Effect of VIP on the respiratory burst in human monocytes ex vivo during prolonged strain and energy deficiency

VIP-stimulated cyclic AMP production and VIP effect on the production of reactive oxygen compounds in human monocytes activated by serum opsonized zymosan (respiratory burst) were studied during a ranger training course lasting for five days with almost continuous physical activity, and deficiency of sleep and energy. Respiratory burst was inhibited and cyclic AMP production was stimulated by VIP on all days. Maximum cyclic AMP production stimulated by VIP (0.1 microM) on the day of control was 148.6% of basal, and 255.3%, 213.8%, 218.9% and 198.7% on Days 1, 2, 3 and 5. Maximum inhibition was observed 20 min after addition of the peptide on the day of control, after 5 min on Days 1, 2 and 3, and after 10 min on Day 5. Inhibition at the 5-min time point was 33.1% on the day of control, and 34.7%, 53.6%, 53.3% and 36.2% on the different days during the training course. The observed increment in VIP effect adds to prior reported data about increased VIP secretion during the training course, and may indicate enhanced physiological significance of VIP during stress.     

Receptors for vasoactive intestinal peptide and secretin on guinea pig pancreatic acini

We investigated the abilities of VIP and secretin to occupy receptors and to increase cellular cyclic AMP using dispersed acini from guinea pig pancreas. The dose-inhibition curve for inhibition of binding of 125I-VIP by VIP was broad with detectable inhibition at 0.1 nM VIP, half-maximal inhibition at 2 nM VIP and complete inhibition at 10 microM VIP. Secretin also inhibited binding of 125I-VIP was compatible with two VIP-preferring receptors with one class having a high affinity for VIP (Kd 1.1 nM) and a low affinity for secretin (Kd 5 microM) and the other class having an intermediate affinity for VIP (Kd 470 nM). The dose inhibition curve for inhibition of binding of 125I-secretin by secretin was not broad. Half-maximal inhibition occurred with 7 nM secretin or with 10 microM VIP. Computer analysis was compatible with a single secretin-preferring receptor with a high affinity for secretin (Kd 7 nM) and a low affinity for VIP (Kd 5.9 microM). Comparison of the ability of VIP to increase cyclic AMP with or without the secretin-receptor antagonist, secretin-5-27, demonstrated only occupation of the high affinity VIP-preferring or high affinity secretin-preferring receptors increase cyclic AMP. Our results demonstrate that, in contrast to previous reports, guinea pig pancreatic acini possess 3 classes of receptors that interact with VIP and secretin. The low affinity receptor seen with 125I-VIP is not the same as the secretin-preferring receptor and does not increase cellular cyclic AMP.     

Homologous regulation of adenylate cyclase-coupled receptors for vasoactive intestinal peptide (VIP) on human mononuclear leucocytes

The effect of agonists on VIP receptor regulation has been investigated in mononuclear human blood leucocytes. VIP receptor number and affinity, as well as VIP-stimulated cyclic AMP accumulation were measured after pretreatment with VIP, PHM-27 or secretin. Pretreatment for 30 min with 0.1 μM VIP caused 28% (S.E.M. = 15) reduction in specific binding, and 52% (S.E.M. = 12) reduction in cyclic AMP accumulation, while 3 h of pretreatment caused 59% (S.E.M. = 10) and 68% (S.E.M. = 12) reduction. Only VIP concentrations at the nanomolar level and higher were shown to have any effect. B_max of the high-affinity receptor was reduced by 66% (S.E.M. = 8) after 30 min, and 95% (S.E.M. = 3) after 3 h of exposure to 0.1 μM VIP. No significant change was observed in receptor affinity, in B_max of the low-affinity receptor, in ED₅₀, or in ED₁₀₀ of VIP-stimulated cyclic AMP accumulation. Pretreatment with PHM-27 (0.1 μM, 3 h) caused 24% reduction in [¹²⁵I]VIP binding and 25% reduction in cyclic AMP accumulation, while no effect was detected after pretreatment with secretin (0.1 μM, 3 h).     

