Studies of hormone-sensitive and -insensitive pools of phosphoinositides in cultured bovine zona fasciculata/reticularis cells. Evidence that acetylcholine and angiotensin II stimulate the breakdown of a common pool of phosphoinositides

Effects of manganese on inositol polyphosphate receptors and nitric oxide synthase activity in rat brain

The neurotoxic effects of excessive exposure to manganese (Mn) include degeneration of dopaminergic neurons, impairment of energy metabolism, and perturbations in phosphoinositide (PI) hydrolysis leading to altered calcium (Ca2+) homeostasis. This study is designed to assess the in vitro and in vivo effects of Mn on Ca2+/calmodulin-dependent neuronal nitric oxide synthase (nNOS) activity and on the regulation of inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) receptors involved in intracellular and extracellular mobilization of Ca2+. In vivo Mn exposure significantly increased 3H-InsP3 and 3H-InsP4 binding in the cerebellum and the cerebral cortex in a dose-dependent manner. However, in vitro Mn decreased 3H-InsP3 binding and increased 3H-InsP4 binding. In vitro and in vivo exposure of Mn inhibited nNOS activity in the cerebellum and the cerebral cortex. Immunohistochemical studies also showed a notable decrease in nNOS immunoreactivity in the granule cell layer of the cerebellum, whereas no significant changes were observed in the cerebral cortex. These data suggest that Mn neurotoxicity may be due to altered calcium homeostasis by its modulation of inositol polyphosphate receptors. Further, the inhibition of nNOS by Mn is of considerable importance because NO regulates a number of neurotransmitter functions.

Carbachol induces homologous steroidogenic refractoriness of bovine fasciculata-reticularis cells

Recently it was shown that the cholinergic agonist carbachol stimulates cortisol production in bovine ZFR cells via muscarinic receptor M(3). In the present study, we investigated the effect of chronic cholinergic stimulation on steroidogenic response and muscarinic receptor regulation in ZFR cells. Cortisol secretion of ZFR cells treated with 10(-4) M of carbachol decreased in a dose- and time-dependent manner. The carbachol-elicited loss of response was associated with a decrease in M(3) receptor number, which was also time- and dose-dependent. The down-regulation of the receptors was not associated with the decrease of M(3) receptor mRNA level. The marked steroidogenic desensitization caused by pretreatment of carbachol did not alter ACTH or angiotensin II activated steroid response. Northern blot analysis showed that carbachol pretreatment did not change the gene expression of P450scc, P450cl7, 3betaHSD and StAR mRNAs. These results suggest that carbachol induces homologous steroidogenic refractoriness of ZFR cells.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

The M3 muscarinic receptor mediates acetylcholine-induced cortisol secretion from bovine adrenocortical zona fasciculata/reticularis cells

In order to characterize the receptor subtype mediating acetylcholine (ACh)-induced cortisol secretion from purified bovine adrenocortical zona fasciculata/reticularis cells in primary culture, the potencies of a range of selective muscarinic antagonists of ACh-induced steroidogenesis were assessed by Schild analysis. Basal secretion of cortisol was 10.2 +/- 1.4 pmol/well/30 min. ACh stimulated a dose-dependent increase in cortisol secretion and was maximally effective at 10(-5) M, at which concentration cortisol secretion was 143.4 +/- 12.9 pmol/well/30 min. Hexahydro-sila-difenidol and para-fluoro-hexa-hydro-sila-difenidol were potent competitive antagonists of ACh-stimulated cortisol secretion, with pA2 values of 8.68 +/- 0.28 and 7.96 +/- 0.29, respectively. Pirenzepine (pA2 = 6.95 +/- 0.28) and methoctramine (pA2 = 6.06 +/- 0.27) were relatively weak competitive antagonists. The pA2 values determined in this study are characteristic of the M3 muscarinic receptor, and we conclude that this receptor subtype mediates ACh-induced cortisol secretion from bovine zona fasciculata/reticularis cells.