Stimulation of aldosterone secretion by benzodiazepines in bovine adrenocortical cells

Targeting the Translocator Protein (18 kDa) in Cardiac Diseases: State of the Art and Future Opportunities

Mitochondria dysfunctions are typical hallmarks of cardiac disorders (CDs). The multiple tasks of this energy-producing organelle are well documented, but its pathophysiologic involvement in several manifestations of heart diseases, such as altered electromechanical coupling, excitability, and arrhythmias, is still under investigation. The human 18 kDa translocator protein (TSPO) is a protein located on the outer mitochondrial membrane whose expression is altered in different pathological conditions, including CDs, making it an attractive therapeutic and diagnostic target. Currently, only a few TSPO ligands are employed in CDs and cardiac imaging. In this Perspective, we report an overview of the emerging role of TSPO at the heart level, focusing on the recent literature concerning the development of TSPO ligands used for fighting and imaging heart-related disease conditions. Accordingly, targeting TSPO might represent a successful strategy to achieve novel therapeutic and diagnostic strategies to unravel the fundamental mechanisms and to provide solutions to still unanswered questions in CDs.

Drug-induced HPA axis alterations during acute critical illness: a multivariable association study

OBJECTIVE

Critical illness is hallmarked by low plasma ACTH in the face of high plasma cortisol. We hypothesized that frequently used drugs could play a role by affecting the hypothalamic-pituitary-adrenal axis.

DESIGN

Observational association study.

PATIENTS

A total of 156 medical-surgical critically ill patients.

MEASUREMENTS

Plasma concentrations of ACTH and total/free cortisol were quantified upon ICU admission and throughout the first 3 ICU days. The independent associations between drugs administered 24 h prior to ICU admission and plasma ACTH and cortisol concentrations upon ICU admission were quantified with use of multivariable linear regression analyses.

RESULTS

Upon ICU admission, compared with healthy subjects, patients had low mean±SEM plasma ACTH concentrations (2·7 ± 0·6 pmol/l vs 9·0 ± 1·6 pmol/l, P < 0·0001) in the face of unaltered total plasma cortisol (336·7 ± 30·4 nmol/l vs 300·8 ± 16·6 nmol/l, P = 0·3) and elevated free plasma cortisol concentrations (41·4 ± 5·5 nmol/l vs 5·5 ± 0·8 nmol/l, P = 0·04). Plasma ACTH concentrations remained low (P < 0·001) until day 3, whereas plasma (free) cortisol concentrations steeply increased and remained high (P < 0·001). No independent correlations with plasma ACTH were found. In contrast, the total admission plasma cortisol concentration was independently and negatively associated with the cumulative opioid (P = 0·001) and propofol (P = 0·02) dose, the use of etomidate (P = 0·03), and positively with the cumulative dobutamine dose (P = 0·0007).

CONCLUSIONS

Besides the known suppressive effect of etomidate, opioids and propofol may also suppress and dobutamine increases plasma cortisol in a dose-dependent manner. The observed independent associations suggest drug effects not mediated centrally via ACTH, but rather peripherally by a direct or indirect action on the adrenal cortex.

Expression and distribution of GABA and GABAB-receptor in the rat adrenal gland

The inhibitory effects of gamma-aminobutyric acid (GABA) in the central and peripheral nervous systems and the endocrine system are mediated by two different GABA receptors: GABAA-receptor (GABAA-R) and GABAB-receptor (GABAB-R). GABAA-R, but not GABAB-R, has been observed in the rat adrenal gland, where GABA is known to be released. This study sought to determine whether both GABA and GABAB-R are present in the endocrine and neuronal elements of the rat adrenal gland, and to investigate whether GABAB-R may play a role in mediating the effects of GABA in secretory activity of these cells. GABA-immunoreactive nerve fibers were observed in the superficial cortex. Some GABA-immunoreactive nerve fibers were found to be associated with blood vessels. Double-immunostaining revealed GABA-immunoreactive nerve fibers in the cortex were choline acetyltransferase (ChAT)-immunonegative. Some GABA-immunoreactive nerve fibers ran through the cortex toward the medulla. In the medulla, GABA-immunoreactivity was seen in some large ganglion cells, but not in the chromaffin cells. Double-immunostaining also showed GABA-immunoreactive ganglion cells were nitric oxide synthase (NOS)-immunopositive. However, neither immunohistochemistry combined with fluorescent microscopy nor double-immunostaining revealed GABA-immunoreactivity in the noradrenaline cells with blue-white fluorescence or in the adrenaline cells with phenylethanolamine N-methyltransferase (PNMT)-immunoreactivity. Furthermore, GABA-immunoreactive nerve fibers were observed in close contact with ganglion cells, but not chromaffin cells. Double-immunostaining also showed that the GABA-immunoreactive nerve fibers were in close contact with NOS- or neuropeptide tyrosine (NPY)-immunoreactive ganglion cells. A few of the GABA-immunoreactive nerve fibers were ChAT-immunopositive, while most of the GABA-immunoreactive nerve fibers were ChAT-immunonegative. Numerous ChAT-immunoreactive nerve fibers were observed in close contact with the ganglion cells and chromaffin cells in the medulla. The GABAB-R-immunoreactivity was found only in ganglion cells in the medulla and not at all in the cortex. Immunohistochemistry combined with fluorescent microscopy and double-immunostaining showed no GABAB-R-immunoreactivity in noradrenaline cells with blue-white fluorescence or in adrenaline cells with PNMT-immunoreactivity. These immunoreactive ganglion cells were NOS- or NPY-immunopositive on double-immunostaining. These findings suggest that GABA from the intra-adrenal nerve fibers may have an inhibitory effect on the secretory activity of ganglion cells and cortical cells, and on the motility of blood vessels in the rat adrenal gland, mediated by GABA-Rs.

