Beta 2-adrenergic stimulation of androgen production by cultured mouse testicular interstitial cells

The present studies characterized the beta-receptor subtype involved in androgen production by cultured mouse testicular interstitial cells and explored the possible stimulation of androgen release by alpha-adrenergic agonists. During a 3-hour incubation period, LH and a non-specific beta-adrenergic agonist, L-isoproterenol steadily increased androgen production with a similar time-course. Isoproterenol, epinephrine, norepinephrine and a specific beta 2-receptor agonist, salbutamol stimulated androgen release in a concentration-dependent manner. The concentrations of the agonists required for half-maximum stimulation (EC50) were approximately 1 nM (isoproterenol), 8 nM (epinephrine), 9 nM (salbutamol) and 2 microM (norepinephrine) giving an order of potency of isoproterenol greater than epinephrine = salbutamol much greater than norepinephrine. L- but not the D-isomer of isoproterenol induced androgen production. A non-selective beta-receptor antagonist, propranolol, abolished androgen production induced by isoproterenol. A selective beta 2-receptor antagonist ICI 118,551 inhibited the isoproterenol effect in a concentration-dependent manner with half-maximum inhibition (IC50) at approximately 23 nM. The beta 1-receptor antagonists, metoprolol and atenolol had no effect on isoproterenol-induced androgen release. The stimulatory effect of norepinephrine (an alpha- and beta-agonist) was completely (100%) abolished by propranolol, unaffected by the alpha-antagonist phentolamine and only partially (35%) inhibited by phenoxybenzamine. Phenoxybenzamine and the alpha 2-agonist, clonidine reduced basal androgen production. These studies indicate that androgen production by primary cultures of mouse testicular interstitial cells occurs exclusively via the beta 2-receptor subtype and that alpha-receptor agonists do not stimulate androgen release by these cells.     

Beta 2-receptors in the rat anterior pituitary mediate adrenergic stimulation of prolactin release

As previously shown, the beta-adrenergic agonists isoproterenol, epinephrine and norepinephrine stimulate prolactin (PRL) release from superfused rat anterior pituitary cell aggregates. In order to further characterize the beta-adrenergic response in this tissue preparation, the effects of various beta-adrenergic agents were investigated. The beta 2-agonist, zinterol, stimulated PRL release at concentrations more than 4 orders of magnitude lower than prenalterol, a beta 1-agonist with high potency in rat heart. The order of potency of the antagonists IPS 339 (beta 2), ICI 118.551 (beta 2), propranolol, sotalol, practolol (beta 1), metoprolol (beta 1) and H 35/25 for inhibition of beta-agonist-stimulated PRL release provided additional support for a beta 2-stimulatory effect. beta-Agonists were also capable of stimulating PRL release from superfused intact pituitaries. The beta-adrenergic response desensitized rapidly during prolonged exposure of the aggregates to beta-agonists.     

Functional characterization of beta-adrenoceptor subtypes in rabbit right atria

The advent of radioligand binding studies has allowed the classification of receptor subtypes in various tissues. However, the presence of a receptor subtype in a heterogenous tissue does not insure that the receptor has a significant physiological role. beta 1- and beta 2-Adrenoceptors have been reported to coexist in the rabbit right atria. The purpose of the present investigation was to determine the physiological role of beta-adrenoceptor subtypes in catecholamine-induced chronotropic responses in the rabbit right atria through comparison of data from functional and radioligand binding studies. Rank order of potency was determined using isoproterenol, epinephrine and norepinephrine for both chronotropic and inotropic responses in the rabbit right atria and right ventricular papillary muscles, respectively. These studies indicated that the beta 1-adrenoceptor was primarily responsible for catecholamine-induced responses. Next, the beta 1-selective antagonist, atenolol, was found to inhibit the chronotropic responses of the nonselective beta-agonist, isoproterenol, and the beta 2-selective agonist, terbutaline, to the same extent. These data indicate that terbutaline produces its chronotropic effects in the rabbit right atria through stimulation of beta 1-, not beta 2-adrenoceptors. Finally, competition studies for [125I]iodocyanopindolol and the relatively selective beta 1- and beta 2-adrenoceptor antagonists (ICI 89406 and ICI 118551, respectively) indicated that the ratio of beta 1- to beta 2-adrenoceptor subtypes is 6:1. It is concluded that while both receptors may be present in the rabbit right atria, the beta 1-adrenoceptor is the predominant subtype both in density and physiological significance, while the beta 2-adrenoceptor plays little, if any role, in the chronotropic responses induced by catecholamines.     

