![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
|
|
|
|
|
||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
CARDIOVASCULAR
Centre for Integrated Systems Biology & Medicine, School of Biomedical Sciences, University of Nottingham Medical School, Queen's Medical Centre, Nottingham, United Kingdom (S.M.G., J.E.M., P.A.K., T.B.); and Pfizer Global Research & Development, Sandwich Laboratories, Kent, United Kingdom (S.A.B., E.H., B.H.)
Received May 28, 2004; accepted September 23, 2004.
 |
Abstract |
|---|
 Top Abstract Materials and MethodsResultsDiscussionReferences |
|---|
) and showed that initially (day 1) UK-357,903 caused modest hypotension together with mesenteric and hindquarters vasodilatation but no significant renal vasodilatation. On the subsequent days of the experiment, the significant effects waned possibly due to activation of vasoconstrictor compensatory mechanisms one of which could have been the renin-angiotensin system (Gardiner et al., 2004). Consistent with this hypothesis, there are reports that PDE5 inhibition in man may lead to activation of the sympathetic nervous system (Phillips et al., 2000) and of the renin-angiotensin system (Chiu and Reid, 2002), although whether or not it is sympathetic activation that drives the renin-angiotensin system, is unknown.
In our previous study, we concluded with the hypothesis that the lack of renal vasodilatation and the transience of the antihypertensive effect of UK-357,903 may have been due to activation of counter-regulatory systems. Therefore, the objective of the present work was to test this hypothesis by assessing the regional hemodynamic responses to the infusion of UK-357,903 alone and in combination with the angiotensin-converting enzyme (ACE) inhibitor enalapril to determine whether or not additional and/or more sustained effects of UK-357903 are apparent when the renin-angiotensin system is inhibited. To provide the best opportunity to observe whether there was an interaction between the PDE5 inhibitor and enalapril, the dose of enalapril was selected to have no overt hypotensive effect itself despite resulting in plasma-free drug concentrations above the Ki for ACE inhibition (Weisser and Schloos, 1991). UK-357,903 was chosen for these studies because it has higher selectivity than sildenafil for PDE5 relative to PDE1 (Ballard et al., 1998; Gardiner et al., 2004). For technical reasons and to allow comparison with our previous study, the experimental protocol ran over 4 days.
|
Materials and Methods |
|---|
|
TopAbstractMaterials and MethodsResultsDiscussionReferences |
|---|
Surgical Preparation. Surgery was performed in two stages under general anesthesia (fentanyl and medetomidine, 300 µg kg-1 of each i.p.). The first stage involved implantation of miniaturised pulsed Doppler flow probes around the left renal artery, the superior mesenteric artery, and the distal abdominal aorta (below the level of the ileocecal artery to monitor flow to the hindquarters). The second stage involved placement of catheters in the distal abdominal aorta (via the caudal artery) to monitor mean arterial blood pressure (BP) and heart rate and in the right jugular vein for drug administrations. After each surgical stage, anesthesia was reversed and analgesia was provided with atipamezole and nalbuphine, respectively (1 mg kg-1 of each s.c.). The two surgical stages were separated by at least 10 days, and prior to the second stage, the fitness of all animals was certified by the named veterinary surgeon.
After catheterization, double-channel fluid-filled swivels (Blair et al., 1980) were used to allow overnight i.v. infusion of drugs or saline (0.4 ml h-1) and i.a. infusion of heparinized (15 U ml-1, 0.4 ml h-1) saline to maintain catheter patency. Experiments began 24 h after catheterization when the animals were fully conscious, freely moving, and with access to food and water ad libitum.
Cardiovascular Recordings. Continuous recordings of cardiovascular variables were made using a customized computer-based system [hemodynamics data acquisition system (HDAS); University of Limburg, Maastricht, The Netherlands] connected to the transducer amplifier (Gould Instrument Systems Inc., Cleveland, OH; model 13-4615-50) and the Doppler flowmeter [Crystal Biotech, Hol-liston, MA; VF-1 mainframe (pulse repetition frequency 125 kHz) fitted with high velocity (HVPD-20) modules]. Raw data were sampled by HDAS every 2 ms, averaged every cardiac cycle, and stored to disc at 5-s intervals. Data were analyzed offline using software (Datview; University of Limburg, Maastricht, The Netherlands) which interfaced with HDAS.
