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

Effects of EDCF and endothelin on phosphatidylinositol hydrolysis and contraction in rat aorta

Endothelium-derived constricting factor (EDCF) and endothelin are peptidergic substances produced and released from endothelial cells that induce contraction of vascular smooth muscle. The purpose of the present study was to investigate possible mechanisms by which EDCF and endothelin elicit contraction. Exposure of rat aorta to EDCF or synthetic endothelin resulted in time- and concentration-dependent increases in tension and levels of inositol monophosphate, a breakdown product of the phosphatidylinositides. A 10-s exposure to endothelin elevated levels of inositol 1,4,5-trisphosphate. Trypsinization or heating of EDCF prevented the contraction and inositol monophosphate formation. To assess whether EDCF and endothelin may act as endogenous agonists of the dihydropyridine-sensitive Ca2+ channel, we evaluated the ability of the dihydropyridine Ca2+ channel agonist (+)-S202-791 to increase the formation of the inositol phosphates. (+)-S202-791 increased inositol monophosphate formation. However, in contrast to that elicited by EDCF and endothelin, the increase in inositol monophosphate because of (+)-S202-791 was abolished by pretreatment with the cyclooxygenase inhibitor indomethacin (10 microM). These results suggest that contractions induced by EDCFs may be mediated through activation of phospholipase C and subsequent production of second messengers.

Altered responses of regenerating hepatocytes to norepinephrine and transforming growth factor type beta

Norepinephrine (NE), acting through the alpha 1-adrenergic receptor, modules the response of rat hepatocytes in primary culture to transforming growth factor type beta 1 (TGF beta) by increasing the amount of TGF beta required for a given degree of inhibition of epidermal growth factor (EGF)-induced DNA synthesis (Houck et al., J. Cell. Physiol. 135:551-555, 1988). This effect was also found in hepatocytes isolated from regenerating livers but was greatly magnified in cells isolated between 12 and 18 hr after two-thirds partial hepatectomy (PHX). During this period of enhanced sensitivity, NE was equally potent in terms of dose but more efficacious in the regenerating hepatocytes. As it did in control hepatocytes (Cruise et al., Science 227:749-751, 1985), the alpha 1-adrenergic receptor mediated the activity of NE in regenerating hepatocytes. Vasopressin (VP) and angiotensin-II (AG) also antagonized the effect of TGF beta and showed increased activity in regenerating hepatocytes but at only 50% or less of the maximal effect reached by NE. Regenerating hepatocytes isolated 24-72 hr after PHX exhibited decreased sensitivity to inhibition by TGF beta, with a nadir in 48-hr-regenerating cells. These findings suggest that NE may be involved in triggering the early phase of DNA synthesis during liver regeneration, with the subsequent acquisition of innate resistance to TGF beta responsible for continued proliferation at a time when TGF beta mRNA is known to be increasing in the liver (Braun et al., Proc. Natl. Acad. Sci. USA 85:1539-1543, 1988). EGF induced increased DNA and protein synthesis in cultures of control hepatocytes; TGF beta inhibited the EGF-induced DNA synthesis but had no effect on protein synthesis. This may be relevant to the latter stages of liver regeneration, when high levels of TGF beta mRNA are detected in liver and cellular hypertrophy predominates over hyperplasia.

Receptor specific for certain nucleotides stimulates inositol phosphate metabolism and Ca2+ fluxes in A431 cells

We have recently reported that extracellular ATP induces a transient rise in cytosolic free Ca2+ [( Ca2+]i) in individual human epidermoid carcinoma A431 cells (Gonzalez et al: Journal of Cellular Physiology 135:269-276, 1988). We have now studied nucleotide specificity and desensitization for several early responses. Extracellular ATP (5-100 microM) caused the rapid formation of inositol trisphosphate and later its metabolites, inositol bisphosphate and inositol monophosphate. ATP also induced the efflux of 45Ca2+ from pre-loaded cells. In addition, an increase in the rate of influx of 45Ca2+ stimulated by extracellular ATP was detected. Based on measurements of 45Ca2+ efflux and influx, desensitization studies, and chlortetracycline fluorimetry, we conclude that ATP mobilizes Ca2+ from internal stores and also stimulates entry across the plasma membrane. These effects were also displayed by UTP and to a lesser extent by ITP, while other nucleoside triphosphates as well as ADP, AMP, and adenosine, were inactive. Furthermore, desensitization of the response to ATP and UTP was seen after prolonged exposure to either nucleotide. This was specific for the nucleotide receptor since a response to bradykinin was not affected by the ATP pretreatment, although pretreatment with phorbol ester inhibited responses to both the nucleotides and bradykinin. Quantitative data on rate of recovery from the desensitized state and the response of desensitized cells to greatly elevated levels of ATP are presented. Extracellular ATP stimulated another early change previously reported for epidermal growth factor, namely, the phosphorylation of an 81-kDa cytoskeletal protein. The stimulation of these events involves an ATP receptor whose properties differ from other ATP receptors that have been described.

Two distinct receptors for ATP can be distinguished in Swiss 3T6 mouse fibroblasts by their desensitization

The stimulation of calcium efflux from Swiss 3T6 mouse fibroblasts by extracellular ATP was studied. It was found that the cells could be desensitized to ATP by a previous exposure to the nucleotide, lending support to the theory that this is a receptor mediated process. Another ATP-receptor mediated process in Swiss 3T6 cells, that is also subject to desensitization, causes the permeabilization of the plasma membrane to nucleotides and other normally impermeant compounds [Gonzalez et al., J. Cell. Physiol. 139:109 (1989)]. Here we demonstrate that selective desensitization of the ATP-dependent calcium mobilization pathway can be achieved without affecting ATP-induced permeabilization. Data are presented in support of the existence of multiple ATP-receptors (purinoceptors) in Swiss 3T6 cells.