Secretin Stimulates Cyclic AMP Formation in the Rat Brain

The effects of secretin on cyclic AMP levels in the rat brain were determined. Incubation of rat brain frontal cortex slices with secretin or the structurally related peptides peptide histidine leucine (PHI) or vasoactive intestinal polypeptide (VIP) in the presence of 10 mM theophylline resulted in a dose-dependent increase in the cyclic AMP levels. The half-maximal increase in cyclic AMP occurred using a 1 microM dose of secretin or a 2 microM dose of PHI or VIP. Preincubation of slices with secretin-(5-27) produced a dose-dependent inhibition of the secretin but not VIP- or PHI-stimulated increase in the cyclic AMP content. Also, in receptor binding studies, secretin-(5-27) produced a dose-dependent inhibition (Ki = 400 nM) of 125I-secretin but not of 125I-VIP binding to rat brain membranes. Guanyl-5'-yl imidodiphosphate decreased the affinity of radiolabelled secretin binding as a result of an increased rate of dissociation of bound 125I-secretin. These data suggest that secretin receptors in the rat brain may be coupled to adenylate cyclase in a stimulatory manner and that secretin-(5-27) may function as a central secretin receptor antagonist.     

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.     

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.     

Effects of growth hormone-releasing factor, somatostatin and dopamine on growth hormone and prolactin secretion from cultured ovine pituitary cells

A synthetic form of human pancreatic growth hormone releasing factor (GRF-44-NH2) was shown to be a potent stimulator of growth hormone (GH) secretion and cellular cyclic AMP levels in cultured sheep pituitary cells. A small dose-dependent stimulation of prolactin secretion was also observed. Somatostatin (0.5 microM) completely blocked the maximal GRF (1 nM)-stimulated secretion without a significant effect on cyclic AMP levels. Dopamine (0.1 microM) inhibited the GRF-elevated GH secretion by 50% and lowered cyclic AMP levels by 30%. Dopamine (0.1 microM) inhibition of basal prolactin secretion was not affected by GRF (1 nM). The data support the hypothesis that cyclic AMP is involved in the action of GRF but suggest that somatostatin can inhibit GRF-induced secretion of GH independently of cyclic AMP.     

The effects of VIP on cyclic AMP and glycogen levels in vertebrate retina

The effects of VIP and related-peptides (PHI, secretin, glucagon) on cyclic AMP formation were investigated in intact pieces of rabbit retina. VIP and PHI increased cyclic AMP levels with EC50 of 160 nM and 300 nM respectively. At 5 microM the peptides increased cyclic AMP 46 fold (VIP) and 38 fold (PHI). Secretin was much less potent and glucagon was totally inactive. VIP was also tested for its effects on glycogen levels under similar experimental conditions. In contrast to its pronounced glycogenolytic action in mouse cerebral cortical slices, VIP at 1 microM decreased only moderately (38.3%) 3H-glycogen newly synthesized from 3H-glucose by pieces of rabbit retina. Furthermore a discrepancy between the efficacy of VIP in increasing cyclic AMP and in promoting glycogenolysis appears to exist. A similar dissociation between these two cellular events was also observed with other neuroactive substances. Thus the pronounced increase in cyclic AMP induced by dopamine and forskolin was accompanied by only a moderate decrease in 3H-glycogen levels. Conversely 50 mM potassium induced a 79.9% decrease in 3H-glycogen levels without any significant increase in cyclic AMP.     

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)     

(N-stearyl, norleucine17) VIP hybrid inhibits the growth of pancreatic cancer cell lines

The effects vasoactive intestinal peptide (VIP) antagonists were investigated on pancreatic cancer cell lines. (N-Stearyl, Norleucine17) VIP hybrid ((SN)VIPhyb) inhibited 125I-VIP binding to human Capan-2 cells with an IC50 value of 0.01 microM whereas VIP hybrid had an IC50 value of 0.2 microM. By RT-PCR and Northern blot, VPAC1 receptor mRNA was detected in CAPAN-2 cells. One microM (SN)VIPhyb and 10 microM VIPhyb inhibited the ability of 30 nM VIP to elevate cyclic AMP and increase c-fos mRNA. (SN)VIPhyb, 1 microM inhibited the clonal growth of CAPAN-2 cells in vitro. In vivo, (SN)VIPhyb (10 microg/day s.c.) inhibited CAPAN-2 xenograft growth in nude mice. These results indicate that (SN)VIPhyb is a pancreatic cancer VPAC receptor antagonist.     

