Cardiovascular Effects of a Specific Nonpeptide Antagonist of Substance P (NK-1) Receptor in DOCA-Salt Hypertension
Abstract The neurotransmitter substance P acts also as a potent vasodilator. Its participation in the pathogenesis of deoxycorticosterone acetate (DOCA)–salt hypertension was evaluated by an acute infusion of a newly synthesized, potent, specific nonpeptide antagonist of substance P at the NK-1 receptor, the agent CP 96 345. In conscious unrestrained rats, CP 96 345 induced significant and sustained increases in mean arterial pressure of DOCA-salt rats but only small, transient, and nonsignificant rises in blood pressure of sham-treated control rats. The rise in blood pressure was not accompanied by changes in heart rate. Maximal blood pressure increase in DOCA-salt rats was 31.7±14.8 mm Hg. In a second series of experiments, the hemodynamic effects of this antagonist were evaluated under anesthesia in both DOCA-salt and sham-treated control rats by the thermodilution method. During CP 96 345 infusion, sustained increases in cardiac index and stroke volume and decreases in total peripheral resistance were observed in both DOCA-salt and control rats. In DOCA-salt rats, cardiac index rose by 79.4%, while total peripheral resistance fell by 27.9% of the baseline values. In control rats, the changes were smaller (+27.2% and −22.5%, respectively). Stroke volume changed in parallel to cardiac output in both groups. The data suggest that acute blockade of NK-1 receptors increases blood pressure in DOCA-salt rats mainly by an increase in cardiac output. We conclude that endogenous substance P tends to counteract the DOCA-salt–induced elevation of blood pressure by modulating both cardiac output and peripheral resistance.
Substance P, an undecapeptide, is the best-characterized member of a family of structurally related peptides known as tachykinins.1 It is widely distributed in the body and is involved in numerous physiological activities such as neurotransmission-neuromodulation,2 3 stimulation of salivary secretion,4 smooth muscle contraction,5 and vasodilation.6 7
The vasodilatory action of substance P is endothelium dependent,8 as it can be significantly inhibited by inhibitors of nitric oxide formation9 and is mediated by the neurokinin NK-1 receptors located on the endothelial cells. Besides its vasodilatory properties, in some vascular beds like rabbit pulmonary artery,10 rat portal vein,11 and rabbit mesenteric vein,12 substance P also exerts a contracting action on the vascular smooth muscle. These vasoactive properties of substance P enable this peptide to participate in the regulation of blood flow that varies among tissues and organs. For example, intra-arterial administration of substance P in dogs increases flow to adipose tissue and skeletal muscle,5 whereas IV infusion increases the hepatic, mesenteric, and portal flows but does not affect the renal circulation.6
The hemodynamic effects of substance P were determined in normal dogs,7 in whom continuous IV infusion of this peptide induced dose-related decreases in total peripheral resistance and BP accompanied by concomitant increases in CO and SV.
The participation of endogenous substance P in hypertension has been evaluated by few investigators, mainly by measurement of the plasma levels of this peptide,13 14 and the pharmacological effects have been explored by administration of synthetic substance P or related analogues.15 16 Reduced plasma levels of this peptide have been described in both stroke-prone spontaneously hypertensive rats13 and in human essential hypertension,14 suggesting that elevated BP in these subjects could be, at least in part, related to the lack of adequate counterregulatory action exerted by this vasodilator.
Recently, a potent, specific nonpeptide antagonist of the NK-1 receptor, devoid of agonistic actions, was synthesized,17 18 allowing more precise and systematic evaluation of the physiological properties of endogenous substance P and its participation in hypertension.
The aim of this study was to evaluate a possible role for endogenous substance P in the pathogenesis of an experimental model of hypertension, DOCA-salt hypertension in its initial phase (benign phase), by determining the BP effects of acute administration of CP 96 345, a potent, specific nonpeptide antagonist of the NK-1 receptor. We also determined the hemodynamic effects of substance P blockade in this experimental model of hypertension by measuring the CO via the thermodilution method.
Male Wistar rats weighing 300 to 370 g were used. Animals were housed in cages containing five rats each. They were maintained in a room with constant temperature, 12-hour dark/light cycles, and free access to regular rat chow and 1% saline as drinking water.
DOCA-salt hypertension was induced in uninephrectomized animals by weekly injections of deoxycorticosterone pivalate 20 mg/kg body wt SC. Control sham-treated rats were also uninephrectomized but were injected weekly with distilled water. All animals were followed for a 6-week period.
Antagonism of substance P receptor was accomplished by the compound CP 96 345 [(2S, 3S)-cis-(diphenilmethyl)-N-[(2-metoxiphenil)-methyl]-1-azabicyclo[2,2,2]octan-3 amine], a potent and specific nonpeptide antagonist of the NK-1 receptor,17 18 infused at a rate of 30 μg · kg−1 · min−1 IV for 60 consecutive minutes. Its inactive (2R, 3R) enantiomer was infused in some hypertensive and sham-treated animals at the same rate as the antagonist as a control experiment. A continuous 60-minute infusion of the same volume (0.9 to 11 mL) of the vehicle solution (distilled water) was used as time control. CP 96 345 dosage was chosen on the basis of previous experiments in our laboratories in which, in four normotensive unanesthetized rats, this compound infused at the rate of 30 μg · kg−1 · min−1 completely abolished the vasodepressor responses to two different IV doses (15 and 30 pmol/kg) of substance P (Sigma Chemical Co) before (−17±4 and −26±7 mm Hg) and during (2±6 and −1±6 mm Hg) CP 96 345 infusion, respectively.
