Alpha-adrenergic inhibition of cyclic AMP accumulation in hamster adipocytes. Similarity of receptor with alpha-2 adrenergic receptors

The present communication shows the effects of several alpha-adrenergic agonists and antagonists on cyclic AMP levels in hamster epididymal adipocytes. In response to ACTH (30 mU/ml) in combination with 1-methyl-3-isobutylxanthine (0.10 mM) or adenosine deaminase (1.0 micrograms/ml), cyclic AMP levels increased to a maximum by 10 min and this level was maintained for another 20 min. Elevated cyclic AMP levels were partially suppressed by the alpha-adrenergic agents clonidine, methoxamine, methyl norepinephrine and phenylephrine. The lowest effective concentration of each of these agonists required to suppress cyclic AMP levels was 10 nM clonidine; 3 microM methoxamine; 10 microM methyl norepinephrine; 10 microM phenylephrine. Clonidine and methoxamine suppressed cyclic AMP levels by nearly 65% while phenylephrine and methyl norepinephrine caused only a 30% decline. Studies of the relative potencies of alpha-adrenergic blocking drugs on prevention of the inhibitor effect of clonidine on cyclic AMP levels disclosed that phentolamine and yohimbine were more potent blockers of clonidine action than phenoxybenzamine and prazosin. The rank order of potencies of agonists at causing suppression of cyclic AMP levels and the rank order of potencies of antagonists of clonidine action suggest similarity of the alpha-adrenergic receptors present on hamster adipocytes, which affect cyclic AMP accumulation to alpha-2 adrenergic receptors.     

Multiple Adrenergic Receptor Subtypes Controlling Cyclic AMP Formation: Comparison of Brain Slices and Primary Neuronal and Glial Cultures

The adrenergic receptor subtypes involved in cyclic AMP responses to norepinephrine (NE) were compared between slices of rat cerebral cortex and primary neuronal and glial cultures from rat brain. In neuronal cultures, NE and the beta-adrenergic receptor agonist isoproterenol (ISO) caused similar increases in cyclic AMP, which were not altered by the alpha-adrenergic receptor antagonist phentolamine. In glial cultures, NE caused a much smaller cyclic AMP response than did ISO, and this difference was reversed by alpha-adrenergic receptor antagonists (phentolamine greater than yohimbine greater than prazosin). alpha 2-Adrenergic receptor agonists partially inhibited the ISO response in glial cultures to a level similar to that observed with NE alone (clonidine = UK 14,304 greater than NE greater than 6-fluoro-NE greater than epinephrine). In slices from cerebral cortex, NE caused a much larger increase in cyclic AMP than did ISO, and this difference was reversed by alpha-adrenergic receptor antagonists with a different order of potency (prazosin greater than phentolamine greater than yohimbine). alpha 1-Adrenergic receptor agonists potentiated the response to ISO to a level similar to that observed with NE alone (epinephrine = NE greater than phenylephrine greater than 6-fluoro-NE greater than methoxamine). In all three tissue preparations, large responses to both alpha 1-receptor activation (increases in inositol phosphate accumulation) and alpha 2-receptor activation (decreases in forskolin-stimulated cyclic AMP accumulation) were observed. These data indicate that all of the major adrenergic receptor subtypes (beta, alpha 1, alpha 2) are present in each tissue preparation.(ABSTRACT TRUNCATED AT 250 WORDS)     

α-adrenergic inhibition of cyclic AMP accumulation in hamster adipocytes similarity of receptor with α-2 adrenergic receptors

