Ryanodine receptor in different malignant hyperthermia-susceptible porcine muscles

The RYR1 g.1843C>T mutation is associated with the effect of the IGF2 intron3-g.3072G>A mutation on muscle hypertrophy

Muscle growth is a complex phenomenon regulated by many factors, whereby net growth results from the combined action of synthesis and turnover. In pigs, two quantitative trait nucleotides (QTN) are known to have an important influence on muscle growth and fat deposition: one QTN is located in the ryanodine receptor 1 (RYR1) gene (RYR1 g.1843C>T) and the other, a paternally expressed QTN, is in the insulin-like growth factor 2 (IGF2) gene (IGF2 intron3-g.3072G>A). The mutation in IGF2 abrogates in vitro interaction with a repressor, which leads to a threefold increase of IGF2 expression in post-natal muscle. The family of the calpains, a family of Ca(2+)-sensitive muscle endopeptidases, and their specific inhibitor calpastatin play an important role in post-natal protein degradation, also influencing muscle and carcass traits. This study investigated the possible interactions between the genotypes of the RYR1 and IGF2 QTN on IGF2 expression. Samples were taken from several muscles and from pigs at several ages, and messenger RNA expression levels were measured using a real-time quantification assay. IGF2 expression in m. longissimus dorsi of animals with mutations in both IGF2 and RYR1 was significantly lower than in animals that inherited the IGF2 mutation but were homozygous wildtype for RYR1.

Characterization of a cGMP-response element in the guanylyl cyclase/natriuretic peptide receptor A gene promoter

Previous studies have shown that atrial natriuretic peptide (ANP) can inhibit transcription of its receptor, guanylyl cyclase A, by a mechanism dependent on cGMP and have suggested the presence of a putative cGMP-response element (cGMP-RE) in the Npr1 gene promoter. To localize and characterize the putative cis-acting element, we have subcloned a 1520-bp fragment of the rat Npr1 promoter in an expression vector containing the luciferase reporter gene. Several fragments, generated by exonuclease III-directed deletions, were transiently transfected into cells to measure their promoter activity. Deletion from -1520 to -1396 of a 1520-bp-long Npr1 promoter led to a 5-fold increase in luciferase activity. Subsequent deletion to the position -1307 resulted in a decrease of luciferase activity by 90%. Preincubation of cells with 100 nM of ANP or 100 microM 8-bromo-cGMP inhibited luciferase activity of the 1520-bp and 1396-bp-long fragments, but not the activity of the 1307-bp fragment, suggesting that the cGMP-RE is localized between positions -1396 and -1307. The cGMP regulatory region was narrowed by gel shift assays and footprinting to position -1372 to -1354 from the transcription start site of Npr1 and indicated its interaction with transcriptional factor(s). Cross-competition experiments with mutated oligonucleotides led to the definition of a consensus sequence (-1372 AaAtRKaNTTCaAcAKTY -1354) for the novel cGMP-RE, which is conserved in the human (75% identity) and mouse (95% identity) Npr1 promoters.

TA repeat variation, Npr1 expression, and blood pressure: impact of the Ace locus

The activity of the atrial natriuretic peptide receptor (Npr1) is altered in spontaneously hypertensive rats (SHR) in relation to its mRNA levels, suggesting abnormal transcriptional control in hypertension. A single-stranded conformational polymorphism caused by a repetitive dinucleotide segment of 10 TA in BN-Lx and of 40 TA in SHR was localized at position -943 relative to the transcription start site of the Npr1 gene, downstream of a putative cGMP-regulatory region, and was the only sequence difference noted between the two strains. Transient transfections of -1520 to -920 Npr1 promoter-SV40-luciferase fusion vector showed that the construct from BN-Lx stimulated the SV40 promoter, whereas that from SHR slightly inhibited it. In contrast to the BN-Lx construct, the activity of the SHR fragment was refractory to downregulation by atrial natriuretic peptide. Genotype-phenotype correlation studies in recombinant inbred strains (RIS) derived from BN-Lx and SHR crosses revealed significant correlations of the TA repeat with basal guanylyl cyclase activity and Npr1 mRNA levels. The correlations were heightened by a locus on chromosome 10 containing the Ace gene. The highest basal guanylyl cyclase activity and Npr1 mRNA values were found in RIS with both genes (Npr1/Ace) of BN genotypes, whereas the lowest were recorded in RIS, with the SHR genotypes at both loci. This was inversely correlated with diastolic blood pressure in these strains. In conclusion, the longer TA repeat unit in the promoter of Npr1 of SHR, in tandem with a putative cGMP responsive element, regulates the transcription of the Npr1 gene with consequences on diastolic blood pressure.

