Rapid breakdown of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate in rat hepatocytes stimulated by vasopressin and other Ca2+-mobilizing hormones

The effects of ACTH, serotonin, K+ and angiotensin analogues on 32P incorporation into phospholipids of the rat adrenal cortex: basis for an assay method using zona glomerulosa cells

The effects of various concentrations of serotonin, ACTH, K+, angiotensin II (AII), angiotensin III (AIII) and [Sar1]angiotensin II (SAII) on steroidogenesis and the incorporation of 32P (after preincubation to near equilibrium with the ATP pool) into phosphatidylinositol (PI), phosphatidic acid (PA) and phosphatidylcholine (PC) in a preparation of capsular cells from rat adrenals, consisting of 95% zona glomerulosa (z.g.) and 5% zona fasciculata plus reticularis (z.f.r.) cells, were investigated. Serotonin and ACTH stimulated steroidogenesis in the usual manner but had little or no effect on 32P incorporation into any of the three phospholipids. However, AII, AIII and SAII stimulated steroidogenesis and also 32P incorporation into PA and PI (maximally to about 280% of control values) but not into PC. These results taken together with other data on effects on the cAMP output and Ca2+ fluxes of z.g. cells suggest that stimulation by ACTH and serotonin is mediated by cAMP as second messenger. However, the angiotensins probably act through Ca2+, with associated changes in phospholipid metabolism. The 32P incorporation into PA as a function of lg concentration of AII was linear and showed a reasonable index of precision (0.36 +/- 0.03, eight experiments, 0.23 +/- 0.02 for a further eight experiments) and correlation with steroidogenesis. The corresponding incorporation into PI showed a maximum effect and a much poorer index of precision (1.02 +/- 0.30 (4.69 +/- 3.7] over the same full range of AII concentration used. The effects of AIII and SAII showed similar characteristics for 32P incorporation into both PA and PI, but, as for stimulation of steroidogenesis, at higher concentrations for AIII than for AII. The effects of different doses of AII, AIII and ACTH on the corticosterone output and 32P incorporation into PA, PI and PC of a preparation of cells, consisting of more than 98% z.f.r. cells, from rat decapsulated adrenals were also studied. ACTH, at low doses, which nevertheless markedly stimulated corticosterone output, had a small (maximally to about 125% of control values) but significant effect on 32P incorporation into PA, PI and PC. The maximum effect was usually at about 10(-10) M ACTH and was not significant at 10(-8) M.

Rapid calcium mobilization by vasopressin and prostaglandin F2 alpha is independent of sodium influx in quiescent 3T3 cells

Vasopressin (VP) rapidly increased 45Ca2+ efflux. A VP antagonist prevented VP from mobilizing Ca2+ and stimulating DNA synthesis. Prostaglandin F2 alpha (PGF2 alpha) also stimulated rapid 45Ca2+ release. The effectiveness of different prostaglandins corresponded to their effectiveness as mitogens. The removal of external Na+ or Ca2+ had no effect on VP-or PGF2 alpha-induced 45Ca2+ release. The present results indicate that the mobilization of intracellular Ca2+ by these hormones is independent of Na+ or Ca2+ influx and that Ca2+ mobilization is important for growth stimulation.

myo-Inositol polyphosphate may be a messenger for visual excitation in Limulus photoreceptors

Photoreceptor excitation begins with the absorption of a photon by rhodopsin and proceeds through an unknown sequence of steps that leads to changes in specific ionic conductances. These conductance changes produce the receptor potential. It has been proposed that hydrolysis of phosphoinositides is involved in the control of a variety of physiological processes. Recent studies have implicated inositol 1,4,5-trisphosphate as an intracellular messenger in the cascade mediating hormone-stimulated secretion. We propose that one of the steps in the excitatory cascade in Limulus ventral photoreceptors may be an increase in intracellular concentration of myo-inositol polyphosphates, derived from hydrolysis of the membrane component phosphatidylinositol bisphosphate by a phospholipase. Here we present biochemical and electrophysiological evidence that an inositol polyphosphate may be an intracellular messenger in the cascade mediating excitation, based on the following criteria: the cells possess the synthetic and degradative metabolism for the messenger; the natural stimulus leads to a change in the concentration of the messenger within the cells; and intracellular injection of exogenous messenger mimics naturally occurring electrophysiological events.

Phospholipid metabolism in rat kidney cortical tubules. II. Effects of hormones on 32P incorporation

To evaluate the effect of hormones on renal phospholipid metabolism and turnover, we studied the changes in 32P-labeling of phospholipids in rat cortical tubule suspension. Angiotensin II, phenylephrine and parathyroid hormone (PTH) stimulate 32P incorporation into PC by 25, 29 and 26% and into PI by 189, 328 and 33% above control rates, respectively, whereas phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate labeling was not affected. However, when phospholipids were prelabeled with [32P]Pi, addition of angiotensin II led to a significant decrease in phosphatidylinositol 4,5-bisphosphate labeling in the first 2 min with no effect on the other phospholipid fractions. The phenylephrine effect on phospholipid labeling was blocked by prazosin but not by yohimbine, indicating an alpha 1-mediated action. In contrast, the effect of angiotensin II was not inhibited by either antagonist. The stimulating effect of substrates on 32P incorporation reported in the preceding paper was additive to that of hormones. Our results confirm previous studies on renal gluconeogenesis that catecholamines act by an alpha 1-type receptor on proximal tubules, and indicate that phenylephrine and angiotensin II act by different receptor sites exerting the same metabolic effect. The additivity of hormone effects with that of renal substrates indicates that the former are not secondary to release of precursors for phospholipid biosynthesis. The rapid decrease in phosphatidylinositol 4,5-bisphosphate labeling after angiotensin II suggests that the polyphosphoinositide is degraded after hormone binding to the receptor and that PI labeling is a secondary event.

