(Hypertension. 2000;35:38.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
From the Departments of Medicine, Physiology and Pharmacology and Toxicology (R.G., J.C., M.D.M., R.D.F.), University of Western Ontario, London, Ontario, Canada; the John P. Robarts Research Institute (S.S.G.F., R.D.F.), London, Ontario, Canada; and the Department of Biochemistry and Molecular Pharmacology (J.L.B.), Kimmel Cancer Institute, Thomas Jefferson University, Philadelphia, Pa.
Correspondence to Dr Ross D. Feldman, Room 6L13B, LHSC-University Campus, 339 Windermere Rd, PO Box 5339, London, Ontario. Canada N6A 5A5. E-mail feldmanr{at}lhsc.on.ca
| Abstract |
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Key Words: G protein adrenergic receptors adenylyl cyclase hypertension, essential cyclic AMP rats
| Introduction |
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To explain this defect in receptor-mediated vasodilation in hypertension, we and others have focused on the receptor/G-protein/adenylyl cyclase transmembrane signaling system. Impaired vascular and renal adenylyl cyclase responsiveness to hormones acting through receptors linked to the stimulatory G protein (Gs) has been reported in rat models of hypertension (reviewed in Reference 22 ).
In human hypertension, we have focused on the lymphocyte ß-adrenoceptor/Gs/adenylyl cyclase complex as a model for the vascular ß-adrenoceptor/Gs/adenylyl cyclase complex. We and others have shown that regulation of the lymphocyte ß-adrenoceptor complex parallels the regulation of functional vascular ß-adrenergic responsiveness.4 5 In a series of studies, we demonstrated that (1) lymphocyte ß-adrenergic responsiveness was impaired in the hypertensive state, (2) this reflected an impairment in receptor/G-protein coupling,6 7 8 and (3) the molecular basis of this defect in receptor/G-protein coupling may relate to an increase in expression of a G-proteincoupled receptor kinase (GRK).9 10 GRKs are a family of enzymes that specifically phosphorylate agonist-occupied G-proteinlinked receptors.11 12 13
The present studies were performed to determine whether, in hypertension, the increase in lymphocyte GRK-2 expression was reflected by a parallel alteration in vascular GRK-2 expression. Data to be presented demonstrate that in both spontaneously hypertensive and Dahl models of hypertension, vascular GRK-2 protein expression is upregulated, providing a potential mechanism for the impairment in vascular responsiveness characteristic of the hypertensive state.
| Methods |
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Lymphocyte Preparation
Whole blood was obtained from rats by cardiac puncture, and
mononuclear leukocytes were separated from heparin-anticoagulated whole
blood by the method of Böyum.14 For assessment of
adenylyl cyclase activity in rat lymphocytes, permeable cells were
prepared as previously described.9 For assessment of GRK-2
protein expression, whole-cell lymphocyte samples were obtained as
described above, pelleted, and frozen at -80°C.
Preparation of Isolated Aortic Vascular Smooth Muscle Cells
and Proteins
Freshly isolated thoracic aortas (prepared with use of our
previously published techniques15 ) were placed into a
Petri dish containing ice-cold physiological salt
solution (mmol/L: NaCl 130, HEPES 20, D-glucose 10, KCl 5,
CaCl2 1, and MgCl2 1, pH
7.4) and extensively cleaned of adventitial fat and connective tissue
by use of fine forceps and scissors. The thoracic aortas were cut into
longitudinal strips
20 to 30 mm long and 5 mm wide. The
aortic strips were then transferred to 2 mL of dissociation solution
(physiological salt solution plus 0.25 mmol/L
EDTA, 1.2 mg/mL BSA, 1 mg/mL papain, 0.8 mg/mL Sigma blend
collagenase, and 0.09 mg/mL dithiothreitol, pH 7.0) and
incubated at 37°C for 1 hour. Tissue was teased apart by triturating
with a Pasteur pipette. Undigested material was removed with forceps.
