Evolution of aortic hyperplasia after reversal of renovascular hypertension in the rat

Unclipping of Goldblatt one-kidney, one clip hypertensive rats leads to a rapid correction of the high blood pressure. Simultaneously, the aortic smooth muscle cell proliferation wave is stopped. Any further proliferation is prevented but the proliferation changes previously established cannot be reversed. Thus, provided clip removal is performed at the earliest phase of hypertension, it can modify the time-course of the aortic proliferation changes. On the contrary, the heart hypertrophy is significantly improved regardless of the unclipping time. Accordingly, the response of the hypertensive cardiovascular modifications to an antihypertensive therapy cannot be regarded as a whole since it is dependent on the cardiovascular target as well as on the onset time of treatment.     

The effect of dietary alteration of prostaglandin synthesis on blood pressure and the reversal of hypertension in the one-kidney, one-clip rat

This study examined the effect of diet-induced changes in prostaglandin synthesis on systolic blood pressure in one-kidney, one clip (1k, 1C) hypertensive rats and on the fall in blood pressure after unclipping. It tested the hypothesis that inhibition of prostaglandin synthesis exacerbates hypertension in this model and prevents complete reversal after unclipping. Rats with sustained hypertension within 8 weeks of renal artery clipping were fed synthetic diets supplemented to 20% of total energy with either safflower oil (linoleic acid) or a mixture of cod liver oil (90%) (containing eicosapentaenoic acid) and linseed oil (10%) (containing linolenic acid) for 4 weeks. The latter oil mixture resulted in a predictable reduction in kidney PGE2 and 6-keto PGF1 alpha (hydrolysis product of PGI2), aortic 6-keto PGF1 alpha and serum TXB2. However, at the end of 4 weeks dietary treatment there were no differences in systolic blood pressure between the two diet groups, and the blood pressure fall 24 hours after unclipping was similar. These findings therefore do not support a role for prostanoids in the maintenance or reversal of 1K, 1C hypertension.     

A new class of antihypertensive neutral lipid: 1-alkyl-2-acetyl-sn-glycerols, a precursor of platelet activating factor

A new type of neutral lipid is described that possesses hypotensive activity in genetic hypertensive (SHR) and normotensive (WKY) rats. 1-Alkyl-2-acetyl-sn-glycerols and 1-alkyl-2-propionyl-sn-glycerols are both equally effective in eliciting the hypotensive response. Requirement for the 1-alkyl and 2-acetyl or 2-propionyl structure of the active isomer was documented by the negative responses obtained with closely related neutral lipid analogs (1-alkyl-2-acyl-, 1-alkyl-3-acetyl-, 1-acyl-2-acetyl-, 1-alkyl-2,3-diacetyl-, and 1-alkyl-glycerols). Although less potent than PAF (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine), the 1-alkyl-2-acetyl-sn-glycerols produce a response of significantly longer duration and may have fewer immediate side effects than PAF. The mechanism for the biological activity is unknown; however, we have demonstrated previously that the enzymatic synthesis of 1-alkyl-2-acetyl-sn-glycerols to PAF occurs via a specific cholinephosphotransferase and therefore the observed blood pressure response might be due to the conversion of the neutral lipid precursor to PAF in vivo.     

The effect of dietary alteration of prostaglandin synthesis on blood pressure and the reversal of hypertension in the one-kidney, one-clip rat

This study examined the effect of diet-induced changes in prostaglandin synthesis on systolic blood pressure in one-kidney, one clip (1K, 1C) hypertensive rats and on the fall in blood pressure after unclipping. It tested the hypothesis that inhibition of prostaglandin synthesis exacerbates hypertension in this model and prevents complete reversal after unclipping. Rats with sustained hypertension within 8 weeks of renal artery clipping were fed synthetic diets supplemented to 20% of total energy with either safflower oil (linoleic acid) or a mixture of cod liver oil (90%) (containing eicosapentaenoic acid) and linseed oil (10%) (containing linolenic acid) for 4 weeks. The latter oil mixture resulted in a predictable reduction in kidney PGE₂ and 6-keto PGF_1α (hydrolysis product of PGI₂), aortic 6-keto PGF_1α and serum TXB₂. However, at the end of 4 weeks dietary treatment there were no differences in systolic blood pressure between the two diet groups, and the blood pressure fall 24 hours after unclipping was similar. These findings therefore do not support a role for prostanoids in the maintenance or reversal of 1K, 1C hypertension.     

