Effects Of Vasoactive Intestinal Polypeptide Research Articles

Effects of PACAP and VIP on hyperglycemia-induced proliferation in murine microvascular endothelial cells

Hyperglycemia is implicated both in micro- and macro-vascular complications in diabetes mellitus. Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are two known nonclassic regulators of angiogenesis, although their biological role on endothelial cell proliferation remains poorly defined. In the present study we hypothesized that either peptides might play an inhibitory role on hyperglycemia-induced cell growth. To this end, we investigated the effect of both PACAP and VIP on cell proliferation in murine microvascular endothelial cells (H5V) cultured both under euglycemic and hyperglycemic conditions (5 and 25 mM glucose, respectively) for 24, 48 h, 7 and 15 days. Results demonstrated that high glucose treatment induced a time-dependent increase in cell viability after 48 h ( p < 0.05), which was much more evident after 7 and 15 days ( p < 0.001). Similar effects were observed in cell proliferation, although significant changes were obtained after prolonged exposures to high glucose (7 and 15 days; p < 0.001). The proliferative response to the glucose-enriched environment was correlated to changes in the expression of PAC1 and, to a minor extent, to VPAC2, but not VPAC1 receptors, as measured by quantitative real-time PCR. These results were further confirmed by Western blot and immunofluorescence analyses. Interestingly, 10 −7 M PACAP or VIP treatment significantly attenuated hyperglycemia-induced increase in cell viability and proliferation after 7 and 15 days. Taken together, our findings demonstrate that both PACAP and VIP peptides exert an inhibitory activity on hyperglycemia-induced endothelial cell proliferation, thus suggesting that the effect might be mediated by PAC1 and VPAC2 receptors.

Vasoactive intestinal polypeptide relaxes isolated rat pulmonary artery rings through two distinct mechanisms

Vasoactive intestinal polypeptide (VIP), an endogenous neuropeptide normally present in lungs and other organs, relaxes pulmonary arteries (PAs) in different species, whereas the underlying mechanisms are still not fully understood. The aim of this study, therefore, is to investigate the signal transduction of VIP in the relaxation of isolated rat PA rings. The isometric tension of the rings was studied in vitro with force-electricity transducers. In endothelium-intact (EI) rings, VIP elicited concentration-dependent relaxation after the rings were pre-contracted by phenylephrine. A similar effect, though smaller, was observed in endothelium-denuded (ED) rings. Inhibition of the endothelial nitric oxide synthase (eNOS) by NG-nitro-L-arginine methyl ester diminished the VIP-induced vasodilatation of PA rings. The VIP-induced vasorelaxation was markedly reduced by the inhibition of the phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling pathway with wortmannin and LY294002, respectively, which was seen in EI rings, but not in ED rings. Western blot analysis revealed that VIP increased the phosphorylation of eNOS at Ser 1177, but did not affect the overall expression of eNOS. In ED rings, the PKA inhibitor H-89 and K(ATP) channel inhibitor glibenclamide almost totally abolished the vasodilatation effect of VIP. The results suggested that the vasodilatation effect of VIP on rat PAs is mediated by both vascular endothelium and smooth muscle, involving respectively the PI3K/Akt-eNOS pathway and the PKA-K(ATP) channel pathway.

Age-related changes in functional NANC innervation with VIP and substance P in the jejunum of Lewis rats

Age-related changes in non-adrenergic, non-cholinergic (NANC) neurotransmission might contribute to differences in gastrointestinal motility. Our aim was to determine age-related changes in functional innervation with vasoactive intestinal polypeptide (VIP) and substance P (Sub P) in rat jejunum. We hypothesized that maturation causes changes in neurotransmission with these two neuropeptides. Longitudinal and circular jejunal muscle strips from young (3 months) and middle-aged (15 months) rats (total: 24 rats) were studied; the response to exogenous VIP and Sub P and the effect of their endogenous release from the enteric nervous system during electrical field stimulation (EFS) were evaluated. In longitudinal muscle, response to exogenous VIP and endogenously released VIP during EFS were increased in middle-aged rats, while the effect of endogenously released Sub P was decreased. In the circular muscle, the response to endogenously released VIP was increased in middle-aged rats, while the effects of exogenous VIP and endogenously released Sub P were unchanged. Response to exogenous Sub P was unaffected by maturation in both muscle layers. Spontaneous contractile activity was increased in the longitudinal and circular muscle of the older rats. In the jejunum of middle-aged rats, participation of VIP in functional NANC innervation was increased, while functional innervation with Sub P was decreased. These changes in the balance of inhibitory and excitatory neurotransmission occur during the year of maturation in rats and demonstrate an age-dependant plasticity of neuromuscular bowel function.

