Cardiovascular effects of antihypertensive polar and neutral renomedullary lipids

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.

Long-term effects of angiotensin-converting enzyme inhibition on renal medullary neutral lipid in spontaneously hypertensive rats

Short-term treatment of young spontaneously hypertensive rats (SHR) with angiotensin-converting enzyme (ACE) inhibitors reduces systolic blood pressure. Renal medullary neutral lipids (RMNLs) have vasodilator properties that may explain the effects of ACE inhibition. We measured RMNL levels of SHR treated between 6 and 10 weeks of age with (1) vehicle, (2) ramipril 1 mg. kg-1. d-1, (3) the bradykinin B2 receptor antagonist icatibant 0.5 mg. kg-1. d-1, or (4) icatibant 0.5 mg. kg-1. d-1 plus ramipril 1 mg. kg-1. d-1. RMNLs were quantified by oil red O fluorescence at 10 and 20 weeks of age. Systolic blood pressure (BP) was measured by tail-cuff plethysmography. Ramipril reduced BP at 10 weeks of age and increased RMNLs compared with controls (0.99+/-0.07% versus 0.56+/-0. 06%, P<0.01). Icatibant alone had no significant effect on RMNLs (0.55+/-0.04%) but attenuated the increase in RMNLs by ramipril (0. 81+/-0.05%). In control SHR, the increase in BP between 10 and 20 weeks of age was associated with a significant increase in RMNLs (0.79+/-0.09%). SHR that had received ramipril had significantly lower BP than controls at 20 weeks of age, but RMNL was not significantly different (0.92+/-0.10%). Therefore, in young SHR, ACE inhibition increases RMNLs and reduces blood pressure, an effect that appears to depend on bradykinin. The changes in RMNLs at the age of 10 weeks paralleled long-term BP effects and may be involved in setting the BP track in SHR.

Plasma PAF acetylhydrolase in non-insulin dependent diabetes mellitus and obesity: effect of hyperinsulinemia and lovastatin treatment

Insulin resistance is characterized principally by impaired insulin-mediated glucose uptake which provokes a compensatory increase in pancreatic beta-cell secretory activity. For a time this may produce well-controlled plasma glucose levels but as the insulin resistance worsens the augmented insulin production becomes inadequate to keep plasma glucose at euglycemia leading to the development of non-insulin dependent diabetes mellitus (NIDDM), accompanied by hyperinsulinemia and hyperglycemia. A number of metabolic defects are associated with NIDDM including obesity, hypercoagulability, cardiovascular disease risk factors such as hypertension and dyslipidemia and these constitute the insulin resistance syndrome. The identity of the biochemical factor that might link all these defects is not yet known. We have hypothesized that platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF) may be such a link. In this study, we measured plasma acetylhydrolase (EC.1.1.48), which degrades PAF to the inactive metabolise lyso-PAF, as a surrogate for PAF activity in three groups of hypercholesterolemic subjects: lean controls (n = 9), non-diabetic obese (n = 6) and NIDDM subjects (n = 6). The ages and body mass indices of the subjects were 46 +/- 3.1 and 24.2 +/- 2.2 for the lean controls, 52 +/- 2.5 and 28.7 +/- 0.9 for the NIDDM subjects and 60 +/- 2 and 27.6 +/- 2.1 for the obese, non-diabetic subjects (mean +/- S.E.M.). The measurements were made before and after therapy with the cholesterol-lowering drug lovastatin, a 3-hydroxy 3 methylglutaryl (HMG) coenzyme. A reductase inhibitor (40 mg/day) for 3 months. Fasting plasma glucose (FPG) levels were 91 +/- 11, 96 +/- 3 and 146 +/- 11 mg/dl, for the lean, obese and NIDDM subjects, respectively, before therapy began. Lovastatin did not affect FPG in any of the three subject groups. Before treatment, the fasting plasma insulin (FPI) levels were 6.1 +/- 0.92, 10.83 +/- 2.03 and 14.68 +/- 3.64 mU/l for the lean, non-diabetic obese and NIDDM subjects, respectively. After lovastatin therapy only the obese group exhibited a significant change in FPI (15.35 +/- 2.47 mU/l) (P < 0.05). Total cholesterol levels were similar in all three groups both before and after lovastatin therapy but within each group lovastatin therapy significantly reduced the total cholesterol by 32, 29 and 34% in the lean, obese and NIDDM subject groups respectively (P < 0.0001). Lovastatin therapy reduced LDL-cholesterol levels by 40, 32 and 46% in the lean, obese and NIDDM subjects, respectively, but produced no significant effect on HDL or triglyceride levels. Before therapy, the plasma acetylyhydrolase activities were 104 +/- 7, 164 +/- 7 and 179 +/- 7 nmol/ml per min in the lean, obese and NIDDM subjects, respectively. Lovastatin therapy reduced plasma acetylhydrolase levels to 70 +/- 7, 87 +/- 6 and 86 +/- 7 nmol/ml per min in the lean, obese and NIDDM subjects, respectively. Plasma acetylhydrolase activity was predominantly (> 80%) associated with LDL cholesterol both before and after lovastatin treatment. Also, plasma acetylhydrolase activity significantly correlated with fasting plasma insulin levels before lovastatin therapy but not after. Taken together, this study clearly implicates PAF metabolism in three defects associated with the insulin resistance syndrome: hypercholesterolemia, obesity and NIDDM. Additionally, we conclude that chronic hyperinsulinemia may play a significant role in the production of plasma acetylhydrolase.

