Phosphorylase phosphatase and flash activation of skeletal muscle glycogen phosphorylase-A tribute to Edmond H. Fischer.

Glycogen is a polymerized form of glucose that serves as an energy reserve in all types of organisms. In animals glycogen synthesis and degradation, especially in liver and skeletal muscle, are regulated by hormonal and physiological signals that reciprocally control the opposing activities of glycogen synthase and glycogen phosphorylase. These enzymes are under allosteric control by binding of metabolites (e.g., ATP, AMP, G6P) and covalent control by reversible phosphorylation by kinase and phosphatase all assembled together on glycogen. More than 50 years ago Edmond Fischer and colleagues showed "flash activation" of phosphorylase in glycogen particles. This involved transient and extensive inhibition of protein phosphatase but even today the phenomenon is not understood. Phosphatase regulation is known to rely on regulatory subunits including glycogen binding subunits that serve as scaffolds, binding catalytic subunit, glycogen, and substrates. This tribute article to Edmond Fischer highlights his thoughts and ideas about the transient inhibition of phosphorylase phosphatase during flash activation of phosphorylase and speculates that phosphatase regulation in glycogen particles might involve a/b hybrids of phosphorylase.

Control of Emi2 activity and stability through Mos-mediated recruitment of PP2A.

Before fertilization, vertebrate eggs are arrested in meiosis II by cytostatic factor (CSF), which holds the anaphase-promoting complex (APC) in an inactive state. It was recently reported that Mos, an integral component of CSF, acts in part by promoting the Rsk-mediated phosphorylation of the APC inhibitor Emi2/Erp1. We report here that Rsk phosphorylation of Emi2 promotes its interaction with the protein phosphatase PP2A. Emi2 residues adjacent to the Rsk phosphorylation site were important for PP2A binding. An Emi2 mutant that retained Rsk phosphorylation but lacked PP2A binding could not be modulated by Mos. PP2A bound to Emi2 acted on two distinct clusters of sites phosphorylated by Cdc2, one responsible for modulating its stability during CSF arrest and one that controls binding to the APC. These findings provide a molecular mechanism for Mos action in promoting CSF arrest and also define an unusual mechanism, whereby protein phosphorylation recruits a phosphatase for dephosphorylation of distinct sites phosphorylated by another kinase.

The inhibitor-1 C terminus facilitates hormonal regulation of cellular protein phosphatase-1: functional implications for inhibitor-1 isoforms.

Inhibitor-1 (I-1) is a selective inhibitor of protein phosphatase-1 (PP1) and regulates several PP1-dependent signaling pathways, including cardiac contractility and regulation of learning and memory. The human I-1 gene has been spliced to generate two alternative mRNAs, termed I-1alpha and I-1beta, encoding polypeptides that differ from I-1 in their C-terminal sequences. Reverse transcription-PCR established that I-1alpha and I-1beta mRNAs are expressed in a developmental and tissue-specific manner. Functional analysis of I-1 in a Saccharomyces cerevisiae strain dependent on human I-1 for viability established that a novel domain encompassing amino acids 77-110 is necessary for PP1 inhibition in yeast. Expression of human I-1 in S. cerevisiae with a partial loss-of-function eukaryotic initiation factor-2alpha (eIF2alpha) kinase (Gcn2p) mutation permitted growth during amino acid starvation, consistent with the inhibition of Glc7p/PP1, the yeast eIF2alpha phosphatase. In contrast, human I-1alpha, which lacks amino acids 83-134, and I-1 with C-terminal deletions were significantly less effective in promoting yeast growth under starvation conditions. These data suggest that C-terminal sequences of I-1 enhance regulation of the eukaryotic eIF2alpha phosphatase. In vitro studies established that C-terminal sequences, deleted in both I-1alpha and I-1beta, enhance PP1 binding and inhibition. Expression of full-length and C-terminally truncated I-1 in HEK293T cells established the importance of the I-1 C terminus in transducing cAMP signals that promote eIF2alpha phosphorylation. This study demonstrates that multiple domains in I-1 target cellular PP1 complexes and establishes I-1 as a cellular regulator of eIF2alpha phosphorylation.

Interaction of maize (Zea mays) protein phosphatase 2A with tubulin.