The present communication shows the effects of several α-adrenergic agonists and antagonists on cyclic AMP levels in hamster epididymal adipocytes. In response to ACTH (30 mU/ml) in combination with 1-methyl-3-isobutylxanthine (0.10 mM) or adenosine deaminase (1.0 μg/ml), cyclic AMP levels increased to a maximum by 10 min and this level was maintained for another 20 min. Elevated cyclic AMP levels were partially suppressed by the α-adrenergic agents clonidine, methoxamine, methyl norepinephrine and phenylephrine. The lowest effective concentration of each of these agonists required to suppress cyclic AMP levels was 10 nM clonidine; 3 μM methoxamine; 10 μM methyl norepinephrine; 10 μM phenylephrine. Clonidine and methoxamine suppressed cyclic AMP levels by nearly 65% while phenylephrine and methyl norepinephrine caused only a 30% decline. Studies of the relative potencies of α-adrenergic blocking drugs on prevention of the inhibitory effect of clonidine on cyclic AMP levels disclosed that phentolamine and yohimbine were more potent blockers of clonidine action than phenoxybenzamine and prazosin. The rank order of potencies of agonists at causing suppression of cyclic AMP levels and the rank order of potencies of antagonists of clonidine action suggest similarity of the α-adrenergic receptors present on hamster adipocytes, which affect cyclic AMP accumulation to α-2 adrenergic receptors.     

Mediation of Norepinephrine-Stimulated Cyclic AMP Accumulation by Adrenergic Receptors in Hypothalamic and Preoptic Area Slices: Effects of Estradiol

Adrenergic receptor agonists and antagonists were employed to establish (a) which receptor subtypes mediate the cyclic AMP response to norepinephrine in hypothalamic and preoptic area slices from gonadectomized female rats and (b) which receptor subtypes might be modulated by the steroid hormone estradiol. Slice cyclic AMP levels were elevated by the beta receptor agonist isoproterenol, but not by alpha 1 (phenylephrine, methoxamine) or alpha 2 (clonidine) agonists. However, the alpha agonist phenylephrine potentiated the effect of the beta agonist isoproterenol on slice cyclic AMP accumulation. In slices from rats given no hormone treatment, the beta antagonist propranolol inhibited norepinephrine-stimulated cyclic AMP production, while the alpha 1 antagonist prazosin was without effect. In contrast, the cyclic AMP response to norepinephrine in slices from estradiol-treated rats was blocked more effectively by prazosin than by propranolol. Estradiol treatment also attenuated the production of cyclic AMP by the beta agonist isoproterenol. The data suggest (a) that norepinephrine induction of cyclic AMP accumulation in hypothalamic and preoptic area slices is mediated by beta receptors and potentiated by alpha receptor activation and (b) that estradiol depresses beta and increases alpha 1 receptor function in slices from brain regions associated with reproductive physiology.     

A cholera toxin substrate regulates cyclic GMP content of rat pinealocytes

The adrenergic regulation of cyclic GMP in isolated pinealocytes was investigated. In this cell, norepinephrine stimulates cyclic GMP and cyclic AMP greater than 100-fold by activating both alpha 1- and beta-adrenoceptors. beta-Adrenergic activation is a requisite event and is potentiated by alpha 1-adrenergic activation (Vanecek, J., Sugden, D., Weller, J. L., and Klein, D. C. (1985) Endocrinology 116, 2167-2173). The current study found that cholera toxin could substitute for beta-adrenergic agonists in stimulating pinealocyte cyclic GMP content, as has been found to be the case for cyclic AMP. Treatment with cholera toxin alone (1 microgram/ml for 90 min) had a small effect (2- to 4-fold increase) on cyclic GMP; addition of the alpha 1-adrenergic agonists, phenylephrine, cirazoline, or methoxamine to cholera toxin-treated cells rapidly (peak at 5 min) caused a further 30- to 300-fold increase. The alpha 1-adrenergic agonists had little effect by themselves at concentrations which potentiated the effects of cholera toxin. The potentiating effect of phenylephrine was inhibited nearly completely by an alpha 1-adrenergic antagonist, but not by either an alpha 2- or beta-adrenergic antagonist. The purified cholera toxin subunits A and B did not stimulate cyclic GMP either alone or in the presence of phenylephrine. Furthermore, the potentiating action of phenylephrine was observed following 90 min but not 20 min of cholera toxin pretreatment. these results suggest that the regulation of cyclic GMP levels in the pineal gland involves an Ns-like GTP-binding regulatory protein. This is of interest because it is the first indication that cyclic GMP is regulated by such a GTP-binding protein in nonretinal tissue. It remains to be determined whether the mechanisms involved in the transmembrane regulation of cyclic AMP and cyclic GMP in any other tissue are similar.     

