Metabolism of inositol phosphates in alpha 1-adrenoceptor-stimulated and homogenized cardiac myocytes of adult rats

Inositol polyphosphates regulate Ca2+ efflux in a cardiac membrane subtype distinct from junctional sarcoplasmic reticulum

The membrane location and mechanism of inositol 1,3,4,5-tetrakisphosphate (InsP4)-regulated Ca2+ uptake in cardiac membrane vesicles was investigated. In canine and rat membranes separated by sucrose density gradient centrifugation, InsP4-regulated Ca2+ uptake was slightly more enriched in low density than in higher density membranes. Membranes supporting InsP4-regulated Ca2+ uptake were correspondingly enriched in type 1 InsP3 receptors. Junctional sarcoplasmic reticulum (J-SR), enriched in sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) and ryanodine receptors, separated predominantly with higher density membranes. In membranes supporting InsP4-regulated Ca2+ uptake, Ca2+ uptake was facilitated by a high Ca2+ affinity carrier that was insensitive to thapsigargin. Ca2+ uptake in J-SR was mediated by thapsigargin-sensitive SERCA2a. Net Ca accumulation was enhanced by oxalate in both SR subtypes. Although Ca2+-carrier-mediated Ca2+ uptake was ATP independent, ATP indirectly regulated net Ca2+ accumulation by modifying Ca2+ efflux via a Ca2+ channel with properties of type 1 InsP3 receptors. In the presence of < or = 0.1 mM ATP, InsP4 enhanced Ca2+ accumulation whereas InsP4 inhibited Ca2+ uptake at higher ATP concentrations. In the presence of 0.15 mM ATP, InsP4 stimulated Ca2+ efflux from vesicles preloaded with Ca. Several other InsP4 isomers and 1,3,4-InsP3 also stimulated Ca2+ efflux but with slightly less potency than 1,3,4,5-InsP4. Ruthenium red enhanced net Ca accumulation by the Ca2+ carrier and reduced the potency of ATP, InsP4, and InsP3 to stimulate Ca2+ efflux in vesicles. In summary, this investigation shows that a Ca2+ carrier facilitates Ca loading in a sarcoplasmic reticulum subtype distinct from J-SR. InsP4 and InsP3 are proposed to regulate Ca2+ efflux in low density SR by acting on an ATP-modulated Ca2+ channel with properties of type 1 InsP3 receptors.

Effect of concomitant beta-adrenoceptor stimulation on alpha 1-adrenoceptor-mediated increase of inositol-1,4,5-trisphosphate mass in adult rat cardiomyocytes

The aim of the present study was to investigate the accumulation of inositol-1,4,5-trisphosphate (IP3) in isolated adult rat ventricular cardiomyocytes after alpha 1- and beta-adrenoceptor stimulation, separate and in combination, in order to elucidate a possible influence of concomitant beta-adrenoceptor stimulation on the alpha 1-adrenoceptor stimulated response. IP3 was measured by a radioligand binding assay based on an (1,4,5)IP3-specific binding protein from bovine adrenal cortex. The basal IP3 content was 4.06 +/- 0.31 pmol/mg protein (N = 56). alpha 1-Adrenoceptor stimulation resulted in a rapid increase in the IP3 level, which reached a plateau, 50-80% above basal level, at 10-30 sec. The plateau lasted at least up to 120 sec., while at 300 sec. there was no significant difference between control values and values after alpha 1-adrenoceptor stimulation. Li+ did not affect either the basal IP3 level, or the magnitude or time course of alpha 1-adrenoceptor-stimulated IP3 accumulation. Combined adrenoceptor stimulation gave a similar response as separate alpha 1-adrenoceptor stimulation, whereas there was no significant change in the IP3 level after beta-adrenoceptor stimulation. No inhibitory influence of simultaneous beta-adrenoceptor stimulation on the alpha 1-adrenoceptor-stimulated increase of IP3 mass was revealed.

