The organ-specific differential roles of rice DXS and DXR, the first two enzymes of the MEP pathway, in carotenoid metabolism in Oryza sativa leaves and seeds

BACKGROUND

Deoxyxylulose 5-phosphate synthase (DXS) and deoxyxylulose 5-phosphate reductoisomerase (DXR) are the enzymes that catalyze the first two enzyme steps of the methylerythritol 4-phosphate (MEP) pathway to supply the isoprene building-blocks of carotenoids. Plant DXR and DXS enzymes have been reported to function differently depending on the plant species. In this study, the differential roles of rice DXS and DXR genes in carotenoid metabolism were investigated.

RESULTS

The accumulation of carotenoids in rice seeds co-expressing OsDXS2 and stPAC was largely enhanced by 3.4-fold relative to the stPAC seeds and 315.3-fold relative to non-transgenic (NT) seeds, while the overexpression of each OsDXS2 or OsDXR caused no positive effect on the accumulation of either carotenoids or chlorophylls in leaves and seeds, suggesting that OsDXS2 functions as a rate-limiting enzyme supplying IPP/DMAPPs to seed carotenoid metabolism, but OsDXR doesn't in either leaves or seeds. The expressions of OsDXS1, OsPSY1, OsPSY2, and OsBCH2 genes were upregulated regardless of the reductions of chlorophylls and carotenoids in leaves; however, there was no significant change in the expression of most carotenogenic genes, even though there was a 315.3-fold increase in the amount of carotenoid in rice seeds. These non-proportional expression patterns in leaves and seeds suggest that those metabolic changes of carotenoids were associated with overexpression of the OsDXS2, OsDXR and stPAC transgenes, and the capacities of the intermediate biosynthetic enzymes might be much more important for those metabolic alterations than the transcript levels of intermediate biosynthetic genes are. Taken together, we propose a 'Three Faucets and Cisterns Model' about the relationship among the rate-limiting enzymes OsDXSs, OsPSYs, and OsBCHs as a "Faucet", the biosynthetic capacity of intermediate metabolites as a "Cistern", and the carotenoid accumulations as the content of "Cistern".

CONCLUSION

Our study suggests that OsDXS2 plays an important role as a rate-limiting enzyme supplying IPP/DMAPPs to the seed-carotenoid accumulation, and rice seed carotenoid metabolism could be largely enhanced without any significant transcriptional alteration of carotenogenic genes. Finally, the "Three Faucets and Cisterns model" presents the extenuating circumstance to elucidate rice seed carotenoid metabolism.

The signal metabolite trehalose-6-phosphate inhibits the sucrolytic activity of sucrose synthase from developing castor beans

In plants, trehalose 6-phosphate (T6P) is a key signaling metabolite that functions as both a signal and negative feedback regulator of sucrose levels. The mode of action by which T6P senses and regulates sucrose is not fully understood. Here, we demonstrate that the sucrolytic activity of RcSUS1, the dominant sucrose synthase isozyme expressed in developing castor beans, is allosterically inhibited by T6P. The feedback inhibition of SUS by T6P may contribute to the control of sink strength and sucrolytic flux in heterotrophic plant tissues.

The Role of Trehalose 6-Phosphate in Crop Yield and Resilience

T6P can be targeted through genetic and chemical methods for crop yield improvements in different environments through the effect of T6P on carbon allocation and biosynthetic pathways.

Trehalose biosynthesis in response to abiotic stresses

Trehalose is a non-reducing disaccharide that is present in diverse organisms ranging from bacteria and fungi to invertebrates, in which it serves as an energy source, osmolyte or protein/membrane protectant. The occurrence of trehalose and trehalose biosynthesis pathway in plants has been discovered recently. Multiple studies have revealed regulatory roles of trehalose-6-phosphate, a precursor of trehalose, in sugar metabolism, growth and development in plants. Trehalose levels are generally quite low in plants but may alter in response to environmental stresses. Transgenic plants overexpressing microbial trehalose biosynthesis genes have been shown to contain increased levels of trehalose and display drought, salt and cold tolerance. In-silico expression profiling of all Arabidopsis trehalose-6-phosphate synthases (TPSs) and trehalose-6-phosphate phosphatases (TPPs) revealed that certain classes of TPS and TPP genes are differentially regulated in response to a variety of abiotic stresses. These studies point to the importance of trehalose biosynthesis in stress responses.

Trehalose 6-phosphate

Trehalose 6-phosphate (T6P) is a sugar signal of emerging significance. It is an essential component of the mechanisms that coordinate metabolism with plant growth adaptation and development. Its significance began to dawn when genetic modification of the trehalose pathway produced dramatic phenotypes, before the genetic proliferation of the trehalose pathway in plants was fully realised. T6P regulates sugar utilization and starch metabolism and interacts with other signalling pathways, including those mediated by plant hormones. Trehalose phosphate synthases (TPSs) and trehalose phosphate phosphatases are regulated at the gene level by sugars, nitrate, cytokinin and abscisic acid. TPSs are also regulated post-translationally. Mechanistic details of how T6P signals are emerging, but still sparse. Nevertheless, even at this stage, targeting central regulators such as T6P offers promise in crop improvement.

The MEP Pathway: A New Target for the Development of Herbicides, Antibiotics and Antimalarial Drugs

Isoprenoids, a diverse group of compounds derived from the five-carbon building units isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP), are essential for survival in all organisms. Animals synthesize their isoprenoids from mevalonic acid (MVA), whereas most pathogenic bacteria and the malaria parasites utilize a completely different pathway for IPP and DMAPP synthesis, the methylerythritol phosphate (MEP) pathway. Plants use both pathways for the synthesis of isoprenoid precursors. The recent elucidation of the MEP pathway has opened the possibility to develop new strategies against microbial pathogens. Novel immunotherapeutic agents can be developed based on the MEP pathway intermediates known to activate the proliferation of human V-delta-9 V-gamma-2 T-cells after infection by many pathogenic bacteria and protozoa. Moreover, the design of specific inhibitors of MEP pathway enzymes (which are highly conserved but show no homology to mammalian proteins) should result in herbicides and drugs with broad-spectrum antimicrobial activity without mechanism-based toxicity to humans. A good example is the cure of bacterial infections and malaria with fosmidomycin, a highly stable inhibitor of the MEP pathway. The use of plants as test systems has led to the identification of additional inhibitors such as ketoclomazone. Biochemical, genetic and crystallographic approaches with the MEP pathway enzymes are now starting to characterize the inhibition kinetics and identify which residues play a structural or catalytic role. Current efforts should eventually contribute to an effective drug designed to fight against microbial pathogens that show resistance to currently available agents.