Experimental Protocol. There were four groups of animals, and the study ran over several weeks with data from equal numbers of rats from each treatment group being collected each week. After a control period of at least a 90-min baseline recording on day 1, rats were randomized to receive vehicle (isotonic saline adjusted to pH 4 with HCl, n = 7), enalapril (10 µg kg-1 h-1, n = 8), UK-357,903 (266 µg kg-1 h-1, n = 8), or UK-357,903 + enalapril (doses as above, n = 8). Thereafter, drugs or vehicle were administered continuously until the end of the experiment. Cardiovascular data were collected for 7 h after the onset of drug administration on day 1 and for periods of 7 h on days 2 to 4. Arterial blood samples were collected prior to any intervention on day 1 and after the recording period of each experimental day for analysis of plasma drug levels, renin activity, and cGMP.
Biochemical and Drug Analyses. The concentration of cGMP in plasma was determined by enzyme immunoassay using Kit RPN 226 from Amersham Biosciences UK, Ltd. (Little Chalfont, Buckingham-shire, UK) as described in Gardiner et al. (2004). Plasma renin activity was determined by monitoring the formation of angiotensin I from endogenous plasma angiotensinogen using the angiotensin I 125I radioimmunoassay kit NEA104,105 (PerkinElmer Life and Analytical Sciences, Boston, MA). The procedure was as described in the kit protocol, except volumes for the angiotensin I generation step were scaled down to accept 100 µl of plasma.
Plasma concentrations of UK-357,903 and enalaprilat (active metabolite of enalapril rapidly formed in rat plasma) were determined by liquid chromatography-mass spectrometry and converted to free (unbound) levels by correcting for plasma protein binding as described previously (Gardiner et al., 2004). As plasma concentrations of enalaprilat were approaching the limit of assay sensitivity, it was necessary to pool samples from two or three animals to obtain sufficient volume for analysis.
Cardiovascular Data Analysis. Details of the procedures for data analysis have been described previously (Gardiner et al., 2004). Briefly, the baseline was taken as the 30- to 45-min period prior to drug administration on day 1 when the animals were settled. For graphical representation, postdosing data are expressed as three sequential averages (
140 min) on day 1 and as four sequential averages (105 min) on days 2 to 4 relative to the original baseline (i.e., on day 1); however, for statistical analyses, postdosing data were averaged across the entire 7-h recording period for each day. The average data for each day were subjected to analysis of covariance (heart rate, mean blood pressure, and free plasma cGMP) allowing for potential week-to-week differences and for differences at baseline or analysis of variance (percentage of changes in Doppler shift and vascular conductance) allowing for potential week-to-week differences. The possibility of a statistical interaction between enalapril and UK-357,903 was assessed using the models described. This interaction can be considered as a comparison of whether the combined action of the two compounds is greater than the sum of the individual compound effects. Estimated treatment differences are also presented from these analyses. These reflect the differences between each treatment group and vehicle compensated for differences at baseline and week-to-week variation. The cGMP and renin data were analyzed on a log scale and, therefore, comparisons of treatment groups are represented by treatment ratios reflecting ratios of estimated means.
To assess the treatment effects over time, an analysis of the postdosing data was performed using a linear mixed model with a Greenhouse-Geisser correction for the repeated measures. This also allowed for potential week-to-week differences and for differences at baseline (heart rate, mean blood pressure, and free plasma cGMP measures only). For measures where there was no evidence of differences in treatment effects over time, overall treatment comparisons (i.e., comparisons averaged over time) were performed and reported in addition to the separate daily analyses. All analyses were carried out using GenStat for Windows, version 6.1. A P value 
0.05 was taken as significant.