Imaging caffeine-induced Ca2+ transients in individual fast-twitch and slow-twitch rat skeletal muscle fibers

Fast-twitch and slow-twitch rat skeletal muscles produce dissimilar contractures with caffeine. We used digital imaging microscopy to monitor Ca2+ (with fluo 3-acetoxymethyl ester) and sarcomere motion in intact, unrestrained rat muscle fibers to study this difference. Changes in Ca2+ in individual fibers were markedly different from average responses of a population. All fibers showed discrete, nonpropagated, local Ca2+ transients occurring randomly in spots about one sarcomere apart. Caffeine increased local Ca2+ transients and sarcomere motion initially at 4 mM in soleus and 8 mM in extensor digitorum longus (EDL; approximately 23 degrees C). Ca2+ release subsequently adapted or inactivated; this was surmounted by higher doses. Motion also adapted but was not surmounted. Prolonged exposure to caffeine evidently suppressed myofilament interaction in both types of fiber. In EDL fibers, 16 mM caffeine moderately increased local Ca2+ transients. In soleus fibers, 16 mM caffeine greatly increased Ca2+ release and produced propagated waves of Ca2+ (approximately 1.5-2.5 microns/s). Ca2+ waves in slow-twitch fibers reflect the caffeine-sensitive mechanism of Ca2(+)-induced Ca2+ release. Fast-twitch fibers possibly lack this mechanism, which could account for their lower sensitivity to caffeine.

Differential distribution and subcellular localization of ryanodine receptor isoforms in the chicken cerebellum during development

The distribution of ryanodine receptor (RyR) isoforms was examined using isoform-specific monoclonal antibodies in the developing chicken brain, from E18 through adulthood, using light and electron microscopic immunocytochemistry. Monoclonal antibody 110F is specific for the alpha-skeletal muscle form of RyR, while monoclonal antibody 110E recognizes both the beta-skeletal muscle and cardiac isoforms, but does not distinguish between the two. Significant differences in the distribution of the alpha- and beta/cardiac forms were observed. Labeling for the alpha-form was restricted to cerebellar Purkinje neurons while the beta/cardiac form was observed in neurons throughout the brain. A major finding was the presence of labeling for the beta/cardiac in presynaptic terminals of the parallel fibers in the molecular layer and the mossy fiber terminals in the granular layer glomeruli in late development and during adulthood. Labeling for the beta/cardiac, but not the alpha-form, underwent a major redistribution in the cerebellum during the course of development. At 1 day of age, beta/cardiac labeling was present mainly in Purkinje neurons. From 1 day to 4 weeks, immunolabeling for the beta/cardiac form gradually disappeared from Purkinje neurons, but increased in granule cells. Within the molecular layer, the labeling pattern changed from being primarily within Purkinje dendrites to a more diffuse pattern. Electron microscopic examination of the cerebellar molecular layer of 2-week-old chicks revealed that beta/cardiac-labeling was mainly present in the axons and presynaptic processes of the parallel fibers. No developmental changes were observed in other brain regions. This study represents the first demonstration of ryanodine receptor immunoreactivity in presynaptic boutons and suggests that the ryanodine receptor may modulate neurotransmitter release through local regulation of intracellular calcium in the parallel fiber synapse.

