Hypothyroidism modulates beta adrenergic receptor adenylate cyclase interactions in rat reticulocytes.

We have investigated alterations in beta adrenergic receptor binding sites of rat reticulocytes occurring in animals rendered hypothyroid by thyroidectomy. Beta adrenergic receptor interactions were assessed by measuring the number of (-)[3H]-dihydroalprenolol binding sites and the ability of an agonist to compete for occupancy of the receptors. The number of receptors was significantly reduced in cells from the hypothyroid animals. In addition, there were significant agonist-specific alterations in binding. Using computer assisted curve fitting techniques, it was found that the ability of (-)isoproterenol to stabilize a high affinity guanine nucleotide sensitive "coupled" form of the receptor was impaired. Reticulocytes from hypothyroid animals have, in addition, a reduction in the concentration of the nucleotide regulatory protein as assessed by the number of 42,000 Mr substrates for cholera toxin catalyzed ADP ribosylation. These alterations are associated with reductions in catecholamine and NaF stimulated adenylate cyclase activity. Diminished coupling of beta adrenergic receptors with other regulatory components of the adenylate cyclase system represents a mechanism by which altered thyroid states modulate beta adrenergic receptor function and beta adrenergic responsiveness of tissues.     

Effects of ethanol in vitro on the beta adrenergic receptor-coupled adenylate cyclase system

The effects of ethanol on the beta adrenergic receptor-coupled adenylate cyclase system were examined in vitro using membranes prepared from S49 lymphoma cells. Ethanol caused a dose-dependent increase in adenylate cyclase activity in membranes prepared from wild-type cells when the activity was measured in the presence of GTP. Activity measured in the presence of isoproterenol was also increased by ethanol, but the fold-stimulation by isoproterenol was lower in the presence of ethanol. Ethanol also shifted the dose-response curve for stimulation of the enzyme by isoproterenol to the right. This shift was due to a decrease in the affinity of the beta adrenergic receptor for isoproterenol. A decrease in the affinity of the receptor for the antagonists [125I]iodopindolol and propranolol was also observed, but the magnitude of this effect was less than that seen with the agonist isoproterenol. The density of binding sites for [125I]iodopindolol was not affected by ethanol. Dose-response curves for NaF and guanosine-5'-O-(3-thiotriphosphate), both of which stimulate adenylate cyclase activity through an effect on the stimulatory guanine nucleotide-binding protein (Gs), were shifted to the left by the addition of ethanol. In membranes prepared from the CYC- variant of S49 cells, which lacks the alpha subunit of Gs, guanosine-5'-O-(3-thiotriphosphate) inhibited forskolin-stimulated adenylate cyclase activity. The inhibition by guanosine-5'-O-(3-thiotriphosphate) was not affected by ethanol. In membranes prepared from both wild-type and CYC- S49 cells, ethanol inhibited forskolin-stimulated adenylate cyclase activity. Whereas the inhibition of this activity by GTP was greatly attenuated in membranes prepared from CYC- S49 cells which had been pretreated with pertussis toxin, the inhibition by ethanol was not affected by pretreatment with pertussis toxin.(ABSTRACT TRUNCATED AT 250 WORDS)     

In Vitro Desensitization of Beta Adrenergic Receptors in Human Neutrophils. ATTENUATION BY CORTICOSTEROIDS

The receptor alterations involved in catecholamine-induced desensitization of adenylate cyclase in human neutrophils have been investigated as has the ability of hydrocortisone to modify such alterations. Incubation of human neutrophils with isoproterenol for 3 h in vitro resulted in an 86% reduction in the ability of isoproterenol to stimulate cyclic AMP accumulation in the cells. Two types of receptor alterations were documented. There was a 40% reduction in the number of beta adrenergic receptors (42 vs. 25 fmol/mg protein, P < 0.005) present after desensitization as assessed by [³H]dihydroalprenolol ([³H]DHA) binding. In addition the receptors appeared to be relatively uncoupled from adenylate cyclase. This uncoupling was assessed by examining the ability of the agonist isoproterenol to stabilize a high-affinity form of the receptor, detected by computer modelling of competition curves for [³H]DHA binding. Desensitized receptors were characterized by rightward-shifted agonist competition curves. When hydrocortisone was added to the desensitizing incubations (combined treatment) there was a statistically significant attenuation in the desensitization process as assessed by the ability of isoproterenol to increase cyclic AMP levels in the cells. Although combined treatment did not prevent the decline in receptor number, it did attenuate the uncoupling of the receptors. Combined treatment resulted in competition curves intermediate between the control and the rightward-shifted desensitization curves. Prednisolone was similar to hydrocortisone in attenuating isoproterenol-induced uncoupling. Thus, steroids appeared to attenuate agonist-induced desensitization of the beta adrenergic receptor-adenylate cyclase system by dampening the ability of agonists to uncouple receptors without modifying their ability to promote down-regulation of beta adrenergic receptors.     