Chronic hyperinsulinemia inhibits platelet-activating factor (PAF) biosynthesis in the rat kidney

A number of risk factors for cardiovascular disease, including hypertension, are associated with the insulin resistance syndrome. The hallmark of this syndrome is an impairment in insulin action which provokes a compensatory increase in pancreatic beta-cell insulin secretion leading to chronic hyperinsulinemia. Indirect studies show that platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine, PAF), a potent antihypertensive lipid produced by the kidney, may be decreased by hyperinsulinemia. The present study was designed to evaluate the effect of chronic hyperinsulinemia on renal PAF metabolism, arterial blood pressure and whole body insulin sensitivity. Chronic catheterized, unstressed rats were infused with saline or insulin plus glucose to create a chronic condition of sustained euglycemic (approximately 130 mg/dl) hyperinsulinemia (approximately 90 mU 1. or 3-fold over basal levels). PAF is a metabolically unstable compound being susceptible to rapid degradation to the biologically inactive lyso-PAF, a metabolite which also serves as a precursor for PAF synthesis. PAF synthesis and counter-regulatory prostaglandins may be derived from the same arachidonate precursor. The enzymes which catalyze these reactions were measured in plasma and in the subcellular fractions of the kidneys. Compared to saline-treated rats, sustained physiologic hyperinsulinemia for 7 days: (i) decreased insulin-mediated glucose disposal by 30%; (ii) caused an increased plasma PAF:acetylhydrolase, which degrades PAF to lyso-PAF, without any change in cytosolic PAF:acetylhydrolase activity; and (iii) completely inhibited microsomal lyso-PAF:acetyl CoA acetyltransferase activity which catalyzes the conversion of lyso-PAF back to bioactive PAF. The increased catabolism of PAF in plasma, combined with decreased renal PAF biosynthesis, would be expected to decrease circulating PAF levels leading to a rise in blood pressure. However, blood pressure remained unchanged. The sustained hyperinsulinemia stimulated plasma membrane CoA-independent transacylase activity, which is responsible for the mobilization of arachidonates into lyso-PAF, to form l-alkylarchidonoyl-glycerophosphocholine. The latter is the stored precursor for the synthesis of PAF and vasodilatory prostaglandins, which may have offset the effect of decreased PAF. We hypothesize that hyperinsulinemia may alter the blood pressure only if the balance between the synthesis/catabolism of PAF and vasodilatory prostaglandins is disrupted.