Pharmacological distinction between dantrolene and ryanodine binding sites: evidence from normal and malignant hyperthermia-susceptible porcine skeletal muscle

Dantrolene inhibits and ryanodine stimulates calcium release from skeletal-muscle sarcoplasmic reticulum (SR), the former by an unknown mechanism, and the latter by activating the ryanodine receptor (RyR), the primary Ca2+-release channel of SR. Dantrolene is used to treat malignant hyperthermia (MH), a genetic predisposition to excessive intracellular Ca2+ release upon exposure to volatile anaesthetics. Porcine MH results from a point mutation in the SR RyR that alters the open probability of the channel, and is reflected in altered [3H]ryanodine binding parameters. Specific binding sites for [3H]dantrolene and [3H]ryanodine co-distribute on SR that has been isolated by discontinuous sucrose gradient centrifugation. If the two drug-binding sites are functionally linked, [3H]dantrolene binding might be affected both by pharmacological and by genetic modulators of the functional state of the RyR. Accordingly, we compared the characteristics of [3H]dantrolene binding to porcine malignant-hyperthermia-susceptible and normal-skeletal-muscle SR, and examined the effects of RyR modulators on [3H]dantrolene binding to these membranes. Additionally, the feasibility of separating the SR binding sites for [3H]dantrolene and [3H]ryanodine was investigated. No significant differences in [3H]dantrolene binding characteristics to SR membranes from the two muscle types were detected, and the Bmax ratio for [3H]dantrolene/[3H]ryanodine was 1.4(+/-0.1):1 in both muscle types. [3H]Dantrolene binding is unaffected by the RyR modulators caffeine, ryanodine, Ruthenium Red and calmodulin, and neither dantrolene nor azumolene have any effect on [3H]ryanodine binding. Additionally, distinct peaks of [3H]dantrolene and [3H]ryanodine binding are detected in SR membranes fractionated by linear sucrose centrifugation, although no differences in protein patterns are detected by SDS/PAGE or Western-blot analysis. We suggest that the binding sites for these two drugs are pharmacologically distinct, and may exist on separate molecules.

Thrombin inhibits atrial natriuretic peptide receptor activity in cultured bovine endothelial cells

Thrombin and the atrial natriuretic peptide (ANP) possess a number of functionally antagonistic properties in vascular endothelial cells. Thus, regulatory interactions that modulate the activity of one or the other could have important sequelae with regard to cardiovascular homeostasis. Thrombin treatment effected a dose- and time-dependent reduction in ANP receptor activity (maximal 70% to 80% inhibition) in cultured bovine aortic endothelial cells. This resulted from a decrease in total receptor number as well as a modest reduction in the affinity of the receptor for its ligand. The inhibition was largely confined to the type C receptor population, in that thrombin had no effect on maximal type A receptor-linked cGMP accumulation. The protein kinase C-activating phorbol ester 12-O-tetradecanoylphorbol 13-acetate effected a similar reduction in binding activity; however, suppression of protein kinase C activity did not reverse the thrombin effect. Pretreatment of endothelial cells with cycloheximide did not completely prevent the thrombin-dependent inhibition, and thrombin did not effect a reduction in type C receptor mRNA levels, findings that argue for a postsynthetic inhibitory locus. The inhibition of receptor activity was effectively irreversible in that suspension of protein synthesis blocked the recovery of receptor density on the cell surface. Reduction in type C receptor density was accompanied by modest increases in the stability of ANP in the culture medium and enhancement of the cellular cGMP response to the peptide, particularly at low ligand concentrations. These findings demonstrate a potentially important interaction between these two agonist systems in regulating endothelial cell function within the vascular wall.

