Differential nature of cross-talk among three G-coupled receptors regulating adenylyl cyclase in rat cardiomyocytes chronically exposed to receptor agonists

Chronic exposure of cells to cognate agonists has been established to cause homologous desensitization of G protein-coupled receptors. In this work, we show that exposure of adult rat cardiomyocytes to isoproterenol (ISO) for 24 h led to the desensitization of beta-adrenoceptor (beta-AR) coupled adenylyl cyclase (AC) activity, which was associated with an increased inhibition of AC by M2-muscarinic receptor (MR) agonist, carbachol (Cch), and a decreased inhibition of AC by A1-adenosine receptor (AdR) agonist, N6-phenylisopropyladenosine (R-PIA). Chronic exposure of cells to Cch caused the desensitization of M2-MR-coupled AC, decreased the inhibitory action of R-PIA on AC and increased ISO-stimulated AC, while chronic exposure to R-PIA caused the desensitization of A1-AdR-coupled AC and modestly increased ISO-stimulated AC without any significant effect on Cch inhibition of the enzyme. Thus, chronic exposure of cardiomyocytes revealed for the first time a more complex and differential nature of cross-talk among the three major G-coupled receptors in modulating AC.

Phosphorylation of inhibitory subunit of troponin and phospholamban in rat cardiomyocytes: modulation by exposure of cardiomyocytes to hydroxyl radicals and sulfhydryl group reagents

Myocytes were isolated from rat heart ventricles and then incubated with [32P]-sodium phosphate to label intracellular ATP stores. Incubations of the [32P]-labelled cardiomyocytes with a beta-adrenoceptor agonist isoproterenol (10 microM) and with a plant diterpene forskolin (100 microM) which directly stimulates adenylyl cyclase increased the phosphorylation of an inhibitory subunit of troponin (TN-I) and phospholamban (PLN). Brief exposure (1 min) of labelled myocytes to the hydroxyl radical generating system (H2O2 plus FeCl2) decreased markedly the stimulatory action of isoproterenol and forskolin on TN-I and PLN phosphorylation. Similar exposure of myocytes to 5-5'-dithiobis-nitrobenzoic acid (DTNB) a sulfhydryl oxidizing reagent exerted little inhibitory effect on the isoproterenol or forskolin stimulated TN-I and PLN phosphorylation. In contrast exposure of myocytes to low concentrations (< 50 microM) of N-ethylmaleimide (NEM) a sulfhydryl alkylating reagent augmented the stimulatory effect of isoproterenol on TN-I and PLN phosphorylation. The results further showed that brief treatment of myocytes to H2O2 plus FeCl2 markedly decreased isoproterenol-, but not forskolin-, stimulated cyclic AMP accumulation in the myocytes. The stimulatory action of NEM on the isoproterenol-stimulated TN-I and PLN phosphorylation appeared related to greater increase in the isoproterenol-stimulated cyclic AMP accumulation in the NEM-treated cardiomyocytes. The results are consistent with the postulate that hydroxyl radical exposure of cardiomyocytes blunts the beta-adrenoceptor-mediated stimulation of adenylyl cyclase leading to decreased phosphorylation of TN-I and PLN and imply that such alterations account in part the reported depressed rate of relaxation of the myocardium exposed to oxygen free radicals.

Protein phosphorylation in rat cardiac microsomes: effects of inhibitors of protein kinase A and of phosphatases

The phosphorylation of rat cardiac microsomal proteins was investigated with special attention to the effects of okadaic acid (an inhibitor of protein phosphatases), inhibitor 2 of protein phosphatase 1 and inhibitor of cyclic AMP-dependent protein kinase (protein kinase A). The results showed that okadaic acid (5 microM) modestly but reproducibly augmented the protein kinase A-catalyzed phospholamban (PLN) phosphorylation, although exerted little effect on the calcium/calmodulin kinase-catalyzed PLN phosphorylation. Microsomes contained three other substrates (M(r) 23, 19 and 17 kDa) that were phosphorylated by protein kinase A but not by calcium/calmodulin kinase. The protein kinase A-catalyzed phosphorylation of these three substrates was markedly (2-3 fold) increased by 5 microM okadaic acid. Calmodulin was found to antagonize the action of okadaic acid on such phosphorylation. Protein kinase A inhibitor was found to decrease the protein kinase A-catalyzed phosphorylation of microsomal polypeptides. Unexpectedly, inhibitor 2 was also found to markedly decrease protein kinase A-catalyzed phosphorylation of phospholamban as well these other microsomal substrates. These results are consistent with the views that protein phosphatase 1 is capable of dephosphorylating membrane-associated phospholamban when it is phosphorylated by protein kinase A, but not by calcium/calmodulin kinase, and that under certain conditions, calcium/calmodulin-stimulated protein phosphatase (protein phosphatase 2B) is also able to dephosphorylate PLN phosphorylated by protein kinase A. Additionally, the observations show that protein phosphatase 1 is extremely active against the three protein kinase A substrates (M(r) 23, 19 and 17 kDa) that were present in the isolated microsomes and whose state of phosphorylation was particularly affected in the presence of dimethylsulfoxide. Protein phosphatase 2B is also capable of dephosphorylating these three substrates.

Altered responsiveness of guanylyl cyclase to nitric oxide following treatment of cardiomyocytes with S-nitroso-D,L-acetylpenicillamine and sodium nitroprusside

The stimulation of cardiomyocyte guanylyl cyclase by nitric oxide (NO)-donor drugs was examined before and after exposure of these cells to the NO-donor drugs: S-nitroso-d,l-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP). Short- (2-hr) and long-term (24-hr) exposure attenuated the maximal stimulation of GC by either SNAP or SNP by up to 80% ("desensitization"). However this "desensitization" of the myocardial GC was atypical in nature in that the reduction in maximal NO-stimulated GC activity was associated with an increase in the affinity of the GC towards either NO-donor, a finding not as yet reported. There was also evidence of "cross-desensitization" of GC (e.g., SNAP exposure decreasing the stimulatory effect of SNP). Further, this is the first time that SNAP-induced desensitization of GC has been observed.

Sarcoplasmic reticulum Ca(2+)-pump dysfunction in rat cardiomyocytes briefly exposed to hydroxyl radicals

The effects of hydroxyl radical exposure of intact cardiomyocytes on sarcoplasmic reticulum (SR) function were investigated. For this purpose, isolated rat heart myocytes were exposed briefly (1 min) to the hydroxyl radical generating system (H2O2/FeCl2 or FeSO4) or 5-5'-dithiobis-nitrobenzoic acid (DTNB), a sulfhydryl oxidizing reagent, and following this a SR-enriched fraction was isolated. Marked decreases in the SR calcium uptake activities were seen in the myocytes exposed to either the hydroxyl radical-generating system or DTNB. The exposure of myocytes to the hydroxyl radical, but not DTNB, markedly increased the amount of malonyldialdehyde (MDA) in the subsequently isolated SR. Total sulfhydryl group content in SR was decreased by exposure of myocytes to DTNB. Further, there was a significant decrease in [3H]-NEM binding to SR isolated from the hydoxyl radical-treated myocytes indicating that sulfhydryl groups are affected (oxidized). Both mannitol and catalase were found to offer complete protection against the inhibitory effect of peroxide +/- iron on calcium uptake. Also the above-mentioned alterations in both MDA and sulfhydryl group content were prevented by mannitol and catalase. Exogenously added cyclic AMP-dependent protein kinase (A-PK) or calmodulin (CAM) increased SR calcium uptake activity. In the SR isolated from the treated myocytes, the stimulatory effects of A-PK and CAM were also seen, although under all assay conditions calcium uptakes were of lower magnitude. The findings are consistent with the view that the damaging effect of the hydroxyl radical and DTNB on the functioning of SR occurs rapidly in the intact cardiomyocytes. The hydroxyl radical-provoked damage involves both protein sulfhydryl and lipid oxidation.

