α-Adrenergic inhibition of cyclic AMP accumulation in epithelial cells isolated from rat small intestine

The present work shows that α-adrenergic agonists induce the suppression of basal and hormone-stimulated cyclic AMP levels in rat intestinal epithelial cells. Epinephrine (100 μM) suppresses by 35% the cyclic AMP levels evoked by the vasoactive intestinal peptide (VIP). The adrenergic agent induces a similar percentage of inhibition at 15, 30 and 37°C. Addition of epinephrine 20 min prior to, on 5 or 20 min after VIP yields the same magnitude of inhibition as when performed together with the stimulus. The α-adrenergic agent does not alter the K_0.5 of VIP in stimulating cyclic AMP production but reduces its efficacy. Epinephrine also suppresses prostaglandin E₁- and E₂-stimulated cyclic AMP levels by about 35%. The lowest effective concentration of epinephrine required to suppress VIP-stimulated cyclic AMP levels is 0.1 μM, half-maximal (K_0.5) and maximal effects being observed at 5 and 100 μM, respectively. Norepinephrine has the same efficacy but a slightly lower potency (K_0.5 = 18 μM) than epinephrine. Phenylephrine acts as a partial agonist of very low potency; clonidine has very little intrinsic activity and antagonizes the inhibition by epinephrine. The inhibition of VIP-stimulated cyclic AMP levels is observed in the absence of any blocking agents. It is not affected by the β blocker propranolol, but is completely reversed with α blockers with the following order of potency: dihydroergotamine>yohimbine>phentolamine. Yohimbine is much more potent than prazosin, which only partially reverses the inhibition induced by epinephrine. It is concluded that α-adrenoreceptors of the α₂ subtype mediate the suppression of VIP-stimulated cyclic AMP levels in intestinal epithelial cells. This effect is likely to be due to the inhibition of adenylate cyclase within intact cells as epinephrine is able to reduce adenylate cyclase activity of intestinal epithelial cell plasma membranes.     

GABAB Receptor-Mediated Enhancement of Vasoactive Intestinal Peptide-Stimulated Cyclic AMP Production in Slices of Rat Cerebral Cortex

Basal and vasoactive intestinal peptide (VIP)-stimulated accumulations of cyclic AMP were measured in slices of rat cerebral cortex. Neither gamma-aminobutyric acid (GABA) nor the selective GABAB receptor agonist (-)-baclofen stimulated basal cyclic AMP accumulation, whereas VIP caused a large dose-dependent increase in cyclic AMP levels. However, in the presence of 100 microM (-)-baclofen, the effects of VIP on cyclic AMP accumulation were significantly enhanced, with the responses to 1 microM and 10 microM VIP being approximately doubled. The enhancing effects of (-)-baclofen was dose related (1-1,000 microM), but an enhancing effect was not observed with 100 microM (+)-baclofen. In the presence of the GABA uptake inhibitor nipecotic acid (1 mM), GABA caused a similar dose-related enhancement of the VIP response. The ability of either GABA or (-)-baclofen to augment VIP-stimulated production of cyclic AMP was not mimicked by the GABAA, agonists isoguvacine and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) and was not antagonized by the GABAA antagonist bicuculline. The putative GABAB antagonist 5-aminovaleric acid (1 mM) significantly reduced the effect of (-)-baclofen. The ability of (-)-baclofen to enhance VIP-stimulated accumulation of cyclic AMP was observed in slices of rat cerebral cortex, hippocampus, and hypothalamus. These results indicate that GABA and (-)-baclofen can enhance VIP-stimulated accumulation of cyclic AMP in rat brain slices via an interaction with specific GABAB receptors.     

Stimulatory effect of vasoactive intestinal peptide (VIP) on cyclic AMP production in rat peritoneal macrophages.

Vasoactive intestinal peptide (VIP) stimulated cyclic AMP production in rat peritoneal macrophages. The stimulatory effect of VIP was dependent on time, temperature and cell concentration, and was potentiated by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). At 15 degrees C, the response occurred in the 0.1-1000 nM range of VIP concentrations. Half maximal stimulation of cellular cyclic AMP (ED50) was obtained at 1.2 +/- 0.5 nM VIP, and maximal stimulation (about 3-fold basal level) was obtained between 100-1000 nM. The cyclic AMP system of rat peritoneal macrophages showed a high specificity for VIP. The order of potency observed in inducing cyclic AMP production was VIP greater than rGRF greater than hGRF greater than PHI greater than secretin. Glucagon, insulin, pancreastatin and octapeptide of cholecystokinin did not modify cyclic AMP levels at concentrations as high as 1 microM. The beta-adrenergic agonist isoproterenol increased the cyclic AMP production and show additive effect with VIP. Somatostatin inhibits the accumulation of cyclic AMP in the presence of both vasoactive intestinal peptide and isoproterenol. The finding of a VIP-stimulated cyclic AMP system in rat peritoneal macrophages, together with the previous characterization of high-affinity receptors for VIP in the same cell preparation, strongly suggest that VIP may be involved in the regulation of macrophage function.     

