Alpha-adrenergic interaction with stimulators of cyclic AMP concentrations in canine thyroid slices.

Thyroid stimulating hormone (TSH) increased cyclic AMP levels approximately 10–20 fold in canine thyroid slices after 30 min incubation. Thereafter the cyclic AMP level declined reaching about 50% of the maximal by 90 min even in the presence of 10 mM theophylline. When phentolamine, an α-adrenergic blocker, was added with TSH to the incubation medium, the decline of cyclic AMP levels that followed the peak was markedly diminished. The maximal effect of phentolamine was observed at a concentration of 10^?6M. A similar decline of the cyclic AMP levels after the peak was observed when the tissues was stimulated by prostaglandin E₁ or cholera toxin and the decline was again prevented by phentolamine. Phentolamine alone had no significant effect on the basal cyclic AMP levels. Phenylephrine, an α-adrenergic agonist, diminished the rise of cyclic AMP levels induced by TSH.Norephinephrine, a physiologic adrenergic stimulator, caused a marked inhibition of the elevation of cyclic AMP levels induced by prostaglandin E₁ or cholera toxin as was the case by TSH (Life Sciences 21, 607, 1977). The norepinephrine effect was abolished by phentolamine, but not by propranolol, a β-adrenergic blocker.These results indicate that α-adrenergic actions may be involved in the counter-regulation of cyclic AMP levels in canine thyroid glands.     

Introduction of cyclic AMP phosphodiesterase into rat submandibular acini prevents isoproterenol-stimulated cyclic AMP rise without affecting mucin secretion

Cyclic AMP phosphodiesterase has been incorporated into isolated rat submandibular acini by hypotonic swelling. This resulted in complete inhibition of the cyclic AMP rise stimulated by isoproterenol (10 microM), but had no effect on the stimulation of mucin secretion. Acini swollen in the absence of cyclic AMP phosphodiesterase showed similar cyclic AMP and mucin secretion responses to those of unswollen acini. The dissociation between cyclic AMP rise and mucin secretion was not due to stimulation of different beta-receptor subtypes since both responses to isoproterenol were inhibited by the beta 1 antagonist atenolol, but not by the beta 2 antagonist, butoxamine. The results are the first to directly demonstrate that a maximally effective concentration of isoproterenol can increase mucin secretion in the absence of a detectable increase in cyclic AMP.     

The interaction of glucagon, gastric inhibitory peptide and somatostatin with cyclic AMP production systems present in rat gastric glands

The effects of glucagon, gastric inhibitory peptide (GIP) and somatostatin on the generation of cyclic AMP have been studied under basal and histamine- or secretin-stimulated conditions in tubular gastric glands isolated by means of EDTA from the rat fundus and antrum. Four types of cell could be identified by electron microscopy; namely, parietal, mucous, peptic and some endocrine cells with a good morphological preservation of the cellular topography as seen in the intact mucosa. Immunoreactive somatostatin was found in antral glands (210 +/- 16 ng/g cell, wet wt., n = 9) as well as in fundic glands, but in smaller concentration (50 +/- 8 ng/g cell, wet wt., n = 9). (1) In rat fundic glands, glucagon, in supraphysiologic doses (3 . 10(-9) -5 . 10(-7) M), raised cyclic AMP levels 46 times above the basal. At maximally effective doses, combination of glucagon plus histamine was not additive whereas glucagon and secretin stimulations resulted in an additive response. Somatostatin (10(-10) -10(-7) M) inhibited both glucagon- and histamine-induced cyclic AMP production, whereas cimetidine specifically blocked the histaminergic stimulation. (2) In the same conditions, 10(-6)M glucagon produced a marginal effect (4-fold increase) in rat antrum, whereas GIP (10(-9) -10(-6)M) was unable to induce a significant rise of cyclic AMP production in either fundic or antral glands, or to prevent cyclic AMP production stimulated by histamine. (3) The present data do not support the view that circulating glucagon or GIP may regulate gastric secretion directly by a cyclic AMP-dependent mechanism in rat gastric glands and raise the possibility that gastric somatostatin may be the final mediator of the inhibitory actions of these hormones on acid secretion. (4) It is proposed that pancreatic glucagon acts through a receptor-cyclic AMP system which is specific for the bioactive peptide enteroglucagon ('oxyntomodulin'), probably in rat parietal cells.     