Expression of IGF receptors on alveolar macrophages: IGF-induced changes in InsPi formation, [Ca2+]i, and pHi

Expression of insulin-like growth factor-I (IGF-I) receptors and insulin-like growth factor-II/mannose-6-phosphate (IGF-II/Man6P) receptors in cultured bovine alveolar macrophages (BAM) was demonstrated by competitive binding studies and crosslinking experiments. Western blotting of protein extracts from cultured BAM using an anti bovine IGF-II/Man6P receptor antiserum (#66416) confirmed the presence of IGF-II/Man6P receptors on BAM. The effects of IGFs and Man6P on generation of inositol phosphates was measured by HPLC analysis of perchloric acid extracts from myo-[3H]inositol-labelled cultured BAM. IGF-I at nanomolar concentrations and Man6P (10[-8]-10[-3] M) stimulated the accumulation of both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 after 30 sec. IGF-II (up to 2.3 x 10[-8] M) had no significant effect on inositol phosphate accumulation under the same conditions. Both IGFs and Man6P induced a rise in [Ca2+]i in cultured BAM. In addition, using the fluorescent dye SNARF-1/AM we could demonstrate rapid but small IGF-II (10[-9] M) triggered acidification (0.07 pH units) of cultured BAM. Taken together, our results indicate not only the presence of both IGF-I and IGF-II/Man6P receptors on BAM, but also provide evidence of the linkage of the IGF-I receptor to the inositol phosphate system.

Inositol-1,4,5-trisphosphate mass content in isolated perfused rat heart during alpha-1-adrenoceptor stimulation

Inositol-1,4,5-trisphosphate (IP3) has been proposed to be a second messenger in response to alpha-1-adrenoceptor stimulation also in myocardial cells. We studied the effect of alpha-1-adrenoceptor stimulation (5 x 10(-5) mol/l phenylephrine or 5 x 10(-5) mol/l noradrenaline both in the presence of 10(-6) mol/l timolol) on IP3 mass content in isolated perfused rat hearts. IP3 content was determined by a specific receptor-binding assay-kit (TRK 1000, Amersham) after validating the method. For comparison also the effect of muscarinic stimulation (10(-4) mol/l carbachol in the presence of 10(-6) mol/l timolol) on IP3 content was measured in corresponding preparations. A basal IP3 level of about 75 pmol/mg protein was found. There were no prominent effects of alpha-1-adrenoceptor stimulation on total IP3 content in isolated perfused rat hearts. Phenylephrine gave a statistically significant increase of about 40% at 1/4 min and a statistically significant decrease of about 25% at 4 min after start of exposure. Noradrenaline, however, gave no statistically significant change of IP3 at the time-points studied. Muscarinic stimulation caused a slight, statistically insignificant, increase of IP3 at 1/4 min. The results are compatible with an assumption that agonist stimulation evokes a localized increase of IP3 which may be masked by a relatively high total IP3 mass content. The IP3 peak after phenylephrine coincided with the early positive inotropic phase of the response reported earlier in perfused rat hearts for alpha-1-adrenoceptor stimulation by phenylephrine. Although this might be compatible with a role for IP3 in this early and transient phase, a mediator function of IP3 in the inotropic response is not established.

Signals mediating stimulation of cardiomyocyte glucose transport by the alpha-adrenergic agonist phenylephrine