The vascular preparation was centrifuged at 500g for
10 minutes at 4°C. The pellet was resuspended in
physiological salt solution and transferred to
microfuge tubes and frozen at -80°C until needed. As an alternative
to isolated vascular smooth muscle cells, we prepared thoracic aortic
lysates as follows: clean thoracic aortic strips were placed in
microfuge tubes with 400 µL lysis buffer (100 mmol/L NaCl, 1%
Triton X-100, 10% glycerol, 50 mmol/L HEPES [pH 7.4], and
1 mmol/L EDTA) and incubated at 4°C for 1 hour. After
incubation, microfuge tubes and tissues were frozen at -80°C until
needed.
Assessment of GRK Protein Expression
Assessment of GRK protein expression was determined by
immunoblotting (as previously described in References 9
and 109 10 ). GRK-2 protein expression was determined by using a 1:100
dilution (for lymphocyte studies) or a 1:10 dilution (for vascular
smooth muscle studies) of a mouse monoclonal antibody 3A10 raised
against purified recombinant bovine GRK-2 (as previously described in
Reference 99 ). Immunoreactivity was detected with the enhanced
chemiluminescence detection system (ECL, Amersham Corp). Submaximally
exposed autoradiographs were assessed densitometrically by using NIH
Image 1.6 software. Initial studies determined that under these
conditions the assay was linear in the range of protein concentrations
used.
Assays of Adenylyl Cyclase Activity in Permeabilized
Lymphocytes
Adenylyl cyclase activity was determined in
permeabilized lymphocytes according to our previously
published methods.8 ß-Adrenoceptor stimulation of
adenylyl cyclase activity was assessed in the presence of isoproterenol
(100 µmol/L) and GTP (100 µmol/L). Maximal catalytic
activity was assessed with forskolin (10 µmol/L). Isoproterenol
and forskolin-stimulated activities were expressed relative to
GTP-stimulated activity. This proportional method of expression was
selected prospectively and is consistent with that used in our
previous studies comparing stimulated levels of adenylyl cyclase
activity in subject groups.8 9 10
Assessment of Tension in the Aortic Ring Segments
Assessment of tension in the aortic ring segments was performed
according to our previously described methods.15 After the
equilibration, rings were maximally preconstricted with
phenylephrine (3 µmol/L) and allowed to reach a
plateau. Relaxation was assessed in response to a single addition of
isoproterenol (10 µmol/L) or sodium nitroprusside (10 nmol/L).
Relaxation response with isoproterenol and nitroprusside was quantified
by determination of the area above the relaxation response curve by use
of a trapezoidal method of analysis as previously
described.15
Analysis
Those parameters expressed as a ratio (eg,
isoproterenol-stimulated adenylyl cyclase activity expressed relative
to GTP-stimulated activity) are log-normally
distributed.16 Therefore, these data were log-transformed
for statistical analysis (t test, correlations). A
value of P<0.05 on a 2-tailed 2-group test was used as a
minimum level of significance.
| Results |
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Alterations in Lymphocyte GRK-2 Protein Expression in SHR
Paralleling the decrease in isoproterenol-stimulated adenylyl
cyclase activity in lymphocytes from 10-week-old SHR, there was a
significant increase in GRK-2 protein expression compared with either
10-week-old Wistar rats (143±10% of expression in Wistar rats,
P<0.05, n=5 in each group; Figure 2) or WKY (131±11% of expression in
WKY, P<0.05, n=5 in each group).
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Alterations in Isoproterenol-Mediated Relaxation in SHR Aortic
Ring Segments
In aortic ring segments isolated from 10-week-old SHR, Wistar
rats, and WKY, there was an age-dependent and blood pressuredependent
impairment in vascular ß-adrenergicmediated response in SHR.
Maximal phenylephrine-mediated constriction was not
significantly different between WKY or Wistar rats and SHR (data not
shown). In phenylephrine-precontracted aortic ring
segments, the addition of isoproterenol resulted in a rapid relaxation
(reaching a nadir by 5 minutes). The extent of maximal relaxation was
significantly attenuated in SHR compared with Wistar rats and WKY
(Table). In contrast, responses to doses of
nitroprusside that mediated an extent of relaxation comparable to that
seen with isoproterenol were not altered between SHR and Wistar rats
(Table).
