Effect of 5-hydroxytryptamine on the membrane potential of endothelial and smooth muscle cells in the pig coronary artery

Characteristics of the actions by which 5-hydroxytryptamine affects electrical and mechanical activities in rabbit jugular vein graft

BACKGROUND AND PURPOSE

The vasomodulating actions of 5-HT in vein grafts, and the underlying mechanisms, remain to be fully clarified. Here, we characterized the actions by which 5-HT affects electrical and mechanical activities in rabbit autologous jugular vein grafts.

EXPERIMENTAL APPROACH

Smooth muscle cell (SMC) membrane potential and isometric tension were measured in vein grafts 4 weeks after implantation into carotid arteries. Changes in the expression of 5-HT receptor subtypes and in myosin heavy chain isoforms (SM1, SM2 and SMemb) were examined by immunohistochemistry and Western blot analysis.

KEY RESULTS

The walls of grafted veins displayed massive increases in the number of SM1- and SM2-positive SMCs. 5-HT induced a large depolarization and contraction that were each reduced by both 5-HT(2A) - and 5-HT(1B/1D) -receptor antagonists. The 5-HT-induced contraction was not modified by a 5-HT₇ -receptor antagonist. The 5-HT₇ -receptor-selective agonist AS 19 did not induce relaxation during the contraction to prostaglandin F(2α) . Immunohistochemical and Western blot analyses revealed that immunoreactive responses against 5-HT(2A) and 5-HT(1B/1D) receptors were increased in the vein graft.

CONCLUSIONS AND IMPLICATIONS

5-HT is able to induce a large contraction in rabbit autologous jugular vein grafts through (i) an increased number of differentiated contractile SMCs; (ii) an increased number of SMCs expressing contractile 5-HT(2A) - and 5-HT(1B/1D) receptors; and (iii) a down-regulation of the function of the relaxant SMC 5-HT₇ receptors. These changes in the vein graft may help it to resist the higher pressure present on the arterial side of the circulation.

Characteristics of the actions by which 5-HT affects electrical and mechanical activities in rabbit jugular vein

BACKGROUND AND PURPOSE

5-HT is known to be a potent vasospasmogenic agonist in various arteries. However, in veins the vasomodulating actions of 5-HT, and the underlying mechanisms, remain to be fully clarified. Here, we characterized the actions by which 5-HT affects electrical and mechanical activities in the rabbit jugular vein.

EXPERIMENTAL APPROACH

Membrane potential and isometric tension were measured in endothelium-intact and -denuded preparations. Localization of 5-HT receptor subtypes was examined immunohistochemically.

KEY RESULTS

5-HT induced a transient then a small, sustained smooth muscle cell hyperpolarization in endothelium-intact strips. In endothelium-denuded strips, 5-HT induced only a sustained hyperpolarization, and this was changed to a depolarization by the selective 5-HT(7) receptor inhibitor SB269970. This depolarization was inhibited by the 5-HT(2A) receptor blocker sarpogrelate. 5-HT induced a relaxation of PGF(2α) -induced contracted strips that was similar in endothelium-intact and -denuded preparations. The latter relaxation was changed to contraction by SB269970 and this contraction was inhibited by sarpogrelate. Immunoreactive responses against endothelial and smooth muscle 5-HT(2A) receptors and smooth muscle 5-HT(7) receptors were identified in the vein. The 5-HT-induced relaxation of the PGF(2α) contraction was inhibited by the cAMP-dependent protein kinase inhibitor Rp-cAMPS and by the AC inhibitor SQ22536.

CONCLUSIONS AND IMPLICATIONS

These results indicate that 5-HT activates both smooth muscle 5-HT(7) receptors (to produce relaxation) and smooth muscle 5-HT(2A) receptors (to produce contraction) in rabbit jugular vein. We suggest that in this particular vein, the 5-HT(2A) receptor-induced depolarization and contraction are masked by the 5-HT(7) receptor-induced responses, possibly via actions mediated by cAMP.