Methodology for development of a physiological model incorporating CYP3A and P-glycoprotein for the prediction of intestinal drug absorption

The small intestine poses a major barrier to the efficient absorption of orally administered therapeutics. Intestinal epithelial cells are an extremely important site for extrahepatic clearance, primarily due to prominent P-glycoprotein-mediated active efflux and the presence of cytochrome P450s. We describe a physiologically based pharmacokinetic model which incorporates geometric variations, pH alterations and descriptions of the abundance and distribution of cytochrome 3A and P-glycoprotein along the length of the small intestine. Simulations using preclinical in vitro data for model drugs were performed to establish the influence of P-glycoprotein efflux, cytochrome 3A metabolism and passive permeability on drug available for absorption within the enterocytes. The fraction of drug escaping the enterocyte (F(G)) for 10 cytochrome 3A substrates with a range of intrinsic metabolic clearances were simulated. Following incorporation of P-glycoprotein in vitro efflux ratios all predicted F(G) values were within 20% of observed in vivo F(G). The presence of P-glycoprotein increased the level of cytochrome 3A drug metabolism by up to 12-fold in the distal intestine. F(G) was highly sensitive to changes in intrinsic metabolic clearance but less sensitive to changes in intestinal drug permeability. The model will be valuable for quantifying aspects of intestinal drug absorption and distribution.

Vasoactive Intestinal Polypeptide Entrains Circadian Rhythms in Astrocytes

Many mammalian cell types show daily rhythms in gene expression driven by a circadian pacemaker. For example, cultured astrocytes display circadian rhythms in Period1 and Period2 expression. It is not known, however, how or which intercellular factors synchronize and sustain rhythmicity in astrocytes. Because astrocytes are highly sensitive to vasoactive intestinal polypeptide (VIP), a neuropeptide released by neurons and important for the coordination of daily cycling, the authors hypothesized that VIP entrains circadian rhythms in astrocytes. They used astrocyte cultures derived from knock-in mice containing a bioluminescent reporter of PERIOD2 (PER2) protein, to assess the effects of VIP on the rhythmic properties of astrocytes. VIP induced a dose-dependent increase in the peak-to-trough amplitude of the ensemble rhythms of PER2 expression with maximal effects near 100 nM VIP and threshold values between 0.1 and 1 nM. VIP also induced dose- and phase-dependent shifts in PER2 rhythms and daily VIP administration entrained bioluminescence rhythms of astrocytes to a predicted phase angle. This is the first demonstration that a neuropeptide can entrain glial cells to a phase predicted by a phase-response curve. The authors conclude that VIP potently entrains astrocytes in vitro and is a candidate for coordinating daily rhythms among glia in the brain.

Neurotrophic effect of vasoactive intestinal polypeptide (VIP) on the ventral spinal cord of rat embryo

Neurotrophic effect of vasoactive intestinal polypeptide (VIP) on the ventral spinal cord of rat embryo

