Activation of the humoral antihypertensive system of the kidney increases diuresis

Diuretic, Natriuretic, and Vasodepressor Activity of a Lipid Fraction Enhanced in Medium of Cultured Mouse Medullary Interstitial Cells by a Selective Fatty Acid Amide Hydrolase Inhibitor

The relationship between the endocannabinoid system in the renal medulla and the long-term regulation of blood pressure is not yet understood. To investigate the possible role of the endocannabinoid system in renomedullary interstitial cells, mouse medullary interstitial cells (MMICs) were obtained, cultured, and characterized for their responses to treatment with a selective inhibitor of fatty acid amide hydrolase, PF-3845 (N-3-pyridinyl-4-[[3-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenyl]methyl]-1-piperidinecarboxamide). Treatment of MMICs with PF-3845 increased cytoplasmic lipid granules detected by Sudan Black B staining and multilamellar bodies identified by transmission electron microscopy. High-performance liquid chromatography (HPLC) analyses of lipid extracts of MMIC culture medium revealed a 205-nm absorbing peak that showed responsiveness to PF-3845 treatment. The biologic activities of the PF-3845-induced product (PIP) isolated by HPLC were investigated in anesthetized, normotensive surgically instrumented mice. Intramedullary and intravenous infusion of PIP at low dose rates (0.5-1 area units under the peak/10 min) stimulated diuresis and natriuresis, whereas these parameters returned toward baseline at higher doses but mean arterial pressure (MAP) was lowered. Whereas intravenous bolus doses of PIP stimulated diuresis, the glomerular filtration rate, and medullary blood flow (MBF) and reduced or had no effect on MAP, an intraperitoneal bolus injection of PIP reduced MAP, increased MBF, and had no effect on urine parameters. These data support a model whereby PF-3845 treatment of MMICs results in increased secretion of a neutral lipid that acts directly to promote diuresis and natriuresis and indirectly through metabolites to produce vasodepression. Efforts to identify the structure of the PF-3845-induced lipid and its relationship to the previously proposed renomedullary antihypertensive lipids are ongoing.

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.

Mechanisms underlying the antihypertensive functions of the renal medulla

There is good evidence that the renal medulla plays a pivotal role in long-term regulation of blood pressure. 'Renal medullary' blood pressure regulating systems have been postulated to involve both exocrine (pressure natriuresis/diuresis) and endocrine [renal medullary depressor hormone (RMDH)] functions. However, recent studies indicate that pressure diuresis/natriuresis dominates the antihypertensive renal response to increased renal perfusion pressure, suggesting little physiological role for a putative RMDH in compensatory responses to acutely increased blood pressure. The medullary circulation appears to play a key role in mediating pressure diuresis, although the precise mechanisms involved remain controversial. Counter-regulatory vasodilator mechanisms (e.g. nitric oxide), at least partly mediated through cross-talk between the vasculature and the tubular epithelium, protect the medullary circulation from the vasoconstrictor effects of hormonal factors such as angiotensin II. These mechanisms also appear to contribute to compensatory responses to increased salt intake in salt-resistant individuals. Failure of these mechanisms predisposes the organism towards the development of hypertension, appears to underlie the development of some forms of experimental hypertension, and may even contribute to the pathogenesis of essential hypertension.

Importance of the renal medullary circulation in the control of sodium excretion and blood pressure

The control of renal medullary perfusion and the impact of alterations in medullary blood flow on renal function have been topics of research interest for almost four decades. Many studies have examined the vascular architecture of the renal medulla, the factors that regulate renal medullary blood flow, and the influence of medullary perfusion on sodium and water excretion and arterial pressure. Despite these studies, there are still a number of important unanswered questions in regard to the control of medullary perfusion and the influence of medullary blood flow on renal excretory function and blood pressure. This review will first address the vascular architecture of the renal medulla and the potential mechanisms whereby medullary perfusion may be regulated. The known extrarenal and local systems that influence the medullary vasculature will then be summarized. Finally, this review will present an overview of the evidence supporting the concept that selective changes in medullary perfusion can have a potent influence on sodium and water excretion with a long-term influence on arterial blood pressure regulation.

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.