Binding and functional characteristics of beta adrenergic receptors in the intact neutrophil

Beta adrenergic receptors have been previously characterized in human neutrophil sonicates. In the present study the intact neutrophil has been assessed for the number and affinity of beta adrenergic binding sites by using the antagonist DNA. Agonist and antagonist potencies, characterized by their effect on DHA binding and cyclic AMP accumulation, are compared with agonist inhibition of lysosomal enzyme (beta glucuronidase) release. Criteria for beta adrenergic receptor identification were successfully demonstrated. At 30 degrees C, beta adrenergic binding was rapid (t 1/2 2 min) and reversible (t 1/2 9 min). Receptor binding was saturable, revealing approximately 900 high-affinity receptors per neutrophil with DHA concentrations of 0.1 to 10 nM. By utilizing both equilibrium and kinetic techniques, the KD was determined to be approximately 0.6 nM. Agonists and antagonists competed for DHA binding in a manner consistent with their effect on cyclic AMP generation. Rank order potency was suggestive of a beta-2 receptor: isoproterenol greater than epinephrine greater than norepinephrine. Stereoselectivity was shown by the greater potency of L-propranolol compared to the D isomer. A high degree of receptor-adenylate cyclase coupling efficiency was suggested by the observation that with only 1% receptor occupancy isoproterenol stimulated 50% maximal cyclic AMP generation. Finally, there was an excellent correlation between the isoproterenol concentration which resulted in 50% of maximal inhibition of beta glucuronidase release (Ki) and that causing 50% maximal cyclic AMP stimulation (Kact), suggestive of a close relationship between beta adrenergic-induced adenylate cyclase activation and beta adrenergic regulation of neutrophil lysosomal enzyme release. The data presented suggest that the use of the intact neutrophil for study of the beta adrenergic receptor is feasible and may provide information which is considerably more closely related to modulation of physiological function by neurohormones than is possible with disrupted cell preparations.     

Effects of agonist and phorbol ester on adrenergic receptors of DDT1 MF-2 cells

The effects of the adrenergic agonist epinephrine (EPI) and of phorbol 12-myristate 13-acetate (PMA) on the regulation of alpha-1 adrenergic receptors (AAR) and beta adrenergic receptors (BAR) were compared in DDT1 MF-2 cells grown in suspension culture. Pretreatment of cells with 10 microM EPI for 30 min at 37 degrees C resulted in homologous desensitization of BAR-coupled adenylate cyclase activity assayed in membranes and induced internalization or sequestration of BAR. Pretreatment of cells with PMA did not alter BAR-coupled adenylate cyclase activity or induce internalization of BAR. EPI pretreatment caused a 50% decrease in the subsequent ability of EPI to stimulate AAR-mediated incorporation of 32P into phosphatidylinositol, whereas PMA pretreatment inhibited incorporation by 95%. Neither EPI nor PMA induced the internalization of AAR. Neither EPI nor PMA altered agonist binding properties of AAR in short-time competition binding assays on intact cells, indicating that pretreatment of cells with these agents does not alter the affinity of AAR for agonist. In control cells, agonists converted AAR from a form exhibiting predominantly high affinity for agonists, detected in short-time assays, to a form, exhibiting low apparent affinity for agonist during the course of equilibrium competition binding assays. PMA pretreatment increased the extent of this subsequent agonist-induced conversion to the low affinity form. These results indicate that PMA can mimic agonist-induced desensitization of AAR, but not BAR, and that the desensitization of AAR-coupled phosphatidylinositol turnover induced by EPI and by PMA is not due to altered receptor affinity for EPI or due to receptor internalization.     

