Dissociation of aldosterone and prostaglandin biosynthesis in the rat adrenal glomerulosa

The role of lipid second messengers in aldosterone synthesis and secretion

Second messengers are small rapidly diffusing molecules or ions that relay signals between receptors and effector proteins to produce a physiological effect. Lipid messengers constitute one of the four major classes of second messengers. The hydrolysis of two main classes of lipids, glycerophospholipids and sphingolipids, generate parallel profiles of lipid second messengers: phosphatidic acid (PA), diacylglycerol (DAG), and lysophosphatidic acid versus ceramide, ceramide-1-phosphate, sphingosine, and sphingosine-1-phosphate, respectively. In this review, we examine the mechanisms by which these lipid second messengers modulate aldosterone production at multiple levels. Aldosterone is a mineralocorticoid hormone responsible for maintaining fluid volume, electrolyte balance, and blood pressure homeostasis. Primary aldosteronism is a frequent endocrine cause of secondary hypertension. A thorough understanding of the signaling events regulating aldosterone biosynthesis may lead to the identification of novel therapeutic targets. The cumulative evidence in this literature emphasizes the critical roles of PA, DAG, and sphingolipid metabolites in aldosterone synthesis and secretion. However, it also highlights the gaps in our knowledge, such as the preference for phospholipase D-generated PA or DAG, as well as the need for further investigation to elucidate the precise mechanisms by which these lipid second messengers regulate optimal aldosterone production.

Cyclo-oxygenase-2 inhibition increases blood pressure in rats

1 It is known that nonselective cyclo-oxygenase (COX) inhibitors have small but significant effects on blood pressure (BP), most notably in hypertensive patients on antihypertensive medication. Whether selective COX-2 inhibitors also interfere with BP regulation is not well understood. Therefore, we aimed to examine the effect of chronic treatment with a selective COX-2 inhibitor (rofecoxib) on systolic blood pressure (sBP) in normotensive Wistar-Kyoto rats (WKY) and on the developmental changes of sBP in young spontaneously hypertensive rats (SHR). In addition, we investigated a possible influence of salt intake on the effects of COX-2 inhibition on BP in these two rat strains. 2 Rofecoxib dose dependently increased sBP and decreased plasma levels of 6-keto prostaglandin (PG)F(1alpha) in WKY rats fed a normal salt diet (0.6% NaCl, wt wt(-1)), without affecting serum thromboxane (TX)B(2) levels. 3 Rofecoxib significantly elevated sBP in both rat strains fed normal salt or high salt diet (8% NaCl, wt wt(-1)), but not in rats on low salt intake (0.02% NaCl, wt wt(-1)). 4 Rofecoxib significantly decreased plasma levels of 6-keto PGF(1alpha) in both rat strains fed normal or high salt diet, but not in rats during low salt intake. 5 Rofecoxib exerted no influence on the changes of body weight nor on water intake. Plasma renin activity (PRA) and renocortical renin mRNA abundance were not changed by rofecoxib, but plasma aldosterone concentration (PAC) was significantly reduced. 6 These results suggest that chronic inhibition of COX-2 causes an increase of blood pressure that depends on prostacyclin synthesis. Furthermore, this increase is independent on genetic predisposition and can be prevented by salt deprivation. Since water intake and body weight gain were not changed by rofecoxib, fluid retention appears not to be a major reason for the development of hypertension. Similarly, an activation of the renin-angiotensin-aldosterone axis appears to be an unlikely candidate mechanism.