The role of nephrectomy and proadifen in blood pressure homeostasis following an acute kinin-induced hypotension in normotensive rats

1. We have, in the present work, studied the importance of the kidneys and the renal hypotensive agent, medullipin, in modulating the blood pressure (BP) response to bradykinin, as well as their ability to influence the balance between the NO- and the adrenergic systems superimposed on a bradykinin-induced hypotension. 2. The rats were pretreated with the NO-synthase inhibitor, N omega-nitro-L-arginine methyl esther (L-NAME) (0.3 g kg-1), proadifen (50 mg kg-1), an inhibitor of the medullipin-system, and nephrectomy (24 h) (Nx) alone, and L-NAME in combination with proadifen, Nx or phentolamine (2 mg kg-1). Subsequent injections of bradykinin (3, 6, 15, 30 micrograms kg-1) induced an acute hypotensive response. The fall in BP was dose-dependent in all groups (P < 0.01), except in the Nx/L-NAME group. No differences in the fall in BP were observed between the groups. 3. The duration of the hypotensive response was abbreviated after L-NAME-treatment (P < 0.05). Proadifen-treatment and Nx had no significant effect on the duration of the hypotension in control rats or in L-NAME-treated rats. Pretreatment with phentolamine prevented the L-NAME-induced rapid restoration of BP (P < 0.001). 4. In L-NAME-treated rats a transient hypertension followed the bradykinin-induced hypotensive response. This hypertensive response was not observed after Nx or proadifen-treatment alone, and addition of Nx or proadifen to L-NAME treatment did not alter the hypertensive response as compared to L-NAME alone. Phentolamine, however, abolished the L-NAME-induced hypertension (P < 0.05). 5. In conclusion, the present results do not support the involvement of the medullipin-system or other hypotensive systems localized in the kidneys, in modulating and counteracting the compensatory adrenergic response following an acute bradykinin-induced hypotension. A hampering modulating effect of the NO-system on this compensatory adrenergic response was confirmed, indicating a close relationship between these two systems in BP homeostasis.

Efferent renal nerve stimulation inhibits the antihypertensive function of the rat renal medulla when studied in a cross-circulation model

The aim of this study was to investigate the effects of renal nerve stimulation on the humoral renal antihypertensive system. An isolated kidney (IK) was perfused at normal or high arterial pressures from a normotensive assay rat by means of a perfusion pump. Perfusion pressure (PP) to the IK was 90 mmHg for a control period of 30 min. In three of five experimental groups PP was then increased to 175 mmHg. In two of the groups the renal nerves were stimulated at 2 (P-175(2Hz)) or 5 Hz (P-175(5Hz)) for 60 min. The remaining group served as a control (P-175C). In two groups IK pressure was maintained at 90 mmHg with 5 Hz nerve stimulation (P-90(5Hz) or without nerve stimulation (P-90C). MAP of the assay rat decreased by 22 and 27% (P < 0.001) in the P-175C and P-175(2Hz) groups, respectively during the 60 min period of nerve stimulation, but remained stable in P-175(5Hz). Renal blood flow increased in the IK when PP was increased in P-175C, but did not change significantly in P-175(2Hz) or P-175(5Hz). Blood pressure remained constant in the assay rat when the IK was perfused at 90 mmHg. The renal excretory functions of the IK decreased in a frequency dependent manner by 2 and 5 Hz renal nerve stimulation compared with P-175C. We conclude that 5 Hz renal nerve stimulation inhibits the pressure dependent release of humoral depressor substances from an IK perfused at 175 mmHg, whereas this is not seen when stimulating at 2 Hz. It is suggested that hte release of antihypertensive substances from the renal medulla requires an increased renomedullary blood flow.

Renal and haemodynamic effects of nitric oxide blockade in a Wistar assay rat during high pressure cross-circulation of an isolated denervated kidney

Blockade of NO synthesis with N-omega-nitro-L-arginine (L-NNA) inhibits the vasodepressor response seen in intact Wistar assay rats in which isolated kidneys perfused via an extracorporeal circuit are perfused at high pressure. This study explores the renal and haemodynamic changes associated with this inhibition. Isolated kidneys (IK) were perfused at high pressure (175 mmHg) by a pump in series with intact Wistar assay rats in which blood pressure (BP), haemodynamics and renal function were studied. Nitric oxide (NO) synthesis was blocked by L-NNA (2.5 mg kg-1) in 13 experiments (175NO) while 14 control experiments (175C) were performed. IK was perfused at 90 mmHg in seven experiments (90C). The BP drop in the 175C assay rat was blocked by L-NNA in 175NO (P < 0.01). However, when the blockade was reversed with L-arginine infusion (20 mg kg-1 min-1) BP declined also in 175NO. Effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) fell dramatically after L-NNA in both the assay rat and in IK despite a high perfusion pressure. The marked increase in filtration fraction (FF) after L-NNA suggests a dominating postglomerular vasoconstriction. The natriuretic response in IK to 175 mmHg was also markedly blunted by L-NNA. We conclude that NO blockade inhibits the renomedullary depressor mechanism probably by restricting renal blood flow, and also blunts the pressure induced natriuretic response as a result of a reduced sodium filtration. Finally, the autoregulation of whole kidney blood flow seems to be more efficient although set at a higher level of vasoconstriction.