The effect of cooling on the contractility of equine digital small lamellar arteries: modulating role of the endothelium

The equine hoof displays thermoregulatory functions, and the blood vessels lying under the hoof wall are temperature sensitive. The aim of this study was to investigate the effect of cooling on the contractile responses to α-adrenoceptor and 5-HT receptor stimulation in equine small lamellar arteries using wire myography. The role of the endothelium in the response mediated by 5-HT was also evaluated. Moderate cooling caused a reduction of the contraction induced by depolarizing Krebs solution (DKS, containing 118 mm KCl) and the maximal contraction caused by UK-14304 (α(2)-adrenoceptor agonist). The potency of methoxamine (α(1)-adrenoceptor agonist) was reduced by cooling [pD(2) (-log EC(50)) at 22°C, 5.7 (5.5-6.0) versus 30°C, 5.9 (5.7-6.1)]; however, the efficacy was maintained. The contractions evoked by sumatripan and α-methyl 5-HT (5-HT receptor agonists) were not modified by moderate cooling. In contrast, a cooling-enhanced contraction was observed in response to 5-HT [maximum response (E(max)) at 22°C, 213.2 ± 13% DKS versus 30°C, 179.9 ± 11% DKS]. Furthermore, this effect was more evident in endothelium-denuded lamellar arteries (E(max) at 22°C, 270.2 ± 26% DKS versus 30°C, 172.2 ± 20% DKS), suggesting a potential modulating role of the endothelium. The L-NAME/ibuprofen-resistant relaxation in response to carbachol was reduced by cooling in small lamellar arteries precontracted with 5-HT but not phenylephrine. Therefore, a moderate reduction of temperature modulates the reactivity of small lamellar arteries by enhancing the 5-HT-mediated contraction, but inhibits the α-adrenoceptor-mediated response. Furthermore, the endothelium of these blood vessels may play an important role in preventing excessive vasoconstriction in response to 5-HT and maintaining digital blood flow in cool environmental temperatures.

Differential inhibition by oxygen radicals of vasoactive amines-induced contractions in the porcine coronary artery

Reactive oxygen species (ROS) play an important role in normal metabolic and signaling processes. Excess ROS, however, can cause severe cardiovascular damage. Thus, the present study was designed to examine effects of H(2)O(2) and xanthine plus xanthin oxidase (X/XO) on the serotonin (5HT), histamine (His) and acetylcholine (ACh)-induced contractions of porcine coronary arteries. In addition, to explore the site of ROS formation and species of it, the inhibitory effects of edaravone and EDTA were also tested. 5HT- and His-induced contractions were suppressed by H(2)O(2) and X/XO treatment. However, these suppressions of ACh-induced contraction by H(2)O(2) treatment was relatively weak and X/XO treatment caused no suppression on ACh-induced contraction. In the presence of edaravone which is thought to be a scavenger for .OH, significant decrease of inhibition of 5HT- and His-induced contractions was observed when coronary artery strips were treated with X/XO, but not H(2)O(2). On the other hand, inhibitory effects by EDTA treatments were also observed in X/XO treatments. These results suggest that 1) ROSs produced by additions of H(2)O(2) or X/XO are considered to be responsible for several physiological functions of coronary artery contractions, 2) the site of ROS produced by X/XO system, probably .OH, was outside the cell, but the inhibitory action of H(2)O(2), was inside the cell, and 3) a low susceptibility of ACh-induced contraction to H(2)O(2) and X/XO may indicates the signal transduction pathway(s) of ACh-induced contraction is different from those of 5HT and His.

Reduced hyperpolarization in endothelial cells of rabbit aortic valve following chronic nitroglycerine administration

This study was undertaken to determine whether long-term in vivo administration of nitroglycerine (NTG) downregulates the hyperpolarization induced by acetylcholine (ACh) in aortic valve endothelial cells (AVECs) of the rabbit and, if so, whether antioxidant agents can normalize this downregulated hyperpolarization. ACh (0.03-3 microM) induced a hyperpolarization through activations of both apamin- and charybdotoxin-sensitive Ca2+-activated K+ channels (K(Ca)) in rabbit AVECs. The intermediate-conductance K(Ca) channel (IK(Ca)) activator 1-ethyl-2-benzimidazolinone (1-EBIO, 0.3 mM) induced a hyperpolarization of the same magnitude as ACh (3 microM). The ACh-induced hyperpolarization was significantly weaker, although the ACh-induced [Ca2+]i increase was unchanged, in NTG-treated rabbits (versus NTG-untreated control rabbits). The hyperpolarization induced by 1-EBIO was also weaker in NTG-treated rabbits. The reduced ACh-induced hyperpolarization seen in NTG-treated rabbits was not modified by in vitro application of the superoxide scavengers Mn-TBAP, tiron or ascorbate, but it was normalized when ascorbate was coadministered with NTG in vivo. Superoxide production within the endothelial cell (estimated by ethidium fluorescence) was increased in NTG-treated rabbits and this increased production was normalized by in vivo coadministration of ascorbate with the NTG. It is suggested that long-term in vivo administration of NTG downregulates the ACh-induced hyperpolarization in rabbit AVECs, possibly through chronic actions mediated by superoxide.

