Metabolism of inositol 1,4,5-trisphosphate and inositol 1,3,4-trisphosphate in rat parotid glands

Automated isocratic high-performance liquid chromatography of inositol phosphate isomers

Isomers of inositol phosphates from biological samples can be analysed by anion-exchange h.p.l.c., by using isocratic elution with phosphate buffers. The method involves the preliminary processing of the extracted samples with conventional soft-gel anion-exchange resins, including the commonly used Dowex resins, followed by direct analysis with h.p.l.c. of a portion of relevant fractions. Run times (up to 20 min) and collected fraction numbers (up to 24) are minimal, so that if the method is used in conjunction with automated h.p.l.c. injection a high throughput of samples is achieved.

Muscarinic and quisqualate receptor-induced phosphoinositide hydrolysis in primary cultures of striatal and hippocampal neurons. Evidence for differential mechanisms of activation

Several neurotransmitters activate polyphosphoinositide (PPI) hydrolysis in CNS neurons as the first step of a transmembrane signalling cascade that may lead to neuronal circuit modulation. Muscarinic and quisqualate receptor-triggered PPI hydrolysis was investigated in neuronal primary cultures. A clear increase in inositol phosphates (Ins-Ps) was detected as early as 15 s after the agonist addition; at this time, the increases of inositol 1,4,5-trisphosphate (measured by HPLC) were relatively larger with respect to the other Ins-Ps. Ins-P accumulation was maintained in part in a Ca2+-free medium, excluding that Ca2+ entry is the fundamental step of the receptor-induced PPI hydrolysis. Acute cell pretreatment with phorbol dibutyrate, an activator of protein kinase C, was able to inhibit 50% of the response to carbachol, and almost completely the quisqualate effect, suggesting a negative feedback modulation by the enzyme. Finally, pertussis toxin failed to inhibit muscarinic responses, whereas it blocked greater than 70% of the quisqualate stimulation. The two receptors therefore appear coupled to phosphodiesterase by two different G proteins. The comparison of the results obtained by stimulating the two receptor systems suggests that the generation of the same intracellular signal at two distinct receptor types may occur by different coupling mechanisms, and be differently regulated even in the same neuronal preparations.

Down-regulation of protein kinase C potentiates angiotensin II-stimulated polyphosphoinositide hydrolysis in vascular smooth-muscle cells

In smooth-muscle cells (SMC) isolated from rat aorta, angiotensin II stimulates a phospholipase C with subsequent formation of inositol trisphosphate (InsP3). Short-term (10 min) pretreatment of SMC with 12-O-tetradecanoylphorbol 13-acetate (TPA; 100 nM) decreases the angiotensin II-induced InsP3 formation. However, this inhibition is not observed after incubating the cells for 2 h with TPA. Longer-term pretreatments even lead to an enhanced generation of InsP3. This increased response to angiotensin II occurs without a significant change in the receptor number or Kd value of angiotensin II binding to the cells. The biologically inactive phorbol ester 4 alpha-phorbol 12,13-didecanoate was without effect on angiotensin II-stimulated InsP3 generation, irrespective of the time of preincubation. In parallel with this potentiation of angiotensin II-induced generation of InsP3 by TPA, a down-regulation of protein kinase C activity is observed. A 24 h pretreatment of SMC with TPA decreases protein kinase C activity to less than 10% of that of control cells. Longer-term pretreatment also increases the angiotensin II-induced release of Ca2+ and delays the decay of the transient Ca2+ increase. All these data suggest that protein kinase C exerts a negative feedback control on angiotensin II-stimulated polyphosphoinositide turnover, and that protein kinase C is an important factor in limiting the production of InsP3 in stimulated cells.

Differences in the metabolism of inositol and phosphoinositides by cultured cells of neuronal and glial origin

Phosphoinositide and inositol metabolism was compared in glioma (C6), neuroblastoma (N1E-115) and neuroblastoma X glioma hybrid (NG 108-15) cells. All cell lines had similar proportions of phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), and phosphatidylinositol 4,5-bisphosphate (PIP2). Neuroblastoma and hybrid cells had almost identical phospholipid and phosphoinositide compositions and similar activities for the enzymes metabolizing polyphosphoinositides (PI kinase, PIP phosphatase, PIP kinase, PIP2 phosphatase, PIP2 phosphodiesterase). Glioma cells differed by having greater proportions of ethanolamine plasmalogen and sphingomyelin, lower PIP kinase, 3-5-fold higher PIP phosphatase activity and 10-15-fold greater PIP2 phosphodiesterase activity. Higher PIP phosphatase and PIP2 diesterase activities appear to be characteristic of cells of glial origin, since similar activities were found in primary cultures of astroglia. Glioma cells also metabolize inositol differently. In pulse and pulse-chase experiments, glioma cells transported inositol into a much larger water-soluble intracellular pool and maintained a concentration gradient 30-times greater than neuroblastoma cells. Label in intracellular inositol was less than in phosphoinositides in neuroblastoma and exchanged rapidly with extracellular inositol. In glioma, labeling of intracellular inositol greatly exceeded that of phosphoinositides. As a consequence, radioactivity in prelabeled phosphoinositides could not be effectively chased from glioma cells by excess unlabeled inositol. Such differences between cells of neuronal and glial origin suggest different and possibly supportive roles for these two cell types in maintaining functions regulated through phosphoinositide-linked signalling systems in the central nervous system.

In vivo determination of 5-hydroxytryptamine receptor-stimulated phosphoinositide turnover in rat brain

Phosphoinositide turnover stimulated by 5-hydroxytryptamine (5-HT) receptors in the intact rat brain was studied using an in vivo method. Phosphoinositides in the rat brain were prelabeled with [3H]inositol injected into the lateral cerebral ventricles. The rats were killed by microwave irradiation after 48 h and the contents in the frontal cortex of 3H-inositol phosphates, [3H]inositol-1-monophosphate [( 3H]IP1), [3H]inositol-1,4-bisphosphate [( 3H]IP2), and a mixture of [3H]inositol-1,4,5-trisphosphate and [3H]inositol-1,3,4-trisphosphate [( 3H]IP3) were assayed by HPLC. Lithium treatment (10 mEq/kg, i.p., 2 h before) increased the content of [3H]IP1 and [3H]IP2. 5-Methoxy-N,N-dimethyltryptamine (5-MeODMT) and quipazine, 5-HT agonists, significantly increased the amount of 3H-inositol phosphates under lithium pretreatment. The response to 5-MeODMT was inhibited by ritanserin, a 5-HT2 antagonist, but not by (-)-propranolol, a 5-HT1 antagonist. These results suggest that phosphoinositide turnover in the rat frontal cortex in vivo is stimulated by 5-HT2 receptor activation. It is considered that this method will be useful for measurement of 5-HT2 receptor-stimulated phosphoinositide turnover in vivo to examine the in vivo effects of various psychotropic drugs such as antidepressants.