Incorporation of hydroxyeicosatetraenoic acids into cellular lipids of adrenal glomerulosa cells: inhibition of aldosterone release by 5-HETE

Metabolites of arachidonic acid appear to be involved in the regulation of aldosterone secretion. Adrenal cells metabolize arachidonic acid to several products including hydroxyeicosatetraenoic acids (HETEs). Since HETEs may be incorporated into the membrane lipids in some cells, we investigated whether HETEs were incorporated into lipids of adrenal glomerulosa cells and tested the influence of incorporation on aldosterone secretion. Cells were incubated with [3H] -arachidonic acid, -5-HETE, -12-HETE, -15-HETE or -LTB4. The cellular lipids were extracted and analyzed by TLC. Arachidonic acid was incorporated into all of the cell lipids with greatest accumulations in phospholipids (22%), cholesterol esters (50%), and triglycerides (21%). Uptake was maximal by 30 min. 5-HETE was incorporated into diglycerides and monoglycerides but not into phospholipids or other neutral lipids. The uptake followed a similar temporal pattern as arachidonic acid. 12-HETE was incorporated to a small extent into phospholipids, predominantly phosphatidylcholine. Neither 15-HETE or LTB4 were associated with cellular lipids. Angiotensin increased the uptake of 5-HETE and arachidonic acid into phosphatidylinositol/phosphatidylserine without altering uptake into the other lipids. When cells were pretreated with 5-HETE and washed to remove the unesterified HETE, basal aldosterone release as well as release stimulated by angiotensin, potassium and ACTH were significantly reduced. 15-HETE, which is not incorporated into cellular lipids, was without effect on aldosterone secretion. These studies indicate that 5-HETE may be incorporated into the cellular lipids of adrenal cells and may modulate steroidogenesis.

Specific action of the lipoxygenase pathway in mediating angiotensin II-induced aldosterone synthesis in isolated adrenal glomerulosa cells

Angiotensin II (AII) in adrenal glomerulosa cells activates phospholipase C resulting in the formation of inositol phosphates and diacylglycerol rich in arachidonic acid (AA). Although glomerulosa cells can metabolize AA via cyclooxygenase (CO), this pathway plays little role in aldosterone synthesis. Recent evidence suggests that the lipoxygenase (LO) pathway may be important for hormonal secretion in endocrine tissues such as the islet of Langerhans. However, the capacity of the glomerulosa cell to synthesize LO products and their role in aldosterone secretion is not known. To study this, the effect of nonselective and selective LO inhibitors on AII, ACTH, and potassium-induced aldosterone secretion and LO product formation was evaluated in isolated rat glomerulosa cells. BW755c, a nonselective LO inhibitor dose dependently reduced the AII-stimulated level of aldosterone without altering AII binding (91 +/- 6 to 36 +/- 4 ng/10(6) cells/h 10(-4) M, P less than 0.001). The same effect was observed with another nonselective LO blocker, phenidone, and a more selective 12-LO inhibitor, Baicalein. In contrast U-60257, a selective 5-LO inhibitor did not change the AII-stimulated levels of aldosterone (208 +/- 11% control, AII 10(-9) M vs. 222 +/- 38%, AII + U-60257). The LO blockers action was specific for AII since neither BW755c nor phenidone altered ACTH or K+-induced aldosterone secretion. AII stimulated the formation of the 12-LO product 12-hydroxyeicosatetraenoic acid (12-HETE) as measured by ultraviolet detection and HPLC in AA loaded cells and by a specific RIA in unlabeled cells (501 +/- 50 to 990 +/- 10 pg/10(5) cells, P less than 0.02). BW755c prevented the AII-mediated rise in 12-HETE formation. In contrast, neither ACTH nor K+ increased 12-HETE levels. The addition of 12-HETE or its unstable precursor 12-HPETE (10(-9) or 10(-8) M) completely restored AII action during LO blockade. AII also produced an increase in 15-HETE formation, but the 15-LO products had no effect on aldosterone secretion. These studies suggest that the 12-LO pathway plays a key role as a new specific mediator of AII-induced aldosterone secretion.

