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[Sar1Ile7]angiotensin III, a new selective antagonist of the pressor effect of angiotensin III in conscious rats
Pergsmon Press
Life Sciences, Vol. 43, pp. 537-543 Printed in the U.S.A.
[SarlIle~lANGIOTENSIN III,
A NEW SELECTIVE ANTAGONIST OF THE PRESSOR EFFECT OF ANGIOTENSIN III IN CONSCIOUS RATS
Reza Tabrizchi,
Graham J. Moore* and Catherine C. Y. PangI
Department of Pharmacology & Therapeutics, Faculty of Medicine, The University of British Columbia, 2176 Health Sciences Mall, Vancouver, B. C., V6T lW5, Canada (Received in final form June 14, 1988) Summary Two analogues of angiotensin III were compared as antagonists of the pressor response to angiotensin II (ANG II) and angiotensin III (ANG III) in conscious, unrestrained rats. Dose-mean arterial sure (MAP) response curves were o tained for ANG II the absen or presence of CI$e9 lAo"l"/k;fI(1.3 x [Sari Ile%ANG III (1.2 x loIn the presence of [Ile71ANG III, the dose-MAP response curve; for Al$G II and ANG III [Ile IANG III behaved were significantly displaced to the right. ANG II but not ANG III receptors. In the the dose-MAP response curve for displaced to the right. a selective antagonist III, on the other hand, antagonizes both ANG II and ANG III receptors. Our results support the hypothesis of the existence of a sub-class of angiotensin receptors activated by ANG III in the vascular smooth muscle. The C-terminal phenylalanine of angiotensin II (ANG II) has been shown to be involved in receptor activation (1,2). This residue has been postulated to be essential for the pressor activity of angiotensin (l-3). Mono-substitutions of the C-terminal phenylalanine residue of ANG II with non-aromatic amino acids have produced antagonists of the pressor actions of ANG II (2,4). More potent antagonists of the pressor actions of ANG II have been produced by the replacement of the N-terminal aspartate with sarcosine and the C-terminal phenylalanine with non-aromatic amino acids (4,5). Recently we reported the presence of a sub-class of angiotensjn receptors in the arterioles that were predominantly stimul ted y [des-Asp 1 angiotensin II or angiotensin III (ANG III) (6). [Sara IleblANG II was found not to act as an antagonist of these receptors (6). Since the C-terminal phenr alanine was shown to be the residue responsible for the activation of ANG II receptors (1,2), it is possible that this residue may play a similar role in the activation of angiotensin receptors which are activated by ANG III. A study was undertaken to examine the actions of two angiotensin analogues, of Medical *Department Alberta, T2N lN4, Canada
Biochemistry,
University
of
IAuthor to whom correspondence should be addressed. 0024-3025188 $3.00 + .OO Copyright (c) 1988 Pergamon Press plc
Calgary,
Calgary,
538
Selective Angiotensin III Antagonist
vol. 43, No. 6, 1988
[IleT]ANG III and [Sat-l Ile7lANG III, as antagonists of the pressor actions of ANG III and ANG II. Dose-response curves for ANG II and ANG III unrestrained rats in the absence and presence of were obtained in consciou [Ile7]ANG III or [Sat-lIleF IANG III. Methods [Sari --
Ile7JAIII
synthesis
[Sari Ile7JANG III was synthesized and purified by methods which have been described in detail previously (81. This peptide antagonizes the contractile response to ANG II in rat isolated uterus with pA2 = 7.5 (Moore et al., unpublished). Surgical preparation and experiment protocol (320-420 gl were anaesthetized with halothane. Sprague-Dawley rats Cannulae were inserted into the iliac artery for the measurement of mean arterial pressure (MAP) by a pressure transducer (P23ID, Gould Statham, CA), and into both iliac veins, for the infusion of drugs. All cannulae were filled with heparinized saline (25 IU/ml) and tunnelled subcutaneously to the back of the neck, exteriorized and secured. The rats were recovered for 24 h before measurements of pressures were made.