Inositol 1,4,5-trisphosphate phosphatase deficiency and malignant hyperpyrexia in swine

The sarcoplasmic reticulum from muscle of swine which are susceptible to malignant hyperpyrexia is deficient in inositol 1,4,5-trisphosphate phosphatase (InsP35-ase) activity, which leads to high intracellular concentrations of inositol 1,4,5-trisphosphate (InsP3) and of calcium ions. Halothane inhibits InsP35-ase and further increases myoplasmic InsP3 and calcium ion concentrations, and produces the clinical features of malignant hyperpyrexia.

Glutamate receptor activation in cultured cerebellar granule cells increases cytosolic free Ca2+ by mobilization of cellular Ca2+ and activation of Ca2+ influx

The Ca2+ sensitive fluorescent probe, fura-2 has been used to monitor cytosolic free calcium levels in mature primary cultures of cerebellar granule cells during exposure to L-glutamate and other excitatory amino acids: quisqualate (QA) kainate (KA) and N-methyl-D-aspartate (NMDA). Glutamate at micromolar concentrations produced a prompt and dose-related increase in the intracellular concentration of free Ca2+, ([Ca2+]i), whereas QA, KA and NMDA had no effect. This increase was also seen in the absence of extracellular Ca2+, suggesting that L-glutamate promotes mobilization of Ca2+ from intracellular stores. In the presence of extracellular calcium, the elevation of [Ca2+]i was, in part, mediated by an increase in the plasma membrane permeability to Ca2+. This Ca2+ influx was not affected by the Ca2+-channel antagonist l-Verapamil. However, L-Verapamil did block the increase in [Ca2+]i seen after depolarization of the cells with potassium. The Ca2+ response elicited by glutamate was partially blocked by the excitatory amino acid antagonist glutamate diethyl ester (GDEE). Furthermore, glutamate stimulated the formation of inositol mono-, bis-, tris- and tetrakisphosphates (IP1, IP2, IP3, and IP4) suggesting a role for these compounds for the increase in [Ca2+]i.

Bradykinin stimulates phosphoinositide hydrolysis and mobilization of arachidonic acid in dorsal root ganglion neurons

Cultures of fetal rat dorsal root ganglion neurons (7 days in culture) were prelabeled with myo-[3H]inositol or [3H]arachidonic acid for 24 h and stimulated with 10 microM bradykinin for time intervals of 5-300 s. The incubation was terminated by addition of 5% perchloric acid to extract inositol phosphates or organic solvent to extract lipids. Inositol phosphates were resolved by anion-exchange HPLC; lipids were resolved by TLC. Bradykinin stimulation resulted in a 10-fold increased accumulation of inositol 1,4,5-trisphosphate (IP3) and inositol bisphosphate (IP2) (fivefold) by 5 s. The increase in IP3 was transient (half maximal by 1 min), whereas stimulated IP2 levels were sustained for several minutes. Even longer term increases were observed in inositol monophosphate. Stimulation also resulted in a threefold increase in arachidonic acid which was preceded by transient increases in diacylglycerol (twofold) and arachidonoyl-monoacylglycerol (threefold). The temporal lag in the accumulation of arachidonic acid with respect to diglyceride and monoglyceride suggested the involvement of di- and monoglyceride lipases in arachidonic acid mobilization. A role for phospholipase A2 is also possible, because pretreatment of cultures with quinacrine partially blocked arachidonic acid release. Bradykinin-stimulated arachidonic acid release was decreased in the presence of calcium channel blockers nifedipine or verapamil (50 microM), or EDTA (2.5 mM). The role of calcium was verified further in that accumulation of phosphatidic acid, diacylglycerol, and arachidonic acid was maximally stimulated by treatment with the calcium ionophore A23187 (20 microM).

Bombesin stimulation of inositol 1,4,5-trisphosphate generation and intracellular calcium release is amplified in a cell line overexpressing the N-ras proto-oncogene

Bombesin stimulation of T15 cells in which the inducible N-ras oncogene is overexpressed caused elevated production of inositol phosphates compared to uninduced cells [Wakelam, Davies, Houslay, McKay, Marshall & Hall (1986) Nature (London) 323, 173-176]. This elevated response is shown here to result from increased generation of inositol 1,4,5-trisphosphate leading to an elevated release of intracellular stored Ca2+. Single-cell analysis of Ca2+ release showed that the elevated response is not a consequence of an increased fraction of responding cells. These amplifications are consistent with p21N-ras acting like a guanine nucleotide coupling protein in this cell line.

Angiotensin II-stimulated cortisol secretion is mediated by a hormone-sensitive phospholipase C in bovine adrenal fasciculata/reticularis cells

Conditions have been established for the incorporation of [3H]inositol ([3H]Ins) into the phosphoinositides of cultured bovine adrenal zona fasciculata/reticularis (ZFR) cells. Stimulation of these prelabelled cells with angiotensin II (10(-11)-10(-7) M AII) resulted in the dose-dependent (max. 16-fold at 10(-7) M AII), time-dependent formation of water-soluble radiolabelled products which show the same chemical and chromatographic properties as [3H]InsP, [3H]InsP2 and [3H]InsP3 standards. The results of the time-course studies of the changes in these products are consistent with the view that AII rapidly (less than 15 s) induces the activation of a polyphosphoinositide-specific phospholipase C. The action of this phospholipase on the polyphosphoinositides is sustained throughout 15 min of stimulation. The dose dependency of this response correlates closely with cortisol output and is reduced (to 52%, P less than 0.00005), but not abolished, in the absence of extracellular Ca2+. To our knowledge these results are the first clear demonstration that AII stimulates a polyphosphoinositide-specific phospholipase C in bovine ZFR cells.

