This study demonstrated enhanced tissue ACE activation in coronary resistance arteries in obesity, a pathological alteration that interferes with BK-mediated regulation of arteriolar diameter. This conclusion is supported by the finding that (1) HFD obese rats showed diminished BK-induced coronary arteriolar dilation, which was restored by in vitro administration of the ACE inhibitor, captopril; (2) similarly, in coronary arterioles from obese patients, in vitro administration of captopril enhanced the dilatory response to BK, which was associated with increased ACE activity of coronary vessels when compared with non-obese subjects; and (3) in our retrospective analysis, obese patients who were taking ACE inhibitors prior to cardiac surgery exhibited enhanced BK-induced dilation in coronary resistance arteries.
Previous studies have shown that systemic administration of ACE inhibitors improves vasodilator function in animals with experimental obesity and insulin resistance. For instance, in obese Zucker rats, oral administration of the ACE inhibitor, imidapril (10 mg/kg), was associated with increased liver and skeletal muscle blood flow.16
Acetylcholine (ACh)-induced dilation in epineural arterioles was improved in obese Zucker rats by treatment with enalapril (20 mg/kg).19
In a study by Duarte et al, endothelium-dependent relaxation of isolated aorta was studied in obese Zucker rats treated orally with either captopril or enalapril (50 mg/kg and 10 mg/kg, respectively). Interestingly, they found that only the sulfhydryl group containing captopril augmented the impaired endothelium-dependent aortic relaxation in obese animals.20
The beneficial effect of ramipril (1 mg/kg) in improving endothelial dysfunction of the aorta was also demonstrated in obese JCR:LA-cp rats.17
In that study, BK-stimulated coronary blood flow was also measured in isolated rat hearts. The authors found enhanced coronary flow in response to BK in ramipril-treated JCR:LA-cp rats, indicating the beneficial effect of ACE inhibitors on myocardial perfusion.17
These studies reveal that part of the beneficial vascular effects of systemic ACE inhibition is indirect and can be related to BP-lowering or improvement of insulin resistance. Indeed, it has been postulated that ACE inhibitors may increase insulin sensitivity in patients with type 2 diabetes.21
It remains unclear whether or not ACE inhibitors have a direct vascular effect, mediated by local inhibition of tissue ACE in the coronary circulation.
In the heart, AngII is primarily synthesized in situ via the conversion of Ang I, a mechanism that appears to be mediated by tissue ACE rather than by the circulating enzyme.22
Although AngII has many adverse, mainly long-term, effects in the heart,11
it preferentially dilates coronary resistance arteries, primarily via activation of AT2R.12,13
The contribution of AngII and AT2R activation to the regulation of coronary arteriolar diameter in obesity is unclear. An earlier study by Zhang et al demonstrated that in dogs fed a HFD, AngII elicited constriction, but dilated the coronary arterioles of control animals.15
The present study shows that AngII-induced dilation in coronary microvessels with no significant difference in the overall magnitude of the dilation between control and HFD rats, as well as between non-obese and obese patients. In order to assess the potential contribution of AT1R and AT2R in this response, AngII-induced dilation was measured in the presence of both the AT1R blocker, losartan, and the AT2R blocker, PD 123,319. We found no significant effect of losartan in AngII-mediated dilation in HFD rats. In the coronary arterioles of obese patients, administration of losartan elicited a trend toward an enhanced AngII-induced dilation, whereas AngII-induced responses were entirely abolished by additional application of AT2R antagonist in both the HFD rats and obese patients. Collectively, these data suggest only a minor contribution of AT1R to the AngII-induced response and indicate preserved AT2R-dependent dilator signaling in obesity.