Adenylate cyclase and beta adrenergic receptor development in the mouse heart

We have demonstrated previously a postnatal peak for the beta adrenergic receptor in the heart and detected the appearance of a beta adrenergic receptor before an (-)-isoproterenol inducible increase in heart rate. The present study examined 1) agonist displaceable [3H] dihydroalprenolol (DHA) binding in the neonatal and adult mouse heart and 2) adenylate cyclase in fetal, neonatal and adult mouse heart. 3[H]DHA binding displaceable by (-)-isoproterenol gave a similar Ki from 1 day neonate through adult. Similar to the result found for antagonist displacement binding, there was a dramatic increase in the agonist displaceable [3H] DHA binding postnatally. The maximum was achieved in 2 weeks and then gradually declined to adult level. Cyclase activity (basal, (-)-isoproterenol- and NaF- stimulated) paralleled beta adrenergic receptor increases before birth. However, no early postnatal peak was present. In the 13 day fetal mouse heart, there is no (-)-isoproterenol increase in heart rate, but beta adrenergic receptor (13 +/- 4% of adult) and (-)-isoproterenol-stimulated adenylate cyclase activity (15 +/- 5% of adult) are present. It is concluded that 1) no significant difference exists between the agonist and antagonist displaceable [3H] DHA binding during development, 2) adenylate cyclase activity increases significantly during the last third of pregnancy in parallel with the beta adrenergic receptor, 3) both the beta adrenergic receptor and adenylate cyclase activity can be detected before the heart rate responses and 4) total adenylate cyclase activity does not increase in parallel with the early postnatal beta adrenergic receptor peak.     

Impaired beta adrenergic receptor binding and function in cystic fibrosis neutrophils.

Cystic fibrosis (CF), a genetic disease characterized by abnormalities of exocrine gland and mucociliary function, has recently been shown to be associated with abnormal adrenergic and cholinergic physiologic responses in addition to decreased beta adrenergic-induced cyclic AMP generation in human leukocytes. In this study we have attempted to elucidate the nature of this hyporesponsiveness by assessing beta adrenergic receptor number and affinity (KD) in the intact neutrophil using the antagonist ligand [3H] dihydroalprenolol and cyclic AMP responses to isoproterenol in addition to histamine, and prostaglandin E1 in CF subjects, CF obligate heterozygotes (CFH), and normal control subjects. CF patients had significantly less (p less than 0.025) cyclic AMP stimulation above basals levels with isoproterenol (0.1 microM to 0.1 mM), compared with control values, but no consistent differences between groups were noted with histamine or PGE1. CF neutrophils had significantly fewer (p less than 0.005) beta adrenergic receptors per neutrophil (398.0 +/- 54.2 vs. 819.4 +/- 67.2) compared with control neutrophils, but the KD (0.740 +/- 0.11 vs. 0.630 +/- 0.05 nM) did not differ significantly (p greater than 0.05). There was no correlation between clinical severity and either cyclic AMP generation or dihydroalprenolol binding (r = 0.27 and 0.24, respectively, p greater than 0.05). The CFH group had approximately 50% of the cyclic AMP stimulation compared with controls, but the number (909.8 +/- 89.3) and KD (0.710 +/- 0.09 nM) of their beta adrenergic receptors were indistinguishable from control subjects. These findings suggest "down regulation" of the beta receptor in the CF patient. The cause of this remains unknown. Although the etiology of the decreased cyclic AMP responses in CFH was not due to decreased beta adrenergic receptors as assessed by antagonist ligand binding, further studies inthe CFH group to include agonist binding, receptor-adenylate cyclase coupling, intrinsic adenylate cyclase activity, and catecholamine metabolism may help determine the basic cause of beta adrenergic hyperesposiveness in both CFH and CF.     

