Acute Stimulation Of Renin Release Research Articles

Expression of the Transmembrane Lysosomal Protein SCARB2/Limp-2 in Renin Secretory Granules Controls Renin Release

Background/Aims: Renin processing and storage is believed to occur in lysosome-like structures in the afferent arteriole. SCARB2/Limp-2 is a transmembrane lysosomal protein responsible for the intracellular trafficking of β-glucocerebrosidase. This study aimed to confirm the expression of SCARB2/Limp-2 in renin secretory granules, and explore its role in renin processing and secretion. Methods: Co-localisation studies of (pro)renin with lysosomal membrane proteins, SCARB2/Limp-2, LAMP-1 and LAMP-2, were performed in mouse and human kidney sections. Intrarenal expression and secretion of (pro)renin in wild-type (WT) and Limp-2^-/- mice were compared with and without stimulation. Results: SCARB2/Limp-2, LAMP-1 and LAMP-2 co-localised with (pro)- renin in mouse and human kidney. Plasma renin concentration was increased in Limp-2^-/- mice when compared to WT littermates. No change in (pro)renin expression, however, was observed in Limp-2^-/- mouse kidney cortex by immunofluorescence microscopy, Western blotting, quantitative RT-PCR or the ultrastructural appearance of renin secretory granules. Acute stimulation of renin release by isoprenaline or hydralazine was similar in WT and Limp-2^-/- mice. Following chronic salt restriction, however, immunofluorescence microscopy showed less (pro)renin expressed in Limp-2^-/- compared with WT mouse kidneys, and there was significantly less prorenin but not renin by Western blotting in Limp-2^-/- mouse kidney cortex, despite no difference in circulating renin levels. Conclusion: Renin secretory granules possess integral lysosomal proteins, confirming that they are indeed modified lysosomes. Limp-2 deficiency leads to a minor increase in circulating renin. Limp-2, however, is not required for acute or chronic stimulation of renin release.

Structural analysis suggests that renin is released by compound exocytosis

The mode of renin release from renal juxtaglomerular cells into circulation is still unsolved in several aspects. Here we studied the intracellular organization of renin-storage vesicles and their changes during controlled stimulation of renin release. This was accomplished using isolated perfused mouse kidneys with 3-dimensional electron microscopic analyses of renin-producing cells. Renin was found to be stored in a network of single granules and cavern-like structures, and dependent on the synthesis of glycosylated prorenin. Acute stimulation of renin release led to increased exocytosis in combination with intracellular fusion of vesicles to larger caverns and their subsequent emptying. Renin release from the kidneys of SCID-beige mice, which contain few but gigantic renin-storage vesicles, was no different from that of kidneys from wild-type mice. Thus, our findings suggest that renin is released by mechanisms similar to compound exocytosis.

Parallel regulation of renin and lysosomal integral membrane protein 2 in renin-producing cells: further evidence for a lysosomal nature of renin secretory vesicles

The protease renin is the key enzyme in the renin-angiotensin system (RAS) that regulates extracellular volume and blood pressure. Renin is synthesized in renal juxtaglomerular cells (JG cells) as the inactive precursor prorenin. Activation of prorenin by cleavage of the prosegment occurs in renin storage vesicles that have lysosomal properties. To characterize the renin storage vesicles more precisely, the expression and functional relevance of the major lysosomal membrane proteins lysosomal-associated membrane protein 1 (LAMP-1), LAMP-2, and lysosomal integral membrane protein 2 (LIMP-2) were determined in JG cells. Immunostaining experiments revealed strong coexpression of renin with the LIMP-2 (SCARB2), while faint staining of LAMP-1 and LAMP-2 was detected in some JG cells only. Stimulation of the renin system (ACE inhibitor, renal hypoperfusion) resulted in the recruitment of renin-producing cells in the afferent arterioles and parallel upregulation of LIMP-2, but not LAMP-1 or LAMP-2. Despite the coregulation of renin and LIMP-2, LIMP-2-deficient mice had normal renal renin mRNA levels, renal renin and prorenin contents, and plasma renin and prorenin concentrations under control conditions and in response to stimulation with a low salt diet (with or without angiotensin-converting enzyme (ACE) inhibition). No differences in the size or number of renin vesicles were detected using electron microscopy. Acute stimulation of renin release by isoproterenol exerted similar responses in both genotypes in vivo and in isolated perfused kidneys. Renin and the major lysosomal protein LIMP-2 are colocalized and coregulated in renal JG cells, further corroborating the lysosomal nature of renin storage vesicles. LIMP-2 does not appear to play an obvious role in the regulation of renin synthesis or release.