In the first set of experiments in unanesthetized animals, direct MAP and HR were recorded directly by a polygraph (Gould Inc) before and every 5 minutes during the 60-minute infusion of either the substance P antagonist (DOCA-salt, n=9; sham, n=8), the inactive enantiomer (DOCA-salt, n=5; sham, n=5), or the vehicle (DOCA-salt, n=6; sham, n=5). For these experiments, animals had indwelling catheters placed in the femoral artery (for MAP measurements) and femoral vein (for drug infusion) under light ether anesthesia on the day before the study.
In the second set of experiments in pentobarbital-anesthetized animals, CO was determined via an automated thermodilution method with the Cardiomax II device (Columbus Inc)19 before and every 15 minutes during the 60-minute IV infusion of either the antagonist (DOCA-salt, n=6; sham, n=50) or the vehicle (DOCA-salt, n=5; sham, n=5). MAP was also recorded in these experiments and the CI, SV, and total peripheral resistance were calculated. For these determinations, under pentobarbital anesthesia, indwelling catheters were placed into the femoral artery (MAP and HR monitoring); femoral vein (drug infusion); right atrium via the right jugular vein (for delivery of the cooled [4°C] saline); and ascending aorta via the left carotid artery (CO probe).
Results are presented as mean±SD. Two-way ANOVA was used for comparison of BP and HR values before and during the antagonist infusion among groups, whereas one-way ANOVA was applied for comparison of the hemodynamic parameter values before and during the NK-1 receptor antagonist infusion for each experimental group.
Effects on BP and HR
Fig 1⇓ shows that administration of the NK-1 receptor antagonist of substance P to sham-treated rats caused only small, transient, and nonsignificant increases in BP. In DOCA-salt rats it produced significant and sustained increases in MAP starting at 15 minutes from the beginning of the infusion and lasting for the duration of the infusion. Peak BP for the whole group was reached at 30 to 45 minutes from the beginning of the infusion. However, further analysis showed that maximal increases were attained at any time between 10 and 40 minutes in individual animals. Maximal increase in BP for this group was +37.7±14.8 mm Hg (Table 1⇓). In the last 15 minutes of the infusion period, BP values of DOCA-salt rats tended to be somewhat lower.
Infusion of the inactive enantiomer at the same rate as the active compound did not significantly change BP in DOCA-salt or sham-treated animals (Fig 1⇑ and Table 2⇓). In both groups MAP remained unchanged during the control period of vehicle infusion (Table 2⇓).
No significant changes in HR were observed throughout the administration of the NK-1 receptor antagonist in either DOCA-salt or sham-treated rats.
In pentobarbital-anesthetized DOCA-salt rats prepared for CO measurements, continuous infusion of the substance P antagonist again induced a significant and sustained increase in BP that reached a maximum +32.5 mm Hg at 45 minutes of the infusion. In sham-treated anesthetized rats, no significant increase in MAP was observed during infusion of the NK-1 receptor antagonist. Again, as in awake animals, HR showed no significant changes during infusion of the antagonist in either DOCA-salt or sham-treated rats.
Fig 2⇓ presents values for the CI (left) and for the total peripheral resistance (right) in DOCA-salt and sham-treated rats before and every 15 minutes during infusion of the NK-1 receptor antagonist. CI was similar at baseline in both groups and rose significantly during administration of the antagonist. However, the rise was significantly greater in the DOCA-salt rats by 79.4% of the baseline value compared with 27.2% in sham-treated rats.
Likewise, SVs that were equal in both groups at baseline (7.3±1.7 and 7.3±1.1 mL per beat in DOCA-salt and sham-treated groups, respectively) showed a significantly greater increase in DOCA-salt rats by 60% of the baseline value in comparison with the 17.8% increase observed in the sham-treated group. Maximal value for SV during the NK-1 receptor antagonist infusion in DOCA-salt rats was 11.7±4.7 mL per beat, whereas in sham rats it was 8.6±1.0 mL per beat.
Total peripheral resistance was significantly higher in DOCA-salt rats at baseline (0.348±0.060 versus 0.295±0.043 mm Hg/min in sham-treated rats, P<.05). It was similarly reduced in both groups by the infusion of substance P antagonist (by −27.9% and −22.5%, respectively, for DOCA-salt and sham-treated rats; Table 1⇑), reaching values of 0.251±0.057 and 0.228±0.053 mm Hg · mL−1 · min−1, respectively, by the end of the experiment.
