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.