Cyclic AMP-dependent protein kinase and Ca2+-calmodulin stimulate the formation of polyphosphoinositides in a sarcoplasmic reticulum preparation of rabbit heart

Cardiac protein phosphorylation: functional and pathophysiological correlates

Protein phosphorylation acts a pivotal mechanism in regulating the contractile state of the heart by modulating particular levels of autonomic control on cardiac force/length relationships. Early studies of changes in cardiac protein phosphorylation focused on key components of the excitation-coupling process, namely phospholamban of the sarcoplasmic reticulum and myofibrillar troponin I. In more recent years the emphasis has shifted towards the identification of other phosphoproteins, and more importantly, the delineation of the mechanistic and signaling pathways regulating the various known phosphoproteins. In addition to cAMP- and Ca(2+)-calmodulin-dependent kinase processes, these have included regulation by protein kinase C and the ever-emerging family of growth factor-related kinases such as the tyrosine-, mitogen- and stress-activated protein kinases. Similarly, the role of protein dephosphorylation by protein phosphatases has been recognized as integral in modulating normal cardiac cellular function. Recent studies involving a variety of cardiovascular pathologies have demonstrated that changes in the phosphorylation states of key cardiac regulatory proteins may underlie cardiac dysfunction in disease states. The emphasis of this comprehensive review will be on discussing the role of cardiac phosphoproteins in regulating myocardial function and pathophysiology based not only on in vitro data, but more importantly, from ex vivo experiments with corroborative physiological and biochemical evidence.

Norepinephrine and thyrotropin stimulation of [Ca++]i in PC C13 a rat thyroid epithelial cell line: effect of transformation by E1A gene adenovirus and polyomavirus middle-T antigen gene

The effect of thyrotropin and norepinephrine on cytosolic calcium levels were evaluated in normal (PC C13) and transformed (PC E1A, PC Py and PC E1APy) rat thyroid epithelial cell lines. A different pattern of response to both norepinephrine and thyrotropin was observed among the distinct cell lines. In PC C13 the cytosolic calcium rise induced by norepinephrine, characterized by an early transient spike followed by a second phase of sustained calcium levels, was greatly enhanced by thyrotropin. The effect of norepinephrine on calcium concentrations was less affected by thyrotropin in PC C13 transformed by the adenovirus E1A oncogene. Conversely, in Polyoma middle-T transformed PC C13 the increase in cytoplasmic calcium was still sensitive to thyrotropin. The most malignant PC E1APy were totally independent of thyrotropin.

Occurrence and functions of the phosphatidylinositol cycle in the myocardium

In the last decade a great deal of attention was awarded to a signal transduction pathway which is utilized primarily by 'Ca2+ mobilizing' signal molecules and which involves the hydrolysis of a quantitatively minor phospholipid, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) by a PtdIns-specific phospholipase C (PLC). The evidence for the existence of receptor-mediated GTP binding protein-coupled PLC in myocardium and its possible functions are briefly summarized. The minireview is concentrated on the following aspects: 1) cellular localization and synthesis of polyphospho-PtdIns from PtdIns, 2) desensitization of the alpha 1-adrenergic agonist and endothelin-1 mediated PtdIns responses, 3) oscillatory Ca2+ transients initiated by PtdIns(4,5)P2 hydrolysis, 4) polyunsaturated fatty acids as constituents of polyphospho-PtdIns and of the protein kinase C activator 1,2-diacylglycerol (DAG), 5) source other than PtdIns(4,5)P2 contributing to the stimulated DAG, 6) role of the PtdIns pathway in cardiomyocyte growth and gene expression during the hypertrophic response.

