PROGESTERONE INDUCES ENDOTHELIUM-INDEPENDENT RELAXATION OF RABBIT CORONARY-ARTERY INVITRO

The effect of progesterone on isolated rabbit coronary arteries and its possible mechanism was investigated by measuring changes of isometric tension. Progesterone (1, 3, 10 and 30-mu-M) induced significant coronary relaxation in K+ (30 mM)-, prostaglandin F2-alpha (3-mu-M)- or Bay K 8644 (1-mu-M plus 15 mM K+)- precontracted arteries. There was no difference between endothelium-intact and -denuded coronary arteries from both male and female rabbits, precontracted with these three agents. Haemoglobin, indomethacin, methylene blue, glibenclamide or barium chloride did not affect the relaxation. In endothelium-denuded rabbit coronary arteries, progesterone shifted calcium concentration-dependent constrictor-response curves to the right, the maximal contraction was also reduced. The -log ED50s were 3.6 in control, and 3.3 and 2.9 after incubation with progesterone (3 and 30-mu-M), respectively. Similar results were obtained in rat aorta. We conclude that progesterone induces significant endothelium-independent relaxation in rabbit coronary arteries in vitro, possibly by affecting calcium influx.