Vasopressin rapidly stimulates Ca2+ efflux from intracellular pool in quiescent Swiss 3T3 cells

Addition of the neurohypophysial nonapeptide [Arg8]vasopressin to 45Ca2+-loaded quiescent cultures of Swiss 3T3 cells or diploid mouse embryo fibroblasts causes a rapid increase (within seconds) in the rate of 45Ca2+ efflux from the cells. This reflects the release of 45Ca2+ from an intracellular store that is partly or entirely derived from mitochondria and leads to a marked decrease (52%) in the intracellular 45Ca2+ content of the cultures under equilibrium 45Ca2+ conditions. The effect is dose dependent, specific, and blocked by a vasopressor antagonist. Prolonged pretreatment with vasopressin that renders the cells unresponsive to the mitogenic effects of the hormone also blocks the Ca2+ response. The results suggest that Ca2+ fluxes may be involved in the mechanism of mitogenic action of vasopressin in Swiss 3T3 cells.

A transient increase in diacylglycerols is associated with the action of vasopressin on hepatocytes

Vasopressin induced a transient increase of 50% in the total concentration of diacylglycerols (determined by g.l.c.) in isolated hepatocytes. The increase was maximal at 0.25 min, and the concentration of diacylglycerols in cells treated with vasopressin had returned to the basal value by 4 min. No change in the concentration of diacylglycerols was observed after the treatment of cells with glucagon. The dependency of this effect on the concentration of vasopressin was similar to that of the effect of the hormone on 45Ca2+ efflux measured at 0.1 mM extracellular Ca2+. Vasopressin increased the proportion of arachidonic acid and stearic acid and decreased the proportion of oleic acid present in the diacylglycerols. In hepatocytes prelabelled with [14C]arachidonic acid, vasopressin increased the amount of [14C]diacylglycerol. The effects of vasopressin on the total concentration of diacylglycerols and [14C]diacylglycerol were mimicked by an exogenous phospholipid phosphodiesterase (phospholipase C) from Clostridium perfringens. The results are consistent with the conclusion that the transient increase in diacylglycerols induced by vasopressin is caused by the rapid hydrolysis of both the phosphoinositides and one or more other phospholipids.

Reduction of epidermal growth factor receptor affinity by heterologous ligands: evidence for a mechanism involving the breakdown of phosphoinositides and the activation of protein kinase C

The tetradecapeptide bombesin converts epidermal growth factor (EGF) receptors on Swiss 3T3 cells from a high affinity state (KD = 9.8 X 10(-11)M) to a lower affinity state (KD = 1.8 X 10(-9)M). This conversion occurs when the cells are incubated with bombesin at 37 degrees C but not when incubated at 4 degrees C. Previously, a number of other (chemically unrelated) cell growth-promoting peptides and polypeptides have been shown to induce a similar indirect, temperature-dependent reduction of EGF receptor affinity. We have now demonstrated that hormones and growth factors which cross-regulate EGF receptor affinity in Swiss 3T3 cells have a common ability to stimulate the breakdown of phosphoinositides in these cells. We propose that the reduction of EGF receptor affinity is a consequence of the activation of protein kinase C by the diacylglycerol generated by this breakdown. In support of this proposal we have found that exogenously added diacylglycerol reduces the affinity of the Swiss 3T3 cell EGF receptor.

Guanine nucleotides decrease the free [Ca2+] required for secretion of serotonin from permeabilized blood platelets. Evidence of a role for a GTP-binding protein in platelet activation

Human platelets containing granule-bound [14C]serotonin were permeabilized, equilibrated at 0 degrees C with ATP and with various Ca2+ buffers and guanine nucleotides, and then incubated at 25 degrees C with or without a stimulatory agonist. Ca2+ alone induced the ATP-dependent secretion of [14C]serotonin (50% at a pCa of 5.1) but the sensitivity of secretion to Ca2+ was greatly enhanced by guanine nucleotides [6-fold by 100 microM GTP, 100-fold by 100 microM guanyl-5'-yl imidodiphosphate and greater than 500-fold by 100 microM guanosine 5'-O-(3-thiotriphosphate)] or by stimulatory agonists (10-fold by 2 units thrombin/ml and 4-fold by 1 microM 1-O-octadecyl-2-O-acetyl-sn-glyceryl-3-phosphorylcholine). When both GTP and a stimulatory agonist were added, they had synergistic effects on secretion. Cyclic GMP and GMP acted similarly to GTP. The effects of all these guanine nucleotides were inhibited by guanosine 5'-O-(2-thiodiphosphate), whereas those of stimulatory agonists were not. Our results demonstrate the presence in platelets of guanine nucleotide-dependent and independent mechanisms regulating the sensitivity of secretion to Ca2+.

Stimulation of phosphatidylinositol turnover by acetylcholine, angiotensin II and ACTH in bovine adrenal fasciculata cells

The effect of acetylcholine, angiotensin II and adrenocorticotropin (ACTH) on phosphatidylinositol (PI) metabolism was examined using bovine adrenocortical fasciculata cell suspensions. The three agents, which acutely stimulate glucocorticoid production by these cells, were all able to increase [32P]Pi incorporation into cellular PI. However, whereas the relative steroidogenic potency (at maximally active concentrations) was ACTH greater than or equal to angiotensin II greater than acetylcholine, the effect on PI labeling was in the order angiotensin II greater than acetylcholine greater than ACTH. The dose-response curves for steroidogenesis and that for PI labeling were superimposable in the case of angiotensin II (ED50 = 1 X 10(-8) M) and of acetylcholine (ED50 = 5 X 10(-7) M), while the two responses were dissociated under graded ACTH challenge. Both steroidogenic response and increased PI labeling elicited by angiotensin II and acetylcholine were respectively inhibited by (Sar1-Ala8)-angiotensin II and muscarinic antagonists. Time-course study showed that in the case of angiotensin II and acetylcholine, the sequence of events was: increased phosphatidic acid labeling, increased PI labeling, activated steroidogenesis. By sharp contrast, under ACTH stimulation, increased steroidogenesis was detected well before activation of PI metabolism. These data suggest that in bovine adrenocortical fasciculata cell, steroidogenesis may be activated by two different pathways. The first one would act mainly through cyclic AMP-dependent intracellular events and is usually accepted in the mechanism of action of ACTH. The other, cyclic AMP-independent pathway, as in the case of angiotensin II and acetylcholine actions, may involve phospholipid-mediated intracellular processes.

