The liver converts the antihypertensive hormone of the kidney. The renohepatic axis

Persistent Hypotension Associated with Hypermedullipinemia: A New Syndrome

A new syndrome is described in a patient with advanced renal insufficiency. This consists of severe and persistent hypotension causing weakness but associated with a clear mental status. Also present is evidence for decreased vascular reactivity. The hypotension was not orthostatic. The hypotension was associated with a circulating vasodepressor substance having the characteristics of medullipin 1. The medullipin appears to have been derived from the remaining right kidney. Hypotension existed despite the presence of major prohypertensive mechanisms, including an endstage kidney, hyperreninemia and hyperaldosteronemia. It is likely that hypotension due to hypermedullipinemia is an entity occurring in the human being.

The vasodepressor function of the kidney: prostaglandin E2 is not the principal vasodepressor lipid of the renal medulla

AIM

Whereas prostaglandin E2 has been characterized as the principal vasodepressor lipid, medullipin remains a hypothetical vasodepressor principle of the renal medulla. Representing the first step towards the isolation of medullipin as a pure compound, the aim of the present study was to determine whether or not the known vasodilator and antihypertensive action of prostaglandins play a role in the antihypertensive activity of renal medulla.

METHODS

A chloroform extract of porcine kidney medulla was fractionated by gradient vacuum liquid chromatography (VLC) and analysed by capillary GC-MS for the presence of prostaglandins (detection limit: 2.2 ppm). The biological activity was determined in spontaneously hypertensive Wistar rats. The particle size of injectable colloids prepared from extract and fractions was controlled by photon correlation spectroscopy.

RESULTS

The extract caused a pronounced blood pressure decline (29.6 +/- 6.3/24.9+/- 5.5 mmHg; P = 0.0078; 10 mg kg(-1) body weight; particle size of 143 +/- 18 nm; n = 7) lasting for more than 1 h. The heart rate remained stable, showing only a slightly decrease. All fractions were shown to be devoid of vasodilator prostanoid substances. The VLC procedure allowed the successful separation of endogenous emulsifiers from the active principle. An extract from the renal cortex did not exhibit a similar vasodepressor effect.

CONCLUSION

Prostaglandins are excluded as the blood pressure-lowering active principle of a total lipid kidney medulla extract. The vasodepressor principle is contained in the kidney medulla, but not in the cortex. It can be separated from endogenous emulsifying substances, is chromatographically stable, and is amenable to purification and chemical characterization.

Medullopressin: a new pressor activity from the renal medulla

The kidney contains blood pressure-lowering substances, such as prostaglandin E2 or prostaglandin A2, and blood pressure-raising substances such as thromboxane A2. Most of the postulated substances, however, have not yet been isolated in a pure state and are of unknown structure. In the present study, we separated a chloroform extract from the medulla of pig kidneys using various chromatography procedures. Each fraction was tested in spontaneously hypertensive Wistar rats. One of these fractions caused a powerful blood pressure increase of 30.1 +/- 7.1 mmHg systolic and 34.7 +/- 6 mmHg diastolic (N=7; p=0.0003), reaching its maximum 55 +/- 27 s after completion of the injection and lasting for 201 +/- 59 s. A long-lasting contractile response in porcine and bovine coronary artery rings was observed. In the mouse aortic rings, the contractile response accounted for 0.38 +/- 0.13 g, ie., 31.9 +/- 10.9% of the maximum potassium response (N=11; p=0.003). Because this activity could not be attributed to any known vasoactive substance, it was considered to arise from a novel underlying active substance in the kidney medulla, which we named medullopressin.

Evidence for a renomedullary vasodepressor hormone

1. Recent physiological experiments have established that increasing the perfusion pressure of the kidney causes the release of 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 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 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.

Purification of class I medullipins from the venous effluent of isolated normal kidneys perfused under high pressure with saline

Medullipin I (Med I) is a vasodepressor prohormone which is continuously elaborated into the renal venous effluent (RVE) of isolated rat kidneys perfused under high pressure. We have improved the yield of Med I by substituting saline for the albumin perfusate previously reported; and considerably improved refinement by directly fractionating the crude lipid extract of the RVE with high pressure liquid chromatography. The results show that Med I, as defined by previous physiologic and pharmacologic criteria, is not a single molecule. The 3 Class I medullipins described here are distinguished by subtle or overt differences in polarity and biologic activity.