Immunological and biochemical evidence has been obtained for an interaction of maize protein phosphatase 2A (PP2A) holoenzyme with tubulin. Tubulin co-purifies with maize seedling PP2A. Affinity chromatography of the maize PP2A preparation on immobilized tubulin revealed two peaks of phosphorylase alpha phosphatase activity. In one of the peaks, the catalytic (C) and constant regulatory (A) subunits of PP2A were identified by Western blotting. The subunits (C and A) of PP2A were co-immunoprecipitated from maize seedlings homogenate by an anti-alpha-tubulin antibody. The interaction of plant PP2A with tubulin indicates a possible role of reversible protein phosphorylation in the dynamic structure of plant cytoskeleton.

The level of the glycogen targetting regulatory subunit R5 of protein phosphatase 1 is decreased in the livers of insulin-dependent diabetic rats and starved rats.

Hepatic glycogen synthesis is impaired in insulin-dependent diabetic rats owing to defective activation of glycogen synthase by glycogen-bound protein phosphatase 1 (PP1). The identification of three glycogen-targetting subunits in liver, G(L), R5/PTG and R6, which form complexes with the catalytic subunit of PP1 (PP1c), raises the question of whether some or all of these PP1c complexes are subject to regulation by insulin. In liver lysates of control rats, R5 and R6 complexes with PP1c were found to contribute significantly (16 and 21% respectively) to the phosphorylase phosphatase activity associated with the glycogen-targetting subunits, G(L)-PP1c accounting for the remainder (63%). In liver lysates of insulin-dependent diabetic and of starved rats, the phosphorylase phosphatase activities of the R5 and G(L) complexes with PP1c were shown by specific immunoadsorption assays to be substantially decreased, and the levels of R5 and G(L) were shown by immunoblotting to be much lower than those in control extracts. The phosphorylase phosphatase activity of R6-PP1c and the concentration of R6 protein were unaffected by these treatments. Insulin administration to diabetic rats restored the levels of R5 and G(L) and their associated activities. The regulation of R5 protein levels by insulin was shown to correspond to changes in the level of the mRNA, as has been found for G(L). The in vitro glycogen synthase phosphatase/phosphorylase phosphatase activity ratio of R5-PP1c was lower than that of G(L)-PP1c, suggesting that R5-PP1c may function as a hepatic phosphorylase phosphatase, whereas G(L)-PP1c may be the major hepatic glycogen synthase phosphatase. In hepatic lysates, more than half the R6 was present in the glycogen-free supernatant, suggesting that R6 may have lower affinity for glycogen than R5 and G(L)

Regulation of glycogen synthase in rat hepatocytes. Evidence for multiple signaling pathways.

We examined the signaling pathways regulating glycogen synthase (GS) in primary cultures of rat hepatocytes. The activation of GS by insulin and glucose was completely reversed by the phosphatidylinositol 3-kinase inhibitor wortmannin. Wortmannin also inhibited insulin-induced phosphorylation and activation of protein kinase B/Akt (PKB/Akt) as well as insulin-induced inactivation of GS kinase-3 (GSK-3), consistent with a role for the phosphatidylinositol 3-kinase/PKB-Akt/GSK-3 axis in insulin-induced GS activation. Although wortmannin completely inhibited the significantly greater level of GS activation produced by the insulin-mimetic bisperoxovanadium 1,10-phenanthroline (bpV(phen)), there was only minimal accompanying inhibition of bpV(phen)-induced phosphorylation and activation of PKB/Akt, and inactivation of GSK-3. Thus, PKB/Akt activation and GSK-3 inactivation may be necessary but are not sufficient to induce GS activation in rat hepatocytes. Rapamycin partially inhibited the GS activation induced by bpV(phen) but not that effected by insulin. Both insulin- and bpV(phen)-induced activation of the atypical protein kinase C (zeta/lambda) (PKC (zeta/lambda)) was reversed by wortmannin. Inhibition of PKC (zeta/lambda) with a pseudosubstrate peptide had no effect on GS activation by insulin, but substantially reversed GS activation by bpV(phen). The combination of this inhibitor with rapamycin produced an additive inhibitory effect on bpV(phen)-mediated GS activation. Taken together, our results indicate that the signaling components mammalian target of rapamycin and PKC (zeta/lambda) as well as other yet to be defined effector(s) contribute to the modulation of GS in rat hepatocytes.

Organization and alternate splice products of the gene encoding nuclear inhibitor of protein phosphatase-1 (NIPP-1).