Inositol trisphosphate formation and calcium mobilization in Swiss 3T3 cells in response to platelet-derived growth factor

Swiss 3T3 cells incubated for 60 h with [3H]inositol incorporated radioactivity into phosphatidylinositol (PI) and the two polyphosphoinositides phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2). On stimulation with platelet-derived growth factor (PDGF) there were significant increases in the levels of inositol 1-phosphate (IP1), inositol 1,4-bisphosphate (IP2) and inositol 1,4,5-trisphosphate (IP3). The effect of PDGF and IP3 on Ca2+ mobilization was studied in both intact cells and in 'leaky' cells that had been permeabilized with saponin. In intact cells, PDGF stimulated the efflux of 45Ca2+, whereas IP3 had no effect. Conversely, IP3 stimulated 45Ca2+ efflux from 'leaky' cells, which were insensitive to PDGF. 'Leaky' cells, which accumulated 45Ca2+ to a steady state within 20 min, were found to release approx. 40% of the label within 1 min after addition of 10 microM-IP3. This stimulation of 45Ca2+ release by IP3 was reversible and was also dose-dependent, with a half-maximal effect at approx. 0.3 microM. It seems likely that an important action of PDGF on Swiss 3T3 cells is to stimulate the hydrolysis of PIP2 to form IP3 and diacylglycerol, both of which may function as second messengers. Our results indicate that IP3 mobilizes intracellular Ca2+, and we propose that diacylglycerol may act through C-kinase to activate the Na+/H+ antiport. By generating two second messengers, PDGF can simultaneously elevate the intracellular level of Ca2+ and alkalinize the cytoplasm by lowering the level of H+.

Rapid accumulation of inositol phosphates in isolated rat superior cervical sympathetic ganglia exposed to V1-vasopressin and muscarinic cholinergic stimuli

An accumulation of 3H-labelled inositol phosphates is observed when prelabelled rat superior cervical sympathetic ganglia are exposed to [8-arginine]vasopressin or to muscarinic cholinergic stimuli. The response to vasopressin is much greater than the response to cholinergic stimuli. The response to vasopressin is blocked by a V1-vasopressin antagonist, and oxytocin is a much less potent agonist than vasopressin. Vasopressin causes no increase in the cyclic AMP content of ganglia. These ganglia therefore appear to have functional V1-vasopressin receptors that are capable of activating inositol lipid breakdown, but no V2-receptors coupled to adenylate cyclase. The first [3H]inositol-labelled products to accumulate in stimulated ganglia are inositol trisphosphate and inositol bisphosphate, suggesting that the initiating reaction in stimulated inositol lipid metabolism is a phosphodiesterase-catalysed hydrolysis of phosphatidylinositol 4,5-bisphosphate (and possibly also phosphatidylinositol 4-phosphate). This response to exogenous vasopressin occurs in ganglia incubated in media of reduced Ca2+ concentration. The physiological functions of the V1-vasopressin receptors of these ganglia remain unknown.

Multiple forms of phosphoinositide-specific phospholipase C of different relative molecular masses in animal tissues. Evidence for modification of the platelet enzyme by Ca2+-dependent proteinase

The Mr distribution of phosphoinositide-specific phospholipase C in the supernatants isolated from a variety of animal tissues was analysed by high-performance gel-filtration chromatography. In most tissues, at least four peaks of activity were resolved. However, different tissues showed quite marked differences in the distribution of activity between these peaks. In rat heart, lung and kidney, the predominant form had Mr approx. 90000, whereas the predominant form in brain had Mr approx. 290000. In liver, the Mr-90000 form predominated, but this tissue also contained relatively large amounts of a form of Mr approx. 150000. Phospholipase C in these tissues from other animal species gave similar distributions of activity between the peaks. In supernatants prepared from platelets sonicated in the presence of leupeptin (0.5 mM) or EGTA (20 mM), the Mr-290000 form predominated. However, when leupeptin or EGTA (inhibitors of Ca2+-dependent proteinase) was omitted from the sonication buffer, the Mr-290000 form appeared to be replaced by a form of Mr 100000. Similar changes in Mr were not demonstrated with the other tissues. These results may be relevant to the intracellular regulation of phospholipase C, since Ca2+-dependent proteolysis has been reported to occur during platelet activation.

The Ca2+-activated polyphosphoinositide phosphodiesterase of human and rabbit neutrophil membranes

Addition of Ca2+ to a plasma-membrane fraction derived from human or rabbit neutrophils led to the specific breakdown of polyphosphoinositides. The degradation products were identified as diacylglycerol and inositol bis- and tris-phosphate, thus demonstrating the presence of a Ca2+-activated phospholipase C. The newly generated diacylglycerol resembled the polyphosphoinositides in its fatty acid composition, and in the presence of MgATP2- it was converted into phosphatidate. These results therefore demonstrate the presence in neutrophil plasma membranes not only of polyphosphoinositide phosphodiesterase but also of diacylglycerol kinase.