Thus, it seems that in obesity there are mechanisms other than increased AngII production that are primarily responsible for the impaired BK-mediated regulation of coronary microcirculation. Indeed, it has been postulated that the effects of ACE inhibitors are mainly attributable to an increase in the tissue level of BK in the microvasculature.23
In low, nanomolar concentrations, BK is converted by ACE into an inactive metabolite BK-(1–7), which is further converted into BK-(1–5).24
BK-(1–5) has no vasoactive effect, although it may inhibit thrombin-induced platelet aggregation.25
Kuga et al demonstrated that in epicardial coronary arteries BK-induced increases in diameter were further enhanced by intracoronary infusion of enalaprilat in patients without significant coronary stenosis.3
Given that, in our study we raised the hypothesis that obesity leads to increased activity of microvascular ACE, which mainly manifests as increased breakdown of the vasodilator BK. To furnish evidence for this scenario, coronary arterioles were dissected from the heart and the BK-induced vasomotor responses were investigated in isolated microvessels ex vivo. In this study design, the function of tissue ACE and its effect on BK-induced responses can be assessed independently of the myocardium and systemic circulation. We found that exogenous BK elicited diminished dilation of isolated coronary arterioles from HFD rats. There were no major changes in the expression of BK receptors (BK1R or BK2R) in lean or obese animals. It is known that BK, via activation of its receptors, may activate the synthesis of several vasoactive molecules in coronary arteries, including nitric oxide (NO), prostaglandins and endothelium-derived hyperpolarizing factor.26
Under certain pathological conditions, the lack of NO production may convert vasodilation to vasoconstriction.27,28
In this context, we have reported earlier that synthesis of endothelial NO is reduced in the coronary microvessels of obese rats.18
In the present study, we found that the NO synthase inhibitor, L-NAME, did not affect BK-induced dilation in isolated human coronary arterioles from either lean or obese patients. These findings are in accordance with previous observations obtained in isolated human coronary arterioles29
and indicate only minor, if any, involvement of NO in BK-induced coronary arteriolar dilation. Similar results were obtained in the control rat coronary arterioles, demonstrating the lack of contribution of NO to BK-induced dilation. Interestingly, in the coronary arterioles of HFD rats, BK-induced constriction was ameliorated in the presence of L-NAME, which suggests that in obesity NO synthase may become a source of a constrictor factor. Indeed, when NO synthase is uncoupled, it generates superoxide anion,30
which may facilitate the production of constrictor prostanoids in vascular endothelial cells.31
Thus, we concluded that BK receptors and changes in their initiated downstream signaling are unlikely to contribute to the diminished BK-induced coronary responses in HFD rats.
When isolated coronary arterioles were incubated with the ACE inhibitor, captopril, the reduced dilation response to BK was restored in HFD rats, close to a similar level observed in the control vessels. In humans, although there was no significant difference in the magnitude of BK-induced coronary dilation between the obese and non-obese patients, we found that captopril significantly enhanced the dilation response to BK preferentially in obese individuals. In association with these functional changes, we detected increased microvascular ACE activity in arterioles obtained from obese subjects. Collectively, these observations are the first evidence that ACE expressed in the wall of coronary resistance arteries interferes with BK-mediated dilation in obesity.
In order to further investigate the effect of obesity-dependent activation of coronary microvascular ACE and to reveal potential therapeutic implications, we retrospectively analyzed the effects of ACE inhibitor therapy on the BK-induced coronary response in obese patients. We found that the use of ACE inhibitors prior to cardiac surgery predicted an enhanced vasodilation response to BK. Statin therapy was also highly predictive of improved vessel dilation. In the multivariable logistic regression analysis, through controlling for statin therapy, we identified ACE inhibitor therapy as an independent predictor of improved vasodilation in obese patients. However, the possible influence of other modifying factors (such as the BP-lowering effect of ACE inhibitors) cannot be entirely excluded, which may limit the interpretation of these data. It should be also noted that in our isolated vessel studies the effect of the ACE inhibitor, captopril, on the BK-induced coronary responses was investigated irrespective of prior ACE inhibitor therapy. Given that, although our results suggest a direct vascular effect of captopril in obese patients, a conclusion in regard to the indirect effect on vasomotor behavior has limitations.