Enhanced forebrain nitric oxide synthase activity in epileptic fowl

Nitric oxide synthase (NOS) activity was examined in forebrain, cerebellum and optic lobes of adult domestic fowl, having a hereditary primary generalized convulsive disorder. NOS was approximately 2-fold higher in only the forebrain of adult epileptic fowl compared to non-epileptic (carrier) hatchmates. A significant increase in NOS was also evident in forebrains of 1-day-old epileptic chicks. Ca(2+)-dependency experiments confirmed that these increments were principally due to type I NOS (NOS-I). Induction of convulsions by intermittent photic stimulation did not affect pre-existing forebrain NOS-I activity. The present data suggest that an enhanced NO signaling may ensue in selected regions of the brain as an adaptive response to hereditary epileptogenesis.

Alterations in inotropy, nitric oxide and cyclic GMP synthesis, protein phosphorylation and ADP-ribosylation in the endotoxin-treated rat myocardium and cardiomyocytes

To evaluate the effects of the in vivo endotoxin treatment of the rat on (1) the contractile responses in the subsequently isolated papillary muscle to adrenergic and cholinergic agonists and (2) the biochemical parameters (cyclic GMP, nitric oxide synthesis, protein phosphorylation and ADP-ribosyslation) in the subsequently isolated cardiomyocytes. Following the in vivo endotoxin treatment (4 mg/kg i.p., 18 h), contractile responses to increasing amounts of isoprenaline or to increasing amounts of oxotremorine in the presence of a fixed amount of isoprenaline were determined in isolated papillary strips. Activities of nitric oxide synthase, guanylyl cyclase, as well as phosphorylation of phospholamban and troponin-inhibitory subunit, and pertussis toxin-catalyzed and endogenous ADP-ribosylations were determined in the intact cardiomyocytes and subcellular fractions. The increase in the force of contraction by isoprenaline was reduced, while its inhibition by oxotremorine was greater in the endotoxin-treated papillary strips. The activities of both nitric oxide synthase, primarily of the inducible form of the enzyme, and cytosolic guanylyl cyclase were higher while the phosphorylations of both phospholamban and troponin-inhibitory subunit were of lesser magnitude in the cardiomyocytes following the in vivo endotoxin treatment. Pertussis toxin-catalyzed ADP-ribosylation of the 41 kDa polypeptide, which is the alpha subunit of Gi, was also decreased. The results of the present study support the postulate that alterations in both the cyclic AMP and cyclic GMP signalling cascade contribute to the myocardial dysfunction caused by endotoxin and cytokines.

Expressed alpha 1-adrenoceptors in adult rat brown adipocytes are primarily of alpha 1A subtype

The main objective of this study was to characterize the alpha 1-adrenoceptors expressed in adult rat brown adipocytes. For this purpose, membrane fractions were prepared from brown adipose tissue as well as from isolated brown adipocytes. The following are major findings: (i) BAT membranes were considerably enriched in alpha 1-adrenoceptors (specific [3H]prazosin binding, Bmax, 79.49 +/- 16.77 fmol/mg protein; KD, 0.24 +/- 0.04 nM); (ii) among the cells that comprise brown adipose tissue, brown adipocytes were enriched in alpha 1-adrenoceptors; (iii) > 95% of total alpha 1-adrenoceptors were resistant to inactivation by 20 microM chloroethylclonidine, which readily and essentially completely inactivated alpha 1B-adrenoceptors in rat liver membranes; (iv) brown adipose tissue membrane alpha 1-adrenoceptors showed high affinity towards 5-methyl urapidil (KD 7.23 +/- 2.49 nM) and WB 4101 (KD 0.66 +/- 0.30 nM) and low affinity towards BMY 7378 (KD 0.34 +/- 0.03 microM); essentially similar affinities for these drugs were seen for membranes prepared from brown adipocytes; and (v) EBDA/LIGAND analysis of 5-methyl urapidil, WB 4101, and BMY 7378 competition curves revealed the presence of a single binding site for these drugs. Recent work has documented that 5-methyl urapidil and WB 4101 interact with high affinity with alpha 1A-adrenoceptors, while BMY 7378 interacts with high affinity with alpha 1D-adrenoceptors. Taken together, these findings are consistent with the view that alpha 1-adrenoceptors expressed in adult rat BAT are mainly of the alpha 1A subtype.

Regulation of phospholamban and troponin-I phosphorylation in the intact rat cardiomyocytes by adrenergic and cholinergic stimuli: roles of cyclic nucleotides, calcium, protein kinases and phosphatases and depolarization

Protein phosphorylation was investigated in [32P]-labeled cardiomyocytes isolated from adult rat heart ventricles. The beta-adrenergic stimulation (by isoproterenol, ISO) increased the phosphorylation of inhibitory subunit of troponin (TN-I), C-protein and phospholamban (PLN). Such stimulation was largely mediated by increased adenylyl cyclase (AC) activity, increased myoplasmic cyclic AMP and increased cyclic AMP dependent protein kinase (A-kinase)-catalyzed phosphorylation of these proteins in view of the following observations: (a) dibutyryl-and bromo-derivatives of cyclic AMP mimicked the stimulatory effect of ISO on protein phosphorylation while (b) Rp-cyclic AMP was found to attenuate ISO-dependent stimulation. Unexpectedly, 8-bromo cyclic GMP was found to markedly increase TN-I and PLN phosphorylation. Both beta 1- and beta 2-adrenoceptors were present and ISO binding to either receptor was found to stimulate myocyte AC. However, the stimulation of the beta 2-AR only marginally increased while the stimulation of beta 1-AR markedly increased PLN phosphorylation. Other stimuli that increase tissue cyclic AMP levels also increased PLN and TN-I phosphorylation and these included isobutylmethylxanthine (non-specific phosphodiesterase inhibitor), milrinone (inhibits cardiotonic inhibitable phosphodiesterase, sometimes called type III or IV) and forskolin (which directly stimulates adenylyl cyclase). Cholinergic agonists acting on cardiomyocyte M2-muscarinic receptors that are coupled to AC via pertussis toxin(PT)-sensitive G proteins inhibited AC and attenuated ISO-dependent increases in PLN and TN-I phosphorylation. The in vivo PT treatment, which ADP-ribosylated Gi-like protein(s) in the myocytes, markedly attenuated muscarinic inhibitory effect on PLN and TN-I phosphorylation on one hand and, increased the beta-adrenergic stimulation, on the other. Controlled exposure of isolated myocytes to N-ethyl maleimide, also led to the findings similar to those seen following the PT treatment. Exposure of myocytes to phorbol, 12-myristate, 13-acetate (PMA) increased the protein phosphorylation, augmenting the stimulation by ISO, and such augmentation was antagonized by propranolol suggesting modulation of the beta-adrenoceptor coupled AC pathway by PMA. Okadaic acid (OA) exposure of myocytes also increased protein phosphorylation with the results supporting the roles for type 1 and 2A protein phosphatases in the dephosphorylation of PLN and TN-I. Interestingly OA treatment attenuated the muscarinic inhibitory effect which was restored by subsequent brief exposure of myocytes to PMA. While the stimulation of alpha adrenoceptors exerted little effect on the phosphorylation of PLN and TN-I, inactivation of alpha adrenoceptors by chloroethylclonidine (CEC), augmented beta-adrenergically stimulated phosphorylation. KCl-dependent depolarization of myocytes was observed to potentiate ISO-dependent increase in phosphorylation (incubation period 15 sec to 1 min) as well as to accelerate the time-dependent decline in this phosphorylation seen upon longer incubation. Verapamil decreased ISO-stimulated protein phosphorylation in the depolarized myocytes. Depolarization was found to have little effect on the muscarinic inhibitory action on phosphorylation. Prior treatment of myocytes with PMA, was found to augment ISO-stimulated protein phosphorylation in the depolarized myocytes. Such augmented increases were completely blocked by propranolol. Forskolin also stimulated PLN and TN-I phosphorylation. Prior exposure of myocytes to forskolin followed by incubation in the depolarized and polarized media showed that PLN was dephosphorylated more rapidly in the depolarized myocytes. The results support the view that both cyclic AMP and calcium signals cooperatively increase the rates of phosphorylation of TN-I and PLN in the depolarized cardiomyocytes during beta-adrenergic stimulation. (ABSTRACT TRUNCATED)

