Effects of exogenic noradrenaline and melatonin on the neurogenic vasoreactivity

The effect of 0.1 microM noradrenaline and melatonin on the response to electrical field stimulation (EFS) of the juvenile rat artery segment was studied. Noradrenaline like melatonin was shown to potentiate the EFS-evoked constriction decreased in the course of experiments or in the acidic solution (pH 6.6), and this potentiation was proportional to the extent of the preceding decrease of the constriction. The effect of these substances was additive. The results suggest that noradrenaline as well as melatonin can serve as a means to restore the diminished neurogenic reactivity of blood vessels.     

Reactivity of blood vessels in vitro: the role of perfusate pH and the initial tone of myocytes

In the rat mesenteric artery ring, acidic solution (pH 7.0) inhibited the electrical field stimulation (EFS)-evoked response in a wide range of stretch. Acidosis potentiated the EFS-evoked constriction of the ring precontracted with 0.5 mu noradrenaline. The EFS dilated rings precontracted with 0.7 mu noradrenaline in control solution but contracted it in the acidic solution. Tone-dependent mechanisms of the acidosis effects on the EFS-evoked responses are discussed.     

Nitric oxide from perivascular nerves modulates cerebral arterial pH reactivity

In the isolated rat middle cerebral artery (MCA) we investigated the role of nitric oxide (NO)/cGMP in the vasodilatory response to extraluminal acidosis. Acidosis increased vessel diameter from 140 +/- 27 microm (pH 7.4) to 187 +/- 30 microm (pH 7.0, P < 0.01). NO synthase (NOS) inhibition by N(omega)-nitro-L-arginine (L-NNA, 10 microM) reduced baseline diameter (103 +/- 20 microm, P < 0.01) and attenuated response to acidosis (9 +/- 8 microm). Application of the NO-donors 3-morpholinosydnonimine (1 microM) or S-nitroso-N-acetylpenicillamine (1 microM), or of 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP, 100 microM) reestablished pre-L-NNA diameter at pH 7.4 and reversed L-NNA-induced attenuation of the vessel response to acidosis. Restoration of pre-L-NNA diameter (pH 7.4) by papaverine (20 microM) or nimodipine (30 nM) had no effect on the attenuated response to acidosis. Guanylyl cyclase inhibition with 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (5 microM) or NOS-inhibition with 7-nitroindazole (7-NI, 100 microM) reduced baseline vessel diameter (109 +/- 8 or 127 +/- 11 microm, respectively) and vasodilation to acidosis, and restoration of baseline diameter with 8-BrcGMP (30 microM) completely restored dilation to pH 7.0. Chronic denervation of NOS-containing perivascular nerves in vivo 14 days before artery isolation significantly reduced pH-dependent reactivity in vitro (diameter increase sham: 48 +/- 14 microm, denervated: 14 +/- 8 microm), and 8-BrcGMP (30 microM) restored dilation to pH 7.0 (denervated: 49 +/- 31 microm). Removal of the endothelium did not change vasodilation to acidosis. We conclude that NO, produced by neuronal NOS of perivascular nerves, is a modulator in the pH-dependent vasoreactivity.     

The effect of noradrenaline on the neurogenic vasoreactivity at various frequencies of the electrical field stimulation

The effect of 0.03-10.0 microM noradrenaline on the response to electrical field stimulation (EFS) of the juvenile rat tail artery segment was studied. At frequencies of the EFS equal to 10 or 40 Hz, noradrenaline was shown to cause much more pronounced potentiation or--at higher concentration--much less pronounced inhibition of the EFS-evoked constriction in arteries characterized by spontaneous decrease in the constriction value in the course of experiments as compared with arteries which were not characterized by such a decrease. At frequencies of the EFS equal to 3 or 5 Hz, the value and/or direction of the change in the neurogenic vasoconstriction in the presence of noradrenaline depends on the presence of the spontaneous decrease in the constriction evoked by EFS at 10 Hz, rather than at 3 or 5 Hz. It is concluded that the character of the change in the neurogenic vasoreactivity is a factor of a great importance for the prediction of the further change in the reactivity in the presence of noradrenaline.     