Characterization, Expression, and Functional Activity of Pituitary Adenylate Cyclase-Activating Polypeptide and Its Receptors in Human Granulosa-Luteal Cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are found in the ovary of mammalian species, although nothing is known about the possible role of PACAP and VIP in the human ovary. We investigated the expression of PACAP and PACAP/VIP receptors in human granulosa-luteal (GL) cells obtained from consenting in vitro fertilization patients attending a private fertility clinic and assessed a possible antiapoptotic effect of these molecules. We measured the expression of PACAP and PACAP/VIP receptor mRNAs in GL cells in response to FSH or LH, as well as the effects of PACAP and VIP on apoptosis. We also evaluated the levels of procaspase-3 in GL cells cultured in the absence of serum. After 7 d in culture, GL cells displayed increased responsiveness to FSH and LH (100 ng/ml). FSH and LH promoted PACAP expression, LH doing so in a time-dependent fashion. VIP receptor (VPAC1-R and VPAC2-R) mRNAs were also induced by gonadotropin stimulation. Although PACAP receptor (PAC1-R) mRNA was barely detectable, Western blot analysis revealed its presence. The apoptotic effect of serum withdrawal from the culture environment was reverted by both PACAP and VIP. Both peptides showed the ability to reverse a decrease in procaspase-3 levels induced by culture in the absence of serum. PACAP and VIP appear to play a role in maintenance of follicle viability as a consequence of the antiapoptotic effect. Further studies are warranted to evaluate the respective roles of PACAP and VIP in ovarian physiology and to identify their mechanism of action.

The Role of Vasoactive Intestinal Polypeptide and Pituitary Adenylate Cyclase-Activating Polypeptide in the Neural Pathways Controlling the Lower Urinary Tract

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are expressed in the neural pathways regulating the lower urinary tract. VIP-immunoreactivity (IR) is present in afferent and autonomic efferent neurons innervating the bladder and urethra, whereas PACAP-IR is present primarily in afferent neurons. Exogenously applied VIP relaxes bladder and urethral smooth muscle and excites parasympathetic neurons in bladder ganglia. PACAP relaxes bladder and urethral smooth muscle in some species (pig) but excites the smooth muscle in other species (mouse). Intrathecal administration of VIP in cats with an intact spinal cord suppresses reflex bladder activity, but intrathecal administration of VIP or PACAP in rats enhances bladder activity and suppresses urethral sphincter activity. PACAP has presynaptic facilitatory effects and direct excitatory effects on lumbosacral parasympathetic preganglionic neurons. Chronic spinal cord transection produces an expansion of VIP-IR (cats) and PACAP-IR (rats) in primary afferent axons in the lumbosacral spinal cord and unmasks spinal excitatory effects of VIP on bladder reflexes in cats. Intrathecal administration of PACAP6-38, a PAC1 receptor antagonist, reduces bladder hyperactivity in chronic spinal-cord-injured rats. These observations raise the possibility that VIP or PACAP have a role in the control of normal or abnormal voiding.

Vasoactive Intestinal Polypeptide Can Excite Gonadotropin-Releasing Hormone Neurons in a Manner Dependent on Estradiol and Gated by Time of Day

A surge of GnRH release signals the LH surge that triggers ovulation. The GnRH surge is dependent on a switch in estradiol feedback from negative to positive and, in rodents, a daily neural signal, likely from the suprachiasmatic nuclei. Vasoactive intestinal polypeptide (VIP) may be involved in suprachiasmatic nuclei-GnRH neuron communication. Here we assessed the effects of acute VIP (5 min treatment) on GnRH neuron function using targeted extracellular recordings of firing activity of GnRH neurons in brain slices. We examined the effect of VIP on firing rate at different times of day using an established ovariectomized, estradiol-treated (OVX+E) mouse model that exhibits daily LH surges timed to the late afternoon. Cells from OVX animals (no estradiol) did not respond to VIP, regardless of time of day. With estradiol, the effect of VIP on GnRH neurons was dependent on the time of recording. During negative feedback, OVX+E cells did not respond. VIP increased firing in cells recorded during surge onset, but this excitatory response was reduced at surge peak. Acute treatment of OVX+E cells during surge peak with a VIP receptor antagonist decreased GnRH neuron firing. This suggests endogenous VIP may both increase GnRH neuron firing during the surge and occlude response to exogenous VIP. These data provide functional evidence for VIP effects on GnRH neurons and indicate that both estradiol and time of day gate the GnRH neuron response to this peptide. VIP may provide an excitatory signal from the circadian clock that helps time the GnRH surge.

