Angiotensin II type 1 receptor: role in renal growth and gene expression during normal development.

To determine whether angiotensin II (ANG II) modulates renal growth and renin and angiotensin type 1 (AT1) gene expression via AT1 during development, weanling rats were given ANG II antagonist losartan (DuP 753) for 3 wk. Body weight (g), kidney weight (g), and kidney weight-to-body weight ratio were lower in losartan-treated rats (162 +/- 7, 1.6 +/- 0.06, and 9.5 +/- 0.1 x 10(-3)) than in control rats (184 +/- 5, 1.8 +/- 0.07, and 10.1 +/- 0.1 x 10(-3); P < 0.05). Renal DNA content (mg/kidney) was lower in losartan-treated (2.4 +/- 0.17) than in control rats (3.3 +/- 0.31; P < 0.05), whereas protein-to-DNA and RNA-to-DNA ratios were similar in losartan-treated and control rats. Renin mRNA levels were sevenfold higher in losartan-treated than in control rats, as determined by quantitative standardized dot blot analysis. In addition, blockade of AT1 with losartan induced recruitment of renin-synthesizing and renin-containing cells in the renal vasculature, as determined by immunocytochemistry and in situ hybridization. To establish whether AT1 blockade has a direct effect on renin gene expression, freshly isolated renin-producing cells were exposed in vitro to losartan (10(-6) M) or culture media (control). Losartan induced a twofold increase in steady-state renin mRNA levels above control (P < 0.05). Intrarenal AT1 mRNA levels were not altered by losartan given either in vivo or in vitro to freshly dispersed cells. To define whether immature renin-secreting cells are responsive to ANG II, renin release was determined by reverse hemolytic plaque assay.(ABSTRACT TRUNCATED AT 250 WORDS)

Neonatal ureteral obstruction stimulates recruitment of renin-secreting renal cortical cells.

Unilateral ureteral obstruction (UUO) in the neonate increases ipsilateral renal renin gene expression, an effect which is mediated by renal nerves. To determine whether neonatal UUO alters the number of renal cortical cells secreting renin and whether this change is modulated by renal nerve activity, newborn Sprague-Dawley rats were subjected to left UUO, right uninephrectomy, or sham operation and studied four weeks thereafter. To evaluate the importance of renal nerves in this response, an additional group of animals underwent chemical sympathectomy with guanethidine. Ureteral obstruction was associated with marked reduction in renal mass in the obstructed kidney and contralateral compensatory hypertrophy, changes which were not altered by sympathectomy. Renin messenger RNA and renal renin content were elevated in the obstructed kidney. The number of cells secreting renin, measured by the reverse hemolytic plaque assay, was markedly increased in the obstructed kidney (45 +/- 18 plaques/slide vs. 11 +/- 1 plaques/slide in sham animals), but not in the opposite kidney or following uninephrectomy. This effect was not significantly altered by sympathectomy. There was no change in the amount of renin secreted per cell or in the secretory response to Ca++. These results show that UUO results in recruitment of cells not previously secreting renin by a mechanism independent of renal nerve activity. This recruitment occurs without alteration of the quantity of renin secreted per cell or in the normal regulatory effect of Ca++ on renin secretion. An increase in the number of renin-secreting cells may contribute to the activation of the renin-angiotensin system, and thus to the vasoconstriction observed following ureteral obstruction.

Effects of angiotensin converting enzyme inhibition, sodium depletion, calcium, isoproterenol, and angiotensin II on renin secretion by individual renocortical cells.

Angiotensin-converting enzyme inhibition with enalapril increases the number of glomeruli with juxtaglomerular cells and the number of cells in the afferent arteriole that express the renin gene and contain renin. However, renin release from these newly recruited renin-containing cells has not been demonstrated. Sodium depletion also has been shown to increase renal renin messenger RNA levels. The aim of these studies was to determine whether increases in renin secretion are a result of altered numbers of cells synthesizing/releasing renin or a change in the amount of renin release per cell, or both. Adult Wistar-Kyoto rats were treated with enalapril or sodium depleted and single cell renin secretion of enzymatically dispersed renal cortical cells was examined by reverse hemolytic plaque assay. Enalapril treatment increased the number of renin secreting cells by approximately 10-fold (P < 0.05). The newly recruited renin-secreting cells were not responsive to changes in extracellular calcium concentration or the presence of isoproterenol. At physiological (2.5 mM) extracellular calcium concentration, the amount of renin secreted per cell was approximately 2-fold greater (P < 0.05) when cells from enalapril-treated rats were compared to controls and sodium depletion increased both the number of renin-secreting cells and the amount of renin secreted by approximately 35% (P < 0.05). Angiotensin II (AII) inhibited the number of cells secreting renin in cortical cells prepared from enalapril-treated and control rats. In conclusion, angiotensin converting enzyme inhibition increased renin secretion predominantly by recruitment of additional renin-secreting cells and, to a lesser extent, by augmentation of the amount of renin released per cell. In contrast, sodium depletion increased renin secretion equally by both mechanisms. Newly recruited renin-secreting cells were not regulated by the extracellular calcium concentration or beta-adrenergic stimulation. Angiotensin II inhibited renin secretion directly by decreasing the number of individual cells releasing renin through a process which was independent of the extracellular calcium concentration.

Colocalization and release of angiotensin and renin in renal cortical cells.