Regulation of Renin Secretion and Expression in Mice Deficient in β1- and β2-Adrenergic Receptors

The present experiments were performed in beta1/beta2-adrenergic receptor-deficient mice (beta1/beta2ADR(-/-)) to assess the role of beta-adrenergic receptors in basal and regulated renin expression and release. On a control diet, plasma renin concentration (in ng angiotensin I per mL per hour), determined in tail vein blood, was significantly lower in beta1/beta2ADR(-/-) than in wild-type (WT) mice (222+/-65 versus 1456+/-335; P<0.01). Renin content and mRNA were 77% and 65+/-5% of WT. Plasma aldosterone (in picograms per mL) was also significantly reduced (420+/-36 in beta1/beta2ADR(-/-) versus 692+/-59 in WT). A low-salt diet (0.03%) for 1 week increased plasma renin concentration significantly in both beta1/beta2ADR(-/-) and WT mice (to 733+/-54 and 2789+/-555), whereas a high-salt diet (8%) suppressed it in both genotypes (to 85+/-24 in beta1/beta2ADR(-/-) and to 676+/-213 in WT). The absolute magnitude of salt-induced changes of plasma renin concentration was markedly greater in WT mice. Acute stimulation of renin release by furosemide, quinaprilat, captopril, or candesartan caused significant increases of plasma renin concentration in both beta1/beta2ADR(-/-) and WT mice, but again the absolute changes were greater in WT mice. We conclude that maintenance of normal levels of renin synthesis and release requires tonic beta-adrenergic receptor activation. In the chronic absence of beta-adrenergic receptor input, the size of the releasable renin pool decreases with a concomitant reduction in the magnitude of the plasma renin concentration changes caused by variations of salt intake or acute stimulation with furosemide, angiotensin-converting enzyme, or angiotensin type 1 receptor inhibition, but regulatory responsiveness is nonetheless maintained.

Plasma noradrenaline and dopamine in renin‐mediated hypertension

Noradrenaline (NA) and dopamine (DA) have opposite effects on the kidney; NA causes vasoconstriction and increased sodium reabsorption while DA promotes vasodilation and natriuresis. In 15 patients investigated for renin-mediated hypertension measurements of plasma renin activity (PRA), NA and DA concentrations were made in arterial and renal venous blood from both kidneys before and after acute stimulation of renin release by i.v. dihydralazine. Nine patients had unilateral renin secretion and were classified as renin-positive, while the remaining six patients were renin-negative. Renin-positive patients had higher arterial and renal venous PRA, NA and DA levels than the negative ones. In the renin-positive group V-A differences for NA and DA were present on both sides despite unilateral secretion of renin. NA but not DA levels were higher in the renin-secreting kidney, which can partly be explained by the reduced plasma flow to the involved kidney. After dihydralazine the arterial NA and DA rose similarly in renin-positive and renin-negative patients, while PRA rose only in the renin-positive cases. In the renin-positive patients where stimulation of renin secretion caused a marked increase of the PRA gradient on the affected side only, renal gradients for NA and DA increased bilaterally. The increase in DA was more pronounced than that of NA yielding a rise in DA/NA ratio on the affected side. Arterial PRA was positively correlated to the plasma concentrations of NA and DA. V-A differences for PRA and NA or DA were positively correlated on the involved renin-secreting side. In summary, patients with renin-dependent hypertension have elevated plasma NA and DA concentrations. Stimulation of renin release by dihydralazine increases the DA/NA ratio in arterial and renal venous blood indicating release of 'precursor dopamine' from noradrenergic fibres and/or activation of dopaminergic nerves. There seems to be a relationship between renal nerve activity and renin release in renin-dependent hypertension.

Preoperative Diagnosis of Renovascular Hypertension

The gain in the diagnostic power of the routine preoperative bilateral renal vein renin investigation by the inclusion of an acute stimulation of renin release was studied in 25 hypertensive patients with fibromuscular (FMD) and 44 with arteriosclerotic (AS) renovascular disease. Nine FMD and 17 AS patients had renal vein renin ratios greater than or equal to 1.50 under unstimulated conditions. Of these 26 patients, 8 in each group improved after surgical treatment. Among the 14 FMD and 24 AS patients with ratios less than 1.50, another 9 FMD and 7 AS patients improved after operation. After acute stimulation of renin release, no less than 18 FMD and 26 AS patients had a unilateral renin secretion from the diseased kidney and of these, 14 FMD and 14 AS patients were successfully treated with operation of the stenosis. Thus the stimulation was necessary for correct preoperative diagnosis in 38% of 32 successfully operated patients, and at the same time the predictive value of a negative test increased from 0.58 to 0.80. It is concluded that acute stimulation of renin release greatly improves the diagnostic power of the renal vein renin investigation in renovascular hypertension.

Effect of furosemide and dietary sodium on kidney and plasma big and small renin.

AbstractRenin was found in mouse plasma in high-molecular-weight forms (big big renin, big renin) and a low-molecular-weight form (small renin). They were measured by a radioimmunoassay procedure for the direct measurement of renin. In the kidney, 89% of total renin was small renin and the rest was big big and big renin. This distribution pattern of renins was not changed when the kidney tissue was homogenized in the presence of protease inhibitors. Low-sodium or high-sodium diets changed renal renin content, but not the distribution pattern of renins in the kidney. Acute stimulation of renin release by furosemide increased small renin but not big big and big renin in plasma. However, dietary sodium depletion for 2 weeks significantly increased big big, big, and small renin in plasma of mice with or without submaxillary glands. In contrast, high-sodium intake significantly decreased big big, big, and small renin in plasma of mice with or without submaxillary glands.