Phenylephrine, a potent stimulator of cardiomyocyte glucose transport (GT), caused a rapid rise in cytosolic Ca2+ by 30%. Agents inducing a similar Ca2+ response did not stimulate (angiotension II, vasopressin) or inhibited GT by 20% (elevated extracellular Ca2+). Stimulation of GT by phorbol myristate acetate was additive to both phases of phenylephrine's effect (4 min, 60 min). Phenylephrine had no influence on the adenosine 3', 5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) levels. Agents raising cAMP (isoproterenol) or cGMP (e.g., nitroprusside) did not stimulate GT. Wortmannin (inhibitor of 1-phosphatidylinositol 3-kinase) suppressed the action of insulin on GT but not that of phenylephrine. In contrast, the Na+/H+ exchange inhibitor amiloride (which blocks phenylephrine-induced cytosolic alkalinization or even lowers cellular pH) depressed the effect of phenylephrine by 50%, whereas insulin-stimulated GT was little affected. However, raising extracellular pH up to 8.4 failed to increase GT. Lowering pH to 6.8 decreased phenylephrine's effect by 40% whereas insulin-dependent GT was not significantly altered. Clorgyline, tranylcypromine (monoamine oxidase inhibitors), and added catalase suppressed the slow phase of phenylephrine's action, whereas amiloride also affected the fast phase. We conclude that 1) stimulation of cardiomyocyte GT by phenylephrine does not involve cAMP, cGMP, or 1-phosphatidylinositol 3-kinase; 2) protein kinase C activation cannot explain the full extent of stimulation; 3) Ca2+ release or cytosolic alkalinization may be required but is not sufficient to trigger phenylephrine's action, and 4) the slow phase of stimulation is mediated by the monoamine oxidase-dependent degradation of phenylephrine and by the resulting H2O2 formation.

Inositol phosphates in the heart: controversy and consensus

Numerous studies have addressed various aspects of inositol phosphate release and metabolism in myocardial preparations, and many different viewpoints have been expressed. The various results and interpretations presented often appear confusing and extracting a consensus view can be difficult. The differences often derive from the differing cardiac preparations used, especially isolated cells versus intact tissue. Despite these problems there are aspects where consensus prevails. Both the metabolism and the functional activity of inositol phosphates in heart appear to differ from those previously described in non-excitable cells. Inositol phosphates do not appear to be of major importance in the control of cardiac function under physiological conditions but may well have greater influence under pathological conditions such as myocardial ischaemia and reperfusion. Hopefully, the near future will see remaining controversies resolved.

Ins 1,4,5-P3 and Ca2+ signaling in quiescent neonatal cardiac myocytes

Activation of alpha 1-adrenergic receptors in neonatal cardiac myocytes results in changes in contractile activity and the induction of hypertrophic growth. The biochemical mechanisms responsible for these diverse effects are not yet established, but presumably involve the associated alpha 1-adrenergic stimulation of phosphatidylinositol (PI) hydrolysis, with concomitant generation of Ins 1,4,5-P3 and diacylglycerol. This study examined whether alpha 1-adrenergic generation of Ins 1,4,5-P3 in intact, quiescent, neonatal cardiac myocytes resulted in a Ca2+ signal. Stimulation of myocytes with norepinephrine in the presence of propranolol caused accumulation of inositol mono-, bis and trisphosphates. However, alpha 1-adrenergic stimulation did not alter cytosolic free Ca2+ levels in 85% of the myocytes examined. Direct generation of Ins 1,4,5-P3, by photolysis of microinjected caged Ins 1,4,5-P3, was also unable to alter cytosolic free Ca2+ levels, despite the presence of Ins 1,4,5-P3 receptors. Taken together, these data indicated that alpha 1-adrenergic stimulation did not initiate Ca2+ signaling because Ins 1,4,5-P3-induced Ca2+ mobilization was not operative in quiescent neonatal cardiac myocytes. Normal excitation-contraction Ca2+ handling mechanisms were present in these cells, as illustrated by depolarization- and caffeine-induced Ca2+ transients. Analysis of these same myocytes following 48 h in the presence of norepinephrine and propranolol showed a 40% increase in the ratio of protein to DNA and a 350% increase in release of atrial naturietic factor, compared to control cells, indicating the normal operation of alpha 1-adrenergic-induced hypertrophic growth. Therefore, the assumption that Ca(2+)-dependent processes will be activated by receptor signaling pathways coupled to enhanced phosphatidylinositol turnover in cardiac cells must be avoided. In addition, the data presented in this study clearly indicated that an increase in cytosolic free Ca2+ was not necessary for the induction of alpha 1-adrenergic-mediated cardiac hypertrophy.

