Effects of TMB-8 and dantrolene on ACTH- and angiotensin-induced steroidogenesis by frog interrenal gland: evidence for a role of intracellular calcium in angiotensin action

Inhibition of cortisol secretion in dispersed head kidney cells of rainbow trout (Oncorhynchus mykiss) by endosulfan, an organochlorine pesticide

The effects of endosulfan, an organochlorine pesticide, on cortisol secretion in vitro were investigated in enzymatically dispersed head kidney cells of rainbow trout, Oncorhynchus mykiss. First, the conditions for maximal cortisol secretion were characterized by selecting the optimal concentrations of adrenocorticotropin (ACTH) (1 IU/ml) and N(6), 2'-o-dibutyryladenosine 3':5'-cyclic monophosphate (dbcAMP) (2 mM), incubation temperature (15 degrees ) and time period (60 min for ACTH, 120 min for dbcAMP), number of cells per incubation well (75 x 10(6)/ml), and osmolarity of the medium (270 mosmol/L). Exposure of head kidney cells to endosulfan decreased ACTH- or dbcAMP-stimulated cortisol secretion and cell viability in a concentration-dependent pattern and the doses required to disrupt cortisol secretion were significantly lower than doses lethal to the head kidney cells. The median effective concentration of endosulfan (EC(50), the dose that inhibits cortisol secretion by 50%) was 17.3 microM while the median lethal concentration (LC(50), the dose that kills 50% of the cells) was 308 microM. Our study identified endosulfan as an environmental endocrine disrupting chemical that interferes with the normal secretory function of teleost interrenal steroidogenic cells. Multiple sites may be affected within the steroidogenic cells since dbcAMP could not restore cortisol secretion in endosulfan exposed cells.

Evidence that basal secretion of relaxin by individual cultured large luteal cells is influenced by mobilization of intracellular calcium: analysis by a reverse hemolytic plaque assay

Ca2+ redistribution from an intracellular site(s) is a key biochemical event associated with relaxin (RLX) secretion by large luteal cells (LLCs) of porcine origin. However, the functional significance of internal stores of Ca2+ to basal rates of RLX secretion is not well understood. In addition, the identity of the intracellular storage site(s) for Ca2+ within LLCs is not known, nor is it clear if all RLX-releasing LLCs are equally dependent on this pool. In the present study, release of RLX from 24 h cultured luteal cells derived from early pregnant swine was monitored by a reverse hemolytic plaque assay (RHPA). Incubation of cultures in the presence of graded concentrations of thapsigargin (1 nM-1 microM), a plant sesquiterpene lactone that inhibits endoplasmic reticulum Ca(2+)-ATPase and thereby increases cytosolic Ca2+ concentrations, resulted in a dose-related increase in basal RLX secretion. The stimulatory effect of thapsigargin on RLX production was not abrogated by culture in Ca(2+)-free medium. Suppression of Ca2+ release from the endoplasmic reticulum of LLCs, achieved by incubating monolayers in medium containing dantrolene (1-100 microM), resulted in dose-related inhibition of basal RLX release. Taken together, these results suggest that the endoplasmic reticulum serves as a major storage site for Ca2+ redistribution within LLCs and, furthermore, that mobilization from this site is functionally coupled to basal secretion of RLX.

Calcium-independent effects of TMB-8. Modification of phospholipid metabolism in neuroblastoma cells by inhibition of choline uptake

TMB-8 [8-(NN-diethylamino)-octyl-3,4,5-trimethoxybenzoate] blocks agonist-stimulated release of Ca2+ from intracellular sites in many cell lines and is often used to distinguish between dependence on extracellular and intracellular Ca2+. In N1E-115 neuroblastoma cells, TMB-8 did not alter the resting cytosolic Ca2+ concentration in unstimulated cells, yet phospholipid metabolism was greatly affected. At concentrations of TMB-8 (25-150 microM) that inhibit Ca2+ release, phosphatidylcholine formation was inhibited, whereas synthesis of phosphatidylinositol, phosphatidylglycerol and phosphatidylserine was stimulated. Unlike other cationic amphipathic compounds, TMB-8 did not inhibit phosphatidate phosphatase or enzymes in the pathway from choline to phosphatidylcholine. Choline transport was the major site of action. TMB-8 was a competitive inhibitor (Ki = 10 microM) of low-affinity (Kt = 20 microM) choline transport. When added at the same time as labelled precursor, TMB-8 also decreased cellular uptake of phosphate and inositol, but not that of ethanolamine or serine. In prelabelled cells, continued uptake and incorporation of phosphate and inositol were not affected. Under these conditions phosphatidylinositol synthesis was increased 2-fold and, like the effect on phosphatidylcholine, reached a plateau at 100 microM-TMB-8. Phosphatidylglycerol synthesis increased linearly with TMB-8 concentration to 40-fold stimulation at 150 microM, suggesting a selective effect on synthesis of phosphatidylglycerol from CDP-diacylglycerol. Phosphatidylserine synthesis was also increased up to 3-fold. These Ca(2+)-independent effects limit the use of TMB-8 in studies of cell signalling that involve stimulated phosphatidylinositol and phosphatidylcholine metabolism.