Effects of chloride substitution in isolated mesenteric blood vessels from Dahl normotensive and hypertensive rats

The purpose of this investigation was to examine the effect of Cl-free medium, nitric oxide synthase inhibitor (N nitro-L-arginine methyl ester; L-NAME), and Cl channel antagonist (niflumic acid), on alpha1-adrenoceptor (cirazoline) mediated responses in the isolated mesenteric blood vessels from Dahl salt-resistant normotensive (SRN) and salt-sensitive hypertensive (SSH) rats on a 4% salt diet for 7 weeks. Cirazoline produced dose-dependent vasoconstriction in blood vessels of SRN and SSH rats. Replacement of extracellular Cl with propionate ions significantly inhibited (P < 0.05) cirazoline-mediated vasoconstriction in SRN but not in SSH rats. Perfusion with L-NAME (10 microM) augmented responses to cirazoline in SRN but not in SSH rats. In Cl-free medium, addition of L-NAME had a biphasic effect on cirazoline responses; potentiation of responses at the lower doses and attenuation at the highest dose. Niflumic acid (10 microM) significantly inhibited cirazoline responses with the inhibition being more pronounced in SRN than SSH rats. The resting Em of smooth muscle cells was -68.0 +/- 4.2 mV (mean +/- SD; n = 87) and -67.2 +/- 4.8 mV (n = 88), in SRN and SSH rats, respectively. Perfusion with Cl-free medium produced a significant depolarization that was larger in smooth muscle cells of SSH (-57.4 +/- 4.8 mV, n = 38) than SRN (-61.3 +/- 5.4 mV, n = 35) rats, while L-NAME depolarized the smooth muscle cells of SRN (-62.1 +/- 6.5 mV, n = 36) but not SSH (-67.5 +/- 4.2 mV, n = 34) rats. The data supports the view that Cl handling and Ca-dependent Cl channels seem to undergo modification as a consequence of salt-induced hypertension. It is also possible that the modified role of nitric oxide on membrane potential may have a direct bearing on the changes observed in Cl handling in blood vessels of SRN versus SSH rats.

Divergent changes to muscarinic and serotonergic signalling following colitis

BACKGROUND

The altered motility of the inflamed intestine derives in part from changes to the contractility of the intestinal smooth muscle cell. While modifications to the muscarinic receptor system are identified, changes to 5-hydroxytryptamine (5-HT; serotonin) receptors that also mediate contraction are less well studied.

METHODS

In the trinitrobenzene sulphonic acid model of rat colitis, we used receptor antagonists to identify changes in receptor utilisation that accompany the selective reversal of the impaired contractile response to acetylcholine (ACh) and 5-HT during colitis (day 4 (D4)) and following resolution of inflammation (day 36 (D36)).

RESULTS

In isolated circular smooth muscle cells, challenged with ACh, the muscarinic 3 receptor (M3R) antagonists 4-DAMP and pF-HSD each showed a 50% decrease in antagonism on D4 while the M2R antagonist methoctramine more than doubled its potency, showing a decreased role of M3R and an increased role of M2R, respectively. These changes were fully reversed by D36. In contrast, the 5-HT2 receptor (5-HT2R) antagonist ketanserin was sharply decreased in effectiveness on D4, with a further decrease by D36, when the contribution of 5-HT(2A)R was only 22% of control. There were no changes in response to the 5-HT4R antagonist SDZ-205-557 at any time. Western blotting identified decreased expression of 5-HT(2A)R on D36 versus controls, further supporting the conclusion that the persistence of the impaired response to 5-HT was due to decreased expression of the excitatory 5-HT(2A)R.

