Attenuation of isoproterenol-stimulated plasma renin activity by chronic estrogen treatment

Renin Cells, the Kidney, and Hypertension

Renin cells are essential for survival perfected throughout evolution to ensure normal development and defend the organism against a variety of homeostatic threats. During embryonic and early postnatal life, they are progenitors that participate in the morphogenesis of the renal arterial tree. In adult life, they are capable of regenerating injured glomeruli, control blood pressure, fluid-electrolyte balance, tissue perfusion, and in turn, the delivery of oxygen and nutrients to cells. Throughout life, renin cell descendants retain the plasticity or memory to regain the renin phenotype when homeostasis is threatened. To perform all of these functions and maintain well-being, renin cells must regulate their identity and fate. Here, we review the major mechanisms that control the differentiation and fate of renin cells, the chromatin events that control the memory of the renin phenotype, and the major pathways that determine their plasticity. We also examine how chronic stimulation of renin cells alters their fate leading to the development of a severe and concentric hypertrophy of the intrarenal arteries and arterioles. Lastly, we provide examples of additional changes in renin cell fate that contribute to equally severe kidney disorders.

Pannexin 1 channels in renin-expressing cells influence renin secretion and blood pressure homeostasis

Kidney function and blood pressure homeostasis are regulated by purinergic signaling mechanisms. These autocrine/paracrine signaling pathways are initiated by the release of cellular ATP, which influences kidney hemodynamics and steady-state renin secretion from juxtaglomerular cells. However, the mechanism responsible for ATP release that supports tonic inputs to juxtaglomerular cells and regulates renin secretion remains unclear. Pannexin 1 (Panx1) channels localize to both afferent arterioles and juxtaglomerular cells and provide a transmembrane conduit for ATP release and ion permeability in the kidney and the vasculature. We hypothesized that Panx1 channels in renin-expressing cells regulate renin secretion in vivo. Using a renin cell-specific Panx1 knockout model, we found that male Panx1 deficient mice exhibiting a heightened activation of the renin-angiotensin-aldosterone system have markedly increased plasma renin and aldosterone concentrations, and elevated mean arterial pressure with altered peripheral hemodynamics. Following ovariectomy, female mice mirrored the male phenotype. Furthermore, constitutive Panx1 channel activity was observed in As4.1 renin-secreting cells, whereby Panx1 knockdown reduced extracellular ATP accumulation, lowered basal intracellular calcium concentrations and recapitulated a hyper-secretory renin phenotype. Moreover, in response to stress stimuli that lower blood pressure, Panx1-deficient mice exhibited aberrant "renin recruitment" as evidenced by reactivation of renin expression in pre-glomerular arteriolar smooth muscle cells. Thus, renin-cell Panx1 channels suppress renin secretion and influence adaptive renin responses when blood pressure homeostasis is threatened.

Regulation of components of the brain and cardiac renin-angiotensin systems by 17β-estradiol after myocardial infarction in female rats

The present study tested the hypothesis that 17β-estradiol (E2) inhibits increases in angiotensin-converting enzyme (ACE) and ANG II type 1 receptor (AT1R) in the brain and heart after myocardial infarction (MI) and, thereby, inhibits development of left ventricular (LV) dysfunction after MI. Age-matched female Wistar rats were treated as follows: 1) no surgery (ovary intact), 2) ovariectomy + subcutaneous vehicle treatment (OVX + Veh), or 3) OVX + subcutaneous administration of a high dose of E2 (OVX + high-E2). After 2 wk, rats were randomly assigned to coronary artery ligation (MI) and sham operation groups and studied after 3 wk. E2 status did not affect LV function in sham rats. At 2–3 wk after MI, impairment of LV function was similar across MI groups, as measured by echocardiography and direct LV catheterization. LV ACE mRNA abundance and activity were increased severalfold in all MI groups compared with respective sham animals and to similar levels across MI groups. In most brain nuclei, ACE and AT1R densities increased after MI. Unexpectedly, compared with the respective sham groups the relative increase was clearest (20–40%) in OVX + high-E2 MI rats, somewhat less (10–15%) in ovary-intact MI rats, and least (<10–15%) in OVX + Veh MI rats. However, because in the sham group brain ACE and AT1R densities increased in the OVX + Veh rats and decreased in the OVX + high-E2 rats compared with the ovary-intact rats, actual ACE and AT1R densities in most brain nuclei were modestly higher (<20%) in OVX + Veh MI rats than in the other two MI groups. Thus E2 does not inhibit upregulation of ACE in the LV after MI and amplifies the percent increases in ACE and AT1R densities in brain nuclei after MI, despite E2-induced downregulation in sham rats. Consistent with these minor variations in the tissue renin-angiotensin system, during the initial post-MI phase, E2 appears not to enhance or hinder the development of LV dysfunction.

