Dexrazoxane protects the heart from acute doxorubicin-induced QT prolongation: a key role for I(Ks)

INTRODUCTION

Doxorubicin, an anthracycline widely used in the treatment of a broad range of tumours, causes acute QT prolongation. Dexrazoxane has been shown to prevent the QT prolongation induced by another anthracycline, epirubicin, but has not yet been reported to prevent that induced by doxorubicin. Thus, the present study was designed to test whether the acute QT effects induced by doxorubicin could be blocked by dexrazoxane and to explore the mechanism. Results were compared with those obtained with a reference human ether-a-go-go (hERG) channel blocker, moxifloxacin.

METHODS

The effects of moxifloxacin (100 microM) and doxorubicin (30 microM), with or without dexrazoxane (from 3 to 30 microM), have been evaluated on the QTc interval in guinea-pig isolated hearts and on I(Kr) (rapid component of the delayed rectifier current) and I(Ks) (slow component of the delayed rectifier current) currents stably expressed in human embryonic kidney 293 cells.

RESULTS

Moxifloxacin (100 microM), a potent hERG blocker, prolonged QTc by 22%, and this effect was not prevented by dexrazoxane. Doxorubicin (30 microM) also prolonged QTc by 13%, did not significantly block hERG channels and specifically inhibited I(Ks) (IC(50): 4.78 microM). Dexrazoxane significantly reduced the doxorubicin-induced QTc prolongation and prevented doxorubicin-induced inhibition of I(Ks).

CONCLUSION AND IMPLICATIONS

Doxorubicin acutely prolonged the QT interval in guinea-pig heart by selective I(Ks) blockade. This effect was prevented by dexrazoxane. This result is important because it illustrates the danger of neglecting I(Ks) in favour of hERG screening alone, for early preclinical testing for possible induction of torsade de pointes.

Impact of serotonin on tumour growth

Several opposite effects of serotonin (5HT) on tumour growth have been reported. On one hand, 5HT is known as a growth factor for several types of nontumoural cells, and it has been proposed to take part in the autocrine loops of growth factors contributing to cell proliferation in aggressive tumours such as small cell lung carcinoma. Depending on the tumour type either 5HT2 or 5HT1 receptor antagonist have been found to inhibit the 5HT-induced increase in tumour growth. In contrast, several authors have also reported that 5HT and 5HT2 agonist can inhibit tumour growth. Most often this effect has been considered to be related with the specific vasoconstrictive effect of 5HT or 5HT2 agonists on the vessels irrigating the tumour, which has been evidenced by intravital microscopy. Intravital microscopy studies have also shown that vessels perfusing the tumour exhibit a specific vasconstrictive response to 5HT1 agonists. In addition, 5HT has been shown to be involved in the effects of several anticancer treatments associated with the reduction of tumour flow. Finally, the specific vasoconstrictive effect of 5HT or 5HT receptor subtype agonists might also be useful in inducing hypoxia in tumours, which could be exploited in a strategy using hypoxia-selective cytotoxins or hypoxia-selective gene therapy.

Effects of nucleotides adenosine monophosphate and adenosine triphosphate in combination with L-arginine on male rabbit corpus cavernosum tissue

Purines and more specifically adenosine monophosphate (AMP) and adenosine triphosphate (ATP) have a strong relaxant effect on smooth muscle cells of the dog, rabbit and human corpus cavernosum, to approximately the same degree as nitric oxide (NO). However, purines are considered as modulators of erectile function rather than key mediators. This suggests that the use of purines combined with NO donors could be effective to treat some specific erectile disorders. The relaxation induced by the combination of l-arginine (Arg), a natural substrate for NO synthase, was assessed with a purine-nucleotide (AMP, ATP) on a rabbit corpus cavernosum model, to determine if these substances could potentiate each other's effect. When a pre-contraction was induced by phenylephrine, AMP alone induced a 43% CC relaxation rate and ATP alone a 26% rate. The relaxation rate induced by Arg was lower in comparison (8% at 5.10(-4) m vs. 25% at AMP 5.10(-4) m and 15% at ATP 5.10(-4) m). NO synthase inhibitor n-nitro-l-arginine did not modify the relaxing effect provoked by AMP suggesting that the mechanism of action of this nucleotide does not involve the NO pathway. The combination of Arg at 5.10(-4) m with either AMP or ATP at different doses ranging from 5.10(-4) to 10(-3) m significantly enhanced the relaxing response reaching rates of 62 and 80% respectively, leading to a synergistic effect. The present data indicate that a 'NO donor' combined with an 'adenosine donor' could be an effective therapeutic approach.