Extracellular ATP is a mitogen for 3T3, 3T6, and A431 cells and acts synergistically with other growth factors

Extracellular ATP in concentrations of 5-50 microM displayed very little mitogenic activity by itself but it caused synergistic stimulation of [3H]thymidine incorporation in the presence of phorbol 12-tetradecanoate 13-acetate, epidermal growth factor, platelet-derived growth factor, insulin, adenosine, or 5'-(N-ethyl)carboxamidoadenosine. Cultures of Swiss 3T3, Swiss 3T6, A431, DDT1-MF2, and HFF cells were used. The percent of cell nuclei labeled with [3H]thymidine and cell number were also increased. ADP was equally mitogenic, while UTP and ITP were much less active. The effect of ATP was not due to hydrolysis by ectoenzymes to form adenosine, a known growth factor. Thus, the nonhydrolyzable analogue adenosine 5'-[beta, gamma-imido]triphosphate was mitogenic. In addition, it was found that ATP showed synergism in 3T6 and 3T3 cells when present for only the first hour of an incorporation assay, during which time no significant hydrolysis occurred. Furthermore, prolonged preincubation of cells with ATP reduced the mitogenic response to ATP but not to adenosine; preincubation with adenosine or N6-(R-phenylisopropyl)adenosine had the reverse effect. Finally, the effect of adenosine, but not of ATP, was inhibited by aminophylline. We conclude that extracellular ATP is a mitogen that interacts with P2 purinoceptors on the plasma membrane.

The effect of acetylcholine on inositol lipid metabolism and intracellular calcium concentrations in bovine anterior pituitary cells

Acetylcholine (ACh) increased the intracellular calcium concentration in bovine anterior pituitary cells. In the presence of the calcium channel antagonists verapamil (20 microM) or nitrendepine (1 microM) the increase in calcium was partially inhibited but showed both transient and sustained components. In the presence of EGTA (2.5 mM) only the transient component was observed. ACh also decreased inositol radioactivity in phosphatidylinositides and increased it in inositol phosphates. It is concluded that the increase in calcium caused by acetylcholine requires both the entry of external calcium and mobilisation of internal calcium. Replacement of external sodium by N-methyl-D-glucamine inhibited the rises in calcium and inositol phosphate labelling in response to ACh. Tetrodotoxin (3 microM) or ouabain (50 microM) did not affect either response to ACh. Verapamil did not affect the calcium rise induced by ACh in the absence of external sodium. The phorbol ester PMA (10 nM) caused a transient rise in calcium and inhibited the calcium rise caused by acetylcholine: it did not modify the effect of acetylcholine on inositol phosphates. The dependence of the stimulation of external calcium entry and inositol phosphate production on external sodium ions and protein kinase C is discussed.

The effects of thyrotropin-releasing hormone and potassium depolarization on phosphoinositide metabolism and cytoplasmic calcium in bovine pituitary cells

Addition of thyrotropin-releasing hormone (TRH) (10 nM to 10 microM) to bovine anterior pituitary cells labelled with [3H]inositol decreased the radioactivity in inositol-containing lipids and increased it in inositol phosphates. TRH also increased the cytoplasmic calcium concentration biphasically. At TRH concentrations below 10 nM, the increase was sustained and sensitive to inhibitors of calcium influx through voltage-gated channels, whereas concentrations over 10 nM elicited in addition a rapid transient increase in calcium, which was relatively insensitive to such inhibition. Incubation of the cells in medium containing 25 mM KCl increased the cytoplasmic calcium concentration by stimulating influx through voltage-gated channels, and markedly enhanced the initial transient increase of calcium seen at TRH concentrations above 10 nM. It did not affect the generation of InsP3 and it also enhanced the calcium response to ionomycin. It is suggested that stimulation of calcium entry through voltage-gated channels can increase the amount of calcium available for mobilisation by TRH.

Role of receptor cycling in the regulation of angiotensin II surface receptor number and angiotensin II uptake in rat vascular smooth muscle cells

In vivo data on the factors controlling angiotensin II (AII) cell surface binding are conflicting. We studied the specific effects of AII on AII binding in rat mesenteric artery vascular smooth muscle cells in culture. Incubation with unlabeled AII at 21 degrees C resulted in time- and concentration-dependent decreases in AII surface binding at 4 degrees C, with a 30% reduction after exposure to 300 nM AII for 15 min. Reductions in cell surface binding were due to decrements in receptor number rather than changes in binding affinity. Loss of surface receptors was mediated by receptor internalization as maneuvers that blocked ligand internalization (cold temperature and phenylarsine oxide [PAO]) attenuated AII-induced loss of surface receptors. After removal of AII, recovery of surface binding was rapid (t1/2 = 15 min) and was mediated by reinsertion of a preexisting pool of receptors into the surface membrane rather than by new receptor synthesis. To determine the role of receptor cycling on AII-induced surface receptor loss, cells were incubated with the endosomal inhibitor chloroquine during exposure to AII at 21 degrees C. Incubation with AII plus chloroquine resulted in a 70% greater loss of surface binding than after incubation with AII alone. To determine the role of receptor cycling on uptake of ligand, cells were incubated with PAO or endosomal inhibitors during exposure to AII at 4 and 21 degrees C. Compared with buffer these agents did not alter AII uptake at 4 degrees C, but decreased uptake by 12-50% at 21 degrees C. These results indicate that after binding AII receptors cycle and that receptor cycling attenuates AII-induced losses of surface receptors and enhances ligand uptake by providing a continuous source of receptors to the cell surface.

Angiotensin II receptors labelled with 125I-[Sar1, Ile8]-AII in albino rabbit ocular tissues

High affinity binding sites for the angiotensin II antagonist 125I-[Sar1,Ile8]-AII have been identified and characterized in membrane suspensions of ocular tissues of albino rabbits. Scatchard analysis of the binding indicated a single class of sites with Kd values of 186, 92, 152, 50, 102 pM for the iris + ciliary body, choroid, ciliary process, retina and cornea, respectively. The corresponding concentrations of binding sites were 22, 68, 35, 22 and 4 fmole/mg of protein. The order of potency for several AII analogs to compete with 125I-[Sar1,Ile8]-AII at its binding sites in iris + ciliary body membranes ([Sar1,Leu8]-AII = [Sar1,Ile8]-AII greater than AII = [Sar1, Ala8]-AII greater than AIII greater than AI) resembled the order of potency found for AII receptors in other tissues. The competition curves for this tissue using AII and AIII were best explained by the existence of two populations of binding sites. The addition of the guanine nucleotide, GppNHp, to the assay resulted in a 6.7-fold and 2.3-fold decrease in the respective affinities of AII and AIII for 125I-[Sar1,Ile8]-AII binding sites without a change in the slope of the competition curves. The GppNHp-induced effect was also observed in ciliary process membranes but not in retinal or choroidal membranes. These results indicate the presence of AII receptors regulated by a GTP-binding protein in both the ciliary process and the iris + ciliary body of the rabbit. They also suggest a difference in the guanine nucleotide regulation of AII receptors in different ocular tissues.