Effects of ACTH and angiotensin II on cytosolic calcium in cultured adrenal glomerulosa cells. Role of cAMP production in the ACTH effect

We have used microspectrofluorometry and video imaging techniques in order to study and compare the changes in intracellular calcium concentrations [( Ca2+]i) of individual Fura-2 loaded glomerulosa cells cultured for three days and stimulated either with angiotensin II (AT), K+, or adrenocorticotropin (ACTH). As previously demonstrated for freshly isolated cells, K+ ion induces an immediate increase in [Ca2+]i, although AT induces a biphasic response, characterized by an initial transient spike, followed by a sustained plateau. In this study, we demonstrate, for the first time, that ACTH is able to induce a [Ca2+]i increase in cultured glomerulosa cells from rat and bovine sources. Moreover, it is clear that the pattern of [Ca2+]i increase elicited by ACTH is different from that observed with AT. In most cases, addition of ACTH leads to a slow increase in [Ca2+]i after a long latency period ranging from 10-15 min, which could be correlated to cAMP time-production. The present results show that: (a) in the absence of extracellular Ca2+, ACTH does not increase [Ca2+]i; (b) the response develops slowly and cases immediately after [Ca2+]e depletion or addition of calcium channel blockers, such as nifedipine or omega-conotoxin; (c) the addition of the calcium channel agonist Bay K 8644 enhances the ACTH response; (d) the cAMP analog, 8-Br-cAMP, induces an increase in [Ca2+]i similar to that observed with ACTH, which is also dependent of the presence of calcium in the extracellular medium; (e) time-production of ACTH-induced cAMP follows quite well the increase in [Ca2+]i; (f) Bay K 8644 also enhances the 8-Br-cAMP induced increase in [Ca2+]i; and (g) ACTH-induced Cai response is inhibited by the specific protein kinase A blocker, HA1004. These observations, combined with previous results obtained on the effects of ACTH on calcium currents and action potentials, suggest that the [Ca2+]i increase induced by ACTH results from a calcium influx through dihydropyridine and omega-conotoxin sensitive calcium channels, which need to be phosphorylated by cAMP for full activation. The use of video-imaging techniques has allowed us to examine the spatial distribution of changes in [Ca2+]i in single cells. The ability to simultaneously record images of a number of cells confirm the heterogeneity of cellular responses, and corroborate results obtained through photocounting only. Our results indicate that ACTH initially increases [Ca2+]i locally beneath the cell membrane and throughout the cell thereafter, whereas angiotensin II elicits a more prominent effect in certain regions of the cell and eventually extends to the entire cell surface.

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

The muscle relaxant dantrolene has been widely used in signal transduction studies as an inhibitor of intracellular calcium release. However, in vivo studies have shown that the drug may inhibit steroidogenesis by a mechanism which is distinct from its effects on calcium mobilization. Using freshly isolated cells and mitochondria from the outermost regions of bovine adrenal cortex we have shown that dantrolene (0.2 mM) significantly inhibits steroid synthesis stimulated by either angiotensin II (AII) or by addition of various precursors. Our results suggest that dantrolene inhibits the rate-limiting steps of adrenocortical steroidogenesis, i.e. the intramitochondrial conversion of cholesterol to pregnenolone (for both aldosterone and cortisol) and the conversion of corticosterone to aldosterone (for aldosterone), by a mechanism independent from its known effects on calcium release. A possible alternative mechanism may involve direct inhibition of cytochrome P450-dependent hydroxylation reactions.

Serotonin stimulates corticosteroid secretion by frog adrenocortical tissue in vitro

The mode of action of serotonin (5-HT) in the regulation of frog adrenal steroidogenesis was studied in vitro using the perifusion system technique. Graded doses of 5-HT (from 10(-8) to 10(-6) M) increased both corticosterone and aldosterone production in a dose-dependent manner. Short pulses (20 min) of 10(-6) M 5-HT, administered at 130 min intervals within the same experiment, did not cause any desensitization phenomenon. Indomethacin (IDM; 5 microM), a cyclooxygenase inhibitor which induced a dramatic decrease in the spontaneous secretion of corticosteroids, did not impair the stimulatory effect of 5-HT on corticosterone and aldosterone production. In the absence of calcium, 5-HT (10(-6) M) was still able to stimulate corticosteroid production. Dantrolene (5 x 10(-5) M), a blocker of calcium mobilization from intracellular pools which significantly inhibited the spontaneous production of corticosteroids, did not suppress 5-HT-evoked corticosteroid secretion. These results show that 5-HT, stored in adrenal chromaffin cells, may act as a paracrine factor to stimulate adrenal steroidogenesis in the frog. Our data also indicate that the mechanism of action of 5-HT does not depend on prostaglandin biosynthesis.