Calcitonin gene-related peptide (CGRP)-induced cyclic AMP, cyclic GMP and vasorelaxant responses in rat thoracic aorta are antagonized by blockers of endothelium-derived relaxant factor (EDRF)

Formic acid up-regulates vascular tension through nitric oxide-cGMP signaling pathway

Formic acid is a common organic acid used in many industrial processes. There is a paucity of research on the direct toxicity of formic acid and how it might affect the cardiovascular system. This study aimed to understand the effect of formic acid on vascular tension in an animal model and the underlying mechanism. Results found that the vasodilation induced by formic acid was related to the endothelium. When the dosage of formic acid was 1 mM or 5 mM, the vasodilation of endothelium-intact rings was partially suppressed by l-NAME, NS-2028 and nifedipine, and vasoconstriction caused by CaCl₂ was inhibited, and the mRNA levels of eNOS, the activity of NOS (tNOS, iNOS and cNOS) and the level of NO and cGMP were significantly increased. Results also found that eNOS protein expression was significantly enhanced by 5 mM of formic acid. These results suggest formic acid can relax the aortic vessels of rats in a dose-dependent pattern. Further, the mechanism of the formic acid-induced vasodilatation likely involved the NO/cGMP pathway. Finally, the current study has revealed that vasodilation induced by high concentrations of formaldehyde may be the effect of the metabolite formic acid. This study will help further inform the etiologies of formic acid-related angiocardiopathies.

The vasorelaxant mechanisms of methanol on isolated rat aortic rings: Involvement of ion channels and signal transduction pathways

It is reported that methanol is generally used as an industrial solvent, antifreeze, windshield washer fluid, cooking fuel and perfume. Methanol ingestion can lead to severe metabolic disturbances, blindness, or even death. So far, few studies about its negative effects on cardiovascular system have been reported. The purpose of this study was to determine the vasoactive effect of methanol and roles of ion channels and signal transduction pathways on isolated rat aorta. The results suggested that the mechanism of methanol-induced vasorelaxation at low concentrations (<500 mM) was mediated by ATP-sensitive K⁺ (K_ATP) and L-type Ca²⁺ channels, but the mechanism at high concentrations (>600 mM) was related to K_ATP, voltage-dependent K⁺, big-conductance Ca²⁺-activated K⁺, L-type Ca²⁺ channels as well as prostacyclin, protein kinase C, β-adrenoceptors pathways. In addition, methanol induced a dose-dependent inhibition of vasoconstrictions caused by calcium chloride, potassium chloride, or norepinephrine. Further work is needed to investigate the relative contribution of each channel and pathway in methanol-induced vasoactive effect.

Chemical mediators of migraine: preclinical and clinical observations

Migraine is a neurovascular disorder, and although the pathophysiology of migraine has not been fully delineated, much has been learned in the past 50 years. This knowledge has been accompanied by significant advancements in the way migraine is viewed as a disease process and in the development therapeutic options. In this review, we will focus on 4 mediators (nitric oxide, histamine, serotonin, and calcitonin gene-related peptide) which have significantly advanced our understanding of migraine as a disease entity. For each mediator we begin by reviewing the preclinical data linking it to migraine pathophysiology, first focusing on the vascular mechanisms, then the neuronal mechanisms. The preclinical data are then followed by a review of the clinical data which support each mediator's role in migraine and highlights the pharmacological agents which target these mediators for migraine therapy.

The vasorelaxant effect and its mechanisms of sodium bisulfite as a sulfur dioxide donor

To study the biological role of bisulfite on vascular contractility and its underlying cellular and molecular mechanisms, to explore whether bisulfite can be used as a sulfur dioxide (SO(2)) donor in the biological experiments, the vasorelaxant effects of sodium bisulfite and sodium sulfite on isolated rat thoracic aortic rings were compared; and the signal transduction pathways and the ion channels involved in the vascular effects of bisulfite were investigated. The results show that: (1) Sodium bisulfite relaxed rat thoracic aortic rings in a concentration-dependent manner (from 100 to 4000 μM); however, sodium sulfite at 500 and 1000 μM caused vasoconstriction, and only at higher concentrations (from 2000 to 4000 μM) it caused vasorelaxation in a concentration-dependent manner. (2) The vasorelaxation caused by the bisulfite at low concentrations (≤500 μM) was endothelium-dependent, but at high concentrations (≥1000 μM) it was endothelium-independent. (3) The vasorelaxation by the bisulfite at the low concentrations was partially mediated by the cGMP pathway and the vasorelaxation was related to big-conductance Ca(2+)-activated K(+) (BK(Ca)) channel, but not due to prostaglandin, protein kinase C (PKC) and cAMP pathways. (4) The vasorelaxation by the bisulfite at high concentrations was partially inhibited by tetraethylammonium (TEA) and glibenclamide, suggesting that the vasorelaxation was related to ATP-sensitive K(+) channel (K(ATP)) and L-type calcium-channel. These results led to the conclusion that bisulfite (HSO(3)(-)) might be a vasoactive factor and sodium bisulfite can be used as a SO(2) donor for the study of SO(2) biology.

The vasodilator mechanisms of sodium metabisulfite on precontracted isolated aortic rings in rats: signal transduction pathways and ion channels

Sodium metabisulfite (SMB) is most commonly used as a food additives, however few study was performed on the vasodilator effect of SMB. In the present paper, the vasodilator effects of SMB and roles of Ca(2+) and K(+) channels as well as the cGMP pathway on isolated rat aortic rings were studied. The results show that: (1) SMB could relax isolated aortic rings precontracted by norepinephrine in a concentration-dependent manner. The maximal endothelium-dependent vasorelaxation was approximately 20% whereas that not depending on the presence of the endothelium was more than 90%. (2) The vasorelaxant effects induced by 50 or 200 μM SMB were partially inhibited by iberiotoxin, NS-2028 or l-NNA. The vasorelaxation of 1000 μM SMB was partially inhibited by nifedipine or glibenclamide. The SMB induced vasorelaxation was partially inhibited by tetraethylammonium. These results led to the conclusions that the vasorelaxation of SMB at low concentrations (<400 μM) was endothelium-dependent and mediated by the cGMP pathway and BK(Ca) channel, but at high concentrations (>500 μM) was endothelium-independent and mediated by K(ATP) channel and L-type Ca(2+) channel. The maximal allowable concentration from China and the acceptable daily intake level from WHO of SMB as a food additive should be revised.

The relevance of preclinical research models for the development of antimigraine drugs: focus on 5-HT(1B/1D) and CGRP receptors

Migraine is a complex neurovascular syndrome, causing a unilateral pulsating headache with accompanying symptoms. The past four decades have contributed immensely to our present understanding of migraine pathophysiology and have led to the introduction of specific antimigraine therapies, much to the relief of migraineurs. Pathophysiological factors culminating into migraine headaches have not yet been completely deciphered and, thus, pose an additional challenge for preclinical research in the absence of any direct experimental marker. Migraine provocation experiments in humans use a head-score to evaluate migraine, as articulated by the volunteer, which cannot be applied to laboratory animals. Therefore, basic research focuses on different symptoms and putative mechanisms, one at a time or in combination, to validate the hypotheses. Studies in several species, utilizing different preclinical approaches, have significantly contributed to the two antimigraine principles in therapeutics, namely: 5-HT(1B/1D) receptor agonists (known as triptans) and CGRP receptor antagonists (known as gepants). This review will analyze the preclinical experimental models currently known for the development of these therapeutic principles, which are mainly based on the vascular and/or neurogenic theories of migraine pathogenesis. These include models based on the involvement of cranial vasodilatation and/or the trigeminovascular system in migraine. Clearly, the preclinical strategies should involve both approaches, while incorporating the newer ideas/techniques in order to get better insights into migraine pathophysiology.