Analysis of [3H]inositol phosphate formation and metabolism in cerebral-cortical slices. Evidence for a dual metabolism of inositol 1,4-bisphosphate

Muscarinic-receptor-mediated phosphoinositide hydrolysis in rat cerebral cortex was investigated by analysis of the kinetics of [3H]inositol phosphate formation and degradation in myo-[2-3H]inositol-labelled tissue slices. Carbachol stimulated rapid (5 s) increases in the concentrations of [3H]Ins(1,4,5)P3, [3H]Ins(1,3,4,5)P4 and [3H]Ins(1,4)P2. Stimulated accumulation of [3H]Ins(1,3,4)P3, [3H]Ins(1,3)P2 and [3H]Ins(3,4)P2 and [3H]Ins(1/3)P or of [3H]Ins(4)P occurred only subsequently and with a sequence indicating formation by successive dephosphorylation of [3H]Ins(1,3,4,5)P4 or of Ins(1,4)P2 respectively. A similar sequence was inferred from the order of rapidity with which the accumulations of [3H]inositol polyphosphates, resulting from sustained (5 min) carbachol stimulation in the presence of LiCl, were reversed when muscarinic receptors were subsequently blocked with atropine. During this latter period of receptor blockade, radiolabel lost from [3H]inositol polyphosphates was quantitively recovered as [3H]inositol monophosphates owing to effective inhibition of monophosphatase by Li+, and the rate of poly- into mono-phosphate conversion was similar to agonist-stimulated rates of monophosphate accumulation. This implies that, even during persistent stimulation, polyphosphoinositide, not PtdIns, is the substrate for phosphoinositidase C. Quantitative comparison of the degradation of [3H]inositol poly- to mono-phosphates after receptor blockade unexpectedly suggests the dual hydrolysis of [3H]Ins(1,4)P2 to [3H]Ins(1)P and [3H]Ins(4)P. This result advises cautious interpretation of the origin of [3H]Ins(1)P in stimulated tissue, but, with other data presented, allows calculation from the observed ratio of [3H]Ins(1/3)P:[3H]Ins(4)P that a minimum of approx. 50% of the [3H]Ins(1,4,5)P3 produced during persistent muscarinic-receptor stimulation is metabolized by Ins(1,4,5)P3 3-kinase.

The alpha 1-adrenergic receptor in human erythrocyte membranes mediates interaction in vitro of epinephrine and thyroid hormone at the membrane Ca(2+)-ATPase

Membrane Ca(2+)-ATPase activity was stimulated in vitro separately by T4 (10(-10) M) and by epinephrine (10(-6) M). In the presence of a fixed concentration of T4, additions of 10(-8) and 10(-6) M epinephrine reduced the T4 effect on the enzyme. beta-Adrenergic blockade with propranolol (10(-6) M) prevented stimulation by epinephrine of Ca(2+)-ATPase activity, but did not prevent the suppressive action of epinephrine on T4-stimulable Ca(2+)-ATPase. In contrast, alpha 1-adrenergic blockade with unlabelled prazosin restored the effect of T4 on Ca(2+)-ATPase activity in the presence of epinephrine. Like propranolol, prazosin prevented enhancement of enzyme activity by epinephrine in the absence of thyroid hormone. Neither prazosin nor propranolol had any effect on the stimulation by T4 of red cell Ca(2+)-ATPase in the absence of epinephrine. Analysis of radiolabelled prazosin binding to human red cell membranes revealed the presence of a single class of high-affinity binding sites (Kd, 1.2 x 10(-8) M; Bmax, 847 fmol/mg membrane protein). Thus, the human erythrocyte membrane contains alpha 1-adrenergic receptor sites that are capable of regulating Ca(2+)-ATPase activity.

