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PubMedDate not supplied

Relationship of cardiac muscle tension to Na+,K+-adenosine triphosphatase activity after chronic digitoxin administration in cats.

To obtain a better understanding of the mechanism of action of the cardiac glycosides, we examined inotropic and biochemical effects of digitoxin in myocardium from cats chronically exposed to the drug. The mechanical function of papillary muscles was tested isometrically and left ventricular tissue was analyzed for Na+,K+-dependent adenosine triphosphatase ATPase activity. Muscles from control cat hearts developed tension at 2.5 +/- 0.7 g/mm2; muscles from cats that received subcutaneous digitoxin--100 micrograms/kg on day 1, followed by 40 micrograms/kg/day for 4 days (group A), and 75 micrograms/kg on day 1, followed by 25 micrograms/kg/day for 9 days (group B)--developed significantly greater (p less than 0.05) tension of 4.8 +/- 0.3 and 3.6 +/- 0.6 g/mm2, respectively. Further, in vitro maximal responsiveness to digitoxin was greater in the muscles from digitalized groups than in controls (p less than 0.05): Muscles from control cats had a maximal response to in vitro addition of digitoxin of 3.5 +/- 0.1 g/mm2; muscles from cats in group A reached 4.9 +/- 0.3 g/mm2, and those from group B, 4.5 +/- 0.7 g/mm2. Specific activity of microsomal Na+,K+-ATPase from hearts of digitalized groups A and B was inhibited by 50-70% (p less than 0.01). Developed tension, specific Na+,K+-ATPase activity, and in vitro maximal responsiveness to digitoxin in a third group (C) of cats receiving the least daily digitoxin (75 micrograms/kg on day 1, followed by 15 micrograms/kg/day for 29 days) were not different from controls. Mean plasma digitoxin concentrations were 33, 16, and 3 ng/ml in groups A, B, and C, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Differential inhibition of calmodulin-sensitive phosphodiesterase and Ca++-adenosine triphosphatase by chlorpromazine-linked calmodulin.

Upon irradiation with UV light, chlorpromazine binds irreversibly to calmodulin and inactivates it. To determine whether this chlorpromazine-calmodulin (CPZ-CaM) complex can inhibit the actions of native calmodulin, we examined its effects on the activity of calmodulin-sensitive cyclic nucleotide phosphodiesterase from rat brain and on the Ca++-adenosine triphosphatase (ATPase) of human erythrocyte membranes. The CPZ-CaM complex was prepared by irradiating purified bovine brain calmodulin in the presence of chlorpromazine and Ca++. The sample was then dialyzed extensively to remove reversibly bound chlorpromazine and then assayed for its ability to activate calmodulin-sensitive phosphodiesterase and Ca++-ATPase, and for its ability to block the stimulatory effects of native calmodulin on these enzymes. The CPZ-CaM complex had no effect on the basal activity of either enzyme; it neither activated nor inhibited the enzymes when assayed in the absence of calmodulin. However, it affected differentially the activation of the two enzymes by native calmodulin. The CPZ-CaM complex totally inhibited calmodulin-stimulated phosphodiesterase but had no effect on the activation of the ATPase by calmodulin. Other studies showed that CPZ-CaM increased the activation constant (Ka) for the interaction of calmodulin with phosphodiesterase but did not affect the maximal activation (Vmax) of the enzyme by calmodulin. Neither calmodulin nor CPZ-CaM altered the Km for the interaction between phosphodiesterase and cyclic AMP. These results suggest that CPZ-CaM inhibits the calmodulin-induced activation of phosphodiesterase by competing with calmodulin for regulatory sites on the enzyme and not by interacting with calmodulin itself or by blocking the interaction of cyclic AMP with the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1976

Location of an oligomycin-insensitive and magnesium ion-stimulated adenosine triphosphatase in rat spleen mitochondria.