Signal transduction and control of lacrimal gland protein secretion: a review

Proteins in lacrimal gland fluid are secreted primarily by the acinar cells. Secretory proteins are synthesized in the endoplasmic reticulum, modified in the Golgi apparatus, stored in secretory granules, and released upon a change in the cellular level of second messenger. The second messenger level is controlled by a process termed signal transduction. Agonists, primarily neurotransmitters in the lacrimal gland, bind to receptors in the basolateral membrane of secretory cells. This interaction activates enzymes in the membrane that cause production of second messengers. It has been hypothesized that second messengers stimulate secretion by activating specific protein kinases to phosphorylate proteins important for secretion. In the lacrimal gland, cholinergic agonists stimulate protein secretion. They act by activating phospholipase C to break down phosphatidylinositol bisphosphate into 1,4,5-inositol trisphosphate (1,4,5-IP3) and diacylglycerol (DAG). 1,4,5-IP3 causes release of Ca2+ from intracellular stores. This Ca2+, perhaps in conjunction with calmodulin, activates specific protein kinases that may be involved in secretion. DAG activates protein kinase C which stimulates protein secretion. alpha 1-Adrenergic agonists also stimulate lacrimal gland protein secretion. These agonists use a pathway that is separate from that utilized by cholinergic agonists and vasoactive intestinal peptide (VIP). The specific pathway has not been identified but may be DAG and protein kinase C. VIP, beta-adrenergic agonists, alpha-melanocyte stimulating hormone, and adrenocorticotropic hormone are lacrimal gland secretagogues. They activate adenylate cyclase to produce cAMP. cAMP stimulates protein kinase A, which perhaps causes protein secretion. Thus, three separate cellular pathways stimulate lacrimal gland protein secretion. Cholinergic agonists and VIP also stimulate lacrimal gland fluid secretion, and the same signal transduction pathways utilized by these agonists to stimulate protein secretion are most likely used for electrolyte and water secretion.

A simple enzymic method to separate [3H]inositol 1,4,5- and 1,3,4-trisphosphate isomers in tissue extracts

A novel method to separate [3H]Ins(1,4,5)P3 and [3H]Ins(1,3,4)P3 in tissue extracts is described. It is based on the selective metabolism of Ins(1,3,4)P3 by a crude cerebral supernatant in a Mg2+-free buffer followed by separation of [3H]inositol trisphosphates using conventional anion-exchange chromatography. Evaluation of the assay was performed using [3H]Ins(1,3,4)P3 standards and tissue extracts containing different proportions of [3H]Ins(1,4,5)P3 and [3H]Ins(1,3,4)P3. Parallel h.p.l.c. separations of extracts established the selective and complete metabolism of [3H]Ins(1,3,4)P3 under the above conditions and demonstrated that the enzymic method provides an accurate estimate of the trisphosphate isomers in rat cerebral cortex, parotid gland and bovine tracheal smooth muscle.

The separation of [32P]inositol phosphates by ion-pair chromatography: optimization of the method and biological applications

We have developed an ion-pair reverse-phase HPLC method to measure inositol phosphates in 32P-labeled cells. The different chromatographic parameters were analyzed to optimize the resolution of the 32P-labeled metabolites. Analysis of inositol phosphates in biological samples was improved by a single charcoal pretreatment which eliminated interfering nucleotides without removing inositol phosphates. The kinetics of production of inositol phosphates in calcium-activated erythrocytes, vasopressin-stimulated hepatocytes, and thrombin-activated platelets were analyzed. Original data on the activation of phosphoinositide phospholipase C were obtained in intact erythrocytes by direct measurement of inositol (1,4,5)P3. Data from agonist-stimulated hepatocytes and platelets were consistent with those from previous studies. In conclusion, this technique offers many advantages over the methodologies currently employed involving anion-exchange chromatography and [3H]inositol labeling: (i) 32P labeling is less expensive and more efficient than 3H labeling and can be used with all types of cells without permeabilization treatments and (ii) ion-pair HPLC gives good resolution of inositol phosphates from nucleotides with shorter retention times, and long reequilibration periods are not required.

Anticholinergic effects of cis-chlorprothixene characterized in rat parotid acini

The anticholinergic effects of the antipsychotic drug, cis-chlorprothixene, on the secretory events underlying the formation of primary saliva were investigated. The neuroleptic, cis-chlorprothixene, is used extensively as a major tranquillizer but shares side-effects such as xerostomia with most antidepressants. The inhibitory effects of cis-chlorprothixene upon the cholinergic-induced rise in Ca2+ as well as on O2 consumption and Cl- loss were investigated in isolated rat parotid acini in order to characterize its anticholinergic effects quantitatively. The cholinergic-induced rise in cytosolic, free Ca2+ was inhibited by cis-chlorprothixene with half-maximal effect at 1.9 microM and maximal inhibition at 10 microM. When the cytosolic, free Ca2+ was enhanced in the presence of 10 microM cis-chlorprothixene by means of the Ca2+ ionophore A23187, a loss of Cl- was observed similar to that observed during cholinergic stimulation in the absence of cis-chlorprothixene. The findings are consistent with the possibility that cis-chlorprothixene exerts its effects on the steps leading from agonist binding to the acetylcholine receptor and to the increase of cytosolic free Ca2+. Thus, measurement of the stimulation-induced rise in cytosolic, free Ca2+ in the presence of neuroleptics such as the thioxanthenes represents a fast and reliable method for detecting inhibitory effects on autonomic receptor activation.

Enhanced inositol trisphosphate response to alpha 1-adrenergic stimulation in cardiac myocytes exposed to hypoxia

Myocardial ischemia elicits an enhanced responsivity to alpha 1-adrenergic stimulation and a reversible increase in alpha 1-adrenergic receptor number. In adult cardiac myocytes, alpha 1-adrenergic receptor number increases two- to threefold after 10 min of hypoxia, an increase similar to that seen during ischemia in vivo. To determine whether this increase in alpha 1-adrenergic receptor number leads to an enhanced synthesis of inositol trisphosphate, the intracellular second messenger for the alpha 1-adrenergic receptor, the mass of inositol trisphosphate was quantified by a novel procedure developed in our laboratory that circumvents problems associated with using labeled precursors. The peak increases in inositol trisphosphate levels of three- to fourfold were measured after 30 s of norepinephrine stimulation and exhibited a 50% effective concentration (EC50) of 7.9 x 10(-8) M. Hypoxia produced a marked leftward shift in the dose-response curve for the production of inositol trisphosphate in response to norepinephrine stimulation (EC50 = 1.2 x 10(-8) M). Hypoxia also induced a 100-fold reduction in the concentration of norepinephrine required to elicit a threshold increase in inositol trisphosphate (10(-9) M), compared with control normoxic myocytes (10(-7) M). Thus, hypoxia, which increases alpha 1-adrenergic receptor density, also leads to an enhanced production of inositol trisphosphate and could account for the enhanced alpha 1-adrenergic responsivity in the ischemic heart in vivo, which is known to facilitate arrhythmogenesis.