Stimulation of CTP: phosphocholine cytidylyltransferase and phosphatidylcholine synthesis by calcium in rat hepatocytes

The effects of Ca2+, ionophore A23187, and vasopressin on CTP:phosphocholine cytidylyltransferase were investigated. Cytidylyltransferase is present in the cytosol and in a membrane-bound form on the microsomes. Digitonin treatment caused release of the cytosolic form rapidly. Addition of 7 mM Ca2+ to hepatocyte medium resulted in a 3-fold decrease in cytidylyltransferase released by digitonin treatment (1.7 +/- 0.1 nmol/min per mg compared to 5.1 +/- 0.2 nmol/min per mg in the control). Verapamil, a calcium channel blocker, partially overcame this effect of Ca2+. Ionophore A23187 and vasopressin both mimicked the effect of Ca2+ and resulted in a decrease in cytidylyltransferase release (2.4 +/- 0.1 nmol/min per mg and 2.5 +/- 0.2 nmol/min per mg, respectively) compared to control (3.4 +/- 0.1 nmol/min per mg). In agreement with the digitonin experiments, incubation with 7 mM Ca2+ resulted in a decrease in cytidylyltransferase in the cytosol (from 4.0 to 1.2 mol/min per mg) and a corresponding increase in the microsomes (from 0.6 to 2.4 nmol/min per mg). Verapamil partially blocked this translocation caused by Ca2+. Ionophore A23187 and vasopressin also caused translocation of the cytidylyltransferase from the cytosol to the microsomes. The addition of Ca2+ also resulted in an increase in PC synthesis. With 7 mM Ca2+ in the medium, the label associated with PC increased to 3.8 +/- 0.1.10(6) dpm/dish from 2.7 +/- 0.1.10(6) dpm/dish after 10 min. PC degradation was also affected, since 7 mM Ca2+ in the medium resulted in an increase in LPC formation both in the cell and the medium. We conclude that high concentrations of calcium in the hepatocyte medium can cause a stimulation of CTP:phosphocholine cytidylyltransferase and PC synthesis in cultured hepatocytes.

Effects of exogenous phospholipase enzymes, arachidonic acid and 1-oleoyl-2-acetyl-sn-glycerol on ketogenesis in isolated rat hepatocytes

Studies were conducted to see whether exogenous phospholipase C from Clostridium perfringens, phospholipase A2 from Crotalus adamanteus venom, arachidonic acid and 1-oleoyl-2-acetyl-sn-glycerol (OAG) mimic the anti-ketogenic action of vasopressin in isolated rat hepatocytes. Exogenous phospholipase C inhibited ketogenesis in the presence of 0.5 mM oleate. Experiments employing [1-14C]oleate, however, indicated that the mechanism involved in the anti-ketogenic action of exogenous phospholipase C is distinct from that of vasopressin. The decreased rate of the production of acid-soluble products from [1-14C]oleate in response to vasopressin could be explained by the sum of the increased rates of 14CO2 formation and [1-14C]oleate esterification. By contrast, exogenous phospholipase C suppressed not only the formation of acid-soluble products but also 14CO2 production and [1-14C]oleate esterification. Indeed, phospholipase C greatly inhibited [1-14C]oleate uptake into hepatocytes. It is suggested that the alteration of the architecture of plasma membrane by exogenous phospholipase C may lead to the disturbance of oleate uptake and consequent general suppression of oleate metabolism. Exogenous phospholipase A2, arachidonic acid and OAG increased ketogenesis regardless of the presence of oleate. The ketogenic effects may be attributed to the supply of fatty acids by these agents to hepatocytes.

Inositol polyphosphate production and regulation of cytosolic calcium during the biphasic activation of adrenal glomerulosa cells by angiotensin II

Stimulation of aldosterone production by angiotensin II in the adrenal glomerulosa cell is mediated by increased phosphoinositide turnover and elevation of intracellular Ca2+ concentration. In cultured bovine glomerulosa cells, angiotensin II caused rapid increases in inositol-1,4,5-trisphosphate (Ins-1,4,5-P3) levels and cytosolic Ca2+ during the first minute of stimulation, when both responses peaked between 5 and 10 s and subsequently declined to above-baseline levels. In addition to this temporal correlation, the dose-response relationships of the angiotensin-induced peak increases in cytosolic Ca2+ concentrations and Ins-1,4,5-P3 levels measured at 10 s were closely similar. However, at later times (greater than 1 min) there was a secondary elevation of Ins-1,4,5-P3, paralleled by increased formation of inositol 1,3,4,5-tetrakisphosphate that was associated with cytosolic Ca2+ concentrations only slightly above the resting value. These results are consistent with the primary role of Ins-1,4,5-P3 in calcium mobilization during activation of the glomerulosa cell by angiotensin II. They also suggest that Ins-1,4,5-P3 participates in the later phase of the target-cell response, possibly by acting alone or in conjunction with its phosphorylated metabolites to promote calcium entry and elevation of cytosolic Ca2+ during the sustained phase of aldosterone secretion.

Importance of the presence of the N-terminal tripeptide of substance P for the stimulation of phosphatidylinositol metabolism in rat parotid gland: a possible activation of phospholipases C and D

In this study, we have compared the effects of Substance P (SP) and an SP deprived of the N-terminal tripeptide, SP(4-11), on phosphoinositide metabolism by measuring phosphoinositide breakdown, inositol phosphate production and inositol incorporation into phosphoinositides. This work shows that SP and SP(4-11) have similar effects on phosphatidylinositol-4.5 bisphosphate (PIP2) metabolism. In fact, SP(4-11), like SP, induces a rapid PIP2 breakdown. On the contrary, SP and SP(4-11) have different effects on phosphatidylinositol (PI) metabolism since SP induces a decrease of radioactivity in PI, whereas SP(4-11) does not. Both peptides stimulate [3H]-inositol mono-, bis- and trisphosphate (respectively IP1, IP2, IP3) production in a time and dose-dependent manner. The kinetic of IP3 production is directly correlated with the one of PIP2 breakdown. The time course of IP1 production after SP(4-11) shows a time delay, while the one after SP does not. Since SP evokes an IP1 production without any delay and a large decrease of radioactivity in PI (which cannot account for the small amount measured in IP1 accumulation) we suggest that SP could activate a PI specific phospholipase C (leading to a PI breakdown) and a phospholipase D. These activations would require the complete structure of SP while the classical PIP2 specific phospholipase C activation (which induces PIP2 breakdown) would only require the carboxamide part of the peptide. So the complete structure of SP would be necessary to have a complete response (stimulation of PIP2 and PI metabolism).