Effects of angiotensinogen antisense oligonucleotides on fluid intake in response to different dipsogenic stimuli in the rat

The role of centrally synthesised angiotensinogen in neural mechanisms subserving water drinking in rats was investigated by injecting antisense oligonucleotides complementary to rat angiotensinogen mRNA into the brain with the aim of inhibiting cerebral angiotensinogen synthesis. Phosphorothioate antisense oligonucleotides (18 mer) encompassing the translation start codon were injected into the lateral ventricle of rats and their responses to a number of dipsogenic stimuli tested. These were: intracerebroventricular (i.c.v.) renin, i.c.v. angiotensin II, i.c.v. carbachol, subcutaneous isoproterenol, intravenous hypertonic saline, water deprivation for 24 h or subcutaneous injection of polyethylene glycol. Antisense treatment significantly reduced (by approximately 50%) the volume of water drunk in response to i.c.v. injection of renin or subcutaneous isoproterenol, but did not reduce water intake elicited by the other dipsogenic stimuli. The i.c.v. administration of mismatch, scrambled or sense oligonucleotides did not inhibit water intake. These data suggest that centrally produced angiotensinogen may have a role in neural pathways subserving isoproterenol-induced drinking.

Angiotensinogen antisense oligonucleotides and fluid intake

The effectiveness of antisense oligonucleotides (ODNs) to angiotensinogen on intracerebrovenricularly injected renin induced thirst was investigated. As a corollary, information would be gained about the role of centrally synthesised angiotensinogen in the neural mechanisms subserving water drinking in rats. Stable, easily synthesised phosphorothioate antisense oligonucleotides (18 mer), one of which included the sequence encompassing the translation start site, were injected into the lateral ventricle of rats. The drinking response to a number of dipsogenic stimuli was tested. Antisense significantly reduced (by about 50%) the volume of water drunk in response to intracerebroventricular (icv) renin or isoproterenol but did not reduce drinking in response to the physiological challenge of icv angiotensin II, icv carbachol, intravenous hypertonic saline, water deprivation or subcutaneous injection of polyethylene glycol. Only one out of four antisense probes gave positive results, while mismatch or scrambled oligonucleotides did not inhibit water intake. This finding reduces the probability that the results observed are non-specific. In these experiments, an ODN specific for angiotensinogen was discovered and was produced easily in large enough amounts and stabilised against intracellular nucleases without floss of cellular access or biological effect.

AT1 receptor density changes during development of hypertension in hyperinsulinemic rats