Abnormalities in cardiac alpha 1-adrenoceptor and its signal transduction in streptozocin-induced diabetic rats

To investigate a mechanism of diabetic cardiomyopathy, we examined an alteration of cardiac alpha 1-adrenoceptor (alpha 1-AR) signaling in streptozotocin-induced diabetic rats. In diabetes, the cell surface alpha 1-AR concentration on isolated cardiac myocytes decreased by 45% without any change in the dissociation constant, and, moreover, norepinephrine (NE)-stimulated ventricular inositol 1,4,5-trisphosphate (IP3) production was also decreased by 34%. In contrast, basal ventricular protein kinase C (PKC) activity was elevated in both cytosolic (by 98%) and membrane (by 41%) fractions in diabetes. All of these abnormalities seen in diabetes were reversed by chronic insulin treatment. Rapid activation of PKC by phorbol ester in the normal rat heart revealed decreases in both receptor number (by 19%) and NE-stimulated IP3 production (by 21%). These results indicate that the impairment of cardiac alpha 1-AR signaling is closely associated with the diabetic state and may be linked, at least in part, with the abnormal activation of cardiac PKC.

Mechanisms for the positive inotropic effect of alpha 1-adrenoceptor stimulation in rat cardiac myocytes

alpha 1-Adrenoceptor activation can enhance myocardial contractility, and two possible inotropic mechanisms are an increase in myofilament Ca2+ sensitivity and action potential prolongation, which can increase net Ca2+ entry into cells. In adult rat ventricular myocytes (bath Ca2+, 1 mM; stimulated at 0.2-0.5 Hz), the drug 4-aminopyridine and the whole-cell voltage clamp have been used to control Ca2+ entry and differentiate between the two mechanisms. At 22-23 degrees C the specific alpha 1-adrenoceptor agonist methoxamine (100 microM) prolonged action potential duration at 50% repolarization from 55 +/- 2 to 81 +/- 5 msec, delayed time to peak contraction, and increased shortening amplitude from 5.3 +/- 0.6 to 7.8 +/- 1 microns (n = 18). Reduction of the transient outward current and other K+ currents by methoxamine was the major cause of action potential prolongation in rat myocytes with little change in the L-type calcium current. Block of the transient outward current with 2 mM 4-aminopyridine prolonged action potential duration from 52 +/- 6 to 98 +/- 12 msec and increased unloaded cell shortening from 2.9 +/- 0.4 to 6.6 +/- 0.6 microns (n = 4). Subsequently, methoxamine no longer increased cell shortening, although significant potentiation of twitch amplitude was still seen after a brief rest interval. In voltage-clamp experiments, with 70-500-msec pulses, although membrane currents were reduced, methoxamine had no positive inotropic effect and reduced cell shortening from 5.3 +/- 0.7 to 4.97 +/- 0.8 microns at pulse potentials positive to -40 mV. Similar alpha 1-adrenoceptor responses were observed at 35 degrees C during action potential and voltage-clamp experiments, which could be blocked by 10 microM prazosin. In myocytes loaded with the Ca2+ indicator indo-1, alpha 1-adrenoceptor stimulation or 4-aminopyridine both increased cell contraction and intracellular Ca2+ transients by similar amounts. As in unloaded cells, prior exposure to 4-aminopyridine prevented any inotropic effect of methoxamine without changing the systolic intracellular Ca2+ transient. The results indicated that under our experimental conditions positive inotropy in rat cardiomyocytes on exposure to alpha 1-adrenoceptor agonists was strongly correlated with the action potential prolongation that accompanied K+ current reduction. In addition, modulation of K+ channels could occur independent of changes in contractility and/or [Ca2+]i.

