Dantrolene inhibits adrenal steroidogenesis by a mechanism independent of effects on stored calcium release

Small-molecule inhibitors of cyclophilin D as potential therapeutics in mitochondria-related diseases

Cyclophilin D (CypD) is a key regulator of mitochondrial permeability transition pore (mPTP) opening. This pathophysiological phenomenon is associated with the development of several human diseases, including ischemia-reperfusion injury and neurodegeneration. Blocking mPTP opening through CypD inhibition could be a novel and promising therapeutic approach for these conditions. While numerous CypD inhibitors have been discovered to date, none have been introduced into clinical practice, mostly owing to their high toxicity, unfavorable pharmacokinetics, and low selectivity for CypD over other cyclophilins. This review summarizes current knowledge of CypD inhibitors, with a particular focus on small-molecule compounds with regard to their in vitro activity, their selectivity for CypD, and their binding mode within the enzyme's active site. Finally, approaches for improving the molecular design of CypD inhibitors are discussed.

Small-molecular inhibitors of Ca²⁺-induced mitochondrial permeability transition (MPT) derived from muscle relaxant dantrolene

A structure consisting of substituted hydantoin linked to a 5-(halophenyl)furan-2-yl group via an amide bond was identified as a promising scaffold for development of low-molecular-weight therapeutic agents to treat vascular dysfunction, including ischemia/reperfusion injury. Among the compounds synthesized, 5-(3,5-dichlorophenyl)-N-{2,4-dioxo-3-[(pyridin-3-yl)methyl]imidazolidin-1-yl}-2-furamide (17) possessed the most potent inhibitory activity against Ca(2+)-induced mitochondrial swelling. The structural development, synthesis and structure-activity relationship of these compounds are described.

Effect of oral administration of dantrolene sodium on serum creatine kinase activity after exercise in horses with recurrent exertional rhabdomyolysis

OBJECTIVE

To determine the effect of oral administration of dantrolene sodium on serum creatine kinase (CK) activity after exercise in horses with recurrent exertional rhabdomyolysis (RER).

ANIMALS

2 healthy horses and 5 Thoroughbreds with RER.

PROCEDURE

3 horses received 2 doses of dantrolene (4, 6, or 8 mg/kg, p.o., with and without withdrawal of food) 2 days apart; 90 minutes after dosing, plasma dantrolene concentration was measured spectrofluorometrically. On the basis of these results, 5 Thoroughbreds with RER from which food was withheld received dantrolene (4 mg/kg) or an inert treatment (water [20 mL]) orally 90 minutes before treadmill exercise (30 minutes, 5 d/wk) during two 3-week periods. Serum CK activity was determined 4 hours after exercise. Plasma dantrolene concentration was measured before and 90 minutes after dosing on the first and last days of dantrolene treatment and before dosing on the first day of the inert treatment period,

RESULTS

90 minutes after dosing, mean +/- SEM plasma dantrolene concentration was 0.62 +/- 0.13 and 0 microg/mL in the dantrolene and inert treatment groups, respectively. Serum CK activity was lower in dantrolene-treated horses (264 +/- 13 U/L), compared with activity in water-treated horses (1,088 +/- 264 U/L). Two horses displayed marked muscle stiffness on the inert treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

In 5 horses with RER from which food had been withheld, 4 mg of dantrolene/kg administered orally provided measurable, though variable, plasma concentrations and significantly decreased serum CK activity after exercise in 4 of those horses.

Stimulation of aldosterone secretion by benzodiazepines in bovine adrenocortical cells