Inhibition of lipolysis in hamster epididymal adipocytes by selective alpha-adrenergic agents. Evidence for cyclic AMP-dependent and independent mechanisms.

In this study the role of cyclic AMP in the antilpolytic effect of the alpha-adrenergic agents methoxamine and phenylephrine in hamster epididymal adipocytes was studied. Both methozamine and phenylephrine lowered the very high levels of cyclic AMP that were produced by high concentrations of isoproterenol (10 muM) or ACTH (100 MU/ml), and partially inhibited lipolysis. When lower concentrations of isoproterenol were used, the antilipolytic effect of phenylephrine and methoxamine was still evident. Under these conditions methoxamine produced a slight suppression of cyclic AMP levels while phenylephrine increased accumulation of cyclic AMP. It follows, therefore, that the inhibition of lipolysis by the alpha agents is most likely unrelated to changes in cyclic AMP levels; in contrast, phenylephrine promoted lipolysis and increased cyclic AMP levels. When the stimulus for lipolysis was provided by methylxanthines a different picture emerged. Methoxamine antagonized lipolysis and lowered cyclic AMP levels. In the presence of propranolol, phenylephrine lowered cyclic AMP levels and suppressed methylxanthine-accelerated lipolysis. It is suggested that when methy xanthines provide the stimulus for lipolysis the antilipolytic effect of methoxamine and phenylephrine (in the presence of propranolol) may be mediated by the suppression in cyclic AMP levels.     

Effect of adrenergic agents on alpha-amylase release and adenosine 3',5'-monophosphate accumulation in rat parotid tissue slices.

The role of cyclic AMP in stimulus-secretion coupling with investigated in rat parotid tissue slices in vitro. Isoproterenol and norepinephrine stimulated a rapid intracellular accumulation of cyclic AMP, which reached a maximum level of 20-30 times the control value by 5 to 10 min after addition of the drug. Isoproterenol was approximately ten times more potent in stimulating both alpha-amylase release and cyclic AMP accumulation than were norepinephrine and epinephrine, which had nearly equal effects on these two parameters. Salbutamol and phenylephrine were less effectivema parallel order of potency and sensitivity was observed for the stimulation of adenylate cyclase activity in a washed particulate fractionmthe results suggest that these drugs are acting on a parotid acinar cell through a beta1-adrenergic mechanismmat the lowest concentrations tested, each of the adrenergic agonists stimulated significant alpha-anylase release with no detectable stimulation of cyclic AMP accumulationmeven in the presence of theophylline, phenylephrine at several concentrations increased alpha-amylase release without a detectable increase in cyclic AMP levels. However, phenylephrine did stimulate adenylate cyclase. These data suggest that, under certain conditions, large increases in the intra-cellular concentration of cyclic AMP may not be necessary for stimulation of alpha-amylase release by adrenergic agonists. Also consistent with this idea was the observation that stimulation of cyclic AMP accumulation by isoproterenol was much more sensitive to inhibition by propranolol than was the stimulation of alpha-amylase release by isoproterenol. Stimulation of alpha-amylase release by phenylephrine was only partially blocked by either alpha- or beta-adrenergic blocking agents, whereas stimulation of adenylate cyclase by phenylephrine was blocked by propranolol and not by phentolaminemphenoxybenzamine and phentolamine potentiated the effects of norepinephrine and isoproterenol on both cyclic AMP accumulation and alpha-amylase release by N-6,O-2'-dibutyryl adenosine 3',5'-monophosphate; These observations may indicate a non-specific action of phenoxybenzamine, and demonstrate the need for caution in interpreting evidence obtained using alpha-adrenergic blocking agents as tools for investigation of alpha- and beta-adrenergic antagonism.     

Stimulation of glutathione absorption in rat small intestine by alpha-adrenergic agonists.