Correlation between inositol phospholipid hydrolysis and substance P receptors in rat CNS

The undecapeptide substance P is a neurotransmitter candidate in the mammalian central and peripheral nervous system. Although the distribution of substance P-like immunoreactivity within the central nervous system (CNS) is well established, the recent identification and autoradiographic localization of specific substance P-binding sites has revealed numerous areas of mismatch between peptide levels and numbers of such sites. Previous studies have shown that substance P stimulates the hydrolysis of inositol phospholipids in peripheral tissues and in the hypothalamus, probably through stimulation of a polyphosphoinositide-specific phospholipase C (refs 9-11). Inositol phospholipid hydrolysis has been implicated in the mobilization of cytosolic calcium following receptor activation in several neurotransmitter and hormonal systems. We have therefore investigated the distribution of 3H-labelled substance P binding sites within various rat brain regions and correlated this with the rate of substance P-induced hydrolysis of inositol phospholipids in the same areas of the CNS. We found that the rate of inositol phospholipid hydrolysis was proportional to the number of binding sites specific for 3H-substance P, suggesting that binding sites revealed by 3H-substance P autoradiography correspond to functional substance P receptors.

Effects of lithium on angiotensin-stimulated phosphatidylinositol turnover and aldosterone production in adrenal glomerulosa cells: a possible causal relationship

Turnover of 32P-labelled phosphatidylinositol (PI) was examined in isolated adrenal glomerulosa cells. Increased incorporation of [32P]phosphate into PI in response to angiotensin II was completely prevented by Li+. A simultaneous accumulation of 32P activity in phosphatidic acid (PA) was also observed. Angiotensin II increased the breakdown of PI despite the presence of Li+. These results suggest that Li is a suitable tool to interrupt the accelerated PI cycle in angiotensin-stimulated cells. Aldosterone production of superfused cells was inhibited by Li+ when the cells were stimulated with angiotensin II. On the other hand, Li+ did not inhibit the aldosterone response of the cells to ACTH, a hormone which acts via cyclic AMP and does not enhance PI turnover in these cells. On the basis of these results, we assume that the inhibitory effect of Li+ on aldosterone production is related to its effect on PI turnover.

Glycogenolytic effects of the calcium ionophore A23187, but not of vasopressin or angiotensin, in foetal-rat hepatocytes

Vasopressin, angiotensin and phenylephrine stimulate glycogenolysis in postnatal rat liver by a Ca2+-mediated mechanism not involving cyclic AMP. To determine whether these hormones promote glycogenolysis in foetal liver, we have examined their effects, and those of the Ca2+ ionophore A23187, on glycogen metabolism in cultured foetal-rat hepatocytes. Vasopressin and angiotensin (0.1 nM-0.1 microM) had no effects on either glycogen synthesis (as assessed by [14C]glucose incorporation into glycogen) or phosphorylase a activity. However, A23187 at 1 and 10 microM inhibited glycogen synthesis by 31.3 and 89.1% respectively (both P less than 0.001) and stimulated phosphorylase a activity by 66.9 and 184.1% respectively (both P less than 0.01). Incubation of cells in Ca2+-deficient medium attenuated the effects of 10 microM-A23187 on glycogen synthesis and abolished the effects of 1 microM-A23187. As in postnatal liver, glucagon (1 and 20 nM) and isoprenaline (1 and 10 microM), which activate adenylate cyclase, inhibited glycogen synthesis and stimulated phosphorylase a activity in foetal hepatocytes. The minimal effective concentration of phenylephrine was 10 times that of isoprenaline. These results indicate striking differences in the ontogeny of cyclic AMP-mediated and Ca2+-mediated processes which regulate hepatic glycogenolysis. Since increases in cytosolic Ca2+ induce glycogenolysis in foetal-rat liver, the weak or absent responses to vasopressin, angiotensin and the alpha-adrenergic agonists may result from defects in hormone-receptor binding or in post-receptor events leading to the mobilization of intracellular Ca2+ stores.

Differential effects of phenylmethanesulfonyl fluoride (PMSF) on carbachol and potassium stimulated phosphoinositide turnover and contraction in longitudinal smooth muscle of guinea pig ileum

Phenylmethanesulfonyl fluoride (PMSF) (2 mM), a putative inhibitor of phosphatidylinositol-specific phospholipase C, almost completely inhibited carbachol-stimulated inositol incorporation into phosphatidylinositol (PI) of longitudinal smooth muscle of guinea pig ileum, while it had no effect on potassium-stimulated inositol incorporation. This suggests that the two stimuli may affect phosphoinositide turnover by different mechanisms, distinguishable by PMSF. In contrast to its specific inhibition of carbachol-stimulated phosphoinositide turnover, PMSF produced a transient inhibition of contraction by both carbachol and potassium. The non-selective effect of PMSF on contraction suggests that it is not the result of its inhibitory effect on phosphoinositide breakdown. PMSF (2 mM) inhibited carbachol-stimulated inositol phosphate accumulation in the presence of Li+ by only 15%-19%, indicating that PMSF inhibition of phosphoinositide turnover was not due to its inhibition of phosphoinositide phosphodiesterase, but to one or more steps following phosphoinositide breakdown.

Determination of mitochondrial calcium content in hepatocytes by a rapid cellular fractionation technique. Vasopressin stimulates mitochondrial Ca2+ uptake

Stimulation of hepatocytes with vasopressin (10 nM) in the presence of 1.25 mM extracellular Ca2+ increased glycogen phosphorylase activity 4-fold within 15s and provoked a rapid efflux of cell-associated Ca2+. Vasopressin also caused a transient increase in the Ca content of a mitochondria-rich fraction separated within seconds of hormone stimulation by a rapid fractionation technique [Shears & Kirk (1984) Biochem. J. 219, 375-382]. The Ca content of this fraction was restored to the control value within 2 min of hormone addition. These results indicate that mitochondria are not the source of the cell-associated Ca which is mobilized in the cytosol of vasopressin-stimulated hepatocytes. Rather, these organelles buffer the increase in cytosol [Ca2+] attributable to Ca mobilization from non-mitochondrial sources.