Phosphatidylinositol(4,5)bisphosphate and Phosphatidylinositol(4)phosphate in Plant Tissues

Pea (Pisum sativum) leaf discs or swimming suspensions of Chlamydomonas eugametos were radiolabeled with [(3)H]myo-inositol or [(32)P]Pi and the lipids were extracted, deacylated, and their glycerol moieties removed. The resulting inositol trisphosphate and bisphosphate fractions were examined by periodate degradation, reduction and dephosphorylation, or by incubation with human red cell membranes. Their likely structures were identified as d-myo-inositol(1,4,5)trisphosphate and d-myo-inositol(1,4,)-bisphosphate. It is concluded that plants contain phosphatidylinositol(4)phosphate and phosphatidylinositol(4,5)bisphosphate; no other polyphosphoinositides were detected.

[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)

Determination of inositol 1,4,5-trisphosphate levels in Dictyostelium by isotope dilution assay

A commercial isotope dilution assay was used for the determination of Ins(1,4,5)P3 levels in the microorganism Dictyostelium discoideum. Cross-reactivity in the assay was detected with extracts from cells and the medium. The compound which induced this cross-reactivity was tentatively identified as Ins(1,4,5)P3 by (i) codegradation with authentic [32P]Ins(1,4,5)P3 by three specific Ins(1,4,5)P3 phosphatases, and (ii) co-chromatography with authentic [32P]Ins(1,4,5)P3 on HPLC columns. The cellular concentration was estimated as 165 +/- 42 pmol/10(8) cells, yielding a mean intracellular Ins(1,4,5)P3 concentration of 3.3 microM. Dictyostelium cells secrete large amounts of Ins(1,4,5)P3 at a rate of about 10% of the cellular content per minute, yielding about 0.13 microM extracellular Ins(1,4,5)P3 after 15 min in a suspension of 10(8) cells/ml. The chemoattractant cAMP induced a transient increase of the Ins(1,4,5)P3 concentration; the data suggest an intracacellular rise from 3.3 to 5.5 microM with a maximum at 6 s after stimulation.

Accumulation of inositol polyphosphate isomers in agonist-stimulated cerebral-cortex slices. Comparison with metabolic profiles in cell-free preparations

1. Basal and carbachol-stimulated accumulations of isomeric [3H]inositol mono-, bis-, tris- and tetrakis-phosphates were examined in rat cerebral-cortex slices labelled with myo-[2-3H]inositol. 2. In control samples the major [3H]inositol phosphates detected were co-eluted on h.p.l.c. with Ins(1)P, Ins(4)P (inositol 1- and 4-monophosphate respectively), Ins(1,4)P2 (inositol 1,4-bisphosphate), Ins(1,4,5)P3 (inositol 1,4,5-tris-phosphate) and Ins(1,3,4,5)P4 (inositol 1,3,4,5-tetrakisphosphate). 3. After stimulation to steady state with carbachol, accumulation of each of these products was markedly increased. 4. Agonist stimulation, however, also evoked much more dramatic increased accumulations of a second [3H]inositol trisphosphate, which was co-eluted on h.p.l.c. with authentic Ins(1,3,4)P3 (inositol 1,3,4-trisphosphate) and of three further [3H]inositol bisphosphates ([3H]InsP2(s]. 5. Examination of the latter by chemical degradation by periodate oxidation and/or h.p.l.c. allowed identification of these as [3H]Ins(1,3)P2, [3H]Ins(3,4)P2 and [3H]Ins(4,5)P2 (inositol 1,3-, 3,4- and 4,5-bisphosphates respectively), which respectively accounted for about 22%, 8% and 3% of total [3H]InsP2 in extracts from stimulated tissue slices. 6. By using a h.p.l.c. method which clearly resolves Ins(1,3,4,5)P4 and Ins(1,3,4,6)P4 (inositol 1,3,4,6-tetrakisphosphate), only the former isomer could be detected in extracts from either control or stimulated tissue slices. Similarly, [3H]inositol pentakis- and hexakis-phosphates were not detectable either in the presence or absence of carbachol under the radiolabelling conditions described. 7. The catabolism of [3H]Ins(1,4,5)P3 and [3H]Ins(1,3,4)P3 by cell-free preparations from cerebral cortex was also studied. 8. In the presence of Mg2+, [3H]Ins(1,4,5)P3 was specifically dephosphorylated via [3H]Ins(1,4)P2 and [3H]Ins(4)P to free [3H]inositol, whereas [3H]Ins(1,3,4)P3 was degraded via [3H]Ins(3,4)P2 and, to a lesser extent, via [3H]Ins(1,3)P2 to D- and/or L-[3H]Ins(1)P and [3H]inositol. 9. In the presence of EDTA, hydrolysis of [3H]Ins(1,4,5)P3 was greater than or equal to 95% inhibited, whereas [3H]Ins(1,3,4)P3 was still degraded, but yielded only a single [3H]InsP2 identified as [3H]Ins(1,3)P2. 10. The significance of these observations with cell-free preparations is discussed in relation to the proportions of the separate isomeric [3H]inositol phosphates measured in stimulated tissue slices.

Diacylglycerol modulates action potential frequency in GH3 pituitary cells: correlative biochemical and electrophysiological studies

We have investigated the involvement of enhanced phosphoinositide metabolism in mediating TRH-induced alteration of electrophysiological events related to prolactin secretion by GH3 cells (a line of pituitary origin). Patch-clamp recording (in the current clamp, whole-cell configuration) showed that a few seconds after TRH application there was a brief period (about 30 s) of membrane hyperpolarization followed by several minutes of increased calcium-dependent action potential frequency. In parallel experiments cells were labeled for 24 h with either [3H]myo-inositol or [3H]arachidonate. Application of TRH resulted in rapid increases in levels of inositol phosphates and diacylglycerol. The time course of elevation of inositol 1,4,5-triphosphate (maximal by 5 s) is compatible with an initial burst of intracellular calcium mobilization associated with a transient phase of TRH-induced prolactin release. Application of TRH was also followed by a rapid but more sustained (several minutes) period of elevated diglyceride accumulation; a time course corresponding to a prolonged period of prolactin release which is dependent on the influx of external calcium. A causal relationship between diglyceride release and increased action potential frequency was demonstrated since local application (via a U-tube apparatus) of either 2 microM phorbol ester (phorbol 12,13-dibutyrate or phorbol 12-myristate 13-acetate) or 60 microM 1-oleoyl-2-acetyl-glycerol to patch-clamped cells could mimic this aspect of the TRH effect. In contrast, the inactive phorbol ester, 4 alpha-phorbol, was unable to elicit this response.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of muscarinic, alpha-adrenergic, and substance P agonists and ionomycin on ion transport mechanisms in the rat parotid acinar cell. The dependence of ion transport on intracellular calcium