Angiotensin is generated within the kidney, but the precise loci for the formation of angiotensin I (ANG I) and angiotensin II (ANG II) have not been demonstrated. We performed electron microscopy immunocytochemistry in kidney sections of 10-day-old (newborn) and adult Wistar-Kyoto (WKY) rats using specific antibodies to renin, ANG I, ANG II, and angiotensinogen (AO). Renin, ANG I, ANG II, and AO were present in juxtaglomerular (JG) cells. Renin was largely confined to cytoplasmic granules; ANG I and ANG II were colocalized to these granules but also were present in the cytoplasm; AO was distributed throughout the cytoplasm. AO also was present in a renal cortical distribution in proximal tubular cells. Northern blot analysis demonstrated AO mRNA in total kidney and liver but not in renal microvessels. Using the reverse hemolytic plaque assay, we demonstrated release of ANG I and renin from individual renocortical cells of adult WKY rats. Under control conditions, the number of releasing cells was 11 +/- 1 for ANG I and 10 +/- 1 for renin. Addition of rat renin inhibitor (RI) (1 x 10(-5) M), which inhibited renin activity in the medium from 37 to 9 pg ANG I.ml-1.h-1, did not alter ANG I plaque number. Addition of rat AO increased ANG I plaque number to 17 +/- 2 (P less than 0.05). Incubation with both RI and AO prevented the increase in ANG I plaque number obtained with AO alone. Enalapril treatment (7 days; n = 5) increased the number of plaque-forming cells to 22 +/- 2 for ANG I (P less than 0.0005) and to 39 +/- 7 for renin (P less than 0.001). The results suggest an intracellular location for AO and angiotensin and release of renin and ANG I by renal cortical cells and suggest that released angiotensin is produced intracellularly and that secretion of ANG I is augmented by converting enzyme inhibition.

Identification of individual renocortical cells that secrete renin.

Successful application of the reverse hemolytic plaque assay was developed to identify individual renocortical cells that secrete renin directly. The plaque assay was validated by a number of established criteria. Using this technique, we demonstrate an increase in renin secretion with beta-adrenergic stimulation and an inhibition of renin secretion with extracellular calcium in groups of renin-secreting cells. Transmission electron microscopy of the cell in the center of a hemolytic plaque demonstrated a modified vascular smooth muscle cell with densely packed secretory granules. Electron microscopy immunocytochemistry demonstrated the presence of renin in the secretory granules, confirming the identity of the cell as a renal juxtaglomerular cell. The technology developed here has allowed the precise identification and study of the individual renin-secreting juxtaglomerular cell.

Cell and molecular studies of renin secretion.

Renin secretion has been studied in the past at the level of the whole kidney, but the control of the genetic basis of renin synthesis is poorly understood. We have studied the regulation of renin gene expression in the fetus and also in the adult rat in response to angiotensin converting enzyme (ACE) inhibition with enalapril in the presence and absence of angiotensin II (AII). In the fetus, vascular smooth muscle cells of the renal afferent arteriole and feeder vessels contain renin mRNA and immunostain for renin. With maturation, these vessels progressively lose the capacity to synthesize renin, and only the juxtaglomerular cells retain this capacity in the adult. However, in response to ACE inhibition, the adult renal feeder vessels acquire the capacity to synthesize and secrete renin within 7 days. This effect is partially reversed with co-administration of AII. In order to study renin biosynthesis and secretion at the cellular level, we have developed a new method of study of individual renin-secreting cells, the reverse hemolytic plaque assay (RHPA). Utilizing this method, we have demonstrated that ACE inhibition with enalapril increases the number of renin secreting cells by over 15-fold at physiologic calcium concentrations. Enalapril also induced a 3-fold increase in the amount of renin released as estimated by the area of the hemolytic plaques formed. Transmission electron microscopy (EM) of the renin-secreting cell at the center of a hemolytic plaque demonstrates modified vascular smooth muscle cells with cytoplasmic granules. In summary, ACE inhibition stimulates renin mRNA accumulation and redistributes renal renin content toward that observed in early fetal life. AII inhibits renal renin mRNA accumulation. ACE inhibition increases the number of renin secreting cells as well as the amount of renin secreted by each cell. The individual renin secreting cell is a modified vascular smooth muscle cell with cytoplasmic secretory granules. Further studies of the cellular pathways for renin secretion can be provided by EM immunocytochemistry of the individual renin secreting cell.

Adenosine-mediated regulation of cyclic AMP levels in isolated incubated retinas.

The effect of adenosine on the modulation of retinal cAMP levels was assessed in several mammalian species including mouse, rat, guinea pig and rabbit. Adenosine had no effect when added to incubated rat, mouse and guinea pig retinas. However, levels of cAMP were elevated in dose-dependent manner by adenosine in both light- and dark-adapted incubated rabbit retinas. Isobutylmethylxanthine (IBMX) blocked the elevation elicited by adenosine. Norepinephrine and dopamine also elevated cAMP in incubated rabbit retinas and these effects were not blocked by IBMX. The elevations of cyclic AMP levels produced by adenosine were additive with the effects of dopamine or norepinephrine. These results indicate that an adenosine-sensitive cAMP system exists in rabbit retina, and it is probably distinct from the dopamine and norepinephrine regulated cyclic AMP systems.