Antihypertensive polar renomedullary lipid, a semisynthetic vasodilator

Bioactive Ether Lipids: Primordial Modulators of Cellular Signaling

The primacy of lipids as essential components of cellular membranes is conserved across taxonomic domains. In addition to this crucial role as a semi-permeable barrier, lipids are also increasingly recognized as important signaling molecules with diverse functional mechanisms ranging from cell surface receptor binding to the intracellular regulation of enzymatic cascades. In this review, we focus on ether lipids, an ancient family of lipids having ether-linked structures that chemically differ from their more prevalent acyl relatives. In particular, we examine ether lipid biosynthesis in the peroxisome of mammalian cells, the roles of selected glycerolipids and glycerophospholipids in signal transduction in both prokaryotes and eukaryotes, and finally, the potential therapeutic contributions of synthetic ether lipids to the treatment of cancer.

The vasodepressor function of the kidney: further characterization of medullipin and a second hormone designated angiolysin

The objective of this study was to further characterize the antihypertensive properties of medullipin and a second hormone designated angiolysin physiologically. Angiolysin and medullipin were tested in coronary and aortic rings from cows, sheep, pigs, mice and rats. In vivo animal experiments were performed using spontaneously hypertensive rats. Medullipin was successfully separated from another antihypertensive agent at the polar end of the polarity continuum. It is an extremely potent vasodilator. With the methods available today, it is not possible to make a galenical preparation of medullipin for in vivo analysis. The newly discovered antihypertensive agent is another extremely potent vasodilator, even stronger than medullipin, and was therefore named angiolysin. The vasodilatory activity of both medullipin and angiolysin persisted for hours, on rat and mouse aortae, and on the coronary arteries of pigs, cows and sheep. Both substances exerted their effects even in animal rings that were precontracted with 100 mmol/l K+. Angiolysin reduced the resting tension in blood vessels from mice and rats even without precontraction. A single injection of angiolysin resulted in a dose-dependent reduction of blood pressure, independent of the initial blood pressure, even to zero if the dosage was sufficient. The effect persisted for several hours. In conclusion, both hormones are extremely potent vasodilators, and are expected to lead to paradigmatic changes in the treatment of hypertension. With regard to potency, only sodium nitroprusside is comparable, but the effects of medullipin and angiolysin persist for hours after a single injection.

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.

Role of platelet-activating factor in cardiovascular pathophysiology

Platelet-activating factor (PAF) is a phospholipid mediator that belongs to a family of biologically active, structurally related alkyl phosphoglycerides. PAF acts via a specific receptor that is coupled with a G protein, which activates a phosphatidylinositol-specific phospholipase C. In this review we focus on the aspects that are more relevant for the cell biology of the cardiovascular system. The in vitro studies provided evidence for a role of PAF both as intercellular and intracellular messenger involved in cell-to-cell communication. In the cardiovascular system, PAF may have a role in embryogenesis because it stimulates endothelial cell migration and angiogenesis and may affect cardiac function because it exhibits mechanical and electrophysiological actions on cardiomyocytes. Moreover, PAF may contribute to modulation of blood pressure mainly by affecting the renal vascular circulation. In pathological conditions, PAF has been involved in the hypotension and cardiac dysfunctions occurring in various cardiovascular stress situations such as cardiac anaphylaxis and hemorrhagic, traumatic, and septic shock syndromes. In addition, experimental studies indicate that PAF has a critical role in the development of myocardial ischemia-reperfusion injury. Indeed, PAF cooperates in the recruitment of leukocytes in inflamed tissue by promoting adhesion to the endothelium and extravascular transmigration of leukocytes. The finding that human heart can produce PAF, expresses PAF receptor, and is sensitive to the negative inotropic action of PAF suggests that this mediator may have a role also in human cardiovascular pathophysiology.

Renal effect of YM435, a new dopamine D1 receptor agonist, in anesthetized dogs

The renal effects of YM435 ((-)-(S)-4-(3,4-dihydroxyphenyl)-7,8-dihydroxy -1,2,3,4-tetrahydroisoquinoline hydrochloride hydrate), a dopamine D1 receptor agonist, were investigated in anesthetized dogs. Intravenous infusion of YM435 (0.1-3 micrograms/kg per min) increased renal blood flow and decreased mean blood pressure in a dose-dependent manner with little effect on heart rate. Glomerular filtration rate, urine flow and urinary sodium excretion were concomitantly increased. The renal effect of YM435 by intravenous infusion at 0.3 microgram/kg per min was completely blocked by treatment with the selective dopamine D1 receptor antagonist SCH 23390 (7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazep ine hydrochloride). Furthermore, intravenous infusion of YM435 (0.3 microgram/kg per min) reversed the angiotensin II-induced decreases in renal blood flow, glomerular filtration rate, urine flow and urinary sodium excretion, and prevented the decrease in renal blood flow, glomerular filtration rate and urine flow induced by renal nerve stimulation and platelet-activating factor (PAF). These results suggest that intravenous administration of YM435 produces renal vasodilating and diuretic/natriuretic effects by stimulation of dopamine D1 receptors, and demonstrate that YM435 can inhibit angiotensin II-, renal nerve stimulation- and PAF-induced renal dysfunction.