Sympathetic denervation fails to produce beta adrenergic supersensitivity in adult rat parotid gland

Parotid acinar cells, prepared from pharmacologically sympathectomized adult rats (reserpine, 0.1 mg/kg/day for 1 week), display decreased responsiveness to beta adrenergic stimulation in vitro compared to cells from control and surgically sympathectomized rats. Both methods of denervation increase amylase content (amylase activity per microgram of DNA). Percent release of amylase activity and percent release of CCl3COOH-precipitable [14C]leucine were used as indicators of protein secretion. Exposure of cells from pharmacologically sympathectomized rats to the beta adrenergic agonist, isoproterenol, resulted in a marked reduction in receptor-coupled secretion (67% and 75% relative to controls, respectively). 8-Bromo-cyclic AMP, like isoproterenol, was unable to surmount this reserpine-induced inhibition of stimulated secretion, suggesting that an alteration in receptor-adenylate cyclase coupling is not responsible for the observed secretion defect. Cells prepared from surgically sympathectomized rats displayed modest decreases in stimulated secretion when the same secretory markers were monitored (30% and 25% relative to controls, respectively). The number of beta adrenoreceptors [( 3H]dihydroalprenolol binding sites) increased (35%), with no change in binding affinity, in membrane preparations from reserpine-treated rats. Thus, the observed inhibition of beta adrenergic agonist-induced secretion is not likely the result of alterations in beta adrenergic receptor characteristics. Short-term (1 week) surgical denervation had no effect on the number of beta adrenergic receptor sites; however, an increase in ligand binding affinity was noted. The decrease in the apparent Kd (30%) was not the result of a shift in receptor subtype as determined by competition studies with specific beta-1 (atenolol) and beta-2 (ICI 118,551) receptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta adrenergic receptor subtypes in the atria: evidence for close coupling of beta-1 and beta-2 adrenergic receptors to adenylate cyclase

The relationship between occupancy of beta adrenergic receptors and stimulation of adenylate cyclase in dog atrial tissue was examined by studying the binding of [125I]iodopindolol and the activation of adenylate cyclase. Computer-assisted nonlinear regression analysis was used to analyze the inhibition of isoproterenol-stimulated adenylate cyclase activity by beta-1- or beta-2-selective antagonists. The Ki values for each subtype of receptor for the selective antagonists resulting from studies of the inhibition of adenylate cyclase activity were similar to those determined in studies of the inhibition of the binding of [125I]iodopindolol. To compare further the occupancy of beta-1 or beta-2 adrenergic receptors with the activation of adenylate cyclase mediated by each class of receptor, computer modeling of the stimulation of adenylate cyclase by the beta-1-selective agonist norepinephrine was carried out. The EC50 values of norepinephrine for each receptor subtype, as measured in studies of norepinephrine-stimulated adenylate cyclase activity, were similar to the Ki values for the inhibition by norepinephrine of the binding of [125I]iodopindolol to each receptor subtype. The data led to the conclusion that beta-1 adrenergic receptors make up about 70% of the total number of beta adrenergic receptors and mediate 70% of the increase in adenylate cyclase activity produced by isoproterenol. These results suggest that the relationship between occupancy of each class of receptor and activation of adenylate cyclase is linear and that, when agonist-stimulated adenylate cyclase activity is used as a functional response, neither spare beta-1 nor spare beta-2 adrenergic receptors exist in the atrium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Down regulation of beta adrenergic receptors in S49 lymphoma cells induced by atypical agonists

The ability of the atypical agonists celiprolol and pindolol to induce sequestration and down regulation of beta adrenergic receptors was investigated in S49 lymphoma cells. Sequestration was measured as the loss of binding sites for [3H]CGP-12177, a hydrophilic radioligand that binds only to surface beta adrenergic receptors at 6 degrees C. Down regulation was measured as the loss of binding sites for [125I]iodopindolol, a lipophilic radioligand which at 37 degrees C binds to both surface and sequestered receptors. Pindolol and celiprolol do not stimulate adenylate cyclase in membranes from wild-type (WT) S49 cells or do they induce the sequestration of beta adrenergic receptors on intact cells. Incubation of WT S49 lymphoma cells with isoproterenol for 24 hr resulted in the loss of 75% of total cellular beta adrenergic receptors (down regulation). Exposure of WT S49 cells to pindolol or celiprolol for 24 hr resulted in the loss of approximately half of the total cellular beta adrenergic receptors. In mutant S49 cells [cyc- (variant of S49 lymphoma cells which lacks Ns activity) and UNC (variant of S49 lymphoma cells in which Ns is present but cannot interact with beta adrenergic receptors)] in which interaction of beta adrenergic receptors with the stimulatory guanine nucleotide binding regulatory protein (Ns) does not occur, a 24 hr incubation with isoproterenol caused the loss of approximately half of the beta adrenergic receptors, whereas pindolol and celiprolol caused no change in the number of receptors. These results suggest that there are two mechanisms of down regulation of beta adrenergic receptors in S49 cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic norepinephrine elicits desensitization by uncoupling the beta-receptor.