Nuclear inhibitor of protein phosphatase-1 (NIPP-1) is one of two major regulatory subunits of protein phosphatase-1 in mammalian nuclei. We report here the cloning and structural characterization of the human NIPP-1 genes, designated PPP1R8P and PPP1R8 in human gene nomenclature. PPP1R8P (1.2 kb) is a processed pseudogene and was localized by in situ hybridization to chromosome 1p33-32. PPP1R8 is an authentic NIPP-1 gene and was localized to chromosome 1p35. PPP1R8 (25.2 kb) is composed of seven exons and encodes four different transcripts, as determined from cDNA library screening, reverse transcriptase-PCR (RT-PCR) and/or EST (expressed sequence tag) database search analysis. NIPP-1alpha mRNA represents the major transcript in human tissues and various cell lines, and encodes a polypeptide of 351 residues that only differs from the previously cloned calf thymus NIPP-1 by a single residue. The other transcripts, termed NIPP-1beta, gamma and delta, are generated by alternative 5'-splice site usage, by exon skipping and/or by alternative polyadenylation. The NIPP-1beta/delta and NIPP-1gamma mRNAs are expected to encode fragments of NIPP-1alpha that differ from the latter by the absence of the first 142 and 224 residues, respectively. NIPP-1gamma corresponds to 'activator of RNA decay-1' (Ard-1) which, unlike NIPP-1alpha, displays in vitro and endoribonuclease activity and lacks an RVXF consensus motif for interaction with protein phosphatase-1. While the NIPP-1alpha/beta/delta-transcripts were found to be present in various human tissues, the NIPP-1gamma transcript could only be detected in human transformed B-lymphocytes.

Identification and characterization of the human HCG V gene product as a novel inhibitor of protein phosphatase-1.

The catalytic subunit of mammalian protein phosphatase-1 (PP1) is known to bind to a number of regulatory subunits, whose functions include the targeting of the catalytic subunit to the molecular proximity of its substrate proteins. In addition, PP1 is potently inhibited by several inhibitory polypeptides that include inhibitor-1 and inhibitor-2. In this study the yeast two-hybrid system was used to screen a human cDNA library for putative PP1-binding proteins. Ten putative positive clones were identified, one of which was found to be a partial cDNA of the hemochromatosis candidate gene V (HCG V) whose function was previously unknown. The full-length protein of 126 amino acid residues was expressed in Escherichia coli as a glutathione S-transferase fusion protein and also as a nonfusion protein. The recombinant protein inhibited recombinant and rabbit muscle protein phosphatase-1 with IC50s of ca. 1 nM, but did not inhibit PP2A. The term inhibitor-3 is proposed for this novel inhibitor. It is extremely hydrophilic, is heat stable, and behaves anomalously on SDS-PAGE with an apparent molecular mass of 23 kDa and on gel filtration with a relative molecular weight of 55 000, in contrast to its calculated molecular mass of 14 kDa. These characteristics are shared by the previously described protein phosphatase-1 inhibitor-2 and inhibitor-1 proteins.

Phosphorylase phosphatase activity in Saccharomyces cerevisiae 257.

A phosphatase, active towards phosphorylase a and phosphorylated proteins casein and histone II-A, was isolated from Saccharomyces cerevisiae 257. The enzyme dephosphorylated glycogen phosphorylase from commercial yeast rendering it inactive. The protein phosphatase activity was not influenced by any metal ions. Phosphorylase phosphatase activity was slightly stimulated by p-nitrophenyl phosphate and inhibited by heparin.

Protein serine/threonine phosphatase 1 and 2A associate with and dephosphorylate neurofilaments.

The phosphorylation state of neurofilaments plays an important role in the control of cytoskeletal integrity, axonal transport, and axon diameter. Immunocytochemical analyses of spinal cord revealed axonal localization of all protein phosphatase subunits. To determine whether protein phosphatases associate with axonal neurofilaments, neurofilament proteins were isolated from bovine spinal cord white matter by gel filtration. approximately 15% of the total phosphorylase a phosphatase activity was present in the neurofilament fraction. The catalytic subunits of PP1 and PP2A, as well as the A and B alpha regulatory subunits of PP2A, were detected in the neurofilament fraction by immunoblotting, whereas PP2B and PP2C were found exclusively in the low molecular weight soluble fractions. PP1 and PP2A subunits could be partially dissociated from neurofilaments by high salt but not by phosphatase inhibitors, indicating that the interaction does not involve the catalytic site. In both neurofilament and soluble fractions, 75% of the phosphatase activity towards exogenous phosphorylase a could be attributed to PP2A, and the remainder to PP1 as shown with specific inhibitors. Neurofilament proteins were phosphorylated in vitro by associated protein kinases which appeared to include protein kinase A, calcium/calmodulin-dependent protein kinase, and heparin-sensitive and -insensitive cofactor-independent kinases. Dephosphorylation of phosphorylated neurofilament subunits was mainly (60%) catalyzed by associated PP2A, with PP1 contributing minor activity (10-20%). These studies suggest that neurofilament-associated PP1 and PP2A play an important role in the regulation of neurofilament phosphorylation.