Drugs. Fentanyl citrate was from Janssen-Cilag (High Wycombe, UK), medetomidine hydrochloride (Domitor) and atipamezole hydrochloride (Antisedan) were from Pfizer Ltd. (Sandwich, Kent, UK), and nalbuphine hydrochloride (Nubain) was from Bristol Myers Squibb Pharmaceuticals Ltd. (Hounslow, UK). UK-357,903 was supplied by Pfizer Ltd., and enalapril was purchased from Sigma Chemical (Poole, Dorset, UK). Drugs and vehicle were infused at a rate of 0.4 ml h-1.
|
Results |
|---|
|
TopAbstractMaterials and MethodsResultsDiscussionReferences |
|---|
|
Figures 1a, 2a, 3a show the mean BP and heart rate (Fig. 1a), percentage changes in Doppler shift (Fig. 2a), and percentage changes in vascular conductances (Fig. 3a), and Figs. 1b, 2b, 3b show the corresponding treatment effects (i.e., adjusted mean differences from vehicle) on each of the 4 study days.
|
|
|
Blood Pressure. In rats treated with enalapril, there was a trend for BP to be lower than in vehicle-treated animals (maximum -7.1 mm Hg on day 4), but the differences were not statistically significant (Fig. 1b). In contrast, rats treated with UK-357,903 alone or in combination with enalapril showed significant reductions in blood pressure up to a maximum of -14.5 mm Hg and -23.0 mm Hg, respectively (Fig. 1b). The falls in BP were significant on days 1, 3, and 4 in the group treated with UK-357,903 alone and on all 4 days in the group treated with the combination of drugs (Fig. 1b). Despite the observation that blood pressure falls tended to be greatest for each treatment on days 3 and 4, a repeated measures analysis did not provide statistical confirmation that treatment effects were changing with time. Therefore, overall BP falls for the 4 days were estimated providing values for enalapril, UK-357,903, and combination groups of -2.4 mm Hg (not significant), -12.1 mm Hg (P < 0.05), and -17.8 mm Hg (P < 0.05), respectively. There was no statistical evidence of an interaction between UK-357,903 and enalapril on BP, i.e., the combined effect was not significantly greater than the sum of the individual effects (P = 0.157).
Heart Rate. Only the group treated with the combination of UK-357,903 and enalapril showed any apparent change in heart rate which differed from that seen with the vehicle, and the effect was small (+17 beats min-1) and only significant on day 1 (Fig. 1b). Furthermore, the repeated measures analysis indicated that for the 4 days overall there was no significant effect of any treatment on heart rate.
Renal Doppler Shift and Vascular Conductance. In rats treated with enalapril alone or with UK-357,903 alone, there were no changes in renal Doppler shift or vascular conductance which differed significantly from those seen with the vehicle (Figs. 2 and 3). However, in the group treated with the combination of drugs, there was an increase in the percentage change in renal Doppler shift (significant on day 2, 10% increase) and vascular conductance (significant on days 1-4, from 9% increase on day 1 to 34% increase on day 4). There was evidence for a statistical interaction between UK-357,903 and enalapril on renal Doppler shift and vascular conductance since the combined effects of the drugs exceeded the sum of the individual effects (significant on day 2, P < 0.05 for Doppler shift and P < 0.01 for conductance). Although not statistically significant, the lower resting renal vascular conductance in the group treated with the combination of drugs (Table 1) could have influenced the vasodilator effects observed. However, consideration of individual responses showed no systematic association between resting renal vascular conductance and response to UK-357,903 plus enalapril. Furthermore, consideration of individual responses within the groups showed that animals having similar baseline values for renal vascular conductance, nonetheless, showed larger increases in the combination group than in any other.
Mesenteric Doppler Shift and Vascular Conductance. In all three treatment groups, there were increases in mesenteric Doppler shift and vascular conductance which differed from those seen with the vehicle. For the group treated with enalapril, the changes were significant on days 2 (Doppler shift) and 4 (Doppler shift and vascular conductance). For the group treated with UK-357,903, the changes in vascular conductance were sustained (20% increase on days 1-4), although the changes in Doppler shift were significant on days 1 and 2 only. For the group treated with the combination of drugs, the changes in Doppler shift and vascular conductance were significant throughout the experiment with the latter increasing from 25% increase on day 1 to 48% increase on day 4 (Figs. 2 and 3). There was no evidence of a statistical interaction between UK-357,903 and enalapril on mesenteric Doppler shift or vascular conductance.