The goal of this study was to determine the mechanism of beta-adrenergic receptor desensitization after chronic elevation of circulating NE levels. Osmotic minipumps containing either NE or saline were implanted subcutaneously in dogs for 3-4 wk. Physiologic desensitization to isoproterenol was confirmed in conscious dogs, i.e., left ventricular dP/dt increased in response to isoproterenol (0.4 micrograms/kg per min) by 5,625 +/- 731 mmHg/s in control dogs with saline pumps, and significantly less, P less than 0.01, by 2,093 +/- 263 mmHg/s in dogs with NE pumps. Myocardial beta-adrenergic receptor density as determined with 125I-cyanopindolol binding was 49% higher (p less than 0.05) in the NE pump group. However, beta-adrenergic receptor agonist binding with isoproterenol demonstrated a significant shift into the low affinity state for the animals with NE pumps. Basal, GTP plus isoproterenol, 5'-guanylylimidodiphosphate, sodium fluoride, and forskolin-stimulated adenylate cyclase activity in the NE pump group were significantly depressed (P less than 0.05) by amounts ranging from 20 to 40%. The functional activity of the guanine nucleotide binding protein Gs was also reduced (P less than 0.05) in animals with NE pumps. Thus, the process of desensitization in response to chronic elevation of NE levels in intact, normal dogs does not involve a decrease in beta-adrenergic receptor density. Rather, it is characterized by reduced adenylate cyclase activation and uncoupling of the beta-adrenergic receptor in association with decreased activity of the GTP-coupling protein Gs.     

The serotonin/norepinephrine-link in brain. II. Role of serotonin in the regulation of beta adrenoceptors in the low agonist affinity conformation

The present studies were undertaken to characterize further the role of serotonin (5-HT) in the regulation of the norepinephrine (NE) beta adrenoceptor coupled adenylate cyclase system in the rat cortex. Although 5-HT in vitro did not influence maximum binding and Kd values of [3H]dihydroalprenolol binding or the IC50 value for isoproterenol as estimated from competition binding curves in cortical tissue from control animals, 5-HT abolished the increase in beta adrenoceptor number and the marked elevation of the IC50 value for isoproterenol in cortical membrane preparations after selective lesions with 5,7-dihydroxytryptamine (5,7-DHT). Nonlinear regression analysis of competition binding curves revealed that the increase in the maximum binding of beta adrenoceptors after 5,7-DHT is due exclusively to an increase in beta adrenoceptors in the agonist low affinity conformation and that it is this receptor population that is reduced by nanomolar concentrations of 5-HT. The increase in the density of beta adrenoceptors in the low affinity conformation occurred approximately 11 days after the lesions and remained elevated throughout the experimental period of 28 days. Ritanserin in a dose that virtually abolished 5-HT2 receptor binding in cortex did not mimic the effect of 5,7-DHT.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta Adrenergic Receptors of Polymorphonuclear Particulates in Bronchial Asthma