beta-Adrenergic receptors in rabbit liver plasma membranes. Predominance of beta 2-receptors and mediation of adrenergic regulation of hepatic glycogenolysis

[3H]Dihydroalprenolol was used to study beta-adrenergic binding sites in plasma membranes isolated from rabbit liver. Specific binding was measured at 25 degrees C as the difference between total binding and binding in the presence of 2 microM dl-propranolol or 10 microM l-isoproterenol. Binding was saturable and stereoselective. The maximum number of binding sites (Bmax) was 434 +/- 41 fmol/mg of protein. The Kd for this binding as determined by Scatchard analysis was 1.39 +/- 0.09 nM. This value agreed well with the Kd value (1.27 +/- 0.12 nM) determined by kinetic analysis. The potency order for the displacement of bound [3H]dihydroalprenolol was isoproterenol greater than epinephrine greater than norepinephrine, indicative of beta 2-receptors. Use of beta 1- and beta 2-subtype-selective inhibitors also supported the interpretation that the binding characteristics are those of beta 2-receptors. Computer-aided analysis of this inhibition indicated that the beta-receptors in this membrane are predominantly, if not exclusively, of the beta 2-subtype. That these receptors are responsible for mediating catecholamine stimulation of hepatic glycogenolysis was deduced from the inhibition of agonist-stimulated glycogenolysis, in isolated hepatocytes, by beta-receptor subtype-selective antagonists. Thus, the hydrochloride of (t-butylamino-3-ol-2-propyl)oximino-9 fluorene, a beta-antagonist which has higher affinity at beta 2-sites than at beta 1-sites, was 3 orders of magnitude more potent in inhibiting isoproterenol-stimulated glycogenolysis than either atenolol or practolol, both of which are beta 1-selective antagonists. These results resemble the inhibition of [3H]dihydroalprenolol binding in plasma membranes. The glycogenolytic effects of catecholamines occurred with the potency order isoproterenol greater than epinephrine greater than norepinephrine. Thus, both by radioligand binding studies and by metabolic studies, the functional adrenergic receptor in the rabbit liver is shown to be of the beta 2-subtype.     

Inhibition of cholinergic neurotransmission in human airways by beta 2-adrenoceptors

The purpose of the study was to determine whether catecholamines modulate cholinergic neurotransmission in isolated human airway smooth muscle. Bronchial rings were suspended in organ baths for isometric measurement of tension, and contractions were induced by either electrical field stimulation (EFS) or exogenous acetylcholine (ACh). Isoproterenol, epinephrine, and norepinephrine in that order of potency produced concentration-dependent inhibition of comparable responses to EFS and ACh. However a potency difference of 100-fold for isoproterenol (IC50 = 4.80 X 10(-8) M for EFS and 3.70 X 10(-6) M for ACh) and 10-fold for both epinephrine and norepinephrine was observed for inhibition of responses to EFS compared with responses to ACh. The inhibitory effects of isoproterenol on responses to EFS were prevented by propranolol and ICI 118551 (a beta 2-antagonist) but not by betaxolol (a beta 1-antagonist). Tyramine had no effect on contractions elicited by EFS. These experiments demonstrate that beta-agonists inhibit cholinergic nerve-induced contractions of human bronchi more potently than contractions induced by exogenous ACh, suggesting modulation of cholinergic neurotransmission by prejunctional beta 2-receptors.     

