Receptor crosstalk: effects of prolonged carbachol exposure on beta 1-adrenoceptors and adenylyl cyclase activity in neonatal rat ventricular myocytes

Decrease in heart adrenoceptor gene expression and receptor number as compensatory tool for preserved heart function and biological rhythm in M(2) KO animals

Muscarinic receptors (MR) are main cardioinhibitory receptors. We investigated the changes in gene expression, receptor number, echocardiography, muscarinic/adrenergic agonist/antagonist changes in heart rate (HR) and HR biorhythm in M(2) KO mice (mice lacking the main cardioinhibitory receptors) in the left ventricle (LV) and right ventricle (RV). We hypothesize that the disruption of M(2) MR, key players in parasympathetic bradycardia, would change the number of receptors with antagonistic effects on the heart (β(1)- and β(2)-adrenoceptors, BAR), while the function of the heart would be changed only marginally. We have found changes in LV, but not in RV: decrease in M(3) MR, β(1)- and β(2)-adrenoceptor gene expressions that were accompanied by a decrease in MR and BAR receptor binding. No changes were found both in LV systolic and diastolic function as assessed by echocardiography (e.g., similar LV end-systolic and end-diastolic diameter, fractional shortening, mitral flow characteristics, and maximal velocity in LV outflow tract). We have found only marginal changes in specific HR biorhythm parameters. The effects of isoprenaline and propranolol on HR were similar in WT and KO (but with lesser extent). Atropine was not able to increase HR in KO animals. Carbachol decreased the HR in WT but increased HR in KO, suggesting the presence of cardiostimulatory MR. Therefore, we can conclude that although the main cardioinhibitory receptors are not present in the heart, the function is not much affected. As possible mechanisms of almost normal cardiac function, the decreases of both β(1)- and β(2)-adrenoceptor gene expression and receptor binding should be considered.

The Effects of Short-Term Immobilization Stress on Muscarinic Receptors, β-Adrenoceptors, and Adenylyl Cyclase in Different Heart Regions

Heart muscarinic receptors (MR) and beta-adrenoceptors (BAR) belong to a large family of G-protein-coupled receptors. Although the role of catecholamines in the stress has been under keen investigation for many years, the effects of immobilization on this pair of receptors, considering their almost completely opposite actions in the heart, are not yet known. We have studied the effects of short-term immobilization (for 120 min) with different times of decapitation after the end of the immobilization period (0, 3, and 24 h) on MR, BAR (beta(1)-AR and beta(2)-AR using radioligand binding studies), and adenylyl cyclase (AC; using high-pressure liquid chromatography detection of cAMP) in different heart regions (left and right atria with or without cardiac ganglion cells [auriculae], septum, and left and right ventricles). The effects of one immobilization period were first apparent after 24 h. Stress brought about a downregulation of MR and BAR with decrease in AC activity. These effects were regionally specific and were predominantly expressed in the right atria, which is rich in ganglia cells, and in the right ventricles. Our results indicate that stressful stimuli can influence not only BAR, but MR, and that AC activity can also be affected. This finding is in good agreement with our previous hypothesis that parallel changes are possible in the number of this pair of receptors on cell membranes.

The interaction of carbachol and strophantin on the electrical and mechanical events in electrically driven strips of guinea pig left atria