Effect of Ginkgo biloba extract (EGb 761) on the vasospastic response of mouse cutaneous arterioles to platelet activation

The effect of intravenously administered Ginkbo biloba extract (EGb 761) on the vasospastic response to platelet activation has been assessed using a cutaneous flap preparation in anaesthetized mice. Arterioles of the axillary artery were observed by intravital microscopy, and platelets were activated by topical application of ADP under two steady state conditions: normothermia (37 degrees C) and hypothermia (24 degrees C). Responses of the cutaneous arterioles to stimulation by topical application of a thromboxane agonist (U46619) were also compared in animals treated intravenously with EGb 761 or with a thromboxane synthesis inhibitor (U63557). ADP induced a 34% constriction of the arterioles in control animals. However, no arteriolar constriction occurred in response to ADP in platelet-depleted animals (collagen-induced thrombocytopenia) or in animals treated with EGb 761 (60 mg/kg, i.v.). Exposure of the arterioles to hypothermia (24 degrees C) for 10 min induced constriction of 7-12% in all experimental groups of animals. Under these hypothermic conditions, either EGb 761 or thrombocytopenia abolished ADP-induced arteriolar constriction which was substituted by arteriolar dilation, indicating that EGb 761 can inhibit the vasospasm that is produced by platelet activation. As topically applied U46619 (10(-5) M) induced arterioles constriction (about 22%) that was abolished by intravenous treatment with EGb 761, the extract appears to act directly rather than as a thromboxane synthase inhibitor. Collectively, these findings indicate that platelet factors can play a significant role in cutaneous vasospasm, and that EGb 761, via an action on the thromboxane pathway, could be useful in treating Raynaud's phenomenon and other vascular disorders which involve increased thromboxane production.

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.

Characterization of the specific response to serotonin of mouse tumour-feeding arterioles

PURPOSE

To investigate the role of tumour versus non-tumour factors in the specific response to serotonin (5-HT) of tumour-feeding arterioles (TFA).

MATERIALS AND METHODS

Using mouse models of intra-vital microscopy, the response to topical administration of 5-HT was studied in arterioles feeding tumours: fibrosarcoma (Meth A), murine mammary adenocarcinoma (EMT6) and human colo-rectal carcinoma (HRT18) intra-cutaneously implanted.

RESULTS

For all types of tumour, 5-HT induced a far more pronounced constriction of TFA than of control arterioles. The presence of a tumour implanted in the connective tissue between the skin and the cremaster muscle also affected the reactivity of muscle arterioles. Conversely, the response to serotonin by neovessels grown after implantation of an exogenous element under the skin did not differ from that of control arterioles.

CONCLUSIONS

Changes in reactivity to serotonin were not dependent on the type of tumour implanted in the skin and were not present for a non-tumour implant. The presence of the tumour can alter the reactivity of vessels from tissue in contact with the tumour even if these vessels did not feed the tumour. This phenomenon is local and was not found in the vessels at a distance from the tumour.

Effects of Ginkgo biloba extract (EGb 761) on arteriolar spasm in a rat cremaster muscle preparation

The effects of an extract of Ginkgo biloba (EGb 761) on arteriolar spasm were confirmed using a preparation of rat cremaster muscle. When vasospasm was induced by rat serum, arteriolar constriction reached 25-30% of the initial diameter after 10 min. Intravenous injection of EGb 761 (30 mg/kg) 5 min after inducing spasm inhibited about 80% of this serum-induced vasoconstriction. As previous studies have shown that EGb 761 has an antiaggregatory effect on platelets, thrombin, serotonin (platelet-derived compounds that are present in the serum) and a thromboxane analogue (U46619) were also used to induce vasospasm. Administration of EGb 761 (30 mg/kg) 5 min after exposure of the preparation to serotonin (10(-3) M) or 10 min after exposure to thrombin (20 units) did not affect vasospasm induced by these agents. In contrast, treatment with this same dose of EGb 761 5 min after exposure of the preparation to U46619 (10(-4) M) abolished the arteriolar constriction induced by this agent in 15 min. The thromboxane/prostaglandin H2 receptor antagonist SQ29548 antagonized serum-induced vasospasm, indicating an involvement of thromboxane. Other experiments indicated that the effects of EGb 761 of counteracting vasospasm may be mediated in part by ginkgolide B, a triterpene constituent of the extract that is an antagonist of platelet-activating factor and in part by an 'NO-like' action of its proanthocyanidin constituents. Taken together, these results have revealed that EGb 761 treatment can antagonize the vasoconstrictor effect of thromboxane on arterioles. As thromboxane is implicated in many cardiovascular disorders, this property of EGb 761 may explain some of its beneficial clinical effects in such pathologies.

