European Journal of Clinical Pharmacology

Europ. J. clin. Pharmacol. 14, 171-176 (1978)

© by Springer-Verlag 1978

The Effect of Angiotensin II Blockade by Saralasin (1-Sar-8-ala-angiotensin II) in Normal Man H. Ibsen, A. M. Kappelgaard, M. Damkja~r Nielsen, and J. Giese Department of Clinical Physiology, Glostrup Hospital, GIostrup, Denmark

Summary. The effects of the angiotensin II analogue saralasin were investigated in 6 normal individuals. Blood pressure, plasma renin (PRC), plasma aldosterone (PAC) and plasma saralasin were measured before and during infusion of saralasin (0.545.4 nmol/kg/min) with the subjects supine. Plasma angiotensin II concentration (PA II) was measured before the infusion. In the sodium replete state, PA II averaged 11pmol/1 (range 5 to 17). Saralasin infusion produced an increase in mean arterial pressure (MAP) and PAC, and a slight fall in PRC, which is consistent with an angiotensin II-like effect or a so-called agonistic effect. After sodium depletion, induced by hydrochlorothiazide 50-100 mg/day for 5 days, PA II was high - 91 pmol/1 on average (range 41 to 217). Angiotensin II blockade produced a fall in MAP in the supine position. The agonistic effect of saralasin on adrenal receptors during sodium depletion was less pronounced or absent. PRC increased sharply during the infusion. Infusion of saralasin at the rate of 5.4 nmol/kg/min produced a plasma saralasin concentration of about 220 nmol/ 1, i. e. in molar terms the plasma concentration of the analogue was 2000 to 10000 times higher than that of the endogenous octapeptide. The relationship between changes in MAP and basal P A I I prior to infusion showed that saralasin exhibited a shift from agonistic to antagonistic properties on vascular receptors when pre-infusion P A I I changed from approximately 20 to 40 pmol/1. A shift from agonistic to antagonistic effect on aldosterone secretion was not consistently seen. It is concluded that angiotensin II does not have a decisive role in the maintenance of normal blood pressure during normal sodium balance. However, after sodium depletion the reninangiotensin system contributes to blood pressure control, even in the supine position. In addition to its antagonistic properties, saralasin possesses a weak agonistic effect on vascular, as well as on renal and

adrenal receptors. This has to be taken into consideration when saralasin infusion is used to define "angiotensin II dependency" in patients with hypertension.

Key words: Angiotensin II, angiotensin II antagonist, aldosterone, renin, saralasin, bloodpressure control.

The exact role of the renin-angiotensin system (RAS) in maintenance of normal blood pressure has been difficult to establish. Until recently it has only been possible in clinical settings to evaluate the activity of the system from measurements of the different components in plasma. By means of specific competitive inhibitors of angiotensin II it is now possible to assess the physiological role of angiotensin II in a more direct way. Such angiotensin II-analogues are used to an increasing extent in evaluation of the role of RAS in different forms of hypertension, but their influence on normal homeostasis remains but little investigated. The aim of the present study has been to investigate the role of the renin-angiotensin system in the maintenance of normal blood pressure by use of 1Sar-8-ala-angiotensin II (saralasin), and to clarify the pharmacological effects of this angiotensin II analogue in normotensive controls, as a background for a better understanding of changes observed during studies on hypertensive patients.

Material and Methods Six healthy normotensive volunteers, 5 males and 1 female, aged 28 to 42 years, were investigated. The 0031-6970/78/0014/0171 /$01.20