The relationship between receptor-mediated increases in the intracellular free calcium concentration [( Ca]i) and the stimulation of ion fluxes involved in fluid secretion was examined in the rat parotid acinar cell. Agonist-induced increases in [Ca]i caused the rapid net loss of up to 50-60% of the total content of intracellular chloride (Cli) and potassium (Ki), which is consistent with the activation of calcium-sensitive chloride and potassium channels. These ion movements were accompanied by a 25% reduction in the intracellular volume. The relative magnitudes of the losses of Ki and the net potassium fluxes promoted by carbachol (a muscarinic agonist), phenylephrine (an alpha-adrenergic agonist), and substance P were very similar to their characteristic effects on elevating [Ca]i. Carbachol stimulated the loss of Ki through multiple efflux pathways, including the large-conductance Ca-activated K channel. Carbachol and substance P increased the levels of intracellular sodium (Nai) to more than 2.5 times the normal level by stimulating the net uptake of sodium through multiple pathways; Na-K-2Cl cotransport accounted for greater than 50% of the influx, and approximately 20% was via Na-H exchange, which led to a net alkalinization of the cells. Ionomycin stimulated similar fluxes through these two pathways, but also promoted sodium influx through an additional pathway which was nearly equivalent in magnitude to the combined uptake through the other two pathways. The carbachol-induced increase in Nai and decrease in Ki stimulated the activity of the sodium pump, measured by the ouabain-sensitive rate of oxygen consumption, to nearly maximal levels. In the absence of extracellular calcium or in cells loaded with the calcium chelator BAPTA (bis[o-aminophenoxy]ethane-N,N,N',N'-tetraacetic acid) the magnitudes of agonist- or ionomycin-stimulated ion fluxes were greatly reduced. The parotid cells displayed a marked desensitization to substance P; within 10 min the elevation of [Ca]i and alterations in Ki, Nai, and cell volume spontaneously returned to near baseline levels. In addition to quantitating the activation of various ion flux pathways in the rat parotid acinar cell, these results demonstrate that the activation of ion transport systems responsible for fluid secretion in this tissue is closely linked to the elevation of [Ca]i.

Stimulation by ATP of inositol trisphosphate accumulation and calcium mobilization in cultured adrenal chromaffin cells

Effects of ATP on accumulation of inositol phosphates and Ca2+ mobilization were investigated in cultured bovine adrenal chromaffin cells. When the cells were stimulated with 30 microM ATP, a rapid and transient rise in intracellular Ca2+ concentration was observed. At the same time, ATP rapidly increased accumulation of inositol phosphates. The concentration-response curve for the ATP-induced Ca2+ mobilization was similar to that for inositol trisphosphate (IP3) accumulation. ATP exerted its maximal effects at 30 microM for either IP3 accumulation or Ca2+ mobilization. The order of the efficacy of the agonists for IP3 accumulation and Ca2+ mobilization at 100 microM was ATP greater than ADP greater than AMP approximately adenosine, AMP (100 microM) and adenosine (300 microM) failed to induce IP3 accumulation and Ca2+ mobilization. Although 100 microM GTP and 100 microM UTP also induced IP3 accumulation and Ca2+ mobilization, their efficacy was less than that of ATP. CTP (100 microM) induced a slight IP3 accumulation, but it did not induce Ca2+ mobilization. Nifedipine (10 microM), a Ca2+ channel antagonist, and theophylline (100 microM), a P1-purinergic receptor antagonist, failed to inhibit the ATP-induced IP3 accumulation and Ca2+ mobilization. The above two cellular responses induced by ATP were also observed in the Ca2+-depleted medium. ATP induced a rapid and transient accumulation of 1,4,5-IP3 (5s), followed by a slower accumulation of 1,3,4-IP3. These results suggest that ATP induces the formation of 1,4,5-IP3 through the P2-purinergic receptor and consequently promotes Ca2+ mobilization from intracellular storage sites in cultured adrenal chromaffin cells.

Colchicine-induced alterations in receptor-stimulated phosphoinositide hydrolysis in the rat hippocampus

Intradentate administration of colchicine has been reported to affect the cholinergic muscarinic system in the hippocampus, causing a reduction in quinclidinylbenzilate binding sites and an increase in choline acetyltransferase activity. Since the cholinergic muscarinic system is coupled to the formation of inositolphosphates in the brain, the effect of intradentate administration of colchicine on the agonist-induced turnover of inositollipid was studied in rats. The animals were sacrificed 12 weeks after injection, and the hippocampi removed and sliced. [3H]Inositol was incorporated into slices in the presence of lithium, and carbachol, a cholinergic muscarinic receptor agonist, was used to study the stimulated turnover of inositollipids. Hippocampal slices taken from colchicine-treated rats showed an increased carbachol-stimulated accumulation of inositolmonophosphate. Thus, intradentate colchicine appears to alter the signal transduction process for the muscarinic cholinergic receptor in the hippocampus, a change that may be associated with compensatory processes following damage to the hippocampus.

Ca2+ dependence of inositol 1,4,5-trisphosphate 3-kinase activity in the cytosol fraction of pig coronary artery

1. The activity of inositol 1,4,5-trisphosphate 3-kinase in subcellular fractions of smooth muscles of the pig coronary artery was examined. 2. Incubation of [3H]inositol 1,4,5-trisphosphate (IP3) with muscle homogenates produced more polar 3H-radioactivity (probably as inositol 1,3,4,5-tetrakisphosphate, IP4) than IP3, in the Mg2+- and ATP-dependent manner, thereby indicating the presence of IP3 3-kinase activity in homogenates of the muscle. 3. Most of the kinase activity was present in the cytosol fraction. The enzyme activity was reversibly activated by Ca2+ with a half-maximal effective concentration of 2.5 x 10(-7) M. 4. The calmodulin antagonists, W-7 and chlorpromazine inhibited the Ca2+-activated enzyme activity.

