Retention and subsequent liberation of glyceryl trinitrate in organ baths influences the relaxation of bovine mesenteric arteries contracted by various agents

Effects of MnDPDP and ICRF-187 on Doxorubicin-Induced Cardiotoxicity and Anticancer Activity

Oxidative stress participates in doxorubicin (Dx)-induced cardiotoxicity. The metal complex MnDPDP and its metabolite MnPLED possess SOD-mimetic activity, DPDP and PLED have, in addition, high affinity for iron. Mice were injected intravenously with MnDPDP, DPDP, or dexrazoxane (ICRF-187). Thirty minutes later, mice were killed, the left atria were hung in organ baths and electrically stimulated, saline or Dx was added, and the contractility was measured for 60 minutes. In parallel experiments, 10 µM MnDPDP or MnPLED was added directly into the organ bath. The effect of MnDPDP on antitumor activity of Dx against two human tumor xenografts (MX-1 and A2780) was investigated. The in vitro cytotoxic activity was studied by co-incubating A2780 cells with MnDPDP, DPDP, and/or Dx. Dx caused a marked reduction in contractile force. In vivo treatment with MnDPDP and ICRF-187 attenuated the negative effect of Dx. When added directly into the bath, MnDPDP did not protect, whereas MnPLED attenuated the Dx effect by approximately 50%. MnDPDP or ICRF-187 did not interfere negatively with the anti-tumor activity of Dx, either in vivo or in vitro. Micromolar concentrations of DPDP but not MnDPDP displayed an in vitro cytotoxic activity against A2780 cells. The present results show that MnDPDP, after being metabolized to MnPLED, protects against acute Dx cardiotoxicity. Both in vivo and in vitro experiments show that cardioprotection takes place without interfering negatively with the anticancer activity of Dx. Furthermore, the results suggest that the previously described cytotoxic in vivo activity of MnDPDP is an inherent property of DPDP.

Adsorption of atrial natriuretic peptide to different materials: a factor influencing results of in vitro experiments?

Studies on atrial natriuretic peptide (ANP) in ex vivo situations, include a risk of adsorption to surrounding materials. In order to investigate this potential source of error, known concentrations of ANP in Krebs solution were prepared in test tubes of different materials. The solutions were analyzed for ANP-concentration by radioimmunoassay (RIA), using a standard-curve of ANP in phosphate buffer supplemented with 0.1% human serum albumin (HSA) and 0.1% Triton X100. A considerable adsorption was seen to the different materials tested, also to siliconized glass and polypropylene. With 1 ml ANP-solution in concentrations from 1 x 10(-9) to 1 x 10(-5)M an adsorption varying between 10 and 31% was seen to a 15 cm2 polystyrene-surface, corresponding to a conventional test tube. With 1 ml of ANP 120 pM in Krebs solution serially dispensed into six empty polystyrene test tubes, 73% of the initial peptide amount was lost due to adsorption. The adsorption could be prevented or partly reversed by adding HSA or Triton X100 to the solutions. These findings indicate that adsorption entails a risk of disturbing the results of in vitro experiments in studies on ANP.

Studies on the biphasic relaxant curve of glyceryl trinitrate in rat aorta: role of GTN metabolites

1. The present study has examined the possibility that one or more metabolites of glyceryl trinitrate (GTN) (i.e. glyceryl-1,2- and -1,3-dinitrate and glyceryl-1- and -2-mononitrate) may be responsible for the second phase of the biphasic relaxant curve to GTN in phenylephrine-contracted rings of rat aorta. 2. The IC50 values for the two phases of the GTN curve were 0.1 mumol/L and 12 mumols/L with the initial phase eliciting 60% of the total relaxation response. The curves for glyceryl-1,2- and -1,3-dinitrate were monophasic with IC50 values of 248 mumols/L and 110 mumols/L, respectively. The mononitrate metabolites elicited relaxant effects at concentrations greater than or equal to 1 mmol/L. 3. The induction of tolerance to GTN or pretreatment with oxyhaemoglobin (5 mumol/L) resulted in a monophasic GTN curve with IC50 values of 16 mumol/L and 18 mumol/L respectively suggesting selective abolition of responses to low concentrations of GTN with little effect on responses to high concentrations of GTN. The relaxant effects of the -1,2- and -1,3-dinitrates, like that to GTN, were essentially unaltered by GTN tolerance or oxyhaemoglobin. 4. Thus while the relaxant effects of the dinitrate metabolites possess similar properties to that of the second phase of relaxation to GTN, a role for these metabolites is unlikely since their IC50 values are 9-20-fold greater than that for the second phase of relaxation to GTN. Whether these differences are due to the 8-10-fold lower lipophilicity of the dinitrates as compared with the parent compound requires further study.