Nitric oxide inhibition does not prevent the hypotensive response to increased renal perfusion in rabbits

1. The involvement of nitric oxide (NO) and platelet activating factor (PAF) in the systemic depressor responses to increased renal perfusion pressure (RPP) were investigated. 2. In anaesthetized rabbits, the left kidney was perfused via an extracorporeal circuit which allowed RPP to be increased from 65 mmHg to 125 mmHg. The response of systemic blood pressure (SBP) to increasing RPP was measured in the same rabbits. 3. One group of rabbits (n = 5) was treated with NG-nitro-L-arginine (NOLA) to inhibit NO synthase activity (20 mg/kg i.v. bolus). Another group (n = 5), received 250 mmol/L NaHCO3 (4 mL/kg bolus) as vehicle treatment. 4. Following an increase in RPP to 125 mmHg, SBP fell at a rate of 0.43 +/- 0.06 mmHg/min in the vehicle treated rabbits. After NO synthase inhibition the rate of fall in SBP of 0.34 +/- 0.07 mmHg/min was not significantly different from that in the vehicle group (P = 0.3). 5. Blockade of NO synthesis did not alter the renal blood flow, renal vascular resistance changes and pressure-related natriuresis and diuresis responses to increased RPP to 125 mmHg. 6. PAF receptor blockade, using WEB 2086 (0.5 mg/kg plus 0.5 mg/kg/h), did not alter the systemic, renal haemodynamic or urinary responses to increasing renal perfusion pressure to 125 mmHg. 7. These findings indicate that neither NO nor PAF play an important role in the blood pressure lowering activity, intrarenal haemodynamics and urinary excretory responses observed when RPP was increased to a level within the physiological range.

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.

Pressure range for release of renomedullary depressor substance in rabbits

We investigated the relation between renal perfusion pressure and the release of a renal vasodepressor substance in vivo to determine whether this substance was released at physiological pressures. We perfused the left kidneys of anesthetized rabbits using an extracorporeal circuit that allowed renal perfusion pressures to be set at 65 mm Hg (control) and increased to 95, 125, 155, or 185 mm Hg for 30-minute experimental periods. Systemic blood pressure did not change significantly when renal perfusion pressure was maintained at 65 mm Hg throughout. When renal perfusion pressure was increased to 95, 125, 155, or 185 mm Hg, systemic blood pressure fell significantly at rates of 0.17 +/- 0.04, 0.79 +/- 0.31, 0.60 +/- 0.11, and 2.18 +/- 0.79 mm Hg/min, respectively (P < .05). Restoration of renal perfusion pressure to 65 mm Hg abruptly reversed the falls in systemic blood pressure in each group. There was a natriuresis and diuresis that were both pressure related and progressive in the face of each constant level of increased renal perfusion pressure. In summary, there was a continuum of arterial vasodepressor responses across a renal perfusion pressure range from resting pressure to 185 mm Hg. We suggest that the threshold level for the release of significant amounts of a renal medullary depressor substance, probably medullipin, is just above normal arterial blood pressure and that the rate of release increases with increasing arterial pressure.

Hypertension and the kidney--an overview

The kidney plays a major role in the genesis of any type of hypertension, as demonstrated by experiments which show that hypertension can be "transplanted" when the kidney itself is transplanted. Hypertension is common in patients with renal disease, and may occur even at normal glomerular filtration rates. The mechanisms that promote hypertension and are involved in renal disease comprise both activation of pressor mechanisms and failure of depressor mechanisms, the latter having been considerably less well studied. The major pressor mechanisms are an abnormal pressure-natriuresis relationship and inappropriate activity of the renin-angiotensin system.

