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Two synthetic penta deca peptides corresponding to the N-terminal portion (amino acid sequence 1-15) and the C-terminal portion (sequence 32-46) of the pro moiety of the human prorenin (PR) molecule were coupled to BSA and used as antigens to generate antibodies against PR. In a RIA system using the 125I-labeled peptide as tracer, it could be shown that antibodies against peptide 32-46 bound the peptide and the native PR in the follicular fluid (FF) to a similar degree, whereas antibodies generated against peptide 1-15 did not specifically recognize native PR. The specificity of the PR-(32-46) antibodies for PR was demonstrated by comparative measurements of PR by RIA and by an indirect procedure (involving trypsin treatment of PR) in different individual FF samples, plasma samples, and fractions of FF obtained by gel filtration or immunoaffinity chromatography. Normal plasma PR levels could not be measured by RIA, since they were below the detection limit of the assay (0.5 micrograms PR/L approximately 10 pmol/L). For measurement of the low PR levels in plasma, a sensitive direct immunoradiometric assay with a detection limit of 0.1 fmol PR/tube was developed. It was based on the combined action of a commercially available solid phase renin antibody and the affinity-purified and 125I-labeled PR-(32-46) antibody. The measurement of 35 individual plasma samples with different PR concentrations showed an excellent correlation (r = 0.99) between the new direct and the conventional indirect assays. The direct assay of PR concentrations in plasma of healthy women during the length of a menstrual cycle resulted in a biphasic pattern of PR concentrations, with peak levels (approximately 3-fold increase) at the time of the LH surge. The intake of monophasic and triphasic contraceptives caused a suppression of normal PR concentrations (96.9 +/- 34 ng/L; early follicular phase; n = 12) by 39% and 25%, respectively, which was also observed in the pill-free phase of the artificial cycle.

Measurements of active and acid-activatable renin concentrations and renin substrate concentrations in plasma and endometrial samples taken from 31 patients are described. All of these patients were taking oestrogen-containing oral contraceptive preparations. Plasma active renin concentration was lower in these cycles than in normal cycles and the substrate concentrations higher. In the endometrium the concentrations of all of the components of the system were raised by oral contraceptives. As with normal cycles, the maximum potential for production of the angiotensins was in the premenstrual stage of the cycle. It is suggested that this increase in potential is associated with the onset of menstruation.

A variety of estrogen- and progestin-containing oral contraceptive agents reduced renal blood flow (RBF) significantly in 23 healthy, nonhypertensive young women, to a mean of 75 +/- 3.3% of the value expected for their age and dietary sodium intake (P less than 0.001). There was also significant activation of the renin-angiotensin system: renin substrate was increased approximately 3-fold in association with a striking increase in the circulating renin activity and angiotensin II levels in relation to sodium intake and excretion. Two observations suggest that the RBF reduction was directly mediated by angiotensin II. A correlation was demonstrable between circulating angiotensin II and RBF (P less than 0.01), and renal vascular responsiveness to angiotensin II infused into the renal artery was reduced significantly (P less than 0.001). Moreover, the oral contraceptive agents modified the basic relationship between sodium balance and vascular responsiveness to angiotensin II, suggesting that the agents acted through some mechanism other than alteration in the state of sodium balance. These observations provide further evidence for an important role of angiotensin II as a determinant of RBF. Renal vasoconstriction may contribute to the genesis of a number of complications, such as sodium retention and hypertension, associated with oral contraceptive use.

Treatment with hormonal contraceptives leads to a considerable stimulation of the renin-angiotensin-aldosterone-system. First of all, there is a 2,5- to 4fold increase of renin substrate synthesis in the liver. As a consequence, more angiotensin I and angiotensin II are released. Angiotensin II stimulates the secretion of aldosterone in the adrenals, thus producing a higher aldosterone concentration in plasma. The urinary excretion of aldosterone is elevated to a lesser degree, probably because of the simultaneously increased binding of aldosterone to plasma proteins. The release of renin is suppressed to 50% by negative feedback mechanisms. Some possible factors in the etiology of hypertension induced by oral contraceptives are discussed.

Plasma renin and angiotensin II levels were measured in nine patients immediately before and at half-hourly intervals in the four hours following therapeutic termination of pregnancy. There was a small fall in renin and angiotensin II levels over the first 1 to 2 hours, followed by a slight increase. The magnitude of these effects was much smaller than those previously seen following normal delivery. It is concluded that in early pregnancy maternal, rather than feto-placental, factors are controlling the renin-angiotensin system.

A method has been developed which allows estimation of the kinetic parameters of the plasma renin-angiotensinogen reaction from data obtained by autologous renin assays at several plasma dilutions. The quantitative aspects of the renin-angiotensin system were examined in 28 plasma samples from 23 healthy, normotensive pregnant women. They were compared with 20 women who were not pregnant, of whom 12 were taking oral contraceptives and eight were not. In the first 3 months of pregnancy, there was a sharp increase in plasma renin activity and concentration. Plasma angiotensinogen rose steadily throughout pregnancy. The higher concentrations of renin and angiotensinogen would lead to an increase in angiotensin formation with potentially adverse consequences. However, this increase may be reduced by the fall in the affinity between renin and angiotensinogen which is suggested by the present observation that the Michaelis-Menten constant (Km) attained values five to six times higher than those seen in women not taking oral contraceptives. The smaller increases in plasma renin and angiotensinogen induced by oral contraceptives were less effectively compensated by lower affinity. Circumstantial evidence is provided which suggests that the observed high Km values may be due to oestrogen/pregnancy-induced synthesis of an angiotensinogen with a lower affinity for renin.