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Alterations in Vascular GRK-2 Protein Expression
GRK-2 protein expression was increased in vascular smooth muscle
cells of 10-week-old SHR versus either 10-week-old Wistar rats or WKY,
with both expressed on an absolute basis (Figures 3 and 4)
and as a proportion of
-actin expression (GRK-2/
-actin ratio
1.00±0.18 normalized arbitrary units [Wistar] and 1.79±0.17
normalized arbitrary units [SHR], P=0.015). Relative
-actin expression was not significantly altered in SHR (data not
shown).
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We also obtained aortas from Dahl salt-sensitive and -resistant
rats after 4 weeks of a high salt diet. Vascular GRK-2 protein
expression was significantly increased (185±14% of expression in Dahl
salt-resistant rats, P<0.05) in aortic tissue
lysates obtained from Dahl salt-sensitive rats compared with Dahl
salt-resistant rats. Relative
-actin expression was not
different (data not shown).
Developmental Regulation of Vascular Reactivity and GRK-2
Expression
To examine the developmental relation among vascular
ß-adrenergic responsiveness, GRK-2 expression, and blood pressure, we
examined SHR and WKY at 5 weeks (prehypertensive), 10 weeks, and 15
weeks of age. In all age groups (5-, 10-, and 15-week-old rats),
maximal phenylephrine-mediated constriction was not
significantly different between SHR and WKY (data not shown). In
contrast to the impairment in ß-adrenergicmediated relaxation in
adult (10-week-old) SHR, no alterations in isoproterenol-mediated
relaxation were observed in 5-week-old rats (Table). Notably,
compared with 5-week-old and 10-week-old WKY, 15-week-old SHR and WKY
exhibited a parallel impairment in both sodium nitroprussidemediated
and isoproterenol-mediated relaxation (P<0.02,
Table). The impairment in both sodium nitroprussidemediated
and isoproterenol-mediated relaxation is consistent with a
defect "downstream" from the receptor/G-protein/enzyme complex.
However, even when normalized for the impairment in sodium
nitroprussidemediated vasodilation, an impairment in
isoproterenol-mediated relaxation was still evident in 15-week-old SHR
compared with WKY (Table).
To examine the developmental regulation of vascular GRK-2 protein
expression, we examined aortic tissue lysates obtained from 5-, 10-,
and 15-week-old SHR versus WKY. GRK-2 protein expression was comparably
increased in hypertensive rats at both 10 and 15 weeks of age in SHR
versus WKY (138±7% of the expression in 10-week-old WKY and 147±11%
of expression in 15-week-old WKY, Figure 4). Notably, GRK-2
protein expression was reduced in 5-week-old (prehypertensive) SHR
compared with 5-week-old WKY (81±10% of the GRK-2 expression observed
in 5-week-old WKY, Figure 4). Overall, in WKY, vascular GRK-2
protein expression decreased with increasing age, whereas vascular
GRK-2 protein levels in SHR did not demonstrate significant alteration
with increasing age (Figure 4). Relative
-actin protein
expression was not significantly altered between SHR and WKY and did
not change from 5 to 15 weeks of age (data not shown).
| Discussion |
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We have previously speculated that the increase in lymphocyte GRK-2 expression in human hypertensive subjects was likely not restricted to lymphocytes (ie, might be generalized to vascular smooth muscle cells), and if so, it might account for the reduction in vascular ß-adrenergic responsiveness demonstrated in both human hypertension9 and in rat models of hypertension.2 The present studies support this hypothesis. SHR have been used commonly for the study of ß-adrenergic receptor systems in hypertension (reviewed in Reference 22 ). Impaired isoproterenol-mediated vasodilation and impaired isoproterenol-stimulated adenylyl cyclase activation in vascular tissues have been previously demonstrated in this model. In the present study, we demonstrate that ß-adrenergicmediated adenylyl cyclase activity in lymphocytes from SHR is impaired to an extent comparable to that previously reported in studies of vascular smooth muscle cells (reviewed in Reference 22 ) and similar to the extent of impairment seen in lymphocytes from human hypertensive subjects.9 13 Thus, this observation reiterates the utility of the lymphocyte as a model for the vascular ß-adrenoceptor complex in hypertension.
