Action Of Vasoactive Intestinal Polypeptide Research Articles

Vasoactive intestinal peptide potentiates and directly stimulates catecholamine secretion from rat adrenal chromaffin cells

The actions of vasoactive intestinal polypeptide (VIP) on catecholamine secretion and changes in [Ca 2+] i in single rat chromaffin cells were studied using amperometry and Indo-1. Application of VIP prior to acetylcholine (ACh) or co-application of VIP and ACh enhanced secretion by 94% and 153% respectively, compared to ACh alone. [Ca 2+] i was increased by 17% when VIP was preapplied and by 73% upon co-application. Exposure to VIP before stimulation with 60 mM K + enhanced secretion by 68%, but not [Ca 2+] i. VIP application prior to DMPP and nicotine had no effect on [Ca 2+] i, but increased [Ca 2+] i signals to muscarine by 18%. VIP co-application potentiated only [Ca 2+] i responses to muscarine, by 28%. The effect of VIP on muscarine-induced [Ca 2+] i signals was mimicked by 8-Br-cAMP, and both were blocked by H-89, a protein kinase A inhibitor. Long-lasting increases in secretion accompanied by a sustained rise in [Ca 2+] i to VIP alone were seen in 55% of cells. Removal of Ca 2+ or addition of La 3+ inhibited both responses, while L-, N- and P-type Ca 2+ channel blockers were ineffective. SK&F 96365 inhibited VIP-induced secretion completely and rises in [Ca 2+] i by 75%. Neither 8-Br-cAMP nor 8-Br-cGMP evoked responses similar to VIP alone. Thus in rat chromaffin cells, VIP acts both directly as a neurotransmitter in provoking sustained catecholamine secretion in a cAMP-independent manner, and also by enhancing ACh-induced secretion, via a cAMP-dependent action involving muscarinic receptors.

Heme oxygenase (HO) inhibitor ZN protoporphyrin IX (ZNPP IX) blocks the actions of vasoactive intestinal polypeptide (VIP) by inhibiting G-protein-coupled receptor binding

Heme oxygenase (HO) inhibitor ZN protoporphyrin IX (ZNPP IX) blocks the actions of vasoactive intestinal polypeptide (VIP) by inhibiting G-protein-coupled receptor binding

Distribution of VIP receptors in the human submandibular gland: an immunohistochemical study.

Distribution of vasoactive intestinal polypeptide (VIP)-immunoreactive nerve fibers and VIP-receptor (VIP-R)-immunoreactive sites in the human submandibular gland were examined by the peroxidase-antiperoxidase method using the specimens taken from patients that had not received radiotherapy. VIP-immunoreactive fibers were found around both serous and mucous acini, the duct system, and those around the mucous acini were more numerous than those around the serous acini. VIP-R immunoreactivity was restricted to the mucous acini and the intercalated duct segment. The serous acini, striated duct, and excretory ducts lacked VIP-R immunoreactivity. These findings suggest that the mucous acinar cells contain VIP-R, and that VIP-R-mediated VIP action is involved in regulating synthesis of viscous saliva and its release.

Direct stimulation of cortisol secretion from the human NCI H295 adrenocortical cell line by vasoactive intestinal polypeptide.

To investigate a possible direct action of vasoactive intestinal polypeptide (VIP) on adrenal cortisol secretion and to define its mechanism of action. The human adrenocortical carcinoma cell line NCI H295, which is not contaminated by medullary chromaffin cells, was used to aid distinction between a direct action of VIP on adrenocortical cells and an indirect mechanism involving VIP-stimulated release of catecholamines. NCI H295 cells were challenged with 10(-11)-10(-7) mol/l VIP for 4 h, with or without prior exposure for 72 h to 10 micromol/l forskolin. Cortisol and cyclic AMP contents of the overlying media were measured using in-house radioimmunoassays. Cells were treated with 10(-8)-10(-6) mol/l adrenaline or 3.3 x 10(-8) mol/l VIP with and without 10(-8)-10(-6) mol/l propranolol to exclude the possibility that an indirect mechanism of action involving beta-adrenoceptors was operating. VIP treatment produced an increase in cortisol secretion without pre-incubation, but this was markedly enhanced by prior exposure of cells to forskolin. VIP was potent, with a threshold of 10(-11) mol/l (n = 4), reaching a maximum 3.9+/-0.9-fold increase in effect on cells pre-exposed to forskolin (n = 4) by 3.3 x 10(-8) mol/l. This increase matched the 4 h response to 10 micromol/l forskolin. Cortisol secretion was accompanied by a parallel, dose-dependent increase in accumulation of cAMP. VIP potently and directly stimulates secretion of cortisol from these adrenocortical cells of human origin via an adenylate cyclase-coupled VIP receptor. These findings raise the possibility of a significant and direct effect of VIP in the control of steroid secretion from the adrenal cortex in humans.