CGRP stimulation of iNOS and NO release from trigeminal ganglion glial cells involves mitogen-activated protein kinase pathways

Clinical and basic science data support an integral role of calcitonin gene-related peptide (CGRP) in the pathophysiology of temporomandibular joint disorders. Recently, we have shown that CGRP can stimulate the synthesis and release of nitric oxide (NO) from trigeminal ganglion glial cells. The goal of this study was to determine the role of mitogen-activated protein kinase (MAPK) signaling pathways in CGRP regulation of iNOS expression and NO release from cultured trigeminal ganglion glial cells from Sprague-Dawley rats. CGRP treatment for 2 h significantly increased activity of the MAPK reporter genes, Elk, ATF-2, and CHOP. In addition, CGRP increased nuclear staining for the active forms of the MAPKs: extracellular signal-regulated kinase, c-Jun amino-terminal kinase, and p38. This stimulatory event was not observed in cultures pre-treated with the CGRP receptor antagonist peptide CGRP(8-37). Similarly, pre-treatment with selective MAPK inhibitors repressed increases in reporter gene activity as well as CGRP-induced increases in iNOS expression and NO release mediated by MAPKs. In addition, over-expression of MAPK kinase 1 (MEK1), MEK3, MEK6, and MEK kinase significantly increased iNOS expression and NO production in glial cells. Results from our study provide evidence that CGRP binding to its receptor can stimulate iNOS gene expression via activation of MAPK pathways in trigeminal ganglion glial cells.

Clinical and preclinical rationale for CGRP-receptor antagonists in the treatment of migraine

Calcitonin gene-related peptide (CGRP) is linked to migraine and other primary headache disorders. It is found in every location described in migraine genesis and processing, including meninges, trigeminal ganglion, trigeminocervical complex, brainstem nuclei, and cortex. It is released in animal models following stimulation of the CNS similar to that seen in migraine, and triptans inhibit this release. Injection of CGRP into migraineurs results in delayed headache similar to migraine. Elevation of CGRP occurs during migraine, resolving following migraine-specific treatment. Finally, and most importantly, CGRP receptor antagonists terminate migraine with efficacy similar to triptans. Both intravenous olcegepant (BIBN 4096 BS) and oral telcagepant (MK-0974) have been effective, safe, and well tolerated in phase I and II studies. Telcagepant is currently in phase III trials, and preliminary results are favorable. The potential for a migraine-specific medication without vasoconstrictive or vascular side effects is enormous. CGRP receptor blockade may also have applications in other pathologic and pain syndromes.

The Vasodilator Effect and Its Mechanism of Sulfur Dioxide-Derivatives on Isolated Aortic Rings of Rats

The vasodilator effect of exogenous sulfur dioxide (SO(2)) derivatives (mixture of sodium bisulfite and sodium sulfite, 3:1 M/M in neutral solution) on rat vascular system was studied in order to explore the mechanism of blood pressure lowered by SO(2) and its derivatives. Isolated rat aortic rings were perfused in bath tubes containing various chemicals and their tensions were recorded. The results showed: (1) The SO(2) derivatives could relax isolated aorta precontracted by norepinephrine (NE) or potassium chloride (KCl) in a dose-dependent manner. (2) This vasodilator effect was attenuated after preincubation with indomethacin, but was not affected by N-L-nitro-arginine, methylene blue, and propranolol, and was independent of the aorta endothelium. (3) The vasoconstriction responses induced by NE, KCl, or Ca(2+) were antagonized by SO(2) derivatives in a noncompetitive manner. (4) The vasoconstrictions of two components (initial fast vasoconstriction induced by intracellular Ca(2+) release and sustained vasoconstriction evoked by extracellular Ca(2+) influx) were also inhibited by SO(2) derivatives. These results led to the conclusions: The SO(2) derivatives could cause vasorelaxation by a direct role of the chemicals on aortic smooth muscle cells. It was not dependent on vascular endothelium and was independent of nitric oxide (NO). It is suggested that SO(2) and its derivatives might be also vasoactive substances that modulate changes of blood pressure, like other gasotransmitters. The vasorelaxation might be related to the inhibition effects of SO(2) derivatives on Ca(2+) entry through both potential-dependent calcium channels and receptor-operating calcium channels, and also to the inhibition of intracellular Ca(2+) release. The vasorelaxation was at partly related to the increase of prostacyclin (PGI(2)) induced by SO(2) derivatives.

Vasodilatation of sulfur dioxide derivatives and signal transduction

The vasodilatation of sulfur dioxide (SO(2)) derivatives on the rat thoracic aortic rings and its cell signal transduction pathway were studied. The levels of cAMP, cGMP, PGI(2), TXA(2) and activities of PKA and adenylyl cyclase (AC) in the rings exposed to SO(2) derivatives were determined. The results indicated that SO(2) derivatives could dose-dependently relax the isolated rat aorta rings with or without endothelium precontracted by NE, no difference was found between the rings with and without endothelium; Levels of cAMP, PGI(2), AC activity and PKA activity in the aortic rings were significantly increased by the derivatives in a dose-related way; No change of cGMP and TXA(2) levels in rings was observed; cAMP/cGMP and PGI(2)/TXA(2) ratios were increased significantly by the SO(2) derivatives. These results led to the conclusions that SO(2) derivatives might cause the endothelium-independent vasorelaxation effect, and the vasorelaxation was mediated in partly by the signal transduction pathway of PGI(2)-AC-cAMP-PKA.

Role of nitric oxide in the vasorelaxant and hypotensive effects of extracts and purified tannins from Geum japonicum

Crude extracts and three purified tannins from Geum japonicum Thunberg (Rosaceae) were examined for relaxant effects in isolated rat thoracic aorta and for hypotensive effects in anesthetized normotensive and hypertensive rats. The acetone extract and the butyl alcohol extract of Geum japonicum at a cumulative concentration of 30mug/ml potently relaxed phenylephrine-precontracted aortic rings by 73+/-5% and 80+/-7%, respectively, without affecting the resting tension of these vessels. Removal of the vascular endothelium, inhibition of nitric oxide (NO) synthase with N(omega)-nitro-l-arginine (l-NA) or inhibition of cGMP biosynthesis with methylene blue all abolished the vasorelaxant effects of the Geum japonicum extracts. Addition of l-arginine, the substrate for NO biosynthesis, reversed the inhibitory effects of l-NA. Similar vasorelaxant effects of 82+/-10%, 61+/-8% and 82+/-14%, were observed with the purified tannins, penta-O-galloyl-beta-glucoside, casuariin and 5-desgalloylstachyurin, respectively, at a cumulative concentration of 10muM. Intravenous injection of the butyl alcohol extract of Geum japonicum at a cumulative dose of 2.5mg/kg into both hypertensive and normotensive rats resulted in a marked reduction in the mean arterial blood pressure by 46+/-6% and 34+/-7%, respectively, which was abolished by prior injection of l-NA. Therefore, these results suggest that tannins may be responsible for the vasorelaxant and hypotensive effects of Geum japonicum, mediated via endogenous NO and subsequent cGMP formation. The data suggest that extracts of Geum japonicum may have potential use as new anti-hypertensive agents for lowering arterial blood pressure in hypertensive patients.

Immunohistochemical study on the distribution of neuronal nitric oxide synthase-immunoreactive neurons in the spinal cord of aged rat

Despite in vivo studies suggesting an important function for nitric oxide (NO) in the spinal cord in the transmission of pain signals, sympathetic nerve activity and presumably other spinal functions, changes of neuronal NO synthase (nNOS)-containing neurons with aging in the spinal cord has not been investigated. In the present study, we demonstrated for the first time that the number of nNOS-immunoreactive neurons was significantly decreased in the central autonomic nucleus and the superficial dorsal horn of spinal cord in aged rats. Morphologically, the number and length of dendritic branches also seemed to be decreased. Combined with our previous studies, age-related decreases in the number of nNOS-immunoreactive neurons in the central autonomic nucleus and the superficial dorsal horn might be associated with the abnormality of micturition function or pain perception encountered in the elderly. However, the mechanisms underlying the decreased immunoreactivity for nNOS, and the functional implications require elucidation.

Role of Nitric Oxide in Mediating In Vivo Vascular Responses to Calcitonin Gene-Related Peptide in Essential and Peripheral Circulations in the Fetus

Background— The role of calcitonin gene-related peptide (CGRP) in cardiovascular regulation is gaining clinical and scientific interest. In the adult, in vivo studies have shown that CGRP-stimulated vasodilation in several vascular beds depends, at least in part, on nitric oxide (NO). However, whether CGRP acts as a vasodilator in the fetus in vivo and whether this effect is mediated via NO have been addressed only minimally. This study tested the hypothesis that CGRP has potent NO-dependent vasodilator actions in essential and peripheral vascular beds in the fetus in late gestation.