Cutting edge: human gamma delta T cells are activated by intermediates of the 2-C-methyl-D-erythritol 4-phosphate pathway of isoprenoid biosynthesis

Activation of V gamma 9/V delta 2 T cells by small nonprotein Ags is frequently observed after infection with various viruses, bacteria, and eukaryotic parasites. We suggested earlier that compounds synthesized by the 2-C:-methyl-D-erythritol 4-phosphate (MEP) pathway of isopentenyl pyrophosphate synthesis are responsible for the V gamma 9/V delta 2 T cell reactivity of many pathogens. Using genetically engineered Escherichia coli knockout strains, we now demonstrate that the ability of E. coli extracts to stimulate gamma delta T cell proliferation is abrogated when genes coding for essential enzymes of the MEP pathway, dxr or gcpE, are disrupted or deleted from the bacterial genome.

Phosphorylation of recombinant human ATP:citrate lyase by cAMP-dependent protein kinase abolishes homotropic allosteric regulation of the enzyme by citrate and increases the enzyme activity. Allosteric activation of ATP:citrate lyase by phosphorylated sugars

Recombinantly expressed human ATP:citrate lyase was purified from E. coli, and its kinetic behavior was characterized before and after phosphorylation. Cyclic AMP-dependent protein kinase catalyzed the incorporation of only 1 mol of phosphate per mole of enzyme homotetramer, and glycogen synthase kinase-3 incorporated an additional 2 mol of phosphate into the phosphorylated protein. Isoelectric focusing revealed that all of the phosphates were incorporated into only one of the four enzyme subunits. Phosphorylation resulted in a 6-fold increase in V(max) and the conversion of citrate dependence from sigmoidal, displaying negative cooperativity, to hyperbolic. The phosphorylated recombinant enzyme is more similar to the enzyme isolated from mammalian tissues than unphosphorylated enzyme with respect to the K(m) for citrate, CoA, and ATP, and the specific activity. Fructose 6-phosphate was found to be a potent activator (60-fold) of the unphosphorylated recombinant enzyme, with half-maximal activation at 0.16 mM, which results in a decrease in the apparent K(m) for citrate and ATP, as well as an increase in the V(max) of the reaction. Thus, human ATP:citrate lyase activity is regulated in vitro allosterically by phosphorylated sugars as well as covalently by phosphorylation.

Cloning and biochemical characterisation of Aspergillus niger hexokinase--the enzyme is strongly inhibited by physiological concentrations of trehalose 6-phosphate

The Aspergillus niger hexokinase gene hxkA has been cloned by heterologous hybridisation using the Aspergillus nidulans hexokinase gene as a probe. The DNA sequence of the gene was determined, and the deduced amino acid sequence showed significant similarity to other eukaryotic hexokinase and glucokinase proteins, in particular to those of the budding yeasts. The encoded protein was purified from a multicopy hxkA transformant, and extensively characterised. The hexokinase protein has a molecular mass of 54090, a pI of 4.9 and is a homodimer. D-Glucose, the glucose analogue 2-deoxy-D-glucose, D-fructose, D-mannose and D-glucosamine are phosphorylated by hexokinase, whereas the hexoses D-galactose, L-sorbose, methyl alpha-D-glucoside and the pentoses L-arabinose and D-xylose are not. The enzyme has high affinity for glucose (Km = 0.35 mM at pH 7.5) and for fructose (Km = 2.0 mM at pH 7.5) and is inhibited by ADP. The enzyme is strongly inhibited by physiological concentrations (0.1-0.2 mM) of trehalose 6-phosphate, which may be of importance for in vivo regulation of the enzyme. Inhibition of A. niger hexokinase by trehalose 6-phosphate is competitive towards the sugar substrate (Ki = 0.01 mM). Based on the kinetic constants of hexokinase and glucokinase their relative contribution to in vivo glucose phosphorylation was calculated and found to be strongly dependent on intracellular pH and glucose concentration. At pH 7.5 glucokinase is predominant, whereas at pH 6.5 hexokinase is predominant at glucose concentrations higher than 0.5 mM. Expression of the hexokinase and the glucokinase gene requires active carbon metabolism. Also on carbon sources which are not substrates for hexokinase or glucokinase, clear expression is observed. The hexokinase and glucokinase enzymes are quite stable in vivo. Even in the absence of transcription, active glucokinase and hexokinase remain present in the cells at almost the same level for at least 3-4 h after depletion of the carbon source.

Opine catabolism and conjugal transfer of the nopaline Ti plasmid pTiC58 are coordinately regulated by a single repressor

The Ti plasmids of Agrobacterium tumefaciens are conjugal elements whose transfer is strongly repressed. Transfer is induced by the conjugal opines, a group of unique carbon compounds synthesized in crown gall tumors. The opines also induce Ti plasmid-encoded genes required by the bacteria for opine catabolism. We have cloned and sequenced a gene from the Ti plasmid pTiC58, whose product mediates the opine-dependent regulation of conjugal transfer and catabolism of the conjugal opines, agrocinopines A and B. The gene, accR, is closely linked to the agrocinopine catabolic locus. A spontaneous mutant Ti plasmid, pTiC58Trac, which constitutively expresses conjugal transfer and opine catabolism, was complemented in trans by a clone of wild-type accR. Comparative sequence analysis identified a 5-base-pair deletion close to the 5' end of the mutant accR allele from pTiC58Trac. Analysis of lacZ fusions in conjugal transfer and opine catabolic structural genes demonstrated that the accR-encoded function is a transcriptional repressor. accR can encode a 28-kDa protein. This protein is related to a class of repressor proteins that includes LacR, GutR, DeoR, FucR, and GlpR that regulate sugar catabolic systems in several bacterial genera.

Suppression of focus formation by bovine papillomavirus-transformed cells by contact with non-transformed cells: involvement of sugar(s) and phosphorylation

Focus formation by bovine papilloma virus-transformed C127 cells was inhibited by direct contact with non-transformed C127 cells. The suppressive capacity of C127 cells was abolished by introduction of the neomycin resistance gene (neor) but not by that of the hygromycin resistance gene (hygrr). Though both genes code for phosphotransferase which inactivates the aminoglycoside antibiotics, their substrates are different, i.e., there is no cross-resistance between them. As the neomycin phosphotransferase phosphorylates the specific hydroxyl group of the sugar in the aminoglycosides, such as 3'OH of the glucose residue of kanamycin A, some specific sugar(s) on the molecules exposed on the cell surface must be responsible for the suppressive signal and their phosphorylation must have resulted in the loss of that signal. The sugar must have the structure shared by kanamycin, neomycin or G418 but not by hygromycin B. Involvement of sugar was also suggested by the observation that concanavalin A partially abrogated the suppressive capacity of C127 cells.