Changes in phosphoinositide turnover in isolated guinea pig hearts stimulated with isoproterenol

The incorporation of 32Pi into phospholamban, troponin I, phosphatidylinositols, and inositol trisphosphates was studied in Langendorff-perfused guinea pig hearts stimulated with isoproterenol. Hearts were perfused with Krebs-Henseleit buffer containing [32P]Pk and freeze-clamped at different times during the positive inotropic response. Exposure of the hearts to 0.1 microM isoproterenol for up to 1 minute was associated with significant (up to threefold) increases in phospholamban and troponin I phosphorylation, but there was no significant increase in 32P incorporation into phospholipids. However, longer exposure (2 minutes or more) to isoproterenol was associated with increases in the degree of 32P labeling of phosphatidylinositols and phosphatidic acid. Examination of 32P labeling of inositol trisphosphates in the same hearts revealed that the radioactivity associated with these compounds decreased with time. The decreases were significant at times of exposure of 2 minutes or longer to beta-adrenergic stimulation. The tissue levels of the inositol 1,4,5-trisphosphate isoform were also measured in hearts perfused with isoproterenol for 3 minutes, and they were found to be significantly lower compared with values obtained in control hearts. The effects of isoproterenol on 32P incorporation into phospholipids and proteins were observed in the presence of prazosin, and they were completely abolished by the beta-receptor blocker propranolol. Examination of the phosphoinositide-specific phospholipase C activity in the perfused hearts revealed that isoproterenol stimulation was associated with a decrease in the membrane-associated enzymatic activity at physiological calcium concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of polyphosphoinositide synthesis in cardiac membranes

The relative distribution of phosphatidylinositol (PI) and phosphatidylinositol-4-phosphate (PIP) kinase activities in enriched cardiac sarcolemma (SL), sarcoplasmic reticulum (SR), and mitochondrial fractions was investigated. PI and PIP kinase activities were assayed by measuring 32P incorporation into PIP and phosphatidylinositol 4,5-bisphosphate (PIP2) from endogenous and exogenous PI in the presence of [gamma-32P]ATP. PI and PIP kinase activities were present in SL, SR, and mitochondrial fractions prepared from atria and ventricles although the highest activities were found in SL. A similar membrane distribution was found for PI kinase activity measured in the presence of detergent and exogenous PI. PI and PIP kinase activities were detectable in the cytosol providing exogenous PI and PIP and Triton X-100 were present. Further studies focused on characterizing the properties and regulation of PI and PIP kinase activities in ventricular SL. Alamethacin, a membrane permeabilizing antibiotic, increased 32P incorporation into PIP and PIP2 4-fold. PI and PIP kinase activities were Mg2+ dependent and plateaued within 15-20 min at 25 degrees C. Exogenous PIP and PIP2 (0.1 mM) had no effect on PIP and PIP2 labeling in SL in the absence of Triton X-100 but inhibited PI kinase activity in the presence of exogenous PI and Triton X-100. Apparent Km's of ATP for PI and PIP kinase were 133 and 57 microM, respectively. Neomycin increased PIP kinase activity 2- to 3-fold with minor effects on PI kinase activity. Calmidazolium and trifluoperazine activated PI kinase activity 5- to 20-fold and completely inhibited PIP kinase activity. Quercetin inhibited PIP kinase 66% without affecting PI kinase activity. NaF and guanosine 5'-O-(3-thiotriphosphate) had no effect on PI and PIP kinase activities, indicating that these enzymes were not modulated by G proteins. The probability that PIP and PIP2 synthesis in cardiac sarcolemma is regulated by product inhibition and phospholipase C was discussed.

Coordinate interactions of cyclic nucleotide and phospholipid metabolizing pathways in calcium-dependent cellular processes