Atrial natriuretic peptide suppresses the transcription of its guanylyl cyclase-linked receptor

Atrial natriuretic peptide (ANP) treatment of rat aortic smooth muscle cells suppressed both 125I-ANP binding and ANP-dependent cGMP accumulation, suggesting reductions in the type C (NPR-C) and type A (NPR-A) natriuretic peptide receptor populations, respectively. NPR-A, but not NPR-C, mRNA levels were reduced in a dose-dependent fashion by ANP. The latter effect appeared to be due, at least in part, to suppression of NPR-A gene promoter activity. ANP effected a dose- and time-dependent reduction in a transiently transfected NPR-A luciferase reporter (-1575LUC). Analysis of 5' deletion mutants of the NPR-A promoter demonstrated that the ANP-dependent sequence lies between -1575 and -1290 relative to the transcription start site. Inhibition of the ANP promoter was also effected by brain natriuretic peptide, type C natriuretic peptide, and 8-bromo-cGMP, but not by the NPR-C-selective ligand cANF. In the case of 8-bromo-cGMP, the responsive element(s) was localized to the same 285-base pair region linked to the ANP effect above. These findings indicate that ANP autoregulates its own receptors in these cells and, at least in the case of NPR-A, it does so through suppression of receptor gene expression and receptor synthesis. This suppression may operate through a cGMP-dependent element located more than a kilobase upstream from the transcription start site.

Ursodeoxycholate-induced hypercholeresis in cirrhotic rats: further evidence for cholehepatic shunting

The aim of the investigation was to explore whether ursodeoxycholate, a tertiary bile acid with potential for treatment of chronic cholestasis in cirrhotic liver disease, has the same physiological effects in cirrhotic as in normal rats. Furthermore, we wanted to investigate whether ductular proliferation, as it occurred in this situation, increases the bicarbonate stimulatory effect of ursodeoxycholate. Rats (n = 16) were rendered cirrhotic by continuous exposure to phenobarbital-carbon tetrachloride; untreated animals (n = 13) served as controls. In cirrhotic rats in vivo, ursodeoxycholate (20 mumoles/min/kg) stimulated bile salt secretion and bile flow less than in controls. Nevertheless, the increment in ursodeoxycholate-induced biliary bicarbonate--the bicarbonate stimulatory potency--was increased by 29% in cirrhotic animals (0.55 +/- 0.08 mmol vs. 0.71 +/- 0.11 mmol; p < 0.05). This finding could be related to ductular proliferation because the volume fraction of bile ductules, determined stereologically, increased from 0.3% +/- 0.1% to 2.7% +/- 0.6% in cirrhotic rats (p < 0.005). To explore further the behavior of ductules during ursodeoxycholate stimulation, we carried out experiments in the in situ perfused rat liver. In the portally perfused organ, replacement of bicarbonate by tricine-acetate abolished ursodeoxycholate-induced hypercholeresis. In the dually perfused organ (perfusion of both portal vein and hepatic artery) perfusion of the hepatic artery with bicarbonate-containing buffer, ursodeoxycholate had a similar stimulatory effect as in vivo in both control and cirrhotic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic ADP-ribose does not affect cardiac or skeletal muscle ryanodine receptors

The cardiac muscle isoform of the ryanodine receptor/Ca2+ release channel (RYR) has been proposed to be an important target of cyclic ADP-ribose (cADPR) action in mammalian cells. However, we now demonstrate that neither cADPR (0.1-5 microM), nor the related metabolites beta-NAD+ (0.1-30 mM) and ADP-ribose (0.1-5 microM), affected cardiac RYR activity as determined by [3H]ryanodine binding to cardiac sarcoplasmic reticulum (SR) vesicles. Similarly, cADPR (1 microM) failed to activate single cardiac RYR channels in planar lipid bilayers. Skeletal muscle SR [3H]ryanodine binding was also unaffected by cADPR (up to 30 microM). These results argue against a direct role for the well-characterized RYRs of cardiac or skeletal muscle in mediating cADPR-activated Ca2+ release.

