Discussion
This study examined the effects of ACE inhibitor and ARB on methylarginines in patients undergoing MHD. We found that short-term treatment with ramipril (1 week) increased ADMA levels and decreased the arginine-to-ADMA ratio compared to placebo and valsartan in patients undergoing MHD. Studies in vitro suggest that increase in ADMA production occur through stimulation of bradykinin B2 receptors.
ADMA levels are increased in patients undergoing MHD and correlate with all-cause mortality [9]. In addition, ADMA levels correlate positively with markers of atherosclerosis such as carotid intima-media thickness, left ventricular hypertrophy, and cardiovascular events [9], [10], [28],[29]. Arginine and ADMA are substrate and inhibitor of endothelial nitric oxide synthase, respectively; thus the ratio of arginine to ADMA may modify the enzyme activity [30]. Accordingly, a lower arginine to ADMA ratio has been also been associated with all-cause mortality and carotid intima-media thickness in the general population [31]–[33]. While the pathogenic role of ADMA in the cardiovascular disease is somewhat unclear, it has been shown to induce endothelial dysfunction, which is involved in the initiation and progression of atherosclerosis [34]. Another possible factor that can contribute to increase cardiovascular risk is the over-activation of the sympathetic nervous system by ADMA. Experiments in humans have shown that nitric oxide inhibition results in the activation of the sympathetic nervous system [35], a known risk factor cardiovascular disease. In fact, ADMA levels strongly correlates with norepinephrine levels in patients with ESRD [36].
We did not find any effect of either ramipril or candersatan on ADMA levels in patients with preserved kidney function. But previous studies demonstrated that both ACE inhibitors and ARBs reduce ADMA levels in patients without kidney disease [19]–[22]. The duration of the treatment, one week vs. several weeks and months, may explain why neither ACE inhibitor nor ARB reduce ADMA levels in our study. Longer treatment with ACE inhibitors or ARBs may decrease ADMA by decreasing angiotensin II effects. Angiotensin II increases ADMA levels in smooth muscle cells by direct stimulation of AT1 receptors and by increasing oxidative stress [18]. The latter is probably mediated through the production of reactive oxygen species by NADPH oxidase [37], [38]. Only one previous study in patients on MHD showed that 6-week treatment with valsartan or amlodipine reduced ADMA and SDMA [39]. Findings from this study were attributed to a reduction in shear stress, which has been shown to increase ADMA release from endothelial cells [40]. In comparison, we did not find reduction in ADMA after 1 week treatment with valsartan. Again, it is possible that a longer treatment with ARB may have an effect on ADMA levels. Nevertheless, our study is the first one to compare the effect of ACE inhibitors and ARBs on ADMA levels in patients on MHD.
In this study we found that ramipril, an ACE inhibitor, but not valsartan, an ARB, increased ADMA level. We found also that ramipril did not increase ADMA level in patients without ESRD, suggesting that other factors pertaining to ESRD, particularly hemodialysis may contribute to the ramipril-induced increase in ADMA levels. A potential explanation as to why ACE inhibitors and ARBs differ in their effect on ADMA levels could be that these medications also differ in their effect on the degradation of bradykinin. Specifically, ACE inhibitors increase bradykinin bioavailability, [41] whereas ARBs does not. We have previously shown that ramipril treatment increased bradykinin levels during hemodialysis [5]. Previous studies have shown that bradykinin increases reactive oxygen species production through stimulation of NADPH oxidases [42]. NADPH activation increases ADMA levels by increasing protein methylation and inhibiting ADMA degradation, [18],[43] which are both mediated by reactive oxygen species. Thus, it is possible that ramipril increases ADMA levels by increasing bradykinin and oxidative stress. Our in vitro studies support this hypothesis. We found that bradykinin contributes to ADMA production in human adenocarcinoma cells. Importantly, we also found that incubation with the bradykinin B2 receptor antagonist HOE-140 reduces intracellular ADMA levels. Bradykinin B1 receptor blockage does not play a role in decreasing ADMA levels suggesting that bradykinin-dependent ADMA production occurs through bradykinin B2 receptor stimulation. A possible mechanism that has not been explored is the role of bradykinin on DDAH, the enzyme responsible for ADMA degradation. Bradykinin may increase ADMA levels by decreasing DDAH activity.