Effect of melatonin on neurogenic vasoreactivity: formation and modulation of the vessel response

The effect of melatonin (0.1 microM) on the contractile response to electrical field stimulation (EFS) of the juvenile rat tail artery segment was studied. The spontaneous decrease in reactivity of the segment observed in the course of our experiments was accompanied with a melatonin-evoked increase in the reactivity which was proportional to this decrease and was not connected with a sensitization of the segment to the substance. Perfusion of the segment with an acidic solution leads to a more pronounced inhibition of the response as well as to a greater melatonin-evoked protentiation of the response. Noradrenaline-evoked response was not augmented by melatonin. The results suggest that the potentiating effect of melatonin on the EFS-evoked response of the juvenile rat tail artery depends on degree of the change in the artery reactivity and was not due to change in sensitivity of postjunctional membrane to noradrenaline.     

Restoring effect of noradrenalne on diminished neurogenic vasoreactivity

The effect of 0.01-1.0 microM noradrenali on response to electrical field stimulation (EFS) of the juvenile rat tail artery segment was studied. Noradrenali was shown to potentiate the EFS-evoked constriction decreased in the course of experiments or in the acidic solution (pH 6.6) and this potentiation was proportional to the extent of the preceding decrease of the constriction. The more decreased was the EFS-evoked constriction the higher was the noradrenali concentration which produced the maximal potentiation and the wider was the potentiative noradrenali concentration range. The potentiative effect of noradrenali was not prevented by the NO synthase inhibitor NG-nitro-L-arginine. The results suggest that noradrenali can restore the diminished neurogenic reactivity of blood vessels, and this effect is not connected with the change in the NO production.     

Mechanism of vascular relaxation by thaligrisine: functional and binding assays

In the present study we examine the mechanism by which thaligrisine, a bisbenzyltetrahydroisoquinoline alkaloid, inhibits the contractile response of vascular smooth muscle. The work includes functional studies on rat isolated aorta and tail artery precontracted with noradrenaline or KCl. In other experiments rat aorta was precontracted by caffeine in the presence or absence of extracellular Ca2+. In order to assess whether thaligrisine interacts directly with calcium channel binding sites or with alpha-adrenoceptors we examined the effect of the alkaloid on [3H]-(+)-cis diltiazem, [3H]-nitrendipine and [3H]-prazosin binding to cerebral cortical membranes. The functional studies showed that the alkaloid inhibited in a concentration-dependent manner the contractile response induced by depolarization in rat aorta (IC50 = 8.9+/-2.9 microM, n=5) and in tail artery (IC50 = 3.04+/-0.3 microM, n=6) or noradrenaline induced contraction in rat aorta (IC50 = 23.0+/-0.39 microM, n=9) and in tail artery (IC50 = 3.8+/-0.9 microM, n=7). In rat aorta, thaligrisine concentration-dependently inhibited noradrenaline-induced contraction in Ca2+-free solution (IC50 = 13.3 microM, n=18). The alkaloid also relaxed the spontaneous contractile response elicited by extracellular calcium after depletion of noradrenaline-sensitive intracellular stores (IC50 = 7.7 microM, n=4). The radioligand receptor-binding study showed that thaligrisine has higher affinity for [3H]-prazosin than for [3H]-(+)-cis-diltiazem binding sites, with Ki values of 0.048+/-0.007 microM and 1.5+/-1.1 microM respectively. [3H]-nitrendipine binding was not affected by thaligrisine. The present work provides evidence that thaligrisine shows higher affinity for [3H]-prazosin binding site than [3H]-(+)-cis-diltiazem binding sites, in contrast with tetrandrine and isotetrandrine that present similar affinity for both receptors. In functional studies thaligrisine, acted as an alpha1-adrenoceptor antagonist and as a Ca2+ channel blocker, relaxing noradrenaline or KCl-induced contractions in vascular smooth muscle. This compound specifically inhibits the refilling of internal Ca2+-stores sensitive to noradrenaline, by blocking Ca2+-entry through voltage-dependent Ca2+-channels.     

