Increase In Contraction Rate Research Articles

Abstract P1028: Elucidating Differential Mechanisms That Underlie Phenotypic Variability In Two Independent Dcm Models

Mutations in components of the cardiac thin filament cause disease via complex, multi-faceted myocellular mechanisms, ultimately presenting as heterogeneous, symptomatic cardiomyopathies. The heterogeneity of both the myocellular mechanism(s) involved and the clinical outcome(s) represents a challenge in ongoing efforts to link genotype to phenotype. Two DCM-associated thin filament mutations (D230N-Tm and R173W-cTnT) with similar clinical phenotypes, were selected explore the molecular basis of disease trajectory. In patient populations, D230N-Tm exhibits an earlier time of onset compared to R173W-cTnT, with children (<1yr) presenting with an often-fatal DCM. We hypothesize that the difference in time of onset can be explained by distinct myocellular mechanisms. We utilized transgenic mice expressing D230N-Tm and R173W-cTnT and defined the progression of pathogenic remodeling across 3 independent timepoints via 2D-echo. At 2 months the %FS of D230N-Tm was significantly reduced (21.79 ± 1.19) compared to NTg littermates (33.45±1.45). R173W-cTnT, however, exhibited no reduction in %FS until 4 months (22.53±0.78), thus phenocopying the patient population. Using these timepoints to guide our myocellular mechanics, contractility measurements were then utilized to assess the myocellular mechanics associated with the mutations. At 2 months D230N-Tm exhibited a decrease in the rate of contraction (4.53±0.24μm/sec) and rate of relaxation (2.88±0.23) compared to NTg. R173W-cTnT did not diverge from NTg until 4 months, exhibiting a later onset of impaired contractility. By 4 months, both mutations displayed the opposite, an increase in rate of contraction and relaxation (rate of contraction, D230N-Tm 4.49±0.18 and R173W-cTnT 5.83±0.27; and rate of relaxation, D230N-Tm 3.14±0.19 and R173W-cTnT 3.70±0.24). Our results suggest that the accelerated time course for the development of D230N-Tm – linked DCM is caused by differential myocellular mechanism(s) that induce the early reduction in contractility and relaxation observed. Ongoing studies aim to further identify possible myocellular mechanism(s) by which these mutations exhibit different myocellular mechanics.

Bile acids and their respective conjugates elicit different responses in neonatal cardiomyocytes: role of Gi protein, muscarinic receptors and TGR5

Bile acids are recognised as bioactive signalling molecules. While they are known to influence arrhythmia susceptibility in cholestasis, there is limited knowledge about the underlying mechanisms. To delineate mechanisms underlying fetal heart rhythm disturbances in cholestatic pregnancy, we used FRET microscopy to monitor cAMP release and contraction measurements in isolated rodent neonatal cardiomyocytes. The unconjugated bile acids CDCA, DCA and UDCA and, to a lesser extent, CA were found to be relatively potent agonists for the GPBAR1 (TGR5) receptor and elicit cAMP release, whereas all glyco- and tauro- conjugated bile acids are weak agonists. The bile acid-induced cAMP production does not lead to an increase in contraction rate, and seems to be mediated by the RI isoform of adenylate cyclase, unlike adrenaline-dependent release which is mediated by the RII isoform. In contrast, bile acids elicited slowing of neonatal cardiomyocyte contraction indicating that other signalling pathways are involved. The conjugated bile acids were found to be partial agonists of the muscarinic M2, but not sphingosin-1-phosphate-2, receptors, and act partially through the Gi pathway. Furthermore, the contraction slowing effect of unconjugated bile acids may also relate to cytotoxicity at higher concentrations.

Abstract P303: Early Changes in Vascular Structure and Function after Bariatric Surgery in an Appalachian Population