in the presence of CIle71ANG III (1.3 x Smed~~,grouf~arP:leSa]tffN;; III (1.2 x 10m7 moles/kg) saline. In the determination of dose-response curves for ANG II ai: ANz"E!l each dose of ANG II and ANG III was infused for 6 min followed by a recovery peripd of 12 min to avoid the development of tachyphylaxls to the drugs. [Sar Ile7JANG III and CIle7lANG III were given b{ i.v. bolus infusion over a 7 min interval at 1.8 x 10-8 and 1.7 x lomole kg/min, respectively, followed by continuous i.v. infusion at 2.0 x lo-V and 2.4 x 10Vg moles/kg/min, respectively. Both of the antagonists and saline were infused at 0.1 ml/min/kg over a 7 min interval followed by 0.01 mlhin/kg for 2 hr. After the infusion of the bolus dose of the antagonist, 12 min was allowed to elapse before a dose-response relationship was obtained for the agonists. Total volume of fluid infused was 2.7 ml/kg over a 2 hr period. iirYe
Drugs All drugs were dissolved in normal saline. The following drugs were used: ANG II (Ciba-Geigy, Canada), ANG III (Sigma Co., Canada) and [Ile7]ANG III (Peninsula Lab., USA). Calculation and statistical analysis Results of MAP were expressed as % maximum of ANG II or ANG III responses in the absence of the antagonists. ED50 values were calculated from individual ANG II and ANG III dose-response curves. All data were analysed by the analysis of variance, complete random design. Duncan's multiple range test was used to compare group means. In all cases, a probability of error of less than 0.05 was preselected as the criterion for statistical significance.
Vol. 43, No. 6, 1988
Selective Angiotensin III Antagonist
539
Results Table
I
shows control MAP in six groups of rats. The infusion of caused a significant increase in MAP whereas the infusion of [SarlIle7]ANG III did not significantly alter MAP.
[Ile7]ANG III
The infusion of CSarlIle71ANG III caused a slight but not significant pressor response as shown by the tracings from a typical experiment (Fig. 1). The bolus infusion of [Ile71ANG III caused a significant pressor response which reached a maximum value 2 min after the start of infusion and gradually declined to a sustafned level (Table 11. TABLE I MAP Before and 17 min After the Start of Infusion of Antagonist or Vehicle in Six Groups of Conscious Rats (n = 6 in each group) MAP
Group
Control
+ Antagonist
(mm Hg) No a tagonist [IleylAN III [SarlIle$lANG
III
112 * 3.0 113 l 4.0 112 * 2.0
121 116
III
111 * 3.0 113 l 3.0 114 f 3.0
121 ; 3.0a 115 * 3.0
No a tagonist [Ile s IANGIII
[SarlIle7JANG
l l
2.0a 3.0
asignificantly different from MAP of control rats (p < 0.05). The infusions of ANG II and ANG III caused dose-dependent increases in MAP which reached similar maximium values of 193 l 5 and 192 f 8 (mean l SE) mmHg, one-ninth of that respectively. The ED50 value for ANG II was approximately for ANG III showing that ANG II is more potent than ANG III in causing a pressor response (Table II).
In the presence of [Ile7lANG III, the dose-MAP response curves for ANG and ANG III were displaced to the right (Fig. 21, with a significant increase in the ED50 value for ANG II as well as ANG III (Table II). The responses to the two lowest doses of ANG II but not ANG III was increased in CSarlIle71ANG III caused a significant the presence of CIle7lANG III. displacement of the ANG III response curve to the right but did not displace the ANG II curve (Fi3. 3). The ED50 values for ANG II in the absence and presence of [SarlIle ]ANG III were not significantly different from each other as opposed to t:e $ o fold increase in the ED5D value of ANG III in The maximum responses of the presence of [Sar Ile ]ANG III (Table II). both ANG II and ANG III were slightly but not significantly decreased by the II
two analogues.
Selective Angiotensin II Antagonist
540
A
150 _
Vol. 43, No. 6, 1988
C
B
-
9 glp;: 0. 200
E
3 ,z
ii
; p 0
z
0
4
I
W-t
t
[Sar111e71ANG
III
200.