The alpha-adrenergic agonist, phenylephrine (1.6 microM), caused a threefold stimulation of glutathione (GSH) transport from the lumen into the vasculature in isolated, vascularly perfused rat small intestine. Stimulation of GSH transport by phenylephrine was blocked by the alpha-adrenergic antagonists, prazosin or phentolamine. Norepinephrine and epinephrine (both alpha and beta agonists) also stimulated GSH absorption but not to the same extent as phenylephrine. Isoproterenol, a strict beta-adrenergic agonist, had no effect on the rate of GSH absorption. Under physiological luminal GSH concentrations, phenylephrine stimulated GSH efflux from the lumen, accumulation in the intestinal mucosa, and transport into the mesenteric vasculature. Phenylephrine did not stimulate the transport of polyethylene glycol, a high molecular weight molecule, and stimulated uptake of cysteine and glycine by 30%. This suggests that the effect of phenylephrine on GSH transport is not due to enhanced bulk flow through paracellular pathways. Studies with isolated small intestinal epithelial cells showed that phenylephrine also stimulated the release of GSH from the cells. Oral administration of phenylephrine with GSH caused a two- to fivefold transient increase in plasma GSH concentrations in rats. Phenylephrine alone or with the amino acid constituents of GSH caused no increase in plasma GSH concentration. Thus, absorption of dietary GSH is under hormonal regulation. The physiological importance of this regulation is not known, although such regulation may function to control utilization of dietary GSH for detoxication and may have therapeutic benefits for individuals with deficient GSH or increased risk of oxidative or chemically induced injury.     

Alpha adrenergic receptor mediated formation of cyclic AMP in rat spinal cord

Methoxamine and phenylephrine (PE), postsynaptic alpha adrenergic agonists stimulated the accumulation of cyclic AMP in spinal cord tissue slices. Naphazoline, oxymetazoline and clonidine, previously shown to have greater efficacy at presynaptic alpha receptors did not alter accumulation and, in fact, blocked the PE response. The PE-stimulation was completely inhibited by postsynaptic alpha antagonists, incompletely by agents which bl ock presynaptic alpha receptors, and slightly by the beta blocker propranolol. Pe-stimulated accumulation was potentiated by phosphodiesterase inhibition (RO 20-1724). In contrast to previous reports on the requirement of the copresence of adenosine for alpha receptor stimulated accumulation of cyclic AMP in neuronal tissue, the PE-stimulation in spinal cord slices was unchanged by adenosine receptor blockade (theophylline), hydrolysis of endogenous adenosine (adenosine deaminase), inhibition of adenosine deaminase (EHNA) or blockade of adenosine uptake (dipyridamole). Added adenosine increased basal accumulation and produced a marked potentiation of the PE response. From this data it is evident that, in spinal cord tissue slices, there occurs a postsynaptic alpha adrenergic receptor linked to cyclic AMP accumulation which does not require the presence of other neurohumoral agents for activation.     

Adenosine and Cyclic AMP in Rat Cerebral Cortical Slices: Effects of Adenosine Uptake Inhibitors and Adenosine Deaminase Inhibitors

The endogenous levels of adenosine functionally linked to cyclic AMP systems in rat cerebral cortical slices are regulated by both adenosine deaminase and adenosine uptake systems. 2'-Deoxycoformycin (2'-DCF), an adenosine deaminase inhibitor, slightly increased basal, adenosine, and norepinephrine-elicited accumulations of cyclic AMP, whereas dipyridamole, an uptake inhibitor, had an even greater effect on cyclic AMP accumulations under the same conditions. Combinations of 2'-DCF and dipyridamole elicited a greater effect than either compound alone. Neither 2'-DCF nor dipyridamole significantly augmented accumulations of cyclic AP elicited by a depolarizing agent, veratridine, suggesting that the adenosine "released" during neuronal depolarization of brain slices is not as subject to inactivation by uptake or deamination as endogenous adenosine in control brain slices. The accumulation of cyclic AMP elicited by a combination of norepinephrine and veratridine was greater than additive. The response to a pure beta-adrenergic agonist, isoproterenol, was not potentiated by 2'-DCF, dipyridamole, or veratridine, consonant with minimal interaction of endogenous adenosine with beta-adrenergic systems.     