Effect of captopril on neurally induced contraction and relaxation of mesenteric arteries of renal hypertensive rats

The effect of captopril treatment on neurally induced vasoconstrictor and vasodilator responses was examined in the isolated mesenteric arterial bed from normotensive and one-kidney, one clip hypertensive (1K1C) rats. In isolated mesenteric beds, electrical field stimulation (EFS) of perivascular nerves at basal tone induced a frequency-dependent increase in perfusion pressure that was greater in preparations from hypertensive rats compared with those from normotensive rats. Captopril treatment was associated with a decrease in vasoconstrictor responses in the hypertensive group compared with its non-treated control. Responses to norepinephrine (320 ng) were greater in hypertensive than normotensive groups; captopril reduced this response only in the hypertensive group. In preconstricted mesenteric arteries perfused with solutions containing guanethidine (5 microM) and atropine (1 microM), EFS elicited a frequency-dependent decrease in perfusion pressure that was abolished by tetrodotoxin (1 microM). Vasodilator responses to EFS were not affected by captopril treatment, although they were smaller in the hypertensive group. Acetylcholine (10 ng) induced similar decreases in perfusion pressure of normotensive and 1K1C groups; captopril did not influence these responses. These results indicate that captopril treatment does not affect the reduced neurogenic vasodilation but normalizes the augmented sympathetic-mediated vasoconstrictor responses of mesenteric resistance vessels of chronic 1K1C hypertensive rats.     

An extract of rat submandibular glands activates platelets and enhances cutaneous vascular permeability in rats and guinea pigs

Washed, [3H]serotonin-labeled platelets from rats and guinea pigs were stimulated in vitro with a novel protein extracted from rat submandibular salivary glands (RS-PAP) and with the phospholipid platelet- activating factor 1-0-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine (AGEPC). Rat platelets, which are refractory to AGEPC stimulation, underwent shape-change, aggregation and secretion of [3H]serotonin in response to graded doses of RS-PAP and AGEPC. Intradermal injections of histamine, RS-PAP and AGEPC caused a dose-related increase in local microvascular permeability in rats, as measured by the extravasation of plasma containing Evans blue dye. Similarly, histamine, RS-PAP and AGEPC increased cutaneous vascular permeability when injected intradermally in guinea pigs. The vascular permeability induced by histamine and RS-PAP, but not by AGEPC, was partially inhibited by pretreatment with an antihistamine (diphenhydramine HCl). Pretreatment of guinea pigs with captopril, an inhibitor of angiotensin-converting enzyme (ACE), partially inhibited cutaneous responses to subsequent intradermal injections of histamine, RS-PAP and AGEPC. Regardless of pretreatment with diphenhydramine or captopril, skin test sites injected with large amounts of RS-PAP became hemorrhagic within minutes and necrotic within 12 hours.     

Regional blood flows and cardiac hemodynamics in renovascular and mineralocorticoid hypertensive rats

Regional blood flows and cardiac hemodynamics were studied in 3 models of hypertensive rats: one-kidney DOC-saline, one-kidney, one-clip and two-kidney, one-clip hypertension and in normotensive control rats. All hypertensive models were characterized by increased peripheral vascular resistance and normal cardiac output. Coronary and cerebral blood flows varied among the hypertensive models but did not significantly differ from the normotensive rats. However, coronary blood flow of one-kidney, one-clip rats (8.4 +/- 1.3 ml X min-1 X g-1) was significantly higher than that of the two-kidney one-clip rats (6.5 +/- 1.2 ml X min.-1 X g-1, P less than 0.05). Cerebral blood flow of DOC-saline rats was lower than that of two-kidney one-clip or one-kidney one-clip renovascular rats. Renal blood flows of the unclipped kidney of two-kidney renovascular rats (3.77 +/- 0.85 ml X min-1 X g-1) and DOC-saline rats (2.95 +/- 0.83 ml X min-1 X g-1) were significantly lower than those of normotensive rats (5.92 +/- 1.16 ml X min-1 X g-1, P less than 0.05). In conclusion, although vascular resistance becomes elevated in all models of experimental hypertension, regional vascular resistance and blood flow distribution may differ depending on the vasoconstrictor mechanisms that participate in each model.     