Tissue specific expression of different human receptor types for pituitary adenylate cyclase activating polypeptide and vasoactive intestinal polypeptide: implications for their role in human physiology.

Pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are two structurally related peptides with pleiotropic physiological effects. Biochemical and cloning experiments have demonstrated that there are two structurally distinct receptors which recognize PACAP and VIP peptides with similar affinities (PACAP/VIP R-1, PACAP/VIP R-2), as well as a receptor that is specific for the PACAP peptide (PACAP-Type 1 receptor). Using a homology-based cloning strategy we have identified PACAP/VIP R-2 receptor in human adipocytes, a tissue which was not previously identified as a target for PACAP and VIP action. This receptor type recognizes PACAP-38 and VIP similar affinity with inhibition concentrations of IC50 = 6.2 +/- 4.8 nM for PACAP-38 and IC50 = 9.4 +/- 4.6 nM for VIP. Like the other two PACAP receptors types, PACAP/VIP R-2 is coupled to cAMP-mediated signal transduction pathway with effective doses ED50 = 3.2 +/- 1.6 nM and ED50 = 2.2 +/- 0.9 nM for PACAP-38 and VIP respectively. Transcripts of the common PACAP/VIP R-2 are also found in human brain and a number of peripheral tissues, such as pancreas, muscle, heart, lund, kidney, stomach and low levels in the liver. Comparison of the tissue distribution of the human PACAP/VIP R-2 to that of the other two types of human PACAP receptors (PACAP-Type 1 and the other common PACAP/VIP R-1) by RNase protection showed that each of the three PACAP receptors is expressed in a unique set of human peripheral tissues. RNA transcripts for all three PACAP receptor types are found in human heart, brain and adipose tissue, while PACAP/VIP R-2 is the only one of these three receptor types that is expressed in the pancreas and skeletal muscle. These results suggest a novel and not yet characterized role for PACAP and VIP peptides in the neuroendocrine regulation of insulin-glucose homeostasis.

Switches in the expression and function of PACAP and VIP receptors during phenotypic interconversion in human neuroblastoma cells.

Clonal human neuroblastoma cells SH-IN undergo a very conspicuous phenotypic change in culture. Large substrate-adherent cells with a slow growth rate give rise to small cells emerging in focal aggregates and growing to high cell densities. This is accompanied by a dramatic switch in the expression of receptors for the structurally related neuropeptides VIP (vasoactive intestinal polypeptide) and PACAP (pituitary adenylate cyclase activating polypeptide). Large cells expressed mainly PACAP-specific receptors that triggered stimulation of intracellular cGMP production. On the other hand, polyvalent VIP/PACAP receptors positively coupled to adenylate cyclase were mostly observed in the small cells. Both neuropeptides stimulated cell proliferation in large and small cells. These data, together with the previous demonstration of autocrine/paracrine actions of VIP and PACAP in human neuroblastomas, support the idea that these neuropeptides may participate in the establishment of the apparent phenotype in these cancer cells.

Vasoactive intestinal polypeptide and neuropeptide Y act indirectly to increase neurite outgrowth of dissociated dorsal root ganglion cells.