Evidence for phosphatidylinositol hydrolysis in pancreatic islets stimulated with carbamoylcholine. Kinetic analysis of inositol polyphosphate metabolism

Anion-exchange h.p.l.c. was used initially to analyse the products formed after addition of either [3H]Ins(1,3,4,5)P4 or [3H]Ins(1,4,5)P3 to homogenates of pancreatic islets. Metabolic routes similar to those of other tissues were established: dephosphorylation of Ins(1,4,5)P3 to Ins(1,4)P2 and then Ins4P; and sequential degradation of Ins(1,3,4,5)P4 to Ins(1,3,4)P3, Ins(3,4)P2 and Ins(3 or 1)P. In addition, there was a limited conversion of Ins(1,3,4)P3 into Ins(1,3)P2. After stimulation of [3H]inositol-prelabelled islets with the muscarinic-receptor agonist carbamoylcholine (carbachol), there was a rapid (10 s) increase in Ins(1,4,5)P3, Ins(1,3,4)P3, Ins(1,4)P2 and Ins4P. In the presence of 10 mM-LiCl, Ins1P was also significantly increased (P less than 0.05) by 5 s, before any increase in Ins4P (10 s), Ins(1,3)P2 (60 s) or Ins(3,4)P2. When carbachol was displaced with atropine, after 1 h pre-stimulation, the maximal decreases in Ins(1,4,5)P3 and Ins1P from the stimulated steady state (5 s) clearly preceded those of the other metabolites. These declines were used to calculate the turnover times and rate of metabolic flux through the various inositol phosphates. These experiments confirmed the relatively minor importance of the Ins(1,3)P2 pathway (less than 10% of the total flux) and demonstrated that Ins(1,4,5)P3 removal was evenly distributed through the Ins(1,4)P2 and Ins(1,3,4,5)P4 routes. They also established that flux through Ins1P was 8-fold greater than that through Ins(1,4,5)P3, indicating that the former could not have been derived from PtdInsP2 hydrolysis. Similarly, in islets pretreated with neomycin, which binds to PtdInsP2 with greater affinity than to PtdIns, the increase in Ins1P caused by 1 min stimulation with carbachol was not affected, despite virtual abolition of the increase in Ins4P, and an overall inhibition of PtdInsP2 hydrolysis by 67%. The results indicate that, in addition to PtdInsP2 breakdown, carbachol also promotes a rapid PtdIns hydrolysis which becomes increasingly predominant with prolonged stimulation.

Isolation of adult cardiomyocytes initiates a return of inositol trisphosphate phosphorylating activity

1. We have previously reported that the addition of noradrenaline to [3H]-inositol-labelled adult rat atria or isolated perfused hearts caused the release of inositol-1,4,5-trisphosphate, which was metabolized by dephosphorylation to inositol-4-monophosphate. Inositol-1,3,4,5-tetrakisphosphate and its dephosphorylation products were not detected. 2. In the current study, the addition of noradrenaline to [3H]-inositol-labelled adult rat cardiomyocytes caused the release of inositol-1,4,5-trisphosphate, which was metabolized in part by phosphorylation to inositol-1,3,4,5-tetrakisphosphate. 3. These results demonstrate that the isolation and culture of rat adult cardiomyocytes initiates enhanced generation of inositol-1,3,4,5-tetrakisphosphate. This change would be expected to enhance the calcium response of the cells to stimulation of alpha 1-adrenoceptors.

Inositol tetrakisphosphates as second messengers induce Ca(++)-dependent chloride currents in Xenopus laevis oocytes

Microinjection of inositol 1,3,4,5-tetrakisphosphate or inositol 1,4,5-trisphosphate induced distinct chloride membrane currents in defolliculated Xenopus laevis oocytes. To decide whether these Cl(-)-currents were due to the injected compounds or their metabolic products, [3H]Ins(1,3,4,5)P4 or [3H]Ins(1,4,5)P3 were injected into oocytes and their metabolites were analyzed by HPLC. Our results indicate that Ins(1,3,4,5)P4 itself or its metabolite Ins(1,3,4,6)P4 is able to induce Cl(-)-membrane currents, most likely by increasing the cytosolic Ca(++)-concentration.

