Plasma converting enzyme activity during the development and maintenance of experimental and spontaneous in rats, 1976

The precise identification of the form of angiotensin recovered from in vitro incubation for plasma renin activity (PRA) determinations has become increasingly important with chemical means for quantitation such as radioimmunoassay. The ability to discriminate A I and A II quantitatively in mixtures of these peptides also provides means for quantitative assessment of angiotensin converting enzyme activity. Consequently, measurement of the ratio of A I and A II recovered as the 37 C incubation product in plasma by radioimmunoassay has been used to examine plasma converting enzyme activity (PCEA). The antibody capture method was used for the determination of A II concentrations in plasma from 3 experimentally induced hypertensive models of male Sprague-Dawley rats. This method provides for the capture of A II formed by the action of the plasma converting enzyme by use of an excess of A II antiserum during the 37 C incubation process. Simultaneous measurements of A I concentrations which demonstrates PRA's were also determined. Whenever an increase or decrease in PRA was observed, similar activities were demonstrated by the plasma converting enzyme. As blood pressure increased in one model of hypertensive rats that had the left renal artery constricted with simultaneous removal of the right kidney, an increase in both PRA and PCEA was observed. Increases in these enzymatic activities, apparently caused by renal artery constriction, are assumed to be due to a negative sodium balance. Sodium depletion of this type is believed to be due to an excessive loss of sodium from the circulating blood by the kidney tubules. These physiologic phenomena apparently result in increased renal enzymatic activities. In contrast to these findings, similar PRA and PCEA activities were observed in one kidney nephrectomized control rats at 10 days. However, decreases in PRA and PCEA at 20 and 30 days indicate that some animals with only one functional kidney presumably have the ability to auto-regulate, and thereby maintain homeostasis. These results indicate an active physiological role of the converting enzyme in the maintenance of blood pressure. Increases or decreases in PCEA result in either increased or decreased concentrations of A II, the vasoconstrictor hormone of the renin-angiotensin system. The same enzymatic activities were significantly reduced in a group of rats given 1.5% saline to drink for 30 days. The results were assumed to be caused by suppression of the renin-angiotensin system due to chronic salt ingestion. In another group of rats that were administered injections (ip) of 5-hydroxytryptophan simultaneously with injections (sc) of estradiol benzoate, PRA and PCEA decreased by 100% and 131.2%, respectively, from that of the control animals. The serotonin hormone precursor, 5-HTP, apparently has a nephrotoxic effect on the kidney, thereby causing decreased enzymatic activities. The ability of the radioimmunoassay procedures used to detect very low concentrations of A I and All was determined prior to the assay on experimental plasma samples. Plasma from 36 hr 2-kidney nephrectomized rats and plasma from a group of SHR's was assayed to make these determinations. PRA and PCEA were significantly decreased in plasma from the 2-kidney nephrectomized rats due to the removal of the source of the renal enzyme (renin). PRA was 60 times greater in the SHR's plasma than that observed in the 2-kidney nephrectomized rats. In the meantime PCEA was increased by approximately 700%. While these results show the validity and effectiveness of the radioimmunoassay, they simultaneously implicate the converting enzyme (CE) as an important factor in regulating the levels of A II in plasma which subsequently regulate blood pressure. These results further imply that it is important to measure converting enzyme activity (CEA) under conditions in which the renin-angiotensin system is suspected of playing a role. This is evident from these results since regulation of CEA might not be reflected in measured PRA. Under these conditions the importance of CEA is a better index of hypertension of a renal orgin than renin, because the action of the CE is the last step in the enzymatic reactions of the renin-angiotensin system, with the subsequent liberation of A II