We compared the electrophysiological and the mechanical actions of the cholinergic agonist carbachol (CARB) and the digitalis glycoside strophantin (STR) in a study on the electrically driven strips of guinea pig left atria. The respective control values for the resting membrane potential (RMP), action potential amplitude (APA), action potential duration at 20%, 50%, and 90% repolarization levels (APD20,50,90), and time to peak tension (TtPt) were of the order of 61.1 +/- 1.69 nmV, 104.64 +/- 1.28 mV, 28.5 +/- 1.22 msec, 51.0 +/- 1.51 msec, 106.91 +/- 3.81 msec, and 60.1 +/- 1.09 msec. Exposure to CARB (2 x 10(-7) M) rapidly reduced contractility, TtPt, APD, and APA to exceptionally lower levels within the initial 5 minutes and gradually caused a slight hyperpolarization in RMP. Strophantin (1 x 10(-7) M) caused contractility and RMP to increase slightly, APD and APA to reduce less markedly, but did not significantly affect TtPt. Under the influence of combined CARB/STR, the effect of CARB on the contraction amplitude (CA), TtPt, APD, and APA was attenuated and that on RMP was significantly potentiated (p < 0.05), whereas the effects of STR on CA, TtPt, APD, and APA were reversed (p < 0.05), and the effect on RMP was augmented. The action potential duration was less responsive to STR at all depolarization levels with respect to those of CARB and combined CARB/STR. Although the correlation between the time course of APD-20 and APD-90 for STR was poor, the correlation for CARB and combined CARB/STR was highly significant (r > 0.95). The correlation between the effects of CARB on CA and APD-90 was high but poor for the effect of combined CARB/STR. Striking to note is that the rate of induction of contractile changes with CARB (d(delta)/dt = 1.39 +/- 0.05%/sec), and its washout was more rapid (p < 0.05) in comparison to the rate of changes in APD-90 (0.98 +/- 0.05%/sec). This result, however, was not observed with combined CARB/STR. From the results we concluded that CARB and STR act as antagonists when used in combination, with the exception that only the effect on RMP was additive, with asignificant discordance between the rates of induction of effects on CA and APD-90, with the former being more rapid.

Reciprocal regulation of beta(1)-adrenergic receptor gene transcription by Sp1 and early growth response gene 1: induction of EGR-1 inhibits the expression of the beta(1)-adrenergic receptor gene

The beta(1)-adrenergic receptor (beta(1)-AR) plays a key role in regulating heart rate and contractility in response to catecholamines. Our studies have focused on defining the factors that regulate the expression of the beta(1)-AR gene. We determined that a 65-base-pair (bp) region in the beta(1)-AR promoter between bp -394 and bp -330 directs basal transcription. An element located between -377 and -365 can bind Sp1 and Sp3. In Drosophila melanogaster SL2 cells, Sp1 stimulated the expression of the beta(1)-AR promoter, whereas Sp3 was unable to activate transcription. Site-directed mutagenesis indicated that an intact Sp1-binding site is essential for maintaining the activity of the basal promoter. In addition to binding Sp family members, the nucleotides between -381 and -367 can bind the zinc-finger transcription factor Egr-1. The Egr-1 and Sp1 binding sites are partially overlapping and their binding sequence is conserved among mammalian beta(1)-AR genes. The induction of Egr-1 in rat neonatal ventricular myocytes with phorbol-12-myristate-13-acetate or in HeLa S3 cells by regulated expression of Egr-1 in a tetracycline-responsive promoter, suppressed expression from the beta(1)-AR promoter. Overexpression of Sp1 in SK-N-MC cells increased beta(1)-AR mRNA by 2.4-fold, whereas overexpression of Egr-1 reduced beta(1)-AR mRNA by 40%. Coexpression of Egr-1 with Sp1 reduced Sp1-mediated up-regulation of beta(1)-AR mRNA by 60%. Mutagenesis revealed that an intact Sp1-binding site is essential for observing transcriptional repression by Egr-1 and that Egr-1 suppressed the transcription of the beta(1)-AR gene by competing with Sp1 for binding to their overlapping sites. These results reveal a novel physiologically relevant transcriptional mechanism for reciprocal regulation of beta(1)-AR gene expression.