E :, ;
1
f
z ;: <
100.
t
t
OJ
1
III
IIk71ANG
FIG.
min
1
Tryings of arterial pressure responses to CSarlIle71ANG III or [Ile IANG III prior to and during the bolus (A) and continuous IB) infusion of an antagonist and 17 min after the start of the infusions (Cl. TABLE II ED50 Values for ANG II and ANG III in Six Groups of Conscious Rats (n = 6 in each group) in the Absence and Presence of CIle71ANG III or [SarlIle71ANG III
Group of rats ED50 MAP
Control
CIle7]ANG III
(1.2 ANG II
4.0
l
0.6
x 10T7 mole/kg) 7.0
l
CSar111e71ANG III (1.3 x 10m7 mole/kg)
0.9a
5.0
l
0.4
(moles/kg/minxlO-lO) ANG III
35 * 9.0
58 * 4.0a
(moles/kg/minxlO-10) aSignificantly different from control (p < 0.05).
73
l
8.0a
Selective
vol. 43, No. 6, 1988
Angiotensin
III
Antagonist
541
100.
60
0.5
1.0
1.5
2.0
Log Oose (mol/kgimin
FIG. % Maximum MAP fesponse to ]ANG III. and absence of [Ile (0 1 and the presence ( A 1 of the absence 1 l I and presence (
0.5
2.5
3.0
x 10”)
2
ANG II and ANG III in the presence Responses to ANG II in the absence antagonist. Response of ANG III in V 1 of antagonist.
1.0
1.S
~oq Dose (mllkglmin
FIG.
2.0
2.5
3.0
x 10")
3
% Maximum MAP respon?e to ANG II and ANG III in the presence and absence of Dar1 Ile IANG III. Responses to ANG II in the absence ( 0 1 and the presence ( A ) of antagonist. Response of ANG III in the absence 1 n 1 and presence ( v 1 of antagonist.
Selective Angiotensin III Antagonist
542
Vol. 43, No. 6, 1988
Discussion The
pressor actions of both ANG II and ANG III were antagonized by but the mode by which the antagonism was achieved appeared to be different. A comparison between the dose-MAP response curves for ANG II in the absence and the presence of 111e71ANG III suggests that CIle71ANG III was acting as a partial agonist on receptors that were activated by ANG II. This inference was supported by two observations. Firstly, the infusion of CIle71ANG III caused a significant increase in MAP and secondly, the responses to the two lowesq doses of ANG II in the presence of [IleTJANG III were increased. Thus [Ile IANG III, at the dose given, showed some intrinsic activity perhaps by act$ng on receptors that were activated by ANG II. However, the actions of [Ile IANG III towards the dose-MAP response curve of ANG III seemed to be entirely different as the antagonist decreased rather than increased the base-line response of ANG III. This suggest that ANG III was activating a sub-class of angiotensin receptors in which CIle7lANG III showed some affinity but little intrinsic activity.
CIle73ANG III
III anfagonized the CSarIIle7lANG In contrast to [Ile71ANG III, Thus CSarlIle IANG III has pressor actions of ANG III but not ANG II. selectivity for receptors activated by ANG III. This analogue, at the dose given, showed very little intrinsic activity in comparison to [Ile71ANG III. The results obtained in this study support our previous hypothesis that in the vascular smooth muscle there exists a sub-class of angiotensin receptors that are predominantly activated by ANG III (61. Even though the replacement of phenylalanine by CIle7lANG III produced an analogue which acts as an antagonist of ANG III in the vasculature, it is apparent that replacements of N-terminal arginine as well as C-terminal phenylalanine of the ANG III molecule are neccessary in order to produce a The replacement of selective antagonist of the pressor actions of ANG III. the N-terminal aspartate with a sarcosine residue in the ANG II molecule has ;;Tn shown to increase binding affinity of the peptide for the ANG II receptor . It seems that replacement of N-terminal arginine of ANG III with sarcoThe basic sine alters the affinity of the peptide towards ANG II receptors. structural requirement of peptides for activating ANG III receptors remains unknown. This makes the predication for more selective and effective antagonists for ANG III more difficult.