The gamma(1)34.5 protein of herpes simplex virus 1 complexes with protein phosphatase 1alpha to dephosphorylate the alpha subunit of the eukaryotic translation initiation factor 2 and preclude the shutoff of protein synthesis by double-stranded RNA-activated protein kinase.

In human cells infected with herpes simplex virus 1 the double-stranded RNA-dependent protein kinase (PKR) is activated but phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF-2) and total shutoff of protein synthesis is observed only in cells infected with gamma(1)z34.5- mutants. The carboxyl-terminal 64 aa of gamma(1)34.5 protein are homologous to the corresponding domain of MyD116, the murine growth arrest and DNA damage gene 34 (GADD34) protein and the two domains are functionally interchangeable in infected cells. This report shows that (i) the carboxyl terminus of MyD116 interacts with protein phosphatase 1alpha in yeast, and both MyD116 and gamma(1)34.5 interact with protein phosphatase 1alpha in vitro; (ii) protein synthesis in infected cells is strongly inhibited by okadaic acid, a phosphatase 1 inhibitor; and (iii) the alpha subunit in purified eIF-2 phosphorylated in vitro is specifically dephosphorylated by S10 fractions of wild-type infected cells at a rate 3000 times that of mock-infected cells, whereas the eIF-2alpha-P phosphatase activity of gamma(1)34.5- virus infected cells is lower than that of mock-infected cells. The eIF-2alpha-P phosphatase activities are sensitive to inhibitor 2. In contrast to eIF-2alpha-P phosphatase activity, extracts of mock-infected cells exhibit a 2-fold higher phosphatase activity on [32P]phosphorylase than extracts of infected cells. These results indicate that in infected cells, gamma(1)34.5 interacts with and redirects phosphatase to dephosphorylate eIF-2alpha to enable continued protein synthesis despite the presence of activated PKR. The GADD34 protein may have a similar function in eukaryotic cells. The proposed mechanism for maintenance of protein synthesis in the face of double-stranded RNA accumulation is different from that described for viruses examined to date.

Stimulation of phosphorylase phosphatase activity of protein phosphatase 2A1 by protamine is ionic strength dependent and involves interaction of protamine with both substrate and enzyme.

The effects of protamine on the phosphorylase phosphatase activity of porcine cardiac protein phosphatase 2A1 (PP2A1) were complex and ionic strength dependent. Under ionic strength conditions that protamine activation was optimal, activation of PP2A1 by either dilution or heparin was prevented. A time-dependent deactivation of the protamine-stimulated phosphatase activity was observed when PP2A1 was preincubated with protamine. Protamine forms a very tight association with phosphorylase a, which is optimal at a 1:1 protamine:phosphorylase a monomer molar ratio. Protamine activation of PP2A1 activity, however, is not substrate-directed since the basic polypeptide did not stimulate either the activity of the catalytic subunit or trypsinolysis of [32P]phosphorylase a. The interaction of protamine with phosphorylase a does not apparently involve the phosphorylation site in the protein substrate (ser 14). The activation of PP2A1 by protamine is proposed to involve part of the basic polypeptide, not associated with phosphorylase a monomer, interacting with the regulatory and/or the catalytic subunit(s) of the phosphatase. A minimal model for the activation of PP2A1 by protamine was tested kinetically. In this model, free PP2A1 binds with decreasing affinities to the protamine:phosphorylase a complex, free phosphorylase a, and free protamine. Protamine decreases the K(m) of PP2A1 for the phosphorylase a monomer 5-fold and increases the Vmax 17-fold. Interaction of free protamine with PP2A1 inhibits the phosphatase activity.

Further evidence that inhibitor-2 acts like a chaperone to fold PP1 into its native conformation.