Heterologous regulation of muscarinic and beta-adrenergic receptors in rat cardiomyocytes in culture

Previous work indicated that hyperstimulation of muscarinic receptors brings about profound changes not only in the density of the muscarinic receptors, but also of the beta-adrenoceptors in rat heart atria in vivo. We have now investigated whether a similar receptor cross-regulation occurs in cardiomyocytes in vitro. Cardiomyocytes from 3-4 day old rats were exposed to chemical agents on days 5-6 in culture. Densities of muscarinic and beta-adrenergic receptors were measured according to the binding of N-[3H]methylscopolamine and [ H]CGP 12177, respectively, to cell surface membranes and cell homogenates. Exposure of cells to the muscarinic agonist carbachol (1 mmol/l) brought about a profound decrease in the number of muscarinic receptors. The number of beta-adrenoceptors displayed biphasic changes, being augmented after 24 h (by 20-45% on the cell surface and by 29% in the homogenate) and diminished after 48 h and 72 h (after 48 h, decrease by 44-75% on the cell surface and by 36% in the homogenate). These effects of carbachol were not prevented by dimethylaminopropyl-bis-indolylmaleimide, the inhibitor of protein kinase C. Exposure of cells to the beta-adrenoceptor agonist isoprenaline (0.1 mmol/l) strongly diminished the number of beta-adrenoceptors on the cell surface and in the homogenate. The density of muscarinic receptors on the cell surface was diminished by 24-43% after 24 h exposure to isoprenaline and unchanged after 48 h, whereas the concentration of muscarinic receptors in the homogenate was unchanged after 24 h and increased by 20% after 48 h. The isoprenaline-induced decrease in the density of cell surface muscarinic receptors could not be simulated by forskolin and was not abolished by the protein kinase A inhibitors Rp-cAMPS and HA-1004. Dibutyryl cyclic AMP diminished the density of cell surface muscarinic receptors more than that of the beta-adrenergic receptors. Our data reveal a novel phenomenon of a biphasic change (an increase followed by a loss) in the density of beta-adrenoceptors during exposure of cardiocytes to carbachol. Activation of beta-adrenoceptors brings about less conspicuous changes in the density of muscarinic receptors. The observed phenomena of receptor cross-regulation cannot be explained by simple activations of protein kinases A and C.

Biphasic changes in the density of muscarinic and beta-adrenergic receptors in cardiac atria of rats treated with diisopropylfluorophosphate

Chronic treatment with organophosphate inhibitors of cholinesterases is known to bring about down-regulation of muscarinic acetylcholine receptors in the heart while its effect on the functionally antagonistic beta-adrenergic receptors is not known. We describe experiments in which rats were exposed to daily injections of diisopropylflurophosphate (DFP) and the density of muscarinic and beta-adrenergic binding sites in their cardiac atria was measured according to the binding of (3H)quinuclidinyl benzilate ((3H)QNB) and (-)-4-(3-tert-butylamino-2-hydroxy)-propoxy-(5,7-3H) benzimidazol-2-one ((3H)CGP 12177) as subtype non-specific muscarinic and beta-adrenergic ligands, respectively. Biphasic course of changes was discovered with both ligands. With the dosage scheme applied, the density of beta-adrenoceptors was augmented 24 h after the first dose of DFP and decreased to about one half of control values after 5 days of treatment with DFP. The density of muscarinic receptors was augmented after two days of treatment with DFP and decreased to about one half of control values after 5 days of treatment. Comparatively small changes in the heart rate were observed during the treatment, they reflected changes in the muscarinic and beta-adrenergic receptor density. The finding of DFP-induced changes in the binding of (3H)CGP 12177 suggests that the mechanisms responsible for the control of the density of muscarinic and beta-adrenergic receptors in the heart cells are interconnected but potential roles of other factors involved in in vivo experiments deserve further analysis. The transient increase in the density of muscarinic receptors after two days of DFP treatment appears related to published data on transient stimulation by cholinergic agonists of the transcription of mRNAs for muscarinic receptors.