A vasopressin-like peptide in the mammalian sympathetic nervous system

Vasopressin was among the first mammalian hormonal peptides to be identified and to have its structure determined. Its only undisputed physiological role is as a circulating neurohypophyseal antidiuretic hormone. Other notable effects of vasopressin on peripheral tissues include contraction of vascular smooth muscle, leading to elevation of blood pressure, and activation of glycogenolysis in liver. It has long been clear that vascular smooth muscle and hepatocytes are relatively insensitive to the low concentrations of vasopressin normally present in the circulation, and the physiological significance of their responses has therefore been in doubt. We now report that a new bioactive and immunoreactive vasopressin-like peptide (VLP) is widely distributed in the sympathetic nervous system of mammals, both in the principal noradrenergic neurones of ganglia and in nerve fibres innervating peripheral tissues. In addition to other peptides described in the mammalian sympathetic nervous system, VLP must be considered as a possible mediator of the non-adrenergic responses to sympathetic activation. Moreover, many of the effects previously attributed to circulating vasopressin may be neurally evoked.

Inositol (1,4,5)trisphosphate-promoted Ca2+ release from microsomal fractions of rat liver

Crude mitochondrial fractions containing a substantial amount of microsomes accumulate Ca2+ in the presence of ATP, ruthenium red and oligomycin. A proportion of this accumulated Ca2+ is released by the addition of low concentrations (ca. 1 microM) of inositol (1,4,5) trisphosphate . Under some conditions the release is transient, and evidence is presented which suggests that this is due to inhomogeneity in the vesicle population. (1,4,5)inositol trisphosphate -induced Ca2+ release can also be demonstrated, under appropriate experimental conditions, in a more purified microsomal fraction essentially free of mitochondria.

Characterization of a rapid cellular-fractionation technique for hepatocytes. Application in the measurement of mitochondrial membrane potential in situ

A rapid cellular-fractionation technique [ Hoek , Nicholls & Williamson (1980) J. Biol. Chem. 255, 1458-1464] was further characterized by using hepatocytes. Of the mitochondrial marker-enzyme activity, 80% was routinely separated from 71-98% of the total cell activities of marker enzymes for plasma membranes, Golgi-membranes, endoplasmic reticulum, lysosomes and cytosol. The mitochondria were contaminated with 53% of cell nuclei. [3H]Triphenylmethylphosphonium ion (TPMP+) was added to hepatocytes in an attempt to measure cellular transmembrane electrical potentials. After rapid cell fractionation the electrical potential between mitochondria in situ and the incubation medium was found to be 202 mV. This value was slightly increased when hepatocytes were treated with oligomycin, but substantially decreased by oligomycin plus an uncoupler of oxidative phosphorylation. Although estimates of TPMP+ binding were obtained, substantial difficulties prevented the accurate measurement of the electrical potential across the plasma membrane. It is concluded that TPMP+ may be employed to demonstrate the integrity of mitochondria during the fractionation procedures. However, the cation is inadequate for the determination of the separate components of the electrical potential between the mitochondrial matrix and the incubation medium.

Phorbol esters inhibit alpha 1 adrenergic stimulation of glycogenolysis in isolated rat hepatocytes

Tumor promoting phorbol esters can stimulate Ca++-phospholipid-dependent protein kinase. It has been suggested that this enzyme may mediate the effects of calcium-dependent hormones. In this paper the effects of phorbol 12-myristate 13-acetate (TPA) on isolated rat hepatocyte metabolism were studied. Phorbol esters completely blocked alpha 1-adrenergic stimulation of glycogenolysis. This effect is quite specific for alpha 1-adrenergic actions, as the stimulations of glycogenolysis by vasopressin, angiotensin II, ionophore A-23187 and glucagon were unaffected by TPA. The potencies of the different phorbol esters used in this study suggests that the inhibitory effects of these agents may be due to activation of protein kinase C. The effect of phorbol esters on alpha 1-adrenergic actions seems to occur at an early step of the alpha 1-adrenergic action. TPA (10(-11) -10(-6)M) was unable to stimulate glycogenolysis. Urea synthesis, which is stimulated by vasopressin and alpha 1-adrenergic agents, was not stimulated by phorbol ester, neither alone nor in combination with the Ca++ ionophore A-23187.

Analysis of the metabolic turnover of the individual phosphate groups of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. Validation of novel analytical techniques by using 32P-labelled lipids from erythrocytes

We have developed methods that yield estimates of the 32P content of each of the individual phosphate groups of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, thus extending the information available from studies of the labelling of these lipids in intact cells or membrane preparations. The analyses are undertaken with the deacylated lipids. Assay of the 5-phosphate of phosphatidylinositol 4,5-bisphosphate is achieved by the use, under conditions of first-order kinetics, of a 5-phosphate-specific phosphomonoesterase present in isolated erythrocyte membranes [Downes, Mussat & Michell (1982) Biochem. J. 203, 169-177]. Assay of the 4-phosphate of phosphatidylinositol 4-phosphate and of the total monoester phosphate content (4-phosphate plus 5-phosphate) of phosphatidylinositol 4,5-bisphosphate employs alkaline phosphatase from bovine intestine. The radioactivity of the 1-phosphate is that remaining as organic phosphate after exhaustive alkaline phosphatase treatment. The methodology has been validated by using lipids from human erythrocytes: these contain no 32P in their 1-phosphate. These methods should be of substantial value in studies of the many cells that show rapid hormonal perturbations of phosphatidylinositol 4,5-bisphosphate metabolism.