The Albert Lasker Medical Awards. Inositol trisphosphate, calcium, lithium, and cell signaling

Inositol lipids play a major role in cell signaling by functioning as precursors of second messengers. Of the three common inositol-containing lipids found in the plasma membrane, phosphatidylinositol (4,5)-bisphosphate is hydrolyzed to give diacylglycerol, which stimulates protein kinase C, and inositol 1,4,5-trisphosphate, which diffuses into the cell to release intracellular calcium. Inositol 1,4,5-trisphosphate is metabolized to give free inositol by two separate pathways. Lithium inhibits the final dephosphorylation step of both pathways, thus reducing the supply of the free inositol required to maintain the lipid precursors used for signaling. An inositol-depletion hypothesis may explain both the teratogenic effects of lithium and its therapeutic action in controlling manic-depressive illness. One of the metabolic pathways generates inositol tetrakisphosphate, which may also play a messenger role by expanding the size of the inositol 1,4,5-trisphosphate-sensitive pool of calcium. Calcium imaging of single cells has begun to reveal that this inositol 1,4,5-trisphosphate/calcium signaling system is organized in complex patterns, which include localization of calcium signals to discrete regions of cells and the generation of both calcium waves and calcium oscillations.

Inositol phosphates and cell signalling

Inositol 1,4,5-trisphosphate is a second messenger which regulates intracellular calcium both by mobilizing calcium from internal stores and, perhaps indirectly, by stimulating calcium entry. In these actions it may function with its phosphorylated metabolite, inositol 1,3,4,5-tetrakisphosphate. The subtlety of calcium regulation by inositol phosphates is emphasized by recent studies that have revealed oscillations in calcium concentration which are perhaps part of a frequency-encoded second-messenger system.

Diacylglycerol, but not inositol 1,4,5-trisphosphate, accounts for platelet-derived growth factor-stimulated proliferation of BALB 3T3 cells

Recently we found that an intracellular event related to phosphatidylinositol 4,5-bisphosphate (PIP2) is crucial for platelet-derived growth factor (PDGF)-induced mitogenesis in fibroblastic cells (Matuoka, K., et al.: Science 239:640-643, 1988). In the present study we examined the mitogenic effects of PIP2 and its hydrolysis products introduced into the cytoplasm of BALB 3T3 cells by micro-injection to confirm the role of PIP2 hydrolysis in PDGF stimulation of cell proliferation. Injection of 1,2-dioleylglycerol (diolein) into serum-deprived quiescent cells induced DNA synthesis with the same time course as that induced by exposure of the cells to PDGF and, in the presence of PDGF, caused no additional increase in the cell population entering S phase. The injection of PIP2, inositol 1,4,5-trisphosphate, or 1,2-dioleylphosphatidic acid into the cells did not induce mitogenesis. Consistent results were obtained in experiments in which the cells were exposed to 1-oleyl-2-acetylglycerol (OAG) and ionomycin; namely, OAG stimulated proliferation of BALB 3T3 cells, but ionomycin did not induce any mitogenesis. Desensitization of the protein kinase C pathway by prolonged exposure of the cells to phorbol ester abolished the induction of cell proliferation by subsequent injection of diolein or exposure to phorbol ester or OAG as well as by PDGF challenge. These findings strongly suggest that activation of the protein kinase C system following formation of diacylglycerol by PIP2 hydrolysis is mainly responsible for the mitogenic action of PDGF on BALB 3T3 cells.

Dissociation of alpha-adrenergic and cholinergic stimulated inositol trisphosphate-dependent calcium mobilization at the "post-receptor" level

Carbachol stimulated inositol trisphosphate (IP3) production and subsequent calcium mobilization in parotid cells are almost completely inhibited by neomycin. In contrast epinephrine stimulated IP3 production and calcium mobilization are much less sensitive to such inhibition. Since neomycin exerts its effects primarily at the level of inositol phosphate production and action, cholinergic and alpha adrenergic stimulation of IP3 dependent calcium mobilization may proceed through different "post-receptor" signal transduction mechanisms in parotid cells.

[Secondary messenger level of inositol triphosphate in fibroblasts after treatment with papaverine]

Ten per cent foetal bovine serum had been added to human embryonic fibroblasts which had previously been incubated in serum free cultures. The effect has been studied on quantity of synthetized DNA in next 30 hours and on inositol-1,4,5-phosphate contents of cells. On the effect of foetal bovine serum synthesis of DNA has increased four times and 1 minute after its addition inositol-1,4,5-phosphate level has doubled. Papaverine given together with FBS has suspended inductive effect of FBS on DNA synthesis and its increasing effect on level of inositol-1,4,5-phosphate.

Inositolphospholipid-linked glutamate receptors mediate cerebellar parallel-fiber-Purkinje-cell synaptic transmission

In slices of adult rat cerebellum inositolphospholipid turnover is stimulated markedly by glutamate and its rigid analogues quisqualate and ibotenate. The drug and amino acid specificity of the response reflects a quisqualate-preferring excitatory amino acid receptor. The absence of glutamate-enhanced inositolphospholipid turnover in mice with Purkinje-cell degeneration indicates that the inositolphospholipid-linked quisqualate receptor mediates parallel fiber-Purkinje cell synaptic transmission. The quantitative prominence of this synapse accounts for the massive enrichment of elements of the inositolphospholipid system in cerebellar Purkinje cells.

Pulsatile intracellular calcium release does not depend on fluctuations in inositol trisphosphate concentration

Many hormones, neurotransmitters and growth factors evoke in their target cells oscillations in the free internal Ca2+ concentration [( Ca2+]i). In electrically non-excitable cells these fluctuations are due to periodic release of Ca2+ from intracellular reservoirs, stimulated by the internal messenger inositol trisphosphate (InsP3). Most models at present invoke fluctuating levels of InsP3 as a key component in generating the oscillations in [Ca2+]i. InsP3 injected into intact cells evokes irregular and transient oscillatory Ca2+-dependent current responses, but the intracellular InsP3 concentration is not constant in such experiments. Here we monitor changes in [Ca2+]i by measuring Ca2+-activated Cl- current in single internally perfused mouse pancreatic acinar cells and show that acetylcholine (ACh), acting through muscarinic receptors, evokes regular and repetitive current pulses which are mimicked by InsP3 applied through a patch pipette. To exclude the possibility that InsP3 is periodically phosphorylated or degraded, we replaced it by the non-metabolizable InsP3 analogue inositol trisphosphorothioate (InsPS3), which also evokes regular pulses of Ca2+-activated Cl- current. These effects are independent of external Ca2+, but abolished by high intracellular concentrations of a Ca2+-chelator. We conclude that repetitive pulses of intracellular Ca2+ release occur even when the concentration of InsP3 is constant.