Renal vascular and sympathetic nerve responses to hypotension induced by platelet-activating factor in anesthetized dogs

This experiment was designed to determine renal sympathetic and renal vascular responses to platelet-activating factor (PAF)-induced hypotension in anesthetized dogs with and without systemic baroreceptor denervation. The left kidney was perfused at a constant flow, and renal perfusion pressure and efferent left renal sympathetic nerve activity were measured simultaneously. Intrarenal injection of PAF (1.25-5.0 x 10(-2) micrograms/kg, n = 6) produced a dose-dependent increase in renal perfusion pressure without any change in systemic blood pressure. An intravenous injection of PAF (10 micrograms/kg) to intact animals (n = 7) caused an initial increase in renal nerve activity (157 +/- 14%) followed by a gradual reduction below baseline (72 +/- 7%) with concomitant systemic hypotension (from 116 +/- 7 to 46 +/- 6 mmHg). Renal perfusion pressure increased significantly from 84 +/- 2 to 161 +/- 33 mmHg concomitant with an increase in renal nerve activity at 1 min and was maintained at this elevated level throughout the experiment. Similar responses of renal nerve activity and renal perfusion pressure were found in animals with complete systemic baroreceptor denervation (n = 7). These results suggest that renal vascular response during PAF-induced hypotension may presumably be mediated by a direct vasoconstrictor effect of PAF on the renal vasculature and that baroreceptor reflex is not involved in either renal sympathetic or renal vascular changes.

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.

Platelet aggregation induced by platelet-activating factor is suppressed by cystine protease inhibitor

The effect of NCO-700, a cystine protease inhibitor, on platelet-activating factor-induced platelet aggregation was determined. A newly synthesized cystine protease inhibitor (calcium-activated neutral protease and cathepsin B inhibitor), NCO-700 (bis[ethyl (2R, 3R)-3-[(S)-methyl-1-[4-(2,3,4- trimethoxyphenylmethyl) piperazine-1-ylcarbonyl]butylcarbonyl]oxiran-2-carboxy late]sulfate), inhibited platelet-activating factor-induced platelet aggregation. The inhibition was dependent on the preincubation time with NCO-700 and on the concentration of the inhibitor. The release of serotonin was also inhibited almost completely by the 20-min preincubation with 10(-4) M NCO-700. Leupeptin also inhibited platelet-activating factor-induced platelet aggregation. But calcium-activated neutral protease inhibitor did not inhibit it. These observations suggest that NCO-700-sensitive protease(s) such as cystine protease may contribute to platelet aggregation induced by platelet-activating factor.

Platelet-activating factor (PAF) and its relation to prostaglandins, leukotrienes and other aspects of arachidonate metabolism

This article summarizes some of the previously reported findings regarding a lipid mediator known as platelet-activating factor (PAF), and briefly describes its effects on cells and tissues. The effects of PAF have also been considered in relation to certain products of arachidonate metabolism released in response to PAF.

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.

Characterization of 3H-labelled platelet activating factor receptor complex solubilized from rabbit platelet membranes

Rabbit platelet membranes, preincubated with 3H-labeled platelet activating factor ([3H]PAF), were solubilized with 2% digitonin. Sedimentation of the detergent extract in a sucrose density gradient revealed a major labeled component with a sedimentation coefficient (s20,omega) of 10.5 S, which was substantially diminished when an excess of unlabeled PAF or L-652,731, (trans-2,5-bis(3,4,5-trimethoxyphenyl)tetrahydrofuran), (PAF antagonist) was present in the preincubation mixture, suggesting that the 10.5 S component is a specific receptor-bound [3H]PAF complex. Gel filtration of the [3H]PAF-receptor complex on Sephacryl S-300 revealed a single radiolabeled fraction with an apparent Stokes' radius of 4.9 nm. The apparent molecular weight and the frictional ratio of the agonist-receptor complex were computed to be 220,000 and 1.13, respectively. Dissociation of [3H]PAF from the radioligand-receptor complex was facilitated by Na+ and Li+, whereas K+ and Cs+ were ineffective. The guanine nucleotide, GTP, was also found to promote the dissociation in a manner that is additive with the effect of Na+, suggestive of the coupling of a guanine nucleotide binding protein to the solubilized PAF-receptor complex.

