F. Coceani et al., CARBON-MONOXIDE FORMATION IN THE DUCTUS-ARTERIOSUS IN THE LAMB - IMPLICATIONS FOR THE REGULATION OF MUSCLE TONE, British Journal of Pharmacology, 120(4), 1997, pp. 599-608
1 We have previously shown that carbon monoxide (GO) potently relaxes
the lamb ductus arteriosus and have ascribed this response to inhibiti
on of a cytochrome P450-based mono-oxygenase reaction controlling the
formation of endothelin-1 (ET-1). In the present study, we have examin
ed whether CO is formed naturally in the vessel. 2 The CO-forming enzy
me, haem oxygenase (HO), was identified in ductal tissue in its consti
tutive (HO-2) and inducible (HO-1) isoforms by Western immunoblotting
and immunological staining procedures (both light and electron microsc
opy). HO-1 was localized to endothelial and muscle cells, while HO-2 w
as found only in muscle cells. Inside the muscle cells, HO-1 and HO-2
immunoreactivity was limited to the perinuclear region, and the Golgi
apparatus in particular. However, upon exposure to endotoxin, HO-1 bec
ame more abundant, and both HO isoforms migrated towards the outer reg
ion of the cytoplasm close to the sarcolemma. 3 CO was formed enzymati
cally from added substrate (hemin, 50 mu M) in the 10,000 g supernatan
t of the ductus and its formation was inhibited by zinc protoporphyrin
IX (ZnPP, 200 mu M). 4 ZnPP (10 mu M) had no effect on the tone of th
e ductus under normal conditions (2.5 to 95% O-2), but it contracted t
he endotoxin-treated ductus (at 2.5% O-2) At the same concentration, Z
nPP also tended to contract the hypoxic vessel (zero O-2) 5 ZnPP (10 m
u M) curtailed the relaxant response of the oxygen (30%)/indomethacin
(2.8 mu M)-contracted ductus to bradykinin (35 nM), while it left the
sodium nitroprusside (35 nM) relaxation unchanged. 6 We conclude that
CO is formed in the ductus and may exert a relaxing influence when its
synthesis is upregulated by an appropriate stimulus.