M1 muscarinic receptors heterologously expressed in cardiac myocytes mediate Ras-dependent changes in gene expression

Stimulation of alpha 1-adrenergic receptors in neonatal ventricular cardiomyocytes induces hypertrophic changes including activation of the atrial natriuretic factor (ANF) gene. This receptor couples to Gq to activate phospholipase C (PLC) and protein kinase C, which have been implicated as mediators of the hypertrophic response. To directly determine whether receptor coupling to Gq/PLC is sufficient to induce ANF expression, we expressed wild-type and chimeric muscarinic cholinergic receptors (mAChRs) with altered G-protein coupling properties in cardiac myocytes and examined their ability to activate an ANF promoter/luciferase reporter gene. The cholinergic agonist carbachol failed to induce transcriptional activation of the ANF reporter gene through endogenous Gi-linked M2mAChRs or in cells transfected with M2mAChRs. In contrast, in cells transfected with M1mAChRs, which effectively couple to Gq/PLC, carbachol increased ANF reporter gene expression 10-fold and also increased ANF protein, as determined by immunofluorescence. Carbachol-mediated ANF gene expression was inhibited by the mAChR antagonist pirenzepine with a Ki value characteristic of an M1mAChR. Studies using chimeric M1- and M2mAChRs demonstrated that the N-terminal 21 amino acids of the third intracellular loop of the M1mAChR were required for receptor coupling to ANF gene expression. This region, previously shown to specify receptor coupling to Gq/PLC, also conferred partial activity to a chimeric M2 receptor. We further demonstrated that M1mAChR coupling to ANF gene expression was Ras-dependent since co-expression of dominant-interfering Ala-15 Ras inhibited M1mAChR-induced ANF expression by 60%. In contrast, ANF expression induced by the chimeric M2 receptor was not blocked by dominant-interfering Ras. We suggest that receptor coupling to Gq/PLC is sufficient to induce ANF expression and that a Ras-dependent pathway contributes additional signals required for maximal M1mAChR-mediated ANF gene expression.

Vasopressin (V1) receptor characteristics in rat aortic smooth muscle cells

We report saturable, high-affinity, specific, reversible binding sites for both [3H]arginine vasopressin ([3H]AVP) and d(CH2)5Tyr(Me)-[3H]AVP, a V1-selective antagonist, in cultured smooth muscle cells obtained from rat aorta (RA) and rat mesenteric artery (RMA). Specific binding of [3H]AVP had the following characteristics in adherent monolayers of RA and RMA smooth muscle cells: dissociation constant (KD) = 1.42 and 1.23 nM and maximal binding capacity (Bmax) = 9,500 and 29,910 sites/cell, respectively. Lower KD and higher Bmax values were detected for 3H-labeled V1 antagonist binding to both types of cells. Complete dissociation of [3H]-AVP binding to RA cells occurred in a biphasic manner with two rate constants of dissociation, suggesting high- and low-affinity states of the binding site for the agonist interaction. Partial dissociation of the antagonist-specific binding occurred, and it was monophasic, suggesting interaction of the 3H-labeled V1 antagonist radioligand to the high-affinity binding state. Inhibition constant (Ki) values determined by competitive inhibition of [3H]AVP binding to RA cells by a series of AVP-related peptide analogues and antagonists were consistent with the saturation data. AVP in a concentration-dependent manner induced the accumulation of inositol phosphates [mean effective concentration (EC50) 1 nM] in the adherent RA cells. The free cytosolic Ca2+ level in the dispersed RA smooth muscle cells was increased by AVP (EC50 8.1 nM). Pretreatment with the V1 antagonist abolished these AVP-evoked responses. The data support the conclusion that the agonist binding occurs at a homogeneous population of V1-subtype receptors in the high-affinity (KD = approximately 1 nM) state and that these receptors are functionally coupled to phospholipase C.

Divalent cation-sensitive antagonist binding to heart muscarinic receptors

1. The binding of (-) [3H]quinuclidinylbenzilate (QNB), a potent muscarinic antagonist, to cardiac muscarinic receptors was examined in washed particles and microsomes isolated from rat heart atria. 2. Addition of divalent cations (Mg2+, Mn2+ and Ca2+) to the binding assays increased (6 to 8-fold) the antagonist binding to microsomes but exerted little or only weak (up to 35%) stimulation of the antagonist binding to washed particles. 3. Mg2+ ions present only in the dilution and washing buffer prior to rapid filtration of the binding reaction were capable of markedly increasing the antagonist binding to microsomes but not to washed particles. 4. Such differential action of Mg2+ on antagonist binding to subcellular fractions was seen when fractions were either isolated and or incubated in media containing either 10 or 50 mM imidazole-Cl, Tris-phosphate or Tris-Cl buffer, each at pH 7.5. While the modulatory effect of Mg2+ was not seen in Na-K-phosphate buffer, pH 7.5, the antagonist binding per se was much higher in Na-K-phosphate buffer compared to other buffers.