Mediators of the hypotensive response to increased renal perfusion in rabbits

We have previously shown that increasing the renal perfusion pressure by using an extracorporeal circuit in anesthetized rabbits resulted in a progressive fall in systemic arterial pressure. Prior ablation of the renal medulla with 2-bromoethylamine abolished the hypotensive response. In the present study, we investigated whether vasodilator prostanoids or platelet activating factor (PAF), both known to be produced in the renal medulla, were responsible for the hypotensive response to increased renal perfusion pressure. Anesthetized animals were treated with indomethacin (5 mg/kg + 0.5 mg/kg per hour), the PAF antagonist WEB 2086 (0.5 mg/kg + 0.5 mg/kg per hour), enalaprilat (2 mg/kg + 10 micrograms/kg per hour), or all three agents. In response to acute elevation of renal artery pressure to 170 mm Hg, systemic mean arterial pressure fell at 0.76 +/- 0.17, 0.59 +/- 0.08, and 0.76 +/- 0.17 mm Hg/min in the indomethacin, WEB 2086, and enalapril groups, respectively. These responses were not significantly different from the rate of 1.00 +/- 0.21 mm Hg/min in a control group that received vehicle infusion alone. Renal blood flow and the diuretic and natriuretic responses were also similar in all groups. Thus, increased renal perfusion pressure resulted in a progressive fall in systemic arterial pressure that was not mediated by PAF, prostaglandins, or suppression of renin release and angiotensin II production.

Biologic differences between vasodilator prostaglandins and medullipin I

Vasodepressor prostaglandins (PGs), PGE2, PGI2, and medullipin I (Med I) are synthesized in the kidney. These vasodilator substances are thought to be involved in the antihypertensive function of the kidney. At issue is whether there are biologic differences between the vasodilator PGs and Med I. Two separate studies have shown that Med I's vasodepressor action is inhibited by four procedures: mixing with Tween 20; treatment with n-butyl boronic acid; treatment of the assay animal with SKF 525A, an inhibitor of cytochrome P-450; and removing the liver from the circulation. These same procedures were applied to the vasodilator PGs. All four failed to inhibit the vasodepressor action of the PG's. It is concluded that Med I and vasodilator PGs of the kidney are separate and distinct biologic entities.

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.

Antihypertensive action of medullipin I given by mouth

Perfusion of normal rat kidneys with 5% human albumin in a balanced salt solution bubbled with oxygen yielded medullipin I (Med I) in the renal venous effluent. The presence of Med I in the renal venous effluent has been established by thin-layer chromatography, by the type of vasodepressor effect when injected intravenously as a bolus into the hypertensive rat, by inhibition of the vasodepressor effect of the renal venous effluent by Tween 20 and SKF 525A (proadifen, inhibitor of cytochrome P-450), and by removal of the liver from the circulation (a procedure that inhibits extracted Med I). Med I so derived lowered blood pressure of spontaneously hypertensive rats when injected into the stomach by an indwelling tube or when given by mouth. The lowering of blood pressure was attended by no change in cardiac output and no change in heart rate. Med I given by mouth to the spontaneously hypertensive rat is a vasodilator that suppresses sympathetic tone, acting in the same way as Med I extracted from renal papillae and given intravenously. Importantly, the antihypertensive action was demonstrated in the spontaneously hypertensive rat, a model of hypertension considered to mimic idiopathic or essential hypertension of humans. Med I is a promising therapeutic agent for hypertension.

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.

Is the humoral renal antihypertensive activity of the spontaneously hypertensive rat (SHR) reset to the high blood pressure?