Beta-adrenergic receptors and stimulatory effects of (-) isoproterenol on testosterone production in fetal mouse Leydig cells

Beta-adrenergic receptors were characterized in freshly excised fetal mouse testis using the radioiodinated antagonist iodocyanopindolol (ICYP). [125I]-CYP bound to a single class of high affinity sites with a KD value of 42.2 +/- 7.0 pM. Adrenergic agonists competed for ICYP binding sites with the following order of potency: (-)isoproterenol greater than (-)epinephrine much greater than (-)norepinephrine which is typical for a beta 2-adrenergic receptor. A selective beta 2-receptor antagonist ICI 118-551 showed an approximately 200 fold higher affinity than the beta 1-selective compound, betaxolol. The beta-adrenergic agonist (-)isoproterenol did not or slightly affect testosterone production by freshly isolated fetal Leydig cells. The ability of fetal Leydig cells to respond to (-)isoproterenol increased during culture. This change in responsiveness was not accompanied either by modification of the number of binding sites or by change in the binding affinity. Taken together these data suggest that i) the stimulatory effect of (-)isoproterenol on testosterone production by cultured fetal Leydig cells is mediated through beta 2-adrenergic receptors ii), the inability of freshly Leydig cells to respond to catecholamines is probably due to post receptor events.     

Characterization of beta-adrenergic binding sites on rodent Leydig cells

A radioligand binding technique was used to study beta-adrenergic binding sites on rodent Leydig cells. Beta-Adrenergic binding sites were found on Leydig cells in both the rat and mouse. Binding of [3H]CGP-12177 [4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H]benzimidazole-2-one] to purified rat Leydig cells was found to be saturable, temperature and time dependent, stereospecific, and readily reversible by the beta-adrenergic antagonist propranolol. Scatchard analysis revealed the presence of high-affinity sites with an apparent dissociation constant (Kd) of 0.79 +/- 0.22 nM and maximal binding capacity (Bmax) of 1716 +/- 245 sites per rat Leydig cell. Competition of various beta-adrenergic agonists and antagonists with [3H]CGP indicates an order of potency of L-isoproterenol greater than epinephrine = salbutamol greater than norepinephrine greater than D-isoproterenol and dl-propranolol = ICI 118,551 much greater than atenolol, respectively. These observations suggest that the binding sites are predominantly of the beta 2-receptor subtype. Incubation of freshly isolated rat Leydig cells with luteinizing hormone (100 ng/ml) caused consistent stimulation of androgen production, but only occasional stimulation by the beta-agonist isoproterenol (10 microM) was observed. However, these cells consistently responded to the beta-agonist after 3 h in primary cultures. These findings indicate that rodent Leydig cells possess beta-adrenergic binding sites and point out a possible dissociation between receptor recognition and physiologic response.     

Demonstration of beta 1-adrenoceptor mediating relaxation of porcine coronary artery by radioligand binding and pharmacological methods

beta-adrenoceptors in the porcine coronary artery were characterized by a radioligand binding assay using (-)-[3H]dihydroalprenolol (DHA) and also by measuring the relaxant response of isolated coronary artery to norepinephrine. Specific (-)-[3H]DHA binding in the porcine coronary artery was saturable, reversible and of high affinity (Kd = 1.6 nM) with a maximal number of binding sites of 63 fmol/mg protein, and it showed a pharmacological specificity as well as stereoselectivity which characterized beta-adrenoceptors. The Hofstee analysis of inhibition of (-)-[3H]DHA binding by atenolol, practolol and ICI 118551 has shown that the averaged concentration of beta 1 and beta 2-adrenoceptors in this tissue was 68% and 32% respectively. The relaxant response of isolated coronary artery to norepinephrine was competitively antagonized by (-)propranolol, (+)propranolol, atenolol, practolol and ICI 118551. The pA2 values of these adrenoceptor antagonists were significantly correlated with the Ki values for beta 1 but not beta 2-adrenoceptors determined by the (-)-[3H]DHA binding assay. Thus, the present study demonstrates that the relaxant response of porcine coronary artery to norepinephrine is predominantly mediated through the stimulation of beta 1-adrenoceptors on vascular smooth muscles.     