Chemoattractant receptor promotion of Ca2+ influx across the plasma membrane of HL-60 cells. A role for cytosolic free calcium elevations and inositol 1,3,4,5-tetrakisphosphate production

The mechanisms by which the chemotactic peptide formyl-methyl-leucyl-phenyl-alanine stimulates Ca2+ influx across the plasma membrane were investigated in the human promyelocytic cell line HL-60, induced to differentiate with dimethyl sulfoxide. Ca2+ influx was determined: (a) from the initial rate of Mn2+ influx, apparent from the quenching of intracellular quin2 or fura-2 fluorescence; (b) from the rate of the elevation of cytosolic free calcium, [Ca2+]i, upon readdition of Ca2+ to cells previously stimulated in the absence of extracellular Ca2+. [3H]Inositol tris-, tetrakis-, and pentakisphosphates were analyzed by a high performance liquid chromatography procedure which was optimized for the separation of inositol tetrakisphosphates, yielding three predominant isomers: inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), inositol 1,4,5,6-tetrakisphosphate, and inositol 1,3,4, 6-tetrakisphosphate. Both the kinetics and agonist dose dependence of Ca2+ influx stimulation correlated closely with the corresponding receptor-mediated variations of [Ca2+]i either in the presence or in the absence of extracellular Ca2+. Of the different inositol phosphates determined in parallel and under the same conditions, accumulation of [3H]Ins(1,3,4,5)P4 correlated best with Ca2+ influx both temporally and in its dose dependence in the presence or in the absence of extracellular Ca2+; inositol 1,3,4-trisphosphate was also correlated but to a lesser extent. Attenuations of [Ca2+]i elevations by decreasing extracellular Ca2+ or by increasing the cytosolic Ca2+ buffering capacity with quin2 led to parallel inhibition of Ca2+ influx and Ins(1,3,4,5)P4 production.

IN CONCLUSION

1) activation of Ca2+ influx by formyl-methionyl-leucyl-phenylalanine depends on the elevation of [Ca2+]i, the latter being initiated by Ca2+ mobilization from intracellular stores; 2) Ins(1,3, 4,5)P4 is a strong candidate for maintaining receptor-mediated activation of Ca2+ influx in differentiated HL-60 cells.

[Effect of vasopressin on the osmotic permeability of the bladder wall in the frog and its content of cAMP, cGMP and inositol triphosphate]

An increase in the water permeability of the frog urinary bladder due to vasopressin, correlates with an increase of cAMP content in the bladder tissue. The osmotic permeability reached its maximum in 15-20 min. The sharp increase of inositol triphosphate content was observed within 20 sec after vasopressin administration, whereas cGMP content significantly decreased within 5 min. The augmentation of cAMP content seems to lead to a rise in water permeability white inositol triphosphate and cGMP acted, probably, as modulators of the vasopressin effect.

The inositol phosphate/diacylglycerol pathway in MA-10 Leydig tumor cells. Activation by arginine vasopressin and lack of effect of epidermal growth factor and human choriogonadotropin

It is now well established that mouse epidermal growth factor (mEGF) modulates the hormonal responsiveness of MA-10 Leydig tumor cells but does not affect cell multiplication. The studies presented herein are the first in a series of experiments designed to characterize the intracellular signaling systems activated by mEGF and their possible roles in mediating the diverse biological actions of this growth factor in MA-10 cells. We show that (i) MA-10 cells express a hormone-sensitive inositol phosphate/diacylglycerol pathway that can be stimulated with arginine vasopressin (AVP), (ii) mEGF does not activate this pathway, and (iii) activation of this pathway with arginine vasopressin does not mimic the biological actions of mEGF. Other data presented show that lutropin/choriogonadotropin, the principal endocrine regulators of Leydig cell function, also do not stimulate the inositol phosphate/diacylglycerol pathway in MA-10 cells.

Inositol trisphosphate and excitation-contraction coupling in skeletal muscle

The role of inositol trisphosphate as a chemical messenger in excitation-contraction coupling is discussed, both in terms of positive and negative results. The evidence presented includes experiments on the effect of inositol trisphosphate in intact and skinned fibers, in calcium release from isolated sarcoplasmic reticulum vesicles, in activation of single calcium release channels incorporated in planar bilayers, and biochemical experiments that have established the presence of all the intermediate steps involved in the metabolism of phosphoinositides, both in intact muscle and in isolated membranes. From these results, it is clear that a role for inositol triphosphate in skeletal muscle function is highly likely; whether this molecule is the physiological messenger in excitation-contraction coupling remains to be established.

[Inositol trisphosphate, a new "second messenger" for positive inotropic effects on the heart?]

Myocardial alpha 1-adrenoceptors mediate a positive inotropic effect and influence the inositol phosphate cycle. The receptor-stimulated, phospholipase C-mediated hydrolysis of phosphatidylinositol bisphosphate (PIP2) results in the generation of two novel second messengers, inositol trisphosphate (IP3) and diacylglycerol (DG). This effect is concentration-dependent and precedes the increase in force of contraction. Recently, it has been shown that the alpha 1-adrenoceptor-mediated increase in IP3 and force of contraction exists in the human heart as well. Possible mechanisms for an inositol phosphate-mediated positive inotropic effect are: (i) release of Ca2+ from the sarcoplasmic reticulum, elicited by IP3, (ii) increase in Ca2+ sensitivity of the contractile proteins, elicited by IP3, inositol tetrakisphosphate (IP4) and/or DG, (iii) increase in slow Ca2+ inward current, elicited directly by IP4 and/or indirectly by DG through a phosphorylation of the protein kinase C substrate in the sarcolemma. In ventricular cardiac preparations muscarinic agonists have a weak positive inotropic effect, but in cardiac atrial preparations they have a negative inotropic effect. In both preparations, these different effects coincide with a concentration-dependent increase in IP3. Thus, the possible positive inotropic effect in atrial preparations is probably masked by an activation of a K+ outward current. The relationship between the inositol phosphate cycle and the positive inotropic effect is in some points still speculative because not all of the mechanisms discussed are well settled yet. However, the stimulation of myocardial phosphoinositide breakdown resulting in an increased IP3 may be involved in the mechanism(s) whereby alpha1-adrenergic and muscarinic receptor stimulation exert an increase in myocardial force of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of inositol 1,4,5-trisphosphate in Ca2+ influx in thrombin stimulated human platelets

By incubating platelets at low temperature (10 degrees C), the relationship between Ca2+ mobilization and formation of inositol 1,4,5-trisphosphate (IP3) in thrombin stimulated platelets could be precisely investigated. In the presence of 1 mM EGTA, time dependent changes in the intracellular free calcium concentration [( Ca2+]i) were closely related to those in IP3 formation. Time course of the influx of external Ca2+, estimated by delta [Ca2+]i obtained by subtracting [Ca2+]i in the presence of 1 mM EGTA from that in the presence of 1 mM CaCl2 was also very similar to that of IP3 formed. Furthermore, the increase in delta [Ca2+]i was extremely well correlated with the amount of IP3 formed (Y = 49X - 34, r = 0.99). Thus, these data indicate that IP3 might be involved not only in intracellular Ca2+ mobilization but in Ca2+ influx of human platelets stimulated by thrombin.