Angiotensin II increases neurogenic nitric oxide metabolism in mesenteric arteries from hypertensive rats

We investigated, in mesenteric arteries from hypertensive rats (SHR), the possible changes in neurogenic nitric oxide (NO) release produced by angiotensin II (AII), and the possible mechanisms involved in this process. In deendothelialized segments the NO synthase inhibitor N(G)-nitro-L-arginine (L-NAME, 10 microM) increased the contractions caused by electrical field stimulation (EFS, 200 mA, 0.3 ms, 1-16 Hz, for 30 s). AII (0.1 nM) enhanced the response to EFS, which was unmodified by the subsequent addition of L-NAME. The AII antagonist receptor saralasine (0.1 microM) prevented the effect of AII, and the subsequent addition of L-NAME restored the contractile response. SOD (25 u/ml) decreased the reponse to EFS and the subsequent addition of L-NAME increased this response. AII did not modify the decrease in EFS response induced by SOD, and the addition of L-NAME increased the response. None of these drugs altered the response to exogenous noradrenaline (NA) or basal tone except SOD, which increased the basal tone, an effect blocked by phentolamine (1 microM). In arteries pre-incubated with [3H]-NA, AII did not modify the tritium efflux evoked by EFS, which was diminished by SOD. AII did not alter basal tritium efflux while SOD significantly increased it. These results suggest that EFS of SHR mesenteric arteries releases neurogenic NO, the metabolism of which is increased in the presence of AII by the generation of superoxide anions.     

Melatonin restores diminished neurogenic reactivity of the juvenile rat tail artery

The effect of melatonin on neurogenic reactivity of the juvenile rat tail artery segment was studied. The electrical field stimulation-evoked contraction of the segment decreased in the course of the experiment. Melatonin (0.1 microM) applied at different time points of the experiment produced an increase in the contraction, which directly correlated with a spontaneous decrease in the electrical field stimulation-evoked response. The increase in the potentiating effect of melatonin in the course of the experiment was not due to sensitization of the segment to this substance. Noradrenaline-evoked contraction of the vessel segment was not changed by melatonin. The data indicate that melatonin restores the diminished neurogenic reactivity of the juvenile rat tail artery probably by potentiation of the contractile response of the vessel, but this effect is hardly due to a change in sensitivity of the postjunctional membrane to noradrenaline.     

Evidence for the presence of GPRC6A receptors in rat mesenteric arteries

In this study, the presence of GPRC6A receptors in rat mesenteric artery was investigated. In artery homogenates, GPRC6A mRNA was detected and Western blotting showed the presence of GPRC6A protein. Immunohistochemical studies revealed GPRC6A in both endothelial cells and myocytes. In whole vessel segments, the GPRC6A activators, 300 microM l-ornithine and 100 microM Al(3+), induced endothelium-dependent myocyte hyperpolarizations sensitive to 10 microM TRAM-34, a blocker of intermediate conductance, Ca(2+)-sensitive K(+) channels (IK(Ca)). Activation of IK(Ca) with calindol (300 nM; a positive allosteric Ca(2+)-sensing receptor - CaR - modulator) was inhibited by 500 nM ouabain (inhibition of rat type 2 and type 3 Na(+)/K(+)-ATPases) but unaffected by 30 microM Ba(2+) (blockade of inwardly rectifying K(+) channels). Neither l-ornithine nor Al(3+) activated CaRs heterologously expressed in CHO or HEK293 cells. In the presence of 300 microM l-ornithine or 100 microM Al(3+), myocyte hyperpolarizations to calindol were potentiated whereas this potentiation and hyperpolarizations to l-ornithine were lost following incubation with an anti-GPRC6A antibody. It is concluded that GPRC6A receptors are present on mesenteric artery endothelial cells and myocytes and that their activation selectively opens IK(Ca) channels. This triggers a ouabain-sensitive myocyte hyperpolarization suggesting a close functional relationship between GPRC6A, the IK(Ca) channel and type 2 and/or type 3 Na(+)/K(+)-ATPases.     