Characterization and Regulation of β1-Adrenergic Receptors in a Human Neuroepithelioma Cell Line

Intact human neuroepithelioma SK-N-MC cells bound the beta-adrenergic antagonist (-)-[3H]-CGP 12177 with a KD of 0.13 nM and a Bmax of 17,500 sites/cell. When the cells were exposed to beta-adrenergic agonists, they accumulated cyclic AMP in the following order of potency: isoproterenol much greater than norepinephrine greater than epinephrine, which is indicative of a beta 1-subtype receptor. Membranes prepared from the cells bound (-)-3-[125I]iodocyanopindolol with a KD of 11.5 pM. Inhibition of agonist-stimulated cyclic AMP production and competition binding experiments indicated that the beta 1-selective antagonists CGP 20712A and ICI 89,406 were much more potent than the beta 2-selective antagonist ICI 118,551. Analysis of the displacement curves indicated that the cells contained only beta 1-adrenergic receptors. Northern blot analysis of SK-N-MC mRNA using cDNA probes for the beta 1- and beta 2-adrenergic receptors revealed the presence of a very strong beta 1-adrenergic receptor mRNA signal, while under the same conditions no beta 2-adrenergic receptor mRNA was observed. Thus, SK-N-MC cells appear to express a pure population of beta 1-adrenergic receptors. When the cells were exposed to isoproterenol, there was no observable desensitization during the first hour. After longer exposure, desensitization slowly occurred and the receptors slowly down-regulated to 50% of control levels by 24 h. Other agents that elevate cyclic AMP levels, such as forskolin, cholera toxin, and cyclic AMP analogues, caused no or little substantial receptor loss.     

Identification of adenylate cyclase-coupled beta-adrenergic receptors in the developing mammalian palate

A direct radioligand binding technique utilizing a beta-adrenergic antagonist [3H]Dihydroalprenolol [( 3H]DHA) was employed in the identification and characterization of fetal palatal beta-adrenergic receptors. [3H]DHA binding was saturable (Bmax 16 fmol/mg protein) with high affinity and an apparent equilibrium dissociation constant (KD) of 1.5 nM. Binding of [3H]DHA was displaced by the competitive beta-adrenergic antagonist propranolol in a concentration-dependent manner. Dissociation kinetic studies demonstrated almost complete reversibility of radioligand binding within 60 min. The functionality of these beta-adrenergic receptors was demonstrated by showing that fetal palatal mesenchymal cells responded to catecholamine agonists with dose-dependent accumulations of intracellular cAMP. This effect could be entirely blocked by the beta-antagonist, propranolol. The relative potency order of catecholamines in eliciting an elevation of cellular cAMP was characteristic of a beta 2-adrenergic receptor-mediated response: (-) isoproterenol greater than (-) epinephrine greater than (-) norepinephrine. In addition, this response was found to be stereospecific with (-) isoproterenol being significantly more potent than (+) isoproterenol. Both the [3H]DHA binding characteristics and the catecholamine sensitivity of fetal palatal tissue support the presence of adenylate cyclase-coupled beta-adrenergic receptors in the developing mammalian secondary palate.     

Beta-adrenoceptors and the regulation of blood pressure and plasma renin during exercise

The relative role of beta 1- and beta 2-adrenoceptors in the regulation of blood pressure and plasma renin at rest and during exercise was studied in 17 normal male volunteers. They performed, in a randomized order and according to a double-blind crossover study design, three graded and uninterrupted exercise tests until exhaustion after being pretreated during 3 consecutive days with a placebo, with a predominantly beta 1-blocker (atenolol, 50 mg once/day), or with a predominantly beta 2-blocker (ICI 118551, 20 mg 3 times/day). Both drugs caused a decrease of heart rate, but the reduction by ICI 118551 was less pronounced at rest and no additional decline occurred at exercise. ICI 118551 did not affect blood pressure at rest, but during exercise diastolic blood pressure was higher than after a placebo. Atenolol lowered systolic blood pressure at rest and suppressed the exercise-induced increase in systolic blood pressure. At rest and during exercise plasma renin activity was lowered by predominantly beta 1-blockade and unchanged during beta 2-antagonism. The exercise-induced increase in plasma renin was, however, not affected by the beta 1-blocker. After atenolol the urinary excretion of aldosterone was decreased but the plasma aldosterone concentration was not changed. ICI 118551 did not alter plasma or urinary aldosterone. Our results therefore provide further evidence that the adrenoceptors mediating the release of renin at rest and during exercise in humans are partially of the beta 1-subtype, whereas beta 2-adrenergic receptors probably play only a minor role in the control of renin secretion, especially at exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Preparation of hormone-sensitive airway smooth muscle adenylate cyclase from dissociated canine trachealis cells