Hindquarters Doppler Shift and Vascular Conductance. In the group treated with enalapril alone, there were no changes in hindquarters Doppler shift or vascular conductance that differed from those seen with the vehicle. In the group treated with UK-357,903 alone, there were increases in hindquarters Doppler shift and vascular conductance and although the estimated treatment differences were approximately constant across the 4 days (supported by repeated measures analysis), they were only significant on days 1 and 2 due to an increase in the variability thereafter (Figs. 2 and 3). The repeated measures analysis showed that overall for the 4-day treatment period the effect of UK-357,903 was significant for both Doppler shift (13% increase) and hindquarters conductance (22% increase). In rats treated with the combination of drugs, increases in hindquarters vascular conductance were different from vehicle on days 1, 2, and 4, although the increase in Doppler shift was only significant on day 1. The overall treatment effect was significant for vascular conductance (24% increase), but not for Doppler shift. There was no evidence of an interaction between the effects of UK-357,903 and enalapril on hindquarters vascular conductance.
Plasma cGMP Concentrations and Renin Activity Levels. UK-357,903 alone caused a significant rise in plasma cGMP to approximately 2- to 3-fold vehicle level (Fig. 4). Cyclic GMP was similarly increased in the combination group (2-fold, Fig. 4). In contrast, enalapril alone was without significant effect (treatment ratios 1, Fig. 4).
|
Plasma renin activity levels (geometric mean in nanograms per milliliter per hour with 95% confidence interval) determined on day 2 of the study were similar for vehicle (n = 7) and UK-357,903 (n = 6) treatment groups [2.5 (2.1, 3.1) and 2.4 (1.9, 3.0), respectively]. In contrast, renin activity was significantly elevated to approximately twice the vehicle level in both enalapril alone (n = 6) and combination groups (n = 6) [4.8 (3.0, 7.7) and 5.67 (4.1, 7.9), respectively].
Plasma Drug Levels. Plasma concentrations of free UK-357,903 (geometric mean in nanomolar concentration with 95% confidence intervals) were relatively stable across the 4 experimental days in both the UK-357,903 alone (range 7.1-10.1 nM) and in the combination treatment groups (range 6.6-10.9 nM; Table 2). Similarly, plasma concentrations of free enalaprilat ranged from 9.8 to 15.2 nM in the enalapril alone group and from 9.3 to 20.1 nM in the combination group (Table 2). There was therefore no evidence that concentrations of either UK-357,903 or enalaprilat were affected when both compounds were administered together. The free levels of UK-357,903 were approximately 5- to 8-fold the IC50 for inhibition of PDE5 (1.3 nM) (Gardiner et al., 2004).
|
|
Discussion |
|---|
|
TopAbstractMaterials and MethodsResultsDiscussionReferences |
|---|
), we found that in conscious SHR UK-357,903 infused for 4 days at doses of 133 and 1330 µg kg-1 h-1 had modest hypotensive effects accompanied by mesenteric and hindquarters vasodilatations that were not dose-related and tended to wane after the first day of infusion. Since the plasma drug levels and cGMP concentrations increased with increasing doses of UK-357,903 and were sustained, we hypothesized that the cardiovascular effects of UK-357,903 might have been influenced by activation of compensatory systems (Gardiner et al., 2004). Here we tested the hypothesis that there was an interaction between the effects of ACE inhibition by enalapril and PDE5 inhibition by UK-357,903. This was found to be the case, but only for the renal vascular bed. Here UK-357,903 had no detectable effect on vascular conductance, and although enalapril showed a trend to increase both flow and conductance, this was not significant. However, in combination, UK-357,903 and enalapril caused hyperemic renal vasodilatation that was significantly greater than the sum of the effects of the individual drugs. This finding is consistent with the reninangiotensin system opposing the renal vasodilator action of UK-357,903 and/or UK-357,903 augmenting a renal vasodilator effect of enalapril.
