THE ROLE OF THE MACULA DENSA IN RENIN SYNTHESIS

1. The role of the macula densa in renin synthesis was studied using mice with one hydronephrotic kidney. 2. Renin synthesis was assessed by measurement of renal renin, renal mRNA for renin and plasma renin. 3. Sodium depletion stimulated mRNA and renal renin to a similar extent in the hydronephrotic and contralateral kidney. 4. Enalapril stimulated mRNA concentration in both kidneys but renal renin did not rise in the hydronephrotic kidney. 5. Propranolol did not alter the response to sodium depletion in either kidney. 6. The macula densa is not crucial for the stimulation of renin synthesis following sodium depletion. However, it may regulate renin production after mRNA synthesis, possibly by controlling the conversion of prorenin to renin.     

Changes in mean arterial pressure predict degranulation of renomedullary interstitial cells

1. Renomedullary interstitial cells (RMIC) are characterized by numerous intracellular granules thought to contain renal medullary antihypertensive substances. However, the nature of the trigger for RMIC degranulation remains to be elucidated. The present study examines the effects of acute alterations in mean arterial pressure (MAP) and medullary blood flow (MBF) on RMIC granulation. 2. Basal MAP and MBF in anaesthetized Sprague-Dawley rats (n = 4/group) were altered by intravenous infusions of vasoactive agents, including angiotensin II alone or with a nitric oxide (NO) synthase inhibitor (N-omega-nitro-l-arginine) or NO donor (sodium nitroprusside), noradrenaline and by carotid artery clamping. Following these treatments, kidneys were examined by electron microscopy and the absolute volume of granules in the renal medulla was calculated using unbiased stereological methods. 3. Acute increases in MAP, regardless of the treatment causing the increase, were associated with a reduction in the absolute volume of granules in the range of 42-67%. Regression analysis revealed that only increases in MAP, but not MBF, strongly predict RMIC degranulation. 4. Despite previous reports that changes in MBF activate renomedullary antihypertensive activity, we conclude that the change in MAP is an important determinant of the activity of the blood pressure-lowering mechanism of the renal medulla, with the assumption that the medullary lipids mediate the antihypertensive property of the renal medulla.     

Imaging of renal medullary interstitial cells in situ by confocal fluorescence microscopy

 Renal medullary interstitial cells are a prevalent and characteristic feature of the inner medulla of the kidney, but the physiological significance of this is unclear. We have developed a method for imaging renal medullary interstitial cells in situ by loading the cells with fluorescent dyes and monitoring their distribution using confocal microscopy. The pH-sensitive probe 2’7’-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester was used as a marker of cytoplasmic volume and therefore of cell morphology. Nile Red was used to demonstrate the presence of renal medullary interstitial cell lipid droplets. Papillae were excised from 100 g Sprague-Dawley rats and loaded with the appropriate dye. The papillae were then examined using a Leica TCS 4D confocal microscope and oil immersion lenses. Fluorescence was excited (488 nm) using an argon laser and emission wavelengths above 515 nm collected using a long pass filter. Images of papillae loaded with 2’7’-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester clearly demonstrate a ladder-like arrangement of renal medullary interstitial cells. More detailed examination revealed the presence of cytoplasmic extensions that appear to make close contact with adjacent loops of Henle. Three-dimensional reconstructions of serial sections revealed spiral arrangements in some ladders of renal medullary interstitial cells. Nile Red-labelled lipid droplets of 0.5–1.0 μm diameter were located throughout the cytoplasm of renal medullary interstitial cells and especially within the cytoplasmic extensions. These experiments highlight the ability of confocal microscopy to allow investigation of renal medullary interstitial cells in situ. Accepted: 3 December 1998     

EVIDENCE FOR A RENOMEDULLARY VASODEPRESSOR HORMONE

1. Recent physiological experiments have established that increasing the perfusion pressure of the kidney causes the release of a vasodepressor substance from the renal medulla. 2. The substance is not a platelet activating factor, a prostaglandin or nitric oxide and the vasodepressor response to increased renal perfusion pressure is not due simply to inhibition of renin release. 3. The mechanisms by which the renomedullary vasodepressor substance lowers arterial pressure remain to be determined. Sympathoinhibition may account for part of the response, but the hypotension still occurs in autonomic ganglion blocked animals. 4. The source of the substance appears to be the renomedullary interstitial cells, though the control of the production and release of the substance remain to be determined. 5. The substance may be a lipid but it is yet to be fully isolated and identified. 6. The threshold for release of the substance appears to be close to normal resting arterial blood pressure. 7. Despite strong evidence that the renal medulla releases a vasodepressor hormone in response to increased renal perfusion pressure, much is still to be determined regarding the physiology of this hormone and its involvement in the aetiology of hypertension.