Intracellular localization of inositol-phospholipid-metabolizing enzymes in rabbit fast-twitch skeletal muscle. Can D-myo-inositol 1,4,5-trisphosphate play a role in excitation-contraction coupling?

Rabbit fast-twitch skeletal muscle microsomes have been separated by isopycnic centrifugation on a linear sucrose gradient into triads and light sarcoplasmic reticulum. In both fractions phosphatidylinositol-kinase activity is found [Varsányi et al. (1986) Biochem. Biophys. Res. Commun. 138, 1395]. In contrast, phosphatidylinositol-4-phosphate kinase is nearly exclusively associated with triads. The phosphatidylinositol-4,5-bisphosphate-phosphodiesterase activity shows a biphasic distribution: approximately 50% of the activity is associated with triads and 50% appears in the overlay. Triads have been broken mechanically into transverse tubules and terminal cisternae, then separated by isopycnic sucrose-gradient centrifugation. Both fractions exhibit phosphatidylinositol-kinase activity; the activities of phosphatidylinositol-4-phosphate kinase and phosphatidylinositol-4,5-bisphosphate phosphodiesterase are associated mainly with the transverse tubules. Consequently, in rabbit fast-twitch skeletal muscle all necessary enzymes for production of D-myo-inositol 1,4,5-trisphosphate are associated with transverse tubules. Phosphatidylinositol-4,5-bisphosphate phosphodiesterase associated with triads shows a pH optimum at 6.8. The enzyme is maximally active between pCa 5 and pCa 4. Mg2+ inhibits the enzyme activity half-maximally at about 1 mM. Guanine-nucleotide-binding proteins seem not to be involved in the regulation of enzyme activity; guanosine 5'-[gamma-thio]triphosphate does not influence phosphatidylinositol-4,5-bisphosphate phosphodiesterase activity. It correlates well with the observation that neither alpha 1-adrenergic nor muscarinic receptors have been found in fast-twitch rabbit skeletal muscle. On basis of the respective enzyme activities estimations on maximal phosphatidylinositol turnover were made and a possible involvement of this signal pathway in excitation-contraction coupling has been discussed. Furthermore, calculations show that during a single twitch D-myo-inositol 1,4,5-trisphosphate concentration does not reach more than 2 nM. However, during a 4-s tetanus D-myo-inositol 1,4,5-trisphosphate can accumulate to a level which could effect force generation [Thieleczek and Heilmeyer (1986) Biochem. Biophys. Res. Commun. 135, 662] and aldolase distribution (Thieleczek et al., unpublished results).

Protein kinase C activators reduce the inositol trisphosphate-induced outward current and the Ca2+-activated outward current in identified neurons of Aplysia

Effects of intracellularly injected activators of protein kinase C on the InsP3-induced K+ current and the Ca2+-activated K+ current recorded from identified neurons (R9-R12) of Aplysia kurodai were investigated with conventional voltage-clamp and pressure-injection techniques. Intracellular injection of InsP3 into identified neurons produced a 4-aminopiridine (4-AP)-resistant, tetraethylammonium (TEA)-sensitive, and quinidine-sensitive K+ current similar to the Ca2+ activated K+ current elicited by direct injection of Ca2+ ions into the same neurons. The diacylglycerol analogue 1,2-oleoylacetylglycerol (OAG) at an intracellular concentration of 65 nM produced irreversible decreases in both the InsP3-induced K+ current and the Ca2+-activated K+ current. The phorbol 12,13-dibutyrate (PDBu) at an intracellular concentration of 150 nM also decreased irreversibly both the InsP3-induced K+ current and the Ca2+-activated K+ current. These results suggest that protein kinase C activators reduce both the InsP3-induced K+ current and the Ca2+-activated K+ current recorded from certain identified neurons of Aplysia and that protein kinase C reduces the ability of Ca2+ to open K+ channels rather than affecting the ability of InsP3 to release Ca2+ from intracellular stores.

Dysbalance of neuronal second messenger function in the aetiology of affective disorders: a pathophysiological concept hypothesising defects beyond first messenger receptors

It is suggested that affective disorders arise from the dysbalance of the two major intraneuronal signal amplification systems, the adenylate cyclase and the phospholipase C system, with depression resulting from underfunction of cyclic adenosine 3',5'-monophosphate-mediated effector cell responses associated with an absolute or relative dominance of the inositoltriphosphate/diacylglycerol-mediated responses and mania resulting from the converse. The usefulness of this hypothesis is discussed with respect to (a) the mechanism of action of current therapeutic agents and (b) the development of novel therapeutic approaches.

Effects of diacylglycerol and inositol trisphosphate on steroidogenesis by ovarian granulosa from pigs

In addition to increasing cyclic adenosine 3',5'-monophosphate (cAMP) levels, luteinizing hormone (LH) stimulation of granulosa results in phosphoinositide hydrolysis producing inositol trisphosphate (IP3) and diacylglycerol. The roles of these putative second messengers were investigated by measuring production of progesterone and inositol phosphates by granulosa from medium-sized porcine follicles (3-7 mm) after 15 min incubation with or without LH (1 microgram/ml), 5 microM dibutyryl cAMP (dbcAMP), or 5 microM 1-oleoyl,2-acetylglycerol (OAG). Compared to a control rate of 5.4 pmoles/10(7) cells/15 min, LH and dbcAMP stimulated progesterone production to 12.8 and 15.9 pmoles, respectively, and OAG decreased progesterone production to 3.7 pmoles. LH also stimulated inositol phosphate (IP) and bisphosphate (IP2) accumulations by approximately 5-fold and IP3 accumulation by 20-fold. In experiments where granulosa were premeabilized with saponin, LH, dbcAMP, and IP3 stimulated progesterone production from 1.3 pmol in control cells to 5.2, 3.2, and 5.1 pmol, respectively, and OAG decreased progesterone production to 1.0 pmol. LH stimulated accumulation of all inositol phosphates in permeabilized cells, whereas the addition of IP3 only increased IP2 and IP3 accumulations. In granulosa preincubated with 0.9 mM [ethylenebis(oxyethylenenitrilo)] tetraacetic acid, A23187 increased progesterone production from 3.7 to 5.8 pmol. Addition of 1-20 nmoles IP3 to 10(7) granulosa incubated in a Ca2+-free medium increased Ca2+ efflux linearly. These data suggest that IP3 may have a role in regulating steroid production in granulosa by regulating intracellular Ca2+.

Inositol trisphosphate-linked calcium mobilization couples substance P receptors to conductance increase in a rat pancreatic acinar cell line

The action of substance P (SP) on a rat pancreatic acinar cell line, AR4-2J, was examined using the whole-cell voltage-clamp technique. Pressure application of SP evoked inward currents accompanying increased membrane conductance. The SP-induced response was suppressed in Ca2+-free or low-Na+ solution. Treatment of cells with caffeine or A23187 produced a transient inward current and depressed the response to SP. Intracellular application of inositol 1,4,5-trisphosphate or guanosine 5'-O-(3-thiotriphosphate) elicited sustained inward currents and suppressed the SP-induced response. The results suggest that activation of SP receptors stimulates the formation of inositol phosphates via a guanine nucleotide-binding protein and leads to the rise in intracellular Ca2+, thereby activating a cation conductance in AR4-2J cells.