Total and active renin concentrations (TRC and ARC) were determined in pregnant women and in women on estrogen-containing oral contraceptives to study the variation of plasma renin forms in pregnancy. TRC was already elevated in the first trimester. After that TRC increased consistently reaching the maximum in the third trimester. The ratio of inactive renin concentrations (IRC) to TRC was between 20 and 30% throughout gestation. Therefore, it was supposed that the development of the placenta or the enlargement of the uterus do not affect the ratio of IRC to TRC too much. In women on oral contraceptives in whom plasma renin activity was increased due to elevation of renin substrate, the ratio of IRC to TRC was almost the same as that in normal controls. From these results, it was suggested that the development of the placenta and the enlargement of the uterus do not play an important role in the variation of plasma renin forms, although remarkable changes are observed in the renin substrate and total amounts of renin in pregnancy.

Plasma renin activity (PRA), Growth hormone (GH) and immunoreactive insulin (IRI) were studied in seven healthy subjects during the ovulatory menstrual cycle and during the first and third cycles of oral contraception with 0.05 mg of ethinylestradiol and 0.100 mg or 0.125 mg of a new progestogen, 17 alpha-ethinyl-18-methylene-4-estren-17 beta-ol. The PRA level in the second half of the control cycle was significantly higher than in the beginning of the cycle. At the end of the treated cycles it was significantly higher than at the end of the control cycle. Neither GH nor IRI showed significant changes during the control cycle. GH was significantly higher at the end of the first and third treated cycles than at the end of the control cycle. IRI was significantly higher both in the beginning and at the end of the first treated cycle than the corresponding IRI levels in the control cycle. IRI at the end of the third treatment cycle was not significantly different from corresponding means at the end of the control cycle or the first treated cycle.

Renin release from the kidneys is controlled by four major mechanisms - renal baroreceptors, the macula densa, the sympathetic nervous system and other humoral and ionic influences. Recently, the importance of the sympathetic nervous system in modulating renin release for a variety of stimuli has become apparent. The sympathetic adrenergic nerves act on a specific beta-receptor to stimulate renin release from the kidney. Those antihypertensive drugs that inhibit adrenergic nerve transmission tend therefore to lower plasma renin. beta-Adrenoreceptor blocking drugs also generally lower plasma renin by specific inhibition of renal renin release both in animals and humans. Diuretics and vasodilators in contrast elevate plasma renin levels. Plasma renin levels in hypertensive patients are the net result of many diverse influences, and similarly in patients treated with antihypertensive drugs the plasma renin level often is the result of opposing influences. Plasma renin levels tend to return towards normal levels after treatment in hypertensive patients. Plasma renin levels may offer a guide to a more rational basis for antihypertensive therapy in the future. However, the primary aim in treatment of the hypertensive patient should be to lower his blood pressure, independent of the effect of plasma renin levels.

Prolonged cold storage of plasma may induce the conversion of plasma prorenin (inactive renin) to renin. This phenomenon is exaggerated in oral contraceptive (OC) users; the titer of Hageman factor (HF, Factor XII) in OC users is higher than in nonusers. The present study relates these observations. The increment in plasma renin activity (PRA) during cold storage, as measured by generation of angiotensin I, correlated strongly with the initial plasma titer of HF. Increasing the HF titer of nonusers to that observed in OC users by addition of purified HF increased cold-induced PRA at least twofold, while reducing the plasma HF titer of OC users correspondingly decreased cold-induced PRA. Thus, in OC users, the enhanced conversion of plasma prorenin to renin during cold storage reflects the elevated plasma titer of HF.

Direct RIA of renin with monoclonal renin antibodies and indirect RIA with angiotensin I antibodies were performed in plasma of 44 pregnant women, 44 women taking an oral contraceptive (OC), and 54 normal women. The following parameters were measured: immunoreactive renin, naturally occurring enzymatically active renin (active renin), trypsin-activatable inactive renin (prorenin), PRA, and renin substrate. Immunoreactive renin (mean, 95% confidence interval) was significantly higher in pregnant women (1090; 420-2800 pg/ml; third trimester) than in normal women (248; 101-562 pg/ml; P less than 0.001) and was lower in OC-treated women (131; 41-415 pg/ml; P less than 0.001). Prorenin and active renin also were increased in pregnant women and decreased in OC-treated women. The fraction of renin that was in the active form was lower in pregnant women (4.8; 1.4-18%) than in OC-treated women (8.8; 3.0-25%; P less than 0.001) and normal women (9.1; 2.9-29%; P less than 0.001). Renin substrate was increased to comparable levels in pregnant women and OC-treated women, but PRA was increased in pregnant women and normal in OC-treated women. The maximum velocity per unit weight of renin was the same for active renal renin as for active plasma renin and trypsin-activated plasma prorenin. Maximum velocity and Km values measured in mixtures of purified active renin and renin substrate and the concentrations of active renin and renin substrate measured in whole plasma were entered into the Michaelis-Menten equation for calculating PRA. The calculated values were similar to the measured results in all three groups, indicating that PRA was determined by the molar concentrations of enzyme and substrate. Thus, we found no evidence of unknown substances in plasma interfering with the enzyme-substrate reaction. The percentage of circulating renin in the active form was much lower during pregnancy than in other conditions where the renal release of active renin is stimulated and prorenin is as high as during pregnancy. This suggests that a smaller fraction of prorenin is intrarenally converted into active renin before its release into the circulation or that a larger fraction of circulating prorenin is of extrarenal origin. The finding that PRA is normal during OC treatment suggests that the estrogen-induced increase in renin substrate is compensated for by suppressed renal release of active renin.