The gamma1-isoform of protein phosphatase-1 expressed in Escherichia coli (PP1gamma) and the native PP1 catalytic subunit (PP1C) isolated from skeletal muscle dephosphorylated Ser-14 of glycogen phosphorylase at comparable rates. In contrast, PP1gamma dephosphorylated several tyrosine-phosphorylated proteins at similar rates to authentic protein tyrosine phosphatases (PTPases), but native PP1C was almost inactive towards these substrates. The phosphorylase phosphatase (PhP) and PTPase activities of PP1gamma were inhibited by vanadate with IC50 values (30-100 microM) comparable to authentic PTPases, whereas the PhP activity of native PP1C was insensitive to vanadate. PP1gamma lost its PTPase activity, and its PhP activity became insensitive to vanadate, after interaction with inhibitor-2, followed by the reversible phosphorylation of inhibitor-2 at Thr-72. These findings support and extend the hypothesis that inhibitor-2 functions like a chaperone to fold PP1 into its native conformation, and suggest that the correct folding of PP1 may be critical to prevent the uncontrolled dephosphorylation of cellular phosphotyrosine residues.

Effects of cantharidin on force of contraction and phosphatase activity in nonfailing and failing human hearts.

1. The effect of the phosphatase inhibitor, cantharidin (3-300 microM) on force of contraction was studied in isolated electrically driven right ventricular trabeculae carneae from human myocardium. 2. The positive inotropic effect of cantharidin started at a concentration of 100 microM with a positive inotropic effect to 199% and to 276% of the predrug value in nonfailing and failing human hearts, respectively. 3. Under basal conditions the contraction time parameters were prolonged in human heart failure vs. nonfailing preparations. However, the positive inotropic effect of cantharidin did not affect contraction time parameters. Thus, time to peak tension, time of relaxation and total contraction time were not shortened by cantharidin in nonfailing and failing preparations. 4. The phosphatase activity was unchanged in preparations from failing hearts compared to nonfailing hearts. 5. Cantharidin inhibited phosphatase activity in a concentration-dependent manner. The IC50 value of cantharidin was about 3 microM in both nonfailing and failing human myocardium. 6. The positive inotropic effect of cantharidin was similar in nonfailing and failing human hearts, accompanied by a similar inhibitory effect of cantharidin on the phosphatase activity. The positive inotropic effect of cantharidin in failing hearts was as strong as the effect of isoprenaline in nonfailing hearts. 7. It is concluded that the treatment with a phosphatase inhibitor may offer a new positive inotropic modality for the treatment of human heart failure.

Mechanisms of the contractile effects of 2,3-butanedione-monoxime in the mammalian heart.

We studied the mechanisms of action of a negative inotropic compound, 2,3-butanedione-monoxime (BDM), which has been suggested to be a cardioprotective agent. In guinea-pig papillary muscles the negative inotropic effect of BDM start at 100 mumol/l amounting to 18.32 +/- 2.09% of predrug value at 10 mmol/l without any effects on time parameters (n = 12, each). 30 mmol/l BDM totally abolished force of contraction; this effect was reversible after washout. In the presence of the phosphatase-inhibitor cantharidin (30 mumol/l) the concentration response curve on force of contraction was shifted to higher concentrations of BDM. 100 mmol/l BDM decreased the phosphorylation state of the inhibitory subunit of troponin (TnI) and phospholamban (PLB) in [32P]-labeled guinea-pig ventricular myocytes to 76.5 +/- 4.7% and 49.7 +/- 4.2%, respectively (n = 7). Furthermore, BDM enhanced the activity of phosphorylase phosphatases in guinea-pig ventricular homogenates amounting to a stimulation to 203.5 +/- 10.4% at 100 mmol/l whereas type 1 phosphorylase phosphatase activity increased only by 24.5% (n = 5). PLB phosphatase activity was enhanced to 155.9 +/- 11.7% by 100 mmol/l BDM (n = 5). It is concluded that the effects of BDM on contractile parameters are accompanied by decreased phosphorylation of the cardiac regulatory proteins TnI and PLB which could in part be due to activation of type 1 or 2A phosphatase activity. Hence, it is suggested that BDM affects the phosphorylation state of TnI and PLB not directly, but via activation of their phosphatases.

The phosphatase inhibitor okadaic acid blocks KCl-depolarization-induced rise of cytosolic calcium of rat insulinoma cells (RINm5F).

