Β-adrenergic System Research Articles

β-Adrenergic regulation of the somatostatinergic system in rat hippocampus

Interaction of β-adrenergic and somatostatinergic systems in the hippocampus has not been investigated fully. We studied the influence of DL-isoproterenol (ISO), a β-adrenergic agonist and DL-propranolol (PRO), a β-adrenergic blocking agent, on the somatostatinergic system in the rat hippocampus. The short- (5 h) and long-term (14 days) administration of ISO (5 mg/kg i.p.) or of PRO (10 mg/kg i.p.) did not affect somatostatin-like immunoreactivity (SSLI) content in the hippocampus of male Wistar rats. Both short- and long-term ISO administration decreased the number of specific [ 125I]Tyr 11-somatostatin ([ 125ITyr 11-SS) receptors in synaptosomes from hippocampus (29%, P < 0.05 and 34%, P < 0.05, after short- and long-term administration, respectively) without changing the affinity constant. This decrease in the number of [ 125I]Tyr 11-SS receptors was not due to a direct effect of ISO on these receptors since no decrease in binding was produced by high concentrations of ISO (10 −5 M) when added in vitro. In addition, this decrease could be blocked by pretreatment with PRO. Short- and long-term administration of PRO alone increased the [ 125I]Tyr 11-SS binding in hippocampus (42%, P < 0.05 and 33%, P < 0.05, after short- or long-term administration, respectively) without changing the affinity constant. Although there is no direct evidence that the regulation of SS receptors by the β-adrenergic system has a physiological significance, this mechanism may provide a means by which the brain environment could modulate SS receptor number and, therefore, sensitivity to SS in a subset of SS-sensitive neurons.

Effects of chronic tachycardia-induced cardiomyopathy on the β-adrenergic receptor system

Chronic supraventricular (or ventricular) tachycardia causes a dilated cardiomyopathy. Effective treatment requires ablation of the tachycardia using antiarrhythmic agents, cryoablation, electroablation, or surgical interruption/excision. However, the underlying pathophysiologic mechanisms responsible for the development of supraventricular tachycardia-induced cardiomyopathy have not been fully identified. We hypothesized that chronic supraventricular tachycardia is associated with significant changes in the beta-adrenergic system that may have implications for the pathophysiology and treatment of supraventricular tachycardia-induced cardiomyopathy. Accordingly, we examined the relationship between left ventricular function, plasma norepinephrine level, beta-receptor number and affinity, and response to a beta-agonist (isoproterenol) infusion in eight control pigs and eight pigs subjected to supraventricular pacing-induced tachycardia (240 beats/min for 3 weeks). Left ventricular function was measured using simultaneous echocardiography and catheterization. Left ventricular end-diastolic dimension and pressure increased in pigs with supraventricular tachycardia (5.1 +/- 0.4 cm and 27 +/- 2 mm Hg) versus control pigs (3.8 +/- 0.3 cm and 8 +/- 2 mm Hg), p < 0.05. Left ventricular fractional shortening decreased in supraventricular tachycardia (10 +/- 1%) versus control pigs (34 +/- 1%), p < 0.05. In addition, in the pigs with supraventricular tachycardia the fractional shortening versus left ventricular end-systolic stress relationship fell below the control relationship. Plasma norepinephrine level (measured by high-performance liquid chromatography) increased in pigs with supraventricular tachycardia (3592 +/- 1606 pg/ml plasma) versus control pigs (323 +/- 74 pg/ml plasma), p < 0.05. beta-Receptor number and affinity (measured by [3H]dihydroalprenolol binding) did not change in supraventricular tachycardia (98.6 +/- 11.5 fmol/mg protein and 7.2 +/- 1.1 nmol) versus control pigs (99.1 +/- 9.4 fmol/mg protein and 6.8 +/- 0.5 nmol). The response to isoproterenol infusion (10 micrograms/kg) in supraventricular tachycardia was blunted: the absolute increase in left ventricular peak (+)dP/dt was reduced in supraventricular tachycardia (833 +/- 233 mm Hg/sec) versus control pigs (2180 +/- 139 mm Hg/sec), p < 0.05. Chronic supraventricular tachycardia caused a decreased contractile state, increased plasma norepinephrine level, and caused no change in beta-receptor number or affinity; however, the response to beta-agonist infusion was blunted. These results suggest that chronic supraventricular tachycardia is associated with uncoupling of the beta-receptor from subsequent intracellular components of the beta-adrenergic system. Therefore medical management of chronic supraventricular tachycardia-induced cardiomyopathy before and immediately after definitive ablation may require use of pharmacologic agents whose actions do not depend on an intact beta-adrenergic pathway.