Cyclic AMP enhances inositol trisphosphate-induced mobilization of intracellular Ca2+ in cultured aortic smooth muscle cells

The effect of cAMP on ATP-induced intracellular Ca+ mobilization in cultured rat aortic smooth muscle cells was investigated. Treatment of cells for 3 min at 37 degrees C with dibutyryl cAMP, a membrane-permeable analogue of cAMP, at concentration up to 500 microM resulted in 1.5- to 1.7-fold increase in the peak cytosolic Ca2+ concentration when cells were stimulated with 3 to 200 microM ATP either in the presence or absence of extracellular Ca2+. Similar results were obtained when 0.5 mM 8-Br-cAMP or 10 microM forskolin was used instead of dibutyryl cAMP. In contrast to the Ca2+ response, dibutyryl cAMP did not affect ATP-induced formation of inositol trisphosphate (IP3). Furthermore, the dibutyryl cAMP treatment did not affect the size of the Ca2+ response elicited by 10 microM ionomycin. These results suggest that intracellular cAMP potentiates the ATP-induced Ca2+ response by enhancing Ca2+ release from the intracellular Ca2+ store(s), rather than by increasing the ATP-induced production of IP3 or by increasing the size of the intracellular Ca2+ store. Using saponin-permeabilized cells, we have shown directly that cAMP enhances Ca2+ mobilization by potentiating the Ca2+-releasing effect of IP3 from the intracellular Ca2+ store.

Insect cardioactive neuropeptides: peptidergic modulation of the intrinsic rhythm of an insect heart is mediated by inositol 1,4,5-trisphosphate

The beat frequency of the myogenic heart of the tobacco hawkmoth, Manduca sexta, markedly increases at adult emergence in response to 2 blood-borne peptide neurohormones, known as the cardioacceleratory peptides (CAP1 and CAP2). Three independent lines of evidence are presented supporting the hypothesis that the CAPs exert their cardiostimulatory effects on the insect myocardium through a change in the intracellular levels of inositol 1,4,5-trisphosphate (InsP3). I show that (1) InsP3 levels increase in response to CAP2 in a timely fashion, (2) exogenous application of InsP3 mimics the effects of CAP2 application, and (3) a blocker of InsP3 metabolism inhibits the effect of CAP2. These results provide strong support for the hypothesis that InsP3 is likely to be the second messenger in the regulation of heart beat activity by CAP2. Besides establishing the nature of the signaling system between CAP2 and the heart, these data also identify a novel role for InsP3, namely, the control of contraction frequency in a myogenic muscle. Given the widespread distribution of cellular systems employing InsP3 as a second messenger, it is suggested that InsP3 may also be involved in the long-term regulation of rhythmic activity in other spontaneously contractile muscles and endogenously active cells.

Cyclic AMP-dependent phosphorylation of a brain inositol trisphosphate receptor decreases its release of calcium

We report the stoichiometric phosphorylation of an inositol 1,4,5-trisphosphate receptor-binding protein from rat brain by the cAMP-dependent protein kinase but not by protein kinase C or Ca2+/calmodulin-dependent protein kinase. This phosphorylation event does not markedly alter [3H]inositol 1,4,5-trisphosphate-binding characteristics. However, inositol 1,4,5-trisphosphate is only 10% as potent in releasing 45Ca2+ from phosphorylated, as compared with native, cerebellar microsomes. Phosphorylation of the inositol 1,4,5-trisphosphate-binding protein by the cAMP-dependent protein kinase may provide a biochemical substrate for second-messenger cross talk.

H+-dependent calcium uptake into an IP3-sensitive calcium pool from rat parotid gland

In permeabilized parotid cells and in isolated membrane vesicles from parotid endoplasmic reticulum (ER), Mg-ATP-dependent Ca2+ uptake was measured using a Ca2+-specific macroelectrode and 45Ca2+, respectively. Mg-ATP-dependent Ca2+ uptake was inhibited by vanadate (2 x 10(-3) mol/l) by approximately 45% in permeabilized cells and by approximately 70% in membrane vesicles from ER during the initial 10 min. After this lag phase, Ca2+ uptake increased and low steady-state free [Ca2+] of approximately 3 x 10(-7) mol/l was still reached in presence of vanadate within 30-40 min. Subsequent addition of inositol 1,4,5-trisphosphate (IP3) caused a similar Ca2+ release compared with control. This indicates that in presence of vanadate an IP3-sensitive Ca2+ pool was filled. However, when protonophores, such as nigericin or carbonyl cyanide-m-chlorophenylhydrazone, were added in addition to vanadate, this low steady-state free [Ca2+] was not reached. 45Ca2+ uptake was reduced by approximately 70% within 60 min, and IP3 did not cause 45Ca2+ release when given subsequently, indicating that filling of an IP3-sensitive Ca2+ pool was prevented. Mg-ATP-driven H+ uptake into ER vesicles, as estimated with acridine orange, was abolished by protonophores and by the H+-ATPase blockers N-ethylmaleimide and Dio 9 but was unaltered by vanadate. Preincubation of ER vesicles in a medium without Ca2+, but with vanadate and with Mg-ATP to generate an H+ gradient, allowed demonstration of 45Ca2+ uptake from a medium that did not contain ATP. The cation sequence in absence of vanadate for support of Mg-ATP-dependent 45Ca2+ uptake was K+ greater than Na+ greater than Li+ = choline+ and, in presence of vanadate, was choline+ greater than Li+ = Na+ greater than K+. A preformed H+ gradient dissipated more rapidly in presence of K+ compared with choline+, probably due to an intrinsic K+ permeability of ER membrane. Our data indicate that both a Ca2+ and a H+ pump are located in a compartment of ER that is also sensitive to IP3. Ca2+ uptake is coupled to an H+ gradient that is generated by the H+ pump and most likely occurs via Mg-ATP-driven Ca2+-H+ countertransport but to some extent can also operate in absence of ATP at the expense of the H+ gradient.