The dose of enalapril was chosen to have submaximal cardiovascular effects when given alone but to provide drug levels within the clinical range. Actual free concentrations in SHR were similar to those found in man at the 24-h trough after a 10-mg dose which is commonly used to treat hypertension (25 nM based on 83% free fraction; Weisser and Schloos, 1993). In our earlier study (Gardiner et al., 2004), we used a dose of enalapril which was 100-fold higher than that used here, and it caused a substantial fall in blood pressure (-39 mm Hg at the end of 4 days). In pilot studies (n = 3) we found a 10-fold higher dose than that used in the present study still caused a marked fall in mean BP (-36 mm Hg at the end of 4 days). The dose of enalapril finally chosen for this study caused significant mesenteric vasodilatation and showed trends to progressively lower blood pressure and increase renal vascular conductance. Although the apparent changes in blood pressure and renal conductance were not statistically significant (mean differences from vehicle -7.1 mm Hg and +14% after 4 days), the chosen dose was ideal for the purpose of the study; inasmuch, as it was pharmacologically active (significant mesenteric vasodilatation and increase in plasma renin activity), but left scope for demonstrating interaction. It was surprising to us that the mesenteric vascular bed showed more consistent vasodilatation in response to enalapril than the renal vascular bed, since using higher doses we (Gardiner et al., 2004) and others (Smits and Struyker-Boudier, 1984; Lappe et al., 1985; Kanagawa et al., 1997) found the vasodilator effects of ACE inhibition in SHR to be most marked in the kidney. It is possible that at the low concentration of enalapril used here, given as an infusion, selective inhibition of local angiotensin II production in the vascular walls occurred and this was counteracted by autoregulatory mechanisms in the kidney (see below). Certainly, plasma renin activity was increased and this would have led to increased angiotensin I levels that might have overcome partial inhibition of ACE, particularly in regions such as the kidney, where ACE activity levels are high. Alternatively, since there was some evidence that the effect of enalapril was increasing with time, it is possible that an effect on renal vascular conductance is slow to develop at the low dose and might have become significant if treatment had continued beyond 4 days.
In this study, the effects of UK-357,903 administration alone were interesting against the background of our earlier study. Thus, the previous results (Gardiner et al., 2004) showed modest hypotensive and mesenteric and hindquarters vasodilator effects of UK-357,903 (133 and 1330 µgkg-1 h-1) that were not dose-dependent and that waned after day 1. In the light of those findings, we chose an intermediate dose of UK-357,903 (266 µg kg-1 h-1) which resulted in plasma-free drug levels that were 5- to 8-fold the IC50 for inhibition of PDE5 (Gardiner et al., 2004) and more closely equated to the relative Cmax free concentrations of sildenafil observed in man after doses that are clinically effective in erectile dysfunction (15-30 nM; 4- to 8-fold PDE5 IC50) (Ballard et al., 1998). Here we confirmed our earlier observations on the regional heterogeneity of the effects of UK-357,903 (Gardiner et al., 2004) that it caused vasodilatation in the mesenteric and hindquarter vascular beds, whereas the renal vascular bed appeared to be relatively resistant. However, the effects of the intermediate dose of UK-357,903 used in the present study were more persistent than those of the higher dose used previously. We have no ready explanation for this unless at the higher dose additional effects of UK-357,903 gave rise to activation of more powerful compensatory mechanisms. In this context, one possible explanation is that accumulation of cGMP in response to UK-357,903 causes up-regulation of ACE (Saijonmaa and Fyhrquist, 1998); however, we did not measure serum or tissue ACE so we cannot confirm whether or not this happened in our experiments.
It is notable that when UK-357,903 was given in combination with enalapril, hyperemic renal vasodilatation occurred that exceeded the sum of effects of the individual agents. Previously, we discussed possible reasons for the lack of effect of UK-357,903 on the renal vascular bed (Gardiner et al., 2004) and predicted that "in the presence of renin-angiotensin blockade, mesenteric vasodilator effects of UK-357,903 might be enhanced and renal vasodilatation might be revealed". The present findings clearly fit with our prediction, at least as far as the renal vascular bed is concerned. Although measurements of plasma renin activity in samples taken at the end of day 2 in the present study showed no significant influence of UK-357,903, this does not preclude a functional involvement of the reninangiotensin system in renal vascular responses to the drug. Thus, it is feasible that the renal vascular bed is less sensitive than other regions to the vasodilator effects of the nitric oxide (NO)-cGMP system, but that removal of a vasoconstrictor influence of angiotensin can uncover an effect. Although there was a trend for blood pressure reduction to be greater in the combination group than with UK-357,903 alone, this was not statistically significant. It would be of interest to determine whether a longer treatment period might reveal a significant difference between the combination and single agents, although in such a protocol it would not be possible to collect detailed hemodynamic data of the sort reported here.