Bradykinin-induced changes in phosphoinositides, inositol phosphate production and intracellular free calcium in cultured bovine aortic endothelial cells

Acute hydrolysis of phosphoinositides has been demonstrated in bovine aortic endothelial cells (BAEC) treated with bradykinin (BK) (10(-7)M). The first phosphoinositide to decrease was phosphatidylinositol-4,5-bisphosphate (PIP2) indicating this to be the initial substrate of phospholipase action. Other lipid changes associated with the stimulation of BAEC were an increase in diacylglycerol (DAG) and arachidonic acid (AA) with a sustained production of phosphatidic acid (PA). The changes in cell phospholipids were accompanied by the release of inositol phosphates. Inositol-1,4,5-trisphosphate (Ins-1,4,5-P3) was produced within 10 s of stimulation with BK. There was no evidence for the production of inositol-1,3,4-trisphosphate. The release of ionic calcium (Ca2+) intracellularly was demonstrated. The timecourse of the rise in intracellular Ca2+ was consistent with the timecourse of production of IP3. Intracellular Ca2+ rose from 127 +/- 21 nM to 462 +/- 27 nM. The Ca2+ peak was at 7.0 +/- 0.4 s and took 3 min to reach a steady state which remained above the basal level. When extracellular Ca2+ was depleted in the extracellular medium a spike of intracellular Ca2+ release was measured with an immediate return to basal. Entry of extracellular Ca2+ into the cell after ionophore A23187 treatment does not induce inositol phosphate release, indicating that phosphoinositide hydrolysis is likely to be the cause rather than consequence of the elevation in cytosolic Ca2+. These data indicate action of phospholipase C (PLC) on PIP2 after BK stimulation of BAEC with the subsequent production of InsP3 causing the resulting intracellular Ca2+ release.(ABSTRACT TRUNCATED AT 250 WORDS)

Anion exchange chromatographic separation of inositol phosphates and their quantification by gas chromatography

The direct measurement of mass of inositol trisphosphate from biologic samples is described. Separation of inositol monophosphate, bisphosphate, trisphosphate, and inositol tetrakisphosphate was achieved using anion exchange chromatography with a sodium sulfate gradient. In addition, separation of the isomers of each inositol phosphate was performed using HPLC procedures. The individual inositol phosphate fractions were subsequently dephosphorylated and desalted. The myo-inositol from each fraction was then derivatized to the hexatrimethylsilyl derivative and the myo-inositol derivatives were quantified by a novel gas chromatographic analysis using the hexatrimethylsilyl derivative of chiro-inositol as an internal concentration reference. This method is a reproducible and relatively rapid procedure for the direct quantification of inositol phosphate mass which overcomes many of the problems associated with the use of radiolabeled precursors. The method is a significant improvement over existing procedures for the quantitative determination of the mass of inositol phosphate by virtue of improved recovery, sensitivity, and technical simplicity. The applicability of this method is illustrated by the quantitative determination of inositol trisphosphate in response to norepinephrine stimulation of adult canine myocytes and cerebral cortical brain slices and by measurement of the isomers of inositol trisphosphate in isolated myocytes.

Inositol 1:2-cyclic,4,5-trisphosphate is only a weak agonist at inositol 1,4,5-trisphosphate receptors

The activity of inositol 2,4,5-trisphosphate and inositol 1:2-cyclic,4,5-trisphosphate relative to inositol 1,4,5-trisphosphate was examined by two assays; firstly, in a binding assay using rat cerebellar membranes, and secondly, in a Ca2+-mobilization assay using permeabilized Swiss 3T3 cells. In both assays the first two phosphates have a potency at least an order of magnitude less than inositol 1,4,5-trisphosphate. The possible reasons for differences between these results and previous data are discussed, as are the implications for any putative physiological role for the cyclic trisphosphate.

Physiological localization of an agonist-sensitive pool of Ca2+ in parotid acinar cells

Muscarinic stimulation of fluid secretion by mammalian salivary acinar cells is associated with a rise in the level of intracellular free calcium ([Ca2+]i) and activation of a calcium-sensitive potassium (K+) conductance in the basolateral membrane. To test in the intact cell whether the rise of [Ca2+]i precedes activation of the K+ conductance (as expected if Ca2+ is the intracellular messenger mediating this response), [Ca2+]i and membrane voltage were measured simultaneously in carbachol-stimulated rat parotid acinar cells by using fura-2 and an intracellular microelectrode. Unexpectedly, the cells hyperpolarize (indicating activation of the K+ conductance) before fura-2 detectable [Ca2+]i begins to rise. This occurs even in Ca2+-depleted medium where intracellular stores are the only source of mobilized Ca2+. Nevertheless, when the increase in [Ca2+]i was eliminated by loading cells with the Ca2+ chelator bis(2-amino-5-methylphenoxy)ethane-N,N,N',N'-tetraacetate (Me2BAPTA) and stimulating in Ca2+-depleted medium, membrane hyperpolarization was also eliminated, indicating that a rise of [Ca2+] is required for the agonist-induced voltage response. Stimulation of Me2BAPTA-loaded cells in Ca2+-containing medium dramatically accentuates the temporal dissociation between the activation of the K+ conductance and the rise of [Ca2+]i. The data are consistent with the hypothesis that muscarinic stimulation results in a rapid localized increase in [Ca2+]i at the acinar basolateral membrane followed by a somewhat delayed increase in total [Ca2+]i. The localized increase cannot be detected by fura-2 but is sufficient to open the Ca2+-sensitive K+ channels located in the basolateral membrane. We concluded that a receptor-mobilized intracellular store of Ca2+ is localized at or near the basolateral membrane.

G-protein involvement in central-nervous-system muscarinic-receptor-coupled polyphosphoinositide hydrolysis

Potentiation of muscarinic-agonist-stimulated polyphosphoinositide (PPI) hydrolysis was demonstrated in a rat cerebral-cortical membrane preparation prelabelled with myo-[3H]inositol. Accumulation of myo-[3H]inositol 1,4-bisphosphate ([3H]IP2) was used to assess brain [3H]phosphatidylinositol 4,5-bisphosphate hydrolysis as its immediate metabolite, myo-[3H]inositol 1,4,5-trisphosphate, was rapidly hydrolysed to [3H]IP2. Inclusion of ATP (100 microM) and Mg2+ (5 mM) in the assay medium was necessary to demonstrate the effect of GTP analogues on carbachol-stimulated brain [3H]PPI turnover. Carbachol (100 microM) induced only a small increment in [3H]IP2 accumulation (142% of control) in 1 min. However, its effect was markedly enhanced, to 800% and 300% of control, by 100 microM-guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) respectively. GTP[S] and p[NH]ppG also stimulated [3H]IP2 accumulation by over 500% and 200% of control, respectively. The GTP-analogue-potentiated carbachol effect was antagonized by 10 microM-atropine, whereas the GTP-analogue stimulation was unaffected. This report confirms the involvement of a G (GTP-binding) protein(s) in brain PPI metabolism and provides new evidence for the role of G protein(s) in the coupling of stimulated muscarinic receptors to PPI hydrolysis in the central nervous system.