β-adrenergic system in myotonic dystrophy

Myotonic dystrophy (DM) is considered an inherited membrane disorder, but the basic defect is unknown. We studies plasma catecholamines as well as lymphocyte and muscle β-adrenergic receptor densities and functioning in 19 DM patients and 15 control subjects. Plasma adrenaline and noradrenaline concentrations were not significantly different between DM patients and controls. In isolated lymphocytes basal cAMP production was 157 ± 26 pmol/mg protein/10 min in DM patients and 91 ± 12 pmol/mg protein/10 min in controls (n.s.), whereas isoproterol-stimulated cAMP production was significantly higher in DM patients with (285 ± 42 pmol/mg protein) compared with control subjects (168 ± 21 pmol/mg protein, P < 0.05). Lymphocyte β-adrenoceptor densities were similar among DM patients (50 ± 3 fmol/mg protein) and controls (47 ± 5 fmol/mg protein). β-Adrenoceptor densities were also similar in muscle biopsy specimens (12.9 ± 1 in DM and 13.1 ± 13.1 ± fmol/mg protein in controls). The correlation between lymphocyte and skeletal muscle recert densities was r = 0.35in DM patients and r = 0.22 in controls. The results do not support the hypothesis of adrenergic dysfunction in DM despite its membrane involvement.

The adenylate cyclase system and prostaglandin production in human decidua parietalis

We studied the β-adrenergic system in the human decidua and its effect on prostaglandin E 2 and F 2a release from dispersed decidual cells in culture. β-adrenergic receptors in decidual membranes were partially characterized using (+)[ 125I]HYP. Specific binding was demonstrated with maximum binding capacity (Bmax) of 70 ± 10. 6 fmol/mg and an affinity ( K D) of 20.85 ± 1.86 pmol. cAMP dependent phosphoproteins in decidual cytosol were also identified. Two phosphoproteins of M r 42 000 and 22 000 were seen in all preparations. Three others ( M r 39 000, 23 000 and 21000) were identified in only some of the preparations. Phosphoproteins of similar M r to those seen in cytosol prepared from decidual homogenates were also identified in cytosol ofcultured decidual cells. Phosphorylation of the 42 000 M r and 22 000 M r proteins was maximal (3.04 ± 0.35 fold and 5.7 ± 0.68 fold) with 10 −6 m cAMP. Cultured decidual cells produced prostaglandin E 2 and F 2a which increased in a dose-dependent manner in response to dbcAMP, forskolin or isoproterenol. The decidua contains an intact β-adrenergic system that, when activated, is capable of phosphorylating specific proteins and modulating prostaglandin release.

Chronic β-blockade transregulates inhibitory A 1 adenosine and muscarinic M 2 receptors of the adenylyl cyclase system