Conventional homogenizing methods produced membrane preparations of canine trachealis airway smooth muscle which contained adenylate cyclase activity that was stimulated by fluoride but not by isoproterenol. We have devised methods using collagenase digestion of minced trachealis which destroy most of the tough connective tissues but leave dissociated canine trachealis cells in suspension. Gentle homogenization of these cells permitted preparation of a particulate fraction containing adenylate cyclase that was readily stimulated by beta-adrenergic agonist of prostaglandin E2. Isoproterenol stimulation was 2.34 +/- 0.58 (S.E.) times basal and 122 +/- 25% of the stimulation induced by NaF. The beta-adrenergic blocking agent propranolol prevented isoproterenol-induced stimulation of the cyclase but had no effect on prostaglandin E2 stimulation. Catecholamine order of potency was isoproterenol greater than epinephrine greater than norepinephrine. These methods enable demonstration of stimulatory effects of hormones in broken cell preparations of airway smooth muscle that are comparable to those when hormone-stimulated cyclic AMP formation is measured in intact muscle strips.     

Effect of catecholamines on Na/H exchange in vascular smooth muscle cells

Catecholamines were found to activate Na/H exchange in a concentration-dependent manner in primary cultures of vascular smooth muscle cells (VSMC). The potency order was found to be epinephrine greater than norepinephrine greater than isoproterenol. The major pathway for catecholamine effects appeared to be via interaction with an alpha 1 adrenergic receptor. In addition, it was found that alpha 1 receptor-mediated Na/H exchange in VSMC was increased by angiotensin II and inhibited by 12-O-tetradecanoyl phorbol-13-acetate (TPA). Adrenergic receptors have been shown to be coupled to both adenylate cyclase and to inositol phosphate release (Leeb-Lundberg, L. M. F., S. Cotecchia, J. W. Lomasney, J. F. DeBernadis, R. J. Lefkowitz, and M. G. Caron, 1985, Proc. Natl. Acad. Sci. USA, 82:5651-5655.). It was found that catecholamines increased AMP levels in the potency order isoproterenol greater than norepinephrine greater than epinephrine and the receptor involved was a beta adrenergic receptor. Since these findings did not parallel the results obtained for catecholamine stimulation of Na/H exchange, an increase in AMP levels was probably not the mechanism by which major pathway for catecholamine-stimulated Na/H exchange in VSMC (via the alpha 1 receptor) was activated. When the effects of catecholamines were measured on inositol phosphate release, the potency order for catecholamine stimulation was epinephrine greater than norepinephrine greater than isoproterenol, and the receptor involved was an alpha 1 adrenergic receptor. In addition, angiotensin II increased and TPA inhibited catecholamine-stimulated inositol phosphate release. Since these findings paralleled the results obtained for catecholamine stimulation of Na/H exchange, inositol phosphate release may be the mechanism by which the major pathway for catecholamine-stimulated Na/H exchange in VSMC (via the alpha 1 receptor) was activated.     

ß-Adrenergic Receptor Activation Increases Acetylcholine Receptor Number in Cultured Skeletal Muscle Myotubes

Treatment of embryonic chick muscle myotubes with the beta-adrenergic agonist isoproterenol increased the number of surface membrane nicotinic cholinergic receptors. Receptor degradation was unaffected by isoproterenol, suggesting that receptor synthesis was increased. The effect of isoproterenol appears to be mediated by the beta-adrenergic receptor adenylate cyclase system for the following reasons: (a) The response to isoproterenol was dose-dependent and stereospecific. (b) The response to catecholamines followed the order isoproterenol greater than epinephrine greater than norepinephrine. (c) Alprenolol, a beta-adrenergic antagonist, but not phentolamine, an alpha-antagonist, abolished the effect. (d) The maximal effects of isoproterenol and cholera toxin, an activator of adenylate cyclase, were not additive. These results suggest that under certain physiological states catecholamines may play an important role in the regulation of cholinergic receptors.     