Hippocampal asymmetry in exploratory behavior to vasoactive intestinal polypeptide

The effects of vasoactive intestinal polypeptide (VIP) microinjected uni- or bilaterally into the CA1 hippocampal area of male Wistar rats at a dose of 10, 50 and 100 ng on exploratory behavior were examined. VIP microinjected bilaterally at a high dose (100 ng) significantly decreased the horizontal movements, while at low doses (10 and 50 ng) had no effect on the exploratory activity. Microinjections of VIP into the left hippocampal CA1 area at doses 50 and 100 ng suppressed the exploratory activity, while right-side VIP administration at a dose 100 ng significantly increased horizontal movements compared to the respective controls. Vertical activity was stimulated only by VIP administered into the right hippocampal CA1 area at the three doses used. Neither bilateral nor left injections of VIP induced changes in the vertical movements. The main finding was the presence of hippocampal asymmetry in exploratory behavior to unilateral microinjections of VIP depending on the dose and the microinjected hemisphere.

Pituitary adenylate cyclase-activating polypeptide, vasoactive intestinal polypeptide and their receptors: distribution and involvement in the secretion of Podarcis sicula adrenal gland

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are regulatory neuropeptides of the hypothalamus-hypophyseal-adrenal axis, acting via the common receptors VPAC(1) and VPAC(2) and the selective PACAP receptor PAC(1). In the adrenal glands of the Italian wall lizard, Podarcis sicula, the presence of VIP in chromaffin cells, and the VIP-stimulated release of catecholamine and aldosterone in vivo, was previously shown. To examine the localization of both peptides and receptors and their mRNAs in the adrenal gland of P. sicula, immunohistochemistry and in situ hybridization were performed: PACAP and its mRNA were detected in chromaffin cells, VPAC(1) was found associated with steroidogenic tissue, VPAC(2) and PAC(1) with chromaffin tissue. Using 'far western blot' technique, we showed the presence of specific binding sites for VIP/PACAP in the adrenal glands of the lizard. The effects of both VIP and PACAP on the adrenal cells of the lizard were examined in vitro in adrenal cell co-cultures: both VIP and PACAP enhanced catecholamine, corticosterone and aldosterone release from adrenal cell co-culture in a time- and dose-dependent manner. The catecholamine release was inhibited by PAC(1) antagonist and in VPAC(2) immunoneutralized adrenal cells. The effects of VIP and PACAP on aldosterone secretion were counteracted by VPAC(1) antagonist administration in vitro. Corticosterone secretion elicited by VIP was not blocked by VPAC(1) antagonist, while the PACAP-induced release of corticosterone was blocked by the antagonist. Overall, our investigations indicate that these neuropeptides of the secretin superfamily can act not only as neurotransmitters but also as autocrine and paracrine regulators on chromaffin and cortical cells, being important mediators of the non-cholinergic system in the lizard adrenal gland.

PKA-dependent activation of the vascular smooth muscle isoform of K ATP channels by vasoactive intestinal polypeptide and its effect on relaxation of the mesenteric resistance artery

Vasoactive intestinal polypeptide (VIP) is a potent vasodilator and has been successfully used to alleviate hypertension. Consistently, disruption of VIP gene in mice leads to hypertension. However, its downstream targets in the vascular regulation are still not well demonstrated. To test the hypothesis that the vascular smooth muscle isoform of K ATP channels is a downstream target of the VIP signaling, we performed the studies on the Kir6.1/SUR2B channel expressed in HEK293 cells. We found that the channel was strongly activated by VIP. Through endogenous VIP receptors, the channel activation was reversible and dependent on VIP concentrations with the midpoint-activation concentration ∼ 10 nM. The channel activation was voltage-independent and could be blocked by K ATP channel blocker glibenclamide. In cell-attached patches, VIP augmented the channel open-state probability with modest suppression of the single channel conductance. The VIP-induced Kir6.1/SUR2B channel activation was blocked by PKA inhibitor RP-cAMP. Forskolin, an adenylyl cyclase activator, activated the channel similarly as VIP. The effect of VIP was further evident in the native tissues. In acutely dissociated mesenteric vascular smooth myocytes, VIP activated the K ATP currents in a similar manner as in HEK293 cells. In endothelium-free mesenteric artery rings, VIP produced concentration-dependent vasorelaxation that was attenuated by glibenclamide. These results therefore indicate that the vascular isoform (Kir6.1/SUR2B) of K ATP channels is a target of VIP. The channel activation relies on the PKA pathway and produces mesenteric arterial relaxation.

