Pituitary adenylate cyclase-activating polypeptide: a novel, long-lasting, endothelium-independent vasorelaxant

Hypersensitivity to PACAP-38 in post-traumatic headache: a randomized clinical trial

Pituitary adenylate cyclase-activating polypeptide-38 (PACAP-38), known for its role in migraine pathogenesis, has been identified as a novel drug target. Given the clinical parallels between post-traumatic headache (PTH) and migraine, we explored the possible role of PACAP-38 in the pathogenesis of PTH. To this end, we conducted a randomized, double-blind, placebo-controlled, two-way crossover trial involving adult participants diagnosed with persistent PTH resulting from mild traumatic brain injury. Participants were randomly assigned to receive a 20-min continuous intravenous infusion of either PACAP-38 (10 pmol/kg/min) or placebo (isotonic saline) on two separate experimental days, with a 1-week washout period in between. The primary outcome was the difference in incidence of migraine-like headache between PACAP-38 and placebo during a 12-h observational period post-infusion. The secondary outcome was the difference in the area under the curve (AUC) for baseline-corrected median headache intensity scores during the same 12-h observational period. Of 49 individuals assessed for eligibility, 21 were enrolled and completed the trial. The participants had a mean age of 35.2 years, and 16 (76%) were female. Most [19 of 21 (90%)] had a migraine-like phenotype. During the 12-h observational period, 20 of 21 (95%) participants developed migraine-like headache after intravenous infusion of PACAP-38, compared with two (10%) participants after placebo (P < 0.001). Furthermore, the baseline-corrected AUC values for median headache intensity scores during the 12-h observational period was higher after PACAP-38 than placebo (P < 0.001). These compelling results demonstrate that PACAP-38 is potent inducer of migraine-like headache in people with persistent PTH. Thus, targeting PACAP-38 signalling might be a promising avenue for the treatment of PTH.

Hypersensitivity to opening of ATP-sensitive potassium channels in post-traumatic headache

OBJECTIVE

To investigate whether levcromakalim (a KATP channel opener) induces migraine-like headache in people with persistent post-traumatic headache who had no known history of migraine.

METHODS

In a randomized, double-blind, placebo-controlled, 2-way crossover trial, participants were randomly assigned to receive a 20-minute continuous intravenous infusion of levcromakalim (50 µg/mL) or placebo (isotonic saline) on two separate experimental days with a 1-week wash-out period in between. The primary endpoint was the difference in incidence of migraine-like headache between levcromakalim and placebo during a 12-hour observational period after infusion start. The secondary endpoint was the difference in area under the curve for baseline-corrected median headache intensity scores between levcromakalim and placebo during the 12-hour observational period.

RESULTS

A total of 21 participants with persistent post-traumatic headache were randomized and completed the trial. During the 12-hour observational period, 12 (57%) of 21 participants reported experiencing migraine-like headache following the levcromakalim infusion, compared with three after placebo (P = 0.013). Moreover, the baseline-corrected median headache intensity scores were higher following the levcromakalim infusion than after placebo (P = 0.003).

CONCLUSION

Our findings suggest that KATP channels play an important role in the pathogenesis of migraine-like headache in people with persistent post-traumatic headache. This implies that KATP channel blockers might represent a promising avenue for drug development. Further research is warranted to explore the potential therapeutic benefits of KATP channel blockers in managing post-traumatic headache.Trial Registration: ClinicalTrials.gov Identifier: NCT05243953.

Parasympathetic innervation of vertebrobasilar arteries: is this a potential clinical target?

This review aims to summarise the contemporary evidence for the presence and function of the parasympathetic innervation of the cerebral circulation with emphasis on the vertebral and basilar arteries (the posterior cerebral circulation). We consider whether the parasympathetic innervation of blood vessels could be used as a means to increase cerebral blood flow. This may have clinical implications for pathologies associated with cerebral hypoperfusion such as stroke, dementia and hypertension. Relative to the anterior cerebral circulation little is known of the origins and neurochemical phenotypes of the parasympathetic innervation of the vertebrobasilar arteries. These vessels normally provide blood flow to the brainstem and cerebellum but can, via the Circle of Willis upon stenosis of the internal carotid arteries, supply blood to the anterior cerebral circulation too. We review the multiple types of parasympathetic fibres and their distinct transmitter mechanisms and how these vary with age, disease and species. We highlight the importance of parasympathetic fibres for mediating the vasodilatory response to sympathetic activation. Current trials are investigating the possibility of electrically stimulating the postganglionic parasympathetic ganglia to improve cerebal blood flow to reduce the penumbra following stroke. We conclude that although there are substantial gaps in our understanding of the origins of parasympathetic innervation of the vertebrobasilar arteries, activation of this system under some conditions might bring therapeutic benefits.

Pituitary Adenylate Cyclase-Activating Polypeptide Is Upregulated in Murine Skin Inflammation and Mediates Transient Receptor Potential Vanilloid-1-Induced Neurogenic Edema

Although pituitary adenylate cyclase-activating polypeptide (PACAP) was described as a key vasoregulator in human skin, little is known about its expression in mouse skin. As it is important to investigate PACAP signaling in translational mouse dermatitis models, we determined its presence, regulation, and role in neurogenic and non-neurogenic cutaneous inflammatory mechanisms. The mRNA of PACAP and its specific receptor PAC1 was detected with real-time PCR in several skin regions at comparable levels. PACAP-38-immunoreactivity measured with radioimmunoassay was similar in plantar and dorsal paw skin and the ear but significantly smaller in the back skin. PACAP and PAC1 mRNA, as well as PACAP-38 and PAC1 protein expression, significantly increased in the plantar skin after intraplantar administration of capsaicin (50 μl, 100 μg ml(-1)), an agonist of the transient receptor potential vanilloid 1 (TRPV1) receptor, evoking chiefly neurogenic inflammation without inflammatory cell accumulation. Intraplantar complete Freund's adjuvant (CFA; 50 μl, 1 mg ml(-1)) also increased PACAP/PAC1 mRNA but not the PACAP peptide. Capsaicin-induced neurogenic paw edema, but not CFA-evoked non-neurogenic swelling, was significantly smaller in PACAP-deficient mice throughout a 24-hour period. To our knowledge, we provide previously unreported evidence for PACAP and PAC1 expression upregulation during skin inflammation of different mechanisms and for its pro-inflammatory function in neurogenic edema formation.

Age-related decline of autocrine pituitary adenylate cyclase-activating polypeptide impairs angiogenic capacity of rat cerebromicrovascular endothelial cells

Aging impairs angiogenic capacity of cerebromicrovascular endothelial cells (CMVECs) promoting microvascular rarefaction, but the underlying mechanisms remain elusive. PACAP is an evolutionarily conserved neuropeptide secreted by endothelial cells and neurons, which confers important antiaging effects. To test the hypothesis that age-related changes in autocrine PACAP signaling contributes to dysregulation of endothelial angiogenic capacity, primary CMVECs were isolated from 3-month-old (young) and 24-month-old (aged) Fischer 344 x Brown Norway rats. In aged CMVECs, expression of PACAP was decreased, which was associated with impaired capacity to form capillary-like structures, impaired adhesiveness to collagen (assessed using electric cell-substrate impedance sensing [ECIS] technology), and increased apoptosis (caspase3 activity) when compared with young cells. Overexpression of PACAP in aged CMVECs resulted in increased formation of capillary-like structures, whereas it did not affect cell adhesion. Treatment with recombinant PACAP also significantly increased endothelial tube formation and inhibited apoptosis in aged CMVECs. In young CMVECs shRNA knockdown of autocrine PACAP expression significantly impaired tube formation capacity, mimicking the aging phenotype. Cellular and mitochondrial reactive oxygen species production (dihydroethidium and MitoSox fluorescence, respectively) were increased in aged CMVECs and were unaffected by PACAP. Collectively, PACAP exerts proangiogenic effects and age-related dysregulation of autocrine PACAP signaling may contribute to impaired angiogenic capacity of CMVECs in aging.