Hepatocyte gap junctions are permeable to the second messenger, inositol 1,4,5-trisphosphate, and to calcium ions

Hepatocytes are well coupled by gap junctions, which allow the diffusion of small molecules between cells. Although gap junctions in many tissues are permeable to molecules larger than cAMP and in several preparations gap junctions pass cAMP itself, little direct evidence supports permeation by other second-messenger species. Ca2+, perhaps the smallest second messenger, would be expected to cross gap junctions, but the issue is complicated because gap-junction channels are closed when intracellular free Ca2+ concentration, [Ca2+]i, is elevated to micromolar levels or above. Inositol 1,4,5-trisphosphate (InsP3), a second messenger that can evoke Ca2+ release, might also reduce junctional permeability by this mechanism. We report here evidence for transjunctional flux of Ca2+ and InsP3 in freshly isolated pairs or small clusters of rat hepatocytes. The Ca2+ indicator fura-2 was used to monitor transjunctional diffusion of Ca2+ directly or to detect passage of InsP3 by localized Ca2+ release. Fura-2 injected as the free acid passed between cells. Injection of InsP3 or CaCl2 immediately increased [Ca2+]i in the injected cell (peak values less than 1 microM), and [Ca2+]i increased rapidly in contacting cells (within seconds). The initial rise in [Ca2+]i induced by InsP3 was greater at discrete regions in the cytoplasm of both injected and uninjected cells and was inconsistent with simple diffusion of Ca2+. In the coupled cells the regions of greatest increase were not necessarily near the contact zone. In contrast, the rise induced in [Ca2+]i by CaCl2 injection when cells were bathed in normal Ca2+ was always more diffuse than with InsP3 injection, and in cells coupled to a cell injected with CaCl2 the earliest and maximal increases occurred at the region of cell contact. This difference in distribution indicates that injected InsP3 (or an active metabolite, but not Ca2+) diffused between cells to cause localized release of Ca2+ from intracellular stores. Ca2+ injection induced a rise in [Ca2+]i in coupled cells even when cells were maintained in Ca2+-free saline, suggesting that changes in [Ca2+]i seen in adjacent cells were due to transjunctional diffusion from the injected cell and not to uptake from the extracellular solution. However, in Ca2+-free saline, [Ca2+]i distribution was nonuniform, indicating that Ca2+-releasing mechanisms contribute to the observed changes. No increase in [Ca2+]i was seen in adjacent cells when Ca2+ was injected after treatment with the uncoupling agent octanol (500 microM), which itself did not change [Ca2+]i. These data provide evidence that the second messengers Ca2+ and InsP3 can be transmitted from cell to cell through gap junctions, a process that may have an important role in tissue function.

Metabolic regulation during early frog development: flow of glycolytic carbon into phospholipids in Xenopus oocytes and fertilized eggs

32P-labeled glucose 6-phosphate, [32P]phosphoenolpyruvate, and [gamma-32P]ATP were injected into oocytes and fertilized eggs of Xenopus laevis, and the incorporation of the 32P label was followed into phospholipids. Several classes of phospholipids incorporated 32P label from the injected glycolytic intermediates, including lysophosphatidic acid, phosphatidic acid, phosphatidylinositol, and phosphatidylinositol phosphates, inferring de novo synthesis of these lipids from dihydroxyacetone phosphate or glycerol 3-phosphate. Injection of [gamma-32P]ATP into oocytes and fertilized eggs led to labeling of phosphatidylinositol phosphate and phosphatidylinositol bisphosphate, indicating an active phosphatidylinositol cycle in resting oocytes and fertilized eggs. Maturation and fertilization of the oocyte led to a qualitative change in phosphatidylinositol metabolism, increased labeling of phosphatidylinositol phosphate compared to phosphatidylinositol bisphosphate (either from glycerol 3-phosphate or from ATP). This change occurs late in the maturation process, and the new pattern of phosphatidylinositol metabolism is maintained during the rapid cleavage stages of early embryogenesis.

Inositol phospholipid metabolism during and following synaptic activation: role of adenosine

The metabolic pathway of inositol phospholipids represents a series of synthetic and hydrolytic reactions with inositol as a by-product. Hence, the rate of [3H]inositol release from prelabeled phospholipids can be used as a reflection of activity of this pathway. In the frog sympathetic ganglion prelabeled with [3H]inositol, we studied the effect of synaptic activity (orthodromic stimulation) on release of 3H-label into the medium. This release was interpreted as [3H]inositol release. The value was low at rest and increased significantly by 32% during orthodromic stimulation (20 Hz for 5 min). However, on cessation of the stimulation, [3H]inositol release increased rapidly by 148% and remained elevated for at least 45 min. This increase in [3H]inositol release during and after the stimulation period was reduced by suffusion of the ganglia with adenosine. We hypothesized that synaptic activation releases a long-lasting stimulatory agonist and a short-lasting inhibitory (adenosine) agonist or agonists affecting [3H]inositol release. To demonstrate the presence of a stimulatory agonist, two sympathetic ganglia were used. One was prelabeled with [3H]inositol, and the other was not. The two ganglia were placed together in a 5-microliter droplet of Ringer's solution containing atropine. Orthodromic stimuli applied to the nonlabeled ganglion elicited release of [3H]inositol from the nonstimulated ganglion. To test whether the adenosine formed during orthodromic stimulation inhibits [3H]inositol release, we destroyed endogenous adenosine by suffusion of the ganglia with adenosine deaminase during the stimulation period. We found that adenosine deaminase induced large increases in [3H]inositol release during the stimulation period, in contrast to an increase seen only during the poststimulation period when adenosine deaminase was omitted. Because [3H]inositol release is assumed to parallel changes in content of inositol phosphates, we anticipated no changes of the levels of these compounds during orthodromic stimulation. However, measurements showed that levels of inositol phosphates and inositol phospholipids were all elevated except for phosphatidylinositol 4-phosphate. On termination of the stimulus, they remained elevated, with a further increase in levels of inositol trisphosphate and phosphatidylinositol 4-phosphate. We conclude that endogenous adenosine inhibits [3H]inositol release, possibly by modulating several of the steps of the inositol phospholipid pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Formation of [3H]inositol metabolites in rat hippocampal formation slices prelabelled with [3H]inositol and stimulated with carbachol