Role of adrenoceptors and cAMP on the catecholamine-induced inhibition of proteolysis in rat skeletal muscle

The role of adrenoceptor subtypes and of cAMP on rat skeletal muscle proteolysis was investigated using a preparation that maintains tissue glycogen stores and metabolic activity for several hours. In both soleus and extensor digitorum longus (EDL) muscles, proteolysis decreased by 15-20% in the presence of equimolar concentrations of epinephrine, isoproterenol, a nonselective beta-agonist, or clenbuterol, a selective beta(2)-agonist. Norepinephrine also reduced proteolysis but less markedly than epinephrine. No change in proteolysis was observed when muscles were incubated with phenylephrine, a nonselective alpha-agonist. The decrease in the rate of protein degradation induced by 10(-4) M epinephrine was prevented by 10(-5) M propranolol, a nonselective beta-antagonist, and by 10(-5) M ICI 118.551, a selective beta(2)-antagonist. The antiproteolytic effect of epinephrine was not inhibited by prazosin or yohimbine (selective alpha(1)-and alpha(2)-antagonists, respectively) or by atenolol, a selective beta(1)-antagonist. Dibutyryl cAMP and isobutylmethylxanthine reduced proteolysis in both soleus and EDL muscles. The data suggest that catecholamines exert an inhibitory control of skeletal muscle proteolysis, probably mediated by beta(2)-adrenoceptors, with the participation of a cAMP-dependent pathway.     

Hormonal regulation of carbamoyl-phosphate synthetase I synthesis in primary cultured hepatocytes and Reuber hepatoma H-35. Defective regulation in hepatoma cells

Regulation of carbamoyl-phosphate synthetase I (CPS) synthesis by various hormones was compared in primary cultured hepatocytes from adult rat and in Reuber hepatoma H-35 by pulse labeling of the cells with [35S]methionine. CPS synthesis in hepatocytes was stimulated 8-fold and 5-fold by dexamethasone and glucagon respectively. CPS synthesis in hepatocytes was synergically (about 50-fold) stimulated by a combination of dexamethasone and glucagon. Less synergic stimulation was observed by combining dexamethasone with N6, O2'-dibutyryladenosine 3',5'-monophosphate (dibutyryl-cAMP) or with isoproterenol. The basal level of CPS synthesis in hepatoma cells was higher than that in hepatocytes. CPS synthesis in hepatoma cells was stimulated by dexamethasone and dibutyryl-cAMP but the extent was only 3-fold and 1.8-fold respectively. The synergic effect of combination of dexamethasone and dibutyryl-cAMP was not observed in hepatoma cells. Neither glucagon nor isoproterenol exhibited an appreciable effect on CPS synthesis in hepatoma cells. Insulin and epinephrine suppressed CPS synthesis both in hepatocytes and hepatoma cells. The effect of epinephrine was indicated to be through alpha-adrenergic receptors. The effects of insulin and epinephrine were additive on CPS synthesis both in hepatocytes and hepatoma cells.     

Differential response of adenylate cyclase to beta-adrenergic agonists in white adipocyte membranes from lean and obese (ob/ob) mice

The quality of the beta-adrenergic response, as measured by the activation of adenylate cyclase, was found to differ in adipocyte membranes from lean and obese mice. In the tissue from lean mice, the response was similar to that in rat adipose tissue and could, by analogy, be classified as beta 1-receptor response. In the tissue preparations from the obese mice, the rank order of potency of the three classical agonists (isoproterenol, epinephrine, and norepinephrine) was more typical of a beta 2-receptor response.     