Neutrophil beta-adrenergic receptor responses are potentiated by acute exposure to phorbol ester without changes in receptor distribution or coupling.

Exposure to the phorbol ester, phorbol 12-myristate, 13-acetate (PMA, 100nM) for 10 minutes enhanced cyclic AMP accumulation in human neutrophils under basal conditions and in response to the beta-adrenergic receptor agonist isoproterenol (ISO), 1 microM) and the adenylate cyclase activator forskolin (FSK, 10mM). Potentiation of responses to ISO by PMA was dose-dependent between 0.1 and 100nM PMA. The diacylglycerol analogue, 1-oleoyl-2-acetylglycerol (OAG) (50 microM) also elevated beta-receptor responses, but 4 beta-phorbol (100nM), lacking the capacity to activate PMA, was ineffective. Short-term exposure (12 seconds) to the peptide n-formylmethionine leucyl-phenylalanine (FMLP, 1 microM) also elevated neutrophil cyclic AMP accumulation. All potentiating effects of PMA on cyclic AMP production were inhibited by the protein kinase inhibitor 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine (H7). Elevation of cyclic AMP by FMLP was insensitive to H7. PMA had no apparent effect on beta-receptor agonist-affinity, distribution between cell-surface and internalised compartments, or the capacity of ISO to induce beta-receptor internalisation. Responses to FSK or ISO in terms of fold-stimulation of basal cyclic AMP accumulation in the presence of PMA were not elevated by PMA. These findings indicate that PMA exerts a potentiating effect on neutrophil adenylate cyclase responses through protein kinase C activation. FMLP elevation of neutrophil cyclic AMP in the absence of other stimuli, appears however, to be insensitive to protein kinase inhibition.     

Vasoactive intestinal peptide enhances phorbol myristate acetate-induced chemiluminescence in human lymphocytes.

Phorbol-myristate-acetate (PMA) induced in lymphocytes the production or reactive oxygen intermediates in a process which was stimulated by the presence of vasoactive intestinal peptide (VIP) in a dose-dependent response at VIP concentrations in the range 10(-11)-10(-8) M. The dissociation constant for the high-affinity receptors of VIP agreed with the ID50 of the activation of adenylate cyclase, and the ID50 for the stimulation by VIP of PMA-induced chemiluminescence, which were close to 0.2 nM VIP. Forskolin produced in lymphocytes an effect quite similar to VIP. A comparison of the response to VIP and forskolin of lymphocytes and monocytes showed that, in contrast to forskolin, VIP failed to induce the above described effect in monocytes. A possible mechanism involving protein kinase C, which is activated by PMA, and an intracellular signal linked to VIP receptors is pointed out. This study further supports a role for VIP as a mediator in the neuroimmune system.     

α2-Adrenoceptors Mediate Inhibition of Cyclic AMP Production in the Spinal Cord After Stimulation of Cyclic AMP with Forskolin but Not After Stimulation with Capsaicin or Vasoactive Intestinal Peptide

In slices obtained from the ventral and the dorsal guinea pig spinal cord both forskolin and vasoactive intestinal peptide (VIP) caused a dose-dependent stimulation of the production of cyclic AMP. By contrast capsaicin stimulated cyclic AMP formation only in the dorsal cord; no effect was observed in the ventral cord. The alpha 2-adrenergic agonist UK-14,304 dose-dependently inhibited the production of cyclic AMP in both the dorsal and ventral aspects of the cord when the formation of cyclic AMP had been stimulated with 3 microM forskolin, the maximal inhibition amounting to 25-32%. Also the basal (i.e., unstimulated) production of cyclic AMP was inhibited, the inhibition amounting to about 16-18%. However, after stimulation of cyclic AMP formation in the dorsal cord with capsaicin, UK-14,304 was virtually ineffective in inhibiting the accumulation of cyclic AMP. Also, when the formation of cyclic AMP was stimulated with VIP, UK-14,304 was virtually ineffective in inhibiting the formation of cyclic AMP both in the ventral and the dorsal parts of the cord. When cyclic AMP production had been stimulated with forskolin the ability of UK-14,304 to inhibit the formation of cyclic AMP was not attenuated by capsaicin, either in the ventral or in the dorsal cord. The results are discussed with the notion that cyclic AMP inhibitory spinal cord alpha 2-adrenoceptors are located on cells accessible to stimulation of cyclic AMP with forskolin but not with capsaicin or VIP.     