Obesity is associated with an elevated risk of cardiovascular disease, metabolic disease, and cardiac mortality. Reactive oxygen species and inflammation in an obese individuals are among a number of suspected driving forces behind obesity’s detrimental health effects. These factors can elevate cardiovascular risk by causing impairments in cardiovascular structure and function. It is hypothesized that gastric bypass surgery will lead to near immediate reversal of metabolic deviations and rapid weight reduction, in addition to remodeling and improvements in arterial function as weight is lost. Patients that meet protocol inclusion criteria are invited to join the study during their 1 week pre-operative clinical appointment. At this time patients are consented and instructed to lie supine for 10 minutes of quite relaxation. Patients then begin a sequence of non-invasive examinations starting with a 12-lead electrocardiogram, followed by an echocardiogram, carotid artery ultrasound, radial applanation tonometry, and endothelial function analysis using EndoPAT technology. Upon completion of the non-invasive procedures a blood sample is taken for analysis. Preliminary results on 12 study subjects suggests the following. As hypothesized, body weight had decreased (pre: 131.8 ±5.6 to post: 125 ±5.2 kg) along with BMI (pre: 45.2 ±2.3 to post: 42.9 ± 2.2) and waist circumference (pre: 122.1 ±5.0 to post: 111.9 ± 3.2 cm) at the 1-week time point. Decrease in body size and weight was aided by a reduction in low-density lipoprotein (pre: 127 ±6.2 to post: 113 ±4.5 mg/dL). In addition, a number of improvements to the cardiovascular system were also marked at the 1-week time point. Estimated left ventricular end systolic pressure and direct measure of left ventricular end systolic volume (pre: 66 ±4 to post: 55 ±3 mmHg) declined post-GBS. This is in consort with an increase in contraction rate (dP/dt pre: 672 ±54 post: 737 ±46, p<0.01), a reduction in LV ejection time (pre: 291 ±10 to post: 211 ± 30ms), and a decrease in myocardial oxygen demand (PTIs pre: 2346 ±120 post: 2194 ±107 area/min, p<0.05). Similar improvements in augmentation index can be seen in assessments of peripheral vessels. Independent of brachial artery blood pressure, central systolic blood pressure (pre: 112 ± 4 to post: 105 ±2 mmHg, p=0.08), augmentation index (pre: 16 ±3 to post: 8 ±3, p=0.06), an indirect measure of arterial stiffness, and central augmentation pressure (pre: 4.9 ±1.1 to post: 2.5 ±0.9 mmHg, p=0.06) tended to decrease 1-week post GBS. The data suggests that vascular changes are happening in the early stages of recovery, prior to dramatic weight loss, thus suggesting vascular changes are a result of an alternate surgical benefit. In conclusion, the preliminary results of this study suggest noteworthy cardiovascular improvements post-gastric bypass surgery, yet prior to significant weight loss.

Spatiotemporal Transitions in Cardiac Neuronal Co-Cultures

A variety of arrhythmias have been attributed to formation of large scale patterns of excitation such as formation and break-up of spiral waves. We investigate the relationship between neuronal activity and evolution of macroscopic spatiotemporal dynamics. We also aim to understand the role of target-derived trophic nerve growth factors (NGF) on the formation and function of synapses between sympathetic neurons and heart tissue.Hearts were isolated from neonatal rat pups, ventricular myocytes were isolated, cultured and after 24h, stellate ganglia were isolated from litter mates enzymatically digested, and neurons were plated on top on myocyte dishes. We used dye-free imaging techniques using to record wave pattern and beat-rate (in the presence/absence of NGF).For the first time, we report wave speeds in cardiac neuron co-cultures. Our results show that (i) basal speed of co-cultures is significantly faster than myocyte only cultures (p<0.001); (ii) nicotine causes an altered behaviour in wave patterns; (iii) application of nicotine to co-culture causes a transient increase in contraction rate; (iv) the spatial organization of cells in myocyte monocultures is significantly altered by the addition of NGF.Our observations may be relevant to understanding how neuronal activity modulates macroscopic cardiac function. The finding that NGF modulates the development and patterning of myocytes raises the question of how NGF and their receptors interact to lead to long term changes in cardiac and synaptic function.

Role of AKAP79/150 Protein in β1-Adrenergic Receptor Trafficking and Signaling in Mammalian Cells