The Croonian lecture, 1988. Inositol lipids and calcium signalling

The response of cells to many external stimuli requires a decoding process at the membrane to transduce information into intracellular messengers. A major decoding mechanism employed by a variety of hormones, neurotransmitters and growth factors depends on the hydrolysis of a unique inositol lipid to generate two key second messengers, diacylglycerol and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3). Here I examine the second messenger function of Ins(1,4,5)P3 in controlling the mobilization of calcium. We know most about how this messenger releases calcium from internal reservoirs but less is known concerning the entry of external calcium. One interesting possibility is that Ins(1,4,5)P3 might function in conjunction with its metabolic product Ins(1,3,4,5)P4 to control calcium entry through a mechanism employing a region of the endoplasmic reticulum as a halfway house during the transfer of calcium from outside the cell into the cytoplasm. The endoplasmic reticulum interposed between the plasma membrane and the cytosol may function as a capacitor to insure against the cell being flooded with external calcium. When stimulated, cells often display remarkably uniform oscillations in intracellular calcium. At least two oscillatory patterns have been recognized suggesting the existence of separate mechanisms both of which may depend upon Ins(1,4,5)P3. In one mechanism, oscillations may be driven by periodic pulses of Ins(1,4,5)P3 produced by receptors under negative feedback control of protein kinase C. The other oscillatory mechanism may depend upon Ins(1,4,5)P3 unmasking a process of calcium-induced calcium release from the endoplasmic reticulum. The function of these calcium oscillations is still unknown. This Ins(1,4,5)P3/calcium signalling system is put to many uses during the life history of a cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular calcium translocation: mechanism of activation by guanine nucleotides and inositol phosphates

The movements of Ca2+ within cells in response to external stimuli are complex. Internal Ca2+ release activated by inositol 1,4,5-trisphosphate (InsP3) is now widely established. However, the mechanism of InsP3-induced Ca2+ release, the identity and control of the InsP3-sensitive Ca2+ pool and its relationship to other internal and external Ca2+ pools all remain uncertain. We have characterized a highly sensitive and specific guanine nucleotide-regulatory mechanism that induces rapid and profound movements of intracellular Ca2+ via a mechanism distinct from that activated by InsP3. Using permeabilized neural or smooth muscle cells, application of submicromolar concentrations of GTP induces rapid release of Ca2+ from a compartment that contains within it the InsP3-releasable Ca2+ pool. Although of similar GTP-sensitivity as G-protein-activated events, the apparent dependence on GTP hydrolysis and blockade by GTP gamma S suggest a mechanism distinct from those mediated by known G-proteins. Recent experiments in the presence of oxalate reveal rapid and profound GTP-activated uptake of Ca2+ via a mechanism with identical nucleotide sensitivity and specificity to GTP-induced Ca2+ release. These results were interpreted to suggest that GTP induces a transmembrane conveyance of Ca2+ between different compartments distinguished by oxalate permeability; GTP-induced release probably occurs via a similar mechanism except involving transfer between closed compartments and nonclosed membranes (perhaps the plasma membrane). Recently, it has been revealed that GTP activates a translocation of Ca2+ into the Ca2+ pool from which InsP3 induces release. This is an important observation suggesting that the GTP-activated Ca2+ translocation process may control entry into and hence the size of the InsP3-releasable Ca2+ pool. Indeed, it is possible that GTP-induced Ca2+ release observed in permeabilized cells reflects a reversal of the pathway that functions in intact cells to permit external Ca2+ entry into the InsP3-releasable pool. This type of process could mediate the longer-term secretory or excitatory responses to external receptors which are known to be dependent on external Ca2+.

Spatial and temporal aspects of cell signalling

As new techniques are developed to measure intracellular messengers it becomes increasingly apparent that there is a remarkable spatial and temporal organization of cell signalling. Cells possess a small discrete hormone-sensitive pool of inositol lipid. In some cells such as Xenopus oocytes and Limulus photoreceptors this phosphoinositide signalling system is highly concentrated in one region of the cell, so establishing localized calcium gradients. Another example is the hydrolysis of inositol lipids in eggs at the point of sperm entry resulting in a localized increase in Ins(1,4,5)P3 and calcium which spreads like a wave throughout the egg. In hamster eggs this burst of calcium at fertilization recurs at 1-3 min intervals for over 100 min, a particularly dramatic example of spontaneous activity. Spontaneous oscillations in intracellular calcium exist in many different cell types and are often induced by agonists that hydrolyse inositol lipids. We have made a distinction between oscillations that are approximately sinusoidal and occur at a higher frequency where free calcium is probably continuously involved in the oscillatory cycle and those where calcium falls to resting levels for many seconds between transients. In the former case, the oscillations are thought to be induced through a cytoplasmic oscillator based on the phenomenon of calcium-induced calcium release. Such oscillations can be induced in Xenopus oocytes after injection with Ins(1,4,5)P3. A receptor-controlled oscillator based on the periodic formation of Ins(1,4,5)P3 is probably responsible for the generation of the widely spaced calcium transients. The function of such calcium oscillations is currently unknown. They may be a reflection of the feedback interactions that operate to control intracellular calcium. Another possibility emerged from observations that in some cells the frequency of calcium oscillations varied with agonist concentration, suggesting that cells might employ these oscillations as a way of encoding information. One advantage of using such a frequency-dependent mechanism may lie in an increase in fidelity, especially at low agonist concentrations. Whatever these functions might be, it is clear that uncovering the mechanisms responsible for such oscillatory activity will greatly enhance our understanding of the relation between the phosphoinositides and calcium signalling.

The localization of calcium release by inositol trisphosphate in Limulus photoreceptors and its control by negative feedback

Microvillar photoreceptors of invertebrates exhibit a light-induced rise in the intracellular concentration of free calcium (Cai) that results in part from release of calcium from an intracellular compartment. This light-induced release of calcium appears to result from a cascade of reactions that involve rhodopsin, a GTP-binding protein and a phospholipase-C which releases inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) from the plasma membrane; the Ins(1,4,5)P3 acts to release calcium from smooth endoplasmic reticulum. In the ventral photoreceptor of the horseshoe crab Limulus polyphemus not all of the endoplasmic reticulum is subject to calcium release by Ins(1,4,5)P3. Only endoplasmic reticulum in the light-sensitive region of the cell is competent to release calcium in response to Ins(1,4,5)P3. The release of calcium by Ins(1,4,5)P3 in ventral photoreceptors appears to be subject to feedback inhibition through elevated Cai. We suggest that this feedback inhibition contributes to sensory adaptation in the photoreceptor and may account for oscillatory membrane responses sometimes observed with large injections of Ins(1,4,5)P3.

Inositol phosphates: proliferation, metabolism and function

After the initial discovery of receptor-linked generation of inositol(1,4,5)trisphosphate (Ins(1,4,5)P3) it was generally assumed that Ins(1,4,5)P3 and its proposed breakdown products inositol(1,4)bisphosphate (Ins(1,4)P2) and Ins1P, along with cyclic inositol monophosphate, were the only inositol phosphates found in significant amounts in animal cells. Since then, three levels of complexity have been introduced. Firstly, Ins(1,4,5)P3 can be phosphorylated to Ins(1,3,4,5)P4, and the subsequent metabolism of these two compounds has been found to be intricate and probably different between various tissues. The functions of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 are almost certainly to regulate cytosolic Ca2+ concentrations, but the reasons for the labyrinth of the metabolic pathways after their deactivation by a specific 5-phosphatase remain obscure. Secondly, inositol pentakis- and hexakisphosphates have been found in many animal cells other than avian erythrocytes. It has been shown that their synthesis pathway is entirely separate from the inositol phosphates discussed above, both in terms of many of the isomers involved and probably in the subcellular localization; some possible functions of InsP5 and InsP6 are discussed here. Thirdly, cyclic inositol polyphosphates have been reported in stimulated tissues; the evidence for their occurrence in vivo and their possible physiological significance are also discussed.