Analysis of responses to sympathetic nerve stimulation in the feline pulmonary vascular bed

The adrenergic receptor subtypes mediating the response to sympathetic nerve stimulation in the pulmonary vascular bed of the cat were investigated under conditions of controlled blood flow and constant left atrial pressure. The increase in lobar vascular resistance in response to sympathetic nerve stimulation was reduced by prazosin and to a lesser extent by yohimbine, the respective alpha 1- and alpha 2-adrenoceptor antagonists. Moreover, in animals pretreated with a beta-adrenoceptor antagonist to prevent an interaction between alpha- and beta 2-adrenoceptors, responses to nerve stimulation were reduced by prazosin, but yohimbine had no significant effect. On the other hand, in animals pretreated with a beta-adrenoceptor antagonist, yohimbine had an inhibitory effect on responses to tyramine and to norepinephrine. Propranolol had no significant effect on the response to nerve stimulation, whereas ICI 118551, a selective beta 2-adrenoceptor antagonist, enhanced responses to nerve stimulation and injected norepinephrine. The present data suggest that neuronally released norepinephrine increases pulmonary vascular resistance in the cat by acting mainly on alpha 1-adrenoceptors and to a lesser extent on postjunctional alpha 2-adrenoceptors but that this effect is counteracted by an action on presynaptic alpha 2-receptors. The present studies also suggest that neuronally released norepinephrine acts on beta 2-adrenoceptors and that the response to sympathetic nerve stimulation represents the net effect of the adrenergic transmitter on alpha 1-, alpha 2-, and beta 2-adrenoceptors in the pulmonary vascular bed.     

Stimulatory and inhibitory effects of catecholamines on DNA synthesis in primary rat hepatocyte cultures: role of alpha 1- and beta-adrenergic mechanisms.

Previous studies suggest that catecholamines may be involved in the regulation of liver growth. Considerable evidence implicates alpha 1-adrenergic mechanisms in the initiation of hepatocyte proliferation, while the role of beta-adrenoceptors is less clear. We have examined further the adrenergic regulation of hepatocyte DNA synthesis, using primary monolayer cultures. In hepatocytes that were also treated with epidermal growth factor and insulin, epinephrine or norepinephrine added early after the seeding strongly accelerated the rate of S phase entry. The beta-adrenergic agonist isoproterenol and the alpha-adrenergic agonist phenylephrine also stimulated the DNA synthesis, but were less efficient than epinephrine and norepinephrine. Experiments with the alpha 1-receptor blocker prazosine and the beta-receptor blocker timolol showed that the stimulatory effect of norepinephrine consisted of both an alpha 1- and a beta-adrenergic component. The alpha 1-component was most prominent in terms of maximal response at high concentrations of the agonist, but the beta-component contributed significantly and predominated at low concentrations (less than 0.1 microM) of norepinephrine. At later stages (about 40 h) of culturing norepinephrine strongly but reversibly inhibited the cells, acting at a point late in the G1 phase. This inhibition was mimicked by isoproterenol and abolished by timolol but was unaffected by prazosine, suggesting a beta-adrenoceptor-mediated effect. The results confirm the alpha 1-adrenoceptor-mediated stimulatory effect, but also show that beta-adrenoceptors may contribute to the growth stimulation by catecholamines. Furthermore, catecholamines, via beta-adrenoceptors and cyclic AMP, inhibit the G1-S transition, and may thus play a role in the termination of hepatic proliferation.     

Effect of adrenergic agonists on human chorionic gonadotropin release by human trophoblast cells obtained from first trimester placenta

The cultured syncytiotrophoblast cells from human first trimester placenta were used to determine the effect of adrenergic agonists on human chorionic gonadotropin (hCG) production in vitro. Beta-adrenergic agonists isoproterenol, ritodrine and isoxsuprine increased the hCG release during the 2 h incubation period, however, alpha-agonists norepinephrine and phenylephrine and a beta 1-agonist dobutamine had no effect. The effect of isoproterenol was blocked by propranolol and butoxamine, but less efficiently by phentolamine and atenolol. These results indicate that placental hCG production can be modulated by stimulation of beta-, possibly beta 2-adrenoceptors but not by alpha-adrenoceptors.