Muscarinic acetylcholine receptors in the sino-atrial node and right atrium of bovine heart

Muscarinic acetylcholine receptors were identified by the specific binding of [H](-)quinuclidinylbenzilate [( 3H](-)QNB) and [3H]oxotremorine-M [( 3H]Oxo-M), to membranes isolated from the sino-atrial (SA) node and right atrium (RA) of bovine heart. The density of [3H](-)QNB binding sites was greater in the SA node compared to the RA. Specific [3H](-)QNB binding was saturable and occurred to a single population of binding sites in both regions. The binding of antagonists, as assessed by competition with [3H](-)QNB, also occurred to a single population of sites; the binding affinities of all antagonists were similar in either region. Agonist competition curves, except for McN-A-343, were complex and computer analyses indicated that agonists bound to at least two populations of binding sites that differed in affinity. The proportion of high-affinity agonist binding sites was consistently greater in the SA nodal, relative to the RA membranes, while the affinity of the high-affinity agonist binding sites to a given agonist was essentially similar in either region. The high-affinity binding of [3H]Oxo-M was saturable and occurred to a single population of sites. The maximal binding of [3H]Oxo-M in the SA nodal membranes was higher than in the RA membranes. Guanine nucleotides and N-ethylmaleimide (NEM) markedly decreased [3H]Oxo-M binding; NEM did not appear to influence guanine nucleotide-dependent decrease in [3H]Oxo-M binding. Phospholipase A2 decreased both [3H](-)QNB and [3H]Oxo-M specific binding, the latter being affected to a greater extent. Phospholipase C also decreased [3H](-)QNB and [3H]Oxo-M binding, although to a lesser degree compared to phospholipase A2. Either lipase, however, increased the guanine nucleotide-sensitive agonist binding. Analysis of [3H](-)QNB binding to microsomal subfractions showed that binding sites were enriched in the light plasma membrane fractions that were also enriched in pertussis toxin sensitive guanine nucleotide binding proteins.

Effects of sulfhydryl agents, trifluoperazine, phosphatase inhibitors and tryptic proteolysis on calcineurin isolated from bovine cerebral cortex

Calcineurin was discovered as an inhibitor of calmodulin stimulated cyclic AMP phosphodiesterase and its ability to act as a calmodulin binding protein largely explains its inhibitory action on calmodulin regulated enzymes. Recent studies establish calcineurin as the enzyme protein phosphatase whose activity is regulated by calmodulin and a variety of divalent metals. In this work, we have investigated the effects of several agents including sulfhydryl agents, trifluoperazine (a calmodulin antagonist), PPi, NaF and orthovanadate and of tryptic proteolysis on the calcineurin inhibition of cyclic AMP phosphodiesterase (called inhibitory activity) and on protein phosphatase activity. Inhibitors for sulfhydryl groups (pHMB, NEM) inhibited phosphatase activity without any effect on the inhibitory activity. Dithioerythritol completely reversed the inhibition by pHMB. Limited proteolysis of calcineurin caused an activation of basal phosphatase activity with a complete loss of inhibitory activity. Phosphatase activity of the proteolyzed calcineurin was not stimulated by calmodulin. The presence of calmodulin along with calcineurin during tryptic digestion appeared to preserve the stimulation of phosphatase by Ca2(+)-calmodulin. [3H]-Trifluoperazine (TFP) was found to be incorporated irreversibly into calcineurin in the presence of ultraviolet light. This incorporation was evident into the A and B subunits of calcineurin. TFP-caused a decrease in the phosphatase activity and an increase in its inhibitory activity. [3H]-TFP incorporation into the A subunit was drastically decreased in the proteolyzed calcineurin. This was also true when the [3H]-TFP incorporated calcineurin was subjected to tryptic proteolysis. The incorporation into the B unit was essentially unaffected in the trypsinized calcineurin. Phosphatase activity was inhibited by orthovanadate, NaF, PPi, and EDTA. Inhibitions by these compounds were more pronounced when the phosphatase was determined in the presence of Ca2(+)-calmodulin than in their absence.

Divalent cation effects on calcineurin phosphatase: differential involvement of hydrophobic and metal binding domains in the regulation of the enzyme activity

The effects of divalent metals, metal chelators (EDTA, EGTA) and sodium dodecyl sulfate were investigated on the phosphatase activity of isolated bovine brain calcineurin assayed in the absence (called intrinsic) and presence of calmodulin. Intrinsic phosphatase was increased by Mn2+, was unaffected by Mg2+, Ca2+, and Ba2+, and was markedly inhibited by Ni2+, Fe2+, Zn2+ and Cu2+. When assayed in the presence of calmodulin, many divalent metals (Ni2+, Zn2+, Pb2+, Cd2+), besides Mn2+, increased modestly the phosphatase activity at low concentrations (10-100 microM) and inhibited it markedly at high concentrations. Ca2(+)-calmodulin stimulated phosphatase activity was antagonized by Ni2+, Zn2+, Fe2+, Cu2+, Pb2+, at low concentrations (50 microM), and by Ba2+, Cd2+ at slightly higher concentrations (greater than 100 microM); Mn2+ and Co2+ (50 microM to 1 mM) in fact augmented it. EDTA and EGTA in a concentration and time dependent fashion inhibited the intrinsic phosphatase activity, particularly that of trypsinized calcineurin. SDS in low concentrations (0.005%) augmented the phosphatase activity and inhibited it at high concentrations. Mn2+ (+/- calmodulin) and Ca2+ only with calmodulin present increased the phosphatase activity assayed with low concentrations of SDS. The EDTA dependent inhibition of intrinsic phosphatase was almost abolished in assays containing SDS. Prior exposure of calcineurin to Mn2+ led to a high activity conformation state of calcineurin that was 'long-lived' or 'pseudo-irreversible'. Such Mn2(+)-activated state of calcineurin exhibited no discernible change in the affinity towards myelin basic protein or its inhibition by trifluoperazine. At alkaline pH, Mg2+ supported the intrinsic phosphatase activity, although to a lesser degree than Mn2+. The latter cation, compared to Mg2+ and Ni2+, was also a more powerful stimulator of the calcineurin phosphatase assayed with histone (III-S) and myosin light chain as substrates.

Pertussis toxin-sensitive G-proteins in the sino-atrial node and right atrium of bovine heart

Three apparently distinct pertussis toxin (PTX)-sensitive substrates, with Mrs of 39, 40 and 41 kDa, were identified in membranes prepared from the sino-atrial (SA) node and right atrium of bovine heart. Based on their biochemical characterization, the effects of guanine nucleotides/MgCl2 on their PTX-catalyzed [32P]ADP ribosylation, and the PTX-induced decrease in radiolabelled agonist high-affinity binding to muscarinic acetylcholine receptors present in these membranes, we tentatively identify these proteins as the alpha-subunits of the G0 and Gi subtypes of G-proteins. These results indicate that PTX alters the G-protein modulation of SA nodal and atrial muscarinic acetylcholine receptors by disrupting at least one of a group of PTX-sensitive G-proteins present in these tissues.

