Increased expression of inhibitory guanine nucleotide binding proteins in membranes from renal proximal tubules, but not brain striata, of the normotensive and the spontaneously hypertensive rat

Increased renal vascular sensitivity to angiotensin II in hypertension is due to decreased response to prostaglandins

OBJECTIVES

An enhanced sensitivity to angiotensin II in the renal circulation has been demonstrated in the pre-hypertensive phase both in the spontaneously hypertensive rat and in man. To further characterize this abnormality and the role of prostanoids, renal haemodynamics in normotensive young men with a positive (PFH) or a negative (NFH) family history of hypertension were studied.

METHODS

Renal vascular reactivity was assessed during infusion of angiotensin II with and without inhibition of prostaglandin synthesis. Normotensive men with PFH (n = 13) and with NFH (n = 10) with a mean age of 38 years were given on two different occasions: (i). angiotensin II infusion i.v. (0.1, 0.5 and 1.0 ng/kg per min) and (ii). angiotensin II infusion after inhibition of prostaglandin synthesis with indomethacin (150 mg daily three consecutive days). Glomerular filtration rate (GFR) and renal plasma flow were measured with renal clearances of chromium edetic acid and para-aminohippuric acid.

RESULTS

Before angiotensin II challenge, the groups did not differ with respect to blood pressure, body mass index, plasma renin activity, GFR, renal blood flow (RBF) or urinary sodium excretion. There was no significant difference in systolic or diastolic blood pressure response to angiotensin II between the two groups. In PFH, the lowest angiotensin II dose caused a significant decrease in RBF and increase in renal vascular resistance (RVR) from baseline (P < 0.01 for both). In NFH, only the highest angiotensin II dose produced a significant decrease in RBF and increase in RVR (P < 0.01 for both). During inhibition of prostaglandin synthesis, all three angiotensin II doses caused a significant decrease in RBF (P < 0.02) and increase in RVR (P < 0.02) also in NFH. The renal haemodynamic difference between PFH and NFH was thus eliminated.

CONCLUSIONS

These findings indicate that young human subjects with a positive family history of hypertension have a defective vasodilator prostaglandin system, which is responsible for increased renal vascular sensitivity to angiotensin II. Enhanced renal vasoconstriction may be an early event leading to the generation of primary hypertension.

Differential expression of D2-like dopamine receptors in the kidney of the spontaneously hypertensive rat

OBJECTIVE

To compare the expression and cellular distribution of D(2)-like dopamine receptors in the kidney of the spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto (WKY) rat.

DESIGN

Renal D(2)-like receptor protein expression and distribution has not been studied in the SHR. Since changes in D(2)-like receptor expression and/or distribution may contribute to the dysregulation of renal dopamine and D(1A) receptor function, we examined the expression of the three subtypes of D(2)-like receptors (D(2), D(3) and D(4)) in SHR and WKY rat kidneys.

METHODS

Western blot analysis and confocal immunocytochemistry with specific polyclonal antipeptide antibodies directed against the receptor subtypes, were used to assess protein expression.

RESULTS

There were no differences in protein expression and cellular immunolocalization of the D(2) receptor subtypes between SHR and WKY rats. Expression of the 50 kDa D(3) receptor was reduced in the cortex of the SHR; no differences in D(3) receptor levels were seen in the inner medulla of SHR and WKY rats. The D(4) receptor polypeptides were overexpressed in the cortex of SHR, while in the inner medulla no difference in expression of the D(4) receptor proteins was observed between SHR and WKY rats. Immunocytochemistry also showed increased immunostaining of D(4) receptors in tubular structures in the cortex, but diminished staining in the SHR inner medulla.

CONCLUSION

The observed differences in expression and distribution of D(3) and D(4) dopamine receptors between cortex and inner medulla of the kidneys of SHR and WKY rats may contribute to the aberrant state of dopaminergic-mediated natriuresis in SHR.

G-proteins in kidneys of spontaneously hypertensive rats

G(s alpha)-, total G(i alpha)- and G(q/11alpha)-protein concentrations were investigated by quantitative immunoblotting in membranes of total kidney, renal cortex and medulla as well as in cortical tubules and glomeruli of Spontaneously Hypertensive Rats (SHR) and normotensive Wistar Kyoto rats (WKY), aged 5 weeks, 3 or 8 months. We found that total kidney of 5 week old SHR possess less G(s alpha)-, G(i alpha)- and G(q/11alpha)-proteins than controls. For G(s alpha)-proteins, differences found in total kidney were mirrored both in cortex (tubules and glomeruli) and in medulla. Decreased G(i alpha)-concentrations were accompanied by lower tubular but higher glomerular levels, while medullar levels were also increased. Decreased G(q/11alpha)-concentrations were reflected in decreased glomerular and medullary concentrations. Kidneys of 3 month old SHR and WKY possessed similar concentrations of all G(alpha)-species. In 8 month old SHR similar G(i alpha)-, but decreased G(s alpha)-and G(q/11alpha)-concentrations were observed. The G(s alpha)-decrease was reflected in cortex and medulla, the G(q/11alpha)-decrease in the medulla. We conclude that the main strain-related differences in G(alpha)-concentrations are seen in prehypertensive SHR.