Pituitary adenylate cyclase-activating polypeptide (PACAP) induces relaxations of peripheral and cerebral arteries, which are differentially impaired by aging

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a well-known neuropeptide, which also has vasomotor effects. However, little is known regarding its age-related and organ-specific vasomotor effects. We hypothesized that the vasomotor effects of PACAP depend on the tissue origin of the vessels and aging substantially modulates its actions. Thus, carotid (CA) and basilar arteries (BA) were isolated from young (2 months old), middle age (12 months old), and old (30 months old) rats. Their vasomotor responses were measured with an isometric myograph (DMT610M) in response to cumulative concentrations of PACAP1-38 (10(-9)-10(-6) M). PACAP1-38 induced (1) significantly greater concentration-dependent relaxations in CA compared to that of BA of young, middle age, and old rats; (2) relaxations of CA significantly decreased, whereas they did not change substantially in BA, as a function of age; (3) sodium nitroprusside (SNP)-induced relaxation did not change after PACAP1-38 administration in any conditions; and (4) inhibition of PAC1 receptors by selective PAC1 receptor blocker (PACAP6-38) completely diminished the responses to PACAP in all age groups of BA and CA. In conclusion, these findings suggest that PACAP1-38 has greater vasomotor effect in CA than that in BA, whereas aging has less effect on PACAP-induced relaxation of cerebral arteries and BA than that in peripheral arteries and CA suggesting that the relaxation to PACAP is maintained in cerebral arteries even in old age.

Pituitary adenylate cyclase activating polypeptide: an important vascular regulator in human skin in vivo

Pituitary adenylate cyclase-activating peptide (PACAP) is an important neuropeptide and immunomodulator in various tissues. Although this peptide and its receptors (ie, VPAC1R, VPAC2R, and PAC1R) are expressed in human skin, their biological roles are unknown. Therefore, we tested whether PACAP regulates vascular responses in human skin in vivo. When injected intravenously, PACAP induced a significant, concentration-dependent vascular response (ie, flush, erythema, edema) and mediated a significant and concentration-dependent increase in intrarectal body temperature that peaked at 2.7°C. Topical application of PACAP induced marked concentration-dependent edema. Immunohistochemistry revealed a close association of PACAP-immunoreactive nerve fibers with mast cells and dermal blood vessels. VPAC1R was expressed by dermal endothelial cells, CD4+ and CD8+ T cells, mast cells, and keratinocytes, whereas VPAC2R was expressed only in keratinocytes. VPAC1R protein and mRNA were also detected in human dermal microvascular endothelial cells. The PACAP-induced change in cAMP production in these cells demonstrated VPAC1R to be functional. PACAP treatment of organ-cultured human skin strongly increased the number of CD31+ vessel cross-sections. Taken together, these results suggest that PACAP directly induces vascular responses that may be associated with neurogenic inflammation, indicating for the first time that PACAP may be a crucial vascular regulator in human skin in vivo. Antagonists to PACAP function may be beneficial for the treatment of inflammatory skin diseases with a neurogenic component.

Maxadilan, the PAC1 receptor, and leishmaniasis

Maxadilan is a vasodilator peptide isolated from sand fly salivary glands. The vasodilator effects of maxadilan are mediated by the PAC1 receptor, although maxadilan and PACAP do not share sequence homology. Sand flies are the vector of the parasitic disease leishmaniasis. The peptide aids the sand fly in obtaining a blood meal while enhancing the infectivity of leishmania parasites transmitted by this arthropod vector. Aspects of maxadilan, PAC1, and leishmaniasis are discussed.

Inhibitory effect of PACAP-38 on acute neurogenic and non-neurogenic inflammatory processes in the rat

Inhibitory actions of pituitary adenylate cyclase activating polypeptide (PACAP) have been described on cellular/vascular inflammatory components, but there are few data concerning its role in neurogenic inflammation. In this study we measured PACAP-like immunoreactivity with radioimmunoassay in the rat plasma and showed a two-fold elevation in response to systemic stimulation of capsaicin-sensitive sensory nerves by resiniferatoxin, but not after local excitation of cutaneous afferents. Neurogenic plasma extravasation in the plantar skin induced by intraplantar capsaicin or resiniferatoxin, as well as carrageenan-induced paw edema were significantly diminished by intraperitoneal PACAP-38. In summary, these results demonstrate that PACAP is released from activated capsaicin-sensitive afferents into the systemic circulation. It diminishes acute pure neurogenic and mixed-type inflammatory reactions via inhibiting pro-inflammatory mediator release and/or by acting at post-junctional targets on the vascular endothelium.

Pituitary adenylate cyclase-activating polypeptide stimulates renin secretion via activation of PAC1 receptors

Besides of its functional role in the nervous system, the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is involved in the regulation of cardiovascular function. Therefore, PACAP is a potent vasodilator in several vascular beds, including the renal vasculature. Because the kidney expresses both PACAP and PACAP-binding sites, it was speculated that PACAP might regulate cardiovascular function by direct vascular effects and indirectly by regulating renin release from the kidneys. PACAP (1-27) stimulated renin secretion from isolated perfused kidneys of rats 4.9-fold with a half-maximum concentration of 1.9 nmol/L. In addition, PACAP stimulated renin release and enhanced membrane capacitance of isolated juxtaglomerular cells, indicating a direct stimulation of exocytotic events. The effect of PACAP on renin release was mediated by the specific PACAP receptors (PAC1), because PACAP (1-27) applied in concentrations in the physiologic range (10 and 100 pmol/L) did not enhance renin release from isolated kidneys of PAC1 receptor knockout mice (PAC1-/-), whereas it stimulated renin release 1.38- and 2.5-fold in kidneys from wild-type mice. Moreover, plasma renin concentration was significantly lower in PAC1-/- compared with their wild-type littermates under control conditions as well as under a low- or high-salt diet and under treatment with the angiotensin-converting enzyme inhibitor ramipril, whereas no differences in plasma renin concentration between the genotypes were detectable after water deprivation. These data show that PACAP acting on PAC1 receptors potently stimulates renin release, serving as a tonic enhancer of the renin system in vivo.

Paradoxical antagonism of PACAP receptor signaling by VIP in Xenopus oocytes via the type-C natriuretic peptide receptor

Atrial natriuretic peptide (ANP) and the closely-related peptides BNP and CNP are highly conserved cardiovascular hormones. They bind to single transmembrane-spanning receptors, triggering receptor-intrinsic guanylyl cyclase activity. The "truncated" type-C natriuretic peptide receptor (NPR-C) has long been called a clearance receptor because it lacks the intracellular guanylyl cyclase domain, though data suggest it might negatively couple to adenylyl cyclase via G(i). Here we report the molecular cloning and characterization of the Xenopus laevis type-C natriuretic peptide receptor (XNPR-C). Analysis confirms the presence of a short intracellular C-terminus, as well as a high similarity to fish and mammalian NPR-C. Injection of XNPR-C mRNA into Xenopus oocytes resulted in expression of high affinity [(125)I]ANP binding sites that were competitively and completely displaced by natriuretic analogs and the unrelated neuropeptide vasoactive intestinal peptide (VIP). Measurement of cAMP levels in mRNA-injected oocytes revealed that XNPR-C is negatively coupled to adenylyl cyclase in a pertussis toxin-sensitive manner. When XNPR-C was co-expressed with PAC(1) receptors for pituitary adenylyl cyclase-activating polypeptide (PACAP), VIP and natriuretic peptides counteracted the cAMP induction by PACAP. These results suggest that VIP and natriuretic peptides can potentially modulate the action of PACAP in cells where these receptors are co-expressed.

