Induction of renin release by exogenous prostaglandins in hyporeninemic hypoaldosteronism

Evidence of prostacyclin deficiency in the syndrome of hyporeninemic hypoaldosteronism

Hyporeninemic hypoaldosteronism is an important cause of hyperkalemia and is characterized by low renin secretion. We found that prostacyclin, a potent vasodilator and renin secretagogue, was markedly reduced--as reflected by its stable urinary metabolite 6-keto-prostaglandin F1 alpha--in seven patients with hyporeninemic hypoaldosteronism as compared with seven matched controls with renal insufficiency and as compared with 12 normal volunteers (mean +/- SE, 42 +/- 7 vs. 185 +/- 37 and 164 +/- 20 ng per gram of creatinine, respectively; P less than 0.001). In contrast, renal prostaglandin E2 excretion was similar in all three groups. A low-dose infusion of calcium or norepinephrine (known stimulants of prostacyclin) increased renal prostacyclin release in normal subjects and controls with renal insufficiency. Neither agonist, however, increased the low basal prostacyclin excretion in the patients (49.6 +/- 11 [basal] vs. 62 +/- 20 [norepinephrine] and 47.5 +/- 16 [calcium]; P greater than 0.8). To evaluate the functional importance of the altered prostacyclin production, we studied the responses of renal blood flow and blood pressure to the calcium infusion. The calcium infusion did not alter blood pressure or renal blood flow in the normal subjects or the controls with renal insufficiency. In contrast, the same dose of calcium in the patients with hyporeninemic hypoaldosteronism produced a rise in mean blood pressure (from 91 +/- 6 to 104 +/- 8 mm Hg, P less than 0.05) and a fall in renal blood flow (from 673 +/- 58 to 560 +/- 42 ml per minute per 1.73 m2, P less than 0.05). These results indicate that a deficiency of prostacyclin could explain the low active-renin concentration and altered vasomotor tone seen in hyporeninemic hypoaldosteronism.

Increased prostaglandin production by glomeruli isolated from rats with streptozotocin-induced diabetes mellitus

Abnormalities in glomerular function have been observed frequently in the early stages of both clinical and experimental diabetes mellitus. Because prostaglandins (PGs) are present in the glomerulus and have profound effects on glomerular hemodynamics, and because abnormalities of PG metabolism have been noted in other tissues from diabetics, we studied PG biosynthesis in glomeruli obtained from rats in the early stages of experimental diabetes mellitus. Streptozotocin, 60 mg/kg, was administered intravenously to male Sprague-Dawley rats. Control rats received an equal volume of the vehicle. Glomeruli were isolated 9-23 d later. Production of eicosanoids was determined by two methods: by direct radioimmunoassay after incubation of glomeruli under basal conditions and in the presence of arachidonic acid (C20:4), 30 microM, and by radiometric high-performance liquid chromatography (HPLC) after incubation of glomeruli with [14C]C20:4. When assessed by radioimmunoassay, mean basal production of both prostaglandin E2 (PGE2) and prostaglandin F2 alpha (PGF2 alpha) was twofold greater in the diabetic animals whereas production of thromboxane B2 (TXB2) was not significantly greater than control. In response to C20:4, both PGE2 and PGF2 alpha were also greater in the diabetic animals, but these differences were not statistically significant. The increased rate of basal PG production did not appear to be related directly to the severity of the diabetic state as reflected by the degree of hyperglycemia at the time of sacrifice. In fact, the rates of glomerular PG production in the individual diabetic animals correlated inversely with the plasma glucose concentration. The increased rate of PG synthesis did not appear to be due to a nonspecific effect of streptozotocin inasmuch as glomerular PG production was not increased significantly in streptozotocin-treated rats which were made euglycemic by insulin therapy. Furthermore, addition of streptozotocin, 1-10 mM, to the incubation media had no effect on PGE2 production by normal glomeruli. PGE2 production by normal glomeruli was also not influenced by varying the glucose concentration in the incubation media over a range of 1-40 mM. When metabolism of [14C]C20:4 was evaluated by high-performance liquid chromatography conversion to labeled PGE2, PGF2 alpha, TXB2, and hydroxyheptadecatrienoic acid by diabetic glomeruli was two- to threefold greater compared with that in control glomeruli, whereas no significant difference in conversion to 12- and 15-hydroxyeicosatetraenoic acid occurred. These findings indicate that glomerular cyclooxygenase but not lipoxygenase activity was increased in the diabetic animals. A concomitant increase in glomerular phospholipase activity may also have been present to account for the more pronounced differences in PG production noted in the absence of exogenous unlabeled C20:4. These abnormalities in PG biosynthesis by diabetic glomeruli may contribute to the altered glomerular hemodynamics in this pathophysiologic setting.

Studies of the renal vasoactive systems in hyporeninemic hypoaldosteronism

Plasma renin activity, total renin, active renin, and aldosterone were measured as well as urinary prostaglandin E2 and kallikrein in a group of patients with hyperkalemia (6.1-7.7 mEq per liter) and hyporeninemic hypoaldosteronism. Plasma renin activity and aldosterone were low and the response was markedly blunted to upright posture, and furosemide. The rise in cortisol but not aldosterone was normal following ACTH stimulation. Active renin was depressed; however, total renin was normal. Urine PGE was variable including some low values, but the mean of the group was normal (p greater than 0.1). Urine kallikrein excretion was markedly diminished and undetectable in most cases. Fludrocortisone normalized potassium but minimally increased kallikrein in the patients. The possibility exists that kallikrein deficiency in these patients may underlie the inability to generate active renin.