It is feasible that our results, rather than reflecting an influence of the renin-angiotensin system on the vasodilator effect of UK-357,903, were due to an influence of UK-357,903 on responses to enalapril. For example, if a component of the hemodynamic response to ACE inhibition is NO-mediated vasodilatation secondary to increased bradykinin levels (Gohlke et al., 1994; Gainer et al., 1998; Kohno et al., 1999; Squire et al., 2000), this effect would be amplified by PDE5 inhibition. Although there was no evidence that ACE inhibition enhanced the increase in plasma cGMP caused by UK-357,903, this does not preclude subtle local effects on cGMP levels that might be physiologically relevant.
Whatever the explanation, we were surprised that the drug interaction was 1) not seen on day 1 and 2) not apparent in the mesenteric vascular bed. Although plasma levels of enalaprilat were lower on day 1 than on subsequent days, they were above the Ki for ACE inhibition throughout the experimental protocol. However, we cannot dismiss the possibility of an enhanced functional effect occurring in association with the higher plasma levels of enalaprilat seen on days 2 to 4.
If activation of the renin-angiotensin system by UK-357,903 occurred, then the lack of interaction between UK-357,903 and enalapril on day 1 suggests that the stimulus for activation of the renin-angiotensin system could not have been the hypotensive effect of UK-357,903, since this was already apparent on day 1. As noted above, it is possible that UK-357,903 caused up-regulation of ACE (Saijonmaa and Fyhrquist, 1998) but that this process took time to develop. However, if our results reflected augmentation by UK-357,903 of a NO-mediated vasodilator effect of enalapril (see earlier), then slow accumulation of the effect of enalapril to enhance bradykinin levels and cause downstream increases in NO and cGMP could be the reason why the effect only became apparent on day 2.
If the latter explanation is correct, it is not obvious why interaction between enalapril and UK-357,903 was not seen in the mesenteric vascular bed unless, for example, mesenteric vasodilator effects of ACE inhibition are more dependent on removal of the vasoconstrictor influence of angiotensin than on bradykinin accumulation. Resolution of this question will require further experiments specifically comparing the effects of angiotensin receptor antagonism with those of ACE inhibition.
In conclusion, the present results indicate that combined administration of UK-357,903 and enalapril might improve renal perfusion over and above that achieved with ACE inhibition alone. The present experiments do not allow us to determine whether the effects observed were due to a renal vasodilator effect of UK-357,903 being uncovered in the presence of enalapril or UK-357,903 augmenting a renal vasodilator effect of enalapril. Nevertheless, if these findings extrapolate to chronic treatment in the clinical setting, it is feasible that combined treatment of the sort used here would be beneficial in patients with compromised renal blood flow.
|
Acknowledgements |
|---|
|
Footnotes |
|---|
ABBREVIATIONS: PDE5, phosphodiesterase type 5; UK-357,903, 1-ethyl-4-{3-[3-ethyl-6,7-dihydro-7-oxo-2-(2-pyridylmethyl)-2H-pyrazolo[4,3-d]pyrimidin-5-yl]-2-(2-methoxyethoxy)-5-pyridylsulphonyl}piperazine; SHR, spontaneously hypertensive rat; ACE, angiotensin-converting enzyme; BP, blood pressure; HDAS, hemodynamics data acquisition system; NO, nitric oxide.