Role of membrane potential in vasomotion of isolated pressurized rat arteries

Vasomotion, the phenomenon of vessel diameter oscillation, regulates blood flow and resistance. The main parameters implicated in vasomotion are particularly the membrane potential and the cytosolic free calcium in smooth muscle cells. In this study, these parameters were measured in rat perfused-pressurized mesenteric artery segments. The application of norepinephrine (NE) caused rhythmic diameter contractions and membrane potential oscillations (amplitude; 5.3 +/- 0.3 mV, frequency; 0.09 +/- 0.01 Hz). Verapamil (1 microM) abolished this vasomotion. During vasomotion, 10(-5) M ouabain (Na(+)-K(+) ATPase inhibitor) decreased the amplitude of the electrical oscillations but not their frequency (amplitude; 3.7 +/- 0.3 mV, frequency; 0.08 +/- 0.002 Hz). Although a high concentration of ouabain (10(-3) M) (which exhibits non-specific effects) abolished both electrical membrane potential oscillations and vasomotion, we conclude that the Na+-K+ ATPase could not be implicated in the generation of the membrane potential oscillations. We conclude that in rat perfused-pressurized mesenteric artery, the slow wave membrane type of potential oscillation by rhythmically gating voltage-dependent calcium channels, is responsible for the oscillation of intracellular calcium and thus vasomotion.     

Adrenomedullin release in the rat mesenteric resistance artery

Adrenomedullin (AM) is a potent vasodilator peptide whose major source is the vascular wall. In the present study, the mechanism of release of AM was investigated in the rat mesenteric resistance artery. The isolated mesenteric vascular bed was perfused with Krebs solution at a constant flow rate (5 ml/min) and AM in the perfusate was measured by a highly sensitive enzyme immunoassay (Immunoenzymometric assay; IEMA) method. In preparations without endothelium, spontaneous release of AM was detected in the perfusate (68.7+/-5.8 fmol/ml, n=45). Periarterial nerve stimulation (PNS, 4 and 8 Hz) caused 11.4+/-3.9% (4 Hz) and 9.1+/-3.5% (8 Hz) decreases in the spontaneous release of AM. Removal of Ca2+ from the medium did not affect the spontaneous AM release, but abolished the PNS-induced inhibition of spontaneous AM release. Perfusion of 10nM calcitonin gene-related peptide (CGRP) or 0.1 microM capsaicin (inducer of CGRP release) inhibited significantly the spontaneous AM release. PNS (8 Hz)-induced inhibition of spontaneous AM release was antagonized by CGRP(8-37) (CGRP receptor antagonist). These results suggest that AM is mainly released from vascular smooth muscle cells of the rat mesenteric artery and endogenous or exogenous CGRP inhibits AM release.     

Role of inwardly rectifying K(+) channels in K(+)-induced cerebral vasodilatation in vivo