In vehicle-infused anesthetized animals, BP, HR, CO, and total peripheral resistance remained unchanged in both the DOCA-salt and sham-treated groups (Table 2⇑).
Our results indicate that in the early phases (benign) of DOCA-salt hypertension, acute inhibition of substance P by a potent and specific nonpeptide antagonist of the NK-1 receptor causes sustained increases in BP. The antagonist was infused at a rate that completely abolishes the BP response to exogenous substance P given to normal rats. These data demonstrated for the first time the effects of endogenous substance P by the use of a specific antagonist and suggest its participation in the pathogenesis of an experimental model of hypertension. In this model, the neurotransmitter substance P,2 3 which also has endothelium-dependent vasodilatory effects,6 7 8 9 appears to be acting as a counterregulatory mechanism against elevation of BP induced by DOCA-salt.
Our results could possibly be considered in agreement with previous data, in which reduced plasma levels of substance P were observed in stroke-prone spontaneously hypertensive rats13 and in human essential hypertension,14 suggesting that at least part of the elevated BP levels of these entities could be due to insufficient synthesis or release of the endogenous vasodilator substance P. The BP effect of the antagonist used in this study was due to blockade of the NK-1 receptor, since the enantiomer of this compound, which has been found to be inactive in vitro, did not affect BP in either DOCA-salt or sham-treated animals. This, of course, does not preclude the possibility of a partial nonspecific effect, eg, one mediated via the action of the compound CP 96 345 on the L-type calcium channels.
An increase in CO of DOCA-salt rats to almost twice the baseline value accounted for the increase in BP during the acute blockade of the NK-1 receptor in these animals, with a drop of nearly 28% total peripheral resistance. This observation appears to be in contrast to previously published data7 showing decrease in peripheral resistance and increase in CO associated with reduction of BP during exogenous infusion of substance P in dogs, and also against data demonstrating the vasodilatory properties of substance P.6 7 Species specificity could account for the difference between our data and those previously published, since wide variation in sensitivity to substance P among species has been demonstrated.1 7 20 A partial agonistic effect of the antagonist used in our study is unlikely since data obtained from experiments in vitro17 18 and in vivo but in another species (cats) demonstrated that this compound is devoid of agonist action at the NK-1 receptor.
The most likely explanation for our results might be that substance P has differential effects on blood flow in various organs. Thus, it has been reported that in dogs7 and humans20 infusion of substance P causes a vasodilatory effect that is more pronounced in the skin and adipose tissue, intermediate in the skeletal muscle, and almost absent in the mesenteric circulation.7 Accordingly, it could be speculated that, although total peripheral resistance is augmented in the DOCA-salt model, regional blood flow is variable in different tissues and organs. Therefore, when substance P receptor is acutely blocked, a redistribution of blood flows may occur with differential alteration in various regions. This could explain the overall increase in CO and SV in the absence of change in HR, as observed in both DOCA-salt and sham-treated animals during administration of the NK-1 antagonist. In such a case, it could be expected that the increase in mean BP reflected mostly an elevated systolic rather than diastolic pressure with a wide pulse pressure. Our BP recording technique did permit recording of systolic and diastolic pressures. The hemodynamic parameters analyzed in this study were obtained in animals under pentobarbital anesthesia, and therefore an interference of the anesthesia on these measurements cannot be completely excluded. However, it appears to be less probable since effects of the NK-1 antagonist on BP of anesthetized animals were of similar magnitude to that obtained in awake animals.
On the basis of these findings we propose the hypothesis that in mild to moderate hypertensive states such as the benign (initial) phase of DOCA-salt hypertension, substance P could play a role as a counterregulatory mechanism against the vasoconstriction characteristic in this model, in an attempt to attenuate the elevation of BP. At later phases of DOCA-salt hypertension, a more severe hypertensive state is known to develop, which could be due at least in part to failure in the counterregulation provided by substance P. Such a sequence would be in accordance with previous observations in another model of severe hypertension, the stroke-prone spontaneously hypertensive rat,13 in which lack of synthesis and/or release of this endogenous vasodilator was suggested to contribute to the elevated BP. At this time, as far as our knowledge goes, there are no data available on the blood levels of endogenous substance P, or BP response to exogenous infusion of substance P at either the benign or the malignant phase of DOCA-salt hypertension to support this hypothesis. Further studies will be necessary to confirm this hypothesis.
In summary, acute blockade of substance P at NK-1 receptors via a potent, specific, nonpeptide antagonist of this receptor increases BP in DOCA-salt hypertensive rats but not in normotensive control animals. This acute effect appeared to be due to increase in CO, probably as a consequence of redistribution of regional blood flows in various organs. An alternative explanation for this effect could be a possible upregulation of NK-2 and NK-3 receptor activation following the NK-1 receptor blockade. Our data suggest that in this hypertensive model substance P may play a role as a counterregulatory mechanism against elevation of BP by modulating both peripheral resistance and CO.
Selected Abbreviations and Acronyms
|MAP||=||mean arterial pressure|
- Received June 18, 1995.
- Revision received August 21, 1995.
- Accepted September 10, 1995.
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