1. When rat spleen mitochondria are incubated with oxidizable substrates, added MgCl2 (greater than 150 muM free concentration) markedly stimulates state-4 respiration and lowers both the respiratory control and ADP/O ratios; this effect is reversible on addition of excess of EDTA. 2. With [gamma-32P]ATP as substrate, an Mg2+-stimulated ATPase (adenosine triphosphate) was identified in the atractyloside-insensitive and EDTA-accessible space of intact rat spleen mitochondria. 3. Oligomycin has no effect on the activity of the Mg2+-stimulated ATPase at a concentration (2.0mug/mg of protein) that completely inhibits the atractyloside-sensitive reaction. Of the two ATPase activities, only the atracytoloside sensitive reaction is stimulated (approx. 40%) by dinitrophenol. 4. On digitonin fractionation the atractyloside-insensitive Mg2+-stimulated ATPase co-purifies with the outer membrane-fraction of rat spleen mitochondria, whereas (as expected) the atractylosidesensitive activity co-purifies with the inner-membrane plus matrix fraction. 5. Stoicheiometric amounts of ADP and Pi are produced as the end products of ATP hydrolysis by purified outer-membrane fragments; no significant AMP production is detected during the time-course of the reaction. 6. The outer-membrane ATPase is present in rat kidney cortex and heart mitochondria as well as in spleen, but is absent from rat liver, thymus, brain, lung, diaphragm and skeletal muscle.

Adenosine Triphosphatases↗
PubMed1991

Involvement of vacuolar H(+)-adenosine triphosphatase activity in multidrug resistance in HL60 cells.

HL60 cells isolated for resistance to vincristine (HL60/Vinc cells) or doxorubicin (HL60/Adr cells) contain enhanced levels of an energy-dependent drug efflux pump. HL60/Vinc cells contain the drug transporter P-glycoprotein, whereas the HL60/Adr isolate does not. In the present study, we examined the possible involvement of vacuolar H(+)-adenosine triphosphatase (H(+)-ATPase) activity in drug resistance in HL60 cells. We utilized bafilomycin A1, an agent which selectively inhibits vacuolar H(+)-ATPase activity at low concentrations. The results showed that bafilomycin A1 induced a major increase in drug accumulation and inhibited drug efflux in both HL60/Adr cells and HL60/Vinc cells. Similar results were obtained with 7-chloro-4-nitrobenz-2-oxa 1,3 diazole, an agent which is also capable of inhibiting vacuolar H(+)-ATPase. Azide, an inhibitor of F1F0 mitochondrial ATPase, and vanadate and ouabain, which are inhibitors of E1E2-type ATPase, did not affect drug levels in resistant cells. We also observed that bafilomycin A1 did not compete with [3H]azidopine binding to P-glycoprotein. Thus, bafilomycin A1 does not appear to function as a substrate for P-glycoprotein. These results suggest an involvement of vacuolar H(+)-ATPase activity in the pathway of drug efflux from HL60/Adr cells and HL60/Vinc cells. The mechanism of this action remains to be determined.

Anti-Bacterial Agents↗
PubMed1994

Age-related changes in fibre number, fibre size, fibre type composition and adenosine triphosphatase activity in rat soleus muscle.

To study the aging of muscle fibres in red skeletal muscle, fibre number, fibre diameter and fibre type composition in the soleus muscle of male rats of 3, 12 and 24 months old were examined. The total number of muscle fibres remained unchanged, while average diameter increased slightly with increasing age. The staining intensity of myosin adenosine triphosphatase (ATPase) activity in the fibres decreased with advancing age. Therefore, observation on the basis of myosin ATPase histochemistry alone is not adequate to study the aging of muscle fibres. In the muscles of 24 month-old animals, four fibre types were recognized; 1) many (52%) type I-O fibres showing weak ATPase and succinate dehydrogenase (SDH) reactions with slight subsarcolemmal aggregates of diformazan (SAD); 2) some (33%) type M fibres showing weak ATPase and intense SDH reactions with marked SAD; 3) a few (12%) type O fibres showing weak ATPase and intense SDH reactions without SAD; and 4) very few (4%) type IIA fibres. Histochemical and morphometric results suggest that type I-O, type M and type O fibres are derived from type I, type I and type IIA fibres, respectively. Furthermore, no transitional fibres from type IIA to type I were observed. Therefore, age-related changes in fibre type composition in the muscle cannot be explained by the simple idea that most type IIA fibres are transformed into type I fibres.

Aging↗
PubMed1997

Alpha 1-adrenergic stimulation inhibits 3,5,3'-triiodothyronine-induced expression of the rat heart sarcoplasmic reticulum Ca2+ adenosine triphosphatase gene.

