Effect of uremic human plasma on renin activity

Increased enzymatic activity of renin and hyperlipidemia

We have previously reported that the in vitro enzymatic activity of exogenous renin, plasma renin reactivity (PRR), is increased in plasma of patients with chronic renal failure, possibly due to the deficiency of a renin inhibitor. To determine whether increases PRR is related to renal failure per se or to hyperlipidemia, PRR was measured in 10 control subjects, 10 patients with renal failure, and 10 hyperlipidemic patients with normal renal function. Compared to that in control subjects (52.6 ng angiotensin I generated per ml/h +/- 3.8 SE) PRR was increased (P < 0.05) in plasma of uremic patients (65.1 +/- 4.3) and hyperlipidemic patients (71.4 +/- 10.7). Renin substrate concentration did not differ among groups, and after denaturation of endogenous substrate by acidification of plasma, PRR was still increased. A "protein-free" extract of plasma from normal subjects inhibited renin, whereas little or no inhibition occurred with a comparable extract from uremic patients and hyperlipidemic patients. Thus, alterations in lipid metabolism may account for the increased enzymatic activity of renin in uremic plasma. Increased PRR may be related to the deficiency of a normally occurring renin inhibitor.

In vitro and in vivo inhibition of renin by fatty acids

Circulating neutral lipids inhibit the in vitro renin reaction. To identify the inhibitor(s), free fatty acids were added to human renin and homologous substrate. Capric, lauric, palmitoleic, linoleic, and arachidonic acids each inhibited the rate of angiotensin I production in vitro (P less than 0.01). Inhibition by polysaturated fatty acids (linoleic and arachidonic) was less (P less than 0.01) after catalytic hydrogenation of the double bonds. To evaluate an in vivo effect of renin inhibition intra-arterial blood pressure responses to infusions of renin and angiotensin II (5.0 microgram) were measured in anephric rats (n = 6) before and after infusion of linoleic acid (10 mg iv). Mean increase of blood pressure to angiotensin II before (75 mmHg +/- 9) and after (90 +/- 12) linoleic acid did not differ (P greater than 0.05). However, the pressor response to renin after linoleic acid (18 +/- 3) was less (P less than 0.00)) than that before (102 +/- 13). In summary, several fatty acids inhibit the in vitro renin reaction, and in part inhibition is dependent on unsaturation. Linoleic acid also inhibits the in vivo pressor response to renin. These results suggest that fatty acids may modify the measurement of plasma renin activity and may also affect angiotensin production in vivo.