Neural function: metabolism and actions of inositol metabolites in mammalian brain

In the nervous system, a variety of cell types respond to external stimuli through the inositol lipid signalling pathways. The stimulus-coupled sequence of intracellular events has been investigated in a homogeneous model system, the cloned mammalian neural cell line NG115-401L. The neural peptide bradykinin stimulates a rapid production of identified inositol phosphate isomers and an intracellular Ca2+ discharge followed by a persistent plasma membrane influx. The temporal sequence suggests that Ins(1,4,5)P3 or Ins(1,3,4,5)P4 or both may coordinate these events in a neuronal cell, as has been suggested in other cell types. Thapsigargin, an irritant and tumour-promoting plant product, produces calcium transients in the absence of inositol phosphate production, and may provide a new tool for investigating the interactions between inositol phosphates and changes in cellular calcium homeostasis. In the 401L line, high levels of radiolabelled InsP5 and InsP6 have been detected, which has led to the evaluation of their possible occurrence and actions in normal brain. Both InsP5 and InsP6 are produced from a radiolabelled myo-inositol precursor in intact mature brain in a region-specific manner. This suggests that both inositol polyphosphates may be end products of regionally regulated biosynthetic pathways. When microinjected into a nucleus of the brainstem, or iontophoretically applied to the dorsal horn of the spinal cord, both InsP5 and InsP6, but not Ins(1,3,4,5)P4 isomers, appear to be potent neural stimulants. These results suggest that the inositol lipid signalling pathways may generate both intracellular and extracellular signals in brain.

Inositol trisphosphate, calcium and muscle contraction

The identity of organelles storing intracellular calcium and the role of Ins(1,4,5)P3 in muscle have been explored with, respectively, electron probe X-ray microanalysis (EPMA) and laser photolysis of 'caged' compounds. The participation of G-protein(s) in the release of intracellular Ca2+ was determined in saponin-permeabilized smooth muscle. The sarcoplasmic reticulum (SR) is identified as the major source of activator Ca2+ in both smooth and striated muscle; similar (EPMA) studies suggest that the endoplasmic reticulum is the major Ca2+ storage site in non-muscle cells. In none of the cell types did mitochondria play a significant, physiological role in the regulation of cytoplasmic Ca2+. The latency of guinea pig portal vein smooth muscle contraction following photolytic release of phenylephrine, an alpha 1-agonist, is 1.5 +/- 0.26 s at 20 degrees C and 0.6 +/- 0.18 s at 30 degrees C; the latency of contraction after photolytic release of Ins(1,4,5)P3 from caged Ins(1,4,5)P3 is 0.5 +/- 0.12 s at 20 degrees C. The long latency of alpha 1-adrenergic Ca2+ release and its temperature dependence are consistent with a process mediated by G-protein-coupled activation of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) hydrolysis. GTP gamma S, a non-hydrolysable analogue of GTP, causes Ca2+ release and contraction in permeabilized smooth muscle. Ins(1,4,5)P3 has an additive effect during the late, but not the early, phase of GTP gamma S action, and GTP gamma S can cause Ca2+ release and contraction of permeabilized smooth muscles refractory to Ins(1,4,5)P3. These results suggest that activation of G protein(s) can release Ca2+ by, at least, two G-protein-regulated mechanisms: one mediated by Ins(1,4,5)P3 and the other Ins(1,4,5)P3-independent. The low Ins(1,4,5)P3 5-phosphatase activity and the slow time-course (seconds) of the contractile response to Ins(1,4,5)P3 released with laser flash photolysis from caged Ins(1,4,5)P3 in frog skeletal muscle suggest that Ins(1,4,5)P3 is unlikely to be the physiological messenger of excitation-contraction coupling of striated muscle. In contrast, in smooth muscle the high Ins(1,4,5)P3-5-phosphatase activity and the rate of force development after photolytic release of Ins(1,4,5)P3 are compatible with a physiological role of Ins(1,4,5)P3 as a messenger of pharmacomechanical coupling.

Metabolism and functions of inositol phosphates

Activation of Ca2+-mobilizing receptors rapidly increases the cytoplasmic Ca2+ concentration both by releasing Ca2+ stored in endoplasmic reticulum and by stimulating Ca2+ entry into the cells. The mechanism by which Ca2+ release occurs has recently been elucidated. Receptor activation of phospholipase C results in the hydrolysis of the plasma membrane lipid, phosphatidylinositol 4,5-bisphosphate (PIP2), to yield two intracellular messengers, diacylglycerol (DAG) and (1,4,5)inositol trisphosphate [(1,4,5)IP3]. DAG remains in the plasma membrane where it stimulates protein phosphorylation via the phospholipid-dependent protein kinase C. (1,4,5)IP3 diffuses to and interacts with specific sites on the endoplasmic reticulum to release stored Ca2+. Receptor stimulation of phospholipase C appears to be mediated by one or more guanine nucleotide-dependent regulatory proteins by a mechanism analogous to hormonal activation of adenylyl cyclase. The actions of (1,4,5)IP3 on Ca2+ mobilization are terminated by two metabolic pathways, sequential dephosphorylation to inositol bisphosphate (IP2), inositol monophosphate (IP) and inositol or by phosphorylation to inositol tetrakisphosphate (IP4) and sequential dephosphorylation to different inositol phosphates. A sustained cellular response also requires Ca2+ entry into the cell from the extracellular space. The mechanism by which hormones increase Ca2+ entry is not known; a recent proposal involving movement of Ca2+ through the endoplasmic reticulum, possibly regulated by IP4, will be considered here.

Heparin inhibits the inositol 1,4,5-trisphosphate-dependent, but not the independent, calcium release induced by guanine nucleotide in vascular smooth muscle

The effects of heparin on the release of intracellular Ca2+, assessed by tension development in saponin-permeabilized rabbit main pulmonary artery, were determined. Heparin inhibited (IC50 = 5 micrograms/ml) inositol 1,4,5-trisphosphate (InsP3)-induced, but not caffeine-induced, Ca2+ release. The initial (InsP3-dependent) component of GTP gamma S-induced Ca2+-release was also inhibited by heparin, but the InsP3-independent component was resistant to both heparin and procaine. These results support the existence of a G protein activated mechanism of Ca2+ release that is not mediated by InsP3 or by Ca2+-induced Ca2+ release.