Methods and Results— Under anesthesia, 5 fetal sheep at 0.8 gestation were instrumented with vascular catheters and Transonic flow probes around an umbilical artery and a femoral artery. Five days later, fetuses received 2- and 5-μg doses of exogenous CGRP intra-arterially in randomized order. Doses were repeated during NO blockade with the NO clamp. This technique permits blockade of de novo synthesis of NO while compensating for tonic production of the gas, thereby maintaining basal cardiovascular function. CGRP resulted in potent and long-lasting NO-dependent dilation in the umbilical and femoral circulations, hypotension, and a positive cardiac chronotropic effect. During NO blockade, the femoral vasodilator response to CGRP was diminished. In contrast, in the umbilical vascular bed, the dilator response was not only prevented but reversed to vasoconstriction.

Conclusions— CGRP has potent NO-dependent vasodilator actions in fetal essential and peripheral vascular beds. CGRP-induced NO-dependent effects in the umbilical vascular bed may provide an important mechanism in the control and maintenance of umbilical blood flow during pregnancy.

Calcitonin gene-related peptide activates different signaling pathways in mesenteric lymphatics of guinea pigs

The effects of calcitonin gene-related peptide (CGRP) on constriction frequency, smooth muscle membrane potential (V(m)), and endothelial V(m) of guinea pig mesenteric lymphatics were examined in vitro. CGRP (1-100 nM) caused an endothelium-dependent decrease in the constriction frequency of perfused lymphatic vessels. The endothelium-dependent CGRP response was abolished by the CGRP-1 receptor antagonist CGRP-(8-37) (1 microM) and pertussis toxin (100 ng/ml). This action of CGRP was also blocked by the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine (L-NNA; 10 microM), an action that was reversed by the addition of L-arginine (100 microM). cGMP, adenylate cyclase, cAMP-dependent protein kinase (PKA), and ATP-sensitive K+ (K+(ATP)) channels were all implicated in the endothelium-dependent CGRP response because it was abolished by methylene blue (20 microM), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 microM), dideoxyadenosine (10 microM), N-[2-(p-bromociannamylamino)-ethyl]-5-isoquinolinesulfonamide-dichloride (H89; 1 microM) and glibenclamide (10 microM). CGRP (100 nM), unlike acetylcholine, did not alter endothelial intracellular Ca2+ concentration or V(m). CGRP (100 nM) hyperpolarized the smooth muscle V(m), an effect inhibited by L-NNA, H89, or glibenclamide. CGRP (500 nM) also caused a decrease in constriction frequency. However, this was no longer blocked by CGRP-(8-37). CGRP (500 nM) also caused smooth muscle hyperpolarization, an action that was now not blocked by L-NNA (100 microM). It was most likely mediated by the activation of the cAMP/PKA pathway and the opening of K+(ATP) channels because it was abolished by H89 or glibenclamide. We conclude that CGRP, at low to moderate concentrations (i.e., 1-100 nM), decreases lymphatic constriction frequency primarily by the stimulation of CGRP-1 receptors coupled to pertussis toxin-sensitive G proteins and the release of NO from the endothelium or enhancement of the actions of endogenous NO. At high concentrations (i.e., 500 nM), CGRP also directly activates the smooth muscle independent of NO. Both mechanisms of activation ultimately cause the PKA-mediated opening of K+(ATP) channels and resultant hyperpolarization.

Mechanism of CGRP-Induced Vasodilation in the Rat Isolated Perfused Kidney

We investigated the intracellular mechanisms involved in calcitonin gene-related peptide (CGRP)-induced vasodilation in rat isolated perfused kidney. CGRP-1 receptor antagonist, CGRP-8-37, abolished the responses. Endothelial denudation by Triton X-100 or nitric oxide (NO) synthase inhibition by NG-nitro-L-arginine attenuated the maximum dilation by about 63 and 55%, respectively. Protein kinase A inhibitor, KT-5720, caused an about 72% inhibition in CGRP-induced maximum dilation. Soluble guanylate cyclase inhibitor, ODQ, and ATP-sensitive potassium channel blocker, glibenclamide, inhibited the CGRP-induced maximum responses by 75 and 55%, respectively. Cyclooxygenase inhibitor, indomethacin, had no effect. Our data suggest that CGRP-1 receptors, endothelium, NO synthase, protein kinase A, soluble guanylate cyclase, and ATP-sensitive potassium channels, but not the cyclooxygenase pathway, may play a role in CGRP-induced vasodilation in rat isolated perfused kidney.

Adrenomedullin induces direct (endothelium-independent) vasorelaxations and cyclic adenosine monophosphate elevations that are synergistically enhanced by brain natriuretic peptide in isolated rings of rat thoracic aorta

Our laboratory previously demonstrated that nitric oxide and natriuretic peptides can synergistically enhance cAMP elevations and vasorelaxations in rat aortic rings induced by calcitonin gene-related peptide, likely involving cyclic guanosine monophosphate (cGMP)-mediated inhibition of type-3 phosphodiesterase (PDE3). It was predicted that this cellular mechanism may also serve as a point of synergism between adrenomedullin (ADM) and brain natriuretic peptide (BNP) in aortic smooth muscle cells. The current study shows that ADM (100 nM)-induced vasorelaxations in isolated aortic rings of Sprague-Dawley rats are dependent on endothelium (34.1 +/- 4.2% relaxation with endothelium versus 3.0 +/- 0.6% relaxation without endothelium; P < 0.001). To determine interactions between ADM and BNP in smooth muscle cells without interference from endothelium-derived factors, further studies used aortic rings denuded of endothelium. Pretreatment with BNP (1 nM), which elevated cGMP levels 1.6 fold, uncovered direct vasorelaxant effects of ADM in endothelium-denuded rings, showing 5.6 +/- 1.8%, 20.9 +/- 6.1%, and 55 +/- 9.4% relaxations with ADM at 1, 10, and 100 nM, respectively (n = 6). ADM (100 nM) significantly (P < 0.05) increased cyclic adenosine monophosphate (cAMP) levels in denuded aortic rings pretreated with BNP (1 nM), but not in denuded rings without BNP. Quazinone (20 microM), a PDE3 inhibitor, caused similar enhancement of direct cAMP elevations to ADM (100 nM). The data indicate vasodilatory synergism between ADM and BNP in aorta, likely mediated by enhanced accumulation of cAMP in smooth muscle cells resulting from BNP/cGMP-induced inhibition of PDE3. This synergistic mechanism may be especially important in subjects with dysfunctional endothelium, in which BNP may uncover direct vasorelaxant effects of ADM in arteries that normally require healthy (nitric oxide-releasing) endothelium for ADM-induced vasorelaxations to occur.

Vasorelaxations induced by calcitonin gene-related peptide, vasoactive intestinal peptide, and acetylcholine in aortic rings of endothelial and inducible nitric oxide synthase-knockout mice

The objective of this study was to determine if vasorelaxant responses to calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), and acetylcholine are altered in aortic rings of mice lacking genetic expression of endothelial nitric oxide synthase (eNOS) or inducible nitric oxide synthase (iNOS) genes (i.e., eNOS- and iNOS-knockout mice) as compared with control (wild-type) mice. Aortic rings from eNOS-knockout (eNOS (-/-)) mice did not relax in response to acetylcholine, thereby confirming previous reports. Aortic rings from iNOS-knockout (iNOS (-/-)) mice relaxed in response to acetylcholine in an endothelium-dependent manner. However, maximum relaxations in endothelium-intact rings were significantly (p < 0.05) larger than in control mice (85.3 +/- 3.1% in iNOS (-/-) mice vs. 67.9 +/- 5.6% in controls). CGRP caused concentration-dependent relaxations in aortas of all three types of mice: control mice, iNOS (-/-) mice, and eNOS (-/-) mice. Vasorelaxant responses to CGRP in control and iNOS (-/-) mice had identical relationships; both were partially dependent on endothelium. In eNOS (-/-) mice, dose-response curves of CGRP in endothelium-intact and endothelium-denuded rings were not significantly different, indicating loss of the partial dependence on endothelium. The vasorelaxant responses to VIP were completely dependent on endothelium in control and iNOS (-/-) mice. Maximum relaxations to VIP in iNOS (-/-) mice (77.4 +/- 2.7%) were significantly greater than in control mice (64.0 +/- 5.5%). Vasorelaxant responses to VIP in eNOS (-/-) aortic rings were also endothelium-dependent, but responses were greatly attenuated compared with wild-type mice. Relaxations induced by VIP (1 x 10 ) in endothelium-intact aortic rings of eNOS (-/-) mice and control mice were 18.3 +/- 5.4% and 64.0 +/- 5.5%, respectively. These findings demonstrated that, in eNOS (-/-) mice, aortic vasorelaxant responses to CGRP were fully present but no longer dependent on the endothelium, and responses to VIP were greatly attenuated compared with control and responses to acetylcholine were abolished. In iNOS (-/-) mice, aortic vasorelaxant responses to VIP and acetylcholine were significantly greater than wild-type control, suggesting that induction of iNOS may have attenuated vascular responses to VIP and acetylcholine in wild-type controls.