Recent studies suggest that rearrangement of synaptic circuitry of primary afferent neurons in the spinal cord may contribute, in part, to hyperalgesia that is often associated with peripheral nerve injury. This study of cultured adult rat dorsal root ganglion cells examined whether vasoactive intestinal polypeptide and neuropeptide Y, which are up-regulated in sensory neurons following nerve transection, possibly contribute to the morphological alterations induced by nerve injury. Neurite outgrowth of dissociated dorsal root ganglion cells was examined two weeks following either sciatic nerve transection or intrathecal administration of test agents via osmotic pumps. Dissociated cells taken from rats with transected sciatic nerve or following intrathecal administration of either vasoactive intestinal polypeptide or neuropeptide Y had a significant increase in the percentage of cells with neurites as compared to dorsal root ganglion cells taken from normal animals. Intrathecal administration, into rats with nerve lesion, of the vasoactive intestinal polypeptide and neuropeptide Y antagonists, vasoactive intestinal polypeptide(10-28) and alpha-trinositol, respectively, significantly attenuated the nerve injury-induced increase in neurite outgrowth. Vasoactive intestinal polypeptide and neuropeptide Y had no influence on neurite outgrowth when applied to normal dissociated dorsal root ganglion cells, however, when added to cells co-cultured with spinal cord explants, both peptides significantly increased the percentage of cells with neurites. K252a, a protein kinase inhibitor, attenuated the trophic action of neuropeptide Y, but not that of vasoactive intestinal polypeptide. The action of vasoactive intestinal polypeptide on neurite outgrowth was attenuated by the protein kinase A inhibitor, the Rp-isomer of adenosine-3',5'-cyclic monophosphorothioate. The results suggest that the peptides may contribute, indirectly, to the nerve injury-induced increase in neurite outgrowth of sensory neurons via separate spinally-derived neurotrophic factors and the study provides further insight into the possible mechanisms underlying hyperalgesia associated with nerve injury.

Role of nitric oxide in control of prolactin release by the adenohypophysis.

Nitric oxide synthase-containing cells were visualized in the anterior pituitary gland by immunocytochemistry. Consequently, we began an evaluation of the possible role of NO in the control of anterior pituitary function. Prolactin is normally under inhibitory hypothalamic control, and in vitro the gland secretes large quantities of the hormone. When hemipituitaries were incubated for 30 min in the presence of sodium nitroprusside, a releaser of NO, prolactin release was inhibited. This suppression was completely blocked by the scavenger of NO, hemoglobin. Analogs of arginine, such as NG-monomethyl-L-arginine (NMMA, where NG is the terminal guanidino nitrogen) and nitroarginine methyl ester, inhibit NO synthase. Incubation of hemipituitaries with either of these compounds significantly increased prolactin release. Since in other tissues most of the actions of NO are mediated by activation of soluble guanylate cyclase with the formation of cyclic GMP, we evaluated the effects of cyclic GMP on prolactin release. Cyclic GMP (10 mM) produced an approximately 40% reduction in prolactin release. Prolactin release in vivo and in vitro can be stimulated by several peptides, which include vasoactive intestinal polypeptide and substance P. Consequently, we evaluated the possible role of NO in these stimulations by incubating the glands in the presence of either of these peptides alone or in combination with NMMA. In the case of vasoactive intestinal polypeptide, the significant stimulation of prolactin release was augmented by NMMA to give an additive effect. In the case of substance P, there was a smaller but significant release of prolactin that was not significantly augmented by NMMA. We conclude that NO has little effect on the stimulatory action of these two peptides on prolactin release. Dopamine (0.1 microM), an inhibitor of prolactin release, reduced prolactin release, and this inhibitory action was significantly blocked by either hemoglobin (20 micrograms/ml) or NMMA and was completely blocked by 1 mM nitroarginine methyl ester. Atrial natriuretic factor at 1 microM also reduced prolactin release, and its action was completely blocked by NMMA. In contrast to these results with prolactin, luteinizing hormone (LH) was measured in the same medium in which the effect of nitroprusside was tested on prolactin release, there was no effect of nitroprusside, hemoglobin, or the combination of nitroprusside and hemoglobin on luteinizing hormone release. Therefore, in contrast to its inhibitory action on prolactin release NO had no effect on luteinizing hormone release. Immunocytochemical studies by others have shown that NO synthase is present in the folliculostellate cells and also the gonadotrophs of the pituitary gland. We conclude that NO produced by either of these cell types may diffuse to the lactotropes, where it can inhibit prolactin release. NO appears to play little role in the prolactin-releasing action of vasoactive intestinal polypeptide and substance P, but mediates the prolactin-inhibiting activity of dopamine and atrial natriuretic factor.