Lyso-PAF:acetyl-CoA acetyltransferase and CDP-choline cholinephosphotransferase activities in the rabbit endometrium

Endometrial platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine) levels change significantly during the pre-implantation period in the rabbit uterus, but under in vitro culture conditions, constitutive PAF biosynthesis by isolated endometrial tissues is not easily demonstrable. Rapid metabolism of PAF relative to its synthesis may account for this disparity because we have recently shown that in stromal cells there is a significant build-up of lyso-PAF suggesting that lyso-PAF-acetyl-CoA acetyltransferase might be a limiting factor. In the glandular epithelial cells however, the lyso-PAF build-up was replaced by a significant accumulation of a neutral lipid which was tentatively identified as 1-O-hexadecyl-2-acetylglycerol. It was hypothesized that, during endometrial growth and development, this lipid might serve as the substrate for the alkylacetylglycerol CDP-choline cholinephosphotransferase enzyme for PAF synthesis via the de novo pathway. We have therefore examined the activities of lyso-PAF:acetyl-CoA acetyltransferase and the CDP-cholinephosphotransferase enzymes. Microsomal preparations containing lyso-PAF:acetyl-CoA acetyltransferase activity catalyzed the incorporation of [3H]acetyl-CoA lyso-PAF into two distinct lipid products. One co-migrated with authentic PAF and the other with 1-O-hexadecyl-2-acetylglycerol, the latter being formed subsequent to PAF formation. The alkylacetylglycerol CDP-choline cholinephosphotransferase enzyme, which would potentially utilize the alkylacetylglycerol synthesized via the remodeling pathway, was also demonstrable. Unlike the species present in other tissues however, it was found to be sensitive to the presence of 10 mM DTT. The diacylglycerol CDP-choline cholinephosphotransferase species was also demonstrable and supported the synthesis of both PAF and phosphatidylcholine, in the absence of DTT, when only the synthesis of phosphatidylcholine was expected. It is hypothesized that the rabbit endometrium possesses active enzymes which may catalyze PAF synthesis via both the de novo and 'remodeling' pathways.

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.

Three-dimensional reconstruction of juxtaglomerular apparatus (JGA) in five-sixth nephrectomized rats

In Wistar male rats, hypertension was induced by five-sixth nephrectomy (5/6N). Body weight, systolic blood pressure (SBP) and plasma renin concentration (PRC) were followed for 12 weeks after 5/6N. Three-dimensional reconstruction and morphometry of the JGA were carried out using a computer program "GLOM". Immunohistochemistry and electron microscopy of the JGA were also investigated. A statistically significant increase in SBP was shown after 5/6N. However, PRC showed no increase and was not correlated with SBP. Renin-containing cells were demonstrated in the afferent and efferent arterioles and the interlobular arteries. Electron microscopy revealed granules of various shapes, sizes and electron densities within the JG cell. The frequency of granulated cells in the efferent arteriole was less than that in the afferent arteriole. The afferent arteriole wall volume of 5/6N rats was significantly increased and positively correlated with SBP. The lack of relationship between PRC and SBP in this model suggests that mechanisms other than the renin-angiotensin system may be involved in the pathogenesis of hypertension.

Vascular capacitance in rats subjected to chemical renal medullectomy

Selective renal medullary destruction is produced in rats by a single injection of 2-bromoethylamine hydrobromide. The object of these studies was to investigate whether destruction of the renal medulla in normal rats would alter vascular capacitance. Conscious bromoethylamine-treated rats (n = 15) were compared with control saline-injected rats (n = 12). Mean circulatory filling pressure was measured during a brief circulatory arrest caused by inflation of a right atrial balloon. Blood volume was determined from plasma volume (iodine-125-labeled albumin) and hematocrit. Mean circulatory filling pressure was measured at resting blood volume and after rapid blood volume changes. Vascular compliance was derived from the mean circulatory filling pressure-blood volume curve. The bromoethylamine-treated rats were significantly hypertensive compared with control rats (mean arterial pressure 133 +/- 2 and 114 +/- 3 mm Hg, respectively, p less than 0.001) and had a significant tachycardia (475 +/- 8 and 443 +/- 10 beats/min, respectively, p = 0.02). Blood volume, plasma volume, hematocrit, and sodium excretion were no different. There was no significant difference in mean circulatory filling pressure (6.5 +/- 0.2 and 6.8 +/- 0.2 mm Hg, respectively, p = 0.4) or vascular compliance (3.64 +/- 0.20 and 3.53 +/- 0.12 ml/kg/mm Hg, respectively, p = 0.7). The position of the vascular pressure-volume curve was unchanged indicating no change in vascular capacity. This would suggest that the destruction of renal medullary vasodepressor mechanisms does not result in alterations in vascular capacitance.