We tested whether activation of inwardly rectifying K(+) (Kir) channels, Na(+)-K(+)-ATPase, or nitric oxide synthase (NOS) play a role in K(+)-induced dilatation of the rat basilar artery in vivo. When cerebrospinal fluid [K(+)] was elevated from 3 to 5, 10, 15, 20, and 30 mM, a reproducible concentration-dependent vasodilator response was elicited (change in diameter = 9 +/- 1, 27 +/- 4, 35 +/- 4, 43 +/- 12, and 47 +/- 16%, respectively). Responses to K(+) were inhibited by approximately 50% by the Kir channel inhibitor BaCl(2) (30 and 100 microM). In contrast, neither ouabain (1-100 microM, a Na(+)-K(+)-ATPase inhibitor) nor N(G)-nitro-L-arginine (30 microM, a NOS inhibitor) had any effect on K(+)-induced vasodilatation. These concentrations of K(+) also hyperpolarized smooth muscle in isolated segments of basilar artery, and these hyperpolarizations were virtually abolished by 30 microM BaCl(2). RT-PCR experiments confirmed the presence of mRNA for Kir2.1 in the basilar artery. Thus K(+)-induced dilatation of the basilar artery in vivo appears to partly involve hyperpolarization mediated by Kir channel activity and possibly another mechanism that does not involve hyperpolarization, activation of Na(+)-K(+)-ATPase, or NOS.     

Contractile function of vessel myocytes: dependence on the stretch magnitude]

The values of the optimal stretch for the maximal responses of the rat mesenteric artery ring to electrical field stimulation (FES) or noradrenaline as well as for the maximal myogenic responses, tone, and maximal changes in these responses produced by low pH and dynamic sinusoidal stretch of the vessel wall were compared. The findings show that the magnitude of stretch corresponding to the maximal effect depend on the character of responses, on the factor, and on the responses occurrence conditions.     

Opposite Effect of Mast Cell Stabilizers Ketotifen and Tranilast on the Vasoconstrictor Response to Electrical Field Stimulation in Rat Mesenteric Artery

Objectives

We analyzed whether mast cell stabilization by either ketotifen or tranilast could alter either sympathetic or nitrergic innervation function in rat mesenteric arteries.

Methods

Electrical field stimulation (EFS)-induced contraction was analyzed in mesenteric segments from 6-month-old Wistar rats in three experimental groups: control, 3-hour ketotifen incubated (0.1 αmol/L), and 3-hour tranilast incubated (0.1 mmol/L). To assess the possible participation of nitrergic or sympathetic innervation, EFS contraction was analyzed in the presence of non-selective nitric oxide synthase (NOS) inhibitor L-NAME (0.1 mmol/L), α-adrenergic receptor antagonist phentolamine (0.1 µmol/L), or the neurotoxin 6-hydroxydopamine (6-OHDA, 1.46 mmol/L). Nitric oxide (NO) and superoxide anion (O₂ ^.-) levels were measured, as were vasomotor responses to noradrenaline (NA) and to NO donor DEA-NO, in the presence and absence of 0.1 mmol/L tempol. Phosphorylated neuronal NOS (P-nNOS) expression was also analyzed.

Results

EFS-induced contraction was increased by ketotifen and decreased by tranilast. L-NAME increased the vasoconstrictor response to EFS only in control segments. The vasodilator response to DEA-NO was higher in ketotifen- and tranilast-incubated segments, while tempol increased vasodilator response to DEA-NO only in control segments. Both NO and O₂ ^- release, and P-nNOS expression were diminished by ketotifen and by tranilast treatment. The decrease in EFS-induced contraction produced by phentolamine was lower in tranilast-incubated segments. NA vasomotor response was decreased only by tranilast. The remnant vasoconstriction observed in control and ketotifen-incubated segments was abolished by 6-OHDA.

Conclusion

While both ketotifen and tranilast diminish nitrergic innervation function, only tranilast diminishes sympathetic innnervation function, thus they alter the vasoconstrictor response to EFS in opposing manners.     