Morphometric studies of the renomedullary interstitial cells

The renomedullary interstitial cell (RIC) is a unique cell found in the renal medullary interstitium among the collecting ducts, Henle's loop, and the vasa recta. A combination of morphologic and physiologic studies has defined some of the properties of this cell, which is the apparent source of the mediators of the antihypertensive function of the kidney. These may include prostaglandins (PGE2), APRL or platelet activating factor, and the incompletely defined neutral lipid, ANRL (or medullipin), the most likely candidate for this function. The lipid droplets in the RIC apparently are the precursors of the mediator(s). Degranulation of the RIC by an experimental maneuver usually indicates increased activity of these cells. The presence of abundant well-granulated RIC in a kidney indicates a resistance of that animal to the induction of salt-sensitive forms of experimental hypertension. There is definite evidence for an extrarenal antihypertensive function of the RIC mediators, but an intrarenal effect also is considered.

Renal effects of platelet-activating factor in the rat

The renal glomerular and tubular effects of the platelet-activating factor (Paf-acether) were studied, by clearance techniques, in acutely thyroparathyroidectomized Brattleboro rats infused with Paf-acether at the rate of 1.25, 2.5 or 5 ng/min/100 g bw. Paf-acether infusion was accompanied by decreases of urinary flow rate, calcium, and magnesium urinary excretion, whereas decreases of mean arterial pressure and glomerular filtration rate did not exceed 20% of control values for the highest perfusion rate of Paf-acether. These changes in tubular function were partially (calcium excretion) or totally (urinary flow rate and magnesium excretion) reversed after Paf-acether infusion was discontinued. Sodium and potassium excretion did not vary significantly during Paf-acether infusion, but increased dramatically after discontinuation of Paf-acether infusion. Infusion of lyso-Paf-acether or ethoxy-Paf-acether, two biologically less active structural analogues of Paf-acether, did not elicit any change in the variables studied. These data suggest that Paf-acether, when locally released by the kidney in pathological conditions, might affect both glomerular and tubular functions, possibly through different mechanisms.

Platelet activating factor vasoconstriction of dog kidney. Inhibition by alprazolam

Systemic administration of platelet activating factor (PAF; acetyl glyceryl ether phosphorylcholine) reduces renal blood flow, but the mechanism responsible for that effect has not been defined. To address that problem, we determined the effects on renal blood flow of PAF administered directly into the renal artery in pentobarbital (30 mg/kg)-anesthetized dogs. Bolus injections of PAF (0.2-0.8 microgram) caused transient renal vasoconstriction, reducing renal blood flow by 20 to 60% without altering systemic blood pressure; lyso-PAF (1 microgram) had no effect. The effects of PAF on renal blood flow were not altered by alpha-adrenergic blockade (phentolamine, 3 mg/kg) or by angiotensin II receptor blockade ([Sar1,Ala8]angiotensin II, 6 micrograms/kg/min), but they were increased in magnitude and duration by meclofenamate (5 mg/kg), a cyclooxygenase inhibitor. Methysergide (3 mg/kg), a serotonin antagonist, slightly reduced PAF effects, but a specific blocker of vascular serotonin receptors did not. Renal venous plasma platelet density was not altered by infusion of PAF into the renal artery at a dose (1-2 micrograms/min) that caused a sustained 20% renal blood flow decrease. Alprazolam, a benzodiazepine that competitively inhibited PAF-induced aggregation in canine platelet-rich plasma, also inhibited the renal vasoconstrictor action of PAF (0.8 mg/min, into the renal artery) but did not alter renal vasoconstrictor effects of norepinephrine or angiotensin II.

Metabolism and function of platelet-activating factor in fetal rabbit lung development

Previously, platelet-activating factor (PAF, PAF-acether, 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) had been identified in association with a lamellar-body-enriched fraction of human amniotic fluid obtained from women in labor. In consideration of the fact that fetal lung is the source of lamellar bodies, we have investigated the capacity of the developing lung to synthesize PAF. The specific activity of the PAF biosynthetic enzyme, 1-alkyl-2-lyso-sn-glycero-3-phosphocholine: acetyl-CoA acetyltransferase, increased from 116 pmol/min per mg protein in day 21 fetal rabbit lung to 332 pmol/min per mg protein by day 31. Although this enzymatic activity in fetal kidney also increased, it never reached the level found in lung. In contrast, the actyltransferase activity decreased by 80% in fetal liver between days 21 and 31. The acetyltransferase activity in lung was primarily localized in the microsomal fraction (105 000 X g pellet); however a significant proportion of the activity was found in the 18 000 X g pellet. The specific activity of acetyltransferase in adult alveolar type II rat pneumonocytes was significantly higher than that of adult rat lung or rat alveolar macrophages, suggesting that type II cells make a significant contribution to the actyltransferase activity of lung tissue. PAF acetylhydrolase remained relatively constant throughout the gestation in all tissues. The concentration of PAF in the fetal lung increased by 3-fold from 12 to 35 fmol/mg protein, between day 21 and day 31 of development. The concentrations of the PAF precursors, 2-lyso-PAF (1-alkyl-2-lyso-sn-glycero-3-phosphocholine) and the 2-acyl derivative, were several orders of magnitude higher than the PAF concentration. The pulmonary glycogen content decreased from 163 at day 21 to 35 micrograms/mg protein at day 31 of gestation. We suggest that the increase in PAF concentration may participate in the regulation of glycogen breakdown in fetal lung as it does in perfused rat liver (Shukla, S.D., Buxton, D.B., Olson, M.S. and Hanahan, D.J. (1983) J. Biol. Chem. 258, 10212-10214). The formation of PAF in the developing lung and its secretion, in association with lamellar bodies, into amniotic fluid is discussed in relation to parturition.