The effect of vasoactive intestinal polypeptide (VIP) on rabbit cavernosal smooth muscle contractility.

Vasoactive intestinal polypeptide (VIP) has emerged as a possible candidate for a nonadrenergic, noncholinergic inhibitory neurotransmitter in penile erection. In this study, the effect of VIP and its relationship to the adrenergic and cholinergic mechanisms were examined using isolated corpus cavernosal strips from the rabbit penis. The mechanism of action of VIP on corporal relaxation was investigated with respect to the activation of cyclic GMP and the mobilization of calcium and potassium ions. VIP caused a dose-dependent relaxation of the cavernosal strip. Pretreatment with VIP had no effect on the contraction induced by norepinephrine, phenylephrine, and clonidine. VIP had no synergistic effect on the relaxation produced by acetylcholine or isoproterenol. Neither atropine nor propranolol had any blocking effect on the VIP-induced relaxation. Methylene blue decreased the VIP-induced relaxation of the cavernosal strip. VIP had no effect on the contraction induced by KCl at either 20 or 80 mM. In calcium-free high-potassium physiologic salt solution, VIP inhibited the calcium-induced contraction. These results suggests that the mechanism of action of VIP is not mediated through classical adrenergic and cholinergic neurotransmission on rabbit cavernosal strips, but that VIP may exert its action by the activation of cyclic GMP, which may be associated, in part, with the inhibition of calcium influx.

Developmental appearance of peptide-binding sites in the small intestine of the neonatal piglet

The ontogeny of the expression of specific binding sites for bombesin (BBS), vasoactive intestinal polypeptide (VIP), and gastrin in the neonatal pig was examined. Piglets were sacrificed at birth, 1 d, 1 wk, 3 wk and 4 wk, and samples of duodenal, jejunal and ileal tissue were harvested for autoradiographic analysis of saturable radioligand binding. Monoiodinated, biologically active 125I-Tyr4-BBS, VIP or gastrin (100 pM) was applied to slide-mounted sections of piglet intestinal tissue and localized using autoradiographic analysis of radioligand binding. Saturable binding sites for 125I-BBS first appeared in duodenal tissue of 7-d-old piglets, with evidence for continued presence in this tissue through 21 d. BBS binding sites were localized in duodenal mucosa, with no detectable binding in any other tissue layers. No evidence for saturable binding of 125I-Tyr4-BBS was found in intestinal sections from either newborn, 1-d-old or 4-wk-old piglets. Saturable binding sites for 125I-VIP were present in duodenal, jejunal and ileal tissues of piglets at all ages studied and were present in both the mucosa and the muscularis externa. No saturable binding sites for 125I-gastrin were observed in intestinal sections from piglets at any of the ages studied. These results suggest potential sites of direct action of VIP in newborn, suckling and newly weaned piglets. Bombesin, however, may have direct actions only on the duodenal mucosa at 7–21 d of age. Key words: Bombesin, vasoactive intestinal polypeptide, gastrin, piglet, small intestine, newborn

Regional haemodynamic responses to pituitary adenylate cyclase‐activating polypeptide and vasoactive intestinal polypeptide in conscious rats