Protein kinase A-anchoring proteins (AKAPs) participate in the formation of macromolecular signaling complexes that include protein kinases, ion channels, effector enzymes, and G-protein-coupled receptors. We examined the role of AKAP79/150 (AKAP5) in trafficking and signaling of the β1-adrenergic receptor (β1-AR). shRNA-mediated down-regulation of AKAP5 in HEK-293 cells inhibited the recycling of the β1-AR. Recycling of the β1-AR in AKAP5 knockdown cells was rescued by shRNA-resistant AKAP5. However, truncated mutants of AKAP5 with deletions in the domains involved in membrane targeting or in binding to calcineurin or PKA failed to restore the recycling of the β1-AR, indicating that full-length AKAP5 was required. Furthermore, recycling of the β1-AR in rat neonatal cardiac myocytes was dependent on targeting the AKAP5-PKA complex to the C-terminal tail of the β1-AR. To analyze the role of AKAP5 more directly, recycling of the β1-AR was determined in ventricular myocytes from AKAP5(-/-) mice. In AKAP5(-/-) myocytes, the agonist-internalized β1-AR did not recycle, except when full-length AKAP5 was reintroduced. These data indicate that AKAP5 exerted specific and profound effects on β1-AR recycling in mammalian cells. Biochemical or real time FRET-based imaging of cyclic AMP revealed that deletion of AKAP5 sensitized the cardiac β1-AR signaling pathway to isoproterenol. Moreover, isoproterenol-mediated increase in contraction rate, surface area, or expression of β-myosin heavy chains was significantly greater in AKAP5(-/-) myocytes than in AKAP5(+/+) myocytes. These results indicate a significant role for the AKAP5 scaffold in signaling and trafficking of the β1-AR in cardiac myocytes and mammalian cells.

Blood Clots Are Rapidly Assembled Hemodynamic Sensors

Blood clots form under flow during intravascular thrombosis or vessel leakage. Prevailing hemodynamics influence thrombus structure and may regulate contraction processes. A microfluidic device capable of flowing human blood over a side channel plugged with collagen (± tissue factor) was used to measure thrombus permeability (κ) and contraction at controlled transthrombus pressure drops. The collagen (κ(collagen)=1.98 × 10(-11) cm(2)) supported formation of a 20-µm thick platelet layer, which unexpectedly underwent massive platelet retraction on flow arrest. This contraction resulted in a 5.34-fold increase in permeability because of collagen restructuring. Without stopping flow, platelet deposits (no fibrin) had a permeability of κ(platelet)=5.45 × 10(-14) cm(2) and platelet-fibrin thrombi had κ(thrombus)=2.71 × 10(-14) cm(2) for ΔP=20.7 to 23.4 mm Hg, the first ever measurements for clots formed under arterial flow (1130 s(-1) wall shear rate). Platelet sensing of flow cessation triggered a 4.6- to 6.5-fold (n=3, P<0.05) increase in contraction rate, which was also observed in a rigid, impermeable parallel-plate microfluidic device. This triggered contraction was blocked by the myosin IIA inhibitor blebbistatin and by inhibitors of thromboxane A2 (TXA(2)) and ADP signaling. In addition, flow arrest triggered platelet intracellular calcium mobilization, which was blocked by TXA(2)/ADP inhibitors. As clots become occlusive or blood pools following vessel leakage, the flow diminishes, consequently allowing full platelet retraction. Flow dilution of ADP and thromboxane regulates platelet contractility with prevailing hemodynamics, a newly defined flow-sensing mechanism to regulate clot function.

Agonist Dose-dependent Phosphorylation by Protein Kinase A and G Protein-coupled Receptor Kinase Regulates β2 Adrenoceptor Coupling to Gi Proteins in Cardiomyocytes

Adrenoceptors receptors (ARs) play a pivotal role in regulating cardiovascular response to catecholamines during stress. beta(2)ARs, prototypical G protein-coupled receptors (GPCRs), expressed in animal hearts, display dual coupling to both G(s) and G(i) proteins to control the adenylyl cyclase-cAMP dependent protein kinase A (PKA) pathway to regulate contraction responses. Here, we showed that the beta(2)AR coupling to G(i) proteins was agonist dose-dependent and occurred only at high concentrations in mouse cardiac myocytes. Both the beta(2)AR-induced PKA activity, measured by fluorescence resonance energy transfer (FRET) imaging, and the increase in myocyte contraction rate displayed sensitivity to the G(i) inhibitor pertussis toxin (PTX). Further studies revealed that activated beta(2)ARs underwent PKA phosphorylation at a broad range of agonist concentrations. Disruption of the PKA phosphorylation sites on the beta(2)AR blocked receptor/G(i) coupling. However, a sufficient beta(2)AR/G(i) coupling was also dependent on the G protein-coupled receptor kinase (GRK)-mediated phosphorylation of the receptors, which only occurred at high concentrations of agonist (> or = 100 nm). Disruption of the GRK phosphorylation sites on the beta(2)AR blocked receptor internalization and coupling to G(i) proteins, probably by preventing the receptor's transportation to access G(i) proteins. Furthermore, neither PKA nor GRK site mutated receptors displayed sensitivity to the G(i)-specific inhibitor, G(i)CT. Together, our studies revealed distinct roles of PKA and GRK phosphorylation of the beta(2)AR for agonist dose-dependent coupling to G(i) proteins in cardiac myocytes, which may protect cells from overstimulation under high concentrations of catecholamines.