Alanine and glutamine synthesis and release from skeletal muscle. IV. beta-Adrenergic inhibition of amino acid release.

Alanine and glutamine formation and release were studied using the intact epitrochlaris preparation of rat skeletal muscle. Epinephrine reduced the release of alanine and glutamine in a concentration-dependent manner. Measurable inhibition was observed at 10(-9) M epinephrine, and maximal inhibition was obtained at 10(-5) M. Norepinephrine also reduced alanine and glutamine formation and release but the concentration required for maximal inhibition was approximately 100-fold greater than for epinephrine. Isoproterenol (beta agonist), but not phenylephrine (alpha agonist), reproduced the effects of epinephrine, and propranolol (beta antagonist), but not phentolamine (alpha antagonist), blocked the effect of the catecholamine. N6,O2'-Dibutyryl adenosine 3':5'-monophosphate reproduced the effects of epinephrine and theophylline potentiated the effect of submaximal concentrations of the hormone. Glucagon and prostaglandin E2 had no observable effect on amino acid release. Insulin did not modify the inhibition of alanine and glutamine release produced by epinephrine. Alanine and glutamine formation from added precursor amino acids was unaffected by epinephrine or cyclic adenosine 3':5'-monophosphate. Epinephrine reduced alanine formation in muscles obtained from diabetic rats or animals treated with thyroxine or cortisone. These findings indicate that physiological levels of catecholamines reduce alanine and glutamine formation and release from skeletal muscle. This effect is mediated by a beta-adrenergic receptor and the adenylate cyclase system and can be accounted for by an inhibition of muscle protein degradation.     

Interactions between catecholamines,methyl xanthines and adenosine in regulation of cyclic AMP accumulation in hamster adipocytes

In the presence of either methyl xanthines or adenosine deaminase, isoproterenol elicited large dramatic increases in accumulation of cyclic AMP. In contrast, cyclic AMP accumulation in response to epinephrine or norepinephrine was not potentiated by either methyl xanthines or by adenosine deaminase. Blocking the alpha adrenergic activity of norepinephrine and epinephrine with phentolamine established synergism between these catecholamines and methyl xanthines and adenosine deaminase. The activity of the particulate phosphodiesterase was not influenced by norepinephrine suggesting that the lack of synergism between the catecholamines norepinephrine and epinephrine and methyl xanthines is unrelated to this enzyme. The data are interpreted to suggest that the alpha adrenergic activity of catecholamines prevents the potentiation of cyclic AMP accumulation that occurs when the action of endogenously produced adenosine is interfered with, either by its degradation with adenosine deaminase or by receptor blockade with methyl xanthine. Because a major action of adenosine on fat cells is to inhibit adenylate cyclase it is suggested that alpha adrenergic receptor activation limits the extent to which the enzyme adenylate cyclase can be activated in a fashion similar to that of adenosine.     

Islet cyclic AMP levels are not lowered during alpha 2-adrenergic inhibition of insulin release

Epinephrine-induced changes in insulin release and cyclic AMP levels were measured simultaneously in isolated rat islets. Forskolin was used to enhance islet cyclic AMP levels. Forskolin (30 microM) stimulated adenylate cyclase activity 10-fold in islet homogenates and raised cyclic AMP levels 5-fold in intact islets (both at low and high glucose). Insulin release was enhanced by forskolin only at high glucose. Epinephrine (0.1 microM) inhibited glucose- and forskolin-induced insulin release to basal rates. At the same time epinephrine potentiated forskolin-elevated cyclic AMP levels. In contrast epinephrine attenuated forskolin-stimulated adenylate cyclase activity in islet homogenates. At low glucose, both alpha 2- and beta-adrenergic blockade counteracted the epinephrine potentiation, each by 50%. At high glucose the effect was mainly beta-adrenergic in nature. The actions of epinephrine in the presence of a beta-blocker were mimicked by the alpha 2-agonist clonidine. Despite the variations in cyclic AMP levels stimulated insulin release was always inhibited by activation of alpha 2-receptors. Finally, insulin release stimulated by exogenous cyclic AMP was abolished by epinephrine. These results suggest that epinephrine inhibits insulin release at a step distal to the generation of cyclic AMP.     

