Both
carbon monoxide (CO), the product of
heme oxygenase (HO), and
nitric oxide (NO) elevate cyclic
guanosine monophosphate levels in smooth muscle cells, leading to relaxation of the vessels. We examined the hypothesis that the effect of CO in regulating blood pressure could be augmented in
hypertension where the function and/or production of NO is impaired. We used two hypertensive models, a spontaneously hypertensive rat (SHR), and a Wistar Kyoto rat (WKY) which was given the
NO synthase (NOS) inhibitor
N(omega)-nitro- L-arginine (L-NNA). In these hypertensive rats, we examined HO gene expression with Northern blot analysis,
guanosine 3',5'-monophosphate (cGMP) levels with
enzyme-linked
immunosorbent assay of each organ, and the response of blood pressure to treatment with an HO substrate (
hemin, 23 micromol/kg
body weight, i.p.) or HO inhibitor (
zinc or
tin protoporphyrin-IX; ZnPP or SnPP, 50 micromol/kg
body weight i.p. or s.c.), for 4 or 8 consecutive days with plethysmography. Northern blot analysis showed that HO-1 and -2
mRNA levels in the left ventricle, aorta, kidney, and soleus muscle in the hypertensive rats were 2-5 times higher than those in control normotensive WKYrats. In contrast, both HO
mRNA levels in the gastrocnemius muscle in the hypertensive rats were similar to those in control WKYrats. As to whether the HO/CO system contributes to the regulation of blood pressure, ZnPP or SnPP increased and
hemin decreased systolic blood pressure (SBP), respectively, in the hypertensive rats (P < 0.01), but not in WKYrats, accompanied with changes in cGMP in each organ of the hypertensive rats. The effect of CO in the regulation of blood pressure is augmented, resulting in increased expression of HO gene when the function and/or production of NO is impaired.