Influence of experimental diabetes on the microcirculation of injured peripheral nerve: functional and morphological aspects

Regeneration of diabetic axons has delays in onset, rate, and maturation. It is possible that microangiopathy of vasa nervorum, the vascular supply of the peripheral nerve, may render an unfavorable local environment for nerve regeneration. We examined local nerve blood flow proximal and distal to sciatic nerve transection in rats with long-term (8 month) experimental streptozotocin diabetes using laser Doppler flowmetry and microelectrode hydrogen clearance polarography. We then correlated these findings, using in vivo perfusion of an India ink preparation, by outlining the lumens of microvessels from unfixed nerve sections. There were no differences in baseline nerve blood flow between diabetic and nondiabetic uninjured nerves, and vessel number, density, and area were unaltered. After transection, there were greater rises in blood flow in proximal stumps of nondiabetic nerves than in diabetic animals associated with a higher number, density, and caliber of epineurial vessels. Hyperemia also developed in distal stumps of nondiabetic nerves but did not develop in diabetic nerves. In these stumps, diabetic rats had reduced vessel numbers and smaller mean endoneurial vessel areas. Failed or delayed upregulation of nerve blood flow after peripheral nerve injury in diabetes may create a relatively ischemic regenerative microenvironment.

Severe impairment of CGRP-induced hypotension in vivo and vasorelaxation in vitro in elderly rats

The aim of this study was to investigate the effects of aging on hypotension in vivo and vasorelaxation in vitro induced by calcitonin gene-related peptide (CGRP), using young (3 months old) and elderly (20 and 28 months old) Sprague-Dawley rats. Vasorelaxant responses were measured in isolated rings of rat thoracic aorta and rat caudal artery, which show endothelium-dependent and endothelium-independent responses to CGRP, respectively. The CGRP-induced vasorelaxations were significantly diminished in 28-month-old male rats in both aorta (39.3% of responses in young controls at 10 nM CGRP) and caudal artery (28.5% of responses in young controls at 10 nM CGRP). Acetylcholine caused vasorelaxations in aortic rings of young male rats, but vasocontractions in aortic rings of 28-month-old male rats. Hypotension induced by CGRP was significantly diminished in both 20-month-old male rats (47.7% of young controls) and 20-month-old female rats (34.4% of young controls). Moreover, ovariectomy, known to decrease CGRP-induced hypotension in young female rats, did not further decrease hypotension to CGRP in elderly female rats. In conclusion, vasorelaxant responses in vitro and hypotensive responses in vivo induced by the neuropeptide CGRP are severely impaired in elderly rats as compared to young rats. The data suggest that the vasodilatory responses to CGRP in both large arteries and the small resistance-sized arteries regulating arterial blood pressure are damaged or down-regulated by the aging process.

Vasorelaxant response to isoprenaline, nitric oxide donor, calcitonin gene-related peptide and vasoactive intestinal peptide in aortic rings of adult C57BL/6J mice

The mouse and tissues from this species are increasingly used as experimental models because of the wide variety of gene deletions and overexpressions available in this species. Yet, very little is known about normal vascular responses in the mouse. We investigated the vasorelaxant responses on thoracic aortic rings from the adult male C57BL/6J mouse. Isoprenaline, acetylcholine, calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP) and sodium nitroprusside all caused concentration-dependent relaxations in aortic rings possessing healthy endothelium and precontracted with phenylephrine. Maximum relaxations were 64.9+/-2.6%, 66.8+/-2.9%, 114.3+/-4.6%, 65.1+/-4.2% and 116.2+/-5.1% with -logEC(50) values of 6.76+/-0.14, 7.04+/-0.11, 8.53+/-0.14, 8.29+/-0.26 and 8.10+/-0.20 for isoprenaline, acetylcholine, CGRP, VIP and sodium nitroprusside, respectively. There were significantly smaller responses to isoprenaline, acetylcholine, CGRP and VIP when the endothelium was denuded. The maximum relaxations for isoprenaline, CGRP and acetylcholine were 48.3+/-5.1%, 99.6+/-4.4% and 5.7+/-1.6% with -logEC(50) values of 6.44+/-0.40 and 8.23+/-0.192, respectively, following endothelium removal. The response to VIP was completely dependent to endothelium. Without precontraction, isoprenaline, at the higher doses, caused small contractions. These experiments provide new information about vascular responses of five vasodilators in aortic rings of adult male C57BL/6J mice.

Modulation of coronary flow and cardiomyocyte size by sensory fibers

Cardiac tissue is densely innervated by sensory neurons that are believed to play important modulatory roles in cardiac functions. In this study, pretreatment of neonate rats with capsaicin was performed. In adult rats, cardiomyocyte size and amount of fibrous tissue in left ventricles as well as in vitro coronary flow were evaluated. The chronotropic and inotropic responses to beta-adrenoceptor agonists (norepinephrine and isoproterenol), muscarinic agonists (carbachol and pilocarpine), and calcitonin gene-related peptide (CGRP) were also investigated with the use of the isolated right atria preparation. Capsaicin pretreatment significantly (P<0.05) reduced both basal coronary flow (18% reduction) and cardiomyocyte size (34% reduction) without affecting the amount of fibrous tissues in the left ventricles. The positive inotropic and chronotropic effects in response to norepinephrine in the isolated rat heart did not significantly differ between control and capsaicin-treated rats. Similarly, the positive chronotropic effects in response to norepinephrine, isoproterenol, and CGRP as well as the negative chronotropic responses to carbachol and pilocarpine in the isolated right atria were not affected by capsaicin pretreatment. Our data are consistent with the suggestion that reductions of both basal coronary flow and cardiomyocyte size seen in hearts from capsaicin-pretreated rats may be consequences of CGRP depletion. The cardiomyocyte size reduction produced by capsaicin treatment may be related to a modulatory role of CGRP as a growth factor.

Mechanism of SNAP potentiating antiproliferative effect of calcitonin gene-related peptide in cultured vascular smooth muscle cells

We previously showed that CGRP inhibits cell proliferation which correlates with an elevation of cAMP levels in rabbit aortic vascular smooth muscle cells (VSMCs). The present study determined the effects of S-nitroso-N-acetylpenicillamine (SNAP, a nitric oxide donor) on CGRP-induced antiproliferative effects and cellular mechanism in cultured rabbit aortic VSMCs. The cells (in fifth-sixth passage) were exposed to 2.5% fetal bovine serum for 24 h in the presence or absence of SNAP, hCGRP or both.(3)H-thymidine incorporation was used to measure DNA synthesis. The results showed that SNAP (60-100 microm) significantly inhibited the proliferation and elevated cGMP levels in cultured rabbit aortic VSMCs. In combination, however, SNAP (30 microm) potentiated hCGRP (10-100 n m)-induced antiproliferation. SNAP (30 microm) and hCGRP (10-100 n m) or forskolin (10 microm), an activator of adenylate cyclase, caused more than additive cAMP elevations, but not cGMP elevations, in these cells. Quazinone, an inhibitor of cGMP-inhibited-phosphodiesterase (cGI-PDE, PDE3), or SNAP plus quazinone caused a similar potentiation as SNAP of the hCGRP-induced elevations of cAMP levels. The data indicate that SNAP-induced potentiation of CGRP's effects likely involves inhibition of cGI-PDE, thus allowing enhanced accumulation of cAMP that mediates the antiproliferative effects of hCGRP in cultured rabbit aortic VSMCs.