VPAC Receptor Modulation of Neuroexcitability in Intracardiac Neurons

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) have been found within mammalian intracardiac ganglia, but the cellular effects of these neuropeptides remain poorly understood. Fluorometric calcium imaging and whole cell patch clamp recordings were used to examine the effects of PACAP and VIP on [Ca2+]i and neuroexcitability, respectively, in intracardiac neurons of neonatal rats. PACAP and VIP evoked rapid increases in [Ca2+]i that exhibited both transient and sustained components. Pharmacological experiments using PAC1 and VPAC receptor-selective antagonists demonstrated that the elevations in [Ca2+]i result from the activation of VPAC receptors. The transient increases in [Ca2+]i were shown to be the product of Ca2+ mobilization from caffeine/ryanodine-sensitive intracellular stores and were not due to inositol 1,4,5-trisphosphate-mediated calcium release. In contrast, the sustained [Ca2+]i elevations were dependent on extracellular Ca2+ and were blocked by the transient receptor channel antagonist, 2-aminoethoxydiphenyl borate, which suggests that they are due to Ca2+ entry via store-operated channels. In addition to elevating [Ca2+]i, both PACAP and VIP depolarized intracardiac neurons, and PACAP was further shown to augment action potential firing in these cells. Depolarization of intracardiac neurons by the neuropeptides was dependent on activation of VPAC receptors and the concomitant increases in [Ca2+]i. Although activation of PAC1 receptors alone had no direct effects on neuroexcitability, PAC1 receptor stimulation potentiated the VPAC receptor-induced depolarizations. Furthermore, enhanced action potential firing was only observed upon concurrent stimulation of PAC1 and VPAC receptors, which indicates that these receptors act synergistically to enhance neuroexcitability in intracardiac neurons.

Effects of pretreatment with PACAP on the infarct size and functional outcome in rat permanent focal cerebral ischemia

PACAP exerts neuroprotective effects under various neurotoxic conditions in vitro. In vivo, it reduces brain damage after global and transient focal ischemia. The present study investigated whether PACAP has neuroprotective effects when applied before the onset of permanent ischemia. Rats were given bolus injections of PACAP38 intracerebroventricularly, and then underwent permanent middle cerebral artery occlusion. The results show that 2 microg of PACAP significantly reduced the infarct size measured 12 and 24h after the onset of ischemia. No further reduction was obtained by a 7-day pretreatment. PACAP also ameliorated certain sensorimotor deficits. Our present study provides further evidence for the neuroprotective effects of PACAP, and implies that it might be a promising preventive therapeutic agent in ameliorating ischemic brain damage.

Heterogeneity of pituitary adenylate cyclase-activating polypeptide and vasoactive intestinal polypeptide receptors in rat intrinsic cardiac neurons

The expression of receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) was investigated in isolated parasympathetic neurons of neonatal rat intracardiac ganglia using single-cell reverse transcription-polymerase chain reaction. Individual neurons were shown to express multiple isoforms of the PACAP receptor, PAC1, including PAC1-short, -HOP1 and -HOP2 variants, which differ in the region encoding the G protein-binding domain. The PAC1-HOP1 isoform was the predominant species, being expressed at higher levels and in a greater number of cells than other PAC1 variants. In addition to PAC1, intrinsic cardiac neurons express transcripts for the VIP receptors, VPAC1 and VPAC2, with VPAC2 being found in a greater proportion of the neurons. These findings may explain the complex effects of PACAP and VIP on neuroexcitability in mammalian intracardiac ganglia.

The vasorelaxant effect of pituitary adenylate cyclase activating polypeptide and vasoactive intestinal polypeptide in isolated rat basilar arteries is partially mediated by activation of nitrergic neurons

The structurally related neuropeptides pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are recognised by two G protein-coupled receptors, termed VPAC(1)-R and VPAC(2)-R, with equal affinity. PACAP and VIP have previously been shown to relax cerebral arteries in an endothelium-independent manner. The aim of the present study was to test if intramural neurons are involved in the mediation of PACAP/VIP-induced vasodilatory responses. Therefore, the vascular tone of isolated rat basilar arteries was measured by means of a myograph. The vasorelaxing effect of PACAP was assessed in arteries precontracted by serotonin in the absence or presence of different test compounds known to selectively inhibit certain signaling proteins. The vasorelaxant effect of PACAP could be significantly reduced by the inhibitor of neuronal N-type calcium channels omega-conotoxin GVIA (omega-CgTx), as well as by 3-bromo-7-nitroindazole (3Br-7-Ni), an inhibitor of the neuronal nitric oxide-synthase (nNOS). The localization of N-type calcium channels and VPAC-Rs within the rat basilar artery was investigated by confocal laser scanning microscopy using omega-CgTx- and VIP-analogs labelled with fluorescent dyes. These findings suggest that activation of intramural neurons may represent an important effector mechanism for mediation of the vasorelaxant PACAP-response.

Characterization of the non-nitrergic NANC relaxation responses in the rabbit vaginal wall

Electrical field stimulation (EFS)-induced non-adrenergic non-cholinergic (NANC) relaxation responses in the rabbit vaginal wall were investigated. These NANC responses were partially inhibited with the nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME; 500 microM), N(G)-nitro-L-arginine (300 microM) or N-iminoethyl-L-ornithine (500 microM) or the selective soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 10 microM). Application of L-NAME and ODQ concomitantly did not increase the degree of inhibition. L-NAME or ODQ were observed to be more effective at low frequencies. The resistant part of the responses was more pronounced at higher frequencies and was completely inhibited by tetrodotoxin (1 microM). Exogenous application of the peptides vasoactive intestinal peptide (VIP), pituitary adenylate cyclase activating peptide (PACAP-27 and PACAP-38), peptide histidine methionine (PHM), peptide histidine valine (PHV), helospectin-I or -II induced a relaxation response. Calcitonin gene-related peptide or substance P did not cause any relaxation. The peptidase alpha-chymotrypsin (type II; 2 units ml(-1)) did not affect non-nitrergic NANC responses, although it did inhibit relaxation responses elicited by exogenous VIP, PACAP-27, PACAP-38, PHM, PHV, helospectin-I or -II. K(+) channel inhibitors apamin (1 microM) or charybdotoxin (100 nM) when used alone or in conjunction did not affect non-nitrergic NANC responses. The non-nitrergic NANC responses were not associated with any increase in intracellular cyclic adenosine-3', 5'-monophosphate (cyclic AMP) or cyclic guanosine-3', 5'-monophosphate (cyclic GMP) concentrations. The peptide-induced relaxations were all associated with increases in cyclic AMP concentrations. These results suggest that a neuronal factor elicits non-nitrergic NANC responses in the rabbit vaginal wall. The identity of this factor remains to be established.

The enhancement of endogenous cAMP with pituitary adenylate cyclase-activating polypeptide protects rat kidney against ischemia through the modulation of inflammatory response

BACKGROUND

Cyclic nucleotide analogue administration improves ischemia-reperfusion damage in several organs. The neuropeptide pituitary adenylate cyclase-activating polypeptide, PACAP-38, is a potent stimulus to enhance cellular cAMP levels. This study tested the protective effect of enhancing endogenous cAMP levels by PACAP-38 in a model of warm renal ischemia.

METHODS

Sprague-Dawley rats underwent 40 min of bilateral warm renal ischemia. PACAP-38 continuous infusion began either before ischemia or at 6 hr or 18 hr after ischemia. A mini-osmotic pump infused PACAP-38 throughout 7 days of follow-up. Groups were constructed with sham, ischemic control, and dibutyryl cAMP treated animals, and four PACAP-38 treatment groups, using 16 pmol/hr or 160 pmol/hr of the compound, or delaying its administration by 6 hr or 18 hr after ischemia. Renal function was assessed by means of serum creatinine levels on days 1, 2, 3, and 7 after ischemia. Conventional histology was performed on day 7. Renal myeloperoxidase (MPO) activity, infiltrating CD45+ cells, plasma and tissue cAMP, and serum IL-6 were measured.

RESULTS

Continuous administration of the high concentration of PACAP-38 ameliorated renal function and morphologic abnormalities induced by warm ischemia. Treatment with dibutyryl cAMP produced morphologic protection but only partial functional effect on the ischemic kidney. A 6-hour delay in the administration of the compound after ischemia offered similar protective effect, whereas an 18-hr delay did not. The neuropeptide clearly increased circulating cAMP after ischemia but not cAMP in renal tissue. PACAP-38 increased circulating IL-6, and minimized renal inflammatory cell infiltration induced by ischemia-reperfusion injury, as evidenced by a reduction of MPO activity and the number of CD45+ cells in ischemic renal tissue.