Role of VIP and Substance P in NANC Innervation in the Longitudinal Smooth Muscle of the Rat Jejunum—Influence of Extrinsic Denervation

This study was designed to determine changes in nonadrenergic, noncholinergic (NANC) neurotransmission mediated by Vasoactive Intestinal Polypeptide (VIP) and Substance P after small bowel transplantation (SBT). Six groups of rats (n > or = 6 per group) were studied: naïve controls (NC); 1 wk after anesthesia/sham celiotomy (SC-1); 1 or 8 wk after jejunal and ileal transection/reanastomosis (TA-1, TA-8), or syngeneic, orthotopic SBT (SBT-1, SBT-8). Jejunal longitudinal muscle strips were studied under NANC-conditions for spontaneous contractile activity, response to exogenous VIP and Substance P, and electrical field stimulation (EFS). Spontaneous activity did not differ between the six groups. VIP inhibited contractile activity in all groups 1 wk postoperatively (P < 0.05), which was prevented by the NO synthase inhibitor L-N(G)-nitro arginine (L-NNA). In contrast, VIP had no effect in the other groups. Precontraction with Substance P exposed an inhibitory effect of VIP in all groups (P < 0.05 each). Substance P increased contractile activity in all groups, but to a lesser extent in SBT-8 compared with NC, TA-8, and SBT-1 (P < 0.05). The inhibitory effect of EFS at 6 Hz was prevented by L-NNA in NC and TA-8; addition of the VIP antagonist ([D-p-Cl-Phe(6), Leu(17)]-VIP) increased contractile activity in NC, but not in TA-8 and SBT-8. The Substance P antagonist ([D-Pro(2), D-Trp(7,9)]-Substance P) decreased contractile activity during EFS at 50 Hz in NC and SBT-8. SBT decreased response to exogenous Substance P, although release of endogenous Substance P (EFS) is preserved. Changes in VIP signaling are acute and reversible and not caused by effects of SBT.

Intravenous vasoactive intestinal polypeptide lowers pulmonary-to-systemic vascular resistance ratio in a neonatal piglet model of pulmonary arterial hypertension

Background Several studies of vasoactive intestinal polypeptide (VIP) demonstrated its potent vasodilative effects on pulmonary and systemic circulation. However, no hemodynamic studies were performed to depict the effects of VIP in an in vivo model of pulmonary arterial hypertension (PAH), thereby limiting a complete understanding of the overall hemodynamic effects of VIP in PAH. Methods and Results The pulmonary and systemic hemodynamic effects of intravenous infusion of 100 ng/kg per minute of VIP in control and pulmonary hypertensive piglets at 6 to 8 weeks of age were assessed. Pulmonary arterial hypertension was induced after the instillation of meconium solution in the subjects' trachea and was characterized by the establishment of a persistently elevated pulmonary arterial pressure, diminished cardiac output, and elevated pulmonary-to-systemic vascular resistance (PVR/SVR) ratio. Conclusions Continuous intravenous infusion of VIP markedly decreased PVR/SVR ratio in pulmonary hypertensive subjects; however, it lowered blood pressure without causing any significant changes in PVR/SVR ratio in control subjects. Collectively, these results suggest an overall pulmonary vasodilative effect of VIP in PAH.