Effects of dopamine on prolactin secretion and cyclic AMP accumulation in the rat anterior pituitary gland

The effects of dopamine on pituitary prolactin secretion and pituitary cyclic AMP accumulation were studied by using anterior pituitary glands from adult female rats, incubated in vitro. During 2h incubations, significant inhibition of prolactin secretion was achieved at concentrations between 1 and 10nm-dopamine. However, 0.1–1μm-dopamine was required before a significant decrease in pituitary cyclic AMP content was observed. In the presence of 1μm-dopamine, pituitary cyclic AMP content decreased rapidly to reach about 75% of the control value within 20min and there was no further decrease for at least 2h. Incubation with the phosphodiesterase inhibitors theophylline (8mm) or isobutylmethylxanthine (2mm) increased pituitary cyclic AMP concentrations 3- and 6-fold respectively. Dopamine (1μm) had no effect on the cyclic AMP accumulation measured in the presence of theophylline, but inhibited the isobutylmethylxanthine-induced increase by 50%. The dopamine inhibition of prolactin secretion was not affected by either inhibitor. Two derivatives of cyclic AMP (dibutyryl cyclic AMP and 8-bromo cyclic AMP) were unable to block the dopamine (1μm) inhibition of prolactin secretion, although 8-bromo cyclic AMP (2mm) significantly stimulated prolactin secretion and both compounds increased somatotropin (growth hormone) release. Cholera toxin (3μg/ml for 4h) increased pituitary cyclic AMP concentrations 4–5-fold, but had no effect on prolactin secretion. The inhibition of prolactin secretion by dopamine was unaffected by cholera toxin, despite the fact that dopamine had no effect on the raised pituitary cyclic AMP concentration caused by this factor. Dopamine had no significant effect on either basal or stimulated somatotropin secretion under any of the conditions tested. We conclude that the inhibitory effects of dopamine on prolactin secretion are probably not mediated by lowering of cyclic AMP concentration, although modulation of the concentration of this nucleotide in some other circumstances may alter the secretion of the hormone.     

Hormonal control of cyclic 3':5'-AMP levels and gluconeogenesis in isolated hepatocytes from fed rats.

Glucagon can stimulate gluconeogenesis from 2 mM lactate nearly 4-fold in isolated liver cells from fed rats; exogenous cyclic adenosine 3':5'-monophosphate (cyclic AMP) is equally effective, but epinephrine can stimulate only 1.5-fold. Half-maximal effects are obtained with glucagon at 0.3 nM, cyclic AMP at 30 muM and epinephrine at 0.2 muM. Insulin reduces by 50% the stimulation by suboptimal concentrations of glucagon (0.5 nM). A half-maximal effect is obtained with 0.3 nM insulin (45 microunits/ml). Glucagon in the presence of theophylline (1 mM) causes a rapid rise and subsequent fall in intracellular cyclic AMP with a peak between 3 and 6 min. Some of the fall can be accounted for by loss of nucleotide into the medium. This efflux is suppressed by probenecid, suggesting the presence of a membrane transport mechanism for the cyclic nucleotide. Glucagon can raise intracellular cyclic AMP about 30-fold; a half-maximal effect is obtained with 1.5 nM hormone. Epinephrine (plus theophylline, 1 mM) can raise intracellular cyclic AMP about 2-fold; the peak elevation is reached in less than 1 min and declines during the next 15 min to near the basal level. Insulin (10 nM) does not lower the basal level of cyclic AMP within the hepatocyte, but suppresses by about 50% the rise in intracellular and total cyclic AMP caused by exposure to an intermediate concentration of glucagon. No inhibition of adenylate cyclase by insulin can be shown. Basal gluconeogenesis is not significantly depressed by calcium deficiency but stimulation by glucagon is reduced by 50%. Calcium deficiency does not reduce accumulation of cyclic AMP in response to glucagon but diminishes stimulation of gluconeogenesis by exogenous cyclic AMP. Glucagon has a rapid stimulatory effect on the flux of 45Ca2+ from medium to tissue.     