CONCLUSIONS

Enhancement of endogenous circulating cAMP with PACAP-38 modulates postischemic inflammatory response and strongly protects from ischemic acute renal failure, even when administration is delayed for 6 hr after injury.

L-type Ca(2+) channel regulation by pituitary adenylate cyclase-activating polypeptide in vascular myocytes from spontaneously hypertensive rats

Pituitary adenylate cyclase-activating polypeptide (PACAP), a vasoactive peptide, modulates the L-type Ca(2+) channel current (L channel current) in vascular smooth muscle cells (VSMC) through activation and integration of two intracellular pathways, protein kinase A and protein kinase C (PKC). In the present study we compared the effects of PACAP on the L channel current in VSMC from the spontaneously hypertensive rats (SHR) and normotensive controls, Wistar Kyoto rats (WKY). We found that compared with WKY, VSMC from SHR had a higher L channel current density. Stimulation by PACAP (10 nM) caused an increase in the amplitude of the whole cell current and prolonged open time in VSMC from SHR and WKY, with the increase greater in SHR. These effects of PACAP on the L channel current was mimicked by an activator of PKC. In contrast, PACAP caused a smaller increase in cAMP accumulation in VSMC from SHR than WKY, and there was no difference in the inhibitory effect of 8-bromo-cAMP on the L channel current from both type of cells. The greater increase in amplitude of the L channel current by PACAP in VSMC from SHR persisted in the presence of adenosine cyclic 3',5'-monophosphothioate, Rp-isomer, a cAMP antagonist, but not calphostin C, a PKC inhibitor. Taken together, our results show an increase in L channel current density and an enhanced PACAP effect on the L channel current in VSMC from SHR compared with WKY. This difference in PACAP response appears to be predominately secondary to an increased PKC sensitivity.

The origin and function of the pituitary adenylate cyclase-activating polypeptide (PACAP)/glucagon superfamily

The pituitary adenylate cyclase-activating polypeptide (PACAP)/ glucagon superfamily includes nine hormones in humans that are related by structure, distribution (especially the brain and gut), function (often by activation of cAMP), and receptors (a subset of seven-transmembrane receptors). The nine hormones include glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, glucose-dependent insulinotropic polypeptide (GIP), GH-releasing hormone (GRF), peptide histidine-methionine (PHM), PACAP, secretin, and vasoactive intestinal polypeptide (VIP). The origin of the ancestral superfamily members is at least as old as the invertebrates; the most ancient and tightly conserved members are PACAP and glucagon. Evidence to date suggests the superfamily began with a gene or exon duplication and then continued to diverge with some gene duplications in vertebrates. The function of PACAP is considered in detail because it is newly (1989) discovered; it is tightly conserved (96% over 700 million years); and it is probably the ancestral molecule. The diverse functions of PACAP include regulation of proliferation, differentiation, and apoptosis in some cell populations. In addition, PACAP regulates metabolism and the cardiovascular, endocrine, and immune systems, although the physiological event(s) that coordinates PACAP responses remains to be identified.

Tachyphylaxis to PACAP-27 after inhibition of NO synthesis: a loss of adenylate cyclase activation

The vasodilator effects of pituitary adenylate cyclase activating polypeptide (PACAP-27) are subject to tachyphylaxis in rats treated with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). This study examined whether this tachyphylaxis is due to the loss of vasodilator potency of cAMP generated by activation of the G(s) protein-coupled PACAP receptors. Five successive treatments with PACAP-27 (2 nmol/kg iv) produced pronounced vasodilator responses in saline-treated rats that were not subject to tachyphylaxis. The first injection of PACAP-27 (2 nmol/kg iv) in L-NAME (50 micromol/kg iv)-treated rats produced vasodilator responses of similar magnitude to those in saline-treated rats, whereas four subsequent injections produced progressively and markedly smaller responses. The hemodynamic effects of the membrane-permeable cAMP analog 8-(4-chlorophenylthiol)-cAMP (8-CPT-cAMP; 5-15 micromol/kg iv) were similar in L-NAME-treated rats and in L-NAME-treated rats that had received the five injections of PACAP-27. In addition, five injections of 8-CPT-cAMP (10 micromol/kg iv) produced pronounced vasodilator responses in saline- and L-NAME-treated rats that were not subject to the development of tachyphylaxis. These results suggest that a loss of biological potency of cAMP is not responsible for tachyphylaxis to PACAP-27 in L-NAME-treated rats. This tachyphylaxis may be due to the inability of the G(s) protein-coupled PACAP receptor to activate adenylate cyclase.

Mast cells contribute to PACAP-induced dermal oedema in mice

In the present study the effect of intradermal PACAP-injection on dermal oedema in mice was investigated and the contribution of mast cells to this response was assessed. The injection of PACAP 1-38 into the ears of C57BL/6 mice evoked a dose-dependent response, which, after higher doses of PACAP 1-38, lasted at least 24 h. Histological examination showed significant mast cell degranulation induced by PACAP. Using mast cell-deficient WBB6F1-Kit(W)/Kit(W-v) mice and the congenic mice, we demonstrated that the the early phase (30 min to 6 h) of PACAP-induced ear swelling response was significantly diminished in mast cell-deficient mice, suggesting that mast cell degranulation contributes to this phase of the response. When mast cell-deficient WBB6F1-Kit(W)/Kit(W-v) mice were locally and selectively reconstituted by adoptive mast cell transfer, the dermal oedema was almost equal to that of control animals in the early phase of PACAP injection. These results show that mast cell degranulation contributes to PACAP-induced dermal oedema in mice.

Rapid tachyphylaxis to hemodynamic effects of PACAP-27 after inhibition of nitric oxide synthesis

The vasodilator effects of pituitary adenylate cyclase-activating polypeptide (PACAP)-27 are subject to tachyphylaxis in rats treated with the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME). We examined whether this tachyphylaxis could be prevented by administration of the putative endothelium-derived nitrosyl factor S-nitroso-L-cysteine (L-SNC) and whether L-SNC may exert its effects via increases in cGMP levels in vascular smooth muscle. Five doses of PACAP-27 (2 nmol/kg iv) produced pronounced vasodilator responses in saline-treated rats. These responses were not subject to tachyphylaxis. The first injection of PACAP-27 (2 nmol/kg iv) in L-NAME-treated (50 micromol/kg iv) rats produced vasodilator responses similar to those in saline-treated rats, whereas subsequent injections produced progressively smaller responses. The injection of L-SNC (1,200 nmol/kg iv) before each injection of PACAP-27 prevented tachyphylaxis to the Gs protein-coupled receptor agonist in L-NAME-treated rats, whereas equihypotensive doses of the NO donor sodium nitroprusside (100 micrograms/kg iv) did not. The injection of the membrane-permeant cGMP analog 8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphate (8-CPT-cGMP; 30 micromol/kg iv) to L-NAME-treated rats restored resting hemodynamic values to pre-L-NAME levels but did not prevent the development of tachyphylaxis to PACAP-27. These results suggest that nitrosyl factors prevent the development of tachyphylaxis to the hemodynamic actions of PACAP-27. These nitrosyl factors may act independently of their ability to generate cGMP in vascular smooth muscle.