Platelet-activating factor stimulates prostaglandin synthesis in cultured cells

The effects of platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) on phospholipase activity were studied in aortic smooth muscle cells and renal epithelial cells. When platelet-activating factor was added to cells prelabeled with [3H]arachidonic acid, it induced rapid hydrolysis of phospholipids. Up to 28% of incorporated [3H]arachidonic acid was released into the medium from both aortic and renal cells. A transient rise of diacylglycerol was also seen after the addition of platelet-activating factor to these cells. The accumulation of diacylglycerol and monoacylglycerol was relatively small when compared with the total amount of released free arachidonic acid. The amount of [3H]arachidonic acid released was comparable to the loss of phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine, which indicates that platelet-activating factor stimulates phospholipase A2 and C activity in aortic smooth muscle cells and renal epithelial cells. Platelet-activating factor also enhanced prostaglandin biosynthesis in these cells.

Effects of ONO-6240, a platelet-activating factor antagonist, on endotoxin shock in unanesthetized sheep

To determine the role of platelet-activating factor (PAF) in endotoxin shock, we studied the effects of ONO-6240, a PAF antagonist, on endotoxin shock in unanesthetized sheep. Changes in hemodynamics, lung lymph balance, leukocyte and platelet counts, and arterial blood gas tensions were measured in four groups; endotoxin alone; endotoxin plus ONO-6240; ONO-6240 alone; vehicle control. Pretreatment with ONO-6240 in sheep given endotoxin significantly prevented the decreases in systemic arterial pressure, left atrial pressure and cardiac output observed in sheep given endotoxin alone. A partial effect on diminishing the magnitude of peripheral leukopenia was also noted. However, pretreatment with ONO-6240 had little effect on pulmonary hypertension and lung lymph balance. We conclude that endotoxin causes two different effects: vascular collapse and direct lung injury; and that PAF is involved only in the circulatory manifestations.

Role of acetyl glyceryl ether phosphorylcholine in blood pressure regulation in rats

The role of an endogenously occurring acetyl glyceryl ether phosphorylcholine (AGEPC) in blood pressure regulation was studied with an AGEPC antagonist in rats with hypertension of various etiologies. The hypotensive activity of an intravenously injected AGEPC was competitively suppressed by the intravenous infusion of 3-(N-n-octadecylcarbamoyloxy)-2-methoxypropyl-2-thiazolioethylphospha te (CV-3988) and was dose-dependent. The CV-3988 was infused intravenously into one- and two-kidney, one clip hypertensive, deoxycorticosterone-salt hypertensive, adrenal regeneration hypertensive, spontaneously hypertensive, and normotensive control rats. The increase in blood pressure caused by CV-3988 infusion in spontaneously hypertensive and normotensive control rats was significant (p less than 0.01 and p less than 0.001, respectively, at 60 min) compared with that caused by vehicle infusion. The increase was not seen in rats with secondary hypertension. In rats with two-kidney, one clip hypertension, the initial rapid decrease in blood pressure seen after unclipping was significantly (p less than 0.05) inhibited by CV-3988 infusion as compared with that by vehicle infusion. These results suggest that endogenous AGEPC may participate in the blood pressure regulation and pathophysiology of some forms of hypertension in rats.

Effects of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet activating factor) on cardiac function in perfused guinea-pig heart

The direct cardiac action of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) was studied in isolated perfused guinea-pig heart preparations. PAF produced a fall in left ventricular pressure, decreases in the rate of rise of the left ventricular pressure (dp/dt) and coronary flow, but had no effect on heart rate. These results indicate that PAF is a cardiodepressant with inotropic selectivity and this effect on heart is blocked by CV-3988, a specific PAF antagonist.

Vascular reactivity in experimental acute renal failure

Vascular reactivity in-vivo and in-vitro was examined in rats with acute renal failure produced by bilateral nephrectomy or intramuscular glycerol injection. Bilaterally nephrectomized rats displayed enhanced pressor responses to noradrenaline and angiotensin. However, the contractile responses to noradrenaline, angiotensin and potassium chloride of aortic rings and portal vein segments from nephrectomized rats were not significantly different from the responses obtained in vessels from sham-operated controls. Rats with glycerol-induced ARF which were pretreated with indomethacin had significantly lower pressor responses to noradrenaline and angiotensin than similarly treated control animals. Aortic rings from glycerol-injected rats produced significantly smaller contractions to noradrenaline than preparations from controls. This difference was not abolished by incubation of vessels with indomethacin. The findings suggest that the absence of kidneys or the presence of damaged renal tissue and not uraemia itself have pronounced but opposite effects on vascular reactivity. The depression of vascular reactivity in glycerol-induced ARF does not appear to be a result of increased production of prostaglandins.