We have tested the beta adrenergic receptor theory of bronchial asthma by determining the number and affinity of binding sites of the beta adrenergic radioligand [(3)H]dihydroalprenolol (DHA) and the activity of adenylate cyclase in broken cell preparations of polymorphonuclear leukocytes (PMN). We studied 31 control subjects (group 1), 30 asthmatics receiving no systemic adrenergic medication (group 2), and 17 asthmatics receiving adrenergic agonists systemically (group 3). Control subjects and asthmatics taking no adrenergic drugs bound similar amounts of DHA at 0.5 nM and 30 nM DHA and had about 900 binding sites per PMN. In contrast, asthmatics receiving adrenergic agonists had a >70% decrease in their number of DHA binding sites per PMN (254+/-57). In a subset of our three groups of subjects (eight from group 1, six from group 2, and five from group 3) we measured DHA binding at several DHA concentrations and found similar values (0.4-0.7 nM) for the dissociation constant of DHA among these subjects.In further studies we examined the interaction of the agonist (-)-isoproterenol with beta adrenergic receptors in 8 normal subjects and 10 asthmatics not receiving adrenergic medication. We tested the ability of isoproterenol to compete for DHA binding sites and to stimulate adenylate cyclase in sonicates prepared from PMN and examined under identical conditions. The dissociation constants for the competition of isoproterenol for DHA binding sites in normal and asthmatic subjects were virtually identical ( approximately 1.0 muM). In addition, the (activation constant) values for stimulation of adenylate cyclase were similar (0.16-0.19 muM) in the two groups of subjects.Thus, these data suggest that asthma per se is not associated with alteration in either the number or affinity of beta adrenergic receptors in PMN. Our findings indicate that previous reports of abnormal beta adrenergic receptor function in asthmatic patients may in part be explained by prior treatment of such patients with adrenergic agonists. Because the asthmatics who received adrenergic agonists in our study tended to be more ill and to receive additional medication compared to subjects in group 2, we cannot rule out unequivocally that severe asthma may be associated with decreased binding to beta adrenergic receptors. Nevertheless, we conclude that beta adrenergic receptors on PMN from asthmatics are relatively normal unless such patients are treated with adrenergic agonists.     

Effects of protein kinase inhibitor, 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, on beta-2 adrenergic receptor activation and desensitization in intact human lymphocytes

To investigate the role of protein kinases in agonist-mediated beta-2 adrenergic receptor regulation, the effects of the protein kinase A and C inhibitor, H-7 [1-(5-isoquinolinylsulfonyl)-2-methylpiperazine], on isoproterenol-induced beta adrenoceptor activation and desensitization have been studied in intact human lymphocytes. In the presence of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, H-7 potentiated 10(-8) to 10(-4) M isoproterenol or prostaglandin E1-induced cyclic AMP (cAMP) accumulation in a dose-dependent manner. We failed to observe any effect of H-7 on forskolin-induced cAMP accumulation. These effects of H-7 are probably not due to its inhibition of phosphodiesterase. In addition, whereas up to 10(-3) M H-7 had no beta adrenergic receptor blocking effect, preincubation of intact cells with 10(-3.5) M H-7 partially prevented 50 nM isoproterenol-induced beta-2 adrenergic receptor desensitization in terms of decreases in beta adrenoceptor density (maximum binding), isoproterenol-mediated cAMP responsiveness and high affinity receptor binding for agonist. Interestingly, 10(-3.5) M H-7 alone treated cells also showed an up-regulation of cell surface beta receptor density (maximum binding) and increased cAMP responsiveness to isoproterenol stimulation. The mechanisms are unclear. If these effects occur as a result of inhibition by H-7 of protein kinase A and/or C, it may suggest an important role of protein kinase A and/or C in agonist-induced beta-2 adrenergic receptor regulation.     

Effect of imipramine and adrenocorticotropin administration on the rat brain norepinephrine-coupled cyclic nucleotide generating system: alterations in alpha and beta adrenergic components

Continuous treatment (1-3 weeks) with imipramine or adrenocorticotropin (ACTH) decreases the responsiveness of the norepinephrine-coupled cyclic nucleotide generating system in rat brain cerebral cortex. Experiments were undertaken to determine which component of the second messenger system is influenced by the hormone and antidepressant. Neither treatment modified the amount or function of extractable stimulatory guanine nucleotide binding protein or the activities of adenylate cyclase or phosphodiesterase. While both imipramine and ACTH treatment decreased the cyclic AMP response to norepinephrine, only imipramine administration influenced the response to isoproterenol. ACTH treatment was found to reduce the alpha adrenergic potentiation of isoproterenol- and 2-chloroadenosine-stimulated cyclic AMP production, as well as reduce the sensitivity of the norepinephrine response to prazosin. These findings indicate that imipramine and ACTH treatments decrease the responsiveness of the rat brain norepinephrine-stimulated cyclic AMP generating system through actions on the alpha and beta adrenergic receptor components. The results suggest that noradrenergic receptor activity may be under the control of adrenal and/or pituitary hormones.     