Amylase secretion by rabbit parotid gland. Role of cyclic AMP and cyclic GMP.

Amylase secretion and changes in the levels of cyclic AMP and GMP were studied in rabbit parotid gland slices incubated in vitro with a variety of neurohumoral transmitters, their analogs and inhibitors. Cyclic GMP levels increased 8-fold 5 min after exposure to carbachol (10(-4) M), without a change in cyclic AMP levels; amylase output also rose. These effects were completely inhibited by muscarinic blockade with atropine, but were unaffected by alpha-adrenergic blockade with phenoxybenzamine. Epinephrine (4 - 10(-5) M) produced a rapid increase in the levels of both cyclic nucleotides and in amylase release. The increase in cyclic GMP level was inhibited by previous exposure of the slices to phenoxybenzamine, while the cyclic AMP rise was prevented by the beta-blocking agent, propranolol. Pure alpha-adrenergic stimulation with methoxamine (4 - 10(-4) M) produced modest elevations in cyclic GMP content and amylase output, effects blocked by pre-treatment of slices with either atropine or phenoxybenzamine. At a concentration of 4 - 10(-6) M, isoproterenol (a beta-agonist) failed to affect cyclic GMP levels, but promptly stimulated increases in cyclic AMP levels, and after a short lag, amylase secretion. At a higher dose (4 - 10(-5) M) isoproterenol produced elevations in the levels of both nucleotides. The carbachol-induced effects on cyclic GMP content and amylase release were greatly potentiated by the addition of isoproterenol (4 - 10(-6) M). These data strongly suggest that cholinergic muscarinic agonists and alpha-adrenergic agonists stimulate amylase output in rabit parotid gland by mechanisms involving cyclic GMP. The atropine-sensitive intracellular events effected by alpha-stimulation may be dependent upon endogenous generation of acetylcholine. Both cyclic nucleotides seem to be required for the early rapid secretion of amylase. The unique responses achieved by the combination of carbachol and isoproterenol suggest that isoproterenol may increase the sensitivity of this tissue to the effects of cholinergic stimuli.     

Endogenous cyclic AMP in thyroid cell culture. Effect of thyroid-stimulating hormone and dibutyryl cyclic AMP.

We have studied the variations of endogenous cyclic AMP levels in thyroid cells cultured over a period of 7 days in several conditions: in the presence of thyroid-stimulating hormone or dibutyryl cyclic AMP which both promote the aggregation of isolated cells into follicles, and in their absence when cells develop as a typical monolayer. In follicle-forming cells, the cyclic AMP level was found to rise during the first day of culture, then to fall rapidly. In monolayer-forming cells, the cyclic AMP content slightly increases attaining the same level as found in other cells at the fourth day, which remains stable till the seventh day. We have investigated the response of these cells cultured in the presence of dibutyryl cyclic AMP retain the capability of increasing their cyclic AMP concentration whereas monolayer-forming cells do not preserve this quality of thyroid cells.     

Regulation of cyclic AMP levels in canine thyroid slices by alpha-adrenergic action.

In an attempt to clarify the role of adrenergic receptors in metabolic responses, interaction of norepinephrine with TSH was studied in canine thyroid slices with regard to cyclic AMP levels. Norepinephrine caused a very rapid (within 1 min), but quite transient increase in cyclic AMP levels. The elevation of cyclic AMP levels induced by TSH was markedly inhibited by norepinephrine. Phentolamine, an α-adrenergic blocker, not only prevented the decline of cyclic AMP levels that followed the rise by norepinephrine, but also abolished the norepinephrine effect on the TSH-induced elevation of cyclic AMP levels. Propranolol, a β-adrenergic blocker, exhibited no such effects. These results indicate that the α-adrenergic receptors control cyclic AMP levels in the thyroid gland.     