CONCLUSIONS

Thus the lasting decrease in receptor expression and resulting impairment of the contractile response will compromise the capacity for an appropriate response to 5-HT, which may contribute to the intestinal dysfunction seen in post-enteritis syndromes.

Cerebral artery reactivity changes during pregnancy and the postpartum period: a role in eclampsia?

Eclampsia is thought to be similar to hypertensive encephalopathy, whereby acute elevations in intravascular pressure cause forced dilatation (FD) of intrinsic myogenic tone of cerebral arteries and arterioles, decreased cerebrovascular resistance, and hyperperfusion. In the present study, we tested the hypothesis that pregnancy and/or the postpartum period predispose cerebral arteries to FD by diminishing pressure-induced myogenic activity. We compared the reactivity to pressure (myogenic activity) as well as factors that modulate the level of tone of third-order branches (<200 μm) of the posterior cerebral artery (PCA) that were isolated from nonpregnant (NP, n = 7), late-pregnant (LP, 19 days, n = 10), and postpartum (PP, 3 days, n = 8) Sprague-Dawley rats under pressurized conditions. PCAs from all groups of animals developed spontaneous tone within the myogenic pressure range (50–150 mmHg) and constricted arteries at 100 mmHg (NP, 30 ± 3; LP, 39 ± 4; and PP, 42 ± 7%; P > 0.05). This level of myogenic activity was maintained in the NP arteries at all pressures; however, both LP and PP arteries dilated at considerably lower pressures compared with NP, which lowered the pressure at which FD occurred from >175 for NP to 146 ± 6.5 mmHg for LP ( P < 0.01 vs. NP) and 162 ± 7.7 mmHg for PP ( P < 0.01 vs. NP). The amount of myogenic tone was also significantly diminished at 175 mmHg compared with NP: percent tone for NP, LP, and PP animals were 35 ± 2, 11 ± 3 ( P < 0.01 vs. NP), and 20 ± 7% ( P < 0.01 vs. NP), respectively. Inhibition of nitric oxide (NO) with 0.1 mM Nω-nitro-l-arginine (l-NNA) caused constriction of all vessel types that was significantly increased in the PP arteries, which demonstrates significant basal NO production. Reactivity to 5-hydroxytryptamine (serotonin) was assessed in the presence of l-NNA and indomethacin. There was a differential response to serotonin: PCAs from NP animals dilated, whereas LP and PP arteries constricted. These results suggest that both pregnancy and the postpartum period predispose the cerebral circulation to FD at lower pressures, a response that may lower cerebrovascular resistance and promote hyperperfusion when blood pressure is elevated, as occurs during eclampsia.

Effects of chloride substitution on electromechanical responses in the pulmonary artery of Dahl normotensive and hypertensive rats

1. We have investigated the in vitro interaction between chloride ions and endothelium as revealed by alterations in vascular contractility and smooth muscle cell membrane potential in isolated pulmonary arteries from Dahl salt-resistant normotensive and salt-sensitive hypertensive rats. 2. Exposure to nitro-l-arginine methyl ester (l-NAME) of tissues from normotensive but not hypertensive rats augmented contractions to cirazoline. While chloride removal did not alter cirazoline-induced contractions, it completely abolished the augmentation by l-NAME in normotensive rats. However, in hypertensive rats, removal of chloride ions significantly attenuated contractions elicited by cirazoline, and l-NAME effectively reversed this inhibition. 3. Methacholine-induced endothelium-dependent relaxations of the same magnitude were evident in both normotensive and hypertensive rats. However, basal cyclic GMP levels were found to be significantly higher (7.8-fold) in blood vessels of normotensive rats compared to hypertensive rats. 4. The resting membrane potential in pulmonary arteries of hypertensive rats (-52.1+/-1.04 mV) revealed a significant hyperpolarisation when compared with that of normotensive rats (-46.4+/-1.58 mV). Cirazoline did not produce a significant depolarisation in blood vessels of either normotensive or hypertensive rats. Perfusion with chloride-free solution resulted in a modest but significant hyperpolarisation (-8.0 mV) in the blood vessels of hypertensive but not in normotensive rats. 5. We conclude that salt-dependent hypertension in Dahl rats is accompanied by functional and biochemical changes in low-pressure blood vessels. These changes can, in part, be attributed to impairment in the basal, but not methacholine-stimulated, release of nitric oxide, and to altered chloride ion handling.