In a previous study we showed that the renin-angiotensin system (RAS) plays a role in the etiology of fructose-induced hypertension. To our knowledge, no previous study has evaluated changes in angiotensin II (Ang II) type I receptor (AT1) density in fructose-fed rats that are insulin resistant and hypertensive. The purpose of this study was to determine the changes in plasma Ang II and AT1 density associated with the elevation of blood pressure in fructose-treated rats. Male Sprague-Dawley rats were divided into two groups and were fed either normal rat chow or a 60% fructose-enriched diet for four weeks. Plasma Ang II and serum insulin levels of the fructose-treated rats were significantly elevated (p < 0.01) by the end of the second week of fructose treatment. Plasma Ang II levels of the fructose-fed rats returned to basal levels by the end of the fourth week of dietary treatment, whereas the serum insulin levels consistently remained elevated. Blood pressure was significantly elevated in the fructose-fed rats within two weeks of fructose treatment. Elevation of blood pressure was associated with left ventricular hypertrophy. Furthermore, there was a significant increase in AT1 receptor density in the ventricles and a significant decrease in AT1 receptor density in the aortas of fructose-fed rats at the end of fourth week. There were no significant changes in receptor density in the hypothalami or adrenal glands of fructose-treated rats. These results suggest that chronic fructose treatment activates the renin-angiotensin system, which is manifested by an increase in plasma Ang II, elevation of blood pressure, cardiac hypertrophy, and changes in AT1 receptor density.

Changes in angiotensin AT1 receptor density during hypertension in fructose-fed rats

Feeding carbohydrate-enriched diets to normal rats has been shown to induce insulin resistance and hyperinsulinemia associated with an elevation of blood pressure. Previously we reported that the renin-angiotensin system (RAS) is likely to be involved in the elevation of blood pressure. The purpose of this study was to determine the changes in plasma angiotensin II (AII) and AII receptor density associated with the elevation of blood pressure in fructose-treated rats. Male Sprague-Dawley rats were divided into two groups and were fed either normal rat chow or a 60% fructose-enriched diet for four weeks. Plasma insulin of fructose-treated rats was significantly elevated (p < 0.05) by the end of first week of fructose treatment and remained elevated throughout the study. Plasma AII levels of fructose-fed rats was 3.5 fold greater than the controls at the end of second week and returned to basal levels at the end of the fourth week of dietary treatment. Blood pressure was significantly elevated in the fructose-fed rats within two weeks of fructose treatment. Elevation of blood pressure was associated with left ventricular hypertrophy. Angiotensin II type I receptor (AT1) density was determined in the left ventricle, aorta, adrenal gland and hypothalamus. There was a significant increase in AT1 receptor density in the ventricle at the end of third and fourth weeks of treatment, whereas there was a significant decrease in the receptor density in the aorta at the end of the fourth week of treatment. Receptor density in the adrenal gland and hypothalamus of fructose-fed rats was similar to their respective controls. The results of this study suggest that the RAS plays a role in the elevation of blood pressure of fructose-fed rats and also contributes to the ventricular hypertrophy observed in these rats.

Prevention of cold-induced increase in blood pressure of rats by captopril

To assess the possibility that the renin-angiotensin system may play a role in the development of cold-induced hypertension, three groups of rats were used. Two groups were exposed to cold (5 +/- 2 degrees C) while the remaining group was kept at 26 +/- 2 degrees C. One group of cold-treated rats received food into which captopril (0.06% by weight) had been thoroughly mixed. The remaining two groups received the same food but without captopril. Systolic blood pressure of the untreated, cold-exposed group increased significantly above that of the warm-adapted, control group within 4 weeks of exposure to cold. In contrast, chronic treatment with captopril prevented the elevation of blood pressure. Rats were killed after 4 months of exposure to cold. At death, the heart, kidneys, adrenal glands, and interscapular brown fat pad were removed and weighed. Although captopril prevented the elevation of blood pressure in cold-treated rats, it did not prevent hypertrophy of the kidneys, heart, and interstitial brown adipose tissue that characteristically accompanies exposure to cold. Thus, chronic treatment with captopril prevented the elevation of blood pressure when administered at the time exposure to cold was initiated. It also reduced the elevated blood pressure of cold-treated rats when administered after blood pressure became elevated. This suggests that the renin-angiotensin system may play a role in the elevation of blood pressure during exposure to cold.