Effects of regression of cardiac hypertrophy on myocardial contractility and ventricular myosin isoenzymes

The effects of regression of cardiac hypertrophy on myocardial contractility and ventricular myosin isoenzymes were investigated in rats with renovascular hypertension. Six-week-old male Wistar rats were made hypertensive by constriction of one renal artery with a silver clip. Regression of cardiac hypertrophy was induced following the lowering of blood pressure by nephrectomy on the affected side 5–6 weeks after constriction of the renal artery and was maintained for 5–6 weeks. In contrast, myocardial hypertrophy was induced by 10–11 weeks of the hypertensive state. Isometric developed tension of isolated left ventricular papillary muscles was measured, while they were being perfused with Tyrode solution. Left ventricular myosin isoenzymes were separated by pyrophosphate gel electrophoresis. The ventricular to body weight ratio of the nephrectomized group was significantly lower than that of the hypertensive group, although it was greater than that of age-matched normal control rats. There were no significant differences in the isometric developed tension among three groups, the nephrectomized, hypertensive, and normal control rats. However, dT/dtmax tended to decrease in the hypertensive rats and recovered to normal in the nephrectomized rats. The left ventricular myosin isoenzyme pattern was shifted toward VM-3 in hypertensive rats and was shifted back toward VM-1 again in nephrectomized rats.     

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.     

Biosynthesis of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet activating factor and a hypotensive lipid) by cholinephosphotransferase in various rat tissues

The unique alkyl phospholipid, 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine, has been reported to exhibit powerful antihypertensive activity (Blank, M.L., Snyder, F., Byers, L.W., Brooks, B. and Muirhead, E.E. (1979) Biochem. Biophys. Res. Commun. 90, 1194-1200) and appears to be an extremely potent platelet-activating factor (Demopoulos, C.A., Pinckard, R.N. and Hanahan, D.J. (1979) J. Biol. Chem. 254, 9355-9358). In the present study, microsomal preparations from several rat tissues were found to catalyze the synthesis of 1-alkyl-1-acetyl-sn-glycero-3-phosphocholine by 1-alkyl-2-acetyl-sn-glycerol:CDPcholine cholinephosphotransferase reaction. Optimal conditions to measure enzyme activity were established. A subcellular survey of this cholinephosphotransferase activity showed that the enzyme was of microsomal origin. Enzyme activity was found in microsomes from several tissues; however, spleen has the highest activity of the tissues examined. Three different species of 1-alkyl-2-acetyl-sn-glycerol were all found to be substrates. The 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine synthesized in the microsomes could be hydrolyzed by adding the 100,000 x g supernatant fraction to the incubation medium. The optimum pH for formation of 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine was 8.0, which was different from the pH optimum of 8.5 observed for the long-chain diacylglycerol cholinephosphotransferases. Activity of cholinephosphotransferase towards 1-alkyl-2-acetyl-sn-glycerol was slightly enhanced and stabilized by dithiothreitol, whereas the activity towards a diacylglycerol was inhibited by dithiothreitol. The possible involvement of two different enzymes in the conversion of 1-alkyl-2-acetyl-sn-glycerol and diacylglycerol to their respective phospholipid products is discussed.     