Effects of nonsteroidal anti-inflammatory drugs in healthy subjects

Nonsteroidal anti-inflammatory drugs inhibit cyclo-oxygenase activity and thereby reduce prostaglandin synthesis. Studies in humans have used these cyclo-oxygenase inhibitors to examine the role of prostaglandins in controlling renal function. Although short-term studies have demonstrated reductions in effective renal plasma flow, glomerular filtration rate, urinary sodium excretion, and plasma renin activity, long-term administration of nonsteroidal anti-inflammatory drugs does not result in significant or clinically important changes in renal function in normal human subjects. If healthy volunteers are placed on low-sodium diets or treated with diuretics, both renal hemodynamics and salt and water excretion can become prostaglandin-dependent. Studies in normal subjects suggest that sulindac, a nonsteroidal anti-inflammatory drug that undergoes biotransformation in the kidney, does not inhibit renal prostaglandin synthesis or urinary sodium excretion under basal conditions but may impair furosemide-stimulated prostaglandin synthesis and changes in renal function. Doses of sulindac that spare basal renal cyclo-oxygenase do inhibit extrarenal cyclo-oxygenase. The mechanism responsible for this biochemical selectivity of sulindac is not related to a differential sensitivity of the renal cyclo-oxygenase to the active metabolite of sulindac, sulindac sulfide. Sulindac sulfide, in concentrations as low as 1 nM, was equipotent to indomethacin as an inhibitor of prostaglandin E2 synthesis in primary cultures of three renal cell lines. Appropriate clinical use of all nonsteroidal anti-inflammatory drugs, including sulindac, requires careful consideration of risk factors that predispose to nephrotoxicity and careful monitoring when administered to patients at risk.

Effects of angiotensin, prostaglandin E2 and indomethacin on the early and late steps of aldosterone biosynthesis in isolated adrenal cells

The involvement of prostaglandins in the regulation of aldosterone biosynthesis was investigated in isolated adrenal glomerulosa cells. Cells were treated with cyanoketone to inhibit the 3 beta-hydroxy-steroid dehydrogenase and isolate the early step of aldosterone synthesis and the late step. Angiotensin II and PGE2 stimulated the synthesis of aldosterone in a concentration-related manner. The stimulation by both compounds was inhibited by indomethacin, a prostaglandin synthesis inhibitor. Indomethacin also inhibited arachidonic acid-stimulation of 6-keto PGF1 alpha synthesis, whereas cyanoketone was without effect. Both angiotensin II and PGE2 stimulated the synthesis of pregnenolone (the early step) in a concentration-related manner. At higher concentrations, angiotensin II also stimulated the conversion of [3H]corticosterone to [3H]aldosterone (the late step). PGE2 did not alter the late step significantly. Indomethacin had no effect on either biosynthetic step when added alone. However, it inhibited the angiotensin- and PGE2-stimulated pregnenolone synthesis by 41 and 59%, respectively (P less than 0.05). Indomethacin did not alter angiotensin stimulation of the conversion of corticosterone to aldosterone. These findings indicate that PGE2 increases the synthesis of aldosterone by stimulating the conversion of cholesterol to pregnenolone. Indomethacin inhibits angiotensin II- and PGE2-induced steroidogenesis at the same early biosynthetic step. These findings suggest that indomethacin may act by a mechanism other than a reduction in the concentration of prostaglandins.

Effect of prostaglandins on renal salt and water excretion

There are several important mechanisms by which renal prostaglandins modulate renal salt and water excretion. The role of endogenous renal prostaglandins in facilitating urinary sodium excretion and the individual nephron segments that are affected by renal prostaglandins are reviewed. The role of the administration of nonsteroidal anti-inflammatory agents on the kidney's ability to excrete salt and water both physiologically and clinically is summarized. The potential role for endogenous prostaglandins to antagonize the effect of antidiuretic hormone and to alter renal water excretion is also described. The clinical consequences of taking nonsteroidal anti-inflammatory drugs in terms of hyperkalemia, sodium retention with associated edema, and possible hyponatremia are all discussed. Although these clinical consequences are quite uncommon statistically, there are certain subsets of patients for whom additional concern is important.