Chronic exposure of NG 108-15 cells to inhibitory acting drugs reduces stimulatory prostaglandin E1 receptor number

Prolonged exposure of neuroblastoma x glioma (NG 108-15) hybrid cells to inhibitory acting drugs results in sensitization of adenylate cyclase. We now report that chronic activation (3 days) of either inhibitory delta-opioid receptors, alpha 2B-adrenoceptors, or muscarinic M4 receptors significantly decreases the number of stimulatory, adenylate cyclase-coupled prostaglandin E1 receptors. Pharmacological characterization further revealed that the loss of [3H]prostaglandin E1-binding sites most likely corresponds to a reduction of the number of high-affinity, G protein-coupled prostaglandin E1 receptors. The decline in functionally active prostaglandin E1 receptors developed in a time- and dose-dependent manner and could be prevented by pretreatment of the cells with pertussis toxin. Heterologous prostaglandin E1 receptor regulation was blocked by concomitant exposure of the cells to antagonists for inhibitory receptors and was rapidly reversed (t 1/2 < 30 min) upon termination of chronic inhibitory drug treatment. The decrease in high-affinity prostaglandin E1 receptors developed regardless of whether full or partial agonists were used for pretreatment. In addition, the concentrations of inhibitory drugs required to maximally affect prostaglandin E1 receptor number closely resembled those mediating maximal adenylate cyclase inhibition. The data demonstrate that chronic inhibitory drug treatment of NG 108-15 hybrid cells reduces the number of functionally active, excitatory prostaglandin E1 receptors. Thus, it is proposed that adaptations at the level of stimulatory receptor systems contribute to the regulatory mechanisms associated with drug dependence.

Atypical regulation of hepatic adenylyl cyclase and adrenergic receptors during a critical developmental period: agonists evoke supersensitivity accompanied by failure of receptor down-regulation

Ordinarily, beta-adrenergic receptors and responses linked to the receptors increase with development but in the liver, beta-receptors are higher in the fetus and neonate than in adulthood. We examined how hepatic beta-receptor signaling mediated through adenylyl cyclase is regulated in rats of different ages. In each case, animals were pretreated with isoproterenol for 4 d, and on the 5th d, hepatic membrane preparations were examined for adenylyl cyclase activity and receptor binding capabilities. Uniquely in 6-d-old animals, the cyclase response to isoproterenol was enhanced by chronic pretreatment, caused by heterologous sensitization mediated through effects on total catalytic activity (increased response to forskolin-Mn2+) and on G-protein coupling (enhanced effect of fluoride and increased GTP dependence of basal activity). Isoproterenol pretreatment failed to cause beta-receptor down-regulation in 6-d-old animals, but by 15 d of age, down-regulation was detected along with slight desensitization of the cyclase response. However, at 25 d, neither effect was present. In adulthood, repeated isoproterenol administration failed to cause cyclase desensitization but did reduce beta-receptor numbers; the loss of receptors was still unusual in that beta-receptor down-regulation could be achieved with either isoproterenol or with methoxamine, an alpha-receptor agonist. The results indicate that, early in development, hepatic beta-receptor-mediated responses are enhanced, not desensitized, after chronic stimulation. These effects would foster responsiveness of hepatic gluconeogenesis in the face of the massive adrenergic stimulation associated with the transition from fetal to neonatal life. In adulthood, when receptor numbers are far lower than in the neonate, the inability to desensitize the signaling cascade despite receptor down-regulation would serve to maintain the response to catecholamines.

Prolonged incubation with neuropeptide Y upregulates beta-adrenoceptors yet does not cause supersensitivity of beta-adrenoceptor signaling

In neonatal rat ventricular myocytes prolonged incubation with 1 microM neuropeptide Y (2 h-48 h) increased beta-adrenoceptor density 16-24% (n = 4--8, all p < 0.05), an effect prevented by pertussis toxin pretreatment. Prolonged incubation with neuropeptide Y had no effect on adenylycylclase activity stimulated by 5'-guanylylimidodiphosphate or (-)-isoprenaline, probably because of a neuropeptide Y-induced decrease in affinity of the beta-adrenoceptor for agonist. Thus, chronic incubation with an inhibitory agonist does not inevitably lead to supersensitivity of the adenylylcyclase pathway.