Digitalis attenuates arterial hypertrophy in experimental hypertension

Several investigators have reported that digitalis administration reduces cardiac hypertrophy in rats with experimental hypertension. To determine whether digitalis similarly affects growth of arteries, we studied young (5- to 14-week-old), male, one-kidney, one-clip hypertensive rats (1K1C; n = 14) and one-kidney normotensive control rats (1K; n = 26). Half of the rats received digoxin (150 mg/kg body wt/day) in chow starting 1-2 weeks before clipping (1K1C-D; 1K-D); the other half were pair-fed (1K1C-C; 1K-C). Serum digoxin levels averaging 488 ng/ml were documented in rats receiving digoxin. After 3-5 weeks of hypertension (conscious tail blood pressures), and at a similar time period in normotensive control rats, we measured direct femoral arterial pressure and weighed standardized segments of the thoracic aorta. At sacrifice body weights of the four groups did not differ. In the one-kidney control rats, mean +/- SE femoral arterial pressure (1K-D, 108 +/- 3; 1K-C, 111 +/- 4, mm Hg), thoracic aortic dry weight (1K-D, 36.6 +/- 0.6; 1K-C, 36.2 +/- 1.1. mg/kg body wt), and aortic water content (1K-D, 62.7 +/- 0.4; 1K-C, 62.4 +/- 0.4, % wet weight) did not differ between rats receiving or not receiving digoxin, respectively. As compared with pooled normotensive control rats, femoral arterial pressure (1K1C-D, 165 +/- 8; 1K1C-C, 153 +/- 5), aortic water content (1K1C-D, 64.8 +/- 0.4; 1K1C-C, 64.9 +/- 0.5), and aortic weight (1K1C-D, 44.8 +/- 2.1; 1K1C-C, 50.1 +/- 1.6) were increased (P less than 0.001) in the one-kidney, one-clip rats, on or off digoxin. Comparison of hypertensive rats receiving to those not receiving digoxin revealed no differences in arterial pressure or aortic water content, but aortic growth was significantly attenuated (-41%, P = 0.02) in the hypertensive rats receiving digoxin. These results provide evidence that digoxin reduces hypertensive arterial growth by a mechanism that does not affect normal growth.     

Humoral factors and the sodium-potassium pump in volume expanded hypertension.

Bioassay studies in the old and recent literature suggest the presence of an unknown slowly acting pressor agent in the blood of animals and man with volume expanded (low renin) hypertension. Recent studies in our laboratories suggest that the sodium-potassium pump activity of blood vessels is suppressed in animals with one-kidney, one wrapped, one-kidney, one clip, and one-kidney, DOCA, salt hypertension. Similar reduction of Na⁺, K⁺-ATPase activity has been observed in the left ventricle of animals with one-kidney, one clip and one-kidney, DOCA salt hypertension. The changes do not appear to result from increased pressure since they have also been observed in veins and right ventricle. Acute volume expansion of the normal rat with saline suppresses pump activity in the tail artery and plasma from these animals suppresses pump activity when applied to a tail artery from another rat. Data in the literature indicate that the adrenergic nerve terminals are depleted of norepinephrine. Suppression of pump activity, with ouabain for example, is known to activate cardiovascular muscle and reduce norepinephrine uptake by nerve terminals. These observations suggest a role for a slowly acting ouabain-like humoral agent, which acts directly on cardiovascular muscle to increase contractility and on nerve endings to reduce reflex compensation, in the genesis of volume expanded hypertension.     

Reduction of blood pressure and increased diuresis and natriuresis during chronic infusion of atrial natriuretic factor (ANF Arg 101-Tyr 126) in conscious one-kidney, one-clip hypertensive rats

Conscious one-kidney, one-clip hypertensive rats and their normotensive controls were infused during 7 days with synthetic ANF (Arg 101-Tyr 126) at 100 ng/hr/rat (35 pmol/hr/rat) by means of osmotic minipumps. The basal blood pressure of 193 +/- 6 mmHg gradually declined to 145 +/- 6 mmHg at day 4 after the infusion was started. No changes in blood pressure were observed in ANF-infused normotensive rats. A significantly higher diuresis and natriuresis was observed in ANF-infused hypertensive rats when compared to the non-treated hypertensive group. No such changes were observed in ANF-treated normotensive animals. No differences in PRA were seen in any group. Atrial immunoreactive ANF was significantly lower in one-kidney, one-clip rats than in the normotensive animals, but whether this is the reflection of an increased release in the circulation remains to be elucidated. It is suggested that the hypotensive response of one-kidney, one-clip animals to ANF may be secondary to a dual mechanism, vasodilatation and volume depletion.     