It has been shown that okadaic acid (OA) diminishes insulin secretion of rat pancreatic islets in response to glucose, glyceraldehyde and KCl. Glucose, glyceraldehyde and KCl cause release of insulin by depolarization and subsequent opening of L-type calcium channels. Calcium entry into cells is thought to be related to protein phosphorylation. To evaluate whether or not OA mediated inhibition of insulin secretion in response to depolarization might be due to an interference with calcium uptake, we studied its effect on KCl (30 mM)-induced increases of cytosolic calcium and discharge of insulin in the insulin secreting clonal tumor cell line RINm5F. OA inhibited KCl-stimulated insulin release in concentrations > or = 1 microM. In intact RINm5F cells similar concentrations of OA decreased the activity of protein phosphates PP-1/PP-2A and inhibited the depolarization-induced rise of cytosolic calcium ([Ca2+]i). The latter action could also be achieved with the protein phosphatase inhibitor calyculin A, whereas the OA analogue 1-nor-okadaone, which is without effect on phosphatases, did not affect [Ca2+]i or insulin release. It is concluded that depression of depolarization-induced insulin secretion by OA is due to inhibition of calcium entry along voltage dependent calcium channels. The data also suggest that in RINm5F cells protein phosphatases PP-1/PP-2A are related to the function of voltage-dependent calcium channels.

Phosphorylation and activation of the ATP-Mg-dependent protein phosphatase by the mitogen-activated protein kinase.

Inhibitor-2 (I-2) is the regulatory subunit of the cytosolic ATP-Mg-dependent form of type 1 serine/threonine protein phosphatase and its phosphorylation at Thr-72 by glycogen synthase kinase-3 results in phosphatase activation. Activation of cytosolic type 1 phosphatase has been observed in cells treated with growth factors. Reported here is the phosphorylation and activation of the ATP-Mg-dependent phosphatase by mitogen-activated protein kinase (MAPK). Recombinant I-2 was phosphorylated by activated MAPK to an extent (approximately 0.3 mol of phosphate/mol of polypeptide) similar to that reported for phosphorylation by the alpha isoform of glycogen synthase kinase-3. The phosphorylation of I-2 by MAPK was exclusively at Thr-72, the site involved in the activation of phosphatase. Incubation of MAPK with purified ATP-Mg-dependent phosphatase resulted in phosphorylation of the I-2 component and activation of the phosphatase. Ribosomal S6 protein kinase II (p90rsk) was also able to phosphorylate the recombinant I-2; however, this phosphorylation occurred on serines and had no effect on phosphatase activation. Our data may explain growth factor-induced activation of the ATP-Mg-dependent phosphatase and suggest that MAPK may of cytosolic type 1 phosphatase in response to insulin and/or other growth factors.

Protein phosphatase type-1 mRNA levels in response to starvation-refeeding and streptozotocin-diabetes.

Levels of the mRNA encoding the catalytic subunit of protein phosphatase type-1 (PP-1cat) were reduced in skeletal muscle but not liver in response to short-term (2h) chow refeeding after prolonged (40h) starvation in the rat. This reduction did not appear to be mediated by insulin per se since streptozotocin-induced diabetes was associated with a reduction in PP-1cat levels in skeletal muscle. It is suggested that glucose levels may be one factor that modulates skeletal muscle PP-1cat mRNA levels. Despite the changes in PP-1cat mRNA levels in skeletal muscle, total protein phosphatase-1 catalytic activity was not altered by either chow refeeding or streptozotocin-diabetes. By contrast, although total hepatic PP-1cat mRNA levels were not altered in response to chow refeeding, there was a marked reduction in glycogen phosphorylase phosphatase activity in the cytosol but not in the glycogen/microsomal fraction.

Mechanisms of the contractile effects of flosequinoxan.

In guinea-pig papillary muscles the positive inotropic effect of flosequinoxan (BTS) starting at 100 mumol/l amounted to 287.6 +/- 34.2% at 300 mumol/l without any effects on time to peak tension (103.9 +/- 2%) and relaxation time (107.1 +/- 6.7% of predrug value, respectively). 10 mumol/l carbachol attenuated the positive inotropic effect of 300 mumol/l to 166.5 +/- 11.6% (n = 10). The phosphorylation state of the inhibitory subunit of troponin (TnI) and phospholamban (PLB) in [32P]-labeled guinea-pig ventricular myocytes was increased starting at 100 mumol/l amounting to 142.5 +/- 12.6% and 130.9 +/- 2.2% at 300 mumol/l, respectively (n = 5). Furthermore, BTS (300 mumol/l) decreased phosphorylase phosphatase activity by 23.1%. It is concluded that the contractile effects of BTS are accompanied by enhanced phosphorylation of regulatory proteins which could in part be due to inhibition of phosphorylase phosphatase activity.