Chronic β-blockade has evolved to an important therapeutic strategy in medicine. Not all its therapeutic effects may be explained by its direct action on the β-adrenergic system. We therefore investigated if chronic β-blockade in vivo or in isolated cell systems may modulate also inhibitory receptors of the adenylyl cyclase system. Chronic treatment with metoprolol for 6 days (10 mg/day) induced an increase of β-adrenergic receptors in rat cardiac plasma membranes (53 ± 8 vs 80 ± 12 fmol/mg protein). Simultaneously the density of cardiac muscarinic M 2 receptors decreased significantly from 150 ± 17 to 110 ± 12 fmol/mg protein without any change of the affinity of the receptors for their agonists or antagonists. By this mechanism chronic β-blockade leads to an unexpected impairment of the muscarinic-mediated inhibition of the adenylyl cyclase. This transregulation of inhibitory receptors by chronic β-blockade was not restricted to the heart but also reduced the muscarinic receptors of rat lung (35 ± 4 vs 24 ± 3 fmol/mg protein). Additionally, other inhibitory receptors of the adenylyl cyclase system such as the A 1 adenosine receptors of rat brain were reduced by chronic β-blockade (532 ± 32 vs 444 ± 26 fmol/mg protein). This transregulation of A 1 adenosine receptors occurred only after chronic β-blockade with the active stereoisomer (−)-metoprolol whereas the (+)-isomer was ineffective. The ability of the remaining A 1 adenosine receptors to form the agonist-promoted high affinity state was unaltered. Their reduction, however, was sufficient to abolish the phenylisopropyl-mediated inhibition of the adenylyl cyclase. To evaluate if this regulation of various inhibitory receptors in different organs may represent a general cellular regulation mechanism, we investigated whether this transregulation also occurred in isolated cells. Isolated smooth muscle cells derived from the vas deferens (DDT 1 MF-2) were cultivated in the presence of the β-blocker atenolol (10 −5 m) for 3 days. Chronic β-blockade in these isolated cells induced an increase of β-adrenergic receptors and concomitantly a significant decrease of A 1 adenosine receptors (460 ± 42 vs 368 ± 18 fmol/mg protein). The affinity of the A 1 adenosine receptors for their agonists and antagonists and the ability of the remaining receptors to form the agonist-promoted high affinity state remained unaltered. In contrast, the reduction of receptor density greatly impaired the adenosine-mediated inhibition of the adenylyl cyclase. These data demonstrate that chronic β-blockade leads to a functionally significant reduction of inhibitory receptors of the adenylyl cyclase system. This newly characterized transregulation of inhibitory receptors of the adenylyl cyclase system by chronic β-blockade represents a new regulation mechanism which involves several inhibitory receptor systems and occurs at the cellular level. This transregulation may explain some of the therapeutic effects of the little understood chronic β-blockade.

Neuroendocrine thymus and β-adrenergic responsiveness in aging mice

It has previously been shown that thymus exerts a regulatory influence on the beta-adrenergic system, controlling, in particular, DNA synthesis in submandibular glands following injection with isoproterenol (IPR). A decreased peak of response found in old mice can be corrected by grafting a neonatal thymus into old animals. In order to clear whether its restoring action needs mutual interrelationships with other control systems or, alternatively, whether even some thymic factor can be effective, the IPR response mentioned above was also assayed in old animals treated with a thymic extract (TME). Among the numerous thymic factors, this extract was chosen due to the effect that it was prepared on the basis of non-immunological tests and had already been shown to be effective in correcting some alterations observed in old rats. Results show that TME is capable of partially restoring the impaired response found in old mice. The location of the peak of thymidine incorporation, however, is shifted towards the right with respect the peak time observed in young, old and thymus-grafted old mice. Without additional studies, it cannot be decided whether the shift and the only partial recovery is due to the particular dose and duration of the treatment or represents a weaker action of the extract with respect to that of thymic graft.

Impairment of the β-Adrenergic System of Peripheral Blood Leukocytes in Atopic Patients with Seasonal Allergic Rhinoconjunctivitis.

Compared to 7 healthy women, an altered β2-adrenoceptor/cyclic adenosine monophosphate (cAMP) system has been observed in a combined study on both expression and function of β2-adrenoceptors on peripheral blood leukocytes (PBL) of 7 atopic women suffering from acute seasonal rhinoconjunctivitis. A reduced affinity of the β-adrenergic radioligand 125Icyanopindolol to its binding site (equilibrium dissociation constant Kd 6.4 ± 1.0 vs. 3.4 ± 0.6 pmol/l in controls; p <0.05) a reduced sensitivity of the intracellular cAMP formation to isoprenaline (concentration necessary to achieve half-maximal effectiveness EC501,088.6 ± 669.0 vs. 71.0 ± 51.0 nmol/l in controls; p <0.01), and a reduced intracellular cAMP increase in response to isoprenaline (Emax as a percentage of the basal cAMP content E₀ 117 ± 3 vs. 145 ± 8% E₀ in controls; p <0.01) point to a subsensitivity of β2-adrenoceptors. A reduced E₀ (4.9 ± 0.8 vs. 8.4 ± 1.3 pmol/l06 cells in controls; p <0.05) suggests an increased activity of the cAMP-degrading enzyme phosphodiesterase. A reduced sympathetic tone in atopy was further confirmed by lower cAMP plasma concentrations (25.7 ± 1.2 vs. 31.3 ± 1.6 nmol/l in controls; p <0.05). The results indicate that more than just one mechanism are involved in the impairment of the PBL β2-adrenoceptor/ cAMP system in atopy.