The effect of ischaemia and reperfusion on sarcolemmal inositol phospholipid and cytosolic inositol phosphate metabolism in the isolated perfused rat heart

In this study the mass of polyphosphoinositides as well as the turnover of [3H]inositol phospholipids and [3H]inositol phosphates during ischaemia and short periods of reperfusion were studied in the isolated perfused rat heart. Since the phosphoinositides located within the sarcolemma are precursors for release of inositoltrisphosphate (InsP3) and diacylglycerol, sarcolemmal membranes (rather than whole tissue) isolated at the end of the experimental procedure, were used. Hearts were prelabelled with [3H]inositol and subsequently perfused with 10 mM LiCl to block the phosphatidylinositol (PI) pathway. The results showed that 20 min of global ischaemia depressed the amount of [3H]inositol present in both sarcolemmal phosphatidylinositol-4-phosphate (PI-4-P) and phsophatidylinositol-4,5-bisphosphate (PI-4,5-P2), as well as in the cytosolic [3H]inositol phosphates, [3H]InsP2 and [3H]InsP3. The mass of the sarcolemmal inositol phospholipids remained unchanged during ischaemia. Reperfusion caused an immediate (within 30 sec) increase in the amount of [3H]inositol in sarcolemmal PI, PI-4-P and PI-4,5-P2. PI-4-P levels showed a transient increase after 30 seconds postischaemic reperfusion, while the mass of the other sarcolemmal inositol phospholipids, PI and PI-4,5-P2, remained unchanged. [3H]InsP, [3H]InsP2 and [3H]InsP3 also increased significantly in comparison to ischaemic hearts after only 30 sec postischaemic reperfusion. In summary, the results obtained indicate inhibition of the PI pathway during ischaemia with an immediate significant stimulation upon reperfusion. In view of the capacity of InsP3 to mobilize Ca2+, the possibility exists that stimulation of this pathway during reperfusion may play a role in the intracellular Ca2+ overload, characteristic of postischaemic reperfusion.

Metabolism of inositol phosphates in ATP-stimulated vascular endothelial cells

The accumulation of InsP1, InsP2, InsP3 and InsP4 isomers was investigated in bovine aortic endothelial cells labelled with [3H]inositol and stimulated with ATP. The separation of these isomers was performed by ion-pairing reverse-phase h.p.l.c. on a mu Bondapack C18 column for the InsP3 and InsP4 isomers and by ion-exchange h.p.l.c. on a Partisil SAX column for the InsP1 and InsP2 isomers. In unstimulated endothelial cells, a large amount of material was co-eluted with InsP5 and InsP6, whereas amounts of InsP3 and InsP4 were small. The addition of ATP (100 microM) induced a striking (35-fold stimulation) and transient increase of Ins(1,4,5)P3 that was maximal around 15 s. This peak was followed by a more sustained accumulation of Ins(1,3,4,5)P4 and Ins(1,3,4)P3, but the amounts of these two metabolites accumulated in response to ATP were much smaller than that of Ins(1,4,5)P3. The increase in InsP2 isomers in response to ATP had similar characteristics: a rapid and transient accumulation of Ins(1,4)P2, followed by an increase of Ins(3,4)P2 and Ins(1,3)P2, which was more sustained but had a smaller magnitude. ATP also induced the accumulation of both Ins1P and Ins4P, but with different time courses: the level of Ins4P was maximal at 1 min (60 times the control value) and returned to baseline after 5 min, whereas the increase in Ins1P was undetectable at 1 min and reached a maximum after 5 min, which represented 240% of the basal level. These data indicate that Ins(1,4,5)P3, which is rapidly formed in aortic endothelial cells as a result of activation of P2Y receptors, is preferentially metabolized at early times (less than 1 min) by a 5-phosphatase, with the sequential formation of Ins(1,4)P2 and Ins4P. Afterwards, a small but sustained increase in the content of Ins(1,3,4)P3, Ins(1,3)P2, Ins(3,4)P2 and Ins1P was observed, reflecting the activation of the Ins(1,4,5)P3 3-kinase.