Oscillations of free cytosolic calcium evoked by cholinergic and catecholaminergic agonists in rat parotid acinar cells

1. In single, dissociated, rat parotid acinar cells the muscarinic agonist carbachol evokes a rapid rise in cytosolic free calcium [( Ca2+]i), from near 100 nM to peak levels of up to 1 microM. In the continued presence of the agonist the response decays to a lower, maintained, level. 2. In most cells, at 22 degrees C, oscillations, with a mean frequency of 0.19 Hz, are superimposed upon this elevation of [Ca2+]i. In voltage-clamped cells oscillations of current occur in phase with the oscillations of [Ca2+]i. 3. The oscillations occur in voltage-clamped cells, and in the absence of extracellular Ca2+, indicating that neither voltage-gated processes, or an influx of Ca2+ is involved. 4. Oscillation frequency is independent of carbachol concentration, in the range 100 nM to 250 microM, and furthermore, shows no relationship to the mean level of [Ca2+]i during the oscillations. 5. Stimulation with the alpha-adrenergic agonist noradrenaline, in the presence of the beta-blocker propanolol, evokes oscillations having the same frequency as those evoked by carbachol. 6. The oscillations show a strong temperature dependence, the frequency increasing with a Q10 of 2.8. In contrast, the amplitude of the oscillations drops from a mean of 33% of the response amplitude at 22 degrees C, and below, to 6% at 33 degrees C. Above the latter temperature oscillations are not resolvable. 7. The phorbol esters, 12-O-tetradecanoyl-phorbol-13-acetate and 12,13-phorbol dibutyrate (1 microM), do not affect the response to carbachol at 22 degrees C, at which temperature the oscillations are of maximum amplitude. Diacylglycerol is, therefore, unlikely to be involved in oscillation generation in these cells. 8. These observations are consistent with a model in which a negative feed-back loop links [Ca2+]i to the mechanisms of Ca2+ elevation, possibly to the inositol 1,4,5-trisphosphate-sensitive Ca2+ release mechanism of the endoplasmic reticulum. If the feed-back path involved an enzymatic step, the slowing of this step at lowered temperatures could give rise to oscillations. At body temperature such a mechanism would act to ensure that [Ca2+]i was elevated in a regulated and dose-dependent manner.

Decreased brain [3H]inositol 1,4,5-trisphosphate binding in Alzheimer's disease

Inositol 1,4,5-trisphosphate (IP3) receptor binding sites were studied in autopsied brains from 10 subjects with dementia of the Alzheimer type (DAT) and 10 age-matched controls. In the parietal cortex and hippocampus, there was a 50-70% loss of [3H]IP3 binding whereas no significant changes were observed in frontal, occipital and temporal cortices, caudate or amygdala. Scatchard analysis confirmed a reduction in receptor density rather than a change in affinity. Since muscarinic receptors are robustly coupled to IP3 formation, our data demonstrate abnormalities beyond the muscarinic receptor recognition site in DAT.

Gradient ion chromatography of inositol phosphates

Inositol phosphates including phytic acid were separated in 30 min by gradient ion chromatography with postcolumn derivatization. All four pentakisphosphates were resolved, while four tetrakisphosphate peaks were detected. The limits of detection for all polyphosphates, including tris- and bisphosphates, were between 1 and 2 nmol. The method was used to compare nonenzymatic dephosphorylation of inositol hexakisphosphate at pH 4.0 versus pH 10.8. The only pentakisphosphate detected in calf brains was identified as myo-inositol 1,3,4,5,6-pentakisphosphate. The major pentakisphosphate in raw soybean seeds was myo-inositol 1,2,4,5,6-pentakisphosphate of unknown enantiomeric composition, while lesser amounts of myo-inositol 1,2,3,4,5-pentakisphosphate of unknown enantiomeric composition, myo-inositol 1,2,3,4,6-pentakisphosphate, and myo-inositol 1,3,4,5,6-pentakisphosphate were also present.

Subcellular localization of the enzymes that dephosphorylate myo-inositol polyphosphates in human platelets

The phosphatase-induced hydrolysis of [3H]inositol 1,4-bisphosphate [Ins(1,4)P2)] and [3H]inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] was studied in platelet subcellular fractions. The activity that hydrolyses Ins(1,4)P2 is cytosolic, whereas the activity that hydrolyses Ins(1,4,5)P3 is present in both particulate and cytosolic fractions. The cytosolic Ins(1,4)P2 phosphatase hydrolyses the 1-phosphate of Ins(1,4)P2, whereas the cytosolic and membrane-bound Ins(1,4,5)P3 phosphatases hydrolyse the 5-phosphate of Ins(1,4,5)P3. In the presence of ATP, it is possible to observe a cytosolic Ins(1,4,5)P3 3-kinase that phosphorylates Ins(1,4,5)P3 to inositol 1,3,4,5-tetrakisphosphate. Apparent Km values for the particulate and the cytosolic Ins(1,4,5)P3 phosphatases are 100 microM and 40 microM respectively. A large proportion of the membrane-associated Ins(1,4,5)P3 phosphatase can be extracted with 1 M-NaCl, and the Mr of this enzyme, as determined by hydrodynamic studies, is 49,000, whereas that of the cytosolic enzyme is 59,000. The Km values for the cytosolic Ins(1,4)P2 phosphatase is 40 microM; this enzyme has an Mr of 49,000. The highest specific activity of the Ins(1,4,5)P3 phosphatase is present in a highly purified plasma-membrane fraction.

