Selective modification of renal alpha 2-adrenergic receptors in Milan hypertensive rat strain

Testosterone dependence of salt-induced hypertension in Sabra rats and role of renal alpha(2)-adrenoceptor subtypes

This study investigated the importance of the male sex hormone testosterone on salt-induced hypertension, renal alpha(2)-adrenoceptor subtype distribution, and gene expression in salt-sensitive (SBH) male Sabra rats. Comparisons of blood pressure and renal alpha(2)-adrenoceptor subtype gene expression and receptor densities have been made among sham-operated rats, and gonadectomized rats treated or not with testosterone and submitted to normal or high salt diet for 6 weeks. In intact rats, only alpha(2B)-adrenoceptors were detected in this rat strain independent of the diet. In these rats, high salt diet increases blood pressure and up-regulates gene expression and density of alpha(2)-adrenoceptors. Gonadectomy abolishes the hypertensive response to salt overload, decreases gene expression and density of alpha(2B)-adrenoceptors, and prevents their salt-induced up-regulation. After gonadectomy, increased gene expression and a detectable density of alpha(2A)-adrenoceptors are observed at similar levels in normal and high salt diet. In gonadectomized rats, testosterone replacement restores salt-induced hypertension, density of renal alpha(2B)-adrenoceptors, and gene expression to the intact levels observed both under normal and high salt diet. Furthermore, the alpha(2A)-adrenoceptor subtype is not detected in these conditions. If the increase in renal alpha(2B)-adrenoceptor subtypes is indicative of the hypertensive phenotype, the presence of the alpha(2A)-adrenoceptor appears associated with a state of salt resistance in male SBH rats. In conclusion, testosterone is needed for the full expression of salt-induced hypertension in male salt-sensitive Sabra rats. Renal densities of alpha(2)-adrenoceptor subtypes are under control of the testicles and are differentially regulated by testosterone.

Hypertension in cafeteria-fed rats: alterations in renal alpha2-adrenoceptor subtypes

Obesity is a major cause of human essential hypertension and there are clear evidences that abnormal kidney functions play a key role in obesity hypertension. Feeding rats a cafeteria diet has been extensively used as an experimental model to study obesity and energy balance expenditure. The present study investigated whether rats fed a cafeteria diet develop hypertension with alterations in renal alpha2-adrenoceptor subtype distribution. Weight gain induced by feeding rats a cafeteria diet during 8 weeks was associated with a marked increase in blood pressure. Insulin levels were higher in these hypertensive rats, leading to a decreased plasma glucose/insulin ratio. Based on radioligand-binding studies using [3H]-RX821002 and selective competitors, a raise in alpha2-adrenoceptor density that was solely due to an increased alpha2B-adrenoceptor subtype density was detected in the kidney of the cafeteria-fed rat. Furthermore, reverse transcription-polymerase chain reaction (RT-PCR) experiments showed an overexpression of the gene encoding the alpha2B-adrenoceptor subtype in these rats. On the other hand, despite a similar mRNA level, the alpha2A-adrenoceptor subtype was no more detectable by radioligand-binding studies in the kidney of the cafeteria-fed rat. In conclusion, cafeteria-fed rats are hypertensive, with renal alterations in alpha2-adrenoceptor distribution. These alterations, which are not related to genetic factors, may play a key role in the onset of hypertension.

Guanine nucleotide regulatory protein alterations in young Milan hypertensive strain rats

Vascular smooth muscle cell membranes from prehypertensive rats of the Milan hypertensive strain (MHS) were used to examine adenylyl cyclase activity and its regulation by guanine nucleotide regulatory proteins (G-proteins). Basal adenylyl cyclase activity was similar in MHS and Milan normotensive strain (MNS) membranes. Forskolin (10(-4) M) produced a significantly greater stimulatory response in MHS membranes, but this was not observed with NaF (10(-2) M). Isoproterenol (10(-4) M) caused a significantly decreased stimulation of adenylyl cyclase activity in MHS membranes, while prostaglandin E1 (10(-5) M) produced similar responses in the two strains. Gi function and GTP responses, as observed by biphasic effects of GTP on isoproterenol-stimulated membranes, were similar in both strains. The levels of Gi2 alpha and Gq alpha/G11 alpha were similar in the two strains, while the levels of Gs alpha (44 and 42 kDa forms) and the beta-subunit were significantly reduced by approximately 20% in MHS membranes. The alpha-subunit of Gi3 was dramatically reduced by approximately 80% in MHS membranes. The affinities of beta-adrenergic receptors for the antagonist, cyanopindolol, were similar in the two strains; however, the number of beta-adrenoceptors was substantially reduced in MHS membranes. These findings may be of relevance to altered vascular reactivity and transmembrane ion distribution observed in the MHS.