Endogenous cyclic AMP in thyroid cell culture. Effect of thyroid-stimulating hormone and dibutyryl cyclic AMP

We have studied the variations of endogenous cyclic AMP levels in thyroid cells cultured over a period of 7 days in several conditions: in the presence of thyroid-stimulating hormone or dibutyryl cyclin AMP which both promote the aggregation of isolated cell into follicles, and in their absence when cells develop as a typical monolayer. In follicle-forming cells, the cyclic AMP level was found to rise during the first day of culture, then to fall rapidly. In monolayer-forming cells, the cyclic AMP content slightly increases attaining the same level as found in other cells at the fourth day, which remains stable till the seventh day. We have investigated the response of these cells to the acute effect of thyroid-stimulating hormone: only cells cultured in the presence of dibutyryl cyclic AMP retain the capability of increasing their cycli AMP concentration whereas monolayer-forming cells do not preserve this quality of thyroid cells.     

Combined effect of phorbol ester and, A23187 or dibutyryl cyclic AMP on pepsinogen secretion from isolated gastric glands

In isolated guinea pig gastric glands, pepsinogen secretion was stimulated by the phorbol ester, 12-0-tetradecanoyl-phorbol-13-acetate (TPA) in a dose dependent manner. Calcium-deprivation from the medium resulted in the decrease in TPA-induced pepsinogen secretion. The combination of 0.4 microM Ca2+ionophore A23187 and TPA stimulated pepsinogen secretion slightly higher than the calculated additive value for each agent. This synergistic effect of the agents supports a role of calcium-activated, phospholipid-dependent protein Kinase (protein Kinase C) in gastric pepsinogen secretion. Furthermore, pepsinogen secretion was also stimulated by dibutyryl cyclic AMP (dbc AMP) and dbc AMP slightly enhanced TPA-induced pepsinogen secretion. Results suggest that gastric chief cells possess at least two different secretory pathways for pepsinogen which are probably dependent on protein kinase C and cyclic AMP, respectively.     

Effects of an ATP analogue (alpha,beta-methylene-adenosine-5'-triphosphate) on cyclic AMP and cyclic GMP levels, 45Ca efflux, and protein secretion from rat pancreas.

The effects of the alpha,beta-methylene analogue of ATP (Ap(CH2)pp), described as a competitive inhibitor of adenylate cyclase (EC 4.6.1.1), were studied in the rat pancreas in vitro. The analogue did not alter basal cyclic AMP production and basal or carbachol-stimulated efflux of 45Ca from isotope-preloaded glands. On the other hand, Ap(CH2)pp reduced the secretory responses to carbachol, carbachol in the presence of dibutyryl cyclic AMP, pancreozymin (PZ), and the calcium ionophore, A-23187. Release of pancreatic protein in response to dibutyryl cyclic AMP itself was unaffected by the ATP analogue, suggesting that the other secretagogues tested share a common, Ap(CH2)pp-inhibitable pathway in their respective stimulatory actions. Though carbachol, PZ, and A-23187 all stimulated a rapid production (30 s) of pancreatic cyclic GMP, these responses were not affected by Ap(CH2)pp. Additional studies with the analogue indicated that it has a slow onset of action (30-45 min), i.e., its effect on secretion is preceded by secretagogue-induced changes in nucleotide levels and calcium efflux. Nonetheless, the analogue may affect cellular calcium homeostasis, if not during the initial events triggering secretion then during those events which maintain continued secretory output in the presence of a stimulus.     

Involvement of calcium and cyclic AMP in controlling ixodid tick salivary fluid secretion.

Catecholamine-stimulated salivary fluid secretion (in vitro) by ixodid ticks is reduced by deletion or lowering the concentration of exogenous bathing medium Ca++. The Ca++ antagonist, verapamil, reversibly inhibits dopamine-stimulated secretion. Ionophore A-23187 is unable to induce glands to secrete. Studies in which labeled and unlabeled Ca++ flux were measured indicate that catecholamines induce release of calcium from intracellular stores during secretion. Cyclic AMP/theophylline-stimulated secretion is inhibited by verapamil, and the exclusion of calcium from the support medium. It is concluded that the primary catecholamine stimulus induces cyclic AMP formation and mobilization of Ca++ (intra- and extracellular). Cyclic AMP and calcium are thought to interact to control secretion within the fluid transporting cells of types II and III alveoli.     

Secretin induces rapid increases in inositol trisphosphate, cytosolic Ca2+ and diacylglycerol as well as cyclic AMP in rat pancreatic acini.