Effects of centrally administered pituitary adenylate cyclase-activating polypeptide on c-fos gene expression and heteronuclear RNA for vasopressin in rat paraventricular and supraoptic nuclei

The effects of intracerebroventricular (i.c.v.) administration of pituitary adenylate cyclase-activating polypeptide (PACAP) on the expression of c-fos gene as well as heteronuclear (hn) RNA for arginine vasopressin (AVP) in paraventricular (PVN) and supraoptic nuclei (SON) of rats were investigated by immunohistochemistry for c-fos protein (Fos) and in situ hybridization histochemistry for c-fos mRNA and AVP hnRNA. The i.c.v. administration of PACAP (200 pmol/rat) caused a marked induction of Fos-like immunoreactivity (LI) in PVN and SON. The nuclear Fos-LI existed in AVP-LI containing cells in the PVN and SON. The expression of the c-fos gene in the PVN and SON was increased in a dose-related manner 30 min after i.c. v. administration of PACAP. PACAP-induced expression of the c-fos gene was significantly reduced by pretreatment with a PACAP receptor antagonist, PACAP-(6-38)-NH2. In addition, Fos-LI and the expression of the c-fos gene were also observed in the periventricular region of the third ventricle after i.c.v. administration of PACAP. The induction of c-fos gene expression in the PVN and SON reached a maximum 30 min after PACAP administration. The expression of c-fos gene in the PVN and SON induced by i.c.v. administration of vasoactive intestinal peptide (200 pmol/rat) was weaker than that induced by PACAP. The hnRNA for AVP in the PVN and SON was significantly increased 30 min after i.c.v. administration of PACAP (200 pmol/rat). Our results suggest that PACAP activates PVN and SON neurons via PACAP receptors and, in parallel, transcription of the AVP gene in the PVN and SON.

Hemodynamic actions of systemically injected pituitary adenylate cyclase activating polypeptide-27 in the rat

The aims of this study were (1) to characterize the hemodynamic mechanisms underlying the hypotensive effects of pituitary adenylate cyclase activating polypeptide-27 (PACAP-27 0.1-2.0 nmol/kg, i.v.) in pentobarbital-anesthetized rats, and (2) to determine the roles of the autonomic nervous system, adrenal catecholamines and endothelium-derived nitric oxide (NO) in the expression of PACAP-27-mediated effects on hemodynamic function. PACAP-27 produced dose-dependent decreases in mean arterial blood pressure and hindquarter and mesenteric vascular resistances in saline-treated rats. PACAP-27 also produced pronounced falls in mean arterial blood pressure in rats treated with the ganglion blocker, chlorisondamine (5 mg/kg, i.v.). The hypotensive and vasodilator actions of PACAP-27 were not attenuated by the beta-adrenoceptor antagonist, propranolol (1 mg/kg, i.v.), or the NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME 50 micromol/kg, i.v.). PACAP-27 produced dose-dependent increases in heart rate whereas the hypotensive response produced by the nitrovasodilator, sodium nitroprusside (10 microg/kg, i.v.), was associated with a minimal tachycardia. The PACAP-27-induced tachycardia was unaffected by chlorisondamine, but was virtually abolished by propranolol. These results suggest that the vasodilator effects of PACAP-27 are due to actions in the microcirculation rather than to the release of adrenal catecholamines and that this vasodilation may not involve the release of endothelium-derived NO. These results also suggest that PACAP-27 produces tachycardia by directly releasing norepinephrine from cardiac sympathetic nerve terminals rather than by direct or baroreceptor reflex-mediated increases in sympathetic nerve activity.

Pituitary adenylate cyclase-activating polypeptide expression and modulation of neuronal excitability in guinea pig cardiac ganglia

Cardiac output is regulated by the coordinate interactions of stimulatory sympathetic and inhibitory parasympathetic signals. Intracardiac parasympathetic ganglia are integrative centers of cardiac regulation, and modulation of the parasympathetic drive on the heart is accomplished by altering intrinsic cardiac ganglion neuron excitability. The pituitary adenylate cyclase-activating polypeptide (PACAP)/vasoactive intestinal peptide (VIP) family of peptides modulates cardiac function, and in guinea pig heart, PACAP appears to act directly on intrinsic parasympathetic cardiac ganglia neurons through PACAP-selective receptors. A multidisciplinary project tested whether cardiac PACAP peptides act through PACAP-selective receptors as excitatory neuromodulators amplifying the parasympathetic inhibition from guinea pig cardiac ganglia. The in vivo sources of regulatory PACAP peptides were localized immunocytochemically to neuronal fibers and a subpopulation of intrinsic postganglionic cardiac neurons. RT-PCR confirmed that cardiac ganglia expressed proPACAP transcripts and have PACAP peptide biosynthetic capabilities. Messenger RNA encoding PACAP-selective PAC1 receptor isoforms were also present in cardiac ganglia. Alternative splicing of PAC1 receptor transcripts produced predominant expression of the very short variant with neither HIP nor HOP cassettes; lower levels of the PAC1HOP2 receptor mRNA were present. Almost all of the parasympathetic neurons expressed membrane-associated PAC1 receptor proteins, localized immunocytochemically, which correlated with the population of cells that responded physiologically to PACAP peptides. PACAP depolarized cardiac ganglia neurons and increased neuronal membrane excitability. The rank order of peptide potency on membrane excitability in response to depolarizing currents was PACAP27>PACAP38>VIP. The PACAP-induced increase in excitability was not a function of membrane depolarization nor was it caused by alterations in action potential configuration. These results support roles for PACAP peptides as integrative modulators amplifying, through PACAP-selective receptors, the parasympathetic cardiac ganglia inhibition of cardiac output.

Rat aortic smooth-muscle cell proliferation is bidirectionally regulated in a cell cycle-dependent manner via PACAP/VIP type 2 receptor

In the cardiovascular system, vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase activating polypeptide (PACAP) have been well characterized as potent vasodepressors or vasodilators. However, their pathophysiological implication in proliferation of vascular smooth muscle cells has not yet been elucidated. In the present study, we have first identified PACAP/VIP type 2 receptor as a dominant type in rat vascular smooth muscle cell (VSMC) by RT-PCR. PACAP and VIP increased cyclic AMP accumulation with similar potency. In 24-h [3H]thymidine incorporation assay, PACAP or VIP exhibited a suppressive effect on the DNA synthesis of rat VSMC stimulated by serum when added at the late G1 phase. In contrast, when added at G0/G1 phase of the cell cycle, PACAP or VIP enhanced the serum-induced DNA synthesis. In 24-h incubation, PACAP alone has little mitogenic activity. However, when incubated up to 48 h, PACAP stimulated significantly the DNA synthesis and the cell proliferation of rat VSMC. These results suggest that PACAP and VIP regulate the proliferation of rat VSMC by enhancing or suppressing in a cell cycle-dependent manner and induce delayed mitogenesis and cell proliferation.

Pituitary adenylate cyclase activating peptides relax human pulmonary arteries by opening of KATP and KCa channels

BACKGROUND

Pituitary adenylate cyclase activating peptides (PACAPs) are potent endothelium independent dilators of human coronary arteries; however, their effects on human pulmonary arteries are unknown.

METHODS

The vasorelaxant effects of PACAP27 on human pulmonary segmental arteries were studied and the specific potassium (K+) channel regulatory mechanisms in the vasorelaxant effects were tested by means of isometric contraction experiments.

RESULTS

PACAP27 produced dose dependent relaxations of 10 microM rings preconstricted with prostaglandin F2 alpha (PGF2 alpha) with half maximal relaxation (IC50) at 17 nM. Pretreatment of the vessels with the ATP sensitive K+ (KATP) channel blocker glibenclamide (1 microM) or with the Ca2+ activated K+ (KCa) channel blocker iberiotoxin (100 nM) inhibited the PACAP27 induced relaxation.

CONCLUSIONS

These results provide evidence that PACAPs are potent vasodilators of human pulmonary arteries and that this relaxation might be mediated by opening of KATP and KCa channels.

The augmentation of pituitary adenylate cyclase-activating polypeptide (PACAP) in streptozotocin-induced diabetic rats

Pituitary adenylate cyclase activating polypeptide (PACAP), which was isolated from ovine hypothalamic extract, has been shown to have a physiological role in the regulation of insulin or islet functions. In streptozotocin (STZ)-induced diabetic rats, we examined the content of PACAP immunoreactivity and gene expression of three specific receptors. Four weeks after administration of STZ (50 mg/kg), plasma glucose levels increased 3.3-fold, and plasma insulin levels decreased to one-tenth as compared with the control. The content of PACAP immunoreactivity in the pancreas potently increased by 30%, but the content of vasoactive intestinal polypeptide (VIP) immunoreactivity was not changed. In the other tissues, the content of PACAP immunoreactivity did not significantly change except in the hypothalamus, which showed a 10% increment. In the expression level of PACAP/VIP receptors, semi-quantitative RT-PCR analysis revealed that VIP1/PACAP receptor mRNA significantly increased as compared with the other two types of receptors in the pancreas of STZ-induced diabetic rats. These findings suggest that PACAP and VIP1/PACAP receptor might be involved in the pathophysiology of diabetes mellitus.

