Physiological effects of the humoral renomedullary antihypertensive system

In experiments on rats, the physiological effects of the antihypertensive humoral system of the renal medulla were explored. We studied the parameters of blood pressure, heart rate, efferent sympathetic nervous activity to the splanchnic region, diuresis and natriuresis. The experimental situations included studies on normotensive rats, injected with extracted renomedullary lipids (medullipin), and normotensive rats extracorporally cross-perfusing an isolated kidney at increased pressure levels, to explore the release of renomedullary depressor substances. Spontaneously hyperactive rat (SHR) kidneys were also cross-perfused to study whether the renomedullary mechanism was reset upwards in this type of primary hypertension. Finally, the acute pressure reductions caused by angiotensin converting enzyme (ACE)-inhibitors were studied in intact and chemically medullectomized (2-bromo-ethylamine hydrobromide) rats to explore whether they were at least in part dependent on an intact renal medulla. In normal kidneys the renomedullary depressor system was initiated already at physiological pressure levels (at or above 100 mm Hg), causing the following humorally induced response pattern: the blood pressure fall was associated with decreases in both heart rate and sympathetic activity, while diuresis and natriuresis increased. In SHR kidneys, perfusion pressure had to be raised between 200 and 250 mm Hg before this depressor pattern was activated, revealing an "upward renomedullary resetting" well beyond the prevailing SHR blood pressure. The acute blood pressure decrease initiated in normotensive rats by an ACE-inhibitor (enalaprilat) was abolished in medullectomized rats, indicating an interaction between the renin-angiotensin and the renomedullary antihypertensive systems.