Renal alpha-adrenergic receptors and genetic hypertension

The hypothesis has been proposed that an increase in the number of renal alpha-adrenergic receptors may contribute to the pathogenesis of genetic hypertension. Herein we review recent findings regarding expression of renal alpha 1 (alpha 1a, alpha 1b)- and alpha 2 (alpha 2a, alpha 2b)-adrenergic subtypes and we provide an updated revision of the above-stated hypothesis. Enhancement in receptor number or in post-receptor components responsible for alpha 1- and alpha 2-adrenergic-mediated sodium reabsorption in proximal tubule may contribute to sodium retention and an elevation in blood pressure. Perhaps such changes contribute to the increase in blood pressure in genetically determined hypertension in humans, although direct tests of this notion have not yet been performed.

Sodium regulation of alpha 2-adrenoreceptors in Dahl rats. Effect of feeding a low or high salt diet

Sodium regulation of alpha 2-adrenoreceptors was investigated in inbred salt-sensitive (S) and inbred salt-resistant (R) rats fed a high or low salt diet. The systolic blood pressure was higher in S rats than in R rats, and this difference was obviously greater on a high salt diet. In rats fed a low or high salt diet, S rats had higher alpha 2-adrenoreceptor density in the kidneys compared with R rats as measured by [3H]yohimbine binding and Scatchard analysis. The affinity of the receptors in the kidney for the antagonist, yohimbine, was nearly the same in these two strains either on a low or high salt diet. In the brain, the affinities or the numbers of receptors were not significantly different whether these two strains were fed a low or high salt diet. Inclusion of NaCl up to 80 mM in the assay medium did not alter the in vitro binding of [3H]yohimbine in the kidney or brain. On the other hand, inclusion of NaCl in the assay medium reduced the ability of epinephrine in competing with [3H]yohimbine for the receptor sites in the kidney and in the brain, and this effect of NaCl was the same in a given tissue between S and R rats, whether they were fed a low or high salt diet. These results suggest that: (1) in the kidneys, the receptor density and not the receptor affinity was different between S and R strains whether they were fed a low or high salt diet; (2) in the brain, the receptor density and affinity were the same between S and R rats regardless of the diet (low or high salt), indicating that the sodium salt diet modulates the peripheral but not the central alpha 2-adrenoreceptors; and this modulatory effect was observed only in S rats; (3) Na+ was able to reduce the affinity of the agonist (epinephrine) for the receptors in both S and R rats, and this effect of Na+ on central and peripheral alpha 2-adrenoreceptors was similar in prehypertensive rats and rats with salt-induced hypertension; and (4) the resistance of R rats to salt-induced hypertension was not due to the absence of Na+ binding component involved in the regulation of alpha 2-adrenoreceptor-adenylate cyclase complex.

Renal nerve contribution to NaCl-exacerbated hypertension in spontaneously hypertensive rats

Previous studies demonstrate that bilateral renal denervation enhances urinary sodium excretion and delays the onset of hypertension in young (7-week-old) spontaneously hypertensive rats (SHR) maintained on ordinary laboratory chow. We interpret these data as suggesting that increased renal nerve activity in this model contributes to hypertension by causing excess sodium retention. More recent studies show that dietary NaCl supplementation increases blood pressure and peripheral sympathetic nervous system activity in NaCl-sensitive SHR (SHR-S). The present study tests the hypothesis that the renal nerves contribute to the rise in arterial pressure caused by dietary NaCl supplementation in this model. SHR-S were fed a high (8%) or basal (1%) NaCl diet beginning at age 7 weeks. Bilateral renal denervation was carried out 2 weeks after the initiation of the diets, at which time systolic blood pressure was significantly higher in the high (compared with the basal) NaCl group. Systolic blood pressure was reduced slightly less in denervated SHR-S on the high (compared with basal) NaCl diet during the following 5 weeks. Renal denervation performed 1 week before initiation of the diets attenuated the subsequent development of hypertension equally in both groups. Both renal denervation and the high NaCl diet increased alpha 2-adrenergic receptor numbers in the kidney; renal denervation caused an approximately equal increase in alpha 2-adrenergic receptor binding in SHR-S on high and basal NaCl diets. The high NaCl diet increased plasma noradrenaline concentration, and renal denervation lowered mean arterial pressure but did not decrease circulating catecholamines in either diet group.(ABSTRACT TRUNCATED AT 250 WORDS)