In conclusion, we have found that [IleTlANG III appears to have similar affinity for both ANG II and ANG III receptors but p sse ses some intrinsic [SaJIleS]ANG III on the activity for receptors activated by ANG II. other hand, shows no intrinsic activity at the dose given and it ha> greater affinity for veceptors activated by ANG III than receptors activated by ANG [SarlIle IANG III is the first ANG III antagonist available for the II. elucidation of vascular effects of ANG III. It is possible that other actions of ANG I II may al so be antagonized by this analogue. Acknowledgements This work was supported by the Medical Research Council of Canada. Reza Tabrizchi was a recipient of a UBC Graduate Fellowship (Hugo E. Malicke Memorial). We would also like to thank Ciba-Giegy Canada Ltd. for the supply of ANG II.
Vol. 43, No. 6, 1988
Selective Angiotensin III Agonist
543
References 1.
2. 3. 4. 5. 6. 7. 8.
M.C. KHOSLA, R.R. SMEBY and F.M. BUMPUS, Hand_Bogk_ofExper &imental Pharmacology: Angiotensin, Vol. 37, eds. I.H. Page and F.M. Bumpus, p. 127, Springer, Heidelberg (19741. 370). P.A. KHAIRALLAH, A. TOTH and F.M. BUMPUS, J. Med. Chem. 13 181-184 (l! F.M. BUMPUS, M.C. KHOSLA, R.A. LEESE, A.T. FERREIRA, R.R. SMEBYAND J. Med. Chem. 15 792-795 (1972). R.K. TURKER, MB. HALL, M. YAMAMOTO, C.S. SWEET and F.M. BUMPUS, Science 177 1203-1205 (1972). ET. PALS, F.D. MASUCCI, G.S. DENNING JR., F. SIPOS and D.C. FESSLER, Circ. Res. 29 673-681 (1971). R. TABRIZCwand C.C.Y. PANG, Eur. J. Pharmacol. 142 359-366 (1987). D. REGOLI, F. RIOUX, W.K. PARK and C. CHOI, Can.J,Physiol. Pharmacol. -52 39-49 (1974) :i;i6FOGHARI, K.J. FRANKLIN and G.J. MOORE, J. Med. Chem. 29 1121-1124
.
Life Sciences, Vol. 43, pp. 537-543 Printed in the U.S.A.
[SarlIle~lANGIOTENSIN III,
A NEW SELECTIVE ANTAGONIST OF THE PRESSOR EFFECT OF ANGIOTENSIN III IN CONSCIOUS RATS
Reza Tabrizchi,
Graham J. Moore* and Catherine C. Y. PangI
Department of Pharmacology & Therapeutics, Faculty of Medicine, The University of British Columbia, 2176 Health Sciences Mall, Vancouver, B. C., V6T lW5, Canada (Received in final form June 14, 1988) Summary Two analogues of angiotensin III were compared as antagonists of the pressor response to angiotensin II (ANG II) and angiotensin III (ANG III) in conscious, unrestrained rats. Dose-mean arterial sure (MAP) response curves were o tained for ANG II the absen or presence of CI$e9 lAo"l"/k;fI(1.3 x [Sari Ile%ANG III (1.2 x loIn the presence of [Ile71ANG III, the dose-MAP response curve; for Al$G II and ANG III [Ile IANG III behaved were significantly displaced to the right. ANG II but not ANG III receptors. In the the dose-MAP response curve for displaced to the right. a selective antagonist III, on the other hand, antagonizes both ANG II and ANG III receptors. Our results support the hypothesis of the existence of a sub-class of angiotensin receptors activated by ANG III in the vascular smooth muscle. The C-terminal phenylalanine of angiotensin II (ANG II) has been shown to be involved in receptor activation (1,2). This residue has been postulated to be essential for the pressor activity of angiotensin (l-3). Mono-substitutions of the C-terminal phenylalanine residue of ANG II with non-aromatic amino acids have produced antagonists of the pressor actions of ANG II (2,4). More potent antagonists of the pressor actions of ANG II have been produced by the replacement of the N-terminal aspartate with sarcosine and the C-terminal phenylalanine with non-aromatic amino acids (4,5). Recently we reported the presence of a sub-class of angiotensjn receptors in the arterioles that were predominantly stimul ted y [des-Asp 1 angiotensin II or angiotensin III (ANG III) (6). [Sara IleblANG II was found not to act as an antagonist of these receptors (6). Since the C-terminal phenr alanine was shown to be the residue responsible for the activation of ANG II receptors (1,2), it is possible that this residue may play a similar role in the activation of angiotensin receptors which are activated by ANG III. A study was undertaken to examine the actions of two angiotensin analogues, of Medical *Department Alberta, T2N lN4, Canada