Rat hippocampal formation slices were prelabelled with [3H]inositol and stimulated with carbachol for times between 7 s and 3 min. The [3H]inositol metabolites in an acid extract of the slices were resolved with anion-exchange HPLC. Carbachol dramatically increased the concentration of [3H]inositol monophosphate, [3H]inositol bisphosphate (two isomers), [3H]inositol 1,3,4-trisphosphate, [3H]inositol 1,4,5-trisphosphate, and [3H]inositol 1,3,4,5-tetrakisphosphate. The levels of [3H]inositol 1,4,5-trisphosphate rose most rapidly; they were maximally elevated after only 7 s and declined toward control levels in 1 min followed by a more sustained elevation in levels for up to 3 min. When [3H]inositol 1,4,5-trisphosphate was incubated with hippocampal formation homogenates in an ATP-containing buffer it was very rapidly metabolised. After 5 min [3H]inositol 1,4-bisphosphate, [3H]inositol 1,3,4-trisphosphate, and [3H]inositol 1,3,4,5-tetrakisphosphate could be detected in the homogenates. Under similar experimental conditions [3H]inositol 1,3,4,5-tetrakisphosphate is metabolised to [3H]inositol 1,3,4-trisphosphate and an inositol bisphosphate isomer that is not [3H]inositol 1,4-bisphosphate. We conclude that like other tissues the primary event in the hippocampus following carbachol stimulation is the activation of phosphatidylinositol 4,5-bisphosphate selective phospholipase C.

Regulation of hepatic parenchymal and non-parenchymal cell function by the diadenine nucleotides Ap3A and Ap4A

The diadenine nucleotides diadenosine 5',5"-P1,P3-triphosphate (Ap3A) and diadenosine 5',5"-P1,P4-tetraphosphate (Ap4A) can be released from platelets and were shown to act as long-lived signal molecules. Accordingly, we studied their potential effect on hepatic metabolism. In isolated perfused rat liver, Ap3A and Ap4A increase the portal pressure, lead to a transient net release of Ca2+, complex net K+ movement across the liver plasma membrane and stimulate hepatic glucose output and 14CO2 production from [1-14C]glutamate. These responses resemble that obtained with extracellular ATP. This and studies on the additivity of ATP and Ap4A effects suggest similar mechanisms mediating the ATP and diadenine nucleotide effects in the liver. Ap3A and Ap4A increased the activity of glycogen phosphorylase a in isolated hepatocyte suspensions by about 100%, pointing to a direct effect of these nucleotides on hepatic parenchymal cells. A response of hepatic non-parenchymal cells to diadenine nucleotide infusion is suggested by a marked stimulation of thromboxane and prostaglandin D2 release from perfused liver. Studies with the thromboxane A2 receptor antagonist BM 13.177 (20 microM) show that the pressure and glucose response to the diadenine nucleotides is partially mediated by this thromboxane formation. Studies with retrograde and sequential liver perfusions suggest a less efficient degradation of the diadenine nucleotides during a single liver passage compared to extracellular ATP. The data suggest that Ap3A and Ap4A are potential regulators of hepatic hemodynamics and metabolism, involving complex interactions between hepatic parenchymal cells and hepatic non-parenchymal cells, including eicosanoids as signal molecules.

Vasopressin transiently stimulates phospholipase C activity in cultured rat hepatocytes

Vasopressin stimulated phospholipase C activity in primary cultures of rat hepatocytes maintained for 18-24 h under serum free conditions. Soluble and membrane-associated phospholipase C activity was determined using exogenous [3H]phosphatidylinositol 4,5-bisphosphate ([3H]PIP2) in the presence of cholate, deoxycholate and NaCl. Exposure of hepatocytes for 5 s to vasopressin (100 nM) stimulated both membrane-associated and soluble phospholipase C activity by 30% and 40%, respectively. However, by 15 s this stimulation had disappeared. Addition of vasopressin to hepatocytes, previously labelled with [3H]inositol, stimulated inositol phosphate production within 5 s, but little further increase was seen over a 5-min incubation. These results indicate that vasopressin rapidly stimulates both soluble and membrane-associated phospholipase C activity.

Subacute and chronic in vivo lithium treatment inhibits agonist- and sodium fluoride-stimulated inositol phosphate production in rat cortex

We have investigated the effects of in vivo lithium treatment on cerebral inositol phospholipid metabolism. Twice-daily treatment of rats with LiCl (3 mEq/kg) for 3 or 16 days resulted in a 25-40% reduction in agonist-stimulated inositol phosphate production, compared with NaCl-treated controls, in cortical slices prelabelled with [3H]inositol. A small effect was also seen with 5-hydroxytryptamine (5-HT) 24 h after a single dose of LiCl (10 mEq/kg). Dose-response curves to carbachol and 5-HT showed that lithium treatment reduced the maximal agonist response without altering the EC50 value. This inhibition was not affected by the concentration of LiCl in the assay buffer. Stimulation of inositol phosphate formation by 10 mM NaF in membranes prepared from cortex of 3-day lithium-treated rats was also inhibited, by 35% compared with NaCl-treated controls. Lithium treatment did not alter the kinetic profile of inositol polyphosphate formation in cortical slices stimulated with carbachol. Muscarinic cholinergic and 5-HT2 bindings were unaltered by lithium, as was cortical phospholipase C activity and isoproterenol-stimulated cyclic AMP formation. [3H]Inositol labelling of phosphatidylinositol 4,5-bisphosphate was significantly enhanced by 3-day lithium treatment. The results, therefore, indicate that subacute or chronic in vivo lithium treatment reduces agonist-stimulated inositol phospholipid metabolism in cerebral cortex; this persistent inhibition appears to be at the level of G-protein-phospholipase C coupling.

Angiotensin II receptors negatively coupled to adenylate cyclase in rat myocardial sarcolemma. Involvement of inhibitory guanine nucleotide regulatory protein

The effect of angiotensin II (AII) on adenylate cyclase was studied in the rat and rabbit heart sarcolemma. AII inhibited adenylate cyclase activity in the rat and rabbit sarcolemma in a concentration-dependent manner. Maximal inhibition of about 35-40% was observed in the rat, with an apparent Ki of about 3 nM; about 30% inhibition, with an apparent Ki of about 6 nM, was noted in rabbit sarcolemma. The inhibitory effect of AII was dependent on the presence of guanine nucleotides and was blocked by saralasin. In addition, AII also inhibited the stimulatory effects of isoproterenol and glucagon on adenylate cyclase. Ninhibin, a sperm factor which has been shown to modify the characteristics of inhibitory guanine nucleotide regulatory protein (Gi), attenuated the inhibitory effects of AII on basal and hormone-sensitive adenylate cyclase. Furthermore, pertussis toxin (PT) treatment of the sarcolemma in the presence of [32P]NAD resulted in ADP-ribosylation of a single 41-kD protein. PT also attenuated the AII-mediated inhibition of basal and hormone-sensitive adenylate cyclase and enhanced the magnitude of the stimulatory effects of isoproterenol and glucagon on adenylate cyclase activity. These data suggest that the rat myocardial sarcolemma contains AII receptors that are negatively coupled to adenylate cyclase through Gi protein.