Cell-free synthesis of active ribulose-1,5-bisphosphate carboxylase in the mesophyll chloroplasts of Sorghum vulgare

In the presence of either methyl xanthines or adenosine deaminase, isoproterenol elicited large dramatic increases in accumulation of cyclic AMPP. In contrast, cyclic AMP accumulation in response to epinephrine or norepinephrine was not potentiated by either methyl xanthines or by adenosine deaminase. Blocking the alpha adrenergic activity of norepinephrine and epinephrine with phentolamine established synergism between these catecholamines and methyl xanthines and adenosine deaminase. The activity of the particulate phosphodiesterase was not influenced by norepinephrine suggesting that the lack of synergism between the catecholamines norepinephrine and epinephrine and methyl xanthines is unrelated to this enzyme. The data are interpreted to suggest that the alpha adrenergic activity of catecholamines prevents the potentiation of cyclic AMP accumulation that occurs when the action of endogenously produced adenosine is interfered with, either by its degradation with adenosine deaminase or by receptor blockade with methyl xanthine. Because a major action of adenosine on fat cells is to inhibit adenylate cyclase it is suggested that alpha adrenergic receptor activation limits the extent to which the enzyme adenylate cyclase can be activated in a fashion similar to that of adenosine.     

Vasoactive-intestinal-polypeptide-stimulated adenosine 3'',5''-cyclic monophosphate accumulation in GH3 pituitary tumour cells. Reversal of desensitization by forskolin.

The interaction between forskolin and vasoactive intestinal polypeptide (VIP) in the regulation of cyclic AMP production in GH3 pituitary tumour cells was investigated. Both forskolin (10nM-10 microns) and VIP (10pM-10nM) increased the cyclic AMP content of GH3 cells. Forskolin (50-100nM) was additive with VIP in stimulating cyclic AMP accumulation when low concentrations (less than 1 nM) of the peptide were used, but exhibited a synergistic interaction with higher VIP concentrations (10-100 nM). These effects on cyclic AMP accumulation were reflected in a leftward shift in the concentration-response curve for VIP-stimulated prolactin release from GH3 cells, a process known to be regulated by intracellular cyclic AMP concentrations. The synergy observed did not appear to be related to changes in cyclic nucleotide phosphodiesterase activity, since it was even more marked in the presence of isobutylmethylxanthine, a phosphodiesterase inhibitor. Studies of the time-course of VIP-induced changes in GH3-cell cyclic AMP content revealed that, with high concentrations of VIP, production ceased within 2 min of addition. This attenuation of cyclic AMP synthesis was still observed in the presence of isobutylmethylxanthine, but was markedly inhibited by low concentrations of forskolin (50-100nM). The results suggest that VIP-induced cyclic AMP production rapidly becomes desensitized. This process, which is prevented by forskolin, may be related to changes in the ability of the guanine nucleotide regulatory protein to couple receptor occupancy to activation of adenylate cyclase.     

Vasoactive intestinal peptide stimulates mucus secretion, but nitric oxide has no effect on mucus secretion in the ferret trachea.

Vasoactive intestinal peptide (VIP) and nitric oxide (NO) are neurotransmitters involved in the regulation of bronchial and pulmonary vascular tone. Published studies of the effects of VIP on airway mucus secretion have yielded conflicting results. The purpose of this study was to determine the effect of VIP on mucus secretion in the ferret trachea and if this effect was influenced by NO. We used a sandwich enzyme-linked lectin assay to measure mucin secretion and a turbidimetric assay to measure lysozyme (serous cell) secretion from ferret tracheal segments. VIP (10(-7) M) increased mucin secretion over 2 h. VIP (10(-9) to 10(-5) M) stimulated mucin secretion in a dose-dependent fashion. VIP-induced mucin secretion was partially blocked by a VIP receptor antagonist (a chimeric VIP-pituitary adenylate cyclase-activating peptide analog, VIP receptor antagonist) at a 10-fold excess concentration. At all concentrations tested, neither NG-nitro-L-arginine methyl ester, an inhibitor of NO synthase, nor S-nitroso-N-acetyl-penicillamine, an NO donor, had any significant effect on constitutive or VIP-induced mucus secretion. We conclude that VIP-stimulated mucin and lysozyme secretion was both time dependent and dose dependent and that NO neither stimulates nor inhibits mucus secretion in the ferret trachea.     