Coronary response to large decreases of hemoglobin-O2 affinity in isolated rat heart

In this study, the consequences of large increases of P50 (O2 partial pressure at 50% oxyhemoglobin saturation) on coronary blood flow (CBF) were investigated in isolated Wistar rat heart. Rightward shifts of the O2 dissociation curve (ODC), obtained by lysing and resealing erythrocytes to encapsulate inositol hexaphosphate (IHP), led to a very large increase in P50 without side effects. Each heart was perfused alternatively with control stored human blood [P50 = 18.8 +/- 0.3 (SE) mmHg] and IHP-treated human blood (P50 = 47.1 +/- 1.7 mmHg), according to the technique of Langendorff (mean perfusion pressure 80 mmHg; hematocrit 25%). Arterial PO2 of 180 mmHg was maintained to keep arterial O2 content identical for both types of blood. When hemoglobin affinity was lowered, CBF decreased from 5.32 +/- 0.20 to 3.40 +/- 0.14 ml X min-1 X g-1, coronary sinus PO2 (PcsO2) rose from 39.9 +/- 0.9 to 69.9 +/- 4.2 mmHg, and myocardial O2 consumption (MVO2) rose slightly from 0.125 +/- 0.005 to 0.149 +/- 0.010 ml O2 X min-1 X g-1 (P less than 0.05). A significant negative correlation was found between CBF and P50 (r = -0.90; n = 32) and a significant positive correlation between PcsO2 and P50 (r = +0.84; n = 28). The coronary blood flow response to high P50 values was not abolished when maximal dilation was induced by adenosine, so this response seems independent of metabolic needs. These experiments have demonstrated that if O2 uptake by erythrocytes remains constant, in the presence of a high P50, sufficient O2 supply may be achieved with substantially less blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific esponse to 5-HT2 agonists by arterioles linked to Meth A tumors in mice

Using intravital microscopy, we compared the responses to serotonin [5-hydroxytryptamine (5-HT)] and to a specific 5-HT2 agonist [1-(2,5-dimethoxy-4-bromo-phenyl)-2-aminopropane (DOB)] by normal arterioles and by the host-modified arterioles feeding a Meth A tumor implanted into the flank of female BALB/c mice. Topical and intravenous administration of 5-HT (10(-6) to 10(-4) M and 0.01-10 micrograms) or DOB (5 x 10(-7) to 5 x 10(-5) M and 0.01-10 micrograms) induced arteriolar constriction, which was far more pronounced for the arterioles feeding the tumor. This larger degree of vasoconstriction in tumor-feeding vs. normal arterioles was not found with norepinephrine. We also compared tumor growth and the mouse life span in three groups of mice, which were given 1 mg of serotonin or 0.74 mg DOB or saline solution. 5-HT or DOB both reduced tumor growth drastically compared with the controls (P less than 0.001), and survival rates were significantly higher in the 5-HT or DOB-treated groups (P less than 0.001). We conclude that 5-HT2-serotoninergic agonists are of particular pharmacological interest, since their vasoconstrictive action on the microvasculature feeding the tumors is much greater than in normal tissue and may interfere with tumor growth.

Role of endothelial factors in the specific response of mouse tumour-feeding arterioles to stimulation of 5-HT1 receptors

PURPOSE

To investigate the possible role of endothelial mediators on the increased vasoconstriction to 5-HT1 receptor stimulation by the host-modified arterioles feeding a Meth-A tumour implanted in the flank of female Balb/c mice.

MATERIALS AND METHODS

Using intravital microscopy, the response to the topical administration of the general 5-HT1 agonist 5-carboxamidotryptamine maleate (5-CT; 10(-6) M to 10(-4) M) by the tumour-feeding arterioles with the responses of tumour-independent arterioles and those of control arterioles from mice without tumour after antagonization or inhibition of the synthesis of endothelial mediators was compared.