Subtype-selective regulation of beta adrenergic receptor-adenylyl cyclase coupling by phorbol esters in 3T3-L1 fibroblasts

To study the epigenetic regulation of beta adrenergic receptor subtypes, we examined the effects of phorbol esters on beta adrenergic receptor coupling to adenylyl cyclase in 3T3-L1 fibroblasts, which express both beta-1 and beta-2 adrenergic receptor subtypes. Pretreatment of intact 3T3-L1 cells with the protein kinase C activator phorbol dibutyrate caused a dose- and time-dependent decrease in subsequent cyclic AMP (cAMP) accumulation mediated by the beta adrenergic agonist isoproterenol. This effect was selective for beta-adrenergic receptor-mediated responses because there was a potentiation of cAMP accumulation caused by other activators such as prostaglandin E1, forskolin or cholera toxin. The inactive phorbol, alpha-phorbol dibutyrate was ineffective at 1 microM in attenuating isoproterenol stimulation, and 25 nM of the protein kinase C inhibitor staurosporine blocked the effects of phorbol ester on beta adrenergic agonist responses. Stimulation of cAMP accumulation by isoproterenol occurred through a greater proportion of beta-2 adrenergic receptors in phorbol dibutyrate-treated cells than in control cells. This was demonstrated using the beta-1 adrenergic selective antagonist ICI 89.406 and the beta-2 adrenergic selective antagonist ICI 118.551 to inhibit competitively isoproterenol-stimulated cAMP accumulation. Beta-2 adrenergic receptor number and subtype in these cells are regulated by glucocorticoids and butyrate. Decreasing the proportion of beta-1 adrenergic receptors and concomitantly increasing beta-2 adrenergic receptors with either glucocorticoids or butyrate decreased the ability of phorbol ester pretreatment to attenuate cAMP accumulation by isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of Digoxin on Ventricular Remodeling and Responsiveness of beta-Adrenoceptors in Chronic Volume Overload

BACKGROUND: Digoxin improves baroreflex function and reduces neurohumoral activation in severe heart failure, but it is uncertain how digoxin affects ventricular remodeling and progression to left ventricular dysfunction. In addition, the effect of digoxin in in vitro beta-adrenoceptor density and function, and contractile reserve in vivo is not well understood. METHODS AND RESULTS: To study this, we compared digoxin with placebo treatment in rats with chronic volume overload induced by aortocaval fistula and in sham-operated control animals. Left ventricular end-diastolic cavity dimensions (LVDd) and wall thickness were measured weekly by in vivo transthoracic echocardiography, and left ventricular mass (LVM) and percent fractional shortening (%FS) were calculated. Six weeks after fistula creation, simultaneous echocardiographic and invasive hemodynamic evaluation at rest and in response to incremental dobutamine (1-10 μg/kg/min intravenously) were measured. Myocardial plasma membrane beta-adrenoceptor density and maximal adenylate cyclase responses (V(max)) to isoproterenol, 5'-guanylylimi dodiphosphate, and forskolin were measured in vitro. Volume overload induced progressive increases in LVDd and LVM over the 6-week study period. Percent fractional shortening at rest, and the change in %FS in response to dobutamine stress were dramatically reduced 6 weeks after fistula creation. Although 6-week fistula animals had unchanged beta-adrenoceptor density (B(max)) and binding affinity (K(d)) as compared with controls, maximal adenylate cyclase responses to stimulation in vitro (V(max)) were markedly reduced. Digoxin treatment prevented this loss of responsiveness of adenylate cyclase but did not affect beta-adrenoceptor density or affinity in vitro. Digoxin had no effect on LVDd, LVM, %FS, or the response to dobutamine infusion in vivo. CONCLUSIONS: Although digoxin prevented beta-adrenoceptor desensitization and improved in vitro myocardial adenylate cyclase response, the cardiac response to adrenergic stimulation in vivo was not significantly improved. These results suggest that the role of beta-adrenoceptor desensitization in the progression from volume overload hypertrophy to left ventricular dysfunction and heart failure may be less important than previously thought. Furthermore, although digoxin treatment did produce modest hemodynamic benefits, it did not prevent progressive remodeling in this model.     