The reduction in ß-adrenergicmediated adenylyl cyclase activity was associated with an increase in GRK-2 protein expression in both lymphocytes and vascular smooth muscle cells. This relative increase in GRK-2 protein expression in hypertensive rats is comparable to the increase in GRK-2 protein expression in lymphocytes from hypertensive subjects. In total, these studies suggest a more generalized defect in GRK-2 expression both in animal models of hypertension and human hypertensive subjects as well as in lymphocytes and myocytes.
The linkage between increased GRK-2 protein expression and increased blood pressure has yet to be determined. In this regard, we examined age-dependent changes in both blood pressure and GRK-2 protein expression in SHR and WKY. The relative increase in GRK-2 protein expression in SHR was apparent only in conjunction with established hypertension and was not apparent in the prehypertensive stage (up to 5 weeks of age). Notably, we observed an age-dependent decrease in vascular GRK-2 expression in normotensive WKY. In contrast, vascular GRK-2 protein expression did not decrease with increasing age in SHR. The reason for the observed developmental decrease in vascular GRK-2 protein expression with age, apparent in normotensive rats, is unknown. However, in this context, the relative increase in GRK-2 protein expression in SHR might be seen as a failure in the developmental downregulation that occurs in normotensive animals.
The mechanisms regulating GRK-2 protein expression are unclear. Our previous studies have suggested that the increase in GRK-2 protein expression was not due to an increase in GRK-2 mRNA expression.13 Developmental regulation of GRK-2 has been reported neonatal rats.17 In addition, recent data have shown that GRK-2 protein expression is regulated via a proteasomal degradation pathway.18 Whether this pathway is altered in hypertension (or with age) is unknown. However, if this pathway is important in developmental regulation of GRK-2 expression and is defective or absent in SHR, the net result might be a relative increase in GRK-2 protein expression in aging SHR compared with aging WKY. This hypothesis is the focus of ongoing studies.
In conclusion, our studies in SHR demonstrate that the increase in GRK-2 expression is evident in both lymphocytes and vascular smooth muscle cells and parallels an impairment of ß-adrenergicmediated adenylyl cyclase activity (comparable to that seen in human hypertension) and ß-adrenergicmediated vasodilation. In addition, increased vascular GRK-2 protein expression was also observed in hypertensive Dahl rats, suggesting a conserved change in GRK-2 protein expression across different rat models of hypertension. Overall, our findings support the hypothesis that an alteration in GRK function could underlie the defect in G-proteincoupled receptor-mediated adenylyl cyclase activation contributing to the impairment in vascular function characteristic of the hypertensive state.
| Acknowledgments |
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Received April 9, 1999; first decision May 10, 1999; accepted August 12, 1999.
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J. Marcil and M. B. Anand-Srivastava Lymphocytes from spontaneously hypertensive rats exhibit enhanced adenylyl cyclase-Gi protein signaling Cardiovasc Res, January 1, 2001; 49(1): 234 - 243. [Abstract] [Full Text] [PDF] |
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W. E. Schutzer, H. Xue, J. F. Reed, J.-B. Roullet, S. Anderson, and S. L. Mader Angiotensin II enhances beta -adrenergic receptor-mediated vasorelaxation in aortas from young but not old rats Am J Physiol Heart Circ Physiol, December 1, 2000; 279(6): H2807 - H2814. [Abstract] [Full Text] [PDF] |
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P. Yu, L. D. Asico, G. M. Eisner, U. Hopfer, R. A. Felder, and P. A. Jose Renal Protein Phosphatase 2A Activity and Spontaneous Hypertension in Rats Hypertension, December 1, 2000; 36(6): 1053 - 1058. [Abstract] [Full Text] [PDF] |
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J. Xu, X. X. Li, F. E. Albrecht, U. Hopfer, R. M. Carey, and P. A. Jose Dopamine1 Receptor, Gs{alpha}, and Na+-H+ Exchanger Interactions in the Kidney in Hypertension Hypertension, September 1, 2000; 36(3): 395 - 399. [Abstract] [Full Text] [PDF] |
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