Stimulation of phosphatidylinositol 4,5-bisphosphate phospholipase C activity by phosphatidic acid

Phosphatidic acid was a potent activator of the phosphatidylinositol 4,5-bisphosphate (PtdIns-P2) phospholipase C activity associated with human platelet membranes. Lysophosphatidic acid was half as active as phosphatidic acid, and shortening the fatty acid chain reduced the effectiveness of the corresponding phosphatidic acid. Compounds lacking either the phosphate group (diacylglycerol or phorbol ester) or the fatty acid (glycerol phosphate) were not activators. When the negative charge was contributed by a carboxyl group (fatty acid or phosphatidylserine), stimulation of phospholipase C was weak but detectable. Structural analogs of phosphatidic acid (lipopolysaccharide, lipid A, and 2,3-diacylglucosamine 1-phosphate) were less effective but also enhanced PtdIns-P2 hydrolysis. Phosphatidic acid potentiated the activation of phospholipase C by alpha-thrombin, chelators, and guanine nucleotides. Phosphatidylinositol 4-phosphate and PtdIns-P2 were also effective activators of PtdIns-P2 degradation. Other phospholipids were without effect. The production of inositol 1,4,5-trisphosphate and diacylglycerol via the activation of phospholipase C provides a rationale for the cellular responses evoked by phosphatidic acid and the ability of this phospholipid to potentiate and initiate hormonal responses.

Enhanced phosphoinositol hydrolysis in response to vasopressin in the septum of the homozygous Brattleboro rat

Arginine8-vasopressin (AVP) receptors in the septum of the Long-Evans rat have been shown to be both pharmacologically (displacement profiles) and functionally (ability to stimulate phosphoinositide hydrolysis) similar to the peripheral V1-type receptor for AVP. Previous binding studies of AVP receptors in the septum of heterozygous (HE) and homozygous (vasopressin-deficient, HO) Brattleboro (BB) rats revealed an increased number of receptors with a lower affinity for AVP in the HO-BB rat when compared to the HE-BB rat. To determine the effect of these receptor changes in the HO-BB rat septum on the postreceptor response of the tissue to AVP, concentration-response relationships for AVP-stimulated phosphoinositide hydrolysis were examined in septal slices from age-matched, adult male HE- and HO-BB rats. AVP-stimulated accumulation of [3H]inositol-1-phosphate (IP1) was significantly greater in the HO-BB (43.7%) than in the HE-BB (13.7%) at AVP concentrations of 10(-08) to 10(-05) M. The two groups did not, however, differ in their ability to stimulate [3H]IP1 accumulation in response to 2.0 mM carbachol. When the AVP-stimulated phosphoinositide response in both genotypes was compared to that obtained for the Long-Evans (LE) rat (the parent strain of the Brattleboro rat) septum under the same assay condition, it was found that the response in the HE-BB was much lower than in the LE. AVP receptor binding capacity (Bmax) correlated (r = 0.975) with release of IP1 ([3H]IP1 accumulation) for all 3 groups studied (LE, HE, HO).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence indicating that the glucagon-induced increase in cytoplasmic free Ca2+ concentration in hepatocytes is mediated by an increase in cyclic AMP concentration

The mechanism whereby glucagon causes an increase in the concentration of cytoplasmic free Ca2+, [Ca2+]c, in isolated hepatocytes has been investigated. There have been proposals of cyclic-AMP-dependent and cyclic-AMP-independent mechanisms. In this work, the inactivation of pyruvate kinase was used as an indicator of increases in the activity of cyclic-AMP-dependent protein kinase, A-kinase. [Ca2+]c was measured using the fluorescent probe indo-1. The decrease in activity of pyruvate kinase caused by an increase in [Ca2+]c alone, i.e. mediated by mechanisms not involving cyclic AMP and exemplified by the effect of vasopressin, was of minimal significance under the conditions of the enzyme assay. Studies of the effects of a wide range of glucagon concentrations indicate that any increase in [Ca2+]c caused by glucagon was always associated with a decrease in pyruvate kinase activity. A similar relationship was obtained if glucagon-receptor occupancy was circumvented by using the 8-bromo-derivative of cyclic AMP to activate the A-kinase. It was also found that the cyclic AMP phosphodiesterase inhibitor isobutylmethylxanthine could potentiate the ability of glucagon to increase [Ca2+]c: no such potentiation was observed when vasopressin was used to raise [Ca2+]c. Together these data indicate that an increase in cyclic AMP concentration, sufficiently great to activate A-kinase, is a mechanism that mediates the glucagon-induced increase in [Ca2+]c.

Inhibition of PI-kinase in rat liver membranes by F-

In a number of membrane preparations GTP or its non-hydrolysable analogues stimulate the breakdown of PIP2 generating the second messengers, inositol triphosphate and diacylglycerol. The G-protein which couples the PIP2-specific phospholipase C with the receptors can also be activated by F-. However, the level of PIP2 is dependent upon the activity of a number of enzymes in the PI-pathway. Besides stimulating the breakdown of PIP2, we report that in rat liver membranes F- also decreases the labelling of the polyphosphoinositides through inhibition of the PI-kinase.

Activity and regulation of calcium-, phospholipid-dependent protein kinase in differentiating chick myogenic cells

The activity of calcium-, phospholipid-dependent protein kinase (PKc) was measured in (a) total extracts, (b) crude membrane, and (c) cytosolic fractions of chick embryo myogenic cells differentiating in culture. Total PKc activity slowly declines during the course of terminal myogenesis in contrast to the activity of cAMP-dependent protein kinase, which was also measured in the same cells. Myogenic cells at day 1 of culture possess high particulate and low soluble PKc activity. A dramatic decline of particulate PKc activity occurs during myogenic cell differentiation and is accompanied, through day 4, by a striking rise of the soluble activity. The difference in the subcellular distribution of PKc between replicating myoblasts and myotubes is confirmed by phosphorylation studies conducted in intact cells. These studies demonstrate that four polypeptides whose phosphorylation is stimulated by the tumor promoter 12-O-tetradecanoyl phorbol 13-acetate in myotubes, are spontaneously phosphorylated in control myoblasts. Phosphoinositide turnover under basal conditions in [3H]inositol-labeled cells is faster in myoblasts than in myotubes, a finding that may in part explain the different distribution of PKc observed during the course of myogenic differentiation.

