Tibolone and its metabolites acutely relax rabbit coronary arteries in vitro.
OBJECTIVE
To compare the acute effects of estradiol, tibolone and its metabolites on coronary arteries in vitro and to investigate possible vascular mechanisms.


METHODS
Coronary artery ring segments from female rabbits were mounted in myographs for isometric tension recordings. Concentration-response curves to tibolone, 3 alpha-OH-tibolone, 3 beta-OH-tibolone, Delta 4-isomer and 17 beta-estradiol were obtained after precontraction with potassium 30 mmol/l and after addition of N omega-nitro-l-arginine methyl ester 10(-4) mol/l (l-NAME, an inhibitor of endothelial nitric oxide (NO) synthase) or tetraethylammonium chloride 10(-2) mol/l (TEA, an unspecific inhibitor of potassium channels). The effects of the different substances to calcium concentration-response curves were evaluated. Responses are expressed as maximal contraction (E max), concentration giving half maximal contraction (log EC50) or area under curve (AUC).


RESULTS
Tibolone and its metabolites induced a concentration-dependent vasodilatation comparable to that of 17 beta-estradiol with the rank of potency: 3 beta-OH-tibolone approximately = to tibolone>3 alpha-OH-tibolone>Delta 4-isomer (ANOVA). l-NAME partly inhibited the relaxation to all substances. TEA induced a slight rightward shift of the relaxation to 3 alpha-OH-tibolone (log EC50: -5.05 versus -5.20; P<0.05; Student's t-test), but not to the other substances. Calcium concentration-dependent contraction curves were inhibited by all substances compared to controls (AUC, P<0.05, ANOVA).


CONCLUSIONS
Our data indicate that the acute relaxation induced by tibolone and its metabolites in coronary arteries in vitro are probably mediated by endothelium independent inhibition of calcium channels but may also involve an endothelium-dependent mechanism via nitric oxide. The effect of tibolone is comparable to that of 17 beta-estradiol in this set-up.
Department of Obstetrics and Gynaecology 537, Hvidovre University Hospital, Kettegaards Allé 30, DK-2650 Hvidovre, Denmark. col@dadlnet.dk
Claus Otto
CO
Lund
Lisbeth
L
Nilas
Susan Helene
SH
Pedersen
Tórur
T
Dalsgaard
Bent
B
Ottesen
D000818:Animals; D019540:Area Under Curve; D003331:Coronary Vessels / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004730:Endothelium, Vascular / Q000187:drug effects / Q000502:physiology; D004958:Estradiol / Q000494:pharmacology; D020847:Estrogen Receptor Modulators / Q000494:pharmacology*; D005260:Female; D009126:Muscle Relaxation / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000502:physiology; D019331:NG-Nitroarginine Methyl Ester; D009652:Norpregnenes / Q000494:pharmacology*; D011817:Rabbits; D014664:Vasodilation / Q000187:drug effects*
D020847:Estrogen Receptor Modulators; D009652:Norpregnenes; D004958:Estradiol; C027385:tibolone; D019331:NG-Nitroarginine Methyl Ester