Renal and myocardial adrenoceptors in steroid contraceptive-induced hypertension in rats

1. Systolic blood pressure (SBP), bodyweight, organ weight, renal beta-adrenoceptor and myocardial beta- and myocardial alpha 1-adrenoceptor characteristics were investigated in female Sprague-Dawley rats after chronic subcutaneous (s.c.) administration of ethynyloestradiol (EE2, 0.2 microgram/day), levonorgestrel (NG, 2.0 micrograms/day) separately and in combination (EE2/NG). 2. EE2 caused a sustained increase in SBP from 6 weeks (maximum at 14 weeks, +22 mmHg compared to control) which was accompanied by increased kidney and ventricle weight after 12 weeks. EE2/NG-treated rats also demonstrated a gradual rise in SBP (maximum at 9 weeks, +18 mmHg compared with control) with renal and ventricular hypertrophy, but were normotensive by week 17 of treatment. In contrast, NG induced only transient SBP increases (maxima at 5 and 10 weeks, +14 mmHg compared with control), unaccompanied by organ hypertrophy. Norethisterone (2 micrograms/day) also produced transient increases (weeks 6-8, +13 mmHg) in SBP. 3. alpha 1- and beta-adrenoceptors were investigated using [3H]-prazosin and (-)-[125I]-iodocyanopindolol (ICYP), respectively. Myocardial alpha 1- and beta-adrenoceptors were unaffected by steroid contraceptive administration for up to 12 weeks. Renal beta-adrenoceptor affinity was markedly reduced in 12 week EE2-treated rats (equilibrium dissociation constant, KD, 53 +/- 7 pmol/L) compared with controls (KD, 31 +/- 4 pmol/L), an effect which was prevented by co-administration of NG (KD, 34 +/- 8 pmol/L). Renal beta-adrenoceptor number was not altered by any treatment. 4. The relatively late onset of organ hypertrophy and beta-adrenoceptor changes appear to result from, rather than cause, EE2-induced hypertension.

Effects of chronic estrogen treatment on water exchange in rats

Intact female rats implanted subcutaneously with Silastic tubes containing estradiol benzoate (EB) (28.7 micrograms X kg-1 X day-1) for 28 wk had a significantly greater daily intake of water, a higher water-to-food intake ratio, and a greater urine output than untreated control rats. Ovariectomized (OVX) rats also implanted for 14 wk with EB tubes (15 and 36 micrograms X kg-1 X day-1) showed identical results. Dipsogenic responses of the EB-treated rats to isoproterenol (25 micrograms/kg sc), angiotensin II (200 micrograms/kg ip), and hypertonic saline (1 M, 1% of body wt ip) were significantly attenuated. Both intact and OVX rats were subjected to a 24-h dehydration to assess renal concentrating ability. EB-treated rats lost significantly more weight and excreted significantly more urine of lower osmolality than controls. Administration of vasopressin to volume-loaded, EB-treated rats revealed no abnormalities in the ability to concentrate urine to the level of controls. Thus, in spite of a reduced responsiveness to several dipsogenic stimuli, EB-treated rats have an increased daily water turnover apparently related to an inability to concentrate their urine. This in turn may be related to abnormalities in either synthesis or release of antidiuretic hormone or both.

Altered peripheral responsiveness to beta-adrenergic stimulation in experimental renal hypertension

Hypertension, induced in male rats by encapsulation of both kidneys with latex envelopes, is associated with a reduction in maximal responsiveness to beta-adrenergic stimulation. Responsiveness was assessed by measurement of both water intake and plasma renin activity following acute subcutaneous administration of graded doses of isoproterenol, a beta-adrenergic agonist. At doses of isoproterenol in excess of 6 micrograms/kg body weight, s.c., the dipsogenic response of rats with renal hypertension was significantly (p less than 0.05) less than that of normotensive controls. Basal, unstimulated plasma renin activities were similar in both the normotensive and renal hypertensive groups. However, following administration of isoproterenol at doses of 5, 10, or 20 micrograms/kg body weight, s.c., plasma renin activity was elevated significantly (p less than 0.01) in the normotensive, but only in the lowest dose in the renal hypertensive group. These results suggest that maximal responsiveness to beta-adrenergic stimulation is reduced in the renal hypertensive rat; however, the mechanism(s) responsible for the reduced beta-adrenergic responsiveness remains speculative.