Phosphatidylinositol metabolism in the adrenal medulla

Effects of phorbol esters and forskolin on basal and histamine-induced accumulation of inositol phosphates in cultured bovine adrenal chromaffin cells

The effect of phorbol esters and forskolin pretreatment on basal and histamine-induced accumulation of inositol phosphates and catecholamine release was examined in cultures of bovine adrenal chromaffin cells. Histamine caused a dose-dependent, Ca2+-dependent accumulation of total inositol phosphates with an EC50 at approximately 1 microM and an eight- to 10-fold increase at 100 microM within 30 min of incubation. Histamine (10 microM) also caused the release of cellular catecholamines amounting to some 2.8% of cellular stores released over a 20-min period. Both the inositol phosphate and catecholamine responses were completely blocked by the H1-antagonist mepyramine and were insensitive to the H2-antagonist cimetidine. Examination of the time course of accumulation of the individual inositol phosphates stimulated by histamine revealed an early and sustained rise in inositol 1,4-bisphosphate content but not inositol 1,4,5-trisphosphate content at 1 min and the overall largest accumulation of inositol monophosphate after 30 min of stimulation. Pretreatment with the tumor-promoting phorbol ester phorbol 12-myristate 13-acetate (PMA) resulted in a dose-dependent, time-dependent inhibition of histamine-induced inositol phosphate formation and catecholamine secretion. In this inhibitory action, PMA exhibited high potency (IC50 of approximately 0.5 nM), an effect not shared by the inactive phorbol ester 4-alpha-phorbol 12,13-didecanoate. Pretreatment with forskolin, on the other hand, only marginally inhibited the histamine-induced inositol phospholipid metabolism and catecholamine secretion. These data suggest that protein kinase C activation in chromaffin cells may mediate a negative feedback control on inositol phospholipid metabolism.

Prolactin induces the formation of inositol bisphosphate and inositol trisphosphate in cultured mouse mammary gland explants

The effect of prolactin on [3H]inositol metabolism in cultured mouse mammary gland explants derived from 12-14-day pregnant mice was determined. In mammary gland explants that were prelabeled by culturing the tissues with 3 microCi/ml myo-[3H]inositol for 48 h, the levels of 3H in inositol derivatives were determined. The temporal effect of prolactin on the quantity of 3H present in phosphatidylinositol (PI), phosphatidylinositol monophosphate (PIP), phosphatidylinositol bisphosphate (PIP2) and various inositol phosphate containing fractions were examined. Prolactin significantly stimulated the accumulation of 3H label in inositol monophosphate (IP1), inositol bisphosphate (IP2) and inositol trisphosphate (IP3) 1-3 h after addition of prolactin. An effect of prolactin on the accumulation of inositol derivatives was not apparent at prolactin-exposure periods of less than 60 min; nor was an effect of prolactin apparent when exposure periods of 4 h or longer were employed. Prolactin did not significantly decrease the 3H label in PI, PIP or PIP2 except at 1 and 2 h. These data when considered with other apropos studies are compatible with the conclusion that the turnover of inositol lipid derivatives may be involved in the mechanism by which prolactin regulates metabolic processes in the mammary gland. The primary action of prolactin on mammary cells, however, would not appear to involve its action on the metabolism of the inositol derivatives in view of the extended time required (1 h) before effects of prolactin on perturbations of inositide metabolism are manifested.

Diacylglycerol breakdown in plasma membranes of bovine chromaffin cells is a two-step mechanism mediated by a diacylglycerol lipase and a monoacylglycerol lipase

The recently identified diacylglycerol lipase activity in membranes of chromaffin cells from bovine adrenal medulla [24] is now shown to consist of two enzymes working in series. First the predominantly saturated fatty acid in the sn-1-position is split by a diacylglycerol lipase (glycerol ester hydrolase, EC 3.1.1.34). Subsequently the resulting sn-2-monoacylglycerol is split by a monoacylglycerol lipase (glycerol-monoester acylhydrolase, EC 3.1.1.23) which prefers sn-2-arachidonoyl-monoacylglycerol to sn-2-palmitoyl-monoacylglycerol. At pH 4.0 only the diacylglycerol lipase is active, whereas the monoacylglycerol lipase is irreversibly inactivated. At pH 6.0 both enzymes are active. Pretreatment of the membranes at pH 10 leads to the selective inactivation of the diacylglycerol lipase. Both enzymes are Ca2+- and calmodulin-independent and both are partially inhibited by p-bromophenacyl bromide, however, only at relatively high concentrations of the inhibitor. Chlorpromazine inhibits the diacylglycerol lipase to about the same extent as p-bromophenacyl bromide but the monoacylglycerol lipase is less sensitive. The specific diacylglycerol lipase inhibitor RHC 80267 (1,6-di(O-(carbamoyl)cyclohexanone oxime)hexane) only interacts with the first step, i.e. the diacylglycerol lipase.

Phospholipase C and diacylglycerol lipase activities associated with plasma membranes of chromaffin cells isolated from bovine adrenal medulla

The plasma membranes of bovine adrenal chromaffin cells were isolated and the activities of enzymes involved in arachidonic acid liberation were investigated. Only a minute activity of phospholipase A2 (phosphatide 2-acylhydrolase, EC 3.1.1.4) could be detected using externally added phosphatidylcholine (PC) and phosphatidylethanolamine (PE) as substrate. When membranes were treated with exogenous phospholipase C (orthophosphoric acid diester phosphohydrolase, EC 3.1.4.1) there was a liberation of free fatty acids from the sn-2 position of PC. The enzyme responsible for this effect could be demonstrated to be a diacylglycerol lipase (glycerol ester hydrolase, EC 3.1.1.3) localized in the plasma membrane. Using phosphatidylinositol (PI) as a substrate, it was found that an endogenous phospholipase C exists which co-purifies with the membrane preparation. The produced diacylglycerol is subsequently hydrolyzed by diacylglycerol lipase liberating arachidonic acid. The two enzymes, phospholipase C and diacylglycerol lipase were characterized. Phospholipase C was found to be calcium dependent and PI specific, showing an activity of 60 pmol/micrograms protein per h (1.2 mM Ca2+), whereas the diacylglycerol lipase was calcium independent hydrolyzing diacylglycerol at a rate of 7.2 pmol/micrograms protein per h. The lipase but not the phospholipase C was inhibited 50% by 1.7 mM para-bromophenacylbromide.

Effects of protease inhibitors and calmodulin inhibitors on Ca2+ influx and catecholamine secretion in isolated bovine adrenal medullary cells

Acetylcholine (ACh) (10(-4) M) induced Ca2+ influx (21.2 nmol/mg cell protein/3 min) and catecholamine (CA) secretion (11.5% of total CA content) in isolated bovine adrenal medullary cells. Both excess K+ (56 mM KCl) and a calcium ionophore (A-23187: 10(-5) M) also induced CA secretion (9.7 and 7.5%, respectively, of total CA content). Protease inhibitors [gabexate mesilate (FOY), TLCK], a synthetic substrate of protease (TAME) and calmodulin inhibitors (W-7, trifluoperazine, promethazine, chlorpromazine) inhibited Ca2+ influx and CA secretion induced by ACh. Protease inhibitors and a substrate of protease had no effect on CA secretion induced by excess K+, and a high concentration of calmodulin inhibitors was required to inhibit excess K+-induced CA secretion. CA secretion induced by A-23187 was not inhibited by these inhibitors and the substrate.