The kidneys have a humoral antihypertensive system, located in the renal medulla and presumably antagonizing the pro-hypertensive renin-angiotensin system. Medullipin I and II and maybe platelet activating factor (PAF), seem to be the mediators of this system, known to be activated after reversal of renovascular hypertension or when the perfusion pressure to a normotensive kidney is suddenly elevated. The present study was undertaken to investigate whether this system is functioning also in the spontaneously hypertensive rat (SHR), and if it is then reset in proportion to the increased mean arterial pressure (MAP). Isolated kidneys from spontaneously hypertensive rats and from Wistar Kyoto rats (WKY) were cross-perfused in vivo from anaesthetized intact Wistar Kyoto rat 'donors'. After 30 min of perfusion at 100 mmHg the perfusion pressure to the isolated kidneys were, for 60 min, either kept unaltered at 100 mmHg or, for the Wistar Kyoto rat kidneys, increased to 150-200 mmHg and, for the spontaneously hypertensive rat kidneys, raised to 200 or 250 mmHg. The results show that the humoral antihypertensive system is present also in spontaneously hypertensive rat kidneys, but is here reset upwards to or even beyond the elevated MAP level. Furthermore, all mean arterial pressure reductions caused by high-pressure perfusion of Wistar Kyoto and spontaneously hypertensive rat kidneys were accompanied by reductions in heart rate (HR) in the 'donors', in agreement with previous observations after reversing renal hypertension and after i.v. medullipin I injection. In fact, in spontaneously hypertensive rat kidneys, the 'incretory' depressor mechanism appears to be more markedly reset upwards than is the 'excretory' depressor mechanism inherent in pressure diuresis with consequent salt-volume elimination. In conclusion spontaneously hypertensive rats, like Wistar Kyoto rats and Wistar rats, have a humoral antihypertensive system in the kidneys, but it is reset upwards even beyond the elevated mean arterial pressure level in spontaneously hypertensive rats. The combination of a depressor response and reduced heart rate in the 'donors' renders further evidence that the medullipins are the principal, though probably not the only, humoral antihypertensive factors released from the cross-circulated kidneys.

Renal and circulatory effects of medullipin I, as studied in the in-vivo cross-circulated isolated kidney and intact Wistar-Kyoto (WKY) rat

The renal medulla harbours powerful humoral antihypertensive mechanisms, as earlier explored in unclipping experiments on renal hypertensive rats or in normotensive isolated kidneys cross-circulated at increased perfusion pressures from 'donor rats', in which renal function also seemed to be affected. Injection of the renomedullary factor medullipin I (Med I; formerly ANRL) mimics these haemodynamic responses, and Med I seems to be one of the most important mediators of the depressor effects. The present study was performed to analyse further the haemodynamic and, particularly, the renal effects of Med I, using anaesthetized intact WKY rats and constant-pressure perfused (90 mmHg) isolated WKY kidneys, cross-circulated by these intact 'donor' rats. Mean arterial pressure (MAP), heart rate (HR) and renal function were followed for one 30-min period before and two 30-min periods after injection of 1 mg Med I (M; n = 7) or an equal volume of saline as control (C; n = 13). In the intact 'donor' WKY, MAP and HR remained largely constant in C during the three periods, being 126 +/- 5, 125 +/- 5, and 120 +/- 5 mmHg, while MAP fell in the M group after Med I, from 121 +/- 5 to 107 +/- 7 and 107 +/- 5 mmHg (P less than 0.05), and also HR tended to decrease in M. Renal resistance (RR) fell while renal plasma flow (RPF) and glomerular filtration rate (GFR) increased significantly (P less than 0.05) after Med I in the M donor rats despite their MAP reduction. However, in the constant-pressure perfused, cross-circulated kidneys the RR, RPF and GFR changes were clearly more pronounced (P less than 0.01) and also diuresis, natriuresis, osmolar excretion and osmolar clearance increased significantly after Med I (P less than 0.01). In conclusion, the present results support the view that Med I not only has important and long-lasting depressor effects but also affects renal function in important ways, inducing vasodilatation and increasing GFR, RPF, diuresis and sodium-osmolar excretion.

Biologic contrasts between medullipin I and vasoactive glyceryl compounds

Medullipin I causes a delayed onset depressor response when injected intravenously into rats. The glyceryl compounds selachyl alcohol (SA) and monoolein (MO) cause similar vasodepression. The neutral lipid 1-O-hexadecyl-2-acetyl-sn-glycerol (HAG) was suggested by Blank et al to be medullipin I (Med I, formerly ANRL). Biologic comparisons were made between Med I and various glyceryl compounds, including SA, MO, HAG, alkyl glyceryl ethers of phosphatidyl choline (termed APRL by us), diacylated SA, and the n-butyl boronic acid derivative of SA and MO. The n-butyl boronic acid derivative of Med I also was evaluated. The delay in onset of the depressor response to Med I was reduced by the injection of Med I into the portal vein; that of SA and MO was not. Med I, SA, and MO were activated by the liver, while APRL and HAG were not. Tween 20 inhibited Med I, SA, and MO, but not APRL and HAG. Proadifen (SKF 525A) inhibited Med I, but not SA and MO. The n-butyl boronic acid derivatives of SA, MO, and Med I were inactive. Med I, like SA and MO, appeared to have two hydroxyl groups in close proximity. It was concluded that Med I is neither HAG, APRL, SA, nor MO.