Regulatory role of retinoic acid on cultured mouse keratinocyte inositol phospholipid metabolism: dose-dependent release of inositol triphosphate

The incorporation of precursor 14C-myoinositol into the three cellular inositol phospholipids (PtdIns, PtdInsP, and PtdInsP2) of cultured, rapidly proliferating keratinocytes is significantly enhanced by the exogenous addition of a high concentration (1 X 10(-7) M) of all-trans retinoic acid or its analog 13-Cis analog, whereas a similar incubation with a low concentration (1 X 10(-10) M) of the same retinoid resulted in an insignificant incorporation of the radio-precursor into the three inositol phospholipids. Incorporation was most marked into the more phosphorylated PtdIns4P and PtdIns4,5P2. These results indicate that retinoic acid affects the biosynthesis of the inositol phospholipids at high concentrations. In contrast, the hydrolysis of 14C-PtdIns4,5P2 and release of 14C-InsP3 from the prelabeled keratinocytes is markedly enhanced by a low physiologic concentration (1 X 10(-10) M) of retinoic acid or its 13-Cis analog. The hydrolysis is rapid, with an accompanying elevated transient release of 14C-InsP3. High concentration (1 X 10(-5) M), on the other hand, supresses 14C-InsP3 release. These results taken together underscore a bifunctional, dose-dependent effect of both the all-trans-RA and its 13-Cis analog on the synthesis and hydrolysis of keratinocyte PtdIns4,5P2. Furthermore, the results suggest that at low physiologic concentrations, these retinoids may function as agonists to perturb the membrane resulting in induced rapid hydrolysis of cellular PtdIns4,5P2, which is coupled to a "transient" generation of InsP3 (an intracellular second messenger). The rapid formation of this putative "second messenger" may in turn play a role in the cellular proliferative or differentiating biochemical events in the murine keratinocytes.

Stimulation of hepatic inositol 1,4,5-trisphosphate kinase activity by Ca2+-dependent and -independent mechanisms

A cytosolic fraction derived from rat hepatocytes was used to investigate the regulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] kinase, the enzyme which converts Ins(1,4,5)P3 to inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. The activity was doubled by raising the free Ca2+ concentration of the assay medium from 0.1 microM to 1.0 microM. A 5 min preincubation of the hepatocytes with 100 microM-dibutyryl cyclic AMP (db.cAMP) plus 100 nM-tetradecanoylphorbol acetate (TPA) resulted in a 40% increase in Ins(1,4,5)P3 kinase activity when subsequently assayed at 0.1 microM-Ca2+. This effect was smaller at [Ca2+] greater than 0.5 microM, and absent at 1.0 microM-Ca2+. Similar results were obtained after preincubation with 100 microM-db.cAMP plus 300 nM-vasopressin (20% increase at 0.1 microM-Ca2+; no effect at 1.0 microM-Ca2+). Preincubation with vasopressin, db.cAMP or TPA alone did not alter Ins(1,4,5)P3 kinase activity. It is proposed that these results, together with recent evidence implicating Ins(1,3,4,5)P4 in the control of Ca2+ influx, could be relevant to earlier findings that hepatic Ca2+ uptake is synergistically stimulated by cyclic AMP analogues and vasopressin.

Activation of the phosphatidylinositol metabolic pathway by low molecular weight B cell growth factor

The possible role of phosphatidylinositol breakdown in the induction of proliferation of human activated B cells by low molecular weight B cell growth factor (LMW-BCGF) was examined. LMW-BCGF was found to induce a rapid rise in the concentration of inositol trisphosphate (InsP3) in [3H]inositol-loaded B cell blasts, obtained by prior anti-mu antibody activation. A concomitant decrease in the concentration of phosphatidylinositol 4,5-bisphosphate could be detected at the same time. Maximum generation of InsP3 occurred within 15-30 s after the addition of the LMW-BCGF ligand to the activated B cells, then was followed by a slow decrease and return to control values. The amount of InsP3 generated by phosphatidylinositol hydrolysis was dependent on the concentration of LMW-BCGF. This effect was only detected in B cells already preactivated by a first signal such as anti-mu antibody and not in resting unstimulated B cells. In contrast, under similar conditions, interleukin 2, another B cell growth-promoting lymphokine, did not alter the rate of formation of the various phosphatidylinositol breakdown products. An augmentation of the [Ca2+]i concentration was also detected in activated B cells upon addition of LMW-BCGF and this increase could be blocked by TMB-8, a specific inhibitor of endoplasmic reticulum calcium release. Hydrolysis of phosphoinositides thus represents an essential component in the mechanism of transduction of the signal provided by LMW-BCGF.

Inositol trisphosphate, inositol phospholipid metabolism, and germinal vesicle breakdown in surf clam oocytes

Others have reported that microinjection of inositol 1,4,5-trisphosphate (InsP3) releases stored intracellular Ca2+ and causes fertilization envelope elevation, part of the activation process normally initiated by fertilization in deuterostome eggs. In the protostome, Spisula solidissima, germinal vesicle breakdown (GVBD) is the first visible response of the egg to fertilization. To test the effects of InsP3 on egg activation in this organism, we microinjected the compound into oocytes. Microinjection of 0.4-7.0 x 10(-21) moles of InsP3 (equivalent to 5-80 pM if distributed throughout the cell) elicited GVBD in a dose-dependent manner, demonstrating that increased oocyte InsP3 can mimic part of the activation process in this protostome. Synthesis of InsP3 occurs in vivo when phosphatidylinositol 4,5-bisphosphate (PtdInsP2) is hydrolyzed by phospholipase C. To determine whether stimulus-induced synthesis of InsP3 occurs after fertilization of Spisula oocytes, we labeled oocyte lipids with [32P]orthophosphate and measured the radioactivity in phospholipids after insemination. Fertilization resulted in a rapid, transient loss of radioactivity from PtdInsP2. Because the radioactivity in phosphatidylinositol 4-phosphate and other phospholipids did not change, the loss of radioactivity from PtdInsP2 is most likely due to its hydrolysis, yielding InsP3 and diacylglycerol. The latter compound activates protein kinase C which has also been shown to be involved in regulating Spisula oocyte GVBD. Since both of these compounds appear to be early products of fertilization, they could coordinately activate Ca2+- and protein kinase C-dependent processes involved in Spisula oocyte GVBD. These data indicate that egg activation in this protostome includes pathways similar to those found in deuterostome eggs and in other eukaryotic cells.