Polyamines inhibit phospholipase C-catalysed polyphosphoinositide hydrolysis. Studies with permeabilized GH3 cells

[3H]Inositol-labelled GH3 rat anterior pituitary tumour cells were permeabilized with digitonin and were incubated at 37 degrees C in the presence of ATP and Mg2+. [3H]Polyphosphoinositide breakdown and [3H]inositol phosphate production were stimulated by hydrolysis-resistant GTP analogues and by Ca2+. Of the nucleotides tested, guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) was the most effective stimulus. Activation by GTP gamma S appeared to be mediated by a guanine nucleotide-binding (G) protein as GTP gamma S-stimulated [3H]inositol phosphate production was inhibited by other nucleotides with a potency order of GTP = GDP = guanosine 5'-[beta-thio]diphosphate greater than ITP greater than GMP greater than UTP = CTP = adenosine 5'-[gamma-thio]triphosphate. The stimulatory effects of 10 microM-GTP gamma S on [3H]inositol phosphate levels were reversed by spermine and spermidine with IC50 values of approx. 0.25 and 2 mM respectively. Putrescine was inhibitory only at higher concentrations. Similarly, GTP gamma S-induced decreases in [3H]polyphosphoinositide levels were reversed by 2.5 mM-spermine. The inhibitory effects of spermine were not overcome by supramaximal concentrations of GTP gamma S. In contrast, [3H]inositol phosphate production stimulated by addition of 0.3-0.6 mM-Ca2+ to incubation media was only partially inhibited by spermine (5 mM), and spermine was not inhibitory when added Ca2+ was increased to 1 mM. These data show that polyamines, particularly spermine, inhibit phospholipase C-catalysed polyphosphoinositide hydrolysis with a marked selectivity towards the stimulatory effects of GTP gamma S.

Plasma from uninephrectomized rats stimulates production of inositol trisphosphates and inositol tetrakisphosphate in renal cortical slices

Metabolism of inositol phosphates in renal cortical slices was investigated in vitro after addition of plasma from uninephrectomized or sham-operated rats. Plasma from uninephrectomized rats stimulated production of InsP3 (inositol trisphosphate) when obtained within the first 3 h after uninephrectomy. With different amounts of added plasma a graded response of InsP3 production was obtained. This effect could be prevented by 0.1 microM-TPA (12-O-tetradecanoylphorbol 13-acetate). When analysis of inositol phosphates was performed by h.p.l.c., plasma from uninephrectomized rats stimulated a rapid increase in Ins(1,4,5)P3 radioactivity, whereas the increase in inositol 1,3,4,5-tetrakisphosphate and Ins(1,3,4)P3 radioactivity was slower. Plasma from uninephrectomized rats decreased cyclic AMP concentration in renal cortical slices. Similar effect was obtained when slices were incubated with TPA (0.05 microM). Plasma from uninephrectomized rats increased cyclic GMP concentration in renal cortical slices, but this effect was abolished when extracellular Ca2+ had been chelated with 4 mM-EGTA. Results indicate that plasma from uninephrectomized rats stimulates phospholipase C, increases cyclic GMP concentration and decreases cyclic AMP concentration in renal cortical slices. Increases in cyclic GMP depend on extracellular Ca2+, whereas the decrease in cyclic AMP concentration is mediated by protein kinase C.

Two components of hormone-evoked calcium release from intracellular stores of pancreatic acinar cells

Dispersed pancreatic acini loaded with Fura 2 were used to study the effect of hormonal stimulation on [Ca2+]i (free cytosolic Ca2+ concentration). Stimulation of acini with cholecystokinin octapeptide or carbachol resulted in two components of increase in [Ca2+]i. The maximal increase in [Ca2+]i and the time to maximum for both components was dependent on hormone concentration. The first component reached a maximum after 2-10 s of stimulation, whereas the second component required 30-60 s of stimulation for maximal effect. Both components of the [Ca2+]i increase can be observed in the presence or absence of Ca2+ in the incubation medium. The two components of Ca2+ release from intracellular stores showed similar dependency on agonist concentration. Termination of cell stimulation with specific antagonist revealed two, kinetically separated, rates of decrease in [Ca2+]i. The initial decrease in [Ca2+]i, was completed within 2.5-7 s, whereas the secondary decrease in [Ca2+]i, back to resting values, required approx. 40 s. The magnitude of the antagonist-induced initial (rapid) and secondary (slow) decrease in [Ca2+]i was dependent on the duration of cell stimulation. Hence it appears that stimulation of pancreatic acinar cells with Ca2+-mobilizing hormones results in two, kinetically separated, components of Ca2+ release from intracellular stores.

Regulation of receptor-mediated calcium influx across the plasma membrane in a human leukemic T-cell line: evidence of its dependence on an initial calcium mobilization from intracellular stores

It has been repeatedly shown that stimulation of a human leukemic T-cell line, JURKAT, by lectins such as phytohaemagglutinin and anti-T3 antibody (OKT3) leads to an elevation in the concentration of cytosolic free Ca2. This Ca2+ transient results from both an intracellular mobilization and an influx of Ca2+ through specific membrane channels. The objective of this study was to investigate the mechanism by which receptor-mediated influx of Ca2+ is regulated in JURKAT cells, which demonstrably lack 'voltage-dependent calcium channels'. It was found that upon increased loading with quin2 or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) there was a pronounced decline of both phytohaemagglutinin-stimulated and OKT3-stimulated influx of 45Ca2+. Using 15 microM quin2/AM or 30 microM BAPTA/AM, agonist-stimulated 45Ca2+ influx was almost totally abolished. At these concentrations of both quin2/AM or BAPTA/AM, phytohaemagglutinin and OKT3 could still induce a rise of cytosolic free Ca2+ above 200 nM. In the presence of La3+ (200 microM), which completely inhibited the agonist-induced 45Ca2+ influx, both phytohaemagglutinin and OKT3 were able to raise the concentrations of cytosolic free Ca2+ to well above 200 nM by merely mobilizing Ca2+ from intracellular stores alone. The data suggest that an agonist-induced increase in the concentration of cytosolic free Ca2+, due to mobilization from intracellular stores, could either directly or indirectly, initiate receptor-mediated Ca2+ influx across the plasma membrane in JURKAT cells.

Mechanism of Ca2+ release in medaka eggs microinjected with inositol 1,4,5-trisphosphate and Ca2+

The reaction time of Ca2+ release from cytoplasmic stores induced by microinjection of inositol 1,4,5-trisphosphate (IP3), calcium ionophore A23187, Ca2+, Sr2+, Ba2+, and cyclic guanosine 5'-monophosphate (cGMP) in Oryzias latipes eggs in Ca2+-free medium was measured by the luminescence of aequorin injected into the egg. Microinjection of IP3 or calcium ionophore induced rapid Ca2+ release without a time lag, while microinjection of either Ca2+ or cGMP required a time lag of 5-30 sec for Ca2+ release. Following microinjection of both IP3 and Ca2+, Ca2+ release commenced in a cytoplasmic region close to the egg surface. These results suggest that in the medaka egg, cytoplasmic Ca2+ induces Ca2+ release from cytoplasmic stores indirectly, probably via a membrane factor such as IP3.