Address correspondence to: Professor S. M. Gardiner, Centre for Integrated Systems Biology and Medicine, School of Biomedical Sciences, Floor E, University of Nottingham Medical School, Queen's Medical Centre, Nottingham NG7 2UH, UK. E-mail: sheila.gardiner{at}nottingham.ac.uk
|
References |
|---|
|
TopAbstractMaterials and MethodsResultsDiscussionReferences |
|---|
Ballard SA, Gingell CJ, Tang K, Turner LA, Price ME, and Naylor AM (1998) Effects of sildenafil on the relaxation of human corpus cavernosum tissue in vitro and on the activities of cyclic nucleotide phosphodiesterase isozymes. J Urol 159: 2164-2171.[CrossRef][Medline]
Blair R, Fishman B, Amit Z, and Weeks JR (1980) A simple double channel swivel for infusion of fluids into unrestrained animals. Pharmacol Biochem Behav 12: 463-466.[CrossRef][Medline]
Chiu YJ and Reid IA (2002) Effect of sildenafil on renin secretion in human subjects. Exp Biol Med (Maywood) 227: 620-625.
Gainer JV, Morrow JD, Loveland A, King DJ, and Brown NJ (1998) Effect of bradykinin-receptor blockade on the response to angiotensin-converting-enzyme inhibitor in normotensive and hypertensive subjects. N Eng J Med 339: 1285-1292.
Gardiner SM, March JE, Kemp PA, Ballard SA, Hawkeswood E, Hughes B, and Bennett T (2004) Haemodynamic effects of the selective phosphodiesterase 5 inhibitor, UK-357,903 in conscious, SHR. Br J Pharmacol 141: 114-122.[CrossRef][Medline]
Gohlke P, Linz W, Scholkens BA, Kuwere I, Bartenbach S, Schnell A, and Unger T (1994) Angiotensin-converting enzyme inhibition improves cardiac function. Role of bradykinin. Hypertension 23: 411-418.
Kanagawa R, Wada T, Sanada T, Ojima M, and Inada Y (1997) Regional hemodynamic effects of candesartan cilexitil (TCV-116), an angiotensin II AT(1)-receptor antagonist, in conscious spontaneously hypertensive rats. Jpn J Pharmacol 73: 185-190.[Medline]
Kohno M, Yokokawa K, Minami M, Yasunari K, Maeda K, Kano H, Hanehira T, and Yoshikawa J (1999) Plasma levels of NO and related vasoactive factors following long-term treatment with angiotensin-converting enzyme inhibitor in patients with essential hypertension. Metabolism 48: 1256-1259.[CrossRef][Medline]
Lappe RW, Todt JA, and Wendt RL (1985) Graded vasodilatation with converting enzyme inhibitors in conscious spontaneously hypertensive rats. J Pharmacol Exp Ther 235: 651-656.
Phillips BG, Kato M, Pesek CA, Winnicki M, Narkiewicz K, Davison D, and Somers VR (2000) Sympathetic activation by sildenafil. Circulation 102: 3068-3073.
Saijonmaa O and Fyhrquist F (1998) Upregulation of angiotensin converting enzyme by atrial natriuretic peptide and cyclic GMP in human endothelial cells. Cardiovasc Res 40: 206-210.
Smits JF and Struyker-Boudier HA (1984) Systemic and regional hemodynamic effects following acute inhibition of angiotensin-1 converting enzyme in the conscious spontaneously hypertensive rat. Prog Pharmacol 5: 40-49.
Squire IB, O'Kane KP, Anderson N, and Reid JL (2000) Bradykinin B(2) receptor antagonism attenuates blood pressure response to acute angiotensin-converting enzyme inhibition in normal men. Hypertension 36: 132-136.
Weisser K and Schloos J (1991) The role of enzyme and substrate concentration in the evaluation of serum angiotensin converting enzyme (ACE) inhibition by enalaprilat in vitro. Biochem Pharmacol 42: 1729-1737.[CrossRef][Medline]
Weisser K and Schloos J (1993) Measurements of serum ACE activity in vitro after administration of enalapril in man cannot reflect inhibition of the enzyme in vivo. Methods Find Exp Clin Pharmacol 15: 413-418.[Medline]
This article has been cited by other articles:
|
|
 |
|
  S. M. Gardiner, J. E. March, P. A. Kemp, S. A. Ballard, and T. Bennett Regional Hemodynamic Effects of Neutral Endopeptidase Inhibition and Angiotensin (AT1) Receptor Antagonism Alone or in Combination in Conscious Spontaneously Hypertensive Rats J. Pharmacol. Exp. Ther., October 1, 2006; 319(1): 340 - 348. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
 |
 |
|
 |
 |
 |