Alpha-adrenergic regulation of phosphoinositide metabolism and protein kinase C in isolated cardiac myocytes

alpha 1-Adrenergic regulation of phosphoinositide metabolism and protein kinase C translocation was studied in isolated rat cardiac myocytes. Exposure of [3H]inositol-labeled myocytes to norepinephrine in the presence of propranolol caused a dose-dependent increase in [3H]inositol phosphates. Norepinephrine also increased the level of membrane-associated protein kinase C from approximately 10% of total activity to 18%, with a dose response similar to that for generation of inositol phosphates. Depolarization of myocytes with 30 mM KCl had no effect on inositol phosphates or membrane-associated protein kinase C but potentiated the effect of submaximal norepinephrine on both parameters. The potentiation of protein kinase C translocation was amplified when extracellular Ca2+ was increased to 4 mM, resulting in membrane association of one-third of the total cellular activity. These data show that activation of protein kinase C occurs during alpha 1-adrenergic stimulation of cardiac myocytes and that elevation of intracellular Ca2+ amplifies this effect at least in part through increased phosphoinositide metabolism.

Kinetics of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate generation in dog-thyroid primary cultured cells stimulated by carbachol

The action of carbachol on the generation of inositol trisphosphate and tetrakisphosphate isomers was investigated in dog-thyroid primary cultured cells radiolabelled with [3H]inositol. The separation of the inositol phosphate isomers was performed by reverse-phase high pressure liquid chromatography. The structure of inositol phosphates co-eluting with inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] standards was determined by enzymatic degradation using a purified Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphatase. The data indicate that Ins(1,3,4,5)P4 was the only [3H]inositol phosphate which co-eluted with a [32P]Ins(1,3,4,5)P4 standard, whereas 80% of the [3H]InsP3 co-eluting with an Ins(1,4,5)P3 standard was actually this isomer. In the presence of Li+, carbachol led to rapid increases in [3H]Ins(1,4,5)P4. The level of Ins(1,4,5)P3 reached a peak at 200% of the control after 5-10 s of stimulation and fell to a plateau that remained slightly elevated for 2 min. The level of Ins(1,3,4,5)P4 reached its maximum at 20s. The level of inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] increased continuously for 2 min after the addition of carbachol. Inositol-phosphate generation was also investigated under different pharmacological conditions. Li+ largely increased the level of Ins(1,3,4)P3 but had no effect on Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Forskolin, which stimulates dog-thyroid adenylate cyclase and cyclic-AMP accumulation, had no effect on the generation of inositol phosphates. The absence of extracellular Ca2+ largely decreased the level of Ins(1,3,4,5)P4 as expected considering the Ca2(+)-calmodulin sensitivity of the Ins(1,4,5)P3 3-kinase. Staurosporine, an inhibitor of protein kinase C, increased the levels of Ins(1,4,5)P3, Ins(1,3,4,5)P4 and Ins(1,3,4)P3. This supports a negative feedback control of diacyglycerol on Ins(1,4,5)P3 generation.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.

Endothelin stimulates phosphatidylinositol turnover in rat right and left atria

The effect of endothelin on phosphatidylinositol (PI) turnover has been investigated in isolated rat atria by measuring the generation of inositol phosphates (IPs) following [3H]inositol phospholipid labelling. In the presence of 10 mM LiCl, endothelin caused dose-dependent increases in the accumulations of inositol mono-, bis- and tris-phosphate (IP1, IP2 and IP3) which were maximal at 10(-6) M endothelin. The dose-response relationship was similar in right and left atria, but right atria showed a higher maximal IP response. Endothelin produced a rapid and transient stimulation of IP3 accumulation, which was maximal at 30 s followed by a slower increase which continued linearly past 20 min. During both the initial phase and the sustained phase the only isomer of IP3 present at detectable levels was the 1,4,5-isomer. As with endothelin, responses to noradrenaline also were higher in right atria compared with left atria and showed a biphasic pattern of release of IP3. These data demonstrate that endothelin receptors in rat atria are coupled to stimulation of the PI turnover pathway in an apparently similar manner to alpha 1-adrenoceptors. The PI pathway may be important in mediating the reported cardiac actions of endothelin.