Anaesthetized mice of one eNOS KO strain showed a paradoxica
Anaesthetized mice of one eNOS-KO strain showed a paradoxical decrease in blood pressure (BP) in response to i.p. administration of the NOS-inhibitor L-NA suggesting a role for non-endothelial isoforms of NOS in maintaining BP . Alternatively, diminution in the activity of the renin-angiotensin system and the autonomic nervous system, which serve as a defense against hypertension and/or involvement of NOS in establishing the baroreceptor setpoint were proposed as an explanation of hypertensive phenotype . However, studies on nNOS-deficient mice revealed that they are normotensive . Furthermore, triple e/i/nNOS knockouts have hypertension and the degree of hypertension is similar to that in the eNOS-gene disrupted single and double e/nNOS-KO . These results clearly demonstrate that among NO-synthases eNOS plays by far the most important role in BP regulation. However, in contrast to large arteries of eNOS-KO where no compensation for the lack of eNOS is observed , , , other endothelium-dependent vasodilators such as prostaglandins, nNOS or endothelium-derived hyperpolarizing factor compensate for the lack of eNOS in coronary, femoral, mesenteric, cerebral and skeletal resistance arteries , , , , , . These data indicate that different vascular-bed specific mechanisms regulate arterial tone in the absence of vascular eNOS and raise the question to what extent hypertension in eNOS-KO is caused by the lack of endothelial eNOS expression.
Discussion Endothelial-specific reintroduction of eNOS in eNOS-KO resulted in a lower eNOS protein expression in different tissues of eNOS-Tg/KO. Therefore, we investigated vascular eNOS activity by acetylcholine-induced endothelium-dependent vasodilation in conductance and resistance vessels, i.e. in aortic rings and in mice in-vivo. We observed no aortic response to cyclin dependent kinases in eNOS-KO ,  but a complete and indistinguishable dilation in C57BL/6 and eNOS-Tg/KO. Thus, aortic eNOS in eNOS-Tg/KO appears to be functionally active and the lower level of expression didn\'t impair the magnitude and sensitivity of endothelium-dependent aortic relaxation. Previous investigations have shown that the sensitivity of the NO-receptor sGC likely adapts to variations of eNOS expression and tonic vascular NO generation inasmuch as NO can induce nitrosylation of sGC which inhibits its activity. This was demonstrated in smooth muscle and endothelial cells upon treatment with the NO-donors or vascular endothelial growth factor  and in aortic and lung tissues of eNOS-Tg or of C57BL/6 treated with L-NA . Such sGC S-nitrosylation might also underlie the hypersensitivity of aortic rings of eNOS-KO to organic nitrates such as glycerol trinitrate , and the spontaneous NO-donor spermine-NONOate . Thus, correction of the NO-hypersensitivity occurring in eNOS-KO can be viewed as further evidence for normal aortic eNOS function in eNOS-Tg/KO. In accordance, the increased sensitivity of aortic tissue of eNOS-KO to alpha-adrenergic stimulation by phenylephrine as described previously  was completely reversed in eNOS-Tg/KO which showed a concentration-dependent vasoconstriction similar to C57BL/6. Taken together, these data suggest a normal function of eNOS in conductance vessels of eNOS-Tg/KO. To investigate endothelium-dependent vasodilation of resistance arteries in-vivo we invasively monitored BP of mice subjected to increasing doses of i.v. acetylcholine as performed previously in rabbits  and rats . Acetylcholine produced a dose-dependent and strong fall of BP in all strains including eNOS-KO albeit with a lower sensitivity to acetylcholine. Therefore, other endothelium-dependent vasodilators appear to compensate for the lack of eNOS in resistance arteries. For example, the dilatory response of coronary arteries to acetylcholine is preserved by compensatory generation of e.g. cyclooxygenase products , . Furthermore, preservation of endothelium-dependent vasodilation has been demonstrated in femoral and mesenteric vessels , cerebral arteries ,  and small skeletal muscle arterioles  of eNOS-KO mice. While these data indicate compensation for the lack of endothelial generation of NO in eNOS-KO, our data suggests that endothelial-specific reintroduction of eNOS to eNOS-KO partially reverses such compensation since there was no statistically significant difference in sensitivity to acetylcholine between eNOS-Tg/KO and C57BL/6. Furthermore, the strong and comparable increase of systolic, diastolic and mean arterial pressure to i.v. l-NAME in C57BL/6 and eNOS-Tg/KO suggests the reversion of compensatory mechanisms and indicates a functional eNOS enzyme in resistance arteries of eNOS-Tg/KO as observed in aortic rings.