Do β-Adrenergic Blockade and Sleep State Affect Cardiorespiratory Control in Neonatal Lambs? Multivariate Autoregressive Modeling Approach

Beta-Blockers are used in pregnancy-associated hypertension and in postnatal cardiac arrhythmias, and the neonate may get them in breast milk. We therefore studied the effects of beta-adrenergic medication on interrelations between heart rate (HR), respiration, and arterial blood pressure (aBP) in newborn lambs. The influence of sleep state on these cardiorespiratory interrelations was also examined. HR, aBP, and respiration (based on transthoracic electrical impedance) were recorded and the sleep state was visually documented in five healthy chronically instrumented newborn lambs before the age of 30 d. Propranolol was given (1 mg/kg). Two-min stationary segments of the three signals were analyzed using a multivariate autoregressive model, which yields oscillations of the signals and intersignal relationships as source contributions. The variabilities of aBP and HR were greatest at the low frequencies (less than 0.25 Hz) and so were their intersignal relationships (including baroreflex). The respiratory variability was greatest at the frequencies corresponding to the respiratory rate. During quiet sleep, the variabilities in HR, aBP, and respiration were lowest. The impact of respiratory oscillations on other signals increased but the impact of aBP variability decreased during quiet sleep. beta-Blockade and sleep state affected separately the cardiovascular and respiratory variables and their interrelations. beta-Blockade reduced HR and increased pulse pressure. The overall heart rate variability and the respiratory low-frequency contribution to heart rate variability decreased due to the beta-blockade. We postulate that the beta-adrenergic system is an important regulator of HR and HR variability in neonatal lambs and also of the low-frequency components of the respiratory sinus arrhythmia.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine and the Control of Adrenergic Regulation of Adipose Tissue Lipolysis During Lactation

Adenosine is a locally active factor that is produced intracellularly and extracellularly in adipose tissue. Adenosine binds to receptors in the plasma membrane of adipocytes; this activates a guanine triphosphate binding protein that inhibits adenylate cyclase activity and, hence, lipolysis. Lactation results in an enhanced responsiveness of adipocytes to β-agonists, which stimulate lipolysis, and, paradoxically, to adenosine, which inhibits lipolysis. These adaptations are partly due to increases in ligand binding and to changes in postreceptor components of the signal transduction systems. Somatotropin is implicated in the chronic adaptations of the β-adrenergic system, whereas insulin, somatotropin, glucocorticoids, and at least one unidentified factor have a role in the chronic control of the adenosine system of adipocytes.

Chronic control of the beta- and alpha 2-adrenergic systems of sheep adipose tissue by growth hormone and insulin.

1. Sheep adipose tissue retained responsiveness to catecholamines when maintained in tissue culture for 48 h; both the rate of basal lipolysis and sensitivity to beta-agonists were increased after tissue culture. 2. Tissue culture in the presence of growth hormone resulted in an increased maximum response and sensitivity to the beta-agonist isoprenaline, but had no effect on basal lipolysis. 3. Tissue culture in the presence of insulin increased the basal rate of lipolysis and increased the ratio of the rate of noradrenaline-stimulated/isoprenaline-stimulated lipolysis, indicating a decrease in the 2-adrenergic effect of noradrenaline. 4. Tissue culture in the presence of growth hormone increased ligand binding to beta-adrenergic receptors. 5. Tissue culture in the absence of exogenous hormones increased ligand binding to alpha 2-adrenergic receptors; this was prevented by actinomycin D and partly prevented by insulin. 6. These studies show that both growth hormone and insulin chronically modulate the adrenergic system of sheep adipose tissue; the effects of growth hormone are primarily on the beta-adrenergic system, whereas insulin modulates the alpha 2-adrenergic system.

Thymus-induced recovery of age-related decrease of brain cortex alpha- and beta-adrenoceptors.

It has previously been demonstrated that thymus exerts a regulatory influence on beta-adrenergic system during aging. In particular, it has been shown that thymus can correct the beta-adrenoceptor density decrease in old mice. In the present paper results of experiments are reported dealing with the influence of the thymus on alpha-adrenoceptors of mouse brain cortex. Both subtypes of alpha-adrenoceptors are studied separately, using different labelled ligands. Results show that alpha 1-adrenoceptor density decreases in old animals, while alpha 2-adrenoceptor density does not change significantly. A neonatal thymus grafted into old recipients is capable of correcting the alteration observed in old mice. The differential impairment of alpha-adrenoceptors resembles that one previously observed on beta-adrenoceptors, where beta 1-type decreases in number during aging with a parallel decrease of adenylyl-cyclase activity, while beta 2-type remains unchanged.