MgCl2-sensitive and Gpp(NH)p-sensitive antagonist binding states of rat heart muscarinic receptors: preferential detection at ambient temperature assay and location in two subcellular fractions

Some novel observations dealing with antagonist binding to cardiac particulate muscarinic receptors are described. Gpp(NH)p increased (2-3 fold) the specific binding of [3H]-QNB or [3H]-NMS, both potent muscarinic antagonists, to washed particles (WP), but not microsomes (MIC), when the binding was conducted at 30 degrees C. Magnesium, on the other hand, increased (2-3 fold) the binding of these antagonists to MIC, but not to WP, under the same condition. The treatment of subcellular fractions with 0.2 mM N-ethylmaleimide (NEM), a sulfhydryl reagent, failed to significantly modify the respective stimulatory actions of either Gpp(NH)p on WP binding or of magnesium on MIC binding of these antagonists; treatment with dithiothreitol (1 mM) was also ineffective in this regard. Gpp(NH)p decreased Kd (WP) while magnesium increased Kd (MIC) for [3H]-QNB. Repeated freezing/thawing of isolated subcellular fractions abolished the stimulatory effect of magnesium on antagonist binding to MIC but not of Gpp(NH)p on WP antagonist binding; the freeze/thaw procedure per se increased MIC binding but not WP binding of these antagonists. When the binding was conducted at 4 degrees C (24 hr), the stimulatory effect of Gpp(NH)p on [3H]-QNB binding was enhanced (6-fold) in the case of WP and was detectable (80%) in the case of MIC. Under this condition, the stimulatory effect of magnesium on [3H]-QNB binding was also enhanced (5-fold) in the case of MIC and became evident (200%) in the case of WP. The results of this work support the following views: (a) antagonist-occupied cardiac muscarinic receptors are capable of interaction with guanine nucleotide binding proteins (G protein like Gi, Go) and such interaction influences antagonist binding properties (e.g. increased affinity) of the cardiac membrane-associated muscarinic receptors (b) magnesium influences (decreased affinity) antagonist binding properties by interacting with multiple sites of which some are likely associated with components other than G proteins of the particulate fractions (c) a pool of NEM-sensitive sulfhydryls involved in the regulation of Gpp(NH)p-sensitive agonist binding to cardiac muscarinic receptors is not involved in the regulation by either Gpp(NH)p or magnesium of antagonist binding in these subcellular fractions and (d) membrane fluidity and microenvironment surrounding the receptor and G proteins contribute to the actions of Gpp(NH)p and magnesium on antagonist binding.

Calcium antagonizes the magnesium-induced high affinity state of the hepatic vasopressin receptor for the agonist interaction

1. The present study describes the role of Ca2+ in the regulation of the hepatic vasopressin V1 receptor. With low concentrations of Ca2+, there was a small increase in [3H]-arginine vasopressin [( 3H]-AVP) binding, but above 10 mM, Ca2+ decreased the binding of this agonist. In contrast, low concentrations of Mg2+ were associated with a dramatic concentration-dependent increase in [3H]-AVP binding, reaching a maximal effect of 650% above control at concentrations ranging between 1-5 mM. At higher concentrations of Mg2+, the stimulatory effect of this cation was less pronounced, falling to 210% of control at 100 mM Mg2+. Strikingly, Ca2(+)-inhibited the stimulatory effect of Mg2+ in a concentration-dependent fashion. 2. Saturation binding data revealed that Ca2+ (2 to 10 mM) per se promotes the high affinity conformation of the V1 receptor for the agonist binding with the KD decreased from a control value of 2.3 nM to 0.5 nM in the presence of 10 mM Ca2+. This effect was attenuated with an increase in Ca2+ above 10 mM. With an increase in Ca2+ to 20 mM, however, the Bmax for [3H]-AVP binding was decreased. Ca2+ also decreased the high affinity/high capacity state (KD 100 pM) of the receptor induced by 1 mM Mg2+ for agonist interaction. 3. [3H]-V1 antagonist binding was inhibited by both Ca2+ and Mg2+. The IC50 values (mean +/- s.e. mean) for Ca2+ and Mg2+ were 32 +/- 8 and 53 +/- 9 mM respectively. Maximal inhibition achieved at 100 mM was 29% for Ca2+ and 42% for Mg2+. Both cations decreased the affinity and increased the capacity of the V1 receptor for the antagonist. 4. The results suggest that the divalent metal ion binding site(s) modulated by Mg2 + is also accessible to Ca2 +. Although Ca2 + opposes the powerful stimulatory effects of Mg2 + on agonist binding, the effects of Ca2+ and Mg2 + on the B,,x of [3H]-AVP binding were different, suggesting that the divalent cations may bind to two different sites, thereby regulating the affinity and the capacity characteristics of the V1 receptor.

Muscarinic cholinergic receptor mediated inhibitory transduction of adenylate cyclase activity in subcellular fractions from rat heart: improved detection in sodium phosphate buffer

Cholinergic inhibition of myocardial adenylate cyclase activity in cell-free fractions has been known for many years, although the reported degrees of inhibition have been rather modest (20-30%), notably in rat heart fractions. The present study conducted with rat heart subcellular fractions document following major findings: (1) Myocardial adenylate cyclase activity and notably its cholinergic inhibition in cell-free fractions are notoriously labile to storage at 4 degrees C whereas its stimulation by beta adrenergic receptor agonists or forskolin are reasonably well preserved during storage. (2) Among four buffers (Tris, glycylglycine, imidazole and sodium phosphate) examined, sodium phosphate buffer afforded the best preservation of cholinergic inhibitory response of adenylate cyclase. (3) The commonly used biochemical buffers, notably imidazole, exerted deleterious effect on the cholinergic inhibition of myocardial adenylate cyclase such that it was considerably attenuated or barely detectable; this explains, in part, the reported poor inhibition of myocardial enzyme by others. (4) Imidazole buffer, on the other hand, augmented beta adrenergic and forskolin stimulated adenylate cyclase activity. The likely significance of these findings is discussed from consideration that the observed differential influence of buffers results from differential actions on the interactions between the components (receptor/coupling G proteins/catalyst) comprising autonomic receptor coupled adenylate cyclase system in rat heart.

Two distinct, divalent cation-sensitive, antagonist binding states of heart muscarinic receptors: differential modulation by guanine nucleotide

1. The binding of [3H]quinuclidinylbenzilate (QNB), a muscarinic antagonist, to cardiac muscarinic receptors was investigated in two subcellular fractions (washed particles and microsomes) isolated from rat heart atria and ventricles. 2. 5'-guanylylimidodiphosphate (Gpp(NH)p, 0.1 mM), increased (2-3-fold) the binding to washed particles, but not to microsomes, whereas Mg2+ (1-20 mM) increased (up to 5-fold) the binding to microsomes, but not to washed particles. Gpp(NH)p modestly increased the affinity while Mg2+ decreased the affinity towards the radiolabelled antagonist. 3. Treatment with N-ethylmaleimide (NEM, 2 mM) increased the antagonist binding to either fraction. The stimulatory effect of Gpp(NH)p was not evident while that of Mg2+ survived in the NEM-treated fractions. 4. The treatment of fractions with divalent cations chelators (EDTA, EGTA; 10 mM) augmented the stimulatory effect of Mg2+ on [3H]QNB binding to microsomes while that of Gpp(NH)p on the washed particle [3H]QNB binding was decreased. Such treatment further revealed an inhibitory action (about 40%) of Mg2+ on the washed particle binding.