Previous studies have shown that the dose-response relationship for secretin-stimulated cyclic AMP accumulation is different from that for secretin-stimulated enzyme secretion in the rat exocrine pancreas. Here we show that secretin concentrations of 10(-10) M and higher stimulated a rise in cyclic AMP levels, with maximum effect on cyclic AMP accumulation being achieved already with 10(-8) M-secretin. However, at this concentration of secretin, enzyme secretion rates were approximately half-maximal. Unexpectedly, at concentrations of secretin greater than 10(-8) M there was evidence suggestive of phosphatidylinositol bisphosphate hydrolysis with rapid increases in inositol trisphosphate, cytosolic free calcium and diacylglycerol content of rat pancreatic acini. Furthermore, there was a dose-response relationship among secretin concentration (in the range 10(-8) M-2 X 10(-6) M), increases in inositol trisphosphate and increases in cytosolic free calcium ([Ca2+]i). Contrary to what has been previously believed, these results clearly indicate that in rat pancreatic acini secretin not only stimulates cyclic AMP accumulation but also raises inositol trisphosphate, [Ca2+]i and diacylglycerol. Thus, two second messenger systems may play a role in the regulation of secretin-induced amylase release.     

Renal effects of cyclic AMP in normal and congenital diabetes insipidus rats

The administration of dibutyryl cyclic AMP to normal rats undergoing water diuresis and to rats with congenital diabetes insipidus resulted in a rise in the excretion of Na⁺ and K⁺. A reduction in free water clearance was also observed in the normal rat, but this could not be entirely attributed to the effect of the nucleotide alone. Infusion of cyclic AMP to Brattleboro rats led to a modest rise in urine osmolality and a fall in urine flow, free water clearance and solute excretion, all of which could be explained on the basis of a fall in GFR. From the present experiments, it may be concluded that at the doses used neither cyclic AMP nor its dibutyryl derivative mimic the effects of ADH on water reabsorption by the kidney in vivo.     

Relationship between endogenous cyclic AMP production and steroid hormone secretion in chick adrenal cells

Dispersed chick adrenal cells were incubated with either ACTH, cholera toxin or forskolin. All three agents stimulated cyclic AMP accumulation and secretion of corticosterone and aldosterone by the dispersed cells. The dose-response to ACTH was similar for cyclic AMP and corticosterone but aldosterone secretion appeared to be more sensitive to ACTH stimulation. Concentrations higher than 10(-8) M of ACTH caused suppression of corticosterone output but not of cyclic AMP accumulation or aldosterone secretion. A significant cyclic AMP accumulation occurred within 30 min of exposure to ACTH whereas significant increases in steroid secretion were observed only after 30 min. An early increase (within 30 min) in cyclic AMP accumulation with both cholera toxin and forskolin was not accompanied by any significant stimulation of steroid secretion, which occurred only after 120 min. The results with the avian adrenal cells are consistent with the thesis that steroid production in the adrenocortical cells is stimulated by cyclic AMP-dependent pathways, whereas steroid release may be modulated by others.     

Production of cyclic AMP and adenosine by the brain and prothoracic glands of Manduca sexta

Relatively large amounts of cyclic AMP are produced by the prothoracic glands (source of the insect moulting hormone or moulting hormone percursor) of the tobacco hornworm, Manduca sexta. Pharate pupal glands produce more cyclic AMP than early fifth instar larval glands, and the addition of aminophylline enhances cyclic AMP accumulation. The much lower cyclic AMP level in the absence of aminophylline indicates the presence of potent cyclic AMP phosphodiesterase activity. Brains (sources of the prothoracicotropic hormone) also produce cyclic AMP but at a lower rate. Brains efficiently produce adenosine from ATP while β-ecdysone inhibits adenosine formation in early fifth instar larval brains. β-Ecdysone stimulates adenyl cyclase in brains of both stages when aminophylline and fluoride are present but has no effect on cyclic AMP accumulation in prothoracic glands. The absence of fluoride greatly reduces the amount of cyclic AMP produced by prothoracic glands when aminophylline is present. No cyclic AMP is accumulated in prothoracic glands when both fluoride and aminophylline are absent or in brains when fluoride is absent, notwithstanding the presence of aminophylline. Other insect tissues were also analysed for cyclic AMP production and none showed levels nearly as high as the prothoracic glands, suggesting a close relationship between cyclic AMP production and the function of the gland.     