Receptors mediating CGRP-induced relaxation in the rat isolated thoracic aorta and porcine isolated coronary artery differentiated by h(alpha) CGRP(8-37)

1 Receptors mediating CGRP-induced vasorelaxation were investigated in rat thoracic aorta and porcine left anterior descending (LAD) coronary artery and anterior interventricular artery (AIA), using CGRP agonists, homologues and the antagonist h(alpha) CGRP(8-37). 2 In the endothelium-intact rat aorta, h(alpha) CGRP, h(beta) CGRP, rat beta CGRP and human adrenomedullin caused relaxation with similar potencies. Compared with h(alpha) CGRP, rat amylin was about 25 fold less potent, while [Cys(ACM2,7)] h(alpha) CGRP and salmon calcitonin were at least 1000 fold weaker. 3 H(alpha) CGRP(8-37) (up to 10(-5) M) did not antagonize responses to h(alpha) CGRP, h(beta) CGRP or rat beta CGRP (apparent pKB <5). Peptidase inhibitors did not increase either the effect of h(alpha) CGRP or [Cys(ACM,2,7)] h(alpha) CGRP, while h(alpha) CGRP(8-37) remained inactive. Endothelium-dependent relaxation produced by h(alpha) CGRP was accompanied by increases in cyclic AMP and cyclic GMP, that were not inhibited by h(alpha) CGRP(8-37) (10(-5) M). 4 In porcine LAD and AIA, h(alpha) CGRP produced relaxation in an endothelium-independent manner. H(alpha) CGRP(8-37) competitively antagonized h(alpha) CGRP responses (pA2 6.3 and 6.7 (Schild slope 0.9+/-0.1, each), in LAD and AIA, respectively). In LAD artery, h(alpha) CGRP-induced relaxation was accompanied by increases in cyclic AMP that were inhibited by h(alpha) CGRP(8-37) (10(-7)-10(5 )). 5 In conclusion, the antagonist affinity for h(alpha) CGRP(8-37) in porcine coronary artery is consistent with a CGRP1 receptor, while the lack of h(alpha) CGRP(8-37) antagonism in rat aorta could suggest either a CGRP receptor different from CGRP1 and CGRP2 type, or a non-CGRP receptor.

Nitric oxide donors enhance calcitonin gene-related peptide-induced elevations of cyclic AMP in vascular smooth muscle cells

Vasorelaxant effects of calcitonin gene-related peptide (CGRP) are dependent on endothelium-derived nitric oxide (NO) in some arteries. The mechanism involved is still not clear. In the present study, we used NO donors (sodium nitroprusside (SNP) and 6-(2-hydroxy-1-methyl-2-nitrisohydrazino)-N-methyl-1-hyxanamine (NOC-9)), cyclic GMP elevator (brain natriuretic peptide (BNP)) and a selective type III (cyclic GMP-inhibited) phosphodiesterase (PDE) inhibitor 5-(4-acetamidophenyl)pyrazin-2(1H)-one (SK&F94120) to investigate involvement of NO, cyclic GMP and type III PDE in CGRP-induced accumulation of cyclic AMP in cultured rat aortic smooth muscle cells. SNP (10 microM), NOC-9 (10 microM) and BNP (1 microM) all increased intracellular cyclic GMP to similar levels (2- to 2.5-fold above basal) and caused significant enhancement of CGRP (10 nM)-induced cyclic AMP accumulation similar to that caused by 10 microM SK&F 94120. The data are therefore consistent with our hypothesis that the mechanism of endothelium-dependent vasorelaxation effect of CGRP involves cyclic GMP-mediated inhibition of type III PDE and subsequent accumulation of cyclic AMP in smooth muscle cells.

Evidence for nitric oxide and nitric oxide synthase activity in proximal stumps of transected peripheral nerves

Nitric oxide may be liberated as an inflammatory mediator within injured peripheral nerve trunks. We evaluated the proximal stumps of injured peripheral nerve stumps that later form neuromas or regenerative nerve sprouts, for evidence of local nitric oxide elaboration and activity. Proximal stumps were created in male Sprague-Dawley rats by sectioning of the sciatic nerve and resection of its distal portions and branches. There was striking physiological evidence of nitric oxide activity at the tips of 48-h and 14-day-old proximal nerve stumps. We detected local nitric oxide-mediated hyperemia of both extrinsic plexus and endoneurial microvessels that was reversible, in a dose-dependent stereospecific fashion, by the broad-spectrum nitric oxide synthase inhibitors, Nomega-nitro-L-arginine-methyl ester or Nomega-nitro-L-arginine, but not by 7-nitroindazole, an inhibitor with relative selectivity for neuronal nitric oxide. Immunohistochemical studies provided evidence for the localization of nitric oxide generators at the same sites. In 48-h but not 14-day stumps increased expression of two isoforms of nitric oxide synthase was detected: endothelial nitric oxide and to a much lesser extent neuronal nitric oxide synthase. Both isoforms appeared in axonal endbulb-like profiles that co-localized with neurofilament immunostaining. Western immunoblots identified a band consistent with endothelial nitric oxide synthase expression. In 14-day stumps with early neuroma formation, but not 48-h stumps, there was staining for immunological nitric oxide synthase in some endoneurial and epineurial macrophages. Total nitric oxide synthase biochemical enzymatic activity, measured by labelled arginine to citrulline conversion, was increased in 14-day but not 48-h stumps. Injured peripheral nerves have evidence of early nitric oxide action, nitric oxide synthase expression and nitric oxide activity in proximal nerve stumps. Nitric oxide may have an important impact on the regenerative milieu.

Interleukin-1beta causes different levels of nitric oxide-mediated depression of contractility in different positions of rat thoracic aorta

Interleukin-1beta (IL-1beta) can be synthesized by macrophages, endothelial cells and vascular smooth muscle cells when stimulated by bacterial lipopolysaccharide (endotoxin) during septic shock. The IL-1beta levels in the blood vessel wall are also elevated in atherosclerosis. IL-1beta can cause induction of inducible nitric oxide synthase (iNOS) expression in vascular smooth muscle cells and produce vasorelaxation, hypotension and ultimately tissue damage. We studied the depressions of vascular smooth muscle contractions at 3 hours after exposure to IL-1beta in different positions of rat thoracic aorta. The data show that the aortic rings from the cranial end of rat thoracic aorta had little response to IL-1beta (0.5 and 1.0 ng/ml) while those from the caudal end of thoracic aorta had larger depressant response. S-methylisothiourea sulfate (SMT), an iNOS inhibitor, completely blocked the depression of contraction caused by IL-1beta in intact aortic rings. If the endothelium was removed from the aortic rings before exposure to IL-1beta, all rings from different parts of the thoracic aorta showed an equal amount of vasodepression. Thus, the difference in the depressant response of IL-1beta in different portions of thoracic aorta is endothelium-dependent and involves induction of NOS.

Brain natriuretic peptide enhances the endothelium-independent cAMP and vasorelaxant responses of calcitonin gene-related peptide in rat aorta

Calcitonin gene-related peptide (CGRP) causes vasorelaxation in rat aorta involving endothelium/nitric oxide (NO)-dependent elevations of both cAMP and cGMP levels. When endothelium is removed, preincubation with exogenous NO uncovers and potentiates direct (endothelium-independent) cAMP elevations and vasorelaxations caused by CGRP. This enhancing effect of NO potentially involves elevation of cGMP and inhibition of Type III (cGMP-inhibitable) phosphodiesterase, causing accumulation of cAMP. However, NO may have other actions. The aim of the present study was to determine if brain natriuretic peptide (BNP), which elevates cGMP levels independent of NO, could enhance cAMP accumulations and vasorelaxations induced by CGRP in rat aortic rings denuded of endothelium. When added separately, neither CGRP (100 nM) nor BNP (10 nM) altered cAMP levels. When added in combination, CGRP (100 nM) and BNP (10 nM) significantly elevated cAMP levels (from control of 0.95+/-0.08 to 1.53+/-0.09 pmol/mg protein) at 2 min. BNP (10 nM) elevated cGMP levels 10-fold at 2 min and this response was not altered by co-administration of CGRP (100 nM). Pretreatment with BNP at concentrations as low as 1 nM in endothelium-denuded aortic rings greatly enhanced the direct vasorelaxant effects of CGRP (100 nM) (from control of 0% to 57.6+/-6.8% relaxation of phenylephrine-precontractions). Our findings indicate that BNP enhances direct (endothelium-independent) cAMP elevations and vasorelaxations caused by CGRP in rat aorta, thus supporting the concept that cGMP inhibits cAMP metabolism and enhances CGRP-induced responses in aortic smooth muscle cells.