Vasopressin stimulates phosphoinositide hydrolysis in LLC-PK1 cells

LLC-PK1 cells have been shown to possess vasopressin (VP) receptors (V2 type) that are coupled to adenyl cyclase to generate adenosine 3,5'-cyclic monophosphate (cAMP). To determine whether VP also stimulates phosphoinositide (PI) hydrolysis to generate inositol phosphate (IP) and diacylglycerol (DAG) messenger system in LLC-PK1 cells, we measured the release of IP in LLC-PK1 cells in the absence and presence of various concentrations of VP. In addition, we also determined the effect of an increase in osmolality of the incubation medium on VP-stimulated PI hydrolysis in LLC-PK1 cells. The methods involved the incubation of LLC-PK1 cells with [3H]inositol for its incorporation into membrane PI and the measurement of the release of [3H]IP in the presence of LiCl which prevents dephosphorylation. The osmolality of the incubation media was increased from 300 to 600, 900, and 1,200 mosmol/kgH2O by the addition of NaCl and urea. In an isosmotic incubation medium, VP (10(-8) M) produced a 100% increase in PI hydrolysis in LLC-PK1 cells. The effect was much greater at higher concentrations of the hormone. There was no effect of osmolality in VP-stimulated PI hydrolysis in LLC-PK1 cells up to 600 mosmol/kgH2O, but PI hydrolysis decreased significantly when the osmolality of the incubation medium was increased to 900 or 1,200 mosmol/kgH2O. Our results suggest that in LLC-PK1 cells, VP stimulates PI hydrolysis probably through VP receptors that are coupled to phospholipase C. Furthermore, VP-stimulated PI messenger system in LLC-PK1 cells is influenced by osmolality of the extracellular fluid.

Effect of vasopressin on the regulation of protein synthesis initiation in liver cells

Vasopressin was found to be an effective inhibitor of protein labelling in isolated liver cells. Its effect shows the following distinct characteristics: (1) in contrast with alpha-adrenergic agonists, its effect is observable under a wide range of cellular Ca2+-loading conditions; (2) it is not influenced by the nutritional state of the animal. The lack of vasopressin effect on valine production, and its ability to decrease protein labelling from near-saturation concentrations of [3H]valine, indicate that the observed variations in protein labelling reflect actual changes in the rate of protein synthesis. The action of vasopressin is primarily exerted on the initiation step of protein synthesis and this effect is accompanied by a decreased activity of eukaryotic initiation factor 2. Activators of protein kinase C showed similar but not additive effects on protein synthesis, as did vasopressin. It seems plausible to conclude that protein kinase C activation may play an important regulatory role in hepatic protein synthesis as a transducer of hormonal and perhaps other type of signals.

Cholera-toxin and corticotropin modulation of inositol phosphate accumulation induced by vasopressin and angiotensin II in rat glomerulosa cells

Vasopressin (VP) and angiotensin II (AT II) stimulate the production of inositol phosphates (IP) in rat glomerulosa cells. Guanosine 5'-[gamma-thio]triphosphate (GTP[S]), but not VP or AT II, stimulates IP production in a myo-[3H]inositol-prelabelled glomerulosa-cell membrane preparation. In combination with GTP[S], these hormones potentiate the response to GTP[S], indicating the existence of a G-protein involved in the coupling of the VP and AT II receptor with the phospholipase C. ADP-ribosylation with pertussis toxin (IAP) revealed the specific labelling of a single molecule of 41 kDa. No significant inhibition of VP- or AT II-stimulated IP accumulation was detected in intact cells when the whole 41 kDa molecule was endogenously ADP-ribosylated by IAP treatment. On the contrary, when glomerulosa cells were infected with cholera toxin (CT), both the VP- and AT II-stimulated IP accumulations were inhibited in a dose-dependent manner. Yet these effects were partial even at high concentrations of CT, and could not be related to the ADP-ribosylation of 'alpha s' molecules. Similarly, when the cells were infected with 1 microgram of CT/ml, the specific binding of VP and AT II decreased by 50-60%. Such results may signify that the treatment primarily affects the densities of the hormone receptors. When glomerulosa cells were incubated for 15 h in the presence of 10 nM-corticotropin (ACTH), a condition in which the intracellular concentration of cyclic AMP was increased 3-fold, the maximum IP response to 0.1 microM-VP or -AT II was decreased by 50%. When similar experiments were carried out only after a 15 min incubation period with the same concentration of ACTH, the increase in cyclic AMP was more pronounced, but no inhibition of hormone-induced IP accumulation was observed. Altogether, these results may suggest that CT exerts its action on the VP- or AT II-sensitive phospholipase C systems via a prolonged increase in intracellular cyclic AMP.

On the source of the vasopressin-induced increases in diacylglycerol in hepatocytes

In hepatocytes pre-labelled with [3H]glycerol, vasopressin increased by 20% the amount of radioactivity present in diacylglycerols. The effect of vasopressin was partially dependent on Ca2+. The magnitude of the increase in [3H]diacylglycerol was 5-times the sum of the radioactivity present in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. No stimulation by vasopressin of the initial rate of incorporation of radioactivity into diacylglycerols was observed in cells incubated in the presence of 10 mM [3H]glycerol. Treatment of hepatocytes labelled with either [3H]ethanolamine or [3H]choline with vasopressin, ionophore A23187 or phospholipase C increased the amount of radioactivity present in trichloroacetic acid extracts of the cells. The effect of vasopressin was dependent on extracellular Ca2+. It is concluded that in hepatocytes vasopressin increases diacylglycerols by a process which does not principally involve the conversion of phosphoinositides to diacylglycerol or the de novo synthesis of diacylglycerol from glycerol 3-phosphate, but does involve the Ca2+-dependent conversion of phosphatidylethanolamine and phosphatidylcholine to diacylglycerol.

Norepinephrine modulates the growth-inhibitory effect of transforming growth factor-beta in primary rat hepatocyte cultures

TGF-beta is a potent inhibitor of EGF-induced DNA synthesis in primary rat hepatocyte cultures. Norepinephrine (NE) was shown to modulate this inhibition of DNA synthesis. It produced a five-fold increase, from 2.8 pM to 14.4 pM, in the ID50 for TGF-beta. The effect was dose-dependent and was significant at concentrations of 10(-6)M NE and greater. The modulation by NE was mediated by the alpha 1-adrenergic receptor as shown by the ability of the alpha 1 antagonist prazosin to block the activity. This effect might be important during liver regeneration in allowing escape of hepatocytes from negative growth control exerted by TGF-beta.