Dual sensitization of the adrenergic system in early myocardial ischemia: Independent regulation of the β-adrenergic receptors and the adenylyl cyclase

Acute myocardial ischemia provokes sensitization of the adenylyl cyclase system. This sensitization can be differentiated in a receptor-specific and an enzyme-specific sensitization. The receptor-linked sensitization is characterized by an increase of β-adrenergic receptors in the plasma membranes after 15 mins of global ischemia (49.8 ± 3.6 to 67 ± 6 fmol/mg protein) followed by a further increase (89 ± 4 fmol/mg protein) after 50 min of ischemia in isolated perfused hearts. Concomitantly functionally coupled receptors which are able to bind the β-agonist with high affinity, increased by 32% after 15 min and by 57% after 50 min of ischemia. The affinities of the receptors for their agonists or their antagonists remain unchanged. Maximally isoproterenol-stimulated adenylyl cyclase activity rose from 66 ± 7 to 101 ± 10 pmol cAMP/min/mg protein after 15 min of global ischemia indicating the β-receptor-specific sensitization of the β-adrenergic system. This sensitization was followed by a gradual decline of the adenylyl cyclase activity after 30 and 50 min of global ischemia. Additionally, 15 min of myocardial ischemia induced an enzyme-linked sesitization of the adenylyl cyclase activity as indicated by an increase of the forskolin-stimulated activity by about 25% (300 ± 20 vs 378 ± 25 pmol cAMP/min/mg protein). In contrast after 50 min of ischemia the total adenylyl cyclase activity declined (232 ± 24 pmol cAMP/min/mg protein) despite the persistent increase of β-adrenergic receptors in the plasma membranes. These data demonstrate that the enzyme-specific sensitization is only transient. The early sensitization and late inactivation of the adenylyl cyclase activity occurred independently of receptor activation and could not be prevented by β-blockade (10 −6 m alprenolol). Cyanide perfusion (1 m m), used to block energy metabolism, lead to energy depletion similar to acute myocardial ischemia. This resulted in an increase of functionally coupled receptors with a time course comparable to that of global ischemia. Additional perfusion with desensitizing concentrations of the β-agonist isoproterenol did not induce uncoupling or internalization of β-adrenergic receptors in cyanide treated hearts, suggesting that the rise in functionally coupled receptors is due to a redistribution in part caused by the abolition of continuous receptor internalization. In contrast, the enzyme-linked sensitization is independent of cellular localization of the β-adrenergic receptors. The increased activity was carried by the enzyme even after partial purification with solubilization and wheat germ affinity chromatography. These data suggest an ischemia-induced, covalent modification of the adenylyl cyclase. This enzyme-specific sensitization or its late inactivation of the enzyme does not occur after cyanide perfusion demonstrating that energy depletion is not solely responsible for this sensitization process. The characterization of these distinct pathomechanisms and time courses of the ischemia-induced activation of the β-adrenergic system allowed to differentiate a receptor- versus an enzyme-specific sensitization of the adenylyl cyclase system in acute myocardial ischemia. Such dual sensitization of the β-adrenergic system may contribute to the increased sensitivity of the infarcted heart to catecholamines.

Age-related alterations of β-adrenergic receptor-adenylate cyclase system in rat parotid gland

Age-related alterations of β-adrenergic receptor-adenylate cyclase system in rat parotid gland