The role of prostaglandins in aldosterone and corticosterone secretion by isolated perfused rat zona glomerulosa cells

The effect of PGE2 and PGA1 on aldosterone and corticosterone biosynthesis by isolated perfused rat zona glomerulosa cells has been studied. Incremental doses of each of the prostaglandins tested produced a progressive rise in aldosterone (3-4 fold increase for PGE2 and 2-3 fold increase for PGA1) and corticosterone (4-8 fold for PGE2 and 2-4 fold for PGA1). The cyclo-oxygenase inhibitors indomethacin and meclofenamate however produced no effect on basal steroidogenesis. PGE2 produced a marked potentiation of angiotensin II-induced aldosterone secretion (45.7 +/- 13.6 to 144.3 +/- 19.4 pg/ml with angiotensin II alone and 78.4 +/- 16.6 to 269.9 +/- 39.5 pg/ml with angiotensin II + PGE2). In contrast, there was no effect of PGE2 on ACTH or serotonin-induced aldosterone secretion. These data show that PGE2 and PGA1 can directly stimulate rat adrenal steroidogenesis and suggest that PGE2 may play a role in mediating angiotensin II-induced aldosterone secretion by rat zone glomerulosa cells.

The effect of angiotensin II on arachidonate metabolism in adrenal glomerulosa cells

The effect of angiotensin II on arachidonate metabolism was examined in rat adrenal glomerulosa cells. Incorporation of both [3H]arachidonate and [32P]phosphate into phosphatidylinositol (PI) were significantly stimulated by angiotensin II. These effects were abolished by lithium, a cation, which was found suitable to prevent increased synthesis of PI in our previous study (T. Balla et al., FEBS Letters 171, 179, 1984). On the other hand, the phospholipase A2 inhibitor mepacrine failed to inhibit the increased labelling of PI. These observations suggest that the increased 3H labelling of PI occurs via CDP-diacylglycerol, and not via enhanced deacylation-reacylation cycle. The validity of this assumption was further supported, since angiotensin II failed to stimulate the formation of lyso-PI, as examined by both [32P]phosphate incorporation and pulse-chase techniques. Angiotensin II decreased the incorporation of [3H]arachidonate into phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Considering that we did not find arachidonate release either from phospholipids or from other possible arachidonate sources this decrease may not be due to dilution of the tracer. Thus we assume that angiotensin II may induce a shift in phospholipid synthesis from PC and PE to phosphoinositides. These observations indicate that the enhanced hydrolysis and synthesis of PI in response to angiotensin II is not associated with increased phospholipase A2 activity in adrenal glomerulosa cells.

The renin-aldosterone system in nephrogenic diabetes insipidus and the influence of hydrochlorothiazide and indomethacin

Three generations of a family with nephrogenic diabetes insipidus were studied. Treatment of a male infant patient with hydrochlorothiazide normalized the serum sodium concentration and improved the clinical condition, but did not influence the polyuria. Although indomethacin alone was without long-term effect, combined therapy with hydrochlorothiazide and indomethacin regulated serum sodium better than hydrochlorothiazide alone. The renin-aldosterone system was not activated in healthy carriers or patients with nephrogenic diabetes insipidus neither in infancy during severe hypernatremic dehydration nor in adult patients.

Role of prostaglandins in the aldosterone response to ACTH in sodium depleted human subjects

It is well established that the response of plasma aldosterone to ACTH is enhanced in the sodium depleted state. The mechanisms for this phenomenon are not clear, however, and the present study was undertaken to determine the possible participation of endogenous prostaglandins. ACTH, 250 ug by i.m. administration, was given to 10 human subjects pretreated in four different ways: 1. Control, receiving a 200 mEq per day sodium diet; 2. Sodium depletion (60 mEq/day sodium plus furosemide) plus indomethacin; 3. Sodium depletion plus indomethacin plus captopril; and 4. Sodium depletion plus captopril. Only in the last group, in which the prostaglandin cyclooxygenase inhibitor, indomethacin, was not given during the sodium depletion, did an exaggerated aldosterone response to ACTH occur (an increase of 468% compared with an increase of 182% during control, P less than 0.005). The angiotensin converting enzyme inhibitor, captopril, did not effect this response. Thus, endogenous prostaglandins appear to be of far greater importance than the renin-angiotensin system in mediating the increased aldosterone response to ACTH administration during the sodium depleted state in man.