It is hoped that his review enables the reader to appreciate the complexities implicit in the interactions among Ca2+, cyclic nucleotides, and phospholipid-metabolizing pathways in cell signal transduction. The interactions are varied and intricate, often involving several levels of cell amplification mechanisms. Upsetting the balance of fatty acids in membrane phospholipids can have detrimental effects on adenylate cyclase. Thus, n - 3 fatty acid enrichment of phospholipids suppresses adenylate cyclase activity. The effects of significant alterations in dietary fatty acids, such as might occur with the current vogue for n - 3 eicosapentaenoic acid and docosahexaenoic acid (fish oil) dietary enrichment regimens, will need to be assessed more fully with regard to stimulus-induced changes in cyclic nucleotide production in various tissues. Since the n - 3 fatty acids have not been demonstrated to affect guanylate cyclase activity, dietary changes in certain of these fatty acids would not be expected to contribute to changes in cGMP generation as much as in cAMP production. Moreover, the ingestion of large quantities of these n - 3 fatty acids can alter the profile of cyclooxygenase and lipoxygenase products produced in cells. According to the paradigm developed in this article, changes in the metabolism of fatty acids are amplified by alterations in cyclic nucleotide production and phospholipase activities, with the eventual physiological impact predicated on the tissue type and the specific stimulus response. There appears to be a rather clear distinction between the regulatory properties of eicosanoids regarding adenylate and guanylate cyclase activities. Whereas prostaglandins often stimulate adenylate cyclase activity, they have little effect on guanylate cyclase activity. On the other hand, the HETE compounds seem to play an important role in guanylate cyclase regulation in certain cells. Moreover, arachidonic acid affects adenylate cyclase activity without prior peroxidation, whereas endoperoxides and hydroperoxides are more effective than arachidonic acid with regard to guanylate cyclase stimulation. However, in the intact cell there is a strong implication that the dual stimulation of guanylate cyclase by Ca2+ and fatty acid evokes optimal enzyme activity. An advantage of multidimensional response mechanisms in cells includes the ability to recognize different stimuli and to respond with specific, coordinated responses modulated in their intensity and/or duration by messenger interaction. Few cell types respond to receptor stimulation in an all-or-none fashion, and the "milieu interior" depends on specific, graded responses to the autonomic nervous system and endocrine stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphorylation of phospholipids in isolated guinea pig hearts stimulated with isoprenaline

Phosphorylation of phospholipids was studied in Langendorff perfused guinea pig hearts subjected to beta-adrenergic stimulation. Hearts were perfused with Krebs-Henseleit buffer containing [32P]Pi and freeze-clamped in a control condition or at the peak of the inotropic response to isoprenaline. 32P incorporation into total phospholipids, individual phospholipids and polyphosphoinositides was analysed in whole tissue homogenates and membranes, enriched in sarcoplasmic reticulum, prepared from the same hearts. Isoprenaline stimulation of the hearts did not result in any significant changes in the levels of phosphate incorporation in the total phospholipid present in cardiac homogenates (11.6 +/- 0.4 nmol of 32P/g for control hearts and 12.4 +/- 0.5 nmol of 32P/g for isoprenaline-treated hearts; n = 6), although there was a significant increase in the degree of phospholipid phosphorylation in sarcoplasmic reticulum (3.5 +/- 0.3 nmol of 32P/mg for control hearts and 6.7 +/- 0.2 nmol of 32P/mg for isoprenaline-treated hearts; n = 6). Analysis of 32P incorporation into individual phospholipids and polyphosphoinositides revealed that isoprenaline stimulation of the hearts was associated with a 2-3-fold increase in the degree of phosphorylation of phosphatidylinositol monophosphate and bisphosphate as well as phosphatidic acid in both cardiac homogenates and sarcoplasmic reticulum membranes. In addition, there was increased phosphate incorporation into phosphatidylinositol in sarcoplasmic reticulum membranes. Thus, perfusion of guinea pig hearts with isoprenaline is associated with increased formation of polyphosphoinositides and these phospholipids may be involved, at least in part, in mediating the effects of beta-adrenergic agents in the mammalian heart.