1. Regional haemodynamic responses to the homologous peptides, pituitary adenylate cyclase-activating peptide (1-27) (PACAP27) and vasoactive intestinal polypeptide (VIP) were assessed by giving 20 min infusions (1.5-15 nmol kg-1 h-1) in conscious, chronically-instrumented, Long Evans rats. 2. PACAP27 caused dose-dependent depressor and tachycardic effects associated with renal, mesenteric and hindquarters vasodilatations, although only in the latter vascular bed was there a sustained increase in flow. 3. VIP caused dose-dependent depressor and tachycardic effects that were not significantly different from those caused by equimolar doses of PACAP27. However, the hindquarters vasodilator effects of VIP (at 7.5 and 15 nmol kg-1 h-1) were greater than those of PACAP27 (at the same doses), and accompanied by reductions in renal and mesenteric flows and conductances. 4. In the presence of the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME; 11 mumol kg-1 h-1), there was significant inhibition of the hindquarters vasodilator effects of PACAP27 and VIP (at 7.5 and 15 nmol kg-1 h-1). Under these circumstances the renal and mesenteric vasoconstrictor effects of VIP were abolished. 5. The beta 2-adrenoceptor antagonist, ICI 118551 (670 nmol kg-1 bolus, 335 nmol kg-1 h-1 infusion), reduced the matched hindquarters vasodilator responses to PACAP27 (15 nmol kg-1 h-1) and VIP (7.5 nmol kg-1 h-1), and also abolished the renal vasoconstrictor effects of VIP. 6. The AT1-receptor antagonist, losartan potassium (20 mumol kg-1), had no significant effect on the haemodynamic response to PACAP27 (15 nmol kg-1 h-1), but augmented the hypotensive action of VIP (7.5 nmol kg-1 h-1). This influence of losartan was associated with conversion of the renal and mesenteric vasoconstrictor effect of VIP to vasodilatation. 7. Our findings show that similar changes in mean systemic arterial blood pressure in response to PACAP27 and VIP conceal substantial differences in their regional haemodynamic actions. Although the hindquarters vasodilator effects of both peptides involve NO- and Beta2-adrenoceptor-mediated mechanisms,it appears that activation of the renin-angiotensin system contributes significantly to the haemodynamic effects of VIP, but not to those of PACAP27.

Properties of Vasoactive Intestinal Polypeptide Receptors in the Anterior Pituitary of Female Chickens.

The membrane fraction of anterior pituitary in laying hens was found to possess a vasoactive intestinal polypeptide (VIP)-binding component having the properties of a receptor. Scatchard analysis of saturation studies revealed that the binding component had a single class of binding site with an equilibrium dissociation constant (Kd) of 0.092 ± 0.008 (mean ± SEM; n=5) nM and the maximum binding capacity (Bmax) of 631 ± 41 fmol/mg protein. The Kd value determined from kinetic analysis (0.091 ± 0.002 nM; n=3) was very similar to that obtained from Scatchard analysis. The Kd value obtained from Scatchard analysis was not significantly different between the laying and nonlaying hens. The Bmax in laying hens was greater than that in nonlaying hens. The administration of chicken VIP to nonlaying hens caused a marked decrease in the VIP receptor binding in the anterior pituitary with a concomitant increase in the blood serum concentrations of prolactin. The results may provide an evidence for a direct action of VIP on the anterior pituitary of female chickens.

Lack of effect of enalaprilat on the action of vasoactive intestinal polypeptide in the human forearm.

It has been proposed that angiotensin converting enzyme (ACE) may play a role in the metabolism of the vasodilator peptide vasoactive intestinal polypeptide (VIP). Reduced metabolism following treatment with ACE inhibitors may cause accumulation of VIP which in turn may mediate some of the beneficial haemodynamic effects of ACE inhibition observed in patients with heart failure. This study has shown that inhibition of local vascular ACE does not interfere with the vascular effects of VIP on forearm resistance vessels when this peptide is infused into the brachial artery of normal volunteers. These results suggest that endothelial ACE plays little part in the metabolism of intravascular VIP.