Bub2 regulation of cytokinesis and septation in budding yeast

BackgroundThe mitotic exit network (MEN) is required for events at the end of mitosis such as degradation of mitotic cyclins and cytokinesis. Bub2 and its binding partner Bfa1 act as a GTPase activating protein (GAP) to negatively regulate the MEN GTPase Tem1. The Bub2/Bfa1 checkpoint pathway is required to delay the cell cycle in response to mispositioned spindles. In addition to its role in mitotic exit, Tem1 is required for actomyosin ring contraction.ResultsTo test the hypothesis that the Bub2 pathway prevents premature actin ring assembly, we compared the timing of actin ring formation in wild type, bub2Δ, mad2Δ, and bub2Δmad2Δ cells both with and without microtubules. There was no difference in the timing of actin ring formation between wild type and mutant cells in a synchronized cell cycle. In the presence of nocodazole, both bub2Δ and mad2Δ cells formed rings after a delay of the same duration. Double mutant bub2Δmad2Δ and bfa1Δmad2Δ cells formed rings at the same time with and without nocodazole. To determine if Bub2 has an effect on actomyosin ring contraction through its regulation of Tem1, we used live cell imaging of Myo1-GFP in a bub2Δ strain. We found a significant decrease in the total time of contraction and an increase in rate of contraction compared to wild type cells. We also examined myosin contraction using Myo1-GFP in cells overexpressing an epitope tagged Bub2. Surprisingly, overexpression of Bub2 also led to a significant increase in the rate of contraction, as well as morphological defects. The chained cell phenotype caused by Bub2 overexpression could be rescued by co-overexpression of Tem1, and was not rescued by deletion of BFA1.ConclusionOur data indicate that the Bub2 checkpoint pathway does not have a specific role in delaying actin ring formation. The observed increase in the rate of myosin contraction in the bub2Δ strain provides evidence that the MEN regulates actomyosin ring contraction. Our data suggest that the overexpression of the Bub2 fusion protein acts as a dominant negative, leading to septation defects by a mechanism that is Tem1-dependent.

Caveolar Localization Dictates Physiologic Signaling of β2-Adrenoceptors in Neonatal Cardiac Myocytes

There is a growing body of evidence that G protein-coupled receptors function in the context of plasma membrane signaling compartments. These compartments may facilitate interaction between receptors and specific downstream signaling components while restricting access to other signaling molecules. We recently reported that beta(1)- and beta(2)-adrenergic receptors (AR) regulate the intrinsic contraction rate in neonatal mouse myocytes through distinct signaling pathways. By studying neonatal myocytes isolated from beta(1)AR and beta(2)AR knockout mice, we found that stimulation of the beta(1)AR leads to a protein kinase A-dependent increase in the contraction rate. In contrast, stimulation of the beta(2)AR has a biphasic effect on the contraction rate. The biphasic effect includes an initial protein kinase A-independent increase in the contraction rate followed by a sustained decrease in the contraction rate that can be blocked by pertussis toxin. Here we present evidence that caveolar localization is required for physiologic signaling by the beta(2)AR but not the beta(1)AR in neonatal cardiac myocytes. Evidence for beta(2)AR localization to caveolae includes co-localization by confocal imaging, co-immunoprecipitation of the beta(2)AR and caveolin 3, and co-migration of the beta(2)AR with a caveolin-3-enriched membrane fraction. The beta(2)AR-stimulated increase in the myocyte contraction rate is increased by approximately 2-fold and markedly prolonged by filipin, an agent that disrupts lipid rafts such as caveolae and significantly reduces co-immunoprecipitation of beta(2)AR and caveolin 3 and co-migration of beta(2)AR and caveolin-3 enriched membranes. In contrast, filipin has no effect on beta(1)AR signaling. These observations suggest that beta(2)ARs are normally restricted to caveolae in myocyte membranes and that this localization is essential for physiologic signaling of this receptor subtype.