Immunohistochemical detection of chloride/bicarbonate anion exchangers in human liver

Sodium-independent Cl-/HCO3- exchange activity has been observed in isolated rat hepatocytes and intrahepatic bile duct epithelial cells, where it is involved in intracellular pH regulation and, possibly, biliary bicarbonate secretion. Monoclonal antibodies to the membrane domain of human chloride/bicarbonate anion exchanger proteins, AE1 and AE2, were prepared so that we might determine by immunohistochemical methods the presence and location of these antiporters in the human liver. To obtain the antibody against AE1, we immunized mice with injections of washed human erythrocytes. The selected monoclonal antibody was found to be specific for the 17-kD proteolytic membrane fragment of AE1 protein. The antibody to AE2 was produced with a 14-mer synthetic peptide, whose sequence corresponds specifically to amino acid residues 871 to 884 in the deduced primary structure of human kidney AE2 protein. When the monoclonal antibody to AE2 peptide was employed for the immunohistochemical study of liver specimens (by both immunofluorescence and immunoperoxidase), a clearly defined staining was present at the canalicular membrane of hepatocytes, as well as the luminal side of the membrane of bile duct epithelial cells from small and medium-sized bile ducts. No staining was observed in the liver parenchyma with the monoclonal antibody to AE1, which instead strongly decorated the erythrocytes in liver blood vessels. We conclude that AE2 immunoreactivity is present in human liver, where it localizes very specifically to the membrane regions, which appear most probably involved in the transport of bicarbonate to bile (i.e., the canalicular membrane of hepatocytes and the apical side of epithelial cells of small and medium bile ducts).

Microtubule-dependent transport of bile salts through hepatocytes: cholic vs. taurocholic acid

Studies with taurine-conjugated bile salts have demonstrated two pathways for hepatocellular delivery of bile salts to bile: a cytosolic, microtubule-independent pathway and a membrane-based, microtubule-dependent pathway. However, a significant portion of circulating bile salts may be unconjugated. To determine whether free bile salts utilize similar pathways, we examined the effect of colchicine on the biliary excretion of intravenously administered cholic acid and taurocholate in intact rats. Basal rats were pretreated with low-dose colchicine or its inactive isomer, lumicolchicine, 1 hr before placement of intravenous and biliary cannulas and 2.75 hr before intravenous injection of [14C]cholic acid and [3H]taurocholate. Superfused rats were prepared as above but with intravenous infusion of taurocholate at 200 nmol/min.100 gm beginning 0.75 hr before [14C]cholic acid/[3H]taurocholate injection. Depleted/reinfused rats were subjected to biliary diversion for 20 hr before colchicine or lumicolchicine pretreatment, infusion of taurocholate and [14C]cholic acid/[3H]taurocholate injection. In each group, biliary excretion of [14C]taurocholate and [3H]taurocholate was inhibited equally by colchicine; for peak excretion rates the respective inhibition values were 33% and 35% in basal rats, 63% and 65% in superfused rats, and 74% and 76% in depleted/reinfused rats. Biliary excretion of [14C]taurocholate occurred consistently later than excretion of [3H]taurocholate, and maximal rates of excretion were reduced. In contrast, plasma uptake rates of [14C]cholic acid and [3H]taurocholate were essentially the same in depleted/reinfused rats. Deconvolution analysis of [14C]taurocholate vs. [3H]taurocholate biliary excretion curves revealed no significant differences among experimental groups. We conclude that conversion of [14C]cholic acid to [14C]taurocholate slightly retards its biliary excretion and diminishes its peak excretion rate compared with exogenous [3H]taurocholate.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of natriuretic peptides on vasopressin neurons mediated through cGMP and cGMP-dependent protein kinase in vitro