Hypotensive mechanism of acetyl glyceryl ether phosphorylcholine (AGEPC) in dogs. Effects on hemodynamics and humoral factors

One-O-hexadecyl-2-O-acetyl-sn-glycero-3-phosphorylcholine (AGEPC) was intravenously administered to anesthetized dogs to study the effects on hemodynamics and several endocrine factors. The effect of AGEPC on local blood flow was also studied by direct intra-arterial injection. Following intravenous injection, blood pressure and cardiac output decreased significantly (p less than 0.001). Changes in total peripheral resistance (TPR) and heart rate were biphasic. TPR increased significantly (p less than 0.01) after an initial slight reduction. Heart rate decreased significantly (p less than 0.01) with only a transient slight elevation. Femoral blood flow was increased (p less than 0.001) by intraarterial injection and decreased (p less than 0.05) by intravenous administration. Plasma norepinephrine (p less than 0.001), epinephrine (p less than 0.01), thromboxane B2 (p less than 0.001), 6-0-PGF1 alpha (p less than 0.01), aldosterone (p less than 0.001) and cortisol (p less than 0.001) were elevated, but plasma renin activity did not change. These results suggest that the hypotensive mechanism of AGEPC is due to both cardiosuppression and vasodilation. AGEPC increased plasma catecholamines, thromboxane A2, PGI2, aldosterone and cortisol which, in turn, may modify hemodynamics.

Effects of acetyl glyceryl ether of phosphorylcholine (platelet activating factor) on ventricular preload, afterload, and contractility in dogs

Acetyl glyceryl ether of phosphorylcholine (AGEPC), platelet activating factor, is a potent hypotensive agent that may mediate changes in blood pressure during anaphylaxis and may be involved in blood pressure variations of renal origin. This study was designed to characterize the hemodynamic mechanisms responsible for hypotension induced by this recently identified phospholipid. Intravenous administration of AGEPC to anesthetized open-chest dogs (n = 5) produced hemodynamic alterations which, for the purpose of analysis, were divided into three phases based on changes in the mean systemic blood pressure. During phase I (5-30 s) mean systemic blood pressure decreased to levels 5 to 10% below baseline values in association with a rise in cardiac output and a decrease in systemic vascular resistance. Phase II (30-90 s) consisted of a substantial reduction in systemic blood pressure to its nadir, 50% of baseline values, together with a decrease of similar magnitude in cardiac output and a rise in systemic vascular resistance. Phase III (90 s-60 min) exhibited a gradual recovery of mean systemic blood pressure toward normal with a several-fold rise in systemic vascular resistance and a continued low cardiac output. On the right side of the circulation, the predominant effect of AGEPC was a marked transient increase in pulmonary artery pressure in phase I, associated with an elevation of pulmonary resistance during phase II. Diethylcarbamazine blocked virtually all of these hemodynamic changes induced by AGEPC; FPL 55712 substantially blocked the rise in systemic vascular resistance in phase III. These results suggest that leukotrienes may mediate at least some of the hemodynamic effects induced by AGEPC, but further studies will be required when more specific leukotriene blocking agents become available. As assessed during phase III with the end-systolic pressure-dimension relation, myocardial performance itself was diminished. The occurrence of an AGEPC-induced negative inotropic effect was further confirmed in isolated Krebs-perfused guinea pig hearts and isolated blood-perfused rabbit hearts. The results indicate that the mechanism of AGEPC-induced hypotension is complex, affecting both vascular tone and the inotropic state of the myocardium.

Cardiovascular and renal action of platelet-activating factor in anesthetized dogs

Platelet-activating factor (PAF) has hypotensive effects similar to those of antihypertensive polar renomedullary lipid (APRL), a potent endogenous hypotensive lipid. In this study the cardiovascular and renal effects of PAF were characterized in anesthetized dogs. Intravenous infusion of PAF at 0.1 micrograms/kg/min for 1 hour caused marked reduction in arterial blood pressure and cardiac output and was accompanied by minimal changes in heart rate. Concomitantly, renal blood flow, glomerular filtration rate, urine flow, and fractional excretion of Na+ and K+ fell significantly. Plasma renin activity was greatly stimulated (11.9 +/- 1.66 vs 3.26 +/- 0.45 ng/angiotensin I/ml/hr for the placebo group). There were no significant alterations in any of these parameters following PAF at a lower dose (0.03 micrograms/kg/min for 1 hour). In a separate study, PAF at 0.1 micrograms/kg/min for 20 minutes produced a decrease in left ventricular myocardial contractile force, concomitant with bradycardia and hypotension, which indicated the presence of a negative inotropic activity. It is concluded that systemic administration of PAF has a deleterious effect on kidney function due to arterial hypotension and diminished cardiac output.