The PDZ Binding Motif of the β1 Adrenergic Receptor Modulates Receptor Trafficking and Signaling in Cardiac Myocytes

Beta(1) and beta(2) adrenergic receptors (AR) regulate the intrinsic contraction rate in neonatal mouse cardiac myocytes through distinct signaling pathways. It has been shown that stimulation of beta(1)ARs leads to a protein kinase A-dependent increase in contraction rate. In contrast, stimulation of beta(2)ARs has a biphasic effect on contraction rate, with an initial protein kinase A-independent increase followed by a sustained decrease that is blocked by pertussis toxin. The beta(2)AR undergoes agonist-induced endocytosis in cardiac myocytes while the beta(1)AR remains on the cell surface. It has been shown that a PDZ domain binding motif at the carboxyl terminus of beta(1)AR interacts with the postsynaptic density protein PSD-95 when both are expressed in HEK293 cells. We found that mutation of this PDZ binding motif in the beta(1)AR (beta(1)AR-PDZ) enabled agonist-induced internalization in cardiac myocytes. Moreover, stimulation of beta(1)AR-PDZ had a biphasic effect on the myocyte contraction rate similar to that observed following stimulation of the beta(2)AR. The secondary decrease in the contraction rate was mediated by G(i) and could be blocked by pertussis toxin. Furthermore, a non-selective endocytosis inhibitor, concanavalin A, inhibited the internalization of wild type beta(2)AR and the mutated beta(1)AR-PDZ, and blocked the coupling of both receptors to G(i). Finally, treating myocytes with a membrane-permeable peptide representing beta(1)AR PDZ motif caused the endogenous beta(1)AR to behave like beta(1)AR-PDZ. These studies suggest that association of the beta(1)AR with PSD-95 or a related protein dictates signaling specificity by retaining the receptor at the cell surface and preventing interaction with G(i).

Human rhabdosphincter cell culture: a model for videomicroscopy of cell contractions.

Physiology of the human rhabdosphincter and its innervation are still a subject to controversy. A better understanding of rhabdosphincter function and anatomy might help to solve important urological problems like urinary incontinence. It was the aim of the present study to develop a human sphincter cell culture model for investigation of contraction mechanisms in vitro. METHODS AND RESULTS Cells were isolated from human rhabdosphincter tissue obtained from prostatectomy and cystoprostatectomy specimens. Cultured cells expressed typical features of striated muscle cells. By means of videomicroscopy with a time lapse videosystem cell contractions could be documented. Under control conditions without any contractile stimulant 8% of the cells were seen to contract. Cholinergic stimulation with 10 mM of acetylcholine induced a significant increase in contraction rate to 49%. These results demonstrate that cholinergic stimulation triggers contraction of cultured human rhabdosphincter cells. This model might help to understand external urethral sphincter physiology and to establish new therapies for the treatment of sphincter dysfunctions. Prostate 47:189-193, 2001.

Signaling Mechanisms Underlying Muscarinic Receptor-mediated Increase in Contraction Rate in Cultured Heart Cells

We have investigated the mechanisms by which stimulation of cardiac muscarinic receptors result in paradoxical stimulatory effects on cardiac function, using cultured neonatal rat ventricular myocytes as a model system. Application of low concentrations of carbachol (CCh) (EC50 = 35 nM) produced an atropine-sensitive decrease in spontaneous contraction rate, while, in cells pretreated with pertussis toxin, higher concentrations of CCh (EC50 = 26 microM) elicited an atropine-sensitive increase in contraction rate. Oxotremorine, an m2 muscarinic acetylcholine receptor (mAChR) agonist, mimicked the negative but not the positive chronotropic response to CCh. Reverse transcription followed by polymerase chain reaction carried out on mRNA obtained from single cells indicated that ventricular myocytes express mRNA for the m1, m2, and, possibly, m4 mAChRs. The presence of m1 and m2 mAChR protein on the surface membranes of the cultured ventricular myocytes was confirmed by immunofluorescence. The CCh-induced positive chronotropic response was significantly inhibited by fluorescein-tagged antisense oligonucleotides directed against the m1, but not the m2 and m4, mAChR subtypes. The response was also inhibited by antisense oligonucleotides against Gqalpha protein. Finally, inhibition of CCh-induced phosphoinositide hydrolysis with 500 microM neomycin or 5 microM U73122 completely abolished the CCh-induced positive chronotropic response. These results are consistent with the stimulatory effects of mAChR activation on the rate of contractions in cultured ventricular myocytes being mediated through the m1 mAChR coupled through Gq to phospholipase C-induced phosphoinositide hydrolysis.