Hepatic vasopressin receptor: differential effects of divalent cations, guanine nucleotides, and N-ethylmaleimide on agonist and antagonist interactions with the V1 subtype receptor

Previously, we reported that magnesium (Mg2+) enhanced the binding affinity of arginine vasopressin [( 3H]AVP) to a single class of sites in rat liver microsomes. In the present study we have examined the effects of divalent cations and guanine nucleotides on the binding characteristics of both the nonselective agonist and the V1 receptor-selective antagonist, d(CH2)5Tyr(Me)-[3H]AVP, to microsomal and plasma membrane fractions of rat liver. At a subsaturating concentration (100 pM) of [3H]AVP, divalent cations increased specific binding in a concentration-dependent manner with the following rank order of potency: Co2+ greater than Mn2+ greater than Ni2+ greater than Mg2+ greater than Ca2+ = control. The maximal effect for Mg2+ was evident at 1 mM, a physiologically relevant concentration. In contrast, binding of the V1 receptor antagonist (at a subsaturating concentration of 10 pM) was inhibited by divalent cations, the rank order of potency being Mn2+ greater than Co2+ greater than Ca2+ greater than Mg2+ greater than Ni2+. The inhibitory effects of divalent cations were of lesser magnitude (up to 60%) compared to the stimulation of agonist binding (up to 700%). Mg2+ enhanced the affinity of [3H]AVP (Kd was decreased from approximately 2 nM to 133 pM), while the affinity of the [3H]V1 antagonist was decreased (Kd was increased from 10 to 95 pM). Scatchard analysis of saturation data (Mg2+ present) revealed similar maximum binding values for the binding of radiolabeled agonist and antagonist, indicating that AVP receptors in rat liver are mostly of the V1 subtype. Competition experiments between V1/V2-specific AVP analogs with either the radiolabeled agonist or antagonist also indicated the presence of predominantly V1 receptor sites in rat liver microsomes. The properties of plasma membrane receptor sites were similar to those of the microsomal sites, except that the density of receptors was higher in the former. In both equilibrium and competitive inhibition experiments GTPase-resistant analogs of guanine nucleotides, GTP gamma S and GDP beta S, decreased the affinity of the agonist for the receptor, but not that of the antagonist. Treatment of membranes with 0.2 mM N-ethylmaleimide (NEM) reduced the maximum binding of [3H]AVP and abolished the GTP gamma S-evoked decrease in agonist-binding affinity. In contrast, antagonist binding was unaffected by NEM. NEM pretreatment failed to influence the divalent cation-dependent increase in agonist-binding affinity. The results provide direct evidence for the existence of a high and a low affinity state of the hepatic V1 receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of a specific, high affinity [3H]arginine8 vasopressin-binding site on liver microsomes from different strains of rat and the role of magnesium

A single class of high affinity, low capacity, specific binding sites for [3H]arginine8 vasopressin (AVP) has been characterized in a plasma membrane-enriched microsomal fraction of the rat liver. Specific binding was saturable, linear with protein concentration, reversible, and 40-65% of the total binding. Binding at 25 C achieved a plateau after 30 min of incubation, whereas at 4 C, equilibrium was reached more slowly, and the level of binding was reduced. The presence of magnesium (Mg2+) in the assay medium enhanced the affinity of specific binding, while calcium and higher levels of sodium and potassium decreased binding. Scatchard analysis of binding in the presence of Mg2+ (5 mM) revealed an apparent mean +/- SE equilibrium dissociation constant (Kd) of 0.29 +/- 0.08 nM, with a maximal site density (Bmax) of 150.4 +/- 25.0 fmol/mg; in contrast, the Kd was 1.93 +/- 0.33 nM and the Bmax was 113.4 +/- 40.0 fmol/mg in the absence of Mg2+. No significant differences in Kd and Bmax were observed among membrane fractions derived from spontaneously hypertensive rats, Wistar-Kyoto rats, Long-Evans rats, and Brattleboro rats. Plasma levels of AVP were similar in spontaneously hypertensive, Wistar-Kyoto, and Long-Evans rats, but AVP was not detectable in the plasma from DI rats. Competitive inhibition of specific [3H]AVP binding by unlabeled AVP and related peptides showed the following Ki values: AVP, 0.19 nM; LVP, 1.7 nM; oxytocin, 41.4 nM; desamino AVP, 0.38 nM; [1-(beta-mercapto-beta, beta-cyclopentamethylene propionic acid) 4-Val,8-D-Arg] VP2-(O-methyl)tyrosine]AVP, 1.8 nM; desglycinamide AVP, 2.2 microM. The neuropeptide metabolite of AVP [[pGlu4,Cyt6] AVP-(4-9)], angiotensin II, and other unrelated peptides did not displace [3H]AVP, demonstrating the specificity of AVP and its related biologically active peptides for this binding site. Moreover, the rank order of potency for displacement of [3H]AVP binding by these various peptides parallels their reported glycogenolytic activity in liver and/or their agonistic or antagonistic potency in vascular smooth muscle. Finally, the Mg2+-induced increase in the affinity of [3H]AVP for this liver binding site is similar to the reported effect of Mg2+ on the contractile responses of vascular smooth muscle to AVP (i.e. increased affinity). The results are consistent with the interpretation that the high affinity receptor site characterized in rat liver microsomes is of the V1 type.

Interaction amongst calcineurin subunits. Stimulatory and inhibitory effects of subunit B on calmodulin stimulation of subunit A phosphatase activity depend on Mn2+ exposure of the holoenzyme prior to its dissociation by urea

Calcineurin was dissociated into subunits A and B by 6 M urea in the presence (method A) and absence (method B) of MnCl2 and dissociated subunits were isolated by gel filtration in urea in the absence (method B) or presence (method A) of MnCl2. Phosphatase activity was associated with the A subunit isolated by either method. The phosphatase activity (nmol/mg) of subunit A isolated by method A was greater (2-5-fold) than by method B. Mn2+ increased subunit A phosphatase and calmodulin further increased the enzyme activity. Subunit B isolated by method A or B increased Mn2+ + calmodulin stimulated subunit A phosphatase prepared by method B but interestingly and unexpectedly inhibited such stimulated activity of the subunit A prepared by method A. These results imply the tightly bound cation (in our case, most likely Mn2+) with subunit A dramatically and differentially influences the effects of two Ca2+-binding proteins, calmodulin and subunit B, on the subunit A phosphatase.

Resolution of bovine brain calcineurin subunits: stimulatory effect of subunit B on subunit A phosphatase activity

Calcineurin was dissociated into subunits A and B by SDS and the dissociated subunits were separated by Sephadex G-100 column chromatography in SDS. The phosphatase activity was associated with the A subunit and was detected only in the presence of MnCl2 of the various divalent cations tested. The Mn2+-dependent phosphatase of A subunit was stimulated (4-5-fold) by calmodulin. The subunit B increased only modestly Mn2+ stimulated phosphatase activity of subunit A but markedly increased it when assay also contained calmodulin. These results support the view that subunit B plays an important role in Mn2+/calmodulin regulation of subunit A phosphatase activity. They also lend further support to our earlier postulate ([1984] FEBS Lett. 169, 251-255) that Mn2+ is a powerful regulator of calcineurin phosphatase.