Calcium-sensitive potassium channel inhibitors antagonize genistein- and daidzein-induced arterial relaxation in vitro

Estradiol-17beta relaxes rabbit coronary artery rings via large conductance Ca2+-activated K+-channels (K(Ca)). Genistein and daidzein are plant-derived estrogen-like compounds. The aim of the present study was to investigate whether potassium channels participate in the genistein- and daidzein-induced arterial relaxation like they do in the case of estradiol-17beta. Endothelium-denuded superior mesenteric arterial rings from non-pregnant Wistar female rats were used. At a concentration of 10 microM, estradiol-17beta, genistein and daidzein relaxed noradrenaline precontracted arterial rings, (58 +/- 4%, 45 +/- 5% and 31 +/- 3%, respectively; (n=6-8)). Genistein- and daidzein-induced relaxations were inhibited both by iberiotoxin (1-10 nM) and charybdotoxin (30 nM), the antagonists of large conductance Ca2+-activated K+-channels (K(Ca)). Estradiol-17beta-induced relaxation was reduced by iberiotoxin (30 nM). Estradiol-17beta- and daidzein-induced relaxations were also decreased by apamin (0.1-0.3 microM), an antagonist of small conductance Ca2+-activated K+-channels. The antagonists of voltage-dependent K+-channels (K(V)) (4-aminopyridine), ATP-sensitive K+-channels (K(ATP)) (glibenclamide), or inward rectifier K+-channels (KIR) (barium) had no effect on the relaxation responses of any of the compounds studied. Estrogen receptor antagonist tamoxifen did not inhibit the relaxations. In conclusion, in the noradrenaline precontracted rat mesenteric arteries, the relaxations caused by estradiol-17beta, genistein and daidzein were antagonized by large and small conductance K(Ca)-channel inhibitors, suggesting the role of these channels as one of the relaxation mechanisms.     

The vasoinhibitory activity of bovine chromogranin A fragment (vasostatin) and its independence of extracellular calcium in isolated segments of human blood vessels.

Endothelium-independent vasoconstrictor responses in isolated segments of human internal thoracic artery (ITA) and saphenous vein (SV) were used as a bioassay system for the vasoinhibitory activity of bovine chromogranin A (CGA). Preincubation with vasostatin (0.8 micrograms/ml), containing the N-terminal domain of CGA, (CGA1-76, CGA1-113 and CGA1-143ff), inhibited the contractile responses evoked by 80 mM K+, 2.6 microM noradrenaline (NA), or 65 nM endothelin-1 (ET-1) in Ca(2+)-free solution in SV but not in ITA. The results demonstrate a vasoinhibitory activity in vasostatin and show that there is a marked difference between the arterial and venous segments in the Ca2+ independent component of the inhibitory response. A vascular role for the N-terminal domain of CGA is indicated, presumably by inhibiting Ca2+ release from intracellular stores in the human vein but not the artery.     

Functional angiotensin II receptors in cultured vascular smooth muscle cells

To study cellular mechanisms influencing vascular reactivity, vascular smooth muscle cells (VSMC) were obtained by enzymatic dissociation of the rat mesenteric artery, a highly reactive, resistance-type blood vessel, and established in primary culture. Cellular binding sites for the vasoconstrictor hormone angiotensin II (AII) were identified and characterized using the radioligand 125I-angiotensin II. Freshly isolated VSMC, and VSMC maintained in primary culture for up to 3 wk, exhibited rapid, saturable, and specific 125I-AII binding similar to that seen with homogenates of the intact rat mesenteric artery. In 7-d primary cultures, Scatchard analysis indicated a single class of high-affinity binding sites with an equilibrium dissociation constant (Kd) of 2.8 +/- 0.2 nM and a total binding capacity of 81.5 +/- 5.0 fmol/mg protein (equivalent to 4.5 x 10(4) sites per cell). Angiotensin analogues and antagonists inhibited 125I-AII binding to cultured VSMC in a potency series similar to that observed for the vascular AII receptor in vivo. Nanomolar concentrations of native AII elicited a rapid, reversible, contractile response, in a variable proportion of cells, that was inhibited by pretreatment with the competitive antagonist Sar1,Ile8-AII. Transmission electron microscopy showed an apparent loss of thick (12-18 nm Diam) myofilaments and increased synthetic activity, but these manifestations of phenotypic modulation were not correlated with loss of 125I-AII binding sites or hormonal responsiveness. Primary cultures of enzymatically dissociated rat mesenteric artery VSMC thus may provide a useful in vitro system to study cellular mechanisms involved in receptor activation-response coupling, receptor regulation, and the maintenance of differentiation in vascular smooth muscle.     