Regional hemodynamic effects of antihypertensive renomedullary lipids in conscious rats

Renomedullary tissue has been proposed to exert an antihypertensive endocrine-like action. The antihypertensive polar renomedullary lipids (APRL) and neutral renomedullary lipids (ANRL) are potential mediators of this action. We evaluated the blood pressure and regional hemodynamic responses to APRL administered peripherally (i.v.) and to the central nervous system (CNS) in normal rats and rats with sinoaortic deafferentation (SAD) to remove baroreflex buffering. Rats were chronically instrumented with Doppler flow probes for measurement of mesenteric, renal, and hind-quarter vascular resistance, with arterial pressure and intravenous catheters, and with lateral cerebroventricular cannuli for intracerebroventricular (i.c.v.) administration. Intravenous APRL (0.01 to 1.0 micrograms) produced a dose-dependent decrease in blood pressure, tachycardia, and dilation of all vascular beds studied. The dose-response relationships were shifted to the left in SAD animals. APRL administered i.c.v. had no effect on intact or SAD rats. Pressor and regional vasoconstrictor responses to norepinephrine, angiotensin, and vasopressin were markedly reduced in SAD animals during constant infusion of APRL. In a second group of conscious SAD animals instrumented for blood pressure and heart rate measurements, intravenous ANRL (500 micrograms) decreased both arterial pressure (-45 +/- 16 mm Hg) and heart rate (-50 +/- 16 bpm). When given i.c.v., however, ANRL (10-100 micrograms) had no significant effect on resting blood pressure or heart rate. These studies suggest that APRL and ANRL produce no significant cardiovascular effects that are mediated through the CNS. However, both lipids are potent depressor agents, and APRL exhibits a strong peripheral vasodilator action and nonspecifically reduces reactivity to vasoconstrictor agents.

Mechanism(s) of the hypotensive effect of synthetic 1-O-octadecyl-2-O-acetyl-glycero-3-phosphorylcholine

The mechanism of the hypotensive effect of 1-O-octadecyl-2-O-acetyl-glycero-3-phosphorylcholine (C18- AGPC ) was examined. Synthetic C18- AGPC caused dose-dependent hypotension in conscious rats. The activity was almost the same in DOCA and renal hypertensive rats. This suggests that it is not a renin inhibitor. Hypotension also appeared in pithed rats. This suggests that the effect is not due to a central mechanism. Hypotension did not result from platelet aggregation or bronchial constriction. Since C18- AGPC suppressed not only the pressor response to noradrenaline but also to angiotensin II and vasopressin, and furthermore, did not disturb the dose-response curve of noradrenaline in the isolated aorta, the possibility of the agent being an alpha-adrenergic antagonist is ruled out. In the PGF2 alpha-contracted rat aorta. C18- AGPC caused marked vasodilation, which disappeared after removal of the endothelium. Perfusion pressure decreased in the blood-perfused rat hindquarters but not in the Tyrode solution-perfused ones. C18- AGPC induced a positive inotropic effect in isolated rat atrium. The hypotensive effect of synthetic C18- AGPC seems to be mainly due to endothelium-dependent vasodilation.

Effect of 1-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine inhibitor on the reduction of one-kidney, one clip hypertension after unclipping in the rat

The role of 1-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine in regulating blood pressure was studied in one-kidney, one clip hypertensive rats using 3-(N-n-octadecylcarbamoyloxy)-2-methoxypropyl-2-thiazolio ethylphosphate, which specifically inhibited the hypotensive activity of exogenously injected 1-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine. The blood pressure of rats with established hypertension produced by clipping one renal artery and contralateral nephrectomy normally decreases rapidly after unclipping the artery, but this rapid decrease was significantly inhibited by intravenous infusion of 3-(N-n-octadecylcarbamoyloxy)-2-methoxypropyl-2-thiazolio ethylphosphate. This shows that endogenous 1-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine participates in the rapid decrease of blood pressure after unclipping the kidney in one-kidney, one clip hypertensive rats.

Antihypertensive polar and neutral renopapillary lipids. Which is a hormone?

Two antihypertensive lipids can be derived from the renal papilla, the antihypertensive polar (APRL) and the antihypertensive neutral (ANRL) renomedullary lipid. The renal venous effluent of the unclipped kidney contains both ANRL and APRL. This effluent lowers the arterial pressure (AP) of the normal rat when infused i.v. As it lowers the AP the heart rate (HR) and sympathetic nerve activity (SNA) are depressed. ANRL infused i.v. also lowers HR and SNA as it depresses the AP. Conversely, APRL elevates HR and SNA as it lowers the AP. Thus, of the two lipids in the renal venous effluent after unclipping, ANRL appears to be dominant. APRL, however, in the renal venous effluent could potentiate the action of ANRL. The net effect of these observations is to support the view that ANRL is an antihypertensive hormone liberated by the kidney after unclipping. The renomedullary interstitial cells (RIC) degranulate after unclipping. ANRL can be derived from these cells. Thus, the RIC, cells known to exert an endocrine-type antihypertensive function, may well be the source of ANRL in the renal venous effluent after unclipping. The hormonal action of ANRL appears as a major cause of the lowering of the AP after unclipping. It is not known what factors modulate the RIC endocrine system. There is a suggestion that angiotensin may be one of these factors based on the ineffectiveness of these cells toward retarding hypertension when the circulating plasma angiotensin level is high, and their effectiveness when the circulating plasma angiotensin level is low.