Evidence for activation of circulating inactive renin by the human kidney.

1. In eight patients with essential hypertension (EHT) and six patients with renovascular hypertension (RVHT) peripheral venous enzymatically active and inactive renin values were followed after acute stimulation of renin release by the vasodilating agent diazoxide (300 mg intravenously). Active renin rose during the first hour after diazoxide and remained high during the following 15 h, but inactive renin fell during the first hour and rose thereafter. Peripheral venous active and inactive renin were not different from arterial values both before and after diazoxide. 2. Sixteen patients with EHT received propranolol, 80 mg, four times a day. Six of them had a first injection of diazoxide the day before propranolol was started and a second one after 10--14 days of propranolol treatment. Peripheral vein active renin was lowered by propranolol, but inactive renin was raised. Both the diazoxide-induced rapid rise of active renin and the fall of inactive renin observed in untreated patients were absent during treatment with propranolol. 3. In four patients with EHT and seven patients with RVHT renal vein sampling was performed before and 30 min after diazoxide. Increased release of active renin from kidneys that were not markedly contracted was associated with a fall of the renal vein to peripheral vein ratio of inactive renin to a value less than one. 4. It is concluded that under certain circumstances stimulated release of active renin is associated with removal of inactive renin from the circulation by the kidney. This may in fact be due to intrarenal transformation of circulating inactive renin into its active counterpart. The findings suggest that a beta-adrenoreceptor might be involved in this activation process.

MEASUREMENT OF PLASMA RENIN CONCENTRATION USING EXOGENOUS HUMAN RENIN SUBSTRATE IN NORMAL SUBJECTS : CORRELATION WITH PLASMA RENIN CONCENTRATION AND PLASMA ALDOSTERONE CONCENTRATION

Measurement of plasma renin concentration (PRC) was done in normal subjects at rest and under acute stimulation of renin release under unrestricted sodium intake. Concurrent measurements of plasma renin activity (PRA) and plasma aldosterone concentration (PA) were carried out. The mean values of PRC at rest and after stimulation of renin release were 12.8 +/- 1.3 (SEM) and 21.7 +/- 4.4 (SEM) ng AT I/ml/h, respectively. These corresponded to renin contents of 3.4 +/- 0.34 (SEM) X 10(-5) Goldblatt units and 5.8 +/- 0.36 (SEM) respectively. The mean percent increase of PRC (82.1 +/- 19.3 (SEM)) %) was almost indentical to that of PA (81.5 +/- 16.4 (SEM) %), but differed from that of PRA (269 +/- 83.1 (SEM) %). A very high correlation between concurrent PRC and PA (r = 0.92, P less than 0.001) was found in normal subjects at rest and under acute stimulation of renin release. A good correlation between PRC and PRA (r = 0.85, P less than 0.001) was also observed. However, a higher correlation between percent increases of PRC and PA (r = 0.92, P less than 0.001) than that of PRA and PA (r = 0.80, 0.01 less than P less than 0.005) was found. Results show that PRA is a good index of the renin content in plasma in normal subjects at rest and PRC reflects actual renin concentration in plasma at rest as well as under stimulation of renin release.

INACTIVE RENIN IN HUMAN PLASMA

Human plasma contains renin, which is enzymatically active at neutral pH (active renin), and a non-dialysable factor, which has renin-like activity after treatment at low pH (inactive renin). In vitro activated plasma-renin and purified human renal renin showed identical enzyme-kinetic properties. Quantitative estimations of inactive renin in renal venous plasma from 5 patients with renal-artery stenosis demonstrated its release by the kidney. Acute stimulation of renin release by isoprenaline, tilting, or diazoxide in 13 normotensive individuals and in 9 patients with essential hypertension increased active plasma-renin and reduced inactive plasma-renin. Inactive plasma-renin was increased and active plasma-renin decreased during suppression of renin release by propranolol in 12 patients with essential hypertension. In 55 patients with various disorders, inactive and active plasma-renin were directly correlated. However, the concentration of inactive renin, for a given value of active renin, varied widely from patient to patient. These results indicate that so-called inactive renin is indeed physiologically related to active renin. They also suggest that inactive renin can be activated not only in vitro, but also in vivo. Different renin assays measure different relative amounts of active and inactive renin. This may call for reinterpretation of results obtained by various methods, especially in situations where changes in plasma concentrations of the two forms of renin are in opposite directions.

Hypertension Due to Renal Tuberculosis: Assessment by Renal Vein Renin Sampling

A 36-year-old man with asymptomatic hypertension was shown to have destruction of the right kidney due to renal tuberculosis. The peripheral renin level was normal, but renal vein renin sampling showed predominant renin secretion from the right kidney both in basal samples and after acute stimulation of renin release with intravenous diazoxide. Nephrectomy has resulted in marked reduction of blood pressure without treatment one year after operation. The findings support the predictive value of renal vein renin sampling when hypertension is associated with renal parenchymal disease, even when peripheral renin is normal.