Effect of VIP on TLR2 and TLR4 Expression in Lymph Node Immune Cells During TNBS‐Induced Colitis

Toll-like receptors (TLRs) are a family of pattern recognition receptors (PRRs), which recognize numerous molecules collectively named pathogen-associated molecular patterns, with an essential role in inflammatory conditions and connecting innate and acquired immune responses. Moreover, a new function of TLRs in the intestinal mucosa has been described. Under homeostatic conditions, TLRs act to protect the intestinal epithelium; but when homeostasis is disrupted, TLRs appear deregulated. Disruption of intestinal homeostasis occurs in disorders, such as Crohn's disease (CD). Trinitrobenzene sulfonic acid (TNBS)-induced colitis is a murine model of human CD and vasoactive intestinal polypeptide (VIP) exerts a beneficial effect, by decreasing both inflammatory and autoimmune components of the disease. Recently, we have demonstrated the constitutive expression of TLR2 and TLR4 at mRNA and protein levels in colon extracts and their upregulation in TNBS-treated mice as well as the effect of VIP treatment, approaching control levels. However, the systemic effect is little known. The present results demonstrate a beneficial role of VIP, restoring homeostatic conditions through the regulation of both lymphoid cell traffic and TLR2/4 expression on macrophages (MØ), dendritic cells (DCs), and CD4 and CD8 T lymphocytes.

VPAC2 Receptor Activation Mediates VIP Enhancement of Population Spikes in the CA1 Area of the Hippocampus

The receptors mediating vasoactive intestinal polypeptide (VIP) enhancement of synaptic transmission to pyramidal cell bodies were investigated. RO 25-1553 (VPAC2agonist) mimicked the excitatory effect of VIP on population spike (PS) amplitude. [K15, R16, L27] VIP (1-7)/GRF (8-27) (VPAC1 agonist) caused only a small increase in PS amplitude. The effect of VPAC2 agonist (but not of the VPAC1 agonist) persisted upon blockade of GABAergic transmission and was strongly attenuated upon inhibition of PKA. In conclusion, VPAC2 receptor activation mediates VIP enhancement of PS amplitude in the hippocampus essentially through a PKA-dependent mechanism.

Vasoactive intestinal polypeptide receptor antagonism enhances the vagally induced increase in cardiac interval of the rat atriumin vitro

The effect of vasoactive intestinal polypeptide (VIP) receptor antagonism on preganglionic vagal electrical stimulation and on vagal postganglionic activation using nicotine and 1,1-dimethyl-4-phenylpiperazinium iodide on cardiac interval was evaluated in the isolated innervated rat right atrium. The vagus was stimulated at 4, 8, 16 and 32 Hz, pulse duration 1 ms, 20 V, for 30 s. All experiments were carried out in the presence of atenolol (4 microM). Vagal stimulation caused a frequency-dependent increase in cardiac interval which was amplified significantly at each frequency, except at 32 Hz, following application of the VIP receptor antagonist VIP(6-28) at 2 nM in 15 rats. Application of the ganglionic antagonist hexmethonium (28 microM, n = 7 rats) prior to 2 nM VIP(6-28) abolished this effect. Increasing the concentration of VIP(6-28) 10-fold to 20 nM did not result in a greater increase in cardiac interval than that obtained at 2 nM. Nicotine (0.1, 0.3, 0.5, 1.0 and 2.0 mM) increased cardiac interval by direct activation of postganglionic vagal fibres, but 2 nM VIP(6-28) did not affect the nicotine concentration response (n = 6 rats). 1,1-Dimethyl-4-phenylpiperazinium iodide (25, 50, 100 and 200 microM; n = 6 rats) was also used to induce an increase in cardiac interval; again it was not significantly altered by 2 nM VIP(6-28). Therefore, VIP receptor antagonism enhances the magnitude of vagally induced cardiac slowing, probably via an action at the preganglionic-postganglionic synapse.