RESULTS

The dramatically higher vasoconstriction to 5-CT observed in tumour-feeding arterioles than in tumour-independent or control arterioles still persisted when either nitric oxide synthase, cyclooxygenase, lipoxygenase, or phospholipase A2 were inhibited or when thromboxane A2 or endothelin were antagonized.

CONCLUSIONS

It was concluded that the higher reactivity to 5-HT1 stimulation by tumour-feeding arterioles is not due to changes in endothelial mediator release but probably due to changes affecting arteriolar smooth muscle.

[Microcirculatory consequences of a venous striction in the rat. Effect of a coumarine-rutine association]

The aim of this study was to investigate the consequences of a venous striction on capillary red blood cell distribution and venular blood return and the effect of a coumarin derivative-rutoside combination. The study was conducted, in vivo, in the rat cremaster muscle using intravital microscopy. The striction lasted thirty minutes and was obtained by clamping the epigastric vein. This mechanical constraint was sufficient to induce microcirculatory modifications without disrupting microvessels. Before the striction (t-5 min), the velocities and diameters of the veins and arteries were comparable in all groups. After the striction (t5 min), in the control group, venous blood flow decreased by 60%, from 0.48 +/- 0.09 mm3/s (t-5 min) to 0.20 +/- 0.06 mm3/s (t5 min). The results showed that after thirty minutes reperfusion, venular blood flow in the control animals was only 34% of initial blood flow. The mean red blood cell velocity dropped by 56%, the percentage of low perfused capillaries increased from 7.5% to 50%. Treatment of animals with a coumarin derivative-rutoside combination, particularly at 4 mg/kg coumarin derivative-100 mg/kg rutoside, has significantly improved the microcirculation. After thirty minutes reperfusion venular blood flow was 60% and the percentage of low perfused capillaries was only 10%. The effect seemed to be more pronounced for rutoside than coumarin derivatives. The interest of this study was to set up an experimental model of a venous striction not too severe to induce micro-hemorrages but enough to modify microcirculation. This model was used to quantify the beneficial effects of a coumarin derivative-rutoside combination.

Effects of adenosine monophosphate used in combination with L-arginine on female rabbit corpus cavernosum tissue

Introduction

Sexual dysfunction is significantly more prevalent in women than in men. However, to date, no satisfactory oral treatment is yet available.

Aim

The aim of this study was to study the effects of adenosine monophosphate (AMP) alone or its combination with L-Arginine on the relaxation of the female rabbit corpus cavernosum.

Methods

Cylinder strips from the corporal body of the excised clitoris from female New Zealand White rabbits were incubated in Krebs solution. Phenylephrine (PE) precontraction was achieved, then the drugs AMP and L-Arginine were administered either independently or in sequential combinations to the strips under precontracted conditions.

Main Outcome Measures

Contraction percentages were compared.

Results

When precontraction was induced by PE 8 μM or 20 μM, AMP was shown to induce relaxation up to 25% in a dose-dependent manner. The relaxation induced by L-Arginine reached 15.6% at 5.10⁻⁴ M vs. 16.5% at AMP 5.10⁻⁴ M under the same experimental conditions. Nitric oxide (NO) synthase inhibitor N-nitro-L-arginine strongly inhibited the relaxing effect provoked by AMP, suggesting that the action mechanism of this nucleotide is related to the NO pathway. The combination of L-Arginine at 5.10⁻⁴ M with AMP at different doses ranging from 5.10⁻⁴ M to 10⁻³ M significantly amplified the relaxing response up to 40.7% and 58%, respectively.

Conclusions

Our results demonstrate that AMP induces a relaxing effect on the female rabbit corpora. They also show that L-Arginine and AMP can potentiate each other and that a synergistic effect can be obtained by their combined use. Because only slight differences exist between both sexes in response to NO donors and/or nucleotide purines or in their use together, it is very likely that close biochemical mechanisms, although not to the same degree and not quite similar, are involved in the engorgement of the penis and the clitoris of New Zealand White rabbits. Stücker O, Pons C, Neuzillet Y, Laemmel E, and Lebret T. Original research-sexual medicine: Effects of adenosine monophosphate used in combination with L-Arginine on female rabbit corpus cavernosum tissue. Sex Med 2014;2:1–7.