Platelet-activating factor in normal rat uterus

Platelet-activating factor (PAF) was found in normal rat uterus and identified as 1-0-hexadecyl/octadecenyl-2-acetyl-sn-glycero-3-phosphocholine. PAF was purified by several successive chromatographic procedures. It showed platelet aggregating activity, which was inhibited by CV 3988, and had no effect on platelets desensitized with 1-0-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine. The tert-butyldimethyl-silylderivative of 1-0-alkyl-2-acetyl-sn-glycerol, which was obtained by hydrolysis of uterine PAF with phospholipase C, was analyzed by gas chromatography-mass spectrometry. One rat uterus contained approximately 21.3 ng of 1-0-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine. This is the first report of the occurrence of a significant amount of PAF in a normal animal tissue.     

Natriuretic response to saline load in one-kidney/one-clip hypertensive rats.

Parameters of renal function were studied in conscious and anesthetized one-kidney (1K) and one-kidney/one-clip (1K-1C) rats. Effective renal blood flow (ERBF) was significantly lower in anesthetized 1K-1C rats than in conscious ones (12.1 +/- 1.6 vs. 16.4 +/- 1.2 ml/min). Renal function was evaluated in two-kidney (2K), 1K and 1K-1C unanesthetized rats. ERBF was lower in 1K and 1K-1C animals than in 2K rats. Glomerular filtration rate (GFR) and urinary sodium excretion (UNa.V) were not affected by uninephrectomy with or without clipping the renal artery. In 1K-1C rats, mean arterial pressure (MAP) increased from 100 +/- 2 to 140 +/- 1 mm Hg. Subsequently, the renal ability of unanesthetized rats to handle Na was studied by a sustained extracellular fluid volume expansion (EFVE) in all groups. During EFVE, MAP remained unchanged in the 2K and 1K groups and decreased significantly in the 1K-1C group, ERBF did not change and GFR increased to the same extent in all groups. The increase in UNa.V was 40% higher in 2K than in 1K or 1K-1C rats. These findings indicate that the relatively smaller natriuretic response to a saline load of 1K rats with or without a clip in the renal artery, as compared with 2K rats, could be ascribed to renal mass reduction. Finally, the study shows the advantage of performing studies of renal function in hypertension in conscious rather than anesthetized rats.     

Antihypertensive and bilateral renal responses to diuretics in 2-kidney, 1-clip Goldblatt hypertensive rats

Experiments were conducted to assess the effect of furosemide or amiloride alone and a combination of both agents on each kidney in anesthetized 2-kidney, 1 clip Goldblatt hypertensive rats (n = 25). Intravenous infusion of furosemide alone (1.02 mg/kg.hr) significantly reduced the blood pressure by 14 +/- 5 mmHg. There were 6- to 10-fold increases in water, absolute sodium and fractional sodium excretions and a 2-fold increase in potassium excretion in the nonclipped kidney. A smaller but significant increase in the excretory function was also observed in the clipped kidney. There was no significant change in GFR of both kidneys. Indomethacin pretreatment (2 mg/kg) failed to significantly alter the vasodepressor and renal responses to furosemide in both hypertensive and normal rats. Removal of the renal artery clip from the hypertensive rats reduced the blood pressure by 12 +/- 3 mmHg and enhanced the function of the ipsilateral, unclipped kidney. Subsequent administration of furosemide further increased the excretory response. Administration of amiloride alone (2.4 mg/kg.hr) or with furosemide into hypertensive rats reduced the arterial pressure and increased excretion rates of urine flow and urinary sodium. Potassium excretion rate decreased bilaterally in amiloride treated rats but did not alter significantly in rats which received a combination of amiloride and furosemide. These results indicate that diuretics ameliorate the excretory function of both the stenotic kidney and the nonstenotic kidney and that the improvement of the kidney function is independent of prostaglandin. Furthermore, removal of the stenosis accentuates the beneficial effect of diuretics on the kidney.     

Role of sodium balance on maintenance of blood pressure in the chronic phase of two-kidney, one-clip hypertension

Blood pressure and sodium balance have been studied after unclipping, nephrectomy and sham operation of ischemic kidney in the chronic phase (16 weeks) of two-kidney, one clip hypertension. In hypertensive rats the fractional excretion of sodium was 85.5 +/- 2.3% and blood pressure (BP) was significantly increased (187.1 +/- 4.5 mmHg, p less than 0.001). Removal of either the constricting clip or ischemic kidney induced a decrease of BP to normal level whereas sham operation did not produce any change. However, rats submitted to these three experimental manipulations showed, at 1, 2, 3 and 4 days, and at 3 weeks, similar changes in sodium excretion. In the groups with unclipping or nephrectomy of ischemic kidney, water intake was less and urine volume smaller than in the sham operated group. These results suggest that a positive sodium balance is not important to maintain hypertension at this stage and that changes in sodium balance, after both unclipping and nephrectomy of ischemic kidney, have no influence in BP normalization.     