Effects of adrenomedullin and calcitonin gene-related peptide on contractions of the rat aorta and porcine coronary artery

1. Effects of adrenomedullin and alpha-calcitonin gene-related peptide (CGRP) on the contractions and cytosolic Ca2+ concentrations ([Ca2+]i) of the rat aorta and porcine coronary artery were investigated. Characteristics of the receptors mediating the effects of adrenomedullin and alpha-CGRP were also investigated. 2. Adrenomedullin and alpha-CGRP caused a concentration-dependent relaxation in the rat aorta contracted with noradrenaline. The IC50 values for adrenomedullin and alpha-CGRP were 2.4 nM and 4.0 nM, respectively. The relaxant effects of these peptides were abolished by removal of the endothelium and significantly attenuated by an inhibitor of nitric oxide synthase, NG-monomethyl-L-arginine (L-NMMA, 100 microM), but not by a cyclo-oxygenase inhibitor, indomethacin (10 microM). 3. Adrenomedullin and alpha-CGRP increased the endothelial [Ca2+]i in the rat aorta with endothelium, whereas they did not change [Ca2+]i in the smooth muscle. 4. An antagonist of the CGRP1 receptor, CGRP (8-37), antagonized the relaxant effects of alpha-CGRP and the beta-isoform of CGRP (beta-CGRP) but not those of adrenomedullin in the rat aorta. 5. In the porcine coronary artery contracted with U46619, adrenomedullin and alpha-CGRP caused a concentration-dependent relaxation with an IC50 of 27.6 and 4.1 nM, respectively. Removal of the endothelium altered neither the IC50 values nor the maximal relaxations induced by adrenomedullin or alpha-CGRP. When the artery was contracted with high K+ solution (72.7 mM), these peptides caused a small relaxation. 6. Adrenomedullin and alpha-CGRP increased cyclic AMP content and decreased the smooth muscle [Ca2+]i in the porcine coronary artery. 7. CGRP (8-37) significantly antagonized the relaxant effects of adrenomedullin and alpha-CGRP in the porcine coronary artery. However, it had little effect on the relaxations induced by the beta-isoform of CGRP (beta-CGRP). 8. These results suggest that in the rat aorta, adrenomedullin and alpha-CGRP increase the endothelial [Ca2+]i, activate nitric oxide synthase and release nitric oxide, without a direct inhibitory action on smooth muscle. In the porcine coronary artery, in contrast, adrenomedullin and alpha-CGRP directly act on smooth muscle, increase cyclic AMP content, decrease the smooth muscle [Ca2+]i and inhibit contraction. The rat aortic endothelium seems to express the CGRP receptor which is sensitive to alpha-CGRP, beta-CGRP and CGRP (8-37) and the adrenomedullin specific receptor. The porcine coronary smooth muscle, in contrast, seems to express two types of CGRP receptor; one of which is sensitive to alpha-CGRP, CGRP (8-37) and adrenomedullin and the other is sensitive only to beta-CGRP.

Pathway-specific effects of calcitonin gene-related peptide on irideal arterioles of the rat

1. Arteriolar diameter and membrane voltage have been measured to investigate the actions of calcitonin gene-related peptide (CGRP) in rat irideal arterioles. 2. Activation of sensory nerves inhibited sympathetic vasoconstriction, reduced the accompanying 40-50 mV depolarization by 90% and caused a 4 mV hyperpolarization. 3. The inhibition of vasoconstriction was prevented by either preincubation in L-NAME (10 microM), to inhibit nitric oxide production, by preincubation in the cell-permeant adenylate cyclase inhibitor dideoxyadenosine (1 mM) or by preincubation in the ATP-sensitive potassium channel blocker glibenclamide (10 microM). The subsequent addition of a nitric oxide donor to the glibenclamide solution inhibited nerve-mediated vasoconstriction, suggesting that the potassium channel involvement preceded the production of nitric oxide. The small hyperpolarization was not affected by L-NAME. 4. Nerve-mediated vasodilatation persisted in the presence of L-NAME (10 microM) but was abolished with the CGRP1 receptor antagonist CGRPS-37. 5. In arterioles preconstricted with the alpha 2-adrenoceptor agonist UK-14304 (100 nM), exogenous CGRP caused a hyperpolarization and a dose-dependent vasodilatation, neither of which was affected by L-NAME (10 microM). 6. In arterioles preconstricted with 30 mM KCl, CGRP (10 nM) caused vasodilatation but not hyperpolarization, suggesting that the hyperpolarization was not causal to the vasodilatation. 7. Forskolin (30 nM), in the presence of L-NAME to present effects due to nitric oxide, caused vasodilatation. 8. These results suggest that CGRP inhibits sympathetic nerve-mediated vasoconstriction through sequential increases in cyclic AMP and nitric oxide, while vasodilatation results from increases in cyclic AMP alone. The production of nitric oxide, but not its mechanism of action, appears to be dependent on the activation of ATP-sensitive potassium channels. The possible sites of action of these two pathways are discussed.

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Lactic acid potentiates bradykinin- and low-pH-induced release of CGRP from rat spinal cord slices

Previous data from our laboratory have shown that calcitonin gene-related peptide (CGRP) is released into the circulation during pathogenesis of endotoxic, hemorrhagic, and septic shock and appears to mediate in part the vascular problems of shock. Elevations in the levels of bradykinin (BK) and lactic acid and lowering of tissue pH also occur during shock and could be involved in CGRP release. In the present study, we have tested whether lactic acid, alone or in combination with BK or low pH, triggers release of CGRP-like immunoreactivity (CGRP-LI) from sensory nerves, using rat spinal cord slices as a tissue model. Lowering media pH from 7.4 to < or = 6.0 increased the release of CGRP-LI. Lactic acid (5 and 10 mM) by itself elevated CGRP-LI release from a control of 6.89 +/- 0.95 to 57.2 +/- 8.2 and 116 +/- 13 pg/mg protein, respectively. The combination of pH 6.0 and lactic acid (5 or 10 mM) caused more than additive stimulation of CGRP-LI release. BK (50 or 100 microM) elevated CGRP-LI release, which was greatly enhanced by lactic acid (2.5 or 5 mM). The data indicate that lactic acid potentiates BK- and low-pH-induced release of CGRP from sensory nerves in spinal cord. Similar mechanisms may occur at peripheral ends of sensory nerves, contributing to CGRP release during septic shock and other conditions with elevated lactic acid levels (e.g., strenuous exercise and tissue ischemia).