Previous studies have indicated that peripheral benzodiazepine receptor (PBR) ligands inhibit aldosterone secretion in isolated adrenal zona glomerulosa cells although positive responses have been demonstrated in other steroidogenic tissues. In the present study, aldosterone secretion was measured in bovine cells after 6 days of primary culture. At this time, basal aldosterone secretion was very low and cells appeared less sensitive to the steroidogenic effects of extracellular [K+] (maximal response required K+ concentration > 32 mmol/L) but were sensitised to angiotensin II (maximal response achieved with 3 nM) when compared with previous studies with freshly isolated cells. Diazepam concentration in the range 0.1 nM to 1 microM increased basal aldosterone secretion, an effect which was not enhanced by pre-treatment with diazepam. The effects were small compared with those of angiotensin II or K+. Over the same concentration range, diazepam also potentiated the stimulatory effects of sub-maximally effective concentrations of angiotensin II. When cells were treated with high-density lipoprotein (HDL-3) as a source of cholesterol, diazepam and the PBR ligands Ro5-4864 and PK11195 also stimulated aldosterone secretion at nanomolar concentrations. In addition, the conversion of added 11-deoxycorticosterone (DOC) to aldosterone was increased by nanomolar concentrations of diazepam and Ro5-4864 but inhibited by high micromolar concentrations of these drugs (100 microM). We conclude that adrenocortical responses to PBR ligands are complex. At high concentrations, inhibitory effects involving competition for steroidogenic enzymes and calcium channel blockage predominate. At low concentrations, an enhancement of basal, angiotensin-II and cholesterol-dependent aldosterone synthesis is revealed which may involve a PBR-mediated mitochondrial uptake of cholesterol and DOC.

Regulation of adrenocortical steroidogenesis by benzodiazepines

Benzodiazepines affect steroidogenesis in at least four ways depending on concentration and adrenocortical cell type. Firstly, at micromolar concentrations, they inhibit steroidogenic enzymes. Competition for microsomal 17- and 21-hydroxylase activity explains the inhibition of ACTH-stimulated aldosterone and cortisol synthesis by diazepam and midazolam. At slightly higher concentrations, we have evidence that 11 beta-hydroxylase activity is also inhibited. Secondly, at sub-micromolar concentrations, calcium influx is inhibited. T-type and L-type calcium channels appear to be blocked, this impairs signal response coupling and, in particular, decreases angiotensin- and K(+)-stimulated aldosterone synthesis in zona glomerulosa cells. Thirdly, the mitochondrion of steroidogenic tissues is a sensitive site for the stimulatory effects of benzodiazepines. Aldosterone synthesis from added HDL-cholesterol by cultured bovine zona glomerulosa cells is stimulated by diazepam, RO5-4864 and PK11195. The fourth site of benzodiazepine's effect on steroidogenesis is particular to zona glomerulosa cells. In addition to cholesterol side chain cleavage, the final part of the aldosterone biosynthetic pathway, the conversion from deoxycorticosterone is controlled. Although high micromolar concentrations of diazepam appear to be inhibitory, lower nanomolar concentrations stimulate the synthesis of aldosterone from added deoxycorticosterone. In vivo, a fifth site of benzodiazepine activity may influence plasma steroid concentrations. Competition between steroids and benzodiazepines for hepatic clearance enzymes may affect half lives of both drugs and hormones.

Dantrolene stimulates renin secretion by rat renal cortical slices but fails to block calcium-dependent inhibition

Calcium (Ca) is an inhibitory second messenger in renin secretion, and it has been proposed that some first messengers, such as angiotensin II (A-II), antidiuretic hormone (ADH), and N6-cyclohexyladenosine (CHA), increase Ca and thereby inhibit renin secretion by mobilizing Ca from intracellular sequestration sites. The present experiments were designed to test this proposal by using dantrolene, an antagonist of intracellular Ca mobilization. Dantrolene stimulated renin secretion by rat renal cortical slices in a concentration dependent manner; at 0.0, 0.1, and 0.5 mM dantrolene, secretory rates were 8.1 +/- 0.6, 9.4 +/- 0.6 (p less than 0.05), and 14.9 +/- 1.2 (p less than 0.0001) GU/g x hr, respectively. These results could be interpreted to mean that Ca mobilization is occurring at a finite rate during the basal state, and that by antagonizing this process, dantrolene lowers intracellular Ca and thereby stimulates renin secretion. However, 0.1 mM dantrolene failed to antagonize the inhibitory effects on renin secretion of A-II, ADH, and CHA, and only CHA-induced inhibition of renin secretion was antagonized by 0.5 mM dantrolene. We conclude that if A-II, ADH, and CHA inhibit renin secretion by mobilizing Ca from an intracellular storage site, then the site is insensitive to dantrolene.