Stimulation by alpha-adrenergic agonists of Ca2+ fluxes, mitochondrial oxidation and gluconeogenesis in perfused rat liver.

Glucose output from perfused livers of 48 h-starved rats was stimulated by phenylephrine (2 microM) when lactate, pyruvate, alanine, glycerol, sorbitol, dihydroxyacetone or fructose were used as gluconeogenic precursors. Phenylephrine-induced increases in glucose output were immediately preceded by a transient efflux of Ca2+ and a sustained increase in oxygen uptake. Phenylephrine decreased the perfusate [lactate]/[pyruvate] ratio when sorbitol or glycerol was present, but increased the ratio when alanine, dihydroxyacetone or fructose was present. Phenylephrine induced a rapid increase in the perfusate [beta-hydroxybutyrate]/[acetoacetate] ratio and increased total ketone-body output by 40-50% with all substrates. The oxidation of [1-14C]octanoate or 2-oxo[1-14C]glutarate to 14CO2 was increased by up to 200% by phenylephrine. All responses to phenylephrine infusion were diminished after depletion of the hepatic alpha-agonist-sensitive pool of Ca2+ and returned toward maximal responses after Ca2+ re-addition. Phenylephrine-induced increases in glucose output from lactate, sorbitol and glycerol were inhibited by the transaminase inhibitor amino-oxyacetate by 95%, 75% and 66% respectively. Data presented suggest that the mobilization of an intracellular pool of Ca2+ is involved in the activation of gluconeogenesis by alpha-adrenergic agonists in perfused rat liver. alpha-Adrenergic activation of gluconeogenesis is apparently accompanied by increases in fatty acid oxidation and tricarboxylic acid-cycle flux. An enhanced transfer of reducing equivalents from the cytoplasmic to the mitochondrial compartment may also be involved in the stimulation of glucose output from the relatively reduced substrates glycerol and sorbitol and may arise principally from an increased flux through the malate-aspartate shuttle.     

Characterization of Dopamine and β-Adrenergic Receptors Linked to Cyclic AMP Formation in Intact Cells of the Clone D384 Derived from a Human Astrocytoma

3,4-Dihydroxyphenylethylamine (dopamine) and beta-adrenergic receptor agonists and antagonists were assessed for their effects on cyclic AMP accumulation in human astrocytoma derived clone D384 cells. Dopamine, SKF 38393, and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene increased cyclic AMP content with Ka values of 2.0, 0.2, and 1.6 microM. The D1-selective antagonists SCH 23390 (Ki, 1.2 nM) and SKF 83566 (Ki, 0.8 nM) were over 5,000-fold more potent than the D2-selective antagonist domperidone (Ki, 6.7 microM) at inhibiting dopamine stimulation of cyclic AMP formation. SCH 23388 (Ki, 560 nM; the S-enantiomer of SCH 23390) was 400-fold less potent than SCH 23390. Isoprenaline, adrenaline, salbutamol, and noradrenaline increased cyclic AMP content with Ka values of 0.13, 0.12, 0.22, and 7.60 microM. The beta 2-selective antagonist ICI 118,551 (Ki,0.8 nM) was almost 8,000-fold more potent than the beta 1-selective antagonist practolol (Ki, 5.9 microM) at inhibiting isoprenaline stimulated cyclic AMP accumulation. These results demonstrate that D384 cells express D1-dopamine and beta 2-adrenergic receptors linked to adenylate cyclase. Furthermore, the dopamine receptor expressed by D384 cells exhibits a pharmacological profile typical of a mammalian striatal D1-receptor and therefore the use of this clone represents another approach to studying central D1-receptors.     

Calcium metabolism in rat hepatocytes.