Inhibition of nitric oxide synthase as a potential therapeutic target

Nitric oxide (NO) regulates numerous physiological processes, including neurotransmission, smooth muscle contractility, platelet reactivity, and the cytotoxic activity of immune cells. Because of the ubiquitous nature of NO, inappropriate release of this mediator has been linked to the pathogenesis of a number of disease states. This provides the rationale for the design of therapies that modulate NO concentrations selectively. A well-characterized family of compounds are the inhibitors of NO synthase, the enzyme responsible for the generation of NO; such agents are potentially beneficial in the treatment of conditions associated with an overproduction of NO, including septic shock, neurodegenerative disorders, and inflammation. This article provides an overview of NO synthase inhibitors, focusing on agents that prevent binding of substrate L-arginine.

Calcium channels, potassium channels and membrane potential of smooth muscle cells of human allantochorial placental vessels

The membrane potential (Um), the main factor of the excitation-contraction coupling, of human allantochorial placental vascular smooth muscle cells (VSMCs) has been previously shown to depend on voltage-sensitive K+ channels. These channels were blocked by high external K+. To characterize other channels which regulated Um, various constrictor or/and vasodilators and channel blockers were used. Serotonin depolarized VSMCs, in normal medium, but induced a more marked depolarization in VSMCs predepolarized by high external K+. This depolarization was inhibited by nifedipine, a blocker of voltage-gated Ca2+ channels. Acetylcholine, sodium nitroprusside (without effect on Um in normal medium), hyperpolarized the predepolarized-high K+ medium VSMCs. This hyperpolarization was inhibited after addition of charybotoxin (a blocker of Ca2+-activated K+ channels) or/and glibenclamide (a blocker of ATP-sensitive K+ channels). A similar effect was obtained with isoproterenol. These results indicated that membrane potential of human placental allantochorial VSMCs was regulated by voltage-gated, Ca2+- and ATP-sensitive K+ channels and by voltage-dependent Ca2+ channels.

Acetylcholine-induced membrane potential changes in endothelial cells of rabbit aortic valve

1. Using a microelectrode technique, acetylcholine (ACh)-induced membrane potential changes were characterized using various types of inhibitors of K+ and Cl- channels in rabbit aortic valve endothelial cells (RAVEC). 2. ACh produced transient then sustained membrane hyperpolarizations. Withdrawal of ACh evoked a transient depolarization. 3. High K+ blocked and low K+ potentiated the two ACh-induced hyperpolarizations. Charybdotoxin (ChTX) attenuated the ACh-induced transient and sustained hyperpolarizations; apamin inhibited only the sustained hyperpolarization. In the combined presence of ChTX and apamin, ACh produced a depolarization. 4. In Ca2+-free solution or in the presence of Co2+ or Ni2+, ACh produced a transient hyperpolarization followed by a depolarization. In BAPTA-AM-treated cells, ACh produced only a depolarization. 5. A low concentration of A23187 attenuated the ACh-induced transient, but not the sustained, hyperpolarization. In the presence of cyclopiazonic acid, the hyperpolarization induced by ACh was maintained after ACh removal; this maintained hyperpolarization was blocked by Co2+. 6. Both NPPB and hypertonic solution inhibited the membrane depolarization seen after ACh washout. Bumetanide also attenuated this depolarization. 7. It is concluded that in RAVEC, ACh produces a two-component hyperpolarization followed by a depolarization. It is suggested that ACh-induced Ca2+ release from the storage sites causes a transient hyperpolarization due to activation of ChTX-sensitive K+ channels and that ACh-activated Ca2+ influx causes a sustained hyperpolarization by activating both ChTX- and apamin-sensitive K+ channels. Both volume-sensitive Cl- channels and the Na+-K+-Cl- cotransporter probably contribute to the ACh-induced depolarization.