Effect of eicosatetraenyl (3-aminopropyl) phosphonate (URI-73A) on platelet aggregation and arachidonic acid metabolism

Eicosatetraenyl (3-aminopropyl) phosphonate (URI-73A), an experimental anti-hypertensive drug, when tested in vitro, inhibits platelet aggregation especially that induced by arachidonic acid (AA) and to a lesser degree by epinephrine. Preincubation of human platelets with this compound in the majority of individuals resulted in a dose-dependent inhibition of the platelet response to AA, mostly in the form of a delay and a reduction of maximal aggregation. Platelets from two smaller groups of donors either exhibited full aggregation or no aggregation. Both the prelabel and pulse label techniques used to study arachidonic acid metabolism yielded complementary results. Production of thromboxane and hydroxy fatty acids was reduced in thrombin-aggregated washed platelets preincubated with URI-73A. In addition, the release of AA from phospholipids was inhibited. Its complex effects on human platelets suggest that URI-73A may, as an AA analog, depress cyclooxygenase and lipoxygenase, and as a phospholipid analog inhibit phospholipase activity.

Hypotensive and vasodilatory activity of (+/-) 1-o-octadecyl-2-acetyl glyceryl-3-phosphorylcholine in the normotensive rat

Synthetic (+/-) 1-O-octadecyl-2-acetyl-glyceryl-3-phosphorylcholine (octadecyl-AGPC) in microgram/kg doses given intravenously effectively and potently lowered mean arterial blood pressure in conscious and anesthetized normotensive rats. The hypotensive activity was much more pronounced in the anesthetized rat than in the conscious rat. The hypotension was associated with a significant elevation in plasma renin activity (PRA). In the rat in which the hindquarters were perfused, octadecyl-AGPC given intraarterially effectively decreased the perfusion and systemic pressures in a dose-dependent manner. Pharmacological blockade with specific cholinergic, histaminergic or beta-adrenergic receptor antagonists, did not block or attenuate the octadecyl-AGPC-induced reduction in perfusion or systemic pressure. These results suggest that the hypotensive activity of octadecyl-AGPC in the normotensive rat is the result of direct vasodilation and not the result of cholinergic, histaminergic or beta-adrenergic receptor interaction.

Cardiovascular effects of antihypertensive polar and neutral renomedullary lipids

Two antihypertensive lipids can be extracted from fresh renal medulla. One is polar (the antihypertensive polar renomedullary lipid, or APRL) and the other is nonpolar (the antihypertensive neutral renomedullary lipid, or ANRL). APRL and ANRL differ in their biologic activities: APRL in bolus intravenous injections causes a very rapid decline in the arterial pressure (AP) while ANRL, after a lag of 2 minutes, causes a slower decline in AP. APRL increases heart rate and sympathetic activity. ANRL decreases heart rate and sympathetic activity. ANRL appears to convert to APRL, under certain in vitro circumstances, suggesting that the structure of the two molecules is related. ANRL and APRL appear in the renal venous effluent after unclipping; biologically, ANRL seems dominant. The renal venous effluent of the unclipped isolated kidney lowers the HR and sympathetic activity of the normal rat. Unclipping degranulates the renomedullary interstitial cells (RIC). The antihypertensive effect of unclipping appears due to the secretion of ANRL and APRL by the kidney. It is concluded that ANRL seems to be the antihypertensive hormone of the RIC.

Antihypertensive effects of I-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine on plasma renin activity and catecholamine responses in spontaneously hypertensive rats

Fourteen 23 week old male spontaneously hypertensive rats (SHR) were randomly divided into saline control or phospholipid (I-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine) treatment groups. Four weeks of baseline systolic blood pressure (SBP) and heart rate (HR) measurements were determined via tail plethysmography. On week 25 of the baseline period a 1.5 ml blood sample was taken by tail clip for analysis of norepinephrine (NE), epinephrine (E), and plasma renin activity (PRA). On the following week, a single injection of phospholipid (11 ug/kg, s.c.) was given to the experimental animals following baseline SBP and HR determinations. A similar procedure was employed for control subjects, except they received an injection of normal saline (0.5 ml, s.c.). Systolic BP and HR responses were monitored for 24 minutes following the injection. A 1.5 ml blood sample was taken at the end of the 4th minute for NE, E, and PRA assays. A significant drop in SBP (202 +/- 5 mmHg to 124 +/- 6 mmHg) and an increase in HR (431 +/- 17 bpm to 519 +/- 21 bpm) were observed for experimental animals, but not for control subjects. Plasma NE increased significantly (446 +/- 42 pg/ml to 1099 +/- 77 pg/ml), but E remained unchanged following treatment with the phospholipid. Plasma renin activity increased for both groups, but this change was only significant for the experimental group (18.1 +/- 5.7 ng Al/ml/hr to 34.3 +/- 3.6 ng Al/ml/hr). Thus, it appears that I-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine is a potent antihypertensive vasodilating agent which stimulates baroreceptor mediated sympathetic discharge to the heart and kidneys of the SHR.