Comparison of some properties of pronethalol and propranolol. 1965.

Comparison of some properties of pronethalol and propranolol. 1965.

Does endogenous adenosine have a role in the cardiac responses to isoprenaline and in the rapid fade of the inotropic response of perfused heart?

The role of endogenous adenosine during the beta-adrenoceptor responses to isoprenaline of guinea-pig isolated cardiac preparations was examined. Insignificant effects of adenosine deaminase (0.3 U.mL-1) on cumulative concentration--response curves for isoprenaline on isolated left and right atria and papillary muscles indicated a negligible depressant effect of endogenous adenosine during these responses. The increase in force of contraction to an infusion of isoprenaline (14 nM) in perfused spontaneously beating hearts rapidly waned while the infusion continued, whereas the increase in rate of contraction remained constant throughout the infusion. The degree of fade was less in paced preparations (5 Hz), indicating that it was only in part due to the rate increase exerting some mechanical constraint on the force of contraction. The P1-purinoceptor antagonist 8-phenyltheophylline (12 microM) and adenosine deaminase (0.3 U.mL-1) did not enhance the peak responses to the isoprenaline infusion. The fade of the inotropic response in both spontaneous and paced hearts was also not attenuated by the presence of 8-phenyltheophylline or adenosine deaminase. The fade was not, therefore, due to release of endogenous adenosine exerting a depressant effect. Whether this declining inotropic response represents a form of rapid desensitization remains to be determined.

Enhanced inotropic responsiveness to alpha 1-adrenoceptor stimulation in isolated working hearts from diabetic rats.

We compared the inotropic responsiveness to the alpha 1-adrenoceptor agonist cirazoline and the calcium entry promoter Bay K 8644 in isolated working hearts from streptozotocin (STZ) diabetic rats and age-matched controls. The maximal rate of contraction and cardiac output (CO) were unaffected by diabetes. The maximal rate of relaxation was significantly decreased in diabetic hearts as compared with controls. Cirazoline (10(-9)-10(-6) M) induced an increase in both maximal rate of contraction and relaxation in diabetic and control hearts. Notably, the maximal rate of contraction was significantly greater in diabetic hearts. Changes in maximal rate of relaxation were similar in both preparations. A significant increase in CO was observed in diabetic hearts, but not in controls. With Bay K 8644 (10(-9)-10(-6) M) added, the increase in maximal rate of contraction and relaxation as well as CO were not significantly different in the two preparations although the changes in maximal rate of relaxation tended to be decreased in diabetic hearts. Our results indicate that cardiac alpha 1-adrenoceptors appear to gain greater functional importance in regulation of myocardial inotropism in diabetes mellitus.

Cardiodynamic effects of dopamine and dobutamine.

On 11 patients undergoing coronary surgery, at the end of the surgical intervention, the inotropic responses to 0.4 and 0.8 microgram x kg-1 x min-1 dopamine and dobutamine given via the aorto-coronary bypass directly into the coronary artery were compared. These dosages correspond to ones 10 times greater applied intravenously. The measurements were made using needle force probes which were implanted into the myocardial offstream area in the left ventricular wall. Bypass flow was measured simultaneously by an electromagnetic flow probe. There is a significant increase in coronary bypass flow induced by both rates of 0.4 and 0.8 microgram x kg-1 x min-1 dobutamine, but there was no significant effect on bypass flow induced by dopamine. Developed myocardial force is raised more by dobutamine medication than by dopamine. However, the rate of contraction increases significantly and relaxation is significantly accelerated by dopamine at both dosages. A significant increase in rate of contraction and relaxation was only induced by the higher dosage of 0.8 microgram x kg-1 x min-1 dobutamine.

Effects of Enoximone and Isobutylmethylxanthine on Contractile Tension and Cyclic Nucleotide Levels in Isolated Blood-Perfused Dog Papillary Muscle