The metabolism of platelet-activating factor in human T-lymphocytes

The metabolism of 1-[3H]alkyl-2-acetyl-sn-glycero-3-phosphocholine (1-[3H]alkyl-2-acetyl-GPC; platelet-activating factor; PAF) was investigated in purified human peripheral blood T-lymphocytes and a human leukemia cell line of T-cell origin (MOLT-4). The major metabolic products of T-lymphocyte PAF metabolism are 1-alkyl-2-acyl-GPC, 1-alkyl-2-lyso-GPC and neutral lipid. The pattern of PAF metabolism in peripheral blood T-lymphocytes and MOLT-4 lymphoblasts was similar, although MOLT-4 lymphoblasts transformed PAF to 1-alkyl-2-acyl-GPC faster than peripheral blood T-lymphocytes (67% vs. 21% of added label after 64 min at 37 degrees C, respectively). Pre-exposure of MOLT-4 lymphoblasts to 1 mM of the serine hydrolase inhibitor phenylmethylsulfonyl fluoride resulted in an inhibition of PAF metabolism. Our results indicate that intact T-lymphocytes actively metabolize this biologically active phospholipid by the deacetylation-transacylation pathway.     

Effect of a potassium-deficient diet on arterial blood pressure, plasma and tissue cations, and tissue norepinephrine in the hypertensive dog

Chronic potassium deficiency in one-kidney one-clip hypertensive dogs significantly reduces blood pressure and plasma potassium, with a simultaneous increase in plasma renin activity. Tissue potassium concentration was decreased and tissue sodium concentration was increased in striated muscle and adrenal glands, which may suggest that the sodium-potassium pump was inhibited. In myocardium the sodium concentration was higher but the potassium concentration was not significantly lower than in control hypertensive dogs on normal diets. Arterial cation concentrations in the potassium-deficient group were not significantly different from those in the control group. Tissue norepinephrine concentration was higher in arteries from potassium-deficient animals, significantly so in the mesenteric and femoral arteries. The conclusion is that potassium deficiency may decrease blood pressure in the one-kidney one-clip hypertensive dogs by impairing the release of norepinephrine.     

Structural analogs of alkylacetylglycerophosphocholine inhibitory behavior on platelet activation

Several analogs of alkylacetylglycerophosphocholine (AGEPC; platelet-activating factor) were investigated as potential selective inhibitors of AGEPC-induced activation of washed rabbit platelets. Two particular compounds, CV-3988 (rac-3-(N-n-octadecylcarbamoyloxy)-2-methoxypropyl-2-thiazolioethyl++ + phosphate) and U66985 (1-O-octadecyl-2-acetyl-sn-glycero-3-phosphoric acid-6'-trimethylammonium-hexyl ester) emerged as particularly active and effective inhibitors. Aggregation and secretion profiles, as well as the degradation of inositol phospholipids and production of phosphatidic acid, were used as monitors of their inhibitory capabilities. U66985 was the most effective inhibitor, giving an IC50 value of 4.1 +/- 1.5 X 10(-8) M against a challenge of 1 X 10(-10) M AGEPC in the secretion assay. Phospholipid turnover was blocked completely at this inhibitor concentration. On the other hand, while CV-3988 was an effective inhibitor, a higher concentration was required and a more restricted range of activity was noted with an IC50 value of 5.9 +/- 1.3 X 10(-7) M against a challenge of 1 X 10(-10) M AGEPC in the secretion assay. While CV-3988 did indeed completely block the turnover of inositol phospholipids and phosphatidic acid formation, these effects were noted at a higher concentration than with U66985. On the basis of data obtained in desensitization experiments with AGEPC and U66985, it appears that each inhibitor occupies the same receptor site as the agonist, AGEPC. These results illustrate the usefulness of these AGEPC analogs in exploring the biochemical characteristics of the interaction of AGEPC with a cell.