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.

The role of the renal medulla in blood pressure control

It is well established that the renal medulla exerts a potent endocrine-like antihypertensive action. The purpose of this paper is to summarize the data that define the extent to which the renomedullary antihypertensive action is involved in blood pressure regulation. It appears that in animals kept under physiological conditions, the renal medulla is not necessary for the maintenance of normotension, since its destruction does not usually result in hypertension. However, in animals exposed to hypertensive stimuli, the presence of renomedullary tissue appears to be the key factor in resisting the increase in blood pressure. Evidence is presented to show that inherited or acquired deficiency of renomedullary antihypertensive function may contribute to the development of various forms of hypertension. It is suggested that inherited differences in the antihypertensive capacity of the renal medulla may account for differing sensitivities of various strains of animals to hypertensive stimuli. Finally, data are presented to show that renomedullary deficiency induced by a decrease in renal perfusion pressure, which could be a consequence of hypertensive damage to renal vessels, may contribute to the increase in blood pressure in various forms of hypertension.

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.

The antihypertensive function of the kidney. Its elucidation by captopril plus unclipping

Unclipping the one-kidney, one-clip hypertensive rat during a free flow of urine caused the blood pressure to return to normal levels within about 3 hours. We found that administration of captopril plus unclipping caused the blood pressure to return to normal in minutes (17 +/- 4). Ureterocaval anastomosis plus captopril plus unclipping also caused the blood pressure to return to normal in minutes (8.8 +/- 2). Thus, the potentiation of the drop in blood pressure does not seem to be due to a volume effect. Administration of indomethacin and aprotinin did not prevent a rapid decline of the blood pressure after unclipping, but the decline was less rapid than that occurring after captopril and unclipping, which suggests that prostaglandin may have some effect on this mechanism. Saralasin administration did not potentiate the antihypertensive action of captopril plus unclipping. Chemical papillectomy prevented the drop in blood pressure after unclipping. The bolus dose of captopril to the hypertensive rat often caused a transient depressor effect resembling that due to the antihypertensive neutral renomedullary lipid, which suggests secretion of this lipid into the blood. The renomedullary interstitial cells accumulated large lipid granules after captopril administration. These cells also degranulated after unclipping. These findings are consistent with the hypothesis that the renal papilla secretes an antihypertensive hormone after unclipping. At present, antihypertensive neutral renomedullary lipid is the main putative hormone.

The morphology and antihypertensive effect of renomedullary interstitial cells derived from Dahl sensitive and resistant rats

The renomedullary interstitial cell (RIC) has been implicated in the antihypertensive action of the kidney. This cell has been isolated in tissue culture and shown to have an antihypertensive action in several models of experimental hypertension. Morphometric studies of RIC in vivo from Dahl rats sensitive and resistant to the hypertensive effects of high-salt diets indicate major differences between the RICs. These cells were therefore isolated from salt-sensitive and salt-resistant strains of rat, grown, and maintained in tissue culture. Major morphologic differences between the two cell lines were noted and persisted for multiple tissue culture passages. The cells from resistant animals were larger and had more lipid granules. These differences were similar to those seen in vivo. In short-term experiments these cells were compared for their antihypertensive effect. The two cell lines were injected subcutaneously into two groups of hypertensive recipient rats, one group of Dahl salt-sensitive rats on a high-salt diet and one group of Wistar rats subjected to the one-kidney, one-clip Goldblatt procedure. In both cases differences were noted between the cell lines. These data support the concept that differences between the Dahl salt-sensitive and salt-resistant rats may be related to variations in their RIC.

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.

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.