Biochemistry,
University
of
IAuthor to whom correspondence should be addressed. 0024-3025188 $3.00 + .OO Copyright (c) 1988 Pergamon Press plc
Calgary,
Calgary,
538
Selective Angiotensin III Antagonist
vol. 43, No. 6, 1988
[IleT]ANG III and [Sat-l Ile7lANG III, as antagonists of the pressor actions of ANG III and ANG II. Dose-response curves for ANG II and ANG III unrestrained rats in the absence and presence of were obtained in consciou [Ile7]ANG III or [Sat-lIleF IANG III. Methods [Sari --
Ile7JAIII
synthesis
[Sari Ile7JANG III was synthesized and purified by methods which have been described in detail previously (81. This peptide antagonizes the contractile response to ANG II in rat isolated uterus with pA2 = 7.5 (Moore et al., unpublished). Surgical preparation and experiment protocol (320-420 gl were anaesthetized with halothane. Sprague-Dawley rats Cannulae were inserted into the iliac artery for the measurement of mean arterial pressure (MAP) by a pressure transducer (P23ID, Gould Statham, CA), and into both iliac veins, for the infusion of drugs. All cannulae were filled with heparinized saline (25 IU/ml) and tunnelled subcutaneously to the back of the neck, exteriorized and secured. The rats were recovered for 24 h before measurements of pressures were made.
in the presence of CIle71ANG III (1.3 x Smed~~,grouf~arP:leSa]tffN;; III (1.2 x 10m7 moles/kg) saline. In the determination of dose-response curves for ANG II ai: ANz"E!l each dose of ANG II and ANG III was infused for 6 min followed by a recovery peripd of 12 min to avoid the development of tachyphylaxls to the drugs. [Sar Ile7JANG III and CIle7lANG III were given b{ i.v. bolus infusion over a 7 min interval at 1.8 x 10-8 and 1.7 x lomole kg/min, respectively, followed by continuous i.v. infusion at 2.0 x lo-V and 2.4 x 10Vg moles/kg/min, respectively. Both of the antagonists and saline were infused at 0.1 ml/min/kg over a 7 min interval followed by 0.01 mlhin/kg for 2 hr. After the infusion of the bolus dose of the antagonist, 12 min was allowed to elapse before a dose-response relationship was obtained for the agonists. Total volume of fluid infused was 2.7 ml/kg over a 2 hr period. iirYe
Drugs All drugs were dissolved in normal saline. The following drugs were used: ANG II (Ciba-Geigy, Canada), ANG III (Sigma Co., Canada) and [Ile7]ANG III (Peninsula Lab., USA). Calculation and statistical analysis Results of MAP were expressed as % maximum of ANG II or ANG III responses in the absence of the antagonists. ED50 values were calculated from individual ANG II and ANG III dose-response curves. All data were analysed by the analysis of variance, complete random design. Duncan's multiple range test was used to compare group means. In all cases, a probability of error of less than 0.05 was preselected as the criterion for statistical significance.
Vol. 43, No. 6, 1988
Selective Angiotensin III Antagonist
539
Results Table
I
shows control MAP in six groups of rats. The infusion of caused a significant increase in MAP whereas the infusion of [SarlIle7]ANG III did not significantly alter MAP.