PACAP-induced plasma extravasation in rat skin

The effects of pituitary adenylate cyclase activating peptide (PACAP) 38, PACAP 27 and vasoactive intestinal peptide (VIP) on plasma extravasation were investigated in vivo in rat skin. PACAP 38, PACAP 27 and VIP, caused concentration-dependent extravasation in rat skin. The order of potency was PACAP 38 > PACAP 27 = VIP, whereas the order of maximal induced extravasation was PACAP 38 = PACAP 27 > VIP, suggesting that PACAP 38 might be the most powerful inducer of plasma extravasation of the three tested members of the secretin-glucagon-VIP family. Substance P (SP) was about 5 times more potent than PACAP 38 and 15 times more potent than PACAP 27. These data indicate that PACAP 38 induced plasma extravasation in concentrations roughly equimolar to SP. Pyrilamine (H1 receptor antagonist) reduced the PACAP 38-induced plasma extravasation more than 50%; cimetidine (H2 receptor antagonist) was without effect. To investigate whether a cAMP-mediated process is involved in the induction of plasma extravasation, the synthetic adenosine 3',5'-cyclic monophosphate (cAMP), dibutyryl adenosine cyclic monophosphate (DBcAMP) and the cAMP-inducing drug, salbutamol, were each injected in the skin; neither of these drugs caused extravasation. We conclude that PACAP 38 and PACAP 27 cause potent plasma extravasation which, at least in part, involves histamine release.

Heterologous desensitization of beta-adrenoceptor signal transduction in vivo

The hemodynamic actions of pituitary adenylate cyclase-activating polypeptide (PACAP-27) rapidly diminish upon repeated i.v. injection in rats treated with the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME). We now report that the beta-adrenoceptor agonist isoproterenol (0.5 microg/kg, i.v.) produces pronounced hypotensive and vasodilator effects in anesthetized rats pretreated with L-NAME (100 micromol/kg, i.v.). However, the hypotensive and vasodilator actions of isoproterenol were markedly diminished in L-NAME-treated rats in which tachyphylaxis to PACAP was induced immediately prior to the injection of the beta-adrenoceptor agonist. This suggests that a reduction in tissue concentrations of nitric oxide-containing factors allows tachyphylaxis to PACAP-27-mediated vasodilation to occur in vivo and that this process leads to the heterologous desensitization of beta-adrenoceptor-mediated responses.

Inhibitory effects of pituitary adenylate cyclase activating polypeptide on histamine-induced respiratory resistance in anesthetized guinea pigs

Asthma is a chronic inflammatory disorder of the airways in which many cells participate. This inflammation causes recurrent episodes and symptoms that are associated with widespread but variable airflow limitation that is at least partly reversible either spontaneously or with treatment. Therefore, an investigation of useful remedies for the treatment of bronchial asthma is proposed. In this study, we determined whether both forms of pituitary adenylate cyclase activating polypeptide (PACAP 38 and PACAP 27) belonging to the vasoactive intestinal peptide (VIP) family of peptides could inhibit the effects of histamine-induced respiratory resistance (Rr) in anesthetized guinea pigs, when compared with VIP. The order for 50% suppression (ED50) of Rr induced by peptides was VIP > PACAP 27 > PACAP 38. The inhibitory effects induced by PACAP 38 on histamine-induced Rr in guinea pigs were more prolonged than with the other two peptides. Moreover, adding the endopeptidase inhibitor phosphoramidon prolonged the inhibitory effects of PACAPs. These results suggested that the exogenous peptides of the inhibitory nonadrenergic noncholinergic nervous (i-NANC) peptides could become a useful remedy for treatment of bronchial asthma, because these belong to an important intrinsic hormone.

Genomic organization of the rat pituitary adenylate cyclase-activating polypeptide receptor gene. Alternative splicing within the 5'-untranslated region

Pituitary adenylate cyclase-activating polypeptide (PACAP) elicits its diverse biological actions by interacting with both PACAP-selective type I PACAP receptors (PACAPRs) and type II PACAPRs that do not distinguish between PACAP and vasoactive intestinal polypeptide. Using long distance polymerase chain reaction, we amplified and characterized the entire coding region of the rat type I PACAPR (rPACAPR) gene, which spans 40 kilobases and contains 15 exons. Mapping of the exons and sequencing of all intron-exon boundaries revealed a structural organization of the rPACAPR gene that is very similar to those encoding other members of the calcitonin/secretin/parathyroid hormone receptor family. Southern blot analysis demonstrated a single copy of the rPACAPR gene. A combination of rapid amplification of cDNA ends and reverse transcriptase polymerase chain reaction revealed an unexpected diversity in the rPACAPR mRNA in the 5'-untranslated (5'-UTR) region. Four rPACAPR cDNAs were identified with 5'-UTR sequences that all diverged from the genomic sequence at a site 76 bp upstream of the ATG start codon, where a consensus 3' slice acceptor sequence was located. Sequence analysis of these amplified transcripts demonstrated that they arise by tissue-specific differential usage of four exons in the 5' noncoding region of the rPACAPR gene. This study is the first to elucidate the structural organization of a PACAPR gene and to demonstrate that alternative splicing generates rPACAPR transcripts with unique 5'-UTRs.

Evidence for a possible synergism between pituitary adenylate cyclase activating polypeptide and calcitonin gene-related peptide in porcine ophthalmic artery

The activities of pituitary adenylate cyclase activating polypeptide (PACAP) and calcitonin gene-related peptide (CGRP) were investigated in isolated segments of porcine ophthalmic artery. In artery segments pre-contracted by prostaglandin F2alpha, PACAP induced a concentration dependent relaxation, but was clearly less potent than CGRP. When 2 x 10(-10) M CGRP (relaxation 4.3 +/- 0.8%, n = 11) and 10(-8) M PACAP (relaxation 12.4 +/- 3.8%, n = 10) were added together, the subsequent relaxation was substantially increased (33.6 +/- 5.6%, p<0.0005). In addition, the rate of relaxation was increased. The results indicate that there is synergism between low concentrations of CGRP and PACAP in isolated porcine ophthalmic artery.

The induction of nitric oxide-mediated relaxation of human isolated pulmonary arteries by PACAP

1. The effects of pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP) were analysed in human isolated circular segments of pulmonary arteries. Guinea-pig pulmonary arteries were used for comparison. The responses obtained were analysed in relation to the vascular endothelium and the nitric oxide (NO) synthase inhibitor NG-monomethyl L-arginine (L-NMMA). 2. PACAP and VIP induced concentration-dependent relaxations of precontracted pulmonary arteries. The maximal dilator response (Imax, %) and the potency (pEC50 value) were the same for both peptides, and there were no differences in the effects obtained on human and guinea-pig segments. PACAP and VIP were both more potent that acetylcholine (ACh). 3. Removal of the vascular endothelium abolished the PACAP induced dilator response in pulmonary arteries from both species. The VIP induced dilatation was unaffected, whereas the response to ACh was abolished. L-NMMA given before PACAP inhibited the dilatation. Furthermore, L-NMMA also reversed the dilatation already induced by PACAP and excess concentrations of L-arginine restored the dilator response of the L-NMMA treated arteries. 4. PACAP is a potent dilator of human pulmonary arteries. Although the dilator effect seems to be similar in amplitude to the one induced by VIP, the present results suggest differences in the underlying mechanisms of action (endothelium-dependency) between the two peptides.

