Usefulness and limitations of saralasin, a partial competitive agonist of angioten II, for evaluating the renin and sodium factors in hypertensive patients

Abstract

Saralasin was infused into 52 untreated hypertensive patients. Immediate, transient pressor responses occurred in 94 per cent followed by a more gradual sustained change in blood pressure reaching an apogee in about 20 minutes. Most (86 per cent) patients with high renin values had sustained depressor responses irrespective of sodium balance. In contrast, during a normal sodium intake, the drug produced a neutral (45 per cent) or mildly pressor (50 per cent) response in patients with normal renin and pressor responses in patients with low renin values. Sodium depletion abolished pressor responses and resulted in depressor responses in 64 per cent of the patients with normal renin values. The pretreatment angiotensin level appeared to determine direction and amplitude of the response to saralasin, since increases and decreases in diastolic pressure exhibited a highly significant relationship to the control renin lever (r = 0.80, p less than 0.001). Above a neutral range of control renin values, from 2 to 7 ng Al/ml/hour, depressor responses were the rule, and below it pressor responses were consistent. Sodium balance also appeared to determine the amplitude of the response. In a subset of patients with similar renin values (range 1.4 to 2.2 ngAl/ml/hour), the induced pressor responses correlated directly with the 24-hour sodium excretion (p less than 0.05). For all patients, the induced pressure change also was related to the rate of sodium excretion (r = 0.53, p less than 0.001). The data suggest that saralasin behaves as a partial competitive agonist of angiotensin II. For this reason, saralasin testing provided only a rough physiologic validation for renin profiling. Thus, depressor responses expose most patients with high renin values. Neutral responses occur in many patients with normal renin and intermediate renin values. But pressor responses occur in subjects with either low or normal renin levels and they may reflect sodium and volume excess associated with a partial or relative absence of renin. Accordingly, due to its partial agonism, saralasin testing under-estimates the renin factor. Hence, the drug cannot be used to identify or exclude renin involvement in the blood pressure in the large majority of hypertensive patients who do not exhibit depressor responses. For them an agent devoid of agonism is required. Moreover, prior sodium depletion as a device to increase the frequency of depressor responses to saralasin does not measure intrinsic renin dependency of the blood pressure but rather the reactivity of the system to sodium depletion.