Evidence for a cyclic AMP system highly sensitive to secretin in gastric glands isolated from the rat fundus and antrum

The effects of secretin and vasointestinal peptide (VIP) on the production of cyclic AMP have been studied in gastric glands isolated by means of EDTA from rat fundic and antral mucosa. (1) In gastric fundus, secretin and VIP caused a time- and temperature-dependent stimulation of cyclic AMP production that was maximal when the test agents were incubated for 60 min at 20 degrees C in the presence of 0.5 mM 3-isobutyl-1-methylxanthine as a phosphodiesterase inhibitor. The dose-response curve was monophasic for both peptides, the production of cyclic AMP being sensitive to 10(-10) M secretin and to 5 . 10(-8) M VIP. Half-maximal stimulation was obtained with 2.9 10(-9) M secretin or 2 . 10(-7) M VIP and the maximal stimulation represented a 21-fold and a 19-fold increase above control for secretin and VIP, respectively. Histamine also stimulated cyclic AMP production, with a Km of about 5 . 10(-4) M. No additive effect on cyclic AMP production was oberved when secretin and VIP were simultaneously added at maximally active concentrations, while an additive effect was observed when secretin and histamine were added together. (2) In gastric antrum, the characteristics of the secretin- and VIP-stimulated cyclic AMP production were similar to those observed in gastric fundus. Histamine nevertheless failed to stimulate the formation of cyclic AMP in antral mucosa. (3) These data demonstrate the existence of a cyclic AMP system highly sensitive to secretin in gastric glands isolated from the rat fundus and antrum and suggest that VIP operates through this system. (4) It is proposed that the pepsinogen- and/or mucous-secreting cells are implicated in the regulation of cyclic AMP production by secretin in gastric glands of the rat.     

Amylase secretion by rabbit parotid gland role of cyclic AMP and cyclic GMP

Amylase secretion and changes in the levels of cyclic AMP and GMP were studied in rabbit parotid gland slices incubated in vitro with a variety of neurohumoral transmitters, their analogs and inhibitors. Cyclic GMP levels increased 8-fold 5 min after exposure to carbachol (10⁻⁴ M), without a change in cyclic AMP levels; amylase output also rose. These effects were completely inhibited by muscarinic blockade with atropine, but were unaffected by α-adrenergic blockade with phenoxybenzamine. Epinephrine (4 · 10⁻⁵ M) produced a rapid increase in the levels of both cyclic nucleotides and in amylase release. The increase in cyclic GMP level was inhibited by previous exposure of the slices to phenoxybenzamine, while the cyclic AMP rise was prevented by the β-blocking agent, propranolol. Pure α-adrenergic stimulation with methoxamine (4 · 10⁻⁴ M) produced modest elevations in cyclic GMP content and amylase output, effects blocked by pre-treatment of slices with either atropine or phenoxybenzamine. At a concentration of 4 · 10^{−6 M}, isoproterenol (a β-agonist) failed to affect cyclic GMP levels, but promptly stimulated increases in cyclic AMP levels, and after a short lag, amylase secretion. At a higher dose (4 · 10⁻⁵ M) isoproterenol produced elevations in the levels of both nucleotides. The carbachol-induced effects on cylcic GMP content and amylase release were greatly potentiated by the addition of isoproterenol (4 · 10⁻⁶ M).These data strongly suggest that cholinergic muscarinic agonists and α-adrenergic agonist stimulate amylase output in rabbit parotid gland by mechanisms involving cyclic GMP. The atropine-sensitive intracellular events effected by α-stimulation may be dependent upon endogenous generation of acetylcholine. Both cyclic nucleotides seem to be required for the early rapid secretion of amylase. The unique responses achieved by the combination of carbachol and isoproterenol suggest that isoproterenol may increase the sensitivity of this issue to the effects of cholinergic stimuli.     