The effects of natriuretic peptides on electrical activity and cellular cGMP levels were studied in neurons of the supraoptic nucleus (SON) of rat hypothalamic slice preparations. Intracellular and extracellular recordings showed that bath application of A type natriuretic peptide (ANP) at 100 nM or B type natriuretic peptide (BNP) at 100 to 300 nM decreased the firing rate and hyperpolarized the membrane potential in phasically firing (putative vasopressin) neurons. Non-phasically firing (putative oxytocin) neurons did not respond to these natriuretic peptides in firing rate or membrane potential. The membrane-permeable cGMP analogue 8-bromo cGMP at 0.5 mM and the phosphodiesterase inhibitor 3/isobutyl-1-methylxanthine (IBMX) at 50 microM mimicked the inhibitory effects of ANP and BNP. The specific inhibitor of cGMP phosphodiesterase 1-(3-chloroanilino)-4-phenylphthalazine+ ++ (MY5445) at 30 microM also decreased the firing rate of SON neurons. The cGMP-dependent protein kinase inhibitor N-(2-(methylamino)ethyl)-5-isoquinoline-sulfonamide dihydrochloride (H8) at 1 microM abolished the inhibition by natriuretic peptides. We measured cGMP and cAMP contents in discrete SON regions and compared the change of contents before and after application of ANP and BNP. The increases in cellular cGMP accumulation were 430% for ANP and 120% for BNP, although they did not cause significant change of cAMP accumulation. The results suggest that the inhibitory effects of natriuretic peptides on putative vasopressin neurons are mediated through cGMP and cGMP-dependent protein kinase.

Phosphorylation of the porcine skeletal and cardiac muscle sarcoplasmic reticulum ryanodine receptor

Porcine skeletal and cardiac muscle sarcoplasmic reticulum (SR) vesicle fractions enriched in the ryanodine receptor were phosphorylated in the presence of [gamma-32P]MgATP and either exogenous cAMP-dependent protein kinase (cAMP-PK), or Ca2+ plus calmodulin. Phosphorylation of the cardiac muscle ryanodine receptor in the presence of either cAMP-PK or calmodulin (6.4 and 10.6 pmol Pi/mg SR respectively) was approximately equal to or twice the [3H]ryanodine binding activity of this preparation (5.2 pmol/mg). Furthermore, cardiac muscle ryanodine receptor Pi incorporation catalyzed by cAMP-PK and calmodulin was approximately additive. In skeletal muscle SR, however, the level of cAMP-PK or calmodulin catalyzed phosphorylation of the intact ryanodine receptor (0.2 or 2.9 pmol Pi/mg SR, respectively) was much less than the [3H]ryanodine binding activity of this fraction (11.6 pmol/mg). Furthermore, Pi incorporation into the intact skeletal muscle ryanodine receptor was 3-8-fold less than that incorporated into a component of slightly lower M(r). Although this latter component comigrated with an immunoreactive fragment of the ryanodine receptor on polyacrylamide gels, it did not appear to be derived from the ryanodine receptor. We conclude that the significant phosphorylation of the cardiac muscle SR ryanodine receptor indicates a likely physiological role for protein kinase-mediated regulation of this Ca(2+)-channel. In contrast, the minimal phosphorylation of the skeletal muscle SR ryanodine receptor indicates that such a role of protein kinases is unlikely in this tissue.

High affinity [3H]ryanodine binding sites in postmortem human brain: regional distribution and effects of calcium, magnesium and caffeine

The pharmacological properties, regional distribution and autoradiographic localization of [3H]ryanodine binding sites were examined in postmortem human brain. Analyses of binding data from labeled ryanodine titration experiments conducted in frontal cortex revealed a single class of high affinity binding sites with a Kd value of 3.6 nM and a Bmax value of 99 fmol/mg protein. In unlabeled ryanodine titration experiments, Kd and Bmax values were 6.5 nM and 132 fmol/mg protein, respectively. Binding was found to be dependent on free Ca2+ (ED50 value, 89 microM) and was decreased by 35% in the presence of 5 mM Mg2+. This Mg2+ inhibition was abolished by the addition of 10 mM caffeine. Analysis of the regional distribution of [3H]ryanodine binding in membrane preparations revealed high levels of sites in putamen and caudate nucleus, intermediate levels in hippocampus and cortex, and low levels in cerebellum. Autoradiographically, the hippocampus displayed a high density of binding sites in the CA3 region and the dentate gyrus. Ryanodine binding sites in human brain exhibit similar, but not identical binding and pharmacological characteristics to ryanodine receptors previously identified in muscle and more recently in rat and rabbit brain and accordingly may be involved in the regulation of intracellular calcium.