Phosphatidylinositol-4,5-bisphosphate phosphodiesterase and phosphomonoesterase activities of rat brain. Some properties and possible control mechanisms

The phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] [and to a lesser extent, the phosphatidylinositol-4-phosphate (PtdIns4P)] phosphodiesterase and monoesterase activities of a rat brain supernatant have been studied by using 32P-labelled substrates prepared from human red blood cells. PtdIns(4,5)P2 monoesterase is maximally stimulated by Mg2+, though some activity is detectable in Ca2+/EDTA (Mg2+-free) buffers. The phosphodiesterase, however, is Ca2+-dependent, and in Ca2+/EDTA buffers with the pure lipid as substrate, shows maximal activity at 100 nM-Ca2+. If PtdIns(4,5)P2 is presented as a component of a lipid mixture of similar composition to that of the inner half of the lipid bilayer of a rat liver plasma membrane, the phosphodiesterase shows considerable activity at 1 microM-Ca2+, and is maximal at 100 microM-Ca2+. However, if it is assayed against the same substrate in Ca2+/EGTA buffers with 3mM-Mg2+ and 80 mM-KCl present (as an approximate parallel with the ionic environment in vivo), it shows no detectable activity below 100 microM-Ca2+, and is maximal at 1 mM-Ca2+. The monoesterase can hydrolyse PtdIns(4,5)P2 in such a lipid mixture at all Ca2+ concentrations with 1 or 3 mM-Mg2+ present. PtdIns(4,5)P2 phosphodiesterase can be induced to attack its substrate under ionic conditions similar to those in vivo (0.1-1 microM-Ca2+; 1 mM-Mg2+; 80 mM-KCl) by the conversion of its substrate into a non-bilayer configuration. If given such a substrate [by mixing PtdIns(4,5)P2 with an excess of phosphatidylethanolamine (PtdEtn)] it shows a shallow Ca2+-dependency curve from 0.1 to 100 microM and then a steep rise to 1 mM-Ca2+. Together these observations lead us to the suggestion that a perturbation in a membrane in vivo equivalent to a non-bilayer configuration would be sufficient to induce phosphodiesterase-catalysed PtdIns(4,5)P2 breakdown. When given substrates mixed with excess PtdEtn at pH 7.25 (or 5.5), 1 microM-Ca2+, 1 mM-Mg2+ and 80 mM-KCl, the rat brain supernatant phosphodiesterase activity hydrolysed PtdIns(4,5)P 50-100-fold faster than it hydrolysed phosphatidylinositol (PtdIns). If the supernatant was presented with such a non-bilayer mixture containing a ten-fold excess of PtdIns over PtdIns(4,5)P2, the latter phospholipid was still hydrolysed by phosphodiesterasic cleavage at nearly ten times the rate of the former. Receptor-stimulated phosphodiesterase cleavage of polyphosphoinositides is an early event in cell activation by many agonists. The properties of PtdIns(4,5)P2 phosphodiesterase in vitro suggest that a change in the presentation of its substrate would be a sensitive and sufficient control on the enzyme's activity in vivo.

Receptor-mediated metabolism of the phosphoinositides and phosphatidic acid in rat lacrimal acinar cells

The metabolism of the inositol lipids and phosphatidic acid in rat lacrimal acinar cells was investigated. The muscarinic cholinergic agonist methacholine caused a rapid loss of 15% of [32P]phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] and a rapid increase in [32P]phosphatidic acid (PtdA). Chemical measurements indicated that the changes in 32P labelling of these lipids closely resembled changes in their total cellular content. Chelation of extracellular Ca2+ with excess EGTA caused a significant decrease in the PtdA labelling and an apparent loss of PtdIns(4,5)P2 breakdown. The calcium ionophores A23187 and ionomycin provoked a substantial breakdown of [32P]PtdIns(4,5)P2 and phosphatidylinositol 4-phosphate (PtdIns4P); however, a decrease in [32P]PtdA was also observed. Increases in inositol phosphate, inositol bisphosphate and inositol trisphosphate were observed in methacholine-stimulated cells, and this increase was greatly amplified in the presence of 10 mM-LiCl; alpha-adrenergic stimulation also caused a substantial increase in inositol phosphates. A23187 provoked a much smaller increase in the formation of inositol phosphates than did either methacholine or adrenaline. Experiments with excess extracellular EGTA and with a protocol that eliminates intracellular Ca2+ release indicated that the labelling of inositol phosphates was partially dependent on the presence of extracellular Ca2+ and independent of intracellular Ca2+ mobilization. Thus, in the rat lacrimal gland, there appears to be a rapid phospholipase C-mediated breakdown of PtdIns(4,5)P2 and a synthesis of PtdA, in response to activation of receptors that bring about an increase in intracellular Ca2+. The results are consistent with a role for these lipids early in the stimulus-response pathway of the lacrimal acinar cell.

Characteristics of the adenohypophyseal Ca2+-phospholipid-dependent protein kinase

The Ca2+-phospholipid-dependent protein kinase, initially described by Takai et al. (J. Biol. Chem. 254, 3692-3695, 1979), has been identified in the anterior pituitary gland of the rat and sheep. The enzyme is essentially undetectable in initial cell extracts but marked activity is manifest following DEAE chromatography, suggesting the potential presence of an endogenous inhibitor of this enzyme. Two forms of this protein kinase exist in both sheep and rat anterior pituitary gland, both of which are similarly dependent upon Ca2+, phosphatidyl serine and diacylglycerol. Several endogenous substrates for this protein kinase have been observed in both the pars distalis and pars tuberalis of the sheep adenohypophysis.