Pertussis toxin stimulates cholecystokinin-induced cyclic AMP formation but is without effect on secretagogue-induced calcium mobilization in exocrine pancreas

The role of a pertussis toxin sensitive GTP-binding protein in mediating between cholecystokinin receptors and phosphatidylinositol 4,5-bisphosphate phosphodiesterase as well as in preventing cholecystokinin from increasing cellular cyclic AMP has been investigated using dispersed acini from rabbit pancreas. Pertussis toxin pretreatment (500 ng/ml, 2 h) did not affect cholecystokinin(octapeptide) (CCK-8)-induced increases in cytosolic free Ca2+ as judged from changes in fluorescence obtained from quin2-loaded acini. Although pretreatment with pertussis toxin was also without effect on resting acinar cell cyclic AMP levels, adenylate cyclase activity was increased, since inhibition of cyclic AMP phosphodiesterase activity by isobutylmethylxanthine (IBMX) resulted in an additional increase in cyclic AMP levels in toxin-treated acini, indicating that acinar cell adenylate cyclase activity is under some tonic inhibitory control by the pertussis toxin-sensitive inhibitory GTP-binding protein (Gi) of the adenylate cyclase system. CCK-8 gave an increase in cyclic AMP levels in both control (1.6-fold) and toxin-treated (2.3-fold) acini, leading to cyclic AMP levels in the toxin-treated acini 2-times as high as those in control acini. In the presence of IBMX, the cyclic AMP response to CCK-8 was again markedly enhanced in acini pretreated with the toxin (3.2- vs. 1.8-fold), resulting in cAMP levels in the toxin-treated acini 3.7-times those in the absence of IBMX, 2.5-times those in control acini in the presence of IBMX and 7.0-times those in control acini in the absence of IBMX. Neither the pretreatment with pertussis toxin, nor the presence of IBMX alone, nor the combination had an effect on basal amylase secretion. However, all three treatments potentiated the stimulatory effect of CCK-8 on amylase secretion and the amount of potentiation was proportional to the cyclic AMP levels reached. Our findings suggest that in the intact pancreatic acinar cell Gi inhibition of the catalytic subunit of the adenylate cyclase may largely be responsible for preventing cholecystokinin from increasing cellular cyclic AMP. They moreover show that cyclic AMP is a modulatory agent in rabbit pancreatic enzyme secretion, not able to stimulate secretion itself, but potentiating effects mediated by the phosphatidylinositol-calcium pathway.     

Effects of ethanol on gastric acid secretion and gastric mucosal cyclic AMP in the rat.

The effect of ethanol on the secretion of gastric acid and the content of cyclic AMP of the gastric mucosa was studied in rats. Intravenously, ethanol (10 to 800 mg/kg) had no effect on the output of acid. Upon local application into the stomach, ethanol (1 to 10%) caused a concentration-dependent inhibition of the output of gastric acid. The effect was evident within 5 min. At the concentration of 1 %,ethanol decreased the rate of acid secretion maximally by about 30%. At the concentration of 3 %, the maximal inhibition was about 70 %. At the concentration of 10 %, ethanol caused a total cessation of the output of acid within 20 to 60 min.Five and 25 min after the administration of 10 % ethanol into the stomach, the gastric mucosal content of cyclic AMP was decreased by approximately 50 %. Also in vitro, the mucosal content of cyclic AMP was decreased by ethanol within 5 min. The decrease was about 30 % with 2.5 % ethanol, approximately 60 % with 10 % ethanol, and approximately 45 % with 20 % ethanol. Alcohol inhibited the activity of the cyclic AMP phosphodiesterase of the gastric mucosa in a competitive manner. The K_i-value was 0.16 M which would correspond to an alcohol concentration of 9.1 % (v/v). Ethanol caused a concentration-dependent inhibition of the activity of the gastric mucosal adenyl cyclase. By 0.166 M (9.4 %) alcohol the inhibition was nearly 100 %.It is concluded that the ethanol-induced decrease of cyclic AMP in the gastric mucosa is due to a decreased formation of the nucleotide. The accompanying inhibition of the output of acid by ethanol is consistent with the view that cyclic AMP is an intracellular regulator of the gastric acid secretion. In view of the role of cyclic AMP in the control of the integrity of the cells, it is suggested that the ethanol-induced damage of gastric mucosa might also be, at least partly, due to the decreased mucosal content of cyclic AMP.