Regional differences in endothelium-dependent relaxation in the rat: contribution of nitric oxide and nitric oxide-independent mechanisms

Relaxant effects of acetylcholine (ACh), histamine, calcitonin gene-related peptide (CGRP) and the calcium ionophore A23187 were examined in rat femoral (phi approximately 0.2 mm), mesenteric (0.2 mm), intrarenal (0.2 mm) and hepatic (0.3 mm) arteries, and aorta (2 mm). Acetylcholine elicited an endothelium-dependent relaxation in all arteries. Histamine induced an endothelium-dependent relaxation in aorta, and mesenteric and intrarenal arteries, whereas a partly endothelium-dependent and mainly endothelium-independent relaxation was observed in hepatic and femoral arteries, respectively. In hepatic, mesenteric and intrarenal arteries, CGRP induced an endothelium-independent relaxation, whereas either small or no relaxation was obtained in aorta and femoral arteries respectively. A23187 induced an endothelium-dependent relaxation in the aorta and hepatic artery, whereas A23187 had no relaxant effect in femoral, mesenteric and intrarenal arteries. N omega-nitro-L-arginine (L-NOARG, 0.3 mM) reduced the maximum ACh-induced relaxation (in the presence of 10 microM indomethacin) by 66% in the aorta, and abolished the relaxation in femoral and intrarenal arteries. A marked L-NOARG/indomethacin-resistant relaxation was obtained in mesenteric and hepatic arteries. Levcromakalim induced a concentration-dependent and almost complete relaxation in all arteries. When contracted by a 60 mM K+ solution, all arteries responded to ACh with a relaxation that was abolished by L-NOARG. These results demonstrate marked regional differences with regard to the vascular effects of ACh, histamine, CGRP and A23187. Whereas nitric oxide appears to mediate endothelium-dependent relaxation regardless of the vascular region, an L-NOARG/indomethacin-resistant relaxation, presumably mediated by an endothelium-deprived hyperpolarizing factor, was observed only in mesenteric and hepatic arteries, and aorta.

Effect of capsaicin on membrane currents in cultured vascular smooth muscle cells of rat aorta

The application of capsaicin (1 microM) produced a minor relaxant effect in endothelium-denuded rat aortae. However, capsaicin caused a greater relaxation of blood vessels precontracted with high K+ or phenylephrine. The effects of capsaicin on the ionic currents were also examined in A7r5 vascular smooth muscle cells. The tight-seal whole-cell voltage clamp technique was used. Capsaicin inhibited the Ba2+ inward current (IBa) through the voltage-dependent L-type Ca2+ channel in a concentration-dependent fashion, whereas calcitonin gene-related peptide and phenylephrine produced a minor increase in IBa. Capsaicin did not alter the overall shape of current-voltage relationship of IBa. However, capsaicin (3 microM) shifted the quasi-steady-state inactivation curve of IBa to more negative membrane potential by about 5 mV. These effects of capsaicin on IBa were reversible. In addition, capsaicin had inhibitory effects on voltage dependent K+ currents. These results suggest that inhibition of the voltage-dependent L-type Ca2+ channel is involved in the capsaicin-induced relaxation of the vascular smooth muscle, whereas capsaicin-induced inhibition of voltage-dependent K+ channels might produce an increase in cell excitability.

Skeletal muscle microcirculatory response to rat alpha-calcitonin gene-related peptide

We used in vivo video microscopy to determine the effect of increasing doses of rat alpha-calcitonin gene-related peptide (rCGRP) on rat cremaster muscle arterioles in the presence or absence of the nitric oxide synthase inhibitor N-omega-nitro-L-arginine (L-NNA). Male Sprague-Dawley rats (118-148 g) were anaesthetized with pentobarbital, and neurovascularly intact cremaster muscles were imaged. Changes in the diameter, erythrocyte velocity and volume flow in second-(A2), third-(A3), and fourth-(A4) order arterioles were determined. To produce uniform arteriolar tone, the cremaster preparation was challenged with norepinephrine (NE: 10(-7) M). L-NNA (10(-4) M), which was shown to inhibit acetylcholine-(ACh: 10(-6) M) induced arteriolar dilations, was added to 16 of the preparations. Preparations were then challenged by adding cumulative log concentrations of rCGRP (10(-12)-10-7) M; n = 16) or an equivalent volume of vehicle (n = 19) to the bath. Following rCGRP challenge, arterioles were maximally dilated with 10(-5) M nitroprusside (NP). rCGRP caused significant dose-dependent increases in erythrocyte velocity and volume flow in A2 arterioles, and in diameter, velocity, and volume flow in A3 and A4 arterioles, by 10(-8) M, when compared with vehicle-treated controls. L-NNA had no significant effect on rCGRP-induced responses. These data indicate that rCGRP causes dose-dependent dilation of skeletal muscle resistance arterioles at a concentration similar to that observed in larger vessels. This dilation does not appear to be dependent on the vascular production of nitric oxide from L-arginine.

Characterization of the CGRP receptor and mechanisms of action in rat mesenteric small arteries

Rat alpha-calcitonin gene-related peptide-induced concentration-dependent (100 pM-10 nM) relaxations in rat mesenteric small arteries (i.d. approximately 220 microns) contracted with noradrenaline, prostaglandin F2 alpha or K+, however, the maximal relaxation depended on the precontractile stimulus, being highest (95%) in arteries contracted with PGF2 alpha and lowest (51%) in arteries contracted with 125 mM K+. The relaxation was inhibited between 10 pM and 1 nM by removal of the endothelium, but was not antagonized by glibenclamide (1 microM), tetraethylammonium (30 mM), apamine (0.3 microM) and 4-aminopyridine (3 mM). The concentration-response curve to rat alpha-CGRP and human beta-CGRP was shifted to the right in the presence of 1 microM human alpha-CGRP(8-37) indicating a receptor affinity, -log(KB[M]), equal to 7.2 and 7.0, respectively. It is concluded that the relaxation induced by CGRP depends minimally on the endothelium and K(+)-channel opening is not a principal process in the relaxing effect of CGRP, thus a third mechanism must mediate the relaxation in these vessels. The main CGRP receptor type mediating relaxation in rat mesenteric small arteries belongs to the CGRP1 subtype.

N omega-nitro-L-arginine inhibits vasodilations and elevations of both cyclic AMP and cyclic GMP levels in rat aorta induced by calcitonin gene-related peptide (CGRP)

Our previous studies showed that vasodilations and elevations of both cyclic AMP and cyclic GMP levels in rat aorta induced by rat calcitonin gene-related peptide (rCGRP) are inhibited by hemoglobin and methylene blue, blockers of the endothelium-derived relaxant factor (EDRF, now recognized as nitric oxide [NO]). In the present study, we used N omega-nitro-L-arginine (L-NNA), a selective inhibitor of nitric oxide synthase, to test whether rCGRP-induced relaxations and cyclic AMP and cyclic GMP responses in rat aorta require de novo synthesis of NO. L-NNA (30 microM, 15 min) inhibited by 84, 76 and 73% the relaxations induced by rCGRP at 1, 10 and 100 nM, respectively. D-NNA (30 microM), which does not inhibit nitric oxide synthase, did not block rCGRP-induced vasorelaxations. Addition of L-arginine (3 mM) 5 min before L-NNA completely prevented the L-NNA-inhibition of CGRP-induced relaxations. L-NNA (30 microM, 15 min) also inhibited the elevations of both cyclic AMP and cyclic GMP levels caused by CGRP (100 nM). The data suggest that de novo synthesis of nitric oxide from its precursor L-arginine is required for rCGRP to induce vasodilations and elevations of both cyclic AMP and cyclic GMP levels in rat aorta.

Nitroglycerin (exogenous nitric oxide) substitutes for endothelium-derived nitric oxide in potentiating vasorelaxations and cyclic AMP elevations induced by calcitonin gene-related peptide (CGRP) in rat aorta

Rat calcitonin gene-related peptide (rCGRP) causes endothelium-dependent vasorelaxations via a dual signal transduction mechanism involving elevations of both cyclic AMP and cyclic GMP levels in rat aorta. These responses are all dependent on de novo synthesis of nitric oxide (NO) in endothelial cells and appear to involve a mechanistic link between cyclic GMP and cyclic AMP responses in smooth muscle cells. The present study determined whether NO from an exogenous source (i.e. added nitroglycerin) could substitute for endogenous NO in rCGRP-induced responses in endothelium-denuded aorta. Nitroglycerin (1 microM) significantly elevated cyclic GMP levels by 20-fold and 3.3-fold and cyclic AMP levels by 26% and 22% at 1 and 2 min, respectively. By itself, rCGRP (100 nM) did not significantly elevate cyclic AMP levels. In combination, however, nitroglycerin and rCGRP caused more-than-additive cyclic AMP elevations (41% above basal at 1 and 2 min). Nitroglycerin also potentiated rCGRP-induced vasorelaxations in endothelium-denuded rings, thus uncovering a direct (endothelium-independent) relaxant effect of rCGRP in rat aorta. The data indicate that exogenous NO can substitute for endogenous NO in rCGRP-induced relaxant and cyclic AMP responses in aorta. This nitroglycerin-induced potentiation of CGRP effects likely involves inhibition of cyclic-GMP-inhibited-phosphodiesterase in smooth muscle cells, thus allowing cyclic AMP to accumulate and mediate the direct vasodilator effects of rCGRP.