1. The total calcium concentration in rat hepatocytes was 7.9 microgram-atoms/g dry wt.; 77% of this was mitochondrial. Approx. 20% of cell calcium exchanged with 45Ca within 2 min. Thereafter incorporation proceeded at a low rate to reach 28% of total calcium after 60 min. Incorporation into mitochondria showed a similar time course and accounted for 20% of mitochondrial total calcium after 60 min. 2. The alpha-adrenergic agonists phenylephrine and adrenaline + propranolol stimulated incorporation of 45Ca into hepatocytes. Phenylephrine was shown to increase total calcium in hepatocytes. Phenylephrine inhibited efflux fo 45Ca from hepatocytes perifused with calcium-free medium. 3. Glucagon, dibutryl cyclic AMP and beta-adrenergic agonists adrenaline and 3-isobutyl-1-methyl-xanthine stimulated calcium efflux from hepatocytes perifused with calcium-free medium. The effect of glucagon was blocked by insulin. Insulin itself had no effect on calcium efflux and it did not affect the response to dibutyryl cyclic AMP. 4. Incorporation of 45Ca into mitochondria in hepatocytes was stimulated by phenylephrine and inhibited by glucagon and by carbonyl cyanide p-trifluoromethoxyphenylhydrazone. The effect of glucagon was blocked by insulin. 5. Ionophore A23187 stimulated hepatocyte uptake of 45Ca, uptake of 45Ca into mitochondria in hepatocytes and efflux of 45Ca into a calcium-free medium.     

Inhibition of lipolysis in hamster adipocytes with selective alpha-adrenergic stimuli. Functional characterization of the alpha-receptor

This communication shows the relative potencies of the alpha-agonists clonidine, methoxamine, methyl norepinephrine and phenylephrine in producing inhibition of lipolysis. At cell densities greater than 15 mg cell/ml lipolysis activated by either 1-methyl-3-isobutyl xanthine or adenosine deaminase was inhibited by alpha-adrenergic stimuli with a rank order of potency of clonidine greater than methoxamine greater than methyl norepinephrine; phenylephrine produced a further stimulation of lipolysis. At the same cell density isoproterenol-accelerated lipolysis was inhibited by alpha-adrenergic stimuli with a rank order of potency of phenylephrine greater than methoxamine greater than clonidine greater than methyl norepinephrine. When the density of fat cells was reduced to less than 5 mg/ml, clonidine was a more effective inhibitor of isoproterenol-activated lipolysis thatn phenylephrine. Lipolysis that was activated by dibutyryl cyclic AMP, ACTH or cholera enterotoxin was not reduced by any alpha-adrenergic agent. Under conditions when clonidine failed to inhibit catecholamine-activated lipolysis (i.e., at cell densities greater than 15 mg/ml), it failed to antagonize the antilipolytic activity of phenylephrine. The antilipolytic activities of clonidine and phenylephrine were most effectively antagonized by the blocking drugs phentolamine and yohimbine; in contrast, phenoxybenzamine and prazosin were less effective blockers. These data indicate that the alpha-adrenergic receptor on hamster fat cells is similar to presynaptic alpha-adrenergic receptors. The data further suggest the possibility that phenylephrine may exert its action through a separate alpha-adrenergic receptor mechanism.     

Activators of Protein Kinase C Act at a Postreceptor Site to Amplify Cyclic AMP Production in Rat Pinealocytes