Effects of PACAP and VIP on cyclic AMP formation in rat neuronal and astrocyte cultures under normoxic and hypoxic condition

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) concentration (0.001-1000 nM)-dependently stimulated cyclic AMP production in rat primary neuronal and glial cell (astrocyte) cultures. The actions of both peptides were much more pronounced in astrocytes than in neuronal cultures. Stimulatory effects of PACAP and VIP on cyclic AMP formation were significantly smaller in cell cultures subjected to 24h lasting hypoxic conditions, induced either chemically (100 microM cobalt chloride) or by low 3% oxygen hypoxia, compared to the normoxic condition (95% air and 5% CO(2)). This picture contrasted with the effects of forskolin that were similar under normoxic and hypoxic conditions. It is suggested that hypoxia leads to changes in PACAP- and VIP-driven cyclic AMP-dependent signaling in the rat brain by influencing molecular processes likely occurring at the level of receptor protein or receptor-Gs protein coupling.     

Forskolin, phosphodiesterase inhibitors, and cyclic AMP analogs inhibit proliferation of cultured bovine aortic endothelial cells

The role of cyclic AMP on endothelial cell proliferation was investigated, since these cells can be exposed to high concentrations of physiological and pharmacological agents that alter cyclic AMP metabolism. Cloned bovine aortic endothelial cells were plated at 25,000 cells/35mm dish and grown for 5 days in the presence of phosphodiesterase (PDE) inhibitors, forskolin, or cyclic AMP analogs. The PDE inhibitors dipyridamole, ZK 62 711, isobutylmethylxanthine (IBMX) and theophylline inhibited cell growth in a concentration-dependent manner. Dipyridamole produced a 30% and a 50% inhibition at 5 microM and 12.5 microM, while higher concentrations were cytotoxic. At its therapeutic plasma concentration range (50-100 microM) theophylline inhibited cell proliferation by 15-25%, while IBMX and the highly specific cyclic AMP phosphodiesterase inhibitor, ZK 62 711 inhibited growth by 60-80% and 40-50%, respectively. Forskolin (5 microM) increased cyclic AMP levels and cyclic AMP-kinase activity ratios by 2.5-fold and 2-fold. In the absence of PDE inhibitors forskolin produced a 20% growth inhibition at 0.5 microM and a 60% inhibition at 10 microM. The forskolin dose-response curve was not altered by theophylline, but was shifted to the left by approximately 10-fold with dipyridamole and ZK 62 711 and 5-fold with IBMX. Forskolin (5 microM), by itself produced a 1.8-fold increase in cyclic AMP. In the presence of 5 microM theophylline, dipyridamole, IBMX, and ZK 62 711, cyclic AMP was increased by forskolin 2.0, 2.6, 3.5, and 6.6-fold, respectively. 8-Bromo cyclic AMP and dibutyryl cyclic AMP produced a 55% and 60% growth inhibition at 100 microM. The cyclic GMP analogs were less effective inhibitors of growth (15-30%). Our results demonstrate that cyclic AMP analogs and pharmacological agents that elevate intracellular cyclic AMP levels inhibit cell growth and suggest that cyclic AMP may be an important endogenous regulator of endothelial cell proliferation.     

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.     

