Effect of prostaglandins on the velocity of the reaction between human renin and homologous renin substrate

Inhibition of renin by plasma linoleic acid

The authors have demonstrated previously that human plasma contains an inhibitor(s) of the enzymatic activity of renin. The purpose of this study is to identify the circulating renin inhibitor. Plasma initially was fractionated with preparative sephacryl S-300 chromatography. A fraction with low protein content inhibited the in vitro enzymatic activity of human renin by 76%. The inhibitory activity of this fraction was not altered by boiling and/or acidification. This fraction was applied to an affinity column, using purified mouse submaxillary renin as ligand. The inhibitor was recovered after elution with hypertonic NaCl. Linoleic acid, a previously identified renin inhibitor, was present in this fraction. The authors conclude that circulating linoleic acid inhibits renin. Conceivably, the overall activity of the renin-angiotensin-aldosterone cascade may be modified by alterations of plasma fatty acid concentrations.

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.

Role of prostaglandins in the pathogenesis of Bartter's syndrome

Increased renal prostaglandins activated by beta-catecholamines could produce renal tubular sodium wasting and angiotensin pressor resistance observed in Bartter's syndrome. We therefore measured plasma renin activity (PRA), aldosterone and prostaglandin A (PGA) by radioimmunoassay, and body composition by isotope dilution prior to and following beta-adrenergic blockade with propranolol (200 mg/day for 4 days) and prostaglandin synthesis inhibition by indomethacin (200 mg/day for 4 days) in a patient with Bartter's syndrome on a 250 meq sodium diet. After the administration of propranolol, body weight increased 3 kg, daily urine sodium decreased within 24 hours from 230 to 64 meq, and urine potassium from 102 to 45 meq, but PRA and the aldosterone level remained elevated. With the administration of indomethacin, body weight increased 5 kg, daily urinary sodium decreased within 24 hours to 11meq and urine potassium to 16 meq, PRA (normal less than 3 ng/100 ml/hour) decreased from 55 to 4.3 ng/ml/hour, plasma aldosterone (normal less than 8 ng/100 ml) from 74.1 to 3.6 ng/100 ml, and whole blood PGA (normal 546 +/- 307 pg/ml) decreased from 1,390 and 945 to 86 pg/ml. After the administration of propranolol or indomethacin, exchangeable sodium, total body water, extracellular volume and plasma volume all increased from less than to greater than predicted, and pressor resistance to angiotensin was normalized. These results suggest that Bartter's syndrome results from beta adrenergic and prostaglandin-mediated proximal tubular rejection of sodium leading to increased distal sodium-potassium exchange.

Big renin: identification, chemical properties and clinical implications

Big renin has a greater molecular weight (63,000 versus 43,000) than normal renin, but it shares the characteristic enzymatic and immunologic properties of normal renin. As it exists in the kidney or plasma of a patient, big renin is less active than normal renin, but its enzymatic activity is greatly enhanced by exposure to pH values of 3.0 to 3.6 or by brief incubation with pepsin or trypsin. Use of the terms prorenin and zymogen might be withheld until big renin is shown to exist in normal tissue or plasma and to be converted to normal renin in vivo. To date, big renin has been found in renal tumors and other abnormal kidney tissues as well as in the plasma of patients with renal disorders. The remarkable activation of big renin at pH levels of 3.3 can be used to detect its presence. If a method involving acidification is used to quantitate plasma renin activity of a patient with circulating big renin, the activated plasma renin activity greatly exceeds that measured in plasma maintained at neutral pH. Gel filtration of plasma is used to prove the presence of big renin. When large amounts of big renin are secreted by a renal tumor, hyperfusion may ensue and be cured by removal of the tumor. The secretion of small amounts of big renin does not necessarily result in any physiologic disorder. However, if there is a concomitant diminution or absence of normal renin a state of apparent hyporeninemia exists, as we have observed in diabetic nephropathy; this may be associated with hypoaldosteronism and hyperkalemia. Big renin does not appear to respond to physiologic changes that stimulate or suppress normal plasma renin activity. The finding of big renin may indicate the presence of certain renin-secreting renal tumors or other renal disorders, especially diabetic nephropathy.