EGTA-sensitive and -insensitive forms of particulate adenylate cyclase in rat cerebral cortex: regulation by divalent cations and GTP

Interactions of several divalent cations (Mn2+, Ca2+, Co2+, Sr2+, and Zn2+) with EGTA-inhibitable adenylate cyclase were investigated in washed membranes (particles) isolated from the gray matter of rat cerebral cortex. The EGTA-inhibitable (called sensitive) enzyme activity was assayed in the presence of Triton X-100 since this detergent caused a marked increase (up to 20-fold) in the enzyme activity. The effects of various divalent metals (all added as chloride salt) indicated the presence of two distinct sites called site I and site II. At low concentrations (less than micromolar) Mn2+, Co2+, and Ca2+ increased (up to 10-fold) the enzyme activity to the same extent and appeared to act via binding to site I (high affinity site). The rank order of affinity was Mn2+ greater than or equal to Co2+ greater than Ca2+. Zn2+ showed the highest affinity and Sr2+ the lowest towards binding to site I; both these metals increased the enzyme activity to lesser extents than Mn2+, Co2+, or Ca2+. GTP was not required for the stimulation of this enzyme by low concentrations of Ca2+. The interaction of Mn2+ with site II (low affinity site) caused further increase in the enzyme activity, whereas Co2+, Ca2+, and Sr2+ were inhibitory at concentrations greater than 10 microM. Isolated fraction contained loosely and tightly associated pools of calmodulin. Myelin basic protein, but not calcineurin, inhibited the EGTA-sensitive adenylate cyclase activity. The EGTA-insensitive enzyme activity was increased by norepinephrine by mechanisms that depended on GTP and was inhibited by Ca2+. The stimulation of the EGTA-insensitive enzyme modulated the Mg2+ requirement such that Mg2+ binding to the low affinity site (site II) apparently occurred with higher affinity. The likely significance of these results is discussed with regard to (i) the presence of two classes of adenylate cyclase in rat cerebral cortex gray matter and (ii) the regulation of their activities by calmodulin-requiring and GTP-requiring mechanisms.

Isolation and characterization of calcineurin from bovine brain

Calcineurin was isolated from bovine cerebrum extracts by sequential chromatography on Affi-Gel blue and calmodulin affinity columns. Calcineurin so isolated was approximately 90% pure and was composed of equimolar amounts of subunit A (Mr = 61 000-63 000) and subunit B (Mr = 15 000-17 000) when examined by sodium dodecyl sulfate gel electrophoresis. A polypeptide (less than 10%) with Mr = 71 000 whose function and role remains to be investigated, was routinely detected in the calcineurin preparation. Both inhibitory activity (towards calmodulin-dependent cAMP phosphodiesterase) and phosphatase activity (with 32P-labelled myelin basic protein as substrate) were associated with calcineurin as evidenced by (i) coelution from Affi-Gel blue, Affi-Gel calmodulin, diethythaminoethyl-Sepharose, and Sephacryl S-200 chromatography columns; (ii) association with the same protein band on nondenaturing gels; (iii) similar stability upon storage at 4 degrees C and with repeated freezing and thawing; and (iv) parallel heat inactivation. Phosphatase activity of calcineurin was maximal with 32P-labelled myelin basic protein as the substrate. Using this substrate, enzyme activity was generally stimulated 5- to 10-fold in the presence of Ca2+ and calmodulin; half-maximal activation (A0.5) was observed with 25 nM calmodulin. Calmodulin increased the Vmax of the reaction without affecting the Km for the substrate. Optimum temperature and pH for the reaction were 45 degrees C and 7, respectively, in both the absence and presence of Ca2+ and calmodulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Phorbol ester inhibits myoblast fusion and activates beta-adrenergic receptor coupled adenylate cyclase

Primary cultures of myoblasts, derived from embryonic chick pectoral muscle, were treated with phorbol ester (TPA) for 8-96 h. TPA treatment blocked the fusion of myoblasts along with the expression of the MM form of creatine kinase. Interestingly, TPA treatment markedly increased the activity of beta-adrenergic receptor coupled adenylate cyclase (AC) activity. The study suggests that TPA treatment augments the functional interaction between a coupling Ns protein and catalytic unit of AC. The likely significance of these results is briefly presented.

Behaviour of cardiac microsomal Ca2+ pump under conditions that may simulate pathological situations

The behaviour of Ca2+ ATPase activity in relation to Ca2+ transport process was studied under different experimental conditions in canine cardiac microsomal fraction predominantly containing sarcoplasmic reticulum. The total Ca2+ concentration required for half maximal activation (Ka) of microsomal Ca2+ ATPase and Ca2+ uptake did not differ significantly, unless 0.1 mmol/l EGTA was present in the incubation media. Pretreatment of cardiac microsomes with membrane disruptive agents like phospholipase A, trypsin as well as deoxycholate strongly increased (2-3 fold) Ca2+ ATPase activity but uptake rate of Ca2+ declined. Only in phospholipase C and beta-glucuronidase pretreatment, a parallel decrease of Ca2+ ATPase and uptake was observed. In presence of excess (free)Ca2+ (greater than 10 mumol/l) both Ca2+ ATPase as well as Ca2+ uptake were inhibited, however, Ca2+ binding process remained unaltered. Likewise, low pH completely altered the relation between Ca2+ binding and ATPase activity; whereas Ca2+ ATPase was inhibited, Ca2+ binding did not change. Our present data provide evidence for some cellular factors that may be involved in producing uncoupling of microsomal Ca2+ ATPase from Ca2+ accumulation process that was previously observed in various pathological situations.

Comparison of cholinergic inhibition and beta-adrenergic stimulation of adenylate cyclase from rat and guinea-pig hearts: effects of guanine nucleotides and monovalent cations

Adenylate cyclase activities and the effects of isoproterenol (a beta-adrenergic receptor agonist), carbachol (a cholinergic receptor agonist) and forskolin (a plant diterpene) were determined in homogenates and washed particulate fractions prepared from rat and guinea-pig hearts. Many interesting differences were noted in the stimulatory and inhibitory effects of isoproterenol and carbachol respectively on the heart enzyme. The likely significance of these results is presented.

Intrinsic phosphatase activity of bovine brain calcineurin requires a tightly bound trace metal

The divalent metal requirement of intrinsic phosphatase activity was investigated using native and trypsinized calcineurin. This was assessed by examining (1) the stimulation of the enzyme by various metals, (2) the inhibition of the enzyme activity by metal chelators (EDTA and EGTA), and (3) the restoration by various metals of the activity of the EDTA-inhibited calcineurin phosphatase. The results supported the view that a tightly bound trace metal is necessary for expression of the phosphatase activity of calcineurin and implicate Mn2+ as the tightly bound metal.

Interaction of EGTA with a hydrophobic region inhibits particulate adenylate cyclase from rat cerebral cortex: a study of an EGTA-inhibitable enzyme by using alamethicin

Washed membranes isolated from rat cerebral cortex (gray matter) showed the presence of EGTA-inhibitable and EGTA-insensitive forms of adenylate cyclase activity. The former activity was stimulated by low concentrations (microM) of various divalent cations (Mn2+, Ca2+, Co2+ and Sr2+) assayed with MgATP2- and MgCl2. At higher concentrations (mM), only Mn2+ stimulated this enzyme whereas Ca2+, Co2+ and Sr2+ were inhibitory. Alamethicin markedly (up to 30-fold) increased the activity of EGTA-inhibitable form and only moderately of EGTA-insensitive form of the enzyme. The increased activity due to alamethicin does not result from solubilization of the enzyme from membranes. Our results suggest the presence of two distinct metal binding sites--one of high (Site I) and other of low (Site II) affinity. Divalent metals via interacting with these produce divergent effects on the enzyme. Site I appears to be located in the hydrophobic region of catalytic unit of the enzyme or of membrane-associated calmodulin. The likely significance of these results is briefly presented.