Comparison of beta adrenergic receptor binding characteristics and coupling to adenylate cyclase in rat pulmonary artery versus aorta

In an effort to define the mechanisms regulating pulmonary vasodilatation and explain the greater in vitro response to iso-proterenol in the pulmonary artery (PA) vs. aorta (AO), we compared beta adrenergic receptor binding characteristics and coupling to adenylate cyclase in PA and AO obtained from adult male rats. Beta adrenergic receptor binding characteristics and affinity for agonists were determined with [125I]-iodocyanopindolol. Agonist displacement studies were characteristic of a beta-2 adrenergic receptor subtype. Receptor density (44.7 +/- 7.3 vs. 39.6 +/- 0.8 fmol/mg of protein means +/- S.E.M., PA vs. AO) and the dissociation constant for the radioligand (10.3 +/- 2.6 vs. 13.4 +/- 3.5 pM) were similar in the two arteries. However, affinity for l-isoproterenol was greater (the inhibition constant was lower) in PA compared to AO (0.08 +/- 0.03 vs. 1.20 +/- 0.18 microM, P less than .05), as was affinity for l-epinephrine (0.89 +/- 0.20 vs. 3.87 +/- 0.62 microM, P less than .05). Affinity was similar for l-norepinephrine (18.93 +/- 3.63 vs. 13.49 +/- 3.12 microM). Base-line cyclic AMP (cAMP) content, basal adenylate cyclase activity and adenylate cyclase activity stimulated by GTP, isoproterenol plus GTP and forskolin were measured by radioimmunoassay for cAMP. Base-line cAMP content was greater in PA than in AO (513.5 +/- 46.9 vs. 125.5 +/- 19.1 pmol of cAMP per mg of protein, P less than .001), as was basal adenylate cyclase activity (10.8 +/- 1.2 vs. 5.7 +/- 1.3 pmol of cAMP per mg of protein per min, P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Norepinephrine and BRL 37344 stimulate adenylate cyclase by different receptors in rat brown adipose tissue

The beta adrenergic activation of adenylate cyclase was examined in membrane homogenates of rat interscapular brown adipose tissue (IBAT). In control membranes, isoproterenol and norepinephrine (NE) stimulated adenylate cyclase with activation constants of about 20 and 300 nM, respectively. Exposure of rats to 4 degrees C for 3 days increased the maximal stimulation of adenylate cyclase to these agonists but did not alter the respective activation constants. The beta 1-selective antagonist 1-(2-cyanophenoxy)-3-beta-(3-phenylureido)ethylamino-2-pr opa nol blocked isoproterenol stimulation of adenylate cyclase in control and cold-exposed membranes at a concentration 100 times lower than did the beta 2-selective antagonist erythro-dl-1-(7-methylindan-4-yloxy)-3-isopropylaminobuta n-2-ol. These data indicate that typical adrenergic agonists stimulate IBAT adenylate cyclase via beta 1 receptors. (R*,R*)-4-[2-[2 [9 3-chlorophenyl)-2-hydroxyethyl]amino)propyl) phenyl]phenoxyacetic acid (BRL 37344), an atypical agonist with activity at the beta 3 receptor, stimulated adenylate cyclase in control membranes with an activation constant of approximately 700 nM. Membranes of cold-exposed rats exhibited a high affinity response to BRL 37344 similar to that seen in control membranes and, in addition, a low affinity response. BRL 37344 stimulation of adenylate cyclase was unaffected by 1-(2-cyanophenoxy)-3-beta-(3-phenylureido)ethyl-amino-2-prop anol, whereas stimulation by NE or epinephrine was potently blocked.(ABSTRACT TRUNCATED AT 250 WORDS)