Tissue distribution of glyceryl trinitrate and the effect on cGMP levels in rat

In order to study distribution in tissue, rats were treated subcutaneously with glyceryl trinitrate, GTN, (50 mg/kg). The concentrations of GTN were measured in plasma, brain, heart, adipose tissue and aortic tissue at different sampling times by a gas chromatographic method with electron-capture detection. The peak GTN-concentration was reached after 2 hours in all tissues examined. The highest concentration of GTN was found in adipose tissue, where the level was approximately forty times higher than in plasma. The concentration of GTN in brain, heart and aortic tissue was about 2-3 times as high as in plasma. The cGMP level was measured in heart and brain. An increase of the cGMP level was found in brain 2 hours after GTN administration. No cGMP increase was found in heart tissue. The results indicate a substantial distribution of GTN to tissue, and an increase of cGMP in brain. The distribution of the substance in the tissues as shown, might have both pharmacokinetic and pharmacodynamic implications.

Biphasic relaxant curves to glyceryl trinitrate in rat aortic rings. Evidence for two mechanisms of action

The concentration-effect curve for the relaxant effects of glyceryl trinitrate (GTN) in rat aortic rings consisted of two phases with IC50 values of 0.1 microM for Phase I and 14 microM for Phase II. Incubation of tissues with oxyhaemoglobin or the induction of tolerance to GTN abolished responses occurring in Phase I but were without effect on Phase II relaxant responses. Both phases of the relaxant curve appeared to involve cyclic GMP since responses were (i) potentiated by the cyclic GMP phosphodiesterase inhibitor zaprinast (M & B 22948) and (ii) inhibited by methylene blue and LY83583, agents which inhibit soluble guanylate cyclase. The latter agents inhibited Phase I responses in a non-surmountable manner while Phase II responses were shifted to the right without effect on the maximal response. Neither phase of relaxation involved stimulation of the Na+/K+ ATPase pump since treatment of tissues with ouabain or K+-free solutions did not alter the GTN biphasic curve. Phase I relaxant responses to GTN resembled those to the endothelium-dependent relaxant acetylcholine, since oxyhaemoglobin and methylene blue were non-surmountable antagonists; however there was no cross tolerance to acetylcholine in GTN tolerant tissues. Phase II relaxant responses resembled those obtained with sodium nitroprusside (SNP) since neither oxyhaemoglobin nor the induction of tolerance to GTN altered the response to SNP. These results indicate that there are two distinct mechanisms of relaxation for GTN in rat aortic rings; however both mechanisms appear to involve cyclic GMP as the second messenger.

Relaxation of bovine mesenteric artery induced by glyceryl trinitrate is attenuated by pertussis toxin

Low concentrations (less than 1 nM) of glyceryl trinitrate (GTN) induced a considerable relaxation of bovine mesenteric arteries (BMA) brought to sustained contraction by the addition of phenylephrine. The concentration-response curve of GTN showed a biphasic pattern with a high and a low affinity component. Pretreatment with pertussis toxin (IAP) attenuated the high affinity relaxant component, but not the low affinity component or the relaxation induced by NaNO2. A polyclonal antibody to IAP counteracted the effect of the toxin on the GTN-response. Low concentrations of GTN increased the cGMP level in BMA via activation of the high affinity pathway. This effect was also inhibited in preparations pretreated with IAP. It is suggested from the present study that GTN induces relaxation of vascular smooth muscle via two separate pathways, the high affinity pathway might involve a receptor-complex interaction with a regulatory protein.