Effects of vasopressin and corticosterone on fatty acid metabolism and on the activities of glycerol phosphate acyltransferase and phosphatidate phosphohydrolase in rat hepatocytes

The effects of vasopressin on the short-term control of fatty acid metabolism were studied in isolated rat hepatocytes. Vasopressin increased the oxidation of oleate to CO2 and decreased the formation of ketones in hepatocytes from Wistar rats, but not from Brattleboro rats. Incubation with vasopressin for 30 min increased the conversion of oleate into triacylglycerol by 17% and 32% in hepatocytes from Wistar and Brattleboro rats respectively. The corresponding increases for the phospholipid fraction were 19% and 42%. When Wistar-rat hepatocytes were incubated with corticosterone for 6 h there was a 19% increase in triacylglycerol synthesis, and a 52% increase if vasopressin was added 30 min before the end of the incubation. Glycerol phosphate acyltransferase activity was not significantly increased by vasopressin. Incubation for 5-60 min with vasopressin increased the Vmax. of phosphatidate phosphohydrolase by 48% and 32% respectively in hepatocytes from Wistar and Brattleboro rats. These increases were antagonized if EGTA was added to the medium used for incubating the hepatocytes. The replacement of vasopressin by 5 microM-ionophore A23187 produced a significant increase of 13% in the phosphohydrolase activity. It is therefore likely that the effects of vasopressin on the phosphohydrolase are mediated by Ca2+. These results are discussed in relation to the possible function of phosphatidate phosphohydrolase in controlling the turnover of phosphoinositides, the synthesis of phosphatidylethanolamine, phosphatidylcholine and triacylglycerol, and the secretion of very-low-density lipoproteins.

Angiotensin-induced changes in the apparent size of rat liver angiotensin receptors

Angiotensin receptors from rat liver were labeled using four different ligands: (Sar1-(3H)Tyr4)-Angiotensin ll ((3H)SarAll); (Sar1-(3H)Tyr4-lle8)-Angiotensin ll ((3H)SarlleAll); (Sar1-(125I) Tyr4-(4'-N3) Phe8)-Angiotensin ll (IN3All); (Sar1-(125I)Tyr4-(4'N3D-Phe)8)-Angiotensin ll (IN3DPheAll). (3H)SarAll and IN3All behaved like agonists and (3H)SarlleAll and IN3DPheAll like antagonists. All four ligands labeled the same population of sites. The azido derivatives allowed covalent labeling of receptors with a high yield (about 40%). Membranes were solubilized by Triton X-100 under experimental conditions which ensured complete solubilization of the liganded receptors in a stable form (less than 40% dissociation after 20 h). The apparent size of liganded angiotensin receptors was determined by gel filtration on Ultrogel ACA-34 columns and by SDS gel electrophoresis (in the case of covalent labeling). The apparent Stokes radius of solubilized angiotensin receptors was different wether the receptor was labeled with an agonist (Stokes radius = 6.2 +/- 0.1 nm (6) after labeling with (3H)SarAll) or with an antagonist (Stokes radii of 5.5 +/- 0.1 (7), and 5.6 +/- 0.1 nm (4) after labeling with (3H)SarlleAll and IN3DPheAll respectively). After covalent labeling with IN3All angiotensin receptors were eluted as a mixture of light and heavy forms. SDS gel electrophoresis revealed only one molecular entity of Mr 64,000. It is concluded that binding of an agonist to liver angiotensin receptors triggers or stabilizes an interaction with another membrane component involved in the coupling of the receptor to its primary effector.

Receptor-induced diacylglycerol formation in permeabilized platelets; possible role for a GTP-binding protein

Exposure of human platelets to 10 discharges from a 4.5 microF capacitor charged at 3 kV permitted isolation of a stable preparation of permeabilized platelets that, after equilibration with Ca2+ buffers (pCa less than 6) for 15 min at O degrees C, secreted 5-hydroxytryptamine (5-HT) at 25 degrees C. Thrombin enhanced the sensitivity to Ca2+ of the secretion of 5-HT by about 10-fold, whereas Arg -vasopressin and the prostaglandin endoperoxide analogue, U-46619, increased sensitivity to Ca2+ by 3 to 4-fold. This action of thrombin was associated with stimulation of diacylglycerol formation, a marked increase in phosphorylation of protein P47 and a smaller increase in phosphorylation of the P-light chain of myosin. Thrombin exerted these effects at a [Ca2+ free] of 0.1 microM, suggesting that the receptor-activated breakdown of platelet phosphoinositides to diacylglycerol may not require prior Ca2+ mobilization in intact platelets. In both the presence and absence of thrombin, a higher [Ca2+ free] was required for optimal secretion than for maximal phosphorylation of P47 and myosin light-chain, indicating that Ca2+ and possibly diacylglycerol have roles in the secretory mechanism additional to activation of the enzymes that phosphorylate these proteins. Stable GTP analogues such as guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S), and to a lesser extent GTP itself, enhanced the Ca2+ sensitivity of the secretion of 5-HT from permeabilized platelets. Moreover, GTP potentiated the stimulatory action of thrombin. These effects of GTP gamma S and GTP were associated with increased diacylglycerol formation and were inhibited by guanosine-5'-O-(2-thiodiphosphate) (GDP beta S) suggesting that a GTP-binding protein may play a role in the receptor-activated breakdown of phosphoinositides. However, as GDP beta S did not inhibit the potentiation of secretion caused by thrombin alone, a GTP-independent pathway of platelet activation may also exist.

The role of phosphatidylinositol 4,5 bisphosphate breakdown in cell-surface receptor activation

The activation of Ca2+-mobilising receptors on hepatocytes and many other cells leads to a prompt reduction in the cellular content of inositol phospholipids. The primary event which underlies these changes is, most probably, a phospholipase C-catalysed attack upon phosphatidylinositol 4,5 bisphosphate. The receptor-mediated breakdown of this lipid in stimulated cells is: (i) not mediated by an increase in cytosol [Ca2+] and (ii) closely coupled to receptor occupation. Phosphatidylinositol 4,5 bisphosphate degradation may be studied by measuring the appearance of the water-soluble product, inositol trisphosphate (and its metabolites: inositol bisphosphate and inositol monophosphate), in stimulated cells. Recent evidence indicates that inositol trisphosphate and the lipid soluble product of phosphatidylinositol 4,5 bisphosphate breakdown, 1,2 diacylglycerol, may act as 'second messengers' which mediate the effects of many extracellular signals in stimulated cells.