Lower Airway Disorders

Purpose of Study The purpose of this study was to assess the relationship between transient tachypnea of the newborn and the incidence of childhood asthma. Study Population The study was done retrospectively. Transient tachypnea of the newborn was diagnosed in children whose gestational age was greater than 34 weeks if they developed a respiratory rate greater than 60/minute for at least 12 hours within 6 hours after birth. All patients had chest x-rays consistent with transient tachypnea of the newborn, and none of the children had hypercapnia or required assisted ventilation. Controls were selected who had the same birth rate and similar gestational age. Total of 58 children with transient tachypnea of the newborn were enrolled as well as 58 controls. Methods Children were examined at 4 to 5 years of age. They were examined in a blind fashion by two physicians. Data accumulated included a history of atopy in the child and in the family as well as pulmonary symptoms and use of bronchodilators. Findings Children who had transient tachypnea of the newborn had a greater incidence of wheezy breathlessness, childhood atopy, and family history of asthma. Reviewer's Comments In their discussion, the authors indicate that the β-adrenergic system plays a role in the absorption of fluid from the newborn's lung at birth. Impairment of the β-adrenergic system, if persistent, would predispose such children to have subsequent difficulty with asthma. This is consistent with the recent prospective study (N Engl J Med. 1988;319:1112-1117) which suggests that diminished lung function is a predisposing factor for the development of wheezing.

Chronic activation of adenosine A1 receptors sensitizes the β-adrenergic system

Chronic activation of adenosine A1 receptors sensitizes the β-adrenergic system

Signal and adaptational changes in gene expression during cardiac overload

Chronic cardiac overload stimulates various quantitative and qualitative mechanisms of adaptation, some of them being species-specific. The signals responsible for these changes in gene expression are still speculative, nevertheless early modifications of the microtubular network have been reported. Soon after overload an increased expression of various genes coding for regulatory proteins has also been observed, this includes various oncogenes and the genes of several heat-shock proteins. Hypertrophy only, is non species-specific and is adaptational because it both multiples the number of contractile units and it lowers wall stress. The slowing of the shortening velocity allows the heart to produce normal tension, at a lower cost, and has different biological explanations depending on the species. In small rodent ventricles, the main but probably not the unique, determinant of this physiological parameter is an isomyosin shift from a high ATPase activity form V1 to a low activity form V3, discovered in our laboratory in 1979. This shift has a transcriptional origin and also occurs in atria in every mammalian including humans; nevertheless it has not been evidenced in the ventricles of humans, dog, cat or guinea-pig. In these species it is necessary to take into account other mechanisms, namely those involved intracellular calcium movements. The number of total, and possibly active, calcium channels is normal in rat overloaded heart suggesting that their synthesis is activated commensurate to the development of hypertrophy. The situation is more complex for other sarcolemma proteins such as the beta-adrenergic system and the Na+, K(+)-ATPase. For the latter there is presently some evidence that an isoenzymatic shift is likely to occur, at least in rats.

Role of thromboxanes in alterations of the diabetic β-adrenergic system

The inotropic effects of isoproterenol (ISO), as well as the β-adrenoceptors population, were measured in cardiac tissues from normal and short-term (3 days) diabetic rats. ISO increased the tension of both normal and diabetic ventricles, but the efficacy ( E max) of the concentration-response curve was greater on ventricles from diabetic rats than in those from the normal control. This phenomenon was accompanied by a decrease in the number of β-adrenoceptor sites ( B max) during diabetes. Insulin-treated diabetic hearts partially reversed the phenomenon. Propanolol blocked, in a competitive manner, the positive inotropic action of ISO in both types of ventricles. Inhibition of the synthesis and receptors of thromboxane (TX) reduced the hyperreactivity to ISO and increased the number of β-adrenoceptors during diabetes, producing B max values almost similar to those of the normal heart. Additionally, the diabetic heart generated and released a greater amount of TXB 2 than the normal heart, even in the presence or absence of ISO. The stimulatory effect of ISO upon TXB 2 release was altered by the specific β-adrenergic blockade and by verapamil. In addition, the drugs able to induce a sustained increase of endogenous cAMP also inhibited the release of TXB 2 by diabetic ventricles. Exogenous TXB 2 exerted the same type of hyperreactivity in diabetic ventricles. This phenomenon was accompanied by an inhibition of Na + + K +-ATPase activity. These results suggest that β-adrenergic inotropic stimulation is secondary to receptor-mediated hydrolysis of arachidonic acid with subsequent release of thromboxanes, which, in turn, may be responsible for both the superreactivity and the decrease in the number of β-adrenoceptors during diabetes. The abnormal reactivity to β-agonists also could be associated with alterations of the diabetic cardiac Na + + K +-ATPase activity induced by TXB 2 whose production is increased during diabetes.