[Ile7]ANG III
The infusion of CSarlIle71ANG III caused a slight but not significant pressor response as shown by the tracings from a typical experiment (Fig. 1). The bolus infusion of [Ile71ANG III caused a significant pressor response which reached a maximum value 2 min after the start of infusion and gradually declined to a sustafned level (Table 11. TABLE I MAP Before and 17 min After the Start of Infusion of Antagonist or Vehicle in Six Groups of Conscious Rats (n = 6 in each group) MAP
Group
Control
+ Antagonist
(mm Hg) No a tagonist [IleylAN III [SarlIle$lANG
III
112 * 3.0 113 l 4.0 112 * 2.0
121 116
III
111 * 3.0 113 l 3.0 114 f 3.0
121 ; 3.0a 115 * 3.0
No a tagonist [Ile s IANGIII
[SarlIle7JANG
l l
2.0a 3.0
asignificantly different from MAP of control rats (p < 0.05). The infusions of ANG II and ANG III caused dose-dependent increases in MAP which reached similar maximium values of 193 l 5 and 192 f 8 (mean l SE) mmHg, one-ninth of that respectively. The ED50 value for ANG II was approximately for ANG III showing that ANG II is more potent than ANG III in causing a pressor response (Table II).
In the presence of [Ile7lANG III, the dose-MAP response curves for ANG and ANG III were displaced to the right (Fig. 21, with a significant increase in the ED50 value for ANG II as well as ANG III (Table II). The responses to the two lowest doses of ANG II but not ANG III was increased in CSarlIle71ANG III caused a significant the presence of CIle7lANG III. displacement of the ANG III response curve to the right but did not displace the ANG II curve (Fi3. 3). The ED50 values for ANG II in the absence and presence of [SarlIle ]ANG III were not significantly different from each other as opposed to t:e $ o fold increase in the ED5D value of ANG III in The maximum responses of the presence of [Sar Ile ]ANG III (Table II). both ANG II and ANG III were slightly but not significantly decreased by the II
two analogues.
Selective Angiotensin II Antagonist
540
A
150 _
Vol. 43, No. 6, 1988
C
B
-
9 glp;: 0. 200
E
3 ,z
ii
; p 0
z
0
4
I
W-t
t
[Sar111e71ANG
III
200.
E :, ;
1
f
z ;: <
100.
t
t
OJ
1
III
IIk71ANG
FIG.
min
1
Tryings of arterial pressure responses to CSarlIle71ANG III or [Ile IANG III prior to and during the bolus (A) and continuous IB) infusion of an antagonist and 17 min after the start of the infusions (Cl. TABLE II ED50 Values for ANG II and ANG III in Six Groups of Conscious Rats (n = 6 in each group) in the Absence and Presence of CIle71ANG III or [SarlIle71ANG III
Group of rats ED50 MAP
Control
CIle7]ANG III
(1.2 ANG II
4.0
l
0.6
x 10T7 mole/kg) 7.0
l
CSar111e71ANG III (1.3 x 10m7 mole/kg)
0.9a
5.0
l
0.4
(moles/kg/minxlO-lO) ANG III
35 * 9.0
58 * 4.0a
(moles/kg/minxlO-10) aSignificantly different from control (p < 0.05).
73
l
8.0a
Selective
vol. 43, No. 6, 1988
Angiotensin
III
Antagonist
541
100.
60
0.5
1.0
1.5
2.0
Log Oose (mol/kgimin
FIG. % Maximum MAP fesponse to ]ANG III. and absence of [Ile (0 1 and the presence ( A 1 of the absence 1 l I and presence (
0.5
2.5
3.0
x 10”)
2
ANG II and ANG III in the presence Responses to ANG II in the absence antagonist. Response of ANG III in V 1 of antagonist.
1.0
1.S
~oq Dose (mllkglmin
FIG.
2.0
2.5
3.0
x 10")
3
% Maximum MAP respon?e to ANG II and ANG III in the presence and absence of Dar1 Ile IANG III. Responses to ANG II in the absence ( 0 1 and the presence ( A ) of antagonist. Response of ANG III in the absence 1 n 1 and presence ( v 1 of antagonist.