In vitro, in vivo 実験からの膀胱頸部及び後部尿道における Vasoactive Intestinal Polypeptide の作用について

The cholinergic and adrenergic neurotransmitters have been shown to play an important role for the contraction as well as the relaxation of the bladder neck and posterior urethra. However, other neurotransmitters were suggested to participate in the action of the lower urinary tract. Therefore, animal experiments were carried out to clarify the physiological function of vasoactive intestinal polypeptide (VIP). In vitro experiments, the smooth muscle obtained from the bladder neck and the urethra of female porcine were exposed to the transmural electric field stimulation with or without anti-cholinergic, anti-adrenergic drugs, VIP or anti-VIP. In vitro experiments, electric stimulation showed the temporary relaxation of the smooth muscle due to the discharge of several kinds of neurotransmitters. Moreover, the muscle relaxation continued with the addition of VIP after the administration of the anticholinergic and the antiadrenergic agents. The urethral pressure was observed using male cat in order to demonstrate the action of VIP in vivo. The intraarterial infusion of VIP showed mainly the reduction in the posterior urethral pressure. Furthermore, the administration of anti-VIP was attempted to counteract the VIP action induced by the electrostimulation. As the result, the intra-urethral pressure tended to decrease after administration of the anti-VIP infusion. From the present investigation, VIP proved to be one of the non-cholinergic and non-adrenergic neurotransmitter through pelvic nerve system, and was thought to play an important part in the muscle relaxation of the bladder neck and the posterior urethra.

Actions of vasoactive intestinal polypeptide (VIP) on neocortical neurons of the rat in vitro

The action of vasoactive intestinal polypeptide (VIP) was studied using intracellular recording techniques in neocortical neurons of the rat in vitro. When added to the perfusion medium, VIP enhanced direct excitability and increased the amplitude of the excitatory postsynaptic potential (EPSP). Effects of VIP were long-lasting and in most cases not fully reversible within the observation period. The present results suggest a role for VIP in long-lasting peptidergic modulation of excitatory synaptic components as well as direct excitability of neocortical pyramidal neurons of the adult rat.

Activity of vasoactive intestinal polypeptide at secretin-preferring receptors on isolated cardiomyocytes

Activity of vasoactive intestinal polypeptide at secretin-preferring receptors on isolated cardiomyocytes

Vasoactive intestinal polypeptide induces neurogenic contraction of guinea-pig ileum. Involvement of acetylcholine and substance P

The effect and mode of action of vasoactive intestinal polypeptide (VIP), a peptidergic neuromodulator in the gastrointestinal nervous system, were investigated in isolated muscle strips of the guinea-pig ileum. VIP induced concentration-dependent (20 nM – 1 μM) contractions of longitudinal ileal strips. TTX (1 μM), a mixture of atropine (3 μM) and spantide (30 μM), a mixture of atropine (3 μM) and ω-conotoxin GVIA (100 nM), somatostatin (60 nM) and dynorphin (100 nM) abolished the effect of VIP. In most cases a small relaxation became evident. Desensitization to substance P in the presence of atropine prevented VIP-induced contraction. A partial inhibition was observed in the presence of atropine (3 μM), spantide (30 μM), ω-conotoxin GVIA (100 nM), β-endorphin (265 nM), met-enkephalin (1100 nM) and a mixture of spantide (30 μM) and ω-conotoxin GVIA (100 nM). The action of VIP was not significantly modified by guanethidine (3 μM) or hexamethonium (150 μM). In circular ileal strips VIP (10–300 nM) caused concentration-dependent relaxations through a direct myogenic effect. These results indicate that the VIP produced contractions of the guinea-pig ileum are exclusively neurally mediated and involve a cholinergic as well as a noncholinergic-nonadrenergic (NANC) pathway. It is concluded that besides acetylcholine (Ach) VIP releases the peptidergic transmitter substance P from postganglionic nerve fibers of myenteric plexus. Opioid peptides and somatostatin modulate the activity of cholinergic and peptidergic nerves in the guinea-pig ileum. The release of substance P appears to depend completely on N-type voltage sensitive calcium channels.

Activation and multiple-site phosphorylation of tyrosine hydroxylase in perfused rat adrenal glands.