Impaired prostaglandin E(2)/prostaglandin I(2) receptor-G(s) protein interactions in isolated renal resistance arterioles of spontaneously hypertensive rats

The protective effect of vasodilator agents linked to the cAMP pathway is less effective for buffering the vasoconstrictor effect of angiotensin II in young animals with genetic hypertension. To determine the underlying cellular mechanism, experiments were performed on freshly isolated preglomerular resistance arterioles obtained from kidneys of 7-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Specific high-affinity saturable binding of (3)H-prostaglandin (PG) E(2) revealed 1 receptor class in renal microvessels; PGE(2) receptor density was similar in SHR and WKY (106 versus 115 fmol/mg; P>0.8), as was receptor affinity (3.6 versus 3.5 nmol/L; P>0.7). Basal cAMP activity was similar in renal arterioles from SHR and WKY. A major finding was that PGE(2), PGI(2), and isoproterenol produced weaker stimulation of cAMP formation in arteriolar cells of SHR (P<0.02). In contrast, GTPgammas and forskolin stimulated cAMP generation to a similar degree in both rat strains, which suggests normal adenylate cyclase activity in hypertension-prone SHR. Immunoblots revealed the presence of 3 classes of G proteins (G(s), G(i), and G(q)) in preglomerular arterioles. The relative amounts of discernible G-protein alpha-subunits in renal resistance vessels did not differ between SHR and WKY. These results extend previous in vivo studies of abnormal renal vascular reactivity in SHR and more directly localize defective coupling of the prostaglandin and beta-adrenergic receptors to a stimulatory G protein and cAMP production in freshly isolated preglomerular arteriolar cells of young SHR. This dysfunction may be due to an abnormal interaction between prostaglandin receptors and G(s) protein that leads to inefficient coupling of initiating steps in the cAMP-protein kinase A cascade during the development of hypertension.

Alteration of association of agonist-activated renal D1(A) dopamine receptors with G proteins in proximal tubules of the spontaneously hypertensive rat

BACKGROUND

Defective D1A dopamine receptor-G protein coupling has been identified in renal proximal tubules of the spontaneously hypertensive rat (SHR).

OBJECTIVE

To determine whether association of D1A dopamine receptors with the alpha subunits of G proteins in kidney of SHR is normal.

METHODS

We analyzed the association of agonist-activated [1251]-labeled D1A dopamine receptors in kidneys of SHR and the normotensive Wistar-Kyoto (WKY) rat through immunoprecipitation, using highly specific antipeptide antibodies directed against alpha subunits of G proteins.

RESULTS

We have shown for the first time that the D1A receptors of renal proximal tubules are associated with the adenylyl cyclase inhibitory G proteins G(i)alpha. The association of WKY rat proximal tubule D1A receptors with Gi1alpha and Gi2alpha in the presence of agonist is significantly (P<0.01) greater (2.4-fold and 3.1-fold greater, respectively) than it is without agonist D1A receptors of WKY rat also exhibit (twofold greater) association with G(s)alpha, consistently with the ability of these receptors to mediate stimulation of adenylyl cyclase. The WKY rat D1A receptors do not associate either with G(o)alpha or with G(q)alpha. The D1A receptors of SHR proximal tubule membranes appear to be resistant to activation by agonist and do not associate with G(s)alpha, G(o)alpha and any of the subunits of G(i)alpha. However, the SHR D1A sites exhibit a modestly (1.7-fold) greater association with G(q)alpha, which was not statistically significant. The differences among associations of the D1A receptors of WKY rat and SHR with these Galpha proteins may be important in understanding renal dopaminergic functions in normal and pathophysiologic states.

Dysfunctional D1A receptor-G-protein coupling in proximal tubules of spontaneously hypertensive rats is not due to abnormal G-proteins

BACKGROUND

Dysfunctional dopamine neurotransmission and defective D1A receptor-G protein coupling exist in renal proximal tubules (RPT) of the spontaneously hypertensive rat (SHR).

OBJECTIVE

To determine whether the G proteins in SHR are abnormal, preventing formation of agonist high affinity sites in SHR.

METHODS

We examined the expression levels of the alpha-subunits of G proteins, as well as D1A receptor receptor coupling to exogenously added normal G proteins, in RPT of SHR and the normotensive Wister-Kyoto (WKY) rat.

RESULTS

In the presence of 110 mmol/l NaCl, the D1A dopamine receptor-selective agonist SKF R-38393 binds both to high- and to low-affinity sites on solubilized and reconstituted D1A receptors extracted from renal proximal tubules of normotensive Wistar-Kyoto (WKY) rats. In the spontaneously hypertensive rat (SHR), SKF R-38393 bound to a single site on the reconstituted receptor with affinity values corresponding to the low-affinity state of the receptor. Western blot analyses indicated that the alpha-subunit of the guanine nucleotide binding protein (G-protein), Gs, was expressed at similar levels, whereas G(o)alpha was not expressed in proximal tubule membranes from WKY rats and SHR. Pretreatment of proximal tubule membranes with the alkylating agent N-ethylmaleimide in the presence of SKF R-38393 inactivated alpha-subunits of endogenous G-proteins, but not D1A receptors, resulting in loss of high-affinity binding sites in WKY rats. These N-ethylmaleimide-treated D1A receptors from WKY rats, when reconstituted with exogenous sources of G-proteins, were able to couple to these exogenous G-proteins, with complete restoration of high-affinity sites. Moreover, the affinity values and the proportion of these hybrid sites were similar to those of untreated receptors, and these affinity sites were regulated by guanine nucleotide analogs. Reconstitution of D1A receptors from SHR with the same exogenous G-proteins failed to similarly induce formation of the high-affinity binding sites in the hybrid reconstituted systems, and SKF R-38393 continued to bind in a single low-affinity state of the receptor.

CONCLUSION

These results demonstrate that the absence of G-protein coupling in SHR is due to intrinsic defects within the receptor protein, rather than to any abnormalities of the endogenous G-proteins themselves.