Comparison of relaxations evoked by photoactivation of NO-containing compounds and nitrergic nerve stimulation in 5-hydroxytryptamine- and potassium-contracted rat gastric fundus

1. The aim of the present study was to further investigate our earlier proposal of liberation of nitric oxide (NO) by photoactivation of molecules containing NO or NO2, which in turn relaxes gastric smooth muscle, and to determine whether presynaptic- and/or postsynaptic NO-mediated relaxation is affected differently by the degree of membrane depolarization in rat gastric fundus smooth muscle. 2. During contraction of rat gastric fundus with 5-hydroxytryptamine (5-HT, 10 microM), low (K+, 25 mM) and high potassium (K+, 65.4 mM), relaxation responses to nitrergic nerve stimulation, photo-activation of caged NO compounds (streptozotocin [STZ], N omega-nitro-L-arginine-methylacetate [L-NAME], N omega-nitro-D-arginine-methylacetate [D-NAME]), and sodium nitroprusside (SNP) were compared. 3. Nitrergic nerve (presynaptic) stimulation and photoactivation (postsynaptic) of all caged NO compounds produced rapid, transient and reversible relaxation of 5-HT and low-K(+)-contracted tissues. However, when contractions were induced by high K+, the relaxation induced by nerve stimulation was abolished, whereas relaxations induced by photoactivated NO compounds were significantly (P < 0.01) reduced. 4. The relaxation induced by sodium nitroprusside (SNP), but not papaverine, was also diminished in high-K(+)-contracted tissues. The magnitude of photoactivated NO-induced relaxation was related to the amount of NO release, light intensity and concentration of compounds. 5. The evidence that photoactivated NO-induced relaxation is mediated by cGMP comes from the observation that zaprinast, but not forskolin, potentiated the relaxation. 6. It is concluded that rat gastric smooth muscle relaxes to photoactivation of NO or NO2-carrying molecules via NO, and it appears that degree of membrane depolarization may be a critical factor in dissociating the response to presynaptic- and postsynaptic NO-mediated relaxation in this muscle.

Hyperpolarizing factors

There is now overwhelming evidence for factors, other than nitric oxide (NO), that mediate endothelium-dependent vasodilation by hyperpolarizing the underlying smooth muscle via activation of Ca2+-activated K+ channels. Although the identity of endothelium-derived hyperpolarizing factor (EDHF) remains to be established, cytochrome P450 (CYP)-dependent metabolites of arachidonic acid (AA), namely, the epoxides, fulfill several of the criteria required for consideration as putative mediators of endothelium-dependent hyperpolarization. They are produced by the endothelium, released in response to vasoactive hormones, and elicit vasorelaxation via stimulation of Ca2+-activated K+ channels. Our studies in the rat indicate that, of the epoxides, 5,6-epoxyeicosatrienoic acid (5,6-EET) is the most likely mediator of NO-independent, but CYP-dependent coronary vasodilation in response to bradykinin. Studies in the rat kidney, however, support the existence of additional EDHFs as acetylcholine also exhibits NO-independent vasodilation that is unaffected by CYP inhibitors in concentrations that attenuate responses to bradykinin. In some blood vessels, NO may tonically suppress the expression of CYP-dependent EDHF. In the event of impaired NO synthesis, therefore, a CYP-dependent vasodilator mechanism may serve as a backup to a primary NO-dependent mechanism, although they may act in concert. In other vessels, particularly microvessels, an EDHF may constitute the major vasodilator mechanism for hormones and other physiological stimuli. EDHFs appear to be important regulators of vascular tone; alterations in this system can be demonstrated in hypertension and diabetes, conditions associated with altered endothelium-dependent vasodilator responsiveness.

Specific response of mouse tumor-feeding arterioles to stimulation by 5-HT1 agonists

Using intravital microscopy, we compared the responses to 5-HT1 receptor stimulation by the host-modified arterioles feeding a Meth-A tumor implanted in the flank of female Balb/c mice with the responses of tumor-independent arterioles (TIA) and those of control arterioles from mice without tumor. Topical administration of 5 x 10(-5) M serotonin in the presence of 10(-4) M ketanserin (5-HT2 receptors inhibitor) induced arteriolar vasodilation in TIA (+13%) and in the control arterioles (+19%), but induced constriction (-14%) in the tumor-feeding arterioles (TFA). Topical administration of the general 5-HT1 agonist 5-carboxamidotryptamine maleate (10(-6) to 10(-4) M) or the 5-HT1A agonist buspirone (2 x 10(-6) to 2 x 10(-4) M) induced vasoconstriction that was dramatically higher in TFA than in TIA or control arterioles (p < 0.0001 in both cases). In addition, topical administration of the 5-HT1B agonist M-trifluoromethylphenylpiperazine (2 x 10(-6) to 2 x 10(-4) M) produced opposite responses, i.e., dose-dependent vasodilation in TIA and control arterioles, and dose-dependent constriction in TFA. Since we observed the same degree of vasodilation in response to 10(-4) M acetylcholine in all three groups of arterioles, the differences between the responses to 5-HT1 receptor stimulation were not due to the absence of endothelial-dependent dilatory mechanisms in the tumor-feeding arterioles. We conclude that 5-HT1 agonists are interesting pharmacologic tools for the modulation of tumoral blood flow, since they more dramatically constrict the microvasculature feeding the tumors than that feeding normal tissue.

