Of all nutrients that may modify arterial pressure, hypertension is most convincingly related to dietary NaCl intake. In some experimental animals and in a significant percentage of hypertensive humans, the development of hypertension is dependent on a high dietary intake of NaCl (Horan et al. 1985). Although it is generally assumed that the effect of NaCl on blood pressure is specifically related to sodium, in 1904 Ambard and Beaujard reported that both urinary chloride excretion and blood pressure were reduced in hypertensive patients consuming a salt restricted diet. Their focus on chloride rather than sodium was related to the ease with which it could be measured. With the advent of techniques for measuring sodium, interest in NaCl dependent hypertension subsequently became focused on sodium. Several years ago, we initiated a series of studies to determine if the anion provided with sodium in the diet is important in the development of salt sensitive hypertension.

The resolution of angiotensinogen (Mr56 800) linked to a decapeptide, angiotensin I (Mr 1295), from the same protein without the decapeptide presents a challenging separation problem. Polyacrylamide gel electrophoresis at an optimized pH of 6.88 is unable to distinguish between the two proteins with statistical significance on the basis of size, but is able to discriminate between them with significance on the basis of their net charge difference. Correspondingly, the proteins are separated in gel electrofocusing with a pI difference of about 0.1 pH unit. For practical purposes, neither the charge separation by gel electrophoresis nor that by steady‐state electrofocusing seems sufficiently convenient. By contrast, transient state electrofocusing provides a ready means for their separation.

Angiotensinogen and the product of its hydrolysis by renin, des-angiotensin I-angiotensinogen, were quantitated in human plasma and in cerebrospinal fluid (CSF) by a direct RIA. This assay was developed using polyclonal antibodies raised against pure human angiotensinogen. The antibodies recognized only primate angiotensinogen and des-angiotensin I-angiotensinogen. Results obtained with the direct RIA were compared with those of the indirect assay which measures angiotensinogen through angiotensin I liberated by an excess of renin. Both assays gave almost identical results in normal subjects whereas in three different conditions characterized by a high renin level (severe hypertension plus low sodium diet, converting enzyme inhibition, and adrenal insufficiency) higher results were obtained by the direct assay. This difference between the results of both methods was attributed to des-angiotensin I-angiotensinogen accumulation which is detected only in the direct assay. CSF angiotensinogen had similar immunochemical properties to plasma angiotensinogen and could also be measured by the direct RIA. Isoelectric focusing of plasma angiotensinogen and des-angiotensin I-angiotensinogen revealed a similar microheterogeneity. Microheterogeneity was also a characteristic of CSF angiotensinogen, but its isoelectric point was more basic than plasma angiotensinogen.

The effects of converting enzyme inhibition on plasma renin substrate concentration were studied in man and rat. This study use new direct radioimmunoassays of angiotensinogen completing the classical enzymatic methods. In human investigation converting enzyme is inhibited after Captopril treatment. Our results demonstrated that resulting increase of plasma renin concentration enhanced the consumption of renin substrate as shown by the fall of angiotensinogen levels measured by indirect method. In the rat, we observed the same drop of renin substrate during MK421 administration. The fall of angiotensinogen levels, measured by indirect method, was not in agreement with results of direct radioimmunoassay. This discrepancy can be explained by the accumulation of des-angiotensin I-angiotensinogen in plasma. These modifications are potentiated by sodium depletion.

In 10 severely hypertensive patients, on a low sodium diet, converting enzyme inhibition increased plasma renin activity and decreased plasma renin substrate. The use of direct radioimmunoassays for both the enzyme and its substrate showed that the number of immunoreactive renin molecules increased from 11.3 +/- 4.9 to 31.7 +/- 25.3 pmol 1(-1) whereas the number of immunoreactive renin substrate molecules decreased from 1.04 +/- 0.35 to 0.74 +/- 0.16 mumol 1(-1). The direct radioimmunoassay for angiotensinogen gave higher values than the direct enzymatic assay, and during converting enzyme inhibition, the difference between both methods increased in proportion to the rise in circulating renin. It is concluded that the difference between the renin substrate radioimmunoassay, which measures angiotensinogen and des-angio I-angiotensinogen, and the renin substrate enzymatic assay which only measures "active" substrate, is an index of the increased consumption of renin substrate, in a situation where the fall in angiotensin II enhances renin release and decreases renin substrate release.