Vasodilator effects of parathyroid hormone, parathyroid hormone-related protein, and calcitonin gene-related peptide in the human fetal-placental circulation

OBJECTIVE

The purpose of our study was to determine the vasoactivity of calcitonin gene-related peptide (CGRP), parathyroid hormone (PTH), and parathyroid hormone-related protein (PTHrP) in the human fetal-placental circulation in vitro.

METHODS

Dually perfused placental cotyledons from term pregnancies were used in this study.

RESULTS

Calcitonin gene-related peptide, PTHrP (both 10(-10)-10(-6) mol/L), and PTH (10(-8)-10(-6) mol/L) demonstrated a significant concentration-dependent vasodilator effect (P = .0007, P = .0172, P = .0063, respectively), following preconstriction with a thromboxane mimetic U46619. The CGRP-1 receptor inhibitor CGRP8-37 (10(-6) mol/L) significantly inhibited (P = .0131) the CGRP-induced vasodilator effect, while the nitric oxide synthesis inhibitor n-nitro-l-arginine showed no inhibitory effect.

CONCLUSIONS

These results demonstrate the vasodilator effects of CGRP, PTH, and PTHrP in the human fetal-placental circulation. Calcitonin gene-related peptide and PTHrP were of equal potency, and both were approximately 100 times more potent than PTH. This study also suggests the CGRP may exert its vasodilator effect through two classes of receptors in the human placenta and may do so independently of nitric oxide.

Calcitonin gene-related peptide innervation and binding sites in rat aorta during development

Indirect immunohistochemistry performed on whole mounts of arch and thoracic part of the rat aorta at six developmental stages (from embryonic day 17 to 6 months, in males and females) revealed that calcitonin gene-related peptide (CGRP) innervation is highest in the arch. The highest density of innervation is found at the three first postnatal ages investigated (day 1, day 3 and 5 weeks; 2.6 +/- 0.6 intercepts/mm in the arch at 1 day); however, all values are low compared to other arteries. The innervation grows from a few short isolated fibres in the embryo to a more complex meshwork in older animals. No striking differences were noticed between males and females. Autoradiographic studies were performed on serial sections at several levels of the aorta but did not reveal binding sites for CGRP in the vascular wall. This might be due to the technique which does not allow visualization of low density of binding sites, or to binding sites of weak affinity. We discuss the possible importance of CGRP in rat aortic smooth muscle development.

Effect of guanine nucleotides and temperature on calcitonin gene-related peptide receptor binding sites in brain and peripheral tissues

Recent data have suggested the existence of at least two major classes of calcitonin gene-related peptide (CGRP) receptors in brain and peripheral tissues [Henke et al., Brain Res., 410 (1987) 404-408; Dennis et al., J. Pharmacol. Exp. Ther., 251 (1989) 718-725; ibid, 254 (1990) 123-128; Quirion et al., Ann. NY Acad. Sci., 657 (1992) 88-105]. However, little is currently known in the structure characteristics of CGRP receptors as cloning as yet to be reported. In the present study, the sensitivity of [125I]humanCGRP alpha binding to guanine nucleotides and temperature was investigated in guinea pig atria (prototypical CGRP1 tissue) guinea pig vas deferens (prototypical CGRP2 tissue) and in the rat brain and cerebellum (mixed assay). Binding isotherms of [125I]hCGRP alpha in those four tissue preparations were curvilinear and best fitted to a two-site model under most assay conditions. The high affinity binding component was highly temperature-sensitive and accounted, under experimental conditions, for up to 18% of the total population of receptors. Moreover, these high affinity sites were also highly sensitive to guanine nucleotides (Gpp(NH)p, 100 microM) in all preparations although to a different extend depending upon assay temperatures. Taken together, this suggests that the different CGRP receptor subtypes present in these tissue all belong to a G-protein coupled receptor family.

Diabetes mellitus prevents capsaicin from inducing hyperaemia in the rat sciatic nerve

Loss of neurogenic inflammation in response to tissue injury may be an important complication of diabetes mellitus. We studied local neurogenic inflammation in the peripheral nerve trunk of Sprague-Dawley rats 4 months following the induction of diabetes by streptozotocin injection. To assess neurogenic inflammation, the epineurial plexus of the sciatic nerve was exposed to topical capsaicin, an agent that releases vasoactive neuropeptides from perivascular afferent terminals. Under normal circumstances, local vasodilation results in endoneurial hyperaemia or a 'flare'. We evaluated the influence of capsaicin in diabetic sciatic nerve by making serial measurements of endoneurial blood flow using microelectrodes sensitive to hydrogen clearance. After 4 months of hyperglycaemia (glucose > 16.0 mmol/l), diabetic animals had slowing of unmyelinated and myelinated sural sensory conduction velocity compared to citrate buffer injected controls. Baseline sciatic endoneurial blood flow was unaltered by diabetes, and was comparable to controls. There was an expected hyperaemic response of endoneurial blood flow to capsaicin in control rat sciatic endoneurium but no consistent 'flare' response in diabetic rats. Our findings indicate that there is loss of capsaicin-related neurogenic inflammation in the vasa nervorum of experimental diabetes. It is possible that a similar deficit following nerve injury could impair the milieu for axonal regeneration in diabetes.

Glibenclamide and L-NG-nitro-arginine methyl ester modulate the ocular and hypotensive effects of calcitonin gene-related peptide

Calcitonin gene-related peptide (CGRP) given i.v. to rabbits induced hypotension and a breakdown of the blood-aqueous barrier. Glibenclamide, a blocker of ATP-sensitive K+ (K+ATP) channels antagonized both these effects. The K+ATP channel opener diazoxide reduced blood pressure but did not damage the blood-aqueous barrier. Inhibition of nitric oxide synthase antagonized the effects of CGRP on the blood-aqueous barrier but did not attenuate the hypotensive response. The results suggest that vasodilatation induced by the opening of K+ATP channels is a prerequisite for the effect of CGRP on the blood-aqueous barrier.

NADPH diaphorase in the spinal cord of rats

To identify spinal neurons that may synthesize nitric oxide, cells and fibers histochemically stained for NADPH diaphorase (a nitric oxide synthase) were studied in the spinal cord of rats. The histochemical reaction gave an image similar to the best Golgi impregnations, staining cells down to their finest processes. Transverse, horizontal, and parasagittal 50 and 100 microns sections were used to follow dendritic and axonal arborizations of stained neurons. Major cell groups were identified in the superficial dorsal horn and around the central canal (at all spinal levels), and in the intermediolateral cell column (at thoracic and sacral levels). Scattered positive cells were also found in deeper dorsal horn, ventral horn, and white matter. In some cases, axons of cells in the dorsal horn could be traced into the white matter; many of these cells resembled neurons projecting to various supraspinal targets. Stained cells in the intermediolateral column, which sent their axons into the ventral root, were presumed to be preganglionic autonomic neurons. Dense plexes of fibers were stained in laminae I and II and in the intermediolateral column. A large number of NADPH diaphorase-positive neurons in the spinal cord appear to be involved in visceral regulation. Fibers of the intermediolateral system had a special relationship with vasculature, suggesting that nitric oxide may help to couple neural activity with regional blood flow in the spinal cord. The abundance of NADPH diaphorase-positive neurons and fibers in the superficial dorsal horn suggests that nitric oxide may also be involved in spinal sensory processing.

Calcitonin gene-related peptide: multiple actions, multiple receptors

Calcitonin gene-related peptide (CGRP) shows diversity both in its effects and its receptors. It is likely to have roles as a neurotransmitter, neuromodulator, local hormone and trophic factor. Its effects include rapid changes in neuronal activity, relaxation of many types of smooth muscle, actions on metabolism and changes in gene expression. Receptor heterogeneity has been revealed from experiments comparing agonist potency ratios and antagonist affinities. The evidence from these approaches is reviewed in this article and a speculative receptor classification scheme is proposed. Some of the likely future directions for CGRP research are discussed.