Peripheral benzodiazepine receptor ligand Ro5-4864 inhibits isoprenaline-induced cardiac hypertrophy in rats

Oxidative stress plays a significant role in the pathogenesis of cardiac hypertrophy. Peripheral benzodiazepine receptors are ubiquitously expressed in various tissues, including the heart. Peripheral benzodiazepine receptors have been reported to be involved in the protection of cells against oxygen radical damage. The present study was designed to determine whether Ro5-4864 (a peripheral benzodiazepine receptor ligand) can inhibit isoprenaline-induced cardiac hypertrophy. Male Wistar rats (body weight 150-200g) were administered, isoprenaline (5mg/kg, body weight, subcutaneously) alone or along with Ro5-4864 (0.1 and 0.5mg/kg, body weight, intraperitoneally) once daily for 14days. Control rats received normal saline subcutaneously (1.0ml/kg). Isoprenaline-induced changes in heart weight to body weight ratio, left ventricular wall thickness (M-mode echocardiography and gross morphometry) and myocyte size were significantly prevented by both the doses of Ro5-4864. Ro5-4864 also attenuated isoprenaline-induced increase in interstitial fibrosis, lipid peroxidation and changes in endogenous antioxidants (glutathione, superoxide dismutase and catalase). Isoprenaline-induced cardiac hypertrophy was associated with increased expression of beta myosin heavy chain, which was also prevented by Ro5-4864. This is the first study to demonstrate a salutary effect of Ro5-4864 in experimental cardiac hypertrophy.

Peripheral-type benzodiazepine receptor: structure and function of a cholesterol-binding protein in steroid and bile acid biosynthesis

Cholesterol transport from the outer to the inner mitochondrial membrane is the rate-determining step in steroid and bile acid biosyntheses. Biochemical, pharmacological and molecular studies have demonstrated that the peripheral-type benzodiazepine receptor (PBR) is a five transmembrane domain mitochondrial protein involved in the regulation of cholesterol transport. PBR gene disruption in Leydig cells completely blocked cholesterol transport into mitochondria and steroid formation, while PBR expression in bacteria, devoid of endogenous PBR and cholesterol, induced cholesterol uptake and transport. Molecular modeling of PBR suggested that cholesterol might cross the membrane through the five helices of the receptor and that synthetic and endogenous ligands might bind to common sites in the cytoplasmic loops. A cholesterol recognition/interaction amino acid consensus (CRAC) sequence in the cytoplasmic carboxy-terminus of the PBR was identified by mutagenesis studies. In vitro reconstitution of PBR into proteoliposomes demonstrated that PBR binds both drug ligands and cholesterol with high affinity. In vivo polymeric forms of PBR were observed and polymer formation was reproduced in vitro, using recombinant PBR protein reconstituted into proteoliposomes, associated with an increase in drug ligand binding and reduction of cholesterol-binding capacity. This suggests that the various polymeric states of PBR might be part of a cycle mediating cholesterol uptake and release into the mitochondria, with PBR functioning as a cholesterol exchanger against steroid product(s) arising from cytochrome P450 action. Taking into account the widespread presence of PBR in many tissues, a more general role of PBR in intracellular cholesterol transport and compartmentalization might be considered.

Elevated K(+) induces myristoylated alanine-rich C-kinase substrate phosphorylation and phospholipase D activation in glomerulosa cells

Elevated extracellular potassium concentrations ([K(+)](e)) are known to stimulate aldosterone secretion from adrenal glomerulosa cells in vivo and in vitro. The mechanism is thought to involve depolarization-elicited activation of voltage-dependent calcium channels and an increase in calcium influx. Until now protein kinase C (PKC) was thought not to play a role in the steroidogenic response to elevated [K(+)](e). In this report, we provide evidence in bovine adrenal glomerulosa cells to suggest that elevated [K(+)](e) increases PKC activity, as shown by an enhancement in the phosphorylation of myristoylated alanine-rich C-kinase substrate (MARCKS). Elevated [K(+)](e)-induced MARCKS phosphorylation was delayed and transient and was not the result of a local production of angiotensin II (AngII). MARCKS phosphorylation in response to elevated [K(+)](e) was not accompanied by phosphoinositide hydrolysis but was inhibited by a selective PKC inhibitor. Elevated [K(+)](e) also activated phospholipase D (PLD) in a delayed but sustained manner. We propose that the observed PLD activation mediates the elevated [K(+)](e)-induced MARCKS phosphorylation via PKC, although other factors may modulate this phosphorylation event.

Local inhibition of nitric oxide temporarily stimulates aldosterone secretion in conscious sheep in vivo

The effect of localized blockage of endogenous nitric oxide (NO) on basal aldosterone secretion was studied in conscious sheep with autotransplanted adrenal glands. We have shown that infusion of the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 130 microg/l blood flow) significantly stimulated basal aldosterone secretion rate (ASR). This stimulatory effect was seen up to 4 h of infusion. Beyond this time point, however, the elevated ASR level was not sustained, and it was seen to drop markedly to lower than control values at 5 h. L-NAME had no effect on cortisol secretion rate (FSR) during the first 4 h of infusion, but a significant reduction in FSR was seen by the 8-h time point. Adrenal blood flow was consistently decreased in association with long L-NAME infusion. Additionally, L-NAME was shown to have no effect on aldosterone secretion when infused systemically. We conclude that the relationship between NO and aldosterone secretion is an inhibitory one, in which NO seems to have a negative effect on basal aldosterone secretion.