 hour of permanent coronary arterial occlusion. Recent data suggest that alpha-adrenergic receptor density increases within 30 to 60 minutes after coronary arterial occlusion in experimental animal models. The administration of catecholamines during the early phases of evolving myocardial injury can result in heightened adrenergic biochemical responses in severely injured compared with normally perfused tissue in the hearts of experimental animals. Thus, there is adequate rationale for anticipating that beta-adrenergic antagonists would protect ischemic myocardium and potentially reduce the incidence of life-threatening arrhythmias in individuals with evolving acute myocardial infarction (AMI). Studies in animal models demonstrate that the administration of beta-adrenergic antagonists in the first few minutes after coronary artery occlusion may reduce the ultimate extent of myocardial necrosis. Clinical data from several different trials in which beta-adrenergic antagonists were administered to (1) protect ischemic myocardium and preserve ventricular function and (2) reduce the severity of serious ventricular arrhythmias in patients with AMI are reviewed. The effects of longer-term administration of beta-adrenergic antagonists in patients after AMI in prolonging life and reducing risk of reinfarction are presented.

Cellular mechanisms of impaired adrenergic responsiveness in neonatal dogs.

The myocardial responsiveness of conscious, instrumental dogs to exogenously administered isoproterenol and norepinephrine was investigated in neonatal, 6-wk-old, and adult animals. Comparable base-line values for peak left ventricular derivative of pressure with respect to time were observed in all age categories. However, when compared with adult responses, the sympathomimetic amine-induced increases in neonatal left ventricular dP/dt were significantly blunted at each concentration of adrenergic agonist examined, whereas the 6-wk-old puppies displayed an intermediate inotropic response. To investigate the cellular mechanisms of this blunted neonatal response, we correlated physiologic and biochemical measurements of the myocardial responses to catecholamines in each age category. When compared with adult myocardial membrane preparations, neonatal cardiac membranes were characterized in vitro by an increased density of beta-adrenergic binding sites, comparable affinity for adrenergic agonists and antagonists, and an enhanced coupling of adenylate cyclase activation to receptor occupancy. Simultaneous changes in either the serum catecholamine concentration or the membrane content of other intrinsic proteins failed to account for the observed neonatal increase in beta-adrenergic receptor density. These findings are most consistent with a compensatory mechanism of the cardiac cell membrane, whereby an inherent depression in the adrenergic responsiveness of the immature myocardium appears to induce the increase in receptor density and activation of adenylate cyclase.