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examination program and aim of the study were explained in detail and all gave their consent. While the subjects were taking their usual diet, 24 h urine collections were made, and exchangeable sodium was determined. In the morning, at the end of these examinations, the first saralasin infusion was carried out. After 60 rain rest in the supine position, an infusion of glucose 5% was given for 30 min, followed by a graded infusion of saralasin. The rate of infusion was 0.54 nmol/kg/min for 10 min, 2.7 nmol/ kg/min for 10 min and 5.4 nmol/kg/min for 15 min, comprising a total infusion period of 35 min. An infusion pump (B. Braun-Melsungen Unita I) was used. Infusion of saralasin at an incremental rate was used to avoid pronounced changes in blood pressure. Blood pressure was recorded at 2 rain intervals during the 30 min control period, during the 35 min of saralasin infusion, and for at least 20 min after cessation of saralasin infusion. During the entire period the subject rested in the supine position. Blood samples for measurement of plasma renin concentration (PRC), plasma aldosterone concentration (PAC) and plasma saralasin concentration were collected at the end of the control period before starting saralasin infusion, and at the end of the infusion period. Samples for measurement of plasma saralasin concentration were also taken at the end of the infusion at the rate of 2.7 nmol/kg/min. Samples for measurements of plasma angiotensin II concentration ( P A I I ) were collected before the saralasin infusion. One week after the initial study the volunteers started treatment with hydroehlorothiazide 5 0 100 mg/day for 5 days, and subsequently the examinations were repeated. Exchangeable sodium (NaE) was measured with 24Na, using an equilibration period of 24 h. Correction for renal loss of 24Na was made. PRC (normal range 6 - 6 0 m I U / 1 in supine position) and P A I I (normal range 3-30 pmol/l) was measured as previously described (Giese et al., 1970; Kappelgaard et al., 1976). PAC (normal range 80-500 pmol/l) was measured by a modificatio{1 of the method of Mayes et al. (1970). Our method for measuring plasma saralasin concentration in plasma has recently been published (Kappelgaard et al., 1978). Blood pressure was measured with an Arteriosonde 1217. Mean arterial pressure (MAP) was calculated as diastolic blood pressure + 1/3 of the amplitude. The control "pre-saralasin" MAP was calculated as the average of the last three measurements over a 6 rain period before infusion of saralasin. MAP during saralasin infusion was determined as the m e a n of the last three measurements before stopping the infusion, or the mean of three consecutive measurements with a lower blood pressure level in

H. Ibsen et al.: Saralasin Blockade

any other period during the infusion period at 5.4 nmol/kg/min. Similarly, the post-infusion blood pressure level was defined as the mean of the last three measurements after discontinuation of the saralasin infusion. For evaluation of the significance of a change in mean MAP during saralasin infusion in a given individual, it was necessary to estimate the variation of MAP measurements in the material. The variance of measurements of MAP was calculated from the last three MAP values in all 12 "pre-saralasin" periods. The variance was tested for homogeneity by Barlett's test. As the SD for MAP determinations in the "pre-saralasin" period was 1.6 mm Hg, it was calculated that during infusion of saralasin a change in mean MAP of more than 4 . 1 m m Hg in a given subject was significant (p < 0.01, two-tailed). Accordingly a change of 5 mm Hg or more has been regarded as significant. Wilcoxon's test for paired differences was used to evaluate changes in the parameters investigated. Figures are presented as mean and range.

Results General Change in Na E between the first and second tests averaged - 223 mmol (range + 18 to -- 480). Body weight decreased by 1.2 kg on average (range - 0.1 to -- 2.0). The 24 h sodium excretion at the first test was 170 mmol (range 113 to 208) and at the second 164 mmol (range 110 to 228). Potassium excretion was 90 mmol (range 61 to 121) and 125 mmol (range 96 to 155), respectively. Plasma sodium concentration was 139 mmol/l on average (range 137 to 140) and did not change during thiazide treatment. Plasma potassium concentration decreased from a mean 3.9 mmol/1 (range 3.6 to 4.2) to 3.2 mmol/1 (range 2.8 to 3.7; p < 0 . 0 5 ) . Plasma Saralasin Concentration During the infusion of 2.7 and 5.4 mmol/kg/min plasma saralasin concentrations averaged 112 nmol/1 (range 98 to 128) and 216 nmol/1, (range 121 to 275) respectively, in the first, 125 nmol/l (range 110 to 138) and 233 nmol/1 (range 179 to 275) respectively, in the second tests. There was no significant difference between plasma saralasin concentration in the first and second tests. Blood Pressure During normal sodium balance there was a small, significant increase in MAP (mean 5.3 mm Hg, range

H. Ibsen et al.: Saralasin Blockade I

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