PACAP-38 activates parasympathetic nerves in isolated, blood-perfused dog atria

A pituitary adenylate cyclase-activating polypeptide (PACAP) activates PACAP and vasoactive intestinal peptide (VIP) receptors. We investigated the effects of PACAP-38 on the sinus rate and atrial contractile force in isolated, blood-perfused dog heart preparations and the stimulation by PACAP-38 of the parasympathetic nerve fibers. PACAP-38 (3-1000 pmol) caused positive and/or negative chronotropic responses and it dose dependently increased atrial and ventricular contractile force. The positive cardiac responses to PACAP-38 unlike those to VIP were much less than the positive responses to norepinephrine. Atropine inhibited the negative chronotropic responses to PACAP-38 and augmented the positive chronotropic and inotropic responses. Physostigmine potentiated the negative cardiac responses to PACAP-38 and acetylcholine. After physostigmine treatment, additionally, tetrodotoxin blocked the negative cardiac responses to PACAP-38 and intracardiac parasympathetic nerve stimulation. Propranolol did not inhibit the positive cardiac responses to PACAP-38 in atropine-treated atria. PACAP-(6-38) (1 and 3 nmol), an antagonist of PACAP-38, did not affect the cardiac responses to 100 pmol of PACAP-38. These results suggest that (1) PACAP-38 directly increases sinus rate and atrial contractile force and (2) PACAP-38 activates parasympathetic nerves and causes negative chronotropic and inotropic responses in the dog heart.

Distribution of two VIP-related peptides, helospectin and pituitary adenylate cyclase activating peptide (PACAP), in the human upper respiratory system

Helospectin (HS) and pituitary adenylate cyclase activating peptide (PACAP) are newly discovered peptides isolated from the salivary gland venom of the lizard Heloderma horridum and the ovine hypothalamus, respectively. They show chemical similarities to vasoactive intestinal polypeptide (VIP), appear to have similar functions and are present in gut, brain, lung, male and female genitourinary tract. In the present study, the distribution of the helospectin and PACAP-27 in the human upper respiratory system was investigated using indirect immunofluorescence and electron-microscopical ABC-pre-embedding methods. Immunohistochemistry revealed helospectin-like (HS-LI) and PACAP-like (PACAP-LI) immunoreactivity in nerve fibers in human nasal, the larynx (vocal cord, ventricular fold, epiglottis), the tongue and the soft palate mucosa. Helospectin-LI and PACAP-LI containing nerve fibers were mainly found in close association to blood vessels and glandular structures. Colocalization studies carried out by application of double immunofluorescence showed that HS and/(or) PACAP-LI coexist with VIP in apparently the same nerve fibers in the upper respiratory system, although single nerve fibers seem to exclusively express helospectin. The localization patterns of helospectin and PACAP-LI in the human upper respiratory system suggests their possible involvement in the regulation of secretory activities and local blood flow.

Pituitary adenylate cyclase activating polypeptide (PACAP): occurrence and vasodilatory effect in the human uteroplacental unit

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuropeptide which was originally isolated from ovine hypothalamus. PACAP exists in at least two biologically active forms, PACAP-38 and PACAP-27. The aim of this study was to establish the distribution, localization and smooth muscle effects of PACAP-38 and PACAP-27 in the human uteroplacental unit. For this purpose we used radioimmunoassay, immunocytochemistry and in vitro studies of the effect of the peptides on smooth muscle activity.

RESULTS

By radioimmunoassay both peptides were detected throughout the uteroplacental unit. The concentrations of PACAP-27 were in general low, ranging from 1/6-1/25 of the corresponding PACAP-38 concentrations. PACAP-immunoreactivity was localized in nerve fibres of the lower segment of the pregnant uterus, but the number of PACAP-immunoreactive nerves was very clearly reduced compared to the corresponding isthmic region of non-pregnant myometrial tissue. PACAP-immunoreactive fibres were not observed in placenta or in the umbilical cord. Both PACAP-38 and PACAP-27 caused a concentration-dependent relaxation on stem villous arteries and on the intramyometrial arteries. Neither of the peptides displayed any effect on non-vascular smooth muscle specimens from the term pregnant myometrium. In conclusion the findings suggest a vasoregulator role of PACAP in the human uteroplacental unit.

Comparative effects of PACAP and VIP on pancreatic endocrine secretions and vascular resistance in rat

1. The effects of pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal peptide (VIP) and secretin on pancreatic endocrine secretions and vascular resistance were investigated and compared in the isolated perfused pancreas of the rat. The PACAP/VIP receptor types involved have been characterized. 2. On insulin secretion, in the range 10(-11) to 10(-8) M, PACAP and VIP elicited a concentration-dependent biphasic response from pancreas perfused with 8.3 mM glucose; the peptides were equipotent. In contrast, secretin was ineffective in the range 10(-11) to 10(-9) M; at 10(-8) and 10(-7) M, it induced only low and transient insulin responses. On the other hand, the peptides did not modify the basal insulin release in the presence of a non stimulating glucose concentration (2.8 mM). 3. On glucagon secretion, PACAP and VIP (10(-11) to 10(-8) M) but also secretin (10(-9) to 10(-7) M) caused a concentration-dependent peak shaped response from pancreas perfused with 2.8 mM glucose; PACAP and VIP were equipotent and 20 times more potent then secretin. On the other hand, the peptides did not affect the glucagon release in the presence of 8.3 mM glucose. 4. On pancreatic vessels, in the range 10(-11) to 10(-9) M, the three peptides were equipotent in inducing a concentration-dependent sustained increase in pancreatic flow rate. On the other hand, at the high concentration of 10(-7) M PACAP but not VIP provoked a transient decrease of flow rate. 5. This study provides evidence for PACAP/VIP type II receptors mediating insulin and glucagon secretion as well as vasodilatation in rat pancreas. In addition, the different efficacies of secretin suggest that these effects are mediated by different PACAP/VIP type II receptor subtypes.

Effects of pituitary adenylate cyclase-activating polypeptide on cardiovascular and respiratory responses in anaesthetised dogs

This study examines some of the cardiovascular and respiratory effects of pituitary adenylate cyclase-activating polypeptide (PACAP) in anaesthetised dogs. Intravenous injection of PACAP 27 caused an increase in arterial blood pressure and an increase in heart rate. The blood pressure response was significantly reduced by adrenoceptor blockade suggesting a mechanism of action mediated in part via catecholamines. The heart rate increase was unaltered by adrenoceptor blockade suggesting a direct effect of PACAP 27. PACAP 27 also caused potentiation of cardiac slowing caused by stimulation of the vagus nerve. In addition, PACAP 27 powerfully stimulated breathing. This was probably evoked by stimulation of arterial chemoreceptors, because bilateral section of the carotid sinus nerves abolished this effect. PACAP 27 had no effect on the ability of the cardiac sympathetic nerve to increase heart rate, nor on the interaction between the sympathetic and parasympathetic systems in the heart.

Pituitary adenylate cyclase-activating polypeptide stimulates cardiodilatin/atrial natriuretic peptide (CDD/ANP-(99-126) secretion from cultured neonatal rat myocardiocytes

It has been reported that the highly homologous neuropeptides pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal polypeptide (VIP) exert similar cardiovascular effects in vivo. In the present study we compared the effects of these neuropeptides on myocardial cyclic AMP content and the release of immunoreactive CDD/ANP-(99-126) (atrial natriuretic peptide). In cultured neonatal rat cardiomyocytes PACAP and VIP evoke concentration-dependent increases in intracellular cyclic AMP content but responses to VIP are markedly less. PACAP stimulates the release of CDD/ ANP-(99-126) in a concentration-dependent manner with a threshold concentration of 1 nM, and up to a 6-fold increase in basal secretion at 1 microM PACAP. In contrast. VIP had no effect on the release of CDD/ANP. Pretreatment of cells with the competitive PACAP-antagonist, PACAP-6-38 (1 microM), significantly reduces the effects of PACAP on intracellular cyclic AMP and on CDD/ANP-(99-126) secretion and abolishes the effects of VIP on cyclic AMP. Pretreatment with VIP-receptor antagonist (1 microM) prevents the cyclic AMP-response to VIP while increases in cyclic AMP as well as stimulation of CDD/ANP-(99-126) release by PACAP are not affected. It is concluded that both neuropeptides directly influence cardiac myocytes through an increase in intracellular cyclic AMP. Release of CDD/ ANP-(99-126) by PACAP may be involved in the decrease in blood pressure that follows intravenous administration of this peptide. The higher potency of PACAP to induce cyclic AMP synthesis, its stimulating effect on the release of CDD/ANP-(99-126) and the finding that the VIP-receptor antagonist inhibits responses to VIP but not to PACAP suggest that PACAP activates cardiac myocytes through a PACAP-specific receptor.