Blood pressure and heart rate effects of alkyl ether phospholipids in conscious renal, spontaneously hypertensive, and normotensive rats

The blood pressure and heart rate effects of a synthetic alkyl ether phospholipid and similar lipids derived from beef heart and egg yolk were studied in normotensive, renal, and spontaneously hypertensive rats. Each agent (1.25 to 10 micrograms/kg) produced a dose-related decrease in blood pressure and increase in heart rate in each model after rapid IV injection. The degree of blood pressure lowering was dependent upon the pretreatment blood pressure. Alkyl ether phospholipid was infused at 10 to 30 micrograms/kg/min to SHR and to platelet depleted SHR; comparable effects (blood pressure decreases and heart rate increases) were noted in both models. Reversal of the epinephrine pressor response was seen in all rats. When administered orally in ascending doses on three consecutive days, 50 and 150 micrograms/kg b.i.d. doses (Day 1 and 2) were inactive whereas 500 micrograms/kg b.i.d. (Day 3) decreased blood pressure by 25 +/- 8 and 21 +/- 8 mm Hg in renal and spontaneously hypertensive rats, respectively. Side effects (by all routes) included limp, cyanotic hind limbs and sedation and were similar in SHR and platelet depleted SHR. Significant, sustained blood pressure lowering was not achieved by any of the alkyl ether phospholipids in the conscious rat models at doses devoid of limiting side effects. The results of these studies show: 1) the BP, HR, and toxic profiles of all three alkyl ether phospholipids were qualitatively similar, 2) at doses causing significant BP lowering there was epinephrine reversal suggesting an alpha-adrenergic blocking component of action, 3) the BP, HR, and toxic effect profiles of alkyl ether phospholipid were comparable in normal and platelet depleted SHR and apparently not platelet-dependent, 4) sustained antihypertensive activity could be achieved by the oral route; however the effects were modest (12 +/- 4% decrease) and associated with limiting side effects.

alpha-Adrenergic blocking action of 1-O-hexadecyl-2-O-acetyl-sn-glycero-3-phosphocholine in rats

The effect of 1-O-hexadecyl-2-O-acetyl-sn-glycero-3-phosphocholine (HAGPC), a major component of antihypertensive polar renomedullary lipid, on the pressor responses to norepinephrine and angiotensin II was investigated in normal Wistar rats. The pressor activity of norepinephrine (1 microgram/kg of body weight (BW)) and angiotensin II (40 ng/kg BW) were markedly suppressed when these substances were injected immediately after intravenous administration of 80 nmol/kg BW of HAGPC. When HAGPC was infused continuously at a rate of 20 nmol/kg BW/min, pressor responses to bolus injection of graded doses (0.5 to 10 micrograms/kg BW) of norepinephrine were significantly lowered, but pressor responses to 10 to 200 ng/kg BW of angiotensin II were not affected. These observations suggest that HAGPC has alpha-adrenergic blocking activity and does not influence angiotensin II receptors.

Blockade of alpha-adrenergic receptors by analogues of phosphatidylcholine

The experimental evidence reviewed in this article suggests that the kidneys may have an additional function in regulating blood pressure besides their role in controlling both blood volume by urine formation and the relative state of vasoconstriction by the renin-angiotensin system. That is, the kidneys may have an additional influence upon the vasculature of a hormonal vasodilating system. The interstitial cells of the renal medulla appear to be mediating this activity and lipid compounds have been extracted from the renal medulla which display depressor activity. One such compound, the antihypertensive polar renomedullary lipid (APRL), has been demonstrated to consist of specific alkyl ether analogues of phosphatidylcholine. The vascular responses to these compounds include vasodilation of both arterioles and venules, rapid lowering of arterial blood pressure with little or no tachycardia, increased depressor activity in hypertensive animals, and blockade of vascular smooth muscle alpha 1-adrenergic receptors. Most recently, APRL and a synthetic analogue, 1-0-octadecyl-2-acetyl-sn-glycero-3-phosphorylcholine, have been used to demonstrate alpha-adrenergic receptor blockade on a smooth muscle cell line (DDT1) by radioligand assays. This action may be due to the insertion of these compounds into cell membranes causing subsequent steric interactions and blockade of the alpha-adrenergic receptor.