Effects of norepinephrine and 5-hydroxytryptamine on phosphoinositide-PO4 turnover in rabbit cornea

We have investigated: (a) Phospholipid composition and phosphoinositide-PO4 turnover in rabbit cornea tissues; and (b) the effects of adrenergic and serotonergic agonists on breakdown of phosphoinositides in the rabbit cornea. The data obtained from these studies can be summarized as follows: (1) in the cornea phosphatidylcholine and phosphatidylethanolamine constitute about 55%, phosphatidylinositol (PI) 10%, and phosphatidylinositol 4-phosphate (PIP), phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid (PA) comprise about 1% each of the total phospholipids; (2) incubation of cornea in 32Pi-containing medium resulted in incorporation of radioactivity in tissue phospholipids. The radioactivity was highest in PIP2 (39%), followed by PI (19%), PIP (16%) and PA (5% of the total radioactivity). When compared with stroma and endothelium, the cornea epithelium was most active in phosphoinositide metabolism; (3) addition of norepinephrine (NE) or 5-hydroxytryptamine (5-HT), 200 microM each, to 32P-labeled cornea resulted in a loss of radioactivity in PIP and PIP2 by about 12- and 20%, respectively. Concomitantly, the radioactivity in PA and PI was increased by 44- and 66%, respectively. The effects of the neurotransmitters were time- and concentration-dependent. When added to the cornea labeled with myo [3H] inositol, NE and 5-HT increased the production of labeled myo-inositol phosphates; (4) prazosin (20 microM), but not yohimbine or propranolol, blocked the effects of NE. Similarly, the effects of 5-HT were antagonized by methysergide (20 microM) and ketanserin (10 microM) but not by prazosin. These data demonstrate that NE and 5-HT stimulate phospholipase C-mediated hydrolysis of PIP2 into diacylglycerol (DG) and myo-inositol trisphosphate (IP3). Furthermore, the effects of NE and 5-HT are mediated by alpha 1-adrenergic and 5-HT2 receptors, respectively. It is suggested that IP3, by releasing Ca2+ from ER, and DG, by activating protein kinase C, may function as second-messenger molecules which may participate in agonist-induced functional responses, including chloride transport, in the cornea epithelium.

Phosphorylation of phosphoinositides in human platelets

32P-labelling of phosphatidylinositol (PI), PI-4-monophosphate (PIP), PI-4,5-bisphosphate (PIP2) and phosphatidic acid (PA) in 32P-labelled intact human platelets was investigated in the presence of various agents which alter intracellular level of cAMP or Ca2+. Addition of dibutyryl cAMP to intact platelets pre- or pulse labelled with 32P resulted in increased 32P-labelling of PIP and in concomitant decreased 32P-labelling of PI without affecting that of PIP2 or PA. Similar changes were observed in intact platelets treated by prostaglandin I2 (PGI2) or a new low Km phosphodiesterase inhibitor (DN-9693). When intracellular Ca2+ was chelated by loading quin 2-AM to intact platelets, 32P-labelling of PIP was significantly increased in a dose related manner. From these observations it was concluded that PI kinase is activated by elevation of cAMP or chelation of Ca2+ in intact platelets.

Phosphatidylinositol 4,5-bisphosphate formation in rabbit skeletal and heart muscle membranes

Incubation of rabbit skeletal muscle microsomes or isolated triads with gamma 32P-ATP/Mg2+ in the absence and in the presence of added phosphatidylinositol resulted in the formation of phosphatidylinositol 4-phosphate catalyzed by phosphatidylinositol kinase. When phosphatidylinositol 4-phosphate was added as exogenous substrate, phosphatidylinositol 4,5-bisphosphate was also formed demonstrating the presence of a membrane bound phosphatidylinositol 4-phosphate kinase. Triads were broken mechanically in a French press and separated on a continuous sucrose gradient. Incubation of these fractions with gamma 32P-ATP/Mg2+ resulted in a rapid labeling of phospholipid in a membrane fraction banding between transverse tubules and the terminal cisternae. Partial triad breakage and triad reformation experiments indicated that this phosphatidylinositol kinase was associated with T-tubules. When exogenous phosphatidylinositol 4-phosphate was employed as substrate phosphatidylinositol 4,5-bisphosphate and phosphatidic acid were formed, indicating the presence of all the enzymes of the polyphosphoinositide signaling system in this special membrane fraction. In contrast, heart muscle microsomes or plasma membranes can catalyze this reaction sequence from endogenous formed phosphatidylinositol 4-phosphate.