A fragment of vasoactive intestinal peptide, VIP(10-28), is an antagonist of VIP in the colon carcinoma cell line, HT29

The 19 amino acid carboxyl terminus fragment of vasoactive intestinal peptide (VIP), VIP(10-28), inhibits [125I]VIP binding in intact HT29 colonic adenocarcinoma cells and in membranes from these cells. However, VIP(10-28) alone has no effect on adenylate cyclase activity (membranes) or cyclic AMP synthesis (intact cells) in HT29 cells although VIP receptor agonists are markedly stimulatory. The indicated antagonist character of VIP(10-28) was confirmed by rightward parallel shifts of VIP dose response curves in the presence of VIP(10-28) in adenylate cyclase and cyclic AMP synthesis experiments. The equilibrium dissociation constant values for VIP(10-28) from these experiments agree with values from inhibition binding studies. The lack of effect of VIP(10-28) on forskolin dose response curves in HT29 adenylate cyclase assays indicates the specificity of the VIP(10-28) antagonism, thus suggesting that VIP(10-28) may be a useful tool in studying VIP receptor regulation and other aspects of the mechanisms of VIP action. The potential regulatory role of a proteolytically generated fragment of VIP acting antagonistically at VIP receptors is discussed.     

β-2-adrenergic stimulation of ornithine decarboxylase activity in porcine granulosa cells in vitro

Epinephrine, norepinephrine, and isoproterenol produced dose-dependent stimulation of ornithine decarboxylase (EC 4.1.1.7) activity in isolated porcine granulosa cells maintained under defined conditions in vitro. β- but not α-receptor-blocking agents prevented enzyme stimulation by catecholamines. Application of preferential β-1 and β-2-receptor antagonists and agonists localized the epinephrine effect to β-2-adrenergic mediation. Epinephrine action was enhanced by the phosphodiesterase inhibitor, 1-methyl-3-isobutyl-xanthine, but not by saturating concentrations of the cyclic AMP analogue, 8-bromocyclic AMP, of follicle-stimulating hormone, or of prostaglandin E₂. However, stimulation by epinephrine was additive to that of luteinizing hormone. Follicular fluid obtained from immature Graafian follicles contined concentrations of norepinephrine and epinephrine active in vitro.Thus, catecholamines may participate in the regulation of ornithine decarboxylase activity in the ovary. Catecholamine effects may be mediated by β-2-receptors linked to the adenylate cyclase system.     

Calcium Dependence of Vasoactive Intestinal Peptide-Stimulated Cyclic AMP Accumulation in Rat Hippocampal Slices

Previous work has shown that incubation of hippocampal slices in medium without added calcium markedly attenuates the capacity of vasoactive intestinal peptide (VIP) to elevate cyclic AMP levels. The present studies examined the mechanism that confers calcium dependence on VIP stimulation of cyclic AMP accumulation in hippocampal slices. Calcium dependence was apparent immediately on slice preparation and was reversible only if calcium ions were added back very early during slice incubation (within 5 min). The cyclic AMP response to VIP was not abolished by preincubating slices in 100 microM adenosine, suggesting that calcium-dependent, VIP-induced release of adenosine does not mediate VIP elevation of cyclic AMP. VIP-stimulated cyclic AMP accumulation was not decreased by agents that block calcium influx (verapamil, nifedipine, magnesium ions), or by calmodulin antagonists (trifluoperazine, calmidozolium). In fact both verapamil (100 microM) and magnesium (14 mM) augmented VIP stimulation of cyclic AMP generation. Incubation of slices with the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (MIX) did not affect VIP activation of cyclic AMP accumulation if slices were incubated without added calcium, but MIX did enhance VIP elevation of cyclic AMP content in slices incubated with calcium. Thus calcium dependence of the cyclic AMP response to VIP in hippocampal slices is unlikely to result from VIP-dependent calcium influx, from interactions with calmodulin, or from calcium-inhibited phosphodiesterase(s).(ABSTRACT TRUNCATED AT 250 WORDS)     

An in vitro evaluation of adrenergic action on rat pancreatic amylase secretion: lack of stimulatory effect.