The effects of enoximone (MDL 17,043) and 3-isobutyl-1-methylxanthine (IBMX) on developed tension, blood flow, and cyclic nucleotide levels were investigated in the isolated blood-perfused dog papillary muscle preparation. Intraarterial infusion of each agent, over 15 s, in doses ranging from 0.001 to 3 mg, produced a dose-dependent positive inotropic effect accompanied by increases in rate of contraction, rate of relaxation, and blood flow. The dose-response relationships for enoximone were always less steep than those for IBMX. Enoximone did not enhance automaticity at any dose, whereas the higher doses of IBMX (0.3, 1, and 3 mg) enhanced automaticity and produced arrhythmic preparations. Both agents produced increases in cyclic AMP during the peak positive inotropic effect (45 s); however, only IBMX produced an increase in cyclic GMP. The increase in cyclic AMP produced by enoximone lagged behind tension development by at least 15 s, whereas the increases in cyclic nucleotides produced by IBMX occurred concurrently with the development of the positive inotropic effect. The evaluation in cyclic AMP produced by enoximone is consistent with the reported property of this agent to inhibit specifically and selectively the particulate high-affinity cyclic AMP phosphodiesterase from dog heart, and supports a cyclic AMP-dependent mechanism of positive inotropism for enoximone.

Studies on the Mechanism of the Positive Inotropic Effect of Piroximone in Cat Papillary Muscle

The mechanism of the inotropic effect of piroximone HCl [MDL 19205A, 4-ethyl-1,3-dihydro-5-(4-pyridinylcarbonyl)-2H-imidozol-2-on e HCl] was studied in the cat papillary muscle paced electrically in vitro. Piroximone produced a concentration-dependent positive inotropic effect accompanied by an increase in rate of contraction and rate of relaxation, but abbreviated time to peak tension and relaxation time. The positive inotropic effect produced by piroximone was antagonized by carbachol, 3 X 10(-6) M, whereas that produced by increasing calcium concentration was not affected by carbachol. In potassium chloride (22 mM) depolarized muscle, piroximone restored contractility, which was not affected by propranolol (10(-6) M) or by tetrodotoxin (2 X 10(-5) M), but was inhibited by nifedipine (10(-7) M). Piroximine also elevated tissue cyclic AMP (cAMP) content in the papillary muscle. Although nifedipine inhibited the restoration of contractility, it did so without altering the increase of cAMP produced by piroximone. These results suggest that piroximone causes an increase in calcium influx that is mediated by an increase in cAMP, and the results are consistent with the hypothesis that specific inhibition of the high affinity cAMP phosphodiesterase (PDE III) plays a role in the positive inotropic effect of piroximone.

Effects of some process temperatures on radiation induced contraction of graphite

The dimensional change that occurs during the thermal processing of graphite and the subsequent dimensional change that occurs when graphite is irradiated have been explored. Raw material bias was removed by fabricating seven sets of graphite from one formulation. A systematic dimensional change during processing was achieved by controlling the graphitization rate to the same final temperature. A second series of samples is used to demonstrate the degree to which binder graphitization (or binder heat-treatment) determines radiation induced dimensional change. The filler for the formulation was graphitized to 2900°C and then mixed with a graphitizable binder. Subsequently, the samples were heattreated to temperatures appropriate for a broad range of crystallite development in the binder phase. As the crystallite size in the filler is the same, differences in radiation induced contraction can be attributed to the binder phase. The dimensional stability of both perpendicular and parallel orientations under irradiation shows a definite effect of the binder graphitization. However, unlike a system in which all constituents have the same graphitization temperature, this series does not show a uniform increase in contraction rate with decreasing graphitization temperature. For this series, there is little effect of heat-treatment temperature down to 2000°C; at heat-treatment temperatures below 2000°C there is a sharp increase in contraction rate for both orientations.

73. Effects of some process temperatures on radiation induced contraction of graphite

Abstract The dimensional change that occurs during the thermal processing of graphite and the subsequent dimensional change that occurs when graphite is irradiated have been explored. Raw material bias was removed by fabricating seven sets of graphite from one formulation. A systematic dimensional change during processing was achieved by controlling the graphitization rate to the same final temperature. A second series of samples is used to demonstrate the degree to which binder graphitization (or binder heat-treatment) determines radiation induced dimensional change. The filler for the formulation was graphitized to 2900°C and then mixed with a graphitizable binder. Subsequently, the samples were heattreated to temperatures appropriate for a broad range of crystallite development in the binder phase. As the crystallite size in the filler is the same, differences in radiation induced contraction can be attributed to the binder phase. The dimensional stability of both perpendicular and parallel orientations under irradiation shows a definite effect of the binder graphitization. However, unlike a system in which all constituents have the same graphitization temperature, this series does not show a uniform increase in contraction rate with decreasing graphitization temperature. For this series, there is little effect of heat-treatment temperature down to 2000°C; at heat-treatment temperatures below 2000°C there is a sharp increase in contraction rate for both orientations.