Activation of alpha 1-adrenoceptors appears to amplify beta-adrenergic stimulation of cyclic AMP (cAMP) accumulation in rat pinealocytes severalfold by a mechanism involving activation of a Ca2+-, phospholipid-dependent protein kinase (protein kinase C). The mechanism of action of protein kinase C was investigated in this report using intact cells. Activation of protein kinase C with 4 beta-phorbol 12-myristate 13-acetate (PMA; 10(-7) M) or the alpha 1-adrenergic agonist phenylephrine (PE; 10(-6) M) did not inhibit cAMP efflux in beta-adrenergically stimulated cells. The amplification of the beta-adrenergic cAMP response by these agents also occurred in the presence of isobutylmethylxanthine (10(-3) M) and Ro 20-1724 (10(-4) M), an observation suggesting that inhibition of cAMP phosphodiesterase activity is not the mechanism of action. Furthermore, although PMA (10(-7) M) caused a sixfold increase in the magnitude of the cAMP response to isoproterenol, it did not alter the EC50 of the response (1.7 X 10(-8) M), a result indicating that protein kinase C activation does not alter beta-adrenoceptor sensitivity. The cAMP response following cholera toxin pretreatment (60-120 min) was rapidly and markedly enhanced by alpha 1-adrenergic agonists (cirazoline greater than PE greater than methoxamine), by phorbol esters (PMA greater than 4 beta-phorbol 12,13,-dibutyrate much greater than 4 alpha-phorbol 12,13-didecanoate), and by synthetic diacylglycerols (1,2-dioctanoylglycerol greater than 1-oleoyl 2-acetylglycerol much greater than diolein). The cAMP response to forskolin (10(-5)-10(-3) M) was also increased by PE (3 X 10(-6) M) and PMA (10(-7) M).(ABSTRACT TRUNCATED AT 250 WORDS)     

A Method to Measure Simultaneously Cyclic AMP and Inositol Phosphate Accumulation in Rat Brain Slices

The simultaneous measurement of the accumulation of cyclic AMP and inositol phosphates in rat cerebral cortical slices is described. After stimulation, the separation of cyclic AMP and inositol phosphates was achieved using ion-exchange chromatography and their concentrations were determined by means of a double-labeling technique, the substrates adenine and inositol being labeled with 14C and 3H, respectively. The recoveries were 70-80% for inositol phosphates and 40-50% for cyclic AMP. To test the applicability of the method, norepinephrine was chosen as an agonist, because it is known to stimulate the production of these two second messengers by interacting with alpha- and beta-adrenergic receptors. This procedure is an improvement over existing methods, because we obtained the simultaneous formation of 3H-inositol phosphates and [14C]cyclic AMP in a concentration-dependent process. EC50 values were similar for the two, 8.5 +/- 3.9 microM for 3H-inositol phosphates and 20.2 +/- 6.3 microM for [14C]cyclic AMP, and close to the values obtained when each process was studied alone. The action of adrenergic antagonists was also tested. Propranolol blocked the norepinephrine stimulation of [14C]cyclic AMP, but did not inhibit the norepinephrine stimulation of 3H-inositol phosphates. The opposite results were observed with prazosin. Our results suggest that this method could be a useful tool to examine the interaction between these two receptor-coupled effectors.     

Antagonism of alpha- and beta-adrenergic-mediated accumulations of cyclic AMP in rat cerebral cortical slices by the beta-antagonist (-)alprenolol.

(?)Alprenolol, a compound reported to bind with a high degree of specificity and stereoselectivity to β-adrenergic receptors from rat cerebral cortex completely inhibited the accumulations of cyclic AMP elicited by maximally effective concentrations of norepinephrine and epinephrine at antagonist concentrations as low as 10^?5M. Other β-adrenergic antagonists such as (?)propranolol, (±)sotalol, and (+)alprenolol only partially antagonized accumulations of cyclic AMP elicited by these catecholamines even at 10-fold higher concentrations. α-Adrenergic antagonists such as phentolamine, phenoxybenzamine and clonidine only partially antagonized inhibited the accumulation of cyclic AMP elicited by methoxamine, a compound shown to stimulate the accumulation of cyclic AMP by interaction with α-adrenergic receptors. The results indicate that in brain tissue containing a mixed population of α- and β- adrenergic linked cyclic AMP generating systems, (?)alprenolol does not exhibit absolute specificity for β-receptors.     