Peptides as neurotransmitters in vascular autonomic neurons

1. Neuropeptides are present in the majority of autonomic neurons projecting to blood vessels, where they are co-localized with non-peptide transmitters and sometimes with other peptides. 2. Neuropeptides are released from vasoconstrictor and vasodilator nerve terminals after high frequency stimulation ( > 2-5Hz) with trains of impulses. 3. Neuropeptides can have potent post-synaptic effects on vascular tone, but often these effects are restricted to selected regions of the vasculature. 4. Post-synaptic effects of neuropeptides tend to be more slowly-developing and more long-lasting than those of non-peptide transmitters. 5. Autonomic vasoconstrictor and vasodilator responses often have multiple phases, with the faster phases being mediated by non-peptide transmitters and the slower phases medicated predominantly by one or more neuropeptides. 6. Some neuropeptides do not seem to have post-synaptic effects in a particular vascular bed, but can have presynaptic actions on neurotransmitter release. 7. Neuropeptides form an important component of the repertoire of neurotransmitters used by vascular autonomic neurons to regulate regional blood flow in response to a range of physiological stimuli.

Vasorelaxant effect of PACAP-27 on canine cerebral arteries and rat intracerebral arterioles

The vasorelaxant effects of pituitary adenylate cyclase activating polypeptide (PACAP)-27 were examined and compared with those of PACAP-38 and vasoactive intestinal polypeptide (VIP) on isolated canine cerebral arteries and rat intracerebral arterioles in vitro. The addition of PACAP-27, PACAP-38 or VIP resulted in similar concentration-dependent relaxations in both canine basilar arteries and rat intracerebral arterioles. There were regional differences in the PACAP-27-induced relaxations measured in canine cerebral arteries. The maximum relaxation induced by PACAP-27 was significantly lower in the basilar arteries (23.0 +/- 5.6%) than in the rostrally located arteries (proximal middle cerebral arteries: 45.4 +/- 5.7%, anterior cerebral arteries: 55.2 +/- 5.8%). The maximum relaxation induced by PACAP-27 in the basilar arteries was significantly enhanced by mechanical removal of the endothelium (16.4 +/- 4.5% vs. 32.7 +/- 5.8%) as well as by pretreatment with indomethacin or aspirin (12.9 +/- 4.1% vs. 48.7 +/- 6.1% and 46.5 +/- 9.2%, respectively). Incubation of canine cerebral arteries with PACAP-27 in vitro resulted in an increased release of prostaglandin F2 alpha in the buffer from 14.5 +/- 2.1 pg/min/1 mg vessel to 31.1 +/- 4.2 pg/min/1 mg vessel, while other cyclooxygenase cascade metabolites such as prostaglandin E2, thromboxane B2 and 6-keto prostaglandin F1 alpha did not change. These data suggest that the PACAP-27-induced relaxation of canine basilar arteries may be associated with prostaglandin F2 alpha or its precursor, prostaglandin H2.

Pituitary adenylate cyclase-activating polypeptide: occurrence and relaxant effect in female genital tract

The distribution, localization, and smooth muscle effects of pituitary adenylate cyclase-activating polypeptide (PACAP) were studied in the human female genital tract. The concentrations of PACAP-38 and PACAP-27 were measured by radioimmunoassays, and both peptides were found throughout the genital tract. The highest concentrations of PACAP-38 were detected in the ovary, the upper part of vagina, and the perineum. The concentrations of PACAP-27 were generally low, in some regions below the detection limit and in other regions 1 to 5% of the PACAP-38 concentrations. Immunocytochemistry revealed that PACAP was located in delicate varicose nerve fibers that were most abundant in the internal cervical os, where they mainly seemed to innervate blood vessels and smooth muscle cells. PACAP-38 and PACAP-27 (10(-10)-10(-6) M) caused a concentration-dependent relaxation of the spontaneous activity of the nonvascular smooth muscle strips from fallopian tube and myometrium in vitro. Likewise, both peptides (10(-10)-10(-6) M) caused relaxation of nonrepinephrine (10(-6) M)-precontracted intramyometrial arteries. No effect of the PACAP sequences, PACAP-(6-27), PACAP-(16-38), and PACAP-(18-27), on fallopian tube was observed. The findings suggest a smooth muscle regulatory role of PACAP in the human female reproductive tract.

The effects of pituitary adenylate cyclase-activating polypeptide on cerebral arteries and vertebral artery blood flow in anesthetized dogs

We investigated and compared the effects of pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) on cerebral circulation in anesthetized dogs. The intracisternal administration of PACAP-27, PACAP-38, and VIP dilated canine cerebral arteries in a dose-dependent manner. A 10 nmol dose of PACAP-27, PACAP-38, and VIP dilated the basilar artery by 23 +/- 3, 27 +/- 3 and 30 +/- 3%, respectively. Rostrally located arteries tended to be more responsive to PACAP-27. Pretreatment with NG-monomethyl-L-arginine did not affect PACAP-27-induced vasodilation. Vertebral artery blood flow was also affected by intra-arterial injection of these peptides in a dose-dependent manner. A 100 pmol dose of PACAP-27, PACAP-38, and VIP increased the vertebral artery blood flow by 42 +/- 10, 29 +/- 4, and 62 +/- 11%, respectively. The VIP receptor antagonist, [Lys1,Pro2,5,Arg3,4,Tyr6]VIP, inhibited both the VIP- and PACAP-38-induced increase in vertebral artery blood flow. These findings suggest that PACAP plays a role in the regulation of cerebral circulation.

Effect of PACAP on gastric acid secretion in rats

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a new VIP-like brain-gut peptide. Its effects on the motility and secretory functions of the gastrointestinal system have been shown in previous studies. In this study we investigated the effect of intravenous PACAP on gastric acid secretion in conscious pylorus-ligated rats and in gastric fistula rats. PACAP showed significant inhibitory effects on pentagastrin- and histamine-stimulated gastric acid secretion, but no effect on basal or carbachol-stimulated secretion in pylorus-ligated rats. It did show dose-related inhibitory effects both on basal gastric acid secretion and on secretion stimulated by pentagastrin, histamine, or carbachol in gastric fistula rats. PACAP did not alter serum gastrin levels. Inhibition of prostaglandin synthesis with indomethacin and immunoneutralization of somatostatin with anti-somatostatin serum did not prevent the inhibitory effect of PACAP on gastric acid secretion in pylorus-ligated rats. We conclude that PACAP most likely has a direct effect on parietal cells and that this effect may be mediated, at least partially, by inhibition of the action of histamine on parietal cells.

Pituitary adenylate cyclase activating peptides are endothelium-independent dilators of human and porcine coronary arteries

The PACAPs have been shown to be potent vasodilators in different animal species. Data in humans are still lacking. Therefore we investigated the effects of PACAP 38, PACAP 27 and VIP on isolated human and porcine coronary arteries (HCA and PCA). Our data show, that the PACAPs are endothelium-independent vasorelaxants, which in HCA are slightly more potent than VIP. The N-terminal shortened peptides PACAP 6-38 and PACAP 6-27 also show relatively potent vasorelaxant effects, acting as partial agonists. Glibenclamide, a selective inhibitor of ATP-sensitive potassium channels, partially reverses the effects of the PACAPs, indicating an involvement of these channels in the mechanism of action.