The influence of hyperthyroidism on pharmacologically induced contractions of isolated resistance arteries

We investigated the effect of hyperthyroidism on the responses of small mesenteric resistance arteries to several contractile and dilator agents. Hyperthyroidism was established by feeding rats for 28 days with 5 mg/kg L-thyroxine-containing rat chow. This treatment produced a stable hyperthyroid state, as indicated by the increased serum T4 levels (236 +/- 7 vs. 60 +/- 2; T4-treated vs. control). Preparations of small mesenteric arteries were mounted in an isometric wire myograph. Subsequently, concentration-effect curves were determined for K+, Ca2+, methoxamine, phenylephrine, 5-hydroxytryptamine (5-HT), 9,11-dideoxy-11 alpha, 9 alpha-epoxymethano-prostaglandin F2 alpha (U46619), methacholine and nitroprusside. Our results indicate that hyperthyroidism does not induce major changes in the sensitivity of isolated resistance vessels to K+, Ca+, the alpha-adrenoceptor agonist, methacholine and nitroprusside. Furthermore, neither the affinity of alpha-receptors for methoxamine, nor the alpha-receptor reserve was influenced by the hyperthyroid state of the animal. A clearly sensitizing influence of hyperthyroidism was found for the vasoconstrictor effects of both 5-HT (6.57 +/- 0.04 vs. 6.29 +/- 0.06; hyperthyroid vs. control) and the thromboxane A2 receptor agonist U46619 (6.78 +/- 0.13 vs. 6.30 +/- 0.09; hyperthyroid vs. control). Sensitization to both 5-HT and U46619 was abolished in the presence of N omega-nitro-L-arginine methylester HCl (L-NAME, 0.1 mM). 5-HT-induced contractions in vessels from hyperthyroid rats were diminished by prior incubation with indomethacin (10 microM). The present results indicate that during hyperthyroidism resistance vessels are sensitized to both 5-HT and U46619. This sensitization involves the nitric oxide/L-arginine pathway and probably also certain steps in the cyclooxygenase pathway.

Development of hyperthermia following intracerebroventricular administration of endotoxin in the rat: effect of kinin B1 and B2 receptor antagonists

1. E. coli lipopolysaccharide (LPS) produced a dose-dependent (dose range: 0.02-150 micrograms) increase in rat core temperature that was maximal 6 h after intracerebroventricular (i.c.v.) administration. LPS (200 ng) increased core temperature by 1.0 +/- 0.2 degrees C, 6 h following administration, as compared to vehicle-treated controls (-0.2 +/- 0.2 degrees C). 2. LPS-induced (200 ng) hyperthermia was prevented by co-administration of the bradykinin (BK) B2 receptor antagonist, Hoe 140 (10 and 30 pmol, i.c.v.) or by indomethacin (10 nmol, i.c.v.). 3. Systemic administration of Hoe 140 at doses up to 1 mumol kg-1, s.c., did not attenuate LPS-induced (200 ng, i.c.v.) hyperthermia. However, LPS hyperthermia was significantly reduced by systemic administration of indomethacin (1 mumol kg-1, i.v.). 4. Co-administration of the selective B1 receptor antagonists, [des-Arg9, Leu8]BK (0.1 - 1 nmol, i.c.v.) or [des-Arg10] Hoe 140 (0.1 - 1 nmol, i.c.v.), did not prevent LPS-induced hyperthermia. 5. It is concluded that the development of hyperthermia following central administration of endotoxin requires activation of central, but not peripheral bradykinin B2 receptors. The formation of kinins within the CNS may be an important initial component of CNS inflammation following infection.