Tryptic digestion of tyrosine hydroxylase (TH) isolated from rat adrenal glands labeled with 32Pi produced five phosphopeptides. Based on the correspondence of these phosphopeptides with those identified in TH from rat pheochromocytoma cells, four phosphorylation sites (Ser8, Ser19, Ser31, and Ser40) were inferred. Field stimulation of the splanchnic nerves at either 1 or 10 Hz (300 pulses) increased 32P incorporation into TH. At 10 Hz, the phosphorylation of Ser19 and Ser40 was increased, whereas at 1 Hz, Ser19, Ser31, and Ser40 phosphorylation was increased. Stimulation at either 1 or 10 Hz also increased the catalytic activity of TH, as measured in vitro (pH 7.2) at either 30 or 300 microM tetrahydrobiopterin. Nicotine (3 microM, 3 min) increased Ser19 phosphorylation, vasoactive intestinal polypeptide (10 microM, 3 min) increased Ser40 phosphorylation, and muscarine (100 microM, 3 min) increased TH phosphorylation primarily at Ser19 and Ser31. Vasoactive intestinal polypeptide, but not nicotine or muscarine, mimicked the effects of field stimulation on TH activity. Thus, the regulation of rat adrenal medullary TH phosphorylation by nerve impulses is mediated by multiple first and second messenger systems, as previously shown for catecholamine secretion. However, different sets of second messengers are involved in the two processes. The action of vasoactive intestinal polypeptide as a secretagogue involves the mobilization of intracellular calcium, whereas its effects on TH phosphorylation are mediated by cyclic AMP. This latter effect of vasoactive intestinal polypeptide and the consequent increase in Ser40 phosphorylation appear to be responsible for the rapid activation of TH by splanchnic nerve stimulation.

Effect of 3-isobutyl-1-methylxanthine and zaprinast on non-adrenergic non-cholinergic relaxation in the rat gastric fundus

Vasoactive intestinal polypeptide (VIP) and nitric oxide (NO) have been proposed as inhibitory non-adrenergic non-cholinergic (NANC) neurotransmitters in the rat gastric fundus. The smooth muscle relaxant actions of VIP and NO are mediated by cAMP and cGMP, respectively; therefore the effect of inhibitors of phosphodiesterases responsible for cyclic nucleotide breakdown on relaxation induced by VIP, NO and electrical field stimulation was investigated. The non-specific phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), the cGMP-specific phosphodiesterase inhibitor, zaprinast, the adenylate cyclase activator, forskolin, and the cyclic nucleotide analog, 8-bromo cGMP, produced concentration-dependent relaxation of rat gastric fundus strips precontracted by PGF 2α. IBMX potentiated isoprenaline-induced relaxation but not relaxation induced by sodium nitroprusside, VIP, NO or electrical field stimulation. Zaprinast potentiated the relaxation induced by sodium nitroprusside, while having no influence on relaxation due to any other stimulus. The combination of both phosphodiesterase inhibitors did not significantly affect the electrically induced relaxation. It is concluded that both cAMP and cGMP mediate relaxation in the rat gastric fundus. Further research is needed to investigate the role of the cyclic nucleotides during NANC relaxation of this tissue.

Role of nitric oxide as a mediator of internal anal sphincter relaxation

The studies were performed in in vitro to examine the role of nitric oxide (NO) in nonadrenergic noncholinergic (NANC) nerve-mediated relaxation of the internal anal sphincter (IAS) smooth muscle strips of opossums. NO caused a concentration-dependent fall in the resting tension of the IAS. The inhibitory action of NO may be exerted directly on the IAS smooth muscle since it was not modified by the neurotoxin tetrodotoxin (1 x 10(-6) M), which abolished the neurally mediated fall in the IAS tension. The inhibitor of NO synthesis NG-nitro-L-arginine (L-NNA) produced concentration-dependent suppression of the neurally mediated fall in the IAS tension. The suppression of the neurally mediated IAS relaxation was stereoselective because D-NNA had no effect on the control responses. The suppressant action of L-NNA was selectively reversed by L-arginine in a concentration-dependent manner. The reversal was complete with 3 x 10(-4) M L-arginine. D-Arginine on the other hand, at the same concentration had no effect on L-NNA-suppressed IAS relaxation. Interestingly, the fall in the IAS tension caused by vasoactive intestinal polypeptide (VIP) (an inhibitory neurotransmitter in the IAS) was also inhibited by L-NNA (3 x 10(-5) M). From these data we conclude that NO or NO-like substances serve as important inhibitory mediators for the NANC nerve-mediated IAS relaxation. A part of the inhibitory action of VIP on the IAS involves NO-synthase pathway. The exact site of formation and release of NO or NO-like substances in response to NANC nerve stimulation remain to be investigated.