VIP and PACAP regulate localized Ca2+ transients via cAMP-dependent mechanism

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) have been suggested as participants in enteric inhibitory neural regulation of gastrointestinal motility. These peptides cause a variety of postjunctional responses including membrane hyperpolarization and inhibition of contraction. Neuropeptides released from enteric motor neurons can elicit responses by direct stimulation of smooth muscle cells as opposed to other transmitters that rely on synapses between motor nerve terminals and interstitial cells of Cajal. Therefore, we studied the responses of murine colonic smooth muscle cells to VIP and PACAP(1-38) with confocal microscopy and patch-clamp technique. Localized Ca2+ transients (Ca2+ puffs) were observed in colonic myocytes, and these events coupled to spontaneous transient outward currents (STOCs). VIP and PACAP increased Ca2+ transients and STOC frequency and amplitude. Application of dibutyryl cAMP had similar effects. The adenylyl cyclase blocker MDL-12,330A alone did not affect spontaneous Ca2+ puffs and STOCs but prevented responses to VIP. Disruption of A-kinase-anchoring protein (AKAP) associations by application of AKAP St-Ht31 inhibitory peptide had effects similar to those of MDL-12,330A. Inhibition of ryanodine receptor channels did not block spontaneous Ca2+ puffs and STOCs but prevented the effects of dibutyryl cAMP. These findings suggest that regulation of Ca2+ transients (which couple to activation of STOCs) may contribute to the inhibitory effects of VIP and PACAP. Regulation of Ca2+ transients by VIP and PACAP occurs via adenylyl cyclase, increased synthesis of cAMP, and PKA-dependent regulation of ryanodine receptor channels.

An Investigation into the Presence of a Vagal Tachycardia and the Effect of Vasoactive Intestinal Polypeptide on Rat Heart Rate in vitro

The presence of the vagal tachycardia and the effect of vasoactive intestinal polypeptide in the isolated innervated rat atrium were investigated. The right vagus, or cardiac branch, were stimulated at 4, 8, 16 and 32 Hz, pulse duration 1 ms, 20 V, 30 s before atropine and for 1 min after atropine (3 µmol/l), experiments were carried out in the presence of atenolol (4 µmol/l). No significant vagal tachycardia was observed in the presence of atropine, the greatest increase in heart rate was at 16 Hz which was 3±1 beats/min (n = 12 rats) (p = 0.052). Baseline heart rates for the control, 226±11 beats/min (n = 12 rats) and atropine experiments, 210±8 beats/min (n = 12 rats), were not significantly different (p = 0.24). VIP (0.06, 0.12, 0.24 µmol/l) caused a maximum increase of 27±13 beats/min (n = 5 rats) after 6 µmol/l VIP which was not significant, two higher concentrations of VIP failed to increase heart rate further. These results show that the vagal tachycardia is not present and that VIP does not cause a significant tachycardia in the rat.

The Relaxant Effect of Vasoactive Intestinal Polypeptide in the Isolated Canine Uterine Artery: The Role of Endothelium

The purpose of this study was to examine the effect of vasoactive intestinal polypeptide (VIP) on the uterine artery obtained from non-pregnant dogs. VIP (3 x 10(-9)-3 x 10(-7) M) induced concentration-dependent relaxation in canine uterine arteries with intact endothelium, pre-contracted with 10(-5) M phenylephrine (pEC(50) = 7.52 +/- 0.02, maximal response was 82.19 +/- 2.15%, n = 36). The administration of the cyclooxygenase inhibitor indomethacin (10(-5) M) or 4-aminopyridine (4-AP), a blocker of potassium channels (10(-5) M), did not modify the relaxation induced by VIP. Contrary to this, N(G)-nitro-L-arginine (L-NOARG) (10(-5) M) inhibited relaxation is evoked by VIP. Indomethacin applied with L-NOARG did not provoke further inhibition of VIP-induced relaxation. In the presence of both L-NOARG and L-NOARG + indomethacin, 4-AP led to the further inhibition of VIP-induced relaxation of canine uterine artery. It is concluded that VIP induces endothelium-dependent relaxation of uterine arteries of non-pregnant dogs, which can be entirely explained by the production of nitric oxide (NO) from the endothelial cells. We proposed that when NO synthesis is inhibited, VIP induces further relaxation, independent of the edothelium-derived relaxing factors, probably through activation of K(+) channels.