Effects of carbachol and pancreozymin (cholecystokinin-octapeptide) on polyphosphoinositide metabolism in the rat pancreas in vitro

We studied the possibility that hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] may be the initiating event for the increase in [32P]Pi incorporation into phosphatidic acid (PtdA) and phosphatidylinositol (PtdIns) during carbachol and pancreozymin (cholecystokinin-octapeptide) action in the rat pancreas. After prelabelling acini for 2h, [32P]Pi incorporation into PtdA, PtdIns(4,5)P2 and phosphatidylinositol 4-phosphate (PtdIns4P) had reached equilibrium. Subsequent addition of carbachol or pancreozymin caused 32P in PtdIns(4,5)P2 to decrease by 30-50% within 10-15 s, and this was followed by sequential increases in [32P]Pi incorporation into PtdA and PtdIns. Similar changes in 32P-labelling of PtdIns4P were not consistently observed. Confirmation that the decrease in 32P in chromatographically-purified PtdIns(4,5)P2 reflected an actual decrease in this substance was provided by the fact that similar results were obtained (a) when PtdIns(4,5)P2 was prelabelled with [2-3H]inositol, and (b) when PtdIns(4,5)P2 was measured as its specific product (glycerophosphoinositol bisphosphate) after methanolic alkaline hydrolysis and ion-exchange chromatography. The secretogogue-induced breakdown of PtdIns(4,5)P2 was not inhibited by Ca2+ deficiency (severe enough to inhibit amylase secretion and Ca2+-dependent hydrolysis of PtdIns), and ionophore A23187 treatment did not provoke PtdIns(4,5)P2 hydrolysis. The increase in the hydrolysis of PtdIns(4,5)P2 and the increase in [32P]Pi incorporation into PtdA commenced at the same concentration of carbachol in dose-response studies. Our findings suggest that the hydrolysis of PtdIns(4,5)P2 is an early event in the action of pancreatic secretogogues that mobilize Ca2+, and it is possible that this hydrolysis may initiate the Ca2+-independent labelling of PtdA and PtdIns. Ca2+ mobilization may follow these responses, and subsequently cause Ca2+-dependent hydrolysis of PtdIns and exocytosis.

Cellular calcium metabolism: activation and antagonism

The role of Ca2+ as a mediator of many cellular responses, including stimulus-secretion and excitation-contraction coupling, is reviewed. Pathways of Ca2+ mobilization are discussed in terms of both intracellular and plasmalemmal processes. The extent to which specific antagonists exist, active at these several sites, is noted, with particular emphasis on the clinically available Ca2+ channel antagonists, including verapamil, nifedipine, and diltiazem.

Breakdown of polyphosphoinositides and not phosphatidylinositol accounts for muscarinic agonist-stimulated inositol phospholipid metabolism in rat parotid glands

The molecular mechanisms underlying the ability of muscarinic agonists to enhance the metabolism of inositol phospholipids were studied using rat parotid gland slices prelabelled with tracer quantities of [3H]inositol and then washed with 10 mM unlabelled inositol. Carbachol treatment caused rapid and marked increases in the levels of radioactive inositol 1-phosphate, inositol 1,4-bisphosphate, inositol 1,4,5-trisphosphate and an accumulation of label in the free inositol pool. There were much less marked changes in the levels of [3H]phosphatidylinositol, [3H]phosphatidylinositol 4-phosphate and [3H]phosphatidylinositol 4,5-bisphosphate. At 5 s after stimulation with carbachol there were large increases in [3H]inositol 1,4-bisphosphate and [3H]inositol 1,4,5-trisphosphate, but not in [3H]inositol 1-phosphate. After stimulation with carbachol for 10 min the levels of radioactive inositol 1,4-bisphosphate and inositol 1,4,5-trisphosphate greatly exceeded the starting level of radioactivity in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate respectively. When carbachol treatment was followed by addition of sufficient atropine to block all the muscarinic receptors the radioactive inositol phosphates rapidly returned towards control levels. The carbachol-evoked changes in radioactive inositol phosphate and phospholipid levels were blocked in the presence of 2,4-dinitrophenol (an uncoupler of oxidative phosphorylation). The results suggest that muscarinic agonists stimulate a polyphosphoinositide-specific phospholipase C and that these lipids are continuously replenished from the labelled phosphatidylinositol pool. [3H]Inositol 1-phosphate in the stimulated glands probably arises via hydrolysis of inositol 1,4-bisphosphate and not directly from phosphatidylinositol.

Sympathetic denervation impairs agonist-stimulated phosphatidylinositol metabolism in rat parotid glands

Substance P, muscarinic and alpha-adrenoceptor agonists stimulated the incorporation of [3H]inositol into phosphatidylinositol in rat parotid gland slices. Surgical denervation of the sympathetic input to the rat parotid gland by superior cervical ganglionectomy produced marked reductions in these responses. The stimulated incorporation of radiolabelled precursors into phosphatidylinositol is a measure of its resynthesis after receptor-mediated breakdown of inositol phospholipids. We therefore examined the enzymic site of the lesion induced by sympathetic denervation using parotid gland slices labelled with either [3H]inositol or [32P]phosphate and stimulated with substance P. Receptor-activated phospholipase C attack upon [3H]inositol phospholipids was assayed by measuring the formation of [3H]inositol 1-phosphate in the presence of 10 mM-Li+ to inhibit further breakdown. It was not affected by denervation. Substance P elicited a rapid breakdown of phosphatidylinositol 4,5-bisphosphate and this response was reduced in the denervated gland. The second step in stimulated phosphatidylinositol turnover, phosphorylation of diacylglycerol to phosphatidate was not affected by denervation. Sympathetic denervation appears to induce a specific enzymic lesion in the parotid gland that impairs receptor-stimulated resynthesis of phosphatidylinositol from phosphatidate. This change in membrane lipid metabolism may be related to a number of the effects of sympathetic denervation, such as agonist supersensitivity, reduced gland cell proliferation and induction of new surface receptors.