Selective Angiotensin III Antagonist
542
Vol. 43, No. 6, 1988
Discussion The
pressor actions of both ANG II and ANG III were antagonized by but the mode by which the antagonism was achieved appeared to be different. A comparison between the dose-MAP response curves for ANG II in the absence and the presence of 111e71ANG III suggests that CIle71ANG III was acting as a partial agonist on receptors that were activated by ANG II. This inference was supported by two observations. Firstly, the infusion of CIle71ANG III caused a significant increase in MAP and secondly, the responses to the two lowesq doses of ANG II in the presence of [IleTJANG III were increased. Thus [Ile IANG III, at the dose given, showed some intrinsic activity perhaps by act$ng on receptors that were activated by ANG II. However, the actions of [Ile IANG III towards the dose-MAP response curve of ANG III seemed to be entirely different as the antagonist decreased rather than increased the base-line response of ANG III. This suggest that ANG III was activating a sub-class of angiotensin receptors in which CIle7lANG III showed some affinity but little intrinsic activity.
CIle73ANG III
III anfagonized the CSarIIle7lANG In contrast to [Ile71ANG III, Thus CSarlIle IANG III has pressor actions of ANG III but not ANG II. selectivity for receptors activated by ANG III. This analogue, at the dose given, showed very little intrinsic activity in comparison to [Ile71ANG III. The results obtained in this study support our previous hypothesis that in the vascular smooth muscle there exists a sub-class of angiotensin receptors that are predominantly activated by ANG III (61. Even though the replacement of phenylalanine by CIle7lANG III produced an analogue which acts as an antagonist of ANG III in the vasculature, it is apparent that replacements of N-terminal arginine as well as C-terminal phenylalanine of the ANG III molecule are neccessary in order to produce a The replacement of selective antagonist of the pressor actions of ANG III. the N-terminal aspartate with a sarcosine residue in the ANG II molecule has ;;Tn shown to increase binding affinity of the peptide for the ANG II receptor . It seems that replacement of N-terminal arginine of ANG III with sarcoThe basic sine alters the affinity of the peptide towards ANG II receptors. structural requirement of peptides for activating ANG III receptors remains unknown. This makes the predication for more selective and effective antagonists for ANG III more difficult.
In conclusion, we have found that [IleTlANG III appears to have similar affinity for both ANG II and ANG III receptors but p sse ses some intrinsic [SaJIleS]ANG III on the activity for receptors activated by ANG II. other hand, shows no intrinsic activity at the dose given and it ha> greater affinity for veceptors activated by ANG III than receptors activated by ANG [SarlIle IANG III is the first ANG III antagonist available for the II. elucidation of vascular effects of ANG III. It is possible that other actions of ANG I II may al so be antagonized by this analogue. Acknowledgements This work was supported by the Medical Research Council of Canada. Reza Tabrizchi was a recipient of a UBC Graduate Fellowship (Hugo E. Malicke Memorial). We would also like to thank Ciba-Giegy Canada Ltd. for the supply of ANG II.
Vol. 43, No. 6, 1988
Selective Angiotensin III Agonist
543
References 1.
2. 3. 4. 5. 6. 7. 8.
M.C. KHOSLA, R.R. SMEBY and F.M. BUMPUS, Hand_Bogk_ofExper &imental Pharmacology: Angiotensin, Vol. 37, eds. I.H. Page and F.M. Bumpus, p. 127, Springer, Heidelberg (19741. 370). P.A. KHAIRALLAH, A. TOTH and F.M. BUMPUS, J. Med. Chem. 13 181-184 (l! F.M. BUMPUS, M.C. KHOSLA, R.A. LEESE, A.T. FERREIRA, R.R. SMEBYAND J. Med. Chem. 15 792-795 (1972). R.K. TURKER, MB. HALL, M. YAMAMOTO, C.S. SWEET and F.M. BUMPUS, Science 177 1203-1205 (1972). ET. PALS, F.D. MASUCCI, G.S. DENNING JR., F. SIPOS and D.C. FESSLER, Circ. Res. 29 673-681 (1971). R. TABRIZCwand C.C.Y. PANG, Eur. J. Pharmacol. 142 359-366 (1987). D. REGOLI, F. RIOUX, W.K. PARK and C. CHOI, Can.J,Physiol. Pharmacol. -52 39-49 (1974) :i;i6FOGHARI, K.J. FRANKLIN and G.J. MOORE, J. Med. Chem. 29 1121-1124
.