Norepinephrine acts as D1-dopaminergic agonist in the embryonic avian retina: late expression of beta1-adrenergic receptor shifts norepinephrine specificity in the adult tissue

Dopamine is the main catecholamine found in the chick retina whereas norepinephrine is only found in trace amounts. We compared the effectiveness of dopamine and norepinephrine in promoting cyclic AMP accumulation in retinas at embryonic day 13 (E13) and from post-hatched chicken (P15). Dopamine (EC(50)=10microM) and norepinephrine (EC(50)=30microM), but not the beta(1)-adrenergic agonist isoproterenol, stimulated over seven-fold the production of cyclic AMP in E13 retina. The cyclic AMP accumulation induced by both catecholamines in embryonic tissue was entirely blocked by 2microM SCH23390, a D(1) receptor antagonist, but not by alprenolol (beta-adrenoceptor antagonist). In P15 retinas, 100microM isoproterenol stimulated five-fold the accumulation of cAMP. This effect was blocked by propanolol (10microM), but not by 2microM SCH23390. Embryonic and adult retina display beta(1) adrenergic receptor mRNA as detected by RT-PCR, but the beta(1) adrenergic receptor protein was detected only in post-hatched tissue. We conclude that norepinephrine cross-reacts with D(1) dopaminergic receptor with affinity similar to that of dopamine in the embryonic retina. In the mature retina, however, D(1) receptors become restricted to activation by dopamine. Moreover, as opposed to the embryonic tissue, norepinephrine seems to stimulate cAMP accumulation via beta(1)-like adrenergic receptors in the mature tissue.     

REGULATION OF GUANOSINE 3'',5'' CYCLIC MONOPHOSPHATE IN THE RAT PINEAL AND POSTERIOR PITUITARY GLANDS

Potassium and norepinephrine stimulate the accumulation of cyclic AMP and cyclic GMP in rat pineal glands and their efflux into the medium. The efflux of both cyclic nucleotides was blocked by probenecid. The accumulation and efflux of cyclic GMP, but not of cyclic AMP, depends upon the presence of intact nerve endings and extracellular calcium. The calcium-dependent release of norepinephrine caused by veratridine was accompanied by the efflux of both cyclic AMP and cyclic GMP. In contrast, the calcium-independent release of norepinephrine caused by tyramine was accompanied by the efflux of cyclic AMP but not cyclic GMP. Changes in cyclic GMP therefore, may be related to exocytosis from the sympathetic nerve endings in the gland. High concentrations of potassium also increased tissue levels of cyclic GMP in the posterior pituitary gland. Veratridine and potassium, but not norepinephrine, stimulated the efflux of cyclic GMP from this neurosecretory gland. Thus, the relationship between cyclic GMP and exocytosis may extend beyond sympathetic nerve endings. The enhanced accumulation of cyclic GMP in the pineal gland after potassium does not appear to be mediated by extracellular (released) norepinephrine. Desmethylimipramine blocked the norepinephrine-stimulated changes in cyclic GMP, but not those caused by potassium. Investigation of the possible relationship between cyclic GMP and release of neurotransmitters is complicated by the apparent seasonal variation in the response of pineal cyclic GMP to potassium or norepinephrine.     

Stimulation-Evoked Ca2+ Fluxes in Cultured Bovine Adrenal Chromaffin Cells Are Enhanced by Forskolin

Forskolin, 1 microM, increased acetylcholine (ACh)-stimulated 45Ca uptake by chromaffin cells. The stimulatory effects of forskolin decreased with increasing concentration of ACh. The attenuation of the effect of forskolin on 45Ca uptake as a function of ACh concentration correlated well with changes in the forskolin effect on ACh-evoked catecholamine (CA) release. Forskolin increased excess KCl- and veratrine-evoked CA release and 45Ca uptake. Forskolin by itself stimulated 45Ca efflux and enhanced ACh-, excess KCl-, and veratrine-stimulated 45Ca efflux. High doses of forskolin inhibited both ACh-evoked 45Ca uptake and CA release. The inhibitory action of forskolin was specific to receptor-mediated response because excess KCl- and veratrine-stimulated 45Ca uptake and CA release were not inhibited. Forskolin, 0.3-30 microM, dose-dependently increased caffeine-stimulated CA release and 45Ca efflux in the absence of Ca2+ in the medium, and the effects were mimicked by dibutyryl cyclic AMP. These results suggest that cyclic AMP increases stimulation-induced CA release by enhancing calcium uptake across the plasma membrane and/or altering calcium flux in an intracellular calcium store.