Bidirectional concentration-dependent effects of glucagon and dibutyryl cyclic AMP on DNA synthesis in cultured adult rat hepatocytes

Glucagon and dibutyryl cyclic AMP exerted both stimulatory and inhibitory effects on hepatocyte DNA synthesis when added to primary monolayer cultures in the presence of serum, dexamethasone, insulin and epidermal growth factor. The stimulation occurred at low concentrations of glucagon (1 pM-1 nM) or dibutyryl cyclic AMP (1 nM-1 microM), while the agents inhibited DNA synthesis at higher concentrations (usually glucagon at over 10 nM or dibutyryl cyclic AMP at over 10 microM). The stimulatory effect was stronger at low cell densities (less than 20 X 10(3) hepatocytes/cm2). When the hepatocytes were cultured at higher densities, stimulatory effects were reduced or absent and the inhibition of (hormone-induced) DNA synthesis by a high concentration of glucagon was much more pronounced than at low cell densities. These results indicate dual, bidirectional, effects of cyclic AMP on hepatocyte DNA synthesis.     

Role of extracellular calcium and calcium efflux in the activation of hepatic glycogenolysis by calcium-dependent hormones

The role of extracellular calcium in the glycogenolytic effects of calcium-dependent hormones was examined in a rat liver perfusion system. Decreasing the perfusate CaCl2 concentration resulted in a concentration-dependent inhibition of glucose output by maximal concentrations of vasopressin (20 nM) and angiotensin II (10 nM), but not of glucagon (1.4 nM), cyclic AMP (100 microM), dibutyryl cyclic AMP (10 microM) or phenylephrine (5 microM). However, the effect of phenylephrine was inhibited when livers were perfused with CaCl2-free perfusate containing 0.5 mM EGTA in a duration-dependent manner. These effects were exerted through the inhibition of the maximal response of each hormone, and were associated with a parallel decrease in phosphorylase activation but not with changes in tissue cyclic AMP concentrations. When livers were preloaded with 45Ca for 45 min and then washed for either 15 min or 45 min, these hormones elicited a rapid and transient 45Ca efflux regardless of the perfusate calcium concentration. The sequential perfusion of two hormones resulted in the loss of 45Ca efflux by the second hormone. These results suggest that the glycogenolytic effects of vasopressin and angiotensin II depend on the extracellular calcium and that of phenylephrine primarily on the cellular calcium. It was also demonstrated that these calcium-dependent hormones mobilize calcium from the same pools. However, the mobilization of cellular calcium does not necessarily correlate directly with the glycogenolytic actions of vasopressin and angiotensin II.     

N6-(Phenylisopropyl)adenosine prevents glucagon both blocking insulin''s activation of the plasma-membrane cyclic AMP phosphodiesterase and uncoupling hormonal stimulation of adenylate cyclase activity in hepatocytes.

Glucagon (10nM) prevented insulin (10nM) from activating the plasma-membrane cyclic AMP phosphodiesterase. This effect of glucagon was abolished by either PIA [N6-(phenylisopropyl)adenosine] (100nM) or adenosine (10 microM). Neither PIA nor adenosine exerted any effect on the plasma-membrane cyclic AMP phosphodiesterase activity either alone or in combination with glucagon. Furthermore, PIA and adenosine did not potentiate the action of insulin in activating this enzyme. 2-Deoxy-adenosine (10 microM) was ineffective in mimicking the action of adenosine. The effect of PIA in preventing the blockade by glucagon of insulin's action was inhibited by low concentrations of theophylline. Half-maximal effects of PIA were elicited at around 6nM-PIA. It is suggested that adenosine is exerting its effects on this system through an R-type receptor. This receptor does not appear to be directly coupled to adenylate cyclase, however, as PIA did not affect either the activity of adenylate cyclase or intracellular cyclic AMP concentrations. Insulin's activation of the plasma-membrane cyclic AMP phosphodiesterase, in the presence of both glucagon and PIA, was augmented by increasing intracellular cyclic AMP concentrations with either dibutyryl cyclic AMP or the cyclic AMP phosphodiesterase inhibitor Ro-20-1724. PIA also inhibited the ability of glucagon to uncouple (desensitize) adenylate cyclase activity in intact hepatocytes. This occurred at a half-maximal concentration of around 3 microM-PIA. However, if insulin (10 nM) was also present in the incubation medium, PIA exerted its action at a much lower concentration, with a half-maximal effect occurring at around 4 nM.     

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.