Mesenteric vascular responsiveness in a rat model of pregnancy-induced hypertension

Reduced perfusion to the placenta in early pregnancy is believed to be the initiating factor in the development of preeclampsia, triggering local ischemia and systemic vascular hyperresponsiveness. This sequence of events creates a predisposition to the development of altered vascular function and hypertension. This study was designed to determine the influence of placental insufficiency on the responsiveness of mesenteric resistance arteries in an animal model of preeclampsia. Placental insufficiency was induced by reduction in uteroplacental perfusion pressure (RUPP) in experimental Sprague-Dawley rat dams. The uterine branches of the ovarian arteries and the abdominal aortae of pregnant rats were surgically constricted on gestational Day 14. Dams in the control group underwent a sham procedure. Rats were euthanized on gestational Day 20, followed by removal of the small intestine and adjacent mesentery. First-order mesenteric resistance arteries were mounted on a small vessel wire myograph and challenged with incremental concentrations of vasoconstrictors and vasorelaxants. Mesenteric arteries in dams with placental insufficiency demonstrated an increased maximal tension to phenylephrine (7.15 +/- 0.15 vs. 5.4 +/- 0.27 mN/mm, P < 0.001); potassium chloride at 60 mM (3.43 +/- 0.11 vs. 2.77 +/- 0.14 mN/mm, P < 0.01) and 120 mM (3.92 +/- 0.18 vs. 2.97 +/- 0.16 mN/mm, P < 0.01); and angiotensin II (2.59 +/- 0.42 vs. 1.51 +/- 0.22 mN/mm, P < 0.05). Maximal relaxation to endothelium-dependent relaxants acetylcholine and calcium ionophore (A23187) was not significantly reduced. Data suggest that placental insufficiency leads to hyperresponsiveness to vasoconstrictor stimuli in mesenteric arteries.     

Distribution of the vasoconstrictor and vasorelaxant effects of norbormide along the vascular tree of the rat

Norbormide is a vasoconstrictor of rat peripheral arteries and a relaxant in rat aorta. To characterise norbormide actions within the rat vascular tree we have investigated its effects on the contractile function of rings from several arteries and veins. A maximal norbormide concentration (50 microM) failed to contract thoracic aorta and carotid artery, whereas in pulmonary artery, abdominal aorta, iliac, caudal, and femoral arteries it induced a contractile effect that was respectively 4.8 +/- 0.6, 18.4 +/- 1.5, 39 +/- 5, 144 +/- 7, and 260 +/- 22% of that induced by 90 mM KCl. In pulmonary, carotid, and iliac arteries, and in thoracic and abdominal aorta, 50 microM norbormide inhibited KCl-induced responses. Norbormide (50 microM) contracted all veins investigated. The effect, expressed as % of KCl-induced contraction, was 121 +/- 25, 154 +/- 14.5, 154 +/- 18.2, 203 +/- 19, and 267 +/- 33 for pulmonary vein, thoracic and abdominal vena cava, iliac and jugular veins, respectively. In jugular vein, as previously shown in rat caudal artery, norbormide contraction was abolished in Ca2+-free medium, was unaffected by the Ca2+ channel blocker nifedipine, and was relaxed by SK&F 96365, a blocker of store-operated Ca2+ channels. In conclusion: i) rat veins represent the main target for contractile norbormide action; ii) in both artery and veins norbormide contractions are generally inversely related to the calibre of the vessel; iii) norbormide-induced contraction is mediated by the same mechanism/s in arteries and veins; iiii) in norbormide-contracted arteries the drug activates both contractile and relaxing mechanisms.