To assess direct evidence of adrenergic stimulation in pancreatic amylase secretion, effects of catecholamines on amylase release and intracellular cyclic AMP accumulation were examined with rat dispersed pancreatic acini. We first carried out control studies with CCK-8 and carbamylcholine to evaluate the usefulness of the material for the examination of amylase secretion, and examined VIP-induced cyclic AMP accumulation to assess the agonist evoked intracellular response. As a result, significant effects of CCK-8, carbamylcholine and VIP were observed, which confirmed that dispersed pancreatic acini used in this study were useful in examining exocrine pancreatic secretion. However, catecholamines failed to stimulate amylase release from pancreatic acini, although a significant increase in intracellular cyclic AMP accumulation was observed. Thus the present study strongly suggests that direct involvement of catecholamine is unlikely in pancreatic amylase secretion, in contrast to results reported previously.     

Cholinergic and VIPergic effects on thyroid hormone secretion in the mouse

The thyroid gland is known to harbor cholinergic and VIPergic nerves. In the present study, the influences of cholinergic stimulation by carbachol, cholinergic blockade by methylatropine and stimulation with various VIP sequences on basal, TSH-induced and VIP-induced thyroid hormone section were investigated in vivo in mice. The mice were pretreated with 125I and thyroxine; the subsequent release of 125I is an estimation of thyroid hormone secretion. It was found that basal radioiodine secretion was inhibited by both carbachol and methylatropine. Furthermore, TSH-induced radioiodine secretion was inhibited already by a low dose of carbachol. Moreover, a high dose of carbachol could inhibit VIP-induced radioiodine secretion. Methylatropine did not influence TSH- or VIP-stimulated radioiodine secretion, but counteracted the inhibitory action of carbachol on TSH- and VIP-induced radioiodine release. In addition, contrary to VIP, six various synthesized VIP fragments had no effect on basal or stimulated radioiodine release. It is concluded that basal thyroid hormone secretion is inhibited by both cholinergic activation and blockade. Furthermore, TSH-induced thyroid hormone secretion is more sensitive to inhibition with cholinergic stimulation than is VIP-induced thyroid hormone secretion. In addition, the VIP stimulation of thyroid hormone secretion seems to require the full VIP sequence.     

Vasoactive Intestinal Peptide (VIP) Inhibits Substrate Adherence Capacity of Rat Peritoneal Macrophages by a Mechanism That Involves cAMP

In this study, vasoactive intestinal peptide (VIP) is shown to inhibit substrate adherence capacity of rat peritoneal macrophages. The inhibitory response occurred in the 0.1-1, 000 nM range of VIP concentrations and it was a time-dependent process. At 15 min, half maximal inhibition (ICw) was obtained at 0.37 ± 0.26 nM and maximal inhibition (53.8%) at 10^-6 M VIP. The inhibitory effect of VIP was correlated with the stimulation by this peptide of cyclic AMP (cAMP) production in rat peritoneal macrophages. Moreover, agents that inhibited VIP-stimulated cAMP production, such as the VIP-antagonist [4-Cl-D-Phe₆ Leu₁₇]-VIP and somatostatin, also decreased the inhibitory effect of VIP on substrate adherence capacity of macrophages. On the contrary, the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) and the lipid-soluble derivative of cAMP N⁶ 2'-O-dibutyryl cAMP (Bu-cAMP) inhibited the adherence of macrophages to substrate and potentiated the inhibitory action of VIP. These results demonstrate that VIP inhibits substrate adherence capacity of rat peritoneal macrophages by a mechanism that involves cAMP, and show, for the first time, an action of VIP on the function of peritoneal macrophages.     

Effects of age and androgens upon functional vasoactive intestinal peptide receptors in rat prostatic epithelial cells

The specific binding of vasoactive intestinal peptide (VIP) and the stimulatory effect of VIP upon cyclic AMP accumulation in isolated epithelial cells of rat ventral prostate were age dependent. The number of VIP receptors decreased but the efficiency of VIP on cyclic AMP accumulation increased in prostatic epithelium when considering the periods 35-65 days and 3-6 months. Since these features could be related to the known age-related decrease of androgen and androgen-receptor levels, we studied the effect of testosterone and its 5 alpha-reduced metabolite dihydrotestosterone upon both steps of VIP action. The two steroid hormones exerted a non-competitive inhibition on VIP-induced cyclic AMP accumulation but did not modify VIP binding to its specific receptors. This modulatory effect of androgens might involve their interaction with specific sites on the cell membrane leading to modifications of membrane activities including adenylate cyclase, as has been suggested by an increasing number of recent reports.     

The role of cyclic AMP in the chemotactic responsiveness and spontaneous motility of rabbit peritoneal neutrophils. The inhibition of neutrophil movement and the elevation of cyclic AMP levels by catecholamines, prostaglandins, theophylline and cholera toxin.