Characterization of Ca2+ release from the cardiac sarcoplasmic reticulum

The characteristics of Ca2+ release in relation to Ca2+ binding were studied in sarcoplasmic reticulum vesicles isolated from canine myocardium. The Ca2+ binding appeared to be dependent on ATP as a 4 fold increase in Ca2+ binding was observed upon the addition of ATP. In the presence of a suboptimal ATP concentration (20 mumol/l; without ATP regenerating system) a rapid release of Ca2+ started within 2 min. The rate of Ca2+ release was increased by increasing the concentration of Ca2+ in the preincubation medium when studied by diluting preloaded vesicles in medium free of Ca2+ and ATP; an apparent saturation was reached at 5 mmol/l Ca2+ but Ca2+ release again increased between 5 and 10 mmol/l Ca2+. High pH (8.0) enhanced the Ca2+ release process. When Ca2+ loaded vesicles were treated with various phospholipases and proteases, an enhanced Ca2+ release was observed in comparison to the control values. The release of Ca2+ was also increased by pharmacological agents like caffeine, ether and halothane. The Ca2+ release rate was stimulated by the p-chloromercurybenzoate treatment, which decreased ATP dependent Ca2+ binding and Ca2+-stimulated ATPase activities of the sarcoplasmic reticulum vesicles. The effect of temperature when evaluated by Arrhenius plots showed a higher energy of activation of Ca2+ release (66.15 kJ/mol) in comparison to that for Ca2+ binding (41.03 kJ/mol). These results indicate that, although Ca2+ release and Ca2+ binding activities of the cardiac sarcoplasmic reticulum appears to be related, Ca2+ release is probably a distinct process and is controlled differently. It seems that the Ca2+ release site in sarcoplasmic reticulum membranes is lipoprotein in nature.

Lowering temperature increases cardiac muscarinic receptor site affinity towards agonists and antagonists

1. The effect of temperature on the binding of agonists and antagonists to cardiac muscarinic receptor sites was investigated. 2. Compared to 37 degrees C, carbachol-[3H]QNB and atropine-[3H]QNB competition curves were shifted to the left at lower temperatures. This was also observed with other agonists and antagonists. 3. Such an effect of temperature persisted in the N-ethylmaleimide (NEM)-and dithiothreitol (DTT)-treated membranes. 4. Dissociation of the membrane-bound [3H]QNB was higher at 37 degrees C 40% at 60 min) compared to 18 degrees C (approximately 20%). Both carbachol and atropine increased the dissociation at 37 degrees C to a similar maximum (to about 60%). 5. The results show marked temperature-dependent alterations in the cardiac muscarinic receptor sites and suggest that these occur either by direct action of the temperature on receptor conformation or its indirect action via membrane fluidity. The likely significance of these results is discussed.

Calcium ion stimulated endogenous protein kinase catalyzed phosphorylation of basic proteins in myelin subfractions and myelin-like membrane fraction from rat brain

Polypeptide composition and endogenous phosphorylation were investigated in the subfractions of rat brain myelin isolated by either discontinuous or continuous sucrose density gradient centrifugation of myelin. Similarly, a myelin-like membrane fraction (SN4) was also studied. Observations were made that strongly indicated the presence of a calcium-stimulated protein kinase in a highly purified myelin preparation and which exclusively phosphorylated myelin basic proteins of the membrane preparation. Adenosine cyclic 3',5'-phosphate (cAMP) stimulated kinase on the other hand was found to be considerably enriched in the myelin-like membrane fraction. Although this latter enzyme is also capable of phosphorylating the basic proteins, its effect was at least 5 times weaker compared to the calcium-stimulated myelin protein kinase. Within the gradient subfractions there appeared a close relation between the amount of basic proteins and their calcium-stimulated phosphorylation; a similar relationship, however, was not obtained in the case of cAMP-dependent phosphorylation of myelin basic proteins. The former (i.e., Ca2+-stimulated phosphorylation) was found to require a protein factor that functionally resembled calmodulin. The results thus raises an interesting possibility of the presence of calmodulin-like proteins and a calcium-stimulated protein kinase in adult myelin membrane from mammalian brain, both of which have been hitherto unrecognized constituents of myelin membranes.

Requirement for sulfhydryl groups in the differential effects of magnesium ion and GTP on agonist binding of muscarinic cholinergic receptor sites in rat atrial membrane fraction

1. The binding of agonist (carbachol) and antagonist (atropine) to the rat heart atrial muscarinic cholinergic receptor sites was investigated. 2. Divalent cations (Mg2+ or Ca2+), in low concentrations, modestly increased the agonist binding affinity of the receptor site without any effect on the antagonist binding affinity. 3. Guanine nucleotides (e.g. GTP), on the other hand, decreased the agonist binding affinity (but not the antagonist binding affinity), and the extent of GTP effect depended on the absence or presence of divalent cation (Mg2+) in the binding assay. 4. Pretreatment of atrial membranes with N-ethylmaleimide (NEM) altered the agonist binding curve (obtained with varying concentrations of carbachol) such that the Hill coefficient (nH) became very close to 1.0, whereas the corresponding nH values for control (untreated) or dithiothreitol (DTT)-treated membranes were much less than 1.0; NEM or DTT treatments failed to show any effect of antagonist binding curve. 5. NEM treatment abolished both divalent cation-induced and guanine nucleotide-induced alterations in the agonist binding affinity of the receptor site. 6. Monovalent cations in low concentrations did not mimic the effects of Mg2+ or Ca2+ on agonist binding. Instead, concentration dependent decreases in both agonist and antagonist binding affinities and abilities were observed. Neither NEM nor DTT treatments failed to alter the monovalent cation effects on carbachol and atropine binding. 7. These observations indicate a likely involvement of -SH groups in the opposing effects of Mg2+ and guanine nucleotides (GTP) on cardiac muscarinic receptor-agonist interaction. The results further suggest some subtle in vitro differences in the brain and heart muscarinic receptor sites with regard to the influence by divalent cations and guanine nucleotides on the receptor-agonist interaction.

Subcellular Ca2+ transport in different areas of dog heart

Calcium transport and ATPase activities were determined in the heavy and mitochondrial fractions isolated from the left and right atria as well as ventricles of dogs. Ultrastructural distribution of these organelles in different areas of the myocardium was also examined. Calcium binding, calcium uptake, and calcium ATPase activities of the atrial microsomes were lower than those of the ventricles. On the other hand, mitochondrial calcium binding and uptake activities in the right atrium were higher than those in other areas. The mitochondrial total ATPase activities in the atria were also higher than those in the ventricles. Mitochondrial as well as microsomal yields from ventricles were significantly higher. Size and number of mitochondria in the ventricles were greater whereas no striking difference in the distribution of sarcoplasmic reticulum was apparent in different areas of the heart. Poorly developed calcium transport functions in the atrial microsomes may be one of the factors responsible for the generation of lower contractile force in this tissue in comparison with the ventricle.

Calcium ion-stimulated phosphorylation of myelin proteins

Myelin isolated from the central and peripheral nervous system contains a Mg2+-dependent protein kinase that catalyses phosphorylation of myelin-specific proteins. This phosphorylation is markedly stimulated by Ca2+ but not by cyclic AMP. Evidence was obtained that suggested an involvement of calmodulin-like protein in the stimulatory effects of Ca2+ on myelin phosphorylation.