Evidence for a renomedullary vasodepressor system in rabbits and dogs

Renal perfusion was increased in anesthetized rabbits and dogs by using an extracorporeal circuit. When left kidney perfusion pressure was raised in rabbits (145-240 mm Hg), arterial pressure fell by 1.34 +/- 0.20 mm Hg/min. Pretreatment of the rabbits with 2-bromoethylamine hydrobromide, which destroyed the renal medulla, abolished the fall in arterial pressure (-0.08 +/- 0.08 mm Hg/min) in response to increased renal perfusion pressure. In dogs (with blockade of autonomic ganglia by pentolinium, converting enzyme inhibition [captopril/enalaprilat], and surgical renal denervation), increasing renal perfusion pressure to 170-220 mm Hg resulted in a fall in arterial pressure by 0.32 +/- 0.03 mm Hg/min (or by 28.9 +/- 3.1 mm Hg over a 90-minute period). Mean arterial pressure did not change significantly in identically prepared dogs not subjected to increased renal perfusion pressure, whereas pretreatment of dogs with bromoethylamine abolished the hypotensive response to increased renal perfusion pressure. Thus, the hypotensive response to increased renal perfusion was dependent on the presence of an intact renal medulla, but hypotension still occurred in the presence of converting enzyme inhibition, autonomic ganglion blockade, and renal denervation. The results provide in vivo evidence in two species that a vasodepressor factor from the renal medulla is released in response to increased renal perfusion.

Secretion of medullipin I by isolated kidneys perfused under elevated pressure

1. Medullipin I (Med I) is a hormone extracted from renal papillae and its renomedullary interstitial cells (RIC). Med I is stimulated by elevation of the renal artery perfusion pressure. 2. When isolated normal rat kidneys were perfused either with oxygenated blood or with 5% albumin bubbled with O2 at elevated perfusion pressures, Med I appeared to be secreted into the renal venous effluent (RVE). Addition of Tween 20, treatment of the assay rat with SKF 525A, inhibitor of cytochrome P-450 and removal of the liver from the systemic circulation prevented vasodepression of both the RVE and extracted Med I. The lipid in the RVE gave the same dose-response as extracted Med I. 3. Lowering the renal artery perfusion pressure below normal inhibited the secretion of Med I. As the perfusion pressure was elevated Med I secretion appeared to increase. 4. Previous observations and the present study support the view that the renin-angiotensin system and the Medullipin system are double feedback systems involved in blood pressure control.

Giraffes, rats and man--what is the importance of the 'structural factor' in normo- and hypertensive states?

1. The normal structural adaptation of heart and vessels to regional changes in load or/and tissue demands is surveyed with respect to its importance for cardiovascular function in normotension as well as in physiological (giraffes) and pathophysiological (e.g. human and rat primary hypertension) variants of high pressure states. 2. At the local level it implies an entirely appropriate adjustment of cardiovascular geometric design according to principles inherent in the LaPlace and Poiseuille laws. However, when generalized to all systemic circuits, as in primary hypertension, it invites to a potentially dangerous positive feedback interaction with even ordinary functional pressor influences. 3. It is further emphasized how resistance vessels, besides the obvious influence of changes in: (i) vascular smooth muscle activity, are greatly affected also by changes of their (ii) geometric design, (iii) wall distensibility, and (iv) transmural pressure, how each of these four parameters can be independently altered and how they interact. Genetic reinforcements and various trophic influences may facilitate the extent and rate of 'structural upward resetting' in primary hypertension, and this resetting also encompasses the barostat mechanisms. 4. Against such a background it is, in fact, from a physiological point of view, more difficult to explain how 85-90% of the population manage to maintain lifelong normotension than to explain why hypertension gradually afflicts the remaining 10-15%. It points to the presence of potentially powerful and durable, negative feedback that are still poorly understood (e.g. Muirhead's renomedullary depressor system).(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effect of the contralateral kidney on the renal pressor reflex evoked by ipsilateral renal artery stenosis

We previously identified a sympathoexcitatory pressor reflex that is evoked by acute (90 min) unilateral renal artery stenosis (RST). The reflex is strongly expressed in conscious rats when the renin-angiotensin system and baroreflexes are diminished by captopril and sinoaortic denervation, and is eliminated by denervation of the stenosed kidney. In the present study, dorsal rhizotomy inhibited the reflex, establishing the role of afferent renal nerves (ARN). During sustained (10 h) RST, the reflex reversed within 4 hours after onset of RST, but persisted after removal of the contralateral kidney. Hematocrit measurements did not support pressure diuresis as a mechanism for inhibition by the contralateral kidney. Other studies suggest that ARN signals and/or release of antihypertensive renomedullary lipid from the contralateral kidney are not involved in inhibition of the reflex. These data suggest that the contralateral kidney can oppose, at least acutely by an as yet undefined mechanism, an ARN-dependent pressor reflex evoked by ipsilateral renal artery stenosis.