Agents known to affect intracellular levels of cyclic AMP in many diverse systems have been tested for their effect on the chemotaxis induced by Escherichia coli culture filtrates, spontaneous motility and cyclic AMP levels of rabbit peritoneal neutrophils. Prostaglandin E1 and A1 but not prostaglandin F2alpha increased neutrophil cyclic AMP levels and, correspondingly, only the former two prostaglandins inhibited chemotaxis. Nevertheless, a quantitative relationship between prostaglandin stimulation of cyclic AMP and inhibition of chemotaxis could not be found. Epinephrine, isoproterenol, and, to a much lesser extent, norepinephrine increased neutrophil cyclic AMP through beta adrenergic stimulation. Only epinephrine and isoproterenol inhibited chemotaxis, but the inhibition was variable and not related to the ability of these catecholamines to increase intracellular cyclic AMP. Cholera toxin increased neutrophil cyclic AMP after a 30-min lag period which paralled its inhibitory effect on chemotaxis and spontaneous motility. However, the effect on chemotaxis require 50 ng/ml of toxin whereas the effect on cyclic AMP was manifested at 2 ng/ml of toxin. Prior to 30-min preincubation there was no effect of even 1250 ng/ml of toxin on either cyclic AMP or chemotaxis. Choleragenoid prevented the effects of toxin on both cyclic AMP and chemotaxis. The bacterial chemotactic factor obtained from E. coli culture filtrates did not effect a measurable change in levels of neutrophil cyclic AMP. The data indicate that even though cyclic AMP is not, in the main sequence of events, triggering the chemotactic response, increases in neutrophil cyclic AMP may modulate the movement and thus the chemotactic responsiveness of the neutrophil.     

Regulation of GH3 pituitary tumour-cell adenylate cyclase activity by activators of protein kinase C.

The influence of protein kinase C (PKC) activation on cyclic AMP production in GH3 cells has been studied. The stimulation of cyclic AMP accumulation induced by forskolin and cholera toxin was potentiated by 4 beta-phorbol 12,13-dibutyrate (PDBu). Moreover, PDBu, which causes attenuation of the maximal response to vasoactive intestinal polypeptide (VIP), also induced a small right shift in the dose-response curve for VIP-induced cyclic AMP accumulation. PDBu-stimulated cyclic AMP accumulation was unaffected by pretreatment of cells with pertussis toxin or the inhibitory muscarinic agonist, oxotremorine. PDBu stimulation of adenylate cyclase activity required the presence of a cytosolic factor which appeared to translocate to the plasma membrane in response to the phorbol ester. The diacylglycerol-generating agents thyroliberin, bombesin and bacterial phospholipase C each stimulated cyclic AMP accumulation, but, unlike PDBu, did not attenuate the stimulation induced by VIP. These results suggest that PKC affects at least two components of the adenylate cyclase complex. Stimulation of cyclic AMP accumulation is probably due to modification of the catalytic subunit, whereas attenuation of VIP-stimulated cyclic AMP accumulation appears to be due to the phosphorylation of a different site, which may be the VIP receptor.     

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)     

Involvement of cyclic AMP and calcium in exocrine protein secretion induced by vasoactive intestinal polypeptide in rat parotid cells

The effects of vasoactive intestinal polypeptide (VIP) on exocrine protein secretion were studied in enzymatically dispersed cell aggregates from rat parotid glands. VIP (10(-9) - 10(-7) M) stimulated secretion of alpha-amylase in a dose-dependent manner. The VIP-induced release of alpha-amylase was potentiated in the presence of a phosphodiesterase inhibitor. Basal levels of cyclic AMP of the dispersed cells were increased 6.7-fold after stimulation for 10 min by VIP (10(-7) M). The VIP-induced release of alpha-amylase was reduced by 40% when cells were incubated in a Ca2+-free medium in the presence of ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA). Efflux of 45Ca2+ was significantly increased over basal levels by stimulation with VIP (10(-8) and 10(-7) M), but this increased efflux was approximately only half the increased efflux induced by carbachol (10(-5) M). VIP had no effect on the incorporation of [14C]leucine into protein by parotid cells, whereas incorporation was reduced to 30% of the control value by carbachol (10(-5) M). Thus, the VIP-ergic secretory response in the rat parotid gland is associated with a raised intracellular cyclic AMP level and the mobilisation of a different intracellular Ca2+ pool than that mobilised by carbachol. It is, therefore, closely analogous to the beta-adrenergic response.