Membrane fluidity of guinea pig lymphocytes and the dysfunction of the respiratory airway and lymphocyte beta-adrenergic systems of the guinea pig

The β-adrenergic receptor responsiveness of isolated guinea pig tracheal spirals can be negatively affected by intraperitoneal administration of the Gram-negative bacterium Haemophilus influenzae , four days prior to the experiment. The reduction in tracheal relaxation is accompanied by a decrease in β-adrenergic receptor binding sites in splenic lymphocyte membranes and by a decrease in the fluidity of these membranes. The H. influenzae -induced dysfunction of both the respiratory airway and lymphocyte β-adrenergic systems can be mimicked by modulating the amount of linoleic acid in the diet. This linoleic acid induced dysfunction of the β-adrenergic system is also accompanied by a decrease in the plasma membrane fluidity of the splenic lymphocyte membranes of the guinea pigs. The role for plasma membrane fluidity in asthma is discussed in relation to current concepts for atopy.

Steroids protect the β-adrenergic system during reperfusion after ischemia: Effects of methylprednisolone on the β-adrenergic response system

Methylprednisolone sodium succinate (MPSS) administered during reperfusion may improve myocardial function. These effects have been related to adrenergic stimulation. The present study investigated (1) the effects of ischemia and reperfusion on the β-adrenergic response system and (2) the ability of MPSS to modify the ischemic effects on the β-adrenergic system. Isolated perfused rat hearts were used. The ischemic protocol consisted of aerobic perfusion (20 minutes) followed by total, global normothermic (37°C) ischemia (30 minutes) and reperfusion (30 minutes) with MPSS (0, 100, 500, or 1,000 mg/L). The non-ischemic protocol consisted of aerobic perfusion (20 minutes) followed by aerobic perfusion (20 minutes) with MPSS (0, 100, 500, or 1,000 mg/L). At the end of the experiments all hearts were rapidly frozen in liquid nitrogen. Crude sarcolemmal membranes were prepared and stimulated at the β-receptor, at the coupling (G s- or N-) protein, or directly at the adenylate cyclase enzyme (AC). Results were assessed by cyclic adenosine monophosphate (cAMP) production. Tissue specimens were analyzed for myocardial content of cAMP and methylprednisolone (MP). In the ischemic protocol, the responsiveness of the β-adrenergic system was significantly reduced at the G s-protein level. The treatment with MPSS (100 or 500 mg/L) during reperfusion preserved the β-adrenergic response. MPSS (1,000 mg/L) offered no protection. In the non-ischemic protocol, MPSS reduced the response of the β-adrenergic system in a dose-dependent manner at the same level. The hearts in the ischemic protocol had significantly higher contents of MP than the hearts in the non-ischemic protocol at corresponding concentrations of MPSS. The present study suggests that postischemic cardiac failure may result in part from β-adrenergic dysfunction. This loss of function, probably at the level of the protein connecting the receptor and AC, can successfully be prevented by an optimal dose of MPSS during reperfusion after ischemia.

Guanine nucleotide regulation of the interconversion of the two-state hepatic glucagon receptor system of rat

To investigate whether guanine nucleotides regulate interconversion of the two-state hepatic glucagon receptor we have utilized kinetic assays of glucagon binding to partially purified rat liver plasma membranes. Dissociation of glucagon at 30 °C exhibited biexponential character in either the absence or presence of GTP, indicating that the system previously seen in intact hepatocytes is independent of intracellular modulators. In each case the receptors underwent a time-dependent conversion from a low affinity to a high affinity state. However, GTP decreased the fraction of receptors in the high affinity state. The rank order for stabilizing the low affinity state was Gpp(NH)p > GTP > GDP ≫ GMP = no nucleotides. Data from competition binding assays with increasing concentrations of GTP allow calculation of equilibrium constants which are 3.32 n m for glucagon and receptor in the absence of GTP, 18.6 n m for glucagon and receptor in the presence of GTP, 1.55 μ m for the association of receptor and GTP presumably linked to an N protein, and 8.86 μ m for the association of the glucagon-receptor complex and GTP again presumably linked to an N protein. Glucagon binding to receptor is noncooperative in both the absence and presence of GTP, distinguishing this system from the β-adrenergic system. With GTP, binding to the low affinity state is favored because of the relative affinities reported. Therefore, GTP regulates the activation by slowing the conversion of the receptor from a low affinity to high affinity form.