Plasma renin activity (PRA) and angiotensin II (A II) concentrations were measured in the first and second half of the menstrual cycle for four months in 27 healthy young women, in three groups. During month II and III group I received lynestrenol 0.5 mg, group II lynestrenol 0.5 mg + ethinyl estradiol 0.05 mg and group III d-norgestrel 0.25 mg + ethinyl estradiol 0.05 mg as contraceptive treatment. In group I no changes of PRA or A II were observed. In group II and III both PRA and A II rose significantly during oral contraceptive treatment. Progestagens may offer an advantage as they do not stimulate the renin-angiotensin axis.

Healthy women (N = 96) were investigated for changes in plasma renin activity (PRA) in abortion. In spontaneous abortion PRA levels were significantly lower than in induced abortion although higher than in healthy women. These findings may be explained by differences in corpus luteum function and renin production of the feto-placental unit. Alternatively, prostaglandins, released by the contracting uterus in spontaneous abortion may cause PRA suppression, as observed in 9 pregnant women with abortion induced by the intra-amniotic use of prostaglandin F2alpha.

Plasma renin activity (PRA), plasma concentrations of cyclic adenosine-3',5'-monophosphate (cAMP) and the serum levels of oestradiol, progesterone, chorionic gonadotrophin, potassium, sodium and calcium were estimated in connection with termination of pregnancy induced in 23 women by sulprostone, a derivative of prostaglandin E2. A rapid decrease in PRA was observed during and after infusion of 1000 micrograms or 1500 micrograms sulprostone. PRA returned to the initial values at 24 h after drug administration. The lowest levels of PRA were 36% and 51% of the initial values (P less than 0.001) in the two drug concentration groups. No significant changes were observed in the control group. The value for plasma cAMP did not correspond completely to the changes in PRA. Serum concentrations of oestradiol, progesterone and chorionic gonadotrophin showed a gradual decrease during drug administration. The changes in serum potassium, sodium and calcium were minimal. The decrease in PRA associated with sulprostone is very surprising. Our finding elucidates in part the still rather poorly known mechanisms and function of renin-angiotensin system in pregnancy.

The effect of contraceptive ingestion and pregnancy on components of the renin-angiotensin-aldosterone system was compared in 17 non-pregnant women, 8 non-pregnant women taking oral contraceptives, and 11 pregnant women. Plasma renin substrate concentrations and the dynamic responses of plasma renin activity and aldosterone concentrations to upright posture and iv saline infusion were evaluated. Renin substrate was significantly higher in those women taking oral contraceptives and among the pregnant subjects than the other non-pregnant group. No significant differences in plasma renin activity or aldosterone concentrations were seen before or after postural stimulation among the 2 non-pregnant groups. After saline loading renin was higher and aldosterone lower in the contraceptive-treated group. In contrast, the pregnant group had significantly higher values at every point. The response to posture and saline among the pregnant subjects was similar in direction and magnitude to those of the non-pregnant groups. The urinary excretion of sodium before and after saline infusion was significantly lower in the pregnant group than in the non-pregnant groups. These observations suggest that estrogen-induced increases in renin substrate do not alone account for the increases in the renin-angiotensin-aldosterone system observed during pregnancy, but rather such increases appear to represent a physiological response to increased sodium need during pregnancy.

The possible relationship between the renal mechanism of volume control and blood pressure regulation is discussed. Expansion of the extracellular fluid (ECF) and plasma volumes was demonstrated following renal artery constriction in the rat; after about one month ECF volume returned to normal although hypertension persisted. Measurements of cardiac output in the unanesthetized rat by an implanted electromagnetic flowmeter showed an initial rise in cardiac output after renal artery constriction, returning to normal in 10 to 15 days. A homeostatic hypothesis for the production of renal hypertension is put forward in which changes in ECF volume, capacity vessel tone and myocardial contractility participate in the development of hypertension by elevating cardiac output. Autoregulation of peripheral flow then occurs and the consequent restoration of blood pressure at a renal pressure receptor results in return to normal of cardiac output by negative feedback. Thus in chronic hypertension the high peripheral resistance is maintained by autoregulation.