We also found that markers of inflammation positively correlate with ADMA levels and inversely with the arginine-to-ADMA ratio. The latter also has an inverse correlation with F2-Isoprostanes. These findings are in agreement with previous studies that showed the correlation between ADMA and inflammatory markers (C-reactive protein and IL-6) [10], [44]. We previously described that ramipril has a greater pro-inflammatory effect in patients with ESRD [5]. Given the interrelation between ADMA and inflammation, it is not surprising that ACE inhibitors also increase ADMA levels in patients on MHD.
Our findings may some interesting implications. First and foremost, these data could explain the relative lack of efficacy of ACE inhibitors in improving cardiovascular outcomes in patients with ESRD, at least to some extent. To date, there are no clear-cut clinical guidelines for the use of ACE inhibitor or ARBs in patients with ESRD, mostly due to lack of randomized clinical trials. If anything, limited epidemiological data on this subject indicate that use of ACE inhibitor could be detrimental in patients with ESRD, at least when used in combination [45]. Accordingly, further studies are necessary to examine the appropriate use of these potentially useful classes of cardiovascular medications. In addition, these data demonstrate that patients with ESRD, particularly ones on MHD behave differently in response to medications commonly used in the general populations. Accordingly, one should not simply assume that what has been observed in response to a medication in the general population can be safely extrapolated to patients with ESRD. Finally, our data once again highlight the importance of the so-called non-traditional cardiovascular risk factors in patients with ESRD. While this particular study focused on ACE inhibitors and ARBs, other interventions that modulate one or more of these metabolic derangements should be aggressively studied in patients with ESRD.
The strengths of the study are its design, a cross-over study that eliminates the within-subject variation, and the biological relevance of the findings. Study limitations include the short-duration of treatment (1 week) and a small sample size. It is notable that we nevertheless had sufficient statistical power to demonstrate significant differences. Finally, the levels of methylarginines are somewhat different from other published studies although there is a wide variability in terms of blood concentrations of ADMA and SDMA in ESRD patient population. In this study, we only used high-flux dialyzers, which may affect the removal of methylarginines. Although a previous study showed the type of dialyzer does not affect plasma ADMA removal, [46] the choice of dialyzer could explain the differential pre-dialysis levels in methylarginines compared to other published studies [9], [10], [47]. Nevertheless, use of same subjects as their own controls in this study should compensate for any difference on baseline level of methylarginines and the comparative effects of ACE inhibitor and ARB administered throughout the study.
DiscussionThis study examined the effects of ACE inhibitor and ARB on methylarginines in patients undergoing MHD. We found that short-term treatment with ramipril (1 week) increased ADMA levels and decreased the arginine-to-ADMA ratio compared to placebo and valsartan in patients undergoing MHD. Studies in vitro suggest that increase in ADMA production occur through stimulation of bradykinin B2 receptors.ADMA levels are increased in patients undergoing MHD and correlate with all-cause mortality [9]. In addition, ADMA levels correlate positively with markers of atherosclerosis such as carotid intima-media thickness, left ventricular hypertrophy, and cardiovascular events [9], [10], [28],[29]. Arginine and ADMA are substrate and inhibitor of endothelial nitric oxide synthase, respectively; thus the ratio of arginine to ADMA may modify the enzyme activity [30]. Accordingly, a lower arginine to ADMA ratio has been also been associated with all-cause mortality and carotid intima-media thickness in the general population [31]–[33]. While the pathogenic role of ADMA in the cardiovascular disease is somewhat unclear, it has been shown to induce endothelial dysfunction, which is involved in the initiation and progression of atherosclerosis [34]. Another possible factor that can contribute to increase cardiovascular risk is the over-activation of the sympathetic nervous system by ADMA. Experiments in humans have shown that nitric oxide inhibition results in the activation of the sympathetic nervous system [35], a known risk factor cardiovascular disease. In fact, ADMA levels strongly correlates with norepinephrine levels in patients with ESRD [36].We did not find any effect of either ramipril or candersatan on ADMA levels in patients with preserved kidney function. But previous studies demonstrated that both ACE inhibitors and ARBs reduce ADMA levels in patients without kidney disease [19]–[22]. The duration of the treatment, one week vs. several weeks and months, may explain why neither ACE inhibitor nor ARB reduce ADMA levels in our study. Longer treatment with ACE inhibitors or ARBs may decrease ADMA by decreasing angiotensin II effects. Angiotensin II increases ADMA levels in smooth muscle cells by direct stimulation of AT1 receptors and by increasing oxidative stress [18]. The latter is probably mediated through the production of reactive oxygen species by NADPH