Rapid non-genomic vasoconstrictor actions of aldosterone in the renal microcirculation

Non-genomic actions of steroid hormones on the contractility of non-vascular smooth muscles

Steroid hormones play an important role in physiological processes. The classical pathway of steroid actions is mediated by nuclear receptors, which regulate genes to modify biological processes. Non-genomic pathways of steroid actions are also known, mediated by cell membrane-located seven transmembrane domain receptors. Sex steroids and glucocorticoids have several membrane receptors already identified to mediate their rapid actions. However, mineralocorticoids have no identified membrane receptors, although their rapid actions are also measurable. In non-vascular smooth muscles (bronchial, uterine, gastrointestinal, and urinary), the rapid actions of steroids are mediated through the modification of the intracellular Ca2+ level by various Ca-channels and the cAMP and IP3 system. The non-genomic action can be converted into a genomic one, suggesting that these distinct pathways may interconnect, resulting in convergence between them. Sex steroids mostly relax all the non-vascular smooth muscles, except androgens and progesterone, which contract colonic and urinary bladder smooth muscles, respectively. Corticosteroids also induce relaxation in bronchial and uterine tissues, but their actions on gastrointestinal and urinary bladder smooth muscles have not been investigated yet. Bile acids also contribute to the smooth muscle contractility. Although the therapeutic application of the rapid effects of steroid hormones and their analogues for smooth muscle contractility disorders seems remote, the actions and mechanism discovered so far are promising. Further research is needed to expand our knowledge in this field by using existing experience. One of the greatest challenges is to separate genomic and non-genomic effects, but model molecules are available to start this line of research.

Aldosterone sensitizes connecting tubule glomerular feedback via the aldosterone receptor GPR30

Increasing Na delivery to epithelial Na channels (ENaC) in the connecting tubule (CNT) dilates the afferent arteriole (Af-Art), a process we call connecting tubule glomerular feedback (CTGF). We hypothesize that aldosterone sensitizes CTGF via a nongenomic mechanism that stimulates CNT ENaC via the aldosterone receptor GPR30. Rabbit Af-Arts and their adherent CNTs were microdissected and simultaneously perfused. Two consecutive CTGF curves were elicited by increasing luminal NaCl in the CNT. During the control period, the concentration of NaCl that elicited a half-maximal response (EC50) was 37.0 ± 2.0 mmol/l; addition of aldosterone 10(-8) mol/l to the CNT lumen caused a left-shift (decrease) in EC50 to 19.3 ± 1.3 mmol/l (P = 0.001 vs. control; n = 6). Neither the transcription inhibitor actinomycin D nor the translation inhibitor cycloheximide prevented the effect of aldosterone (control EC50 = 34.7 ± 1.9 mmol/l; aldosterone+actinomycin D EC50 = 22.6 ± 1.6 mmol/l; P < 0.001 and control EC50 = 32.4 ± 4.3 mmol/l; aldosterone+cycloheximide EC50 = 17.4 ± 3.3 mmol/l; P < 0.001). The aldosterone antagonist eplerenone prevented the sensitization of CTGF by aldosterone (control EC50 = 33.2 ± 1.7 mmol/l; aldosterone+eplerenone EC50 = 33.5 ± 1.3 mmol/l; n = 7). The GPR30 receptor blocker G-36 blocked the sensitization of CTGF by aldosterone (aldosterone EC50 = 16.5 ± 1.9 mmol/l; aldosterone+G-36 EC50 = 29.0 ± 2.1 mmol/l; n = 7; P < 0.001). Finally, we found that the sensitization of CTGF by aldosterone was mediated, at least in part, by the sodium/hydrogen exchanger (NHE). We conclude that aldosterone in the CNT lumen sensitizes CTGF via a nongenomic effect involving GPR30 receptors and NHE. Sensitized CTGF induced by aldosterone may contribute to renal damage by increasing Af-Art dilation and glomerular capillary pressure (glomerular barotrauma).

Moderate antiproteinuric effect of add-on aldosterone blockade with eplerenone in non-diabetic chronic kidney disease. A randomized cross-over study

Background

Reduction of proteinuria and blood pressure (BP) with blockers of the renin-angiotensin system (RAS) impairs the progression of chronic kidney disease (CKD). The aldosterone antagonist spironolactone has an antiproteinuric effect, but its use is limited by side effects. The present study evaluated the short-term antiproteinuric effect and safety of the selective aldosterone antagonist eplerenone in non-diabetic CKD.

Study Design

Open randomized cross-over trial.

Setting and Participants

Forty patients with non-diabetic CKD and urinary albumin excretion greater than 300 mg/24 hours.

Intervention

Eight weeks of once-daily administration of add-on 25–50 mg eplerenone to stable standard antihypertensive treatment including RAS-blockade.

Outcomes & Measurements

24 hour urinary albumin excretion, BP, p-potassium, and creatinine clearance.

Results

The mean urinary albumin excretion was 22% [CI: 14,28], P<0.001, lower during treatment with eplerenone. Mean systolic BP was 4 mmHg [CI: 2,6], P = 0.002, diastolic BP was 2 mmHg [CI: 0,4], P = 0.02, creatinine clearance was 5% [CI: 2,8], P = 0.005, lower during eplerenone treatment. After correction for BP and creatinine clearance differences between the study periods, the mean urinary albumin excretion was 14% [CI: 4,24], P = 0.008 lower during treatment. Mean p-potassium was 0.1 mEq/L [CI: 0.1,0.2] higher during eplerenone treatment, P<0.001. Eplerenone was thus well tolerated and no patients were withdrawn due to hyperkalaemia.

Limitations

Open label, no wash-out period and a moderate sample size.

Conclusions

In non-diabetic CKD patients, the addition of eplerenone to standard antihypertensive treatment including RAS-blockade caused a moderate BP independent fall in albuminuria, a minor fall in creatinine clearance and a 0.1 mEq/L increase in p-potassium.

Trial Registration

Clinicaltrials.gov NCT00430924

Classic versus non-classic receptors for nongenomic mineralocorticoid responses: emerging evidence

Mineralocorticoids, which are synthesized locally in the central nervous system in addition to their adrenal production, trigger both genomic and nongenomic responses. Several functions of mineralocorticoids in the CNS are known to date, which are reviewed along with nongenomic responses in other tissues. A controversy regarding the identity of receptors that mediate nongenomic, transcription-independent cellular responses to steroids is presently attracting considerable scientific interest. While there is strong evidence for classic receptors belonging to the nuclear receptor superfamily to mediate nongenomic steroid effects in some cases, it does not exist for others. Recent findings on new and unexpected properties of classic receptors have partially withdrawn the interest from novel, non-classic membrane receptors, which are being progressively identified at present. This has been facilitated by the robust and elaborate toolkit for classic receptor studies in contrast to the comparably immature research tools for alternative receptors. To know the nature of receptors involved may be the key to beneficial medical translation of specific and targeted steroid responses.