The interactions between the beta-adrenergic system and thyroid hormone (T3) on cardiac function have been investigated in detail. In addition to beta-adrenoceptors, alpha 1-adrenergic receptors are present in the mammalian heart. The interactions between T3 and the alpha 1-adrenergic system remain, however, poorly understood. T3 stimulates the expression and transcription of the sarcoplasmic reticulum Ca2+ adenosine triphosphatase (SERCA2) gene, a protein vital in the control of cardiac calcium transients and contractility. We show that in rat cardiac myocytes, the stimulatory effect of T3 on SERCA2 messenger RNA expression and gene transcription is inhibited by an alpha 1-adrenergic agonist. We demonstrate that direct activation of the alpha 1-adrenergic signaling pathway, using a mutant constitutively active G protein (Gq) similarly down-regulated the T3 effect on SERCA2 transcription. The combined effect of thyroid hormone receptor and retinoid X receptors on T3-stimulated SERCA2 gene transcription was also markedly attenuated by alpha 1-adrenergic stimulation. These results suggested that activation of the alpha 1-adrenergic signaling pathway has an inhibitor effect on T3-dependent SERCA2 gene transcription. As this inhibitory effect of alpha 1-adrenergic stimulation occurs when only one thyroid hormone response element (TRE) drives reporter expression, it is most likely mediated by an alteration of the nuclear factors binding to the TRE or by influencing the interaction of the TRE complex with the basal transcriptional machinery.

Animals↗
PubMed1983

Effect of reserpine pretreatment on calcium transport and Ca++-Mg++ adenosine triphosphatase activity of guinea-pig cardiac microsomes.

A previous report from this laboratory showed that reserpine pretreatment, in appropriate doses and under restricted conditions, increased the inotropic responsiveness of guinea-pig hearts to calcium. The enhanced responsiveness was characterized by a selective increase in the rate of ventricular relaxation (-dP/dt). We therefore hypothesized that reserpine might alter calcium uptake or (Ca++-Mg++) adenosine triphosphatase (ATPase) activity of guinea-pig ventricular sarcoplasmic reticulum (SR). Pretreatment of guinea pigs with reserpine (2.5 mg/kg/day, 2 days) significantly elevated ATP-dependent, Tris oxalate-facilitated SR Ca++ uptake and increased the calcium-sensitive component of the SR (Ca++) ATPase activity. These changes appeared to be functionally related to a reserpine-induced potentiation of ventricular relaxation rate, as estimated by the relationship between negative and positive left ventricular dP/dt of isolated working guinea-pig hearts. An alternative dose of reserpine (5 mg/kg, -24 hr), which had been demonstrated to produce an equivalent degree of catecholamine depletion, had no effect on either the inotropic responsiveness to calcium or on the SR calcium uptake or ATPase activities. The exact mechanism for these reserpine-induced alterations in calcium homeostasis remains to be elucidated.

Animals↗
PubMed1991

Aldose reductase inhibition with imirestat-effects on impulse conduction and insulin-stimulation of Na+/K(+)-adenosine triphosphatase activity in sciatic nerves of streptozotocin-diabetic rats.

This study describes reduced motor nerve conduction velocity and increased resistance to hypoxia-induced conduction failure in sciatic nerves of rats after four weeks of streptozotocin-induced diabetes (both effects were significant at p less than 0.05). These changes occurred in the absence of any deficit in the steady-state ouabain-sensitive adenosine triphosphatase (ATPase) activity of sciatic nerve endoneurial homogenates. The addition of 10 nmol/l insulin to endoneurial homogenates from control animals resulted in a 34% increase in ouabain-sensitive ATPase activity and a 19% reduction in ouabain-insensitive ATPase activity (both p less than 0.01). This stimulation of ouabain-sensitive ATPase activity by insulin did not occur in homogenates from diabetic rats. Treating diabetic rats daily with the aldose reductase inhibitor, imirestat (1 mg/kg) improved nerve conduction velocity (p less than 0.05) but was without effect upon the resistance to hypoxic conduction blockade or the deficit in insulin-stimulated ouabain-sensitive ATPase activity. These data suggest that in streptozotocin-diabetic rats the functional disorders of reduced motor nerve conduction velocity and increased resistance to hypoxic conduction blockade do not share a common aetiology and that impaired nerve conduction is not related to reduced maximal potential ouabain-sensitive ATPase activity.

Action Potentials↗