oxidase [37], [38]. Only one previous study in patients on MHD showed that 6-week treatment with valsartan or amlodipine reduced ADMA and SDMA [39]. Findings from this study were attributed to a reduction in shear stress, which has been shown to increase ADMA release from endothelial cells [40]. In comparison, we did not find reduction in ADMA after 1 week treatment with valsartan. Again, it is possible that a longer treatment with ARB may have an effect on ADMA levels. Nevertheless, our study is the first one to compare the effect of ACE inhibitors and ARBs on ADMA levels in patients on MHD.In this study we found that ramipril, an ACE inhibitor, but not valsartan, an ARB, increased ADMA level. We found also that ramipril did not increase ADMA level in patients without ESRD, suggesting that other factors pertaining to ESRD, particularly hemodialysis may contribute to the ramipril-induced increase in ADMA levels. A potential explanation as to why ACE inhibitors and ARBs differ in their effect on ADMA levels could be that these medications also differ in their effect on the degradation of bradykinin. Specifically, ACE inhibitors increase bradykinin bioavailability, [41] whereas ARBs does not. We have previously shown that ramipril treatment increased bradykinin levels during hemodialysis [5]. Previous studies have shown that bradykinin increases reactive oxygen species production through stimulation of NADPH oxidases [42]. NADPH activation increases ADMA levels by increasing protein methylation and inhibiting ADMA degradation, [18],[43] which are both mediated by reactive oxygen species. Thus, it is possible that ramipril increases ADMA levels by increasing bradykinin and oxidative stress. Our in vitro studies support this hypothesis. We found that bradykinin contributes to ADMA production in human adenocarcinoma cells. Importantly, we also found that incubation with the bradykinin B2 receptor antagonist HOE-140 reduces intracellular ADMA levels. Bradykinin B1 receptor blockage does not play a role in decreasing ADMA levels suggesting that bradykinin-dependent ADMA production occurs through bradykinin B2 receptor stimulation. A possible mechanism that has not been explored is the role of bradykinin on DDAH, the enzyme responsible for ADMA degradation. Bradykinin may increase ADMA levels by decreasing DDAH activity.We also found that markers of inflammation positively correlate with ADMA levels and inversely with the arginine-to-ADMA ratio. The latter also has an inverse correlation with F2-Isoprostanes. These findings are in agreement with previous studies that showed the correlation between ADMA and inflammatory markers (C-reactive protein and IL-6) [10], [44]. We previously described that ramipril has a greater pro-inflammatory effect in patients with ESRD [5]. Given the interrelation between ADMA and inflammation, it is not surprising that ACE inhibitors also increase ADMA levels in patients on MHD.Our findings may some interesting implications. First and foremost, these data could explain the relative lack of efficacy of ACE inhibitors in improving cardiovascular outcomes in patients with ESRD, at least to some extent. To date, there are no clear-cut clinical guidelines for the use of ACE inhibitor or ARBs in patients with ESRD, mostly due to lack of randomized clinical trials. If anything, limited epidemiological data on this subject indicate that use of ACE inhibitor could be detrimental in patients with ESRD, at least when used in combination [45]. Accordingly, further studies are necessary to examine the appropriate use of these potentially useful classes of cardiovascular medications. In addition, these data demonstrate that patients with ESRD, particularly ones on MHD behave differently in response to medications commonly used in the general populations. Accordingly, one should not simply assume that what has been observed in response to a medication in the general population can be safely extrapolated to patients with ESRD. Finally, our data once again highlight the importance of the so-called non-traditional cardiovascular risk factors in patients with ESRD. While this particular study focused on ACE inhibitors and ARBs, other interventions that modulate one or more of these metabolic derangements should be aggressively studied in patients with ESRD.The strengths of the study are its design, a cross-over study that eliminates the within-subject variation, and the biological relevance of the findings. Study limitations include the short-duration of treatment (1 week) and a small sample size. It is notable that we nevertheless had sufficient statistical power to demonstrate significant differences. Finally, the levels of methylarginines are somewhat different from other published studies although there is a wide variability in terms of blood concentrations of ADMA and SDMA in ESRD patient population. In this study, we only used high-flux dialyzers, which may affect the removal of methylarginines. Although a previous study showed the type of dialyzer does not affect plasma ADMA removal, [46] the choice of dialyzer could explain the differential pre-dialysis levels in methylarginines compared to other published studies [9], [10], [47]. Nevertheless, use of same subjects as their own controls in this study should compensate for any difference on baseline level of methylarginines and the comparative effects of ACE inhibitor and ARB administered throughout the study.
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