Dog Papillary Muscle Research Articles

Characterization of Spontaneous Vascular Findings in the Papillary Muscles of Beagle Dogs.

Standard histology trimming practices for toxicity studies in dogs include preparation of a section of the papillary muscle associated with the left ventricular free wall-the anteriolateral or anterior muscle. In contrast, the posteromedial or posterior papillary muscle, associated with the interventricular septum, is not commonly evaluated. In humans, the posterior papillary muscle is more often affected in ischemic myocardial disease, in large part due to the absence of collateral circulation, in contrast to the anterior muscle. Due to the differential vascular supply to the papillary muscles, we sought to determine whether there is a higher incidence of spontaneous coronary vascular changes in the posterior papillary muscle versus the anterior muscle in dogs. The hearts of 30 vehicle-treated or untreated beagle dogs that were euthanized for other purposes were collected and sectioned in a consistent manner to include both papillary muscles for histologic evaluation. The posterior muscle consistently had higher incidences of intramural coronary arteries affected by vascular medial hypertrophy, medial arteriosclerosis, intimal hyperplasia, and/or disruption or loss of the internal elastic lamina. This observation may have significant implications for the evaluation and characterization of spontaneous and xenobiotic-induced cardiovascular lesions in dogs.

Interaction between titin and thin filaments in intact cardiac muscle.

A 'freeze break' technique and immunoelectron microscopy were used to study the elastic properties of cardiac titin filaments. Small bundles consisting of a few fibres from freshly prepared dog papillary muscle were quickly frozen and broken under liquid nitrogen to fracture sarcomeres in planes perpendicular to the filament axes. Breaks occurred at each of several regions along the sarcomeres. The still-frozen specimens were thawed during fixation to allow elastic filaments to retract. The broken muscle segments were then treated with monoclonal titin antibody 9D10 which labelled a unique epitope in the I-band. In sarcomeres broken at the A-I junction, the titin filaments reacted toward the Z-line, independently of the thin filaments. The retracted epitopes did not reach the Z-line; retraction stopped at the N1-line level. In sarcomeres broken near the Z-line, the titin filaments retracted in the opposite direction, to the tip of the thick filaments. When the break occurred in the A-band, by contrast, the titin-epitope position was unaffected. On the basis of these results, and despite the reported interaction of titin and actin in vitro, it appears that cardiac titin molecules form elastic filaments that are functionally independent of the thin filaments. Near the Z-line, however, the titin filaments seem to associate firmly with the thin filaments.

Role of extracellular calcium on heart muscle energetics: effects of verapamil

The mechanical and energetic effects of verapamil (VER) and reduction of extracellular Ca concentration ([Ca]o) were studied in the interventricular rabbit septa and the dog papillary muscle. Even though the negative inotropic effects of VER [i.e., decrease in developed tension (T), maximal rates of contraction (+T) and relaxation (-T), and tension time integral] qualitatively resemble [Ca]o reduction, VER also elicited an anti-relaxant effect (decrease in -T/T and prolongation of the last phase of relaxation) that was not found with [Ca]o reduction. Resting heat production was similar in both preparations and remained unaffected either by changes in [Ca]o or by the presence of VER. The ratio between T and active heat production per beat (H'a) under constant fiber length decreased with VER, and this decreased economy of contraction was more marked with the increase in contraction frequency. Conversely, the T/H'a remained unaltered with changes in [Ca]o. Tension-independent heat decreased in the presence of VER and, although muscle economy can be improved by increasing muscle length in a VER-treated muscle, it is not possible to achieve either the maximal T or the maximal contraction economy that can be obtained by stretching a nontreated muscle. It may be concluded that at constant fiber length and frequency of contraction VER decreases myocardial contractile force, impairs relaxation, and decreases contraction economy. Neither the mechanical nor the energetic effects of VER can be explained solely on the basis of a reduced extracellular Ca availability, so that either the density of the Ca that enters through the channel is different from that of other sources of Ca or VER has an effect on the cross-bridge cycling mechanism.

Pharmacology and Pharmacokinetics of Enoximone

Enoximone is an inotropic vasodilating agent. Its principal effects are positive inotropism and vasodilation, which are not accompanied by changes in myocardial oxygen consumption. An inotropic dose of enoximone increases the level of cyclic AMP in the isolated, blood-perfused dog papillary muscle owing to its selective inhibition of the one isoform of cyclic AMP phosphodiesterase from the dog heart that is inhibited by cyclic GMP. Studies on the metabolism and pharmacokinetic profile of enoximone have been carried out in the rat, dog and monkey. Enoximone is metabolized mainly by oxidation to enoximone sulphoxide in all species studied, and this is reversible. In congestive heart failure patients, approximately 74% of a rapidly administered intravenous dose of enoximone is excreted in a 24-hour urine collection as the sulphoxide metabolite; only about 0.49% is recoverable as intact drug. Enoximone sulphoxide has the same inotropic and vasodilator activities as enoximone but is 0.13-0.14 times as potent and has a 13 times longer duration of inotropic action in the dog. It is suggested that the metabolite may contribute to some of the effects that follow enoximone administration.

Pharmacology of enoximone

Enoximone is a new cardiotonic agent, active by both intravenous and oral routes of administration, that is being studied clinically for the treatment of patients with congestive heart failure. The animal pharmacology pertinent to the clinical development of enoximone is reviewed. Direct positive inotropic, positive chronotropic and vasodilator properties have been demonstrated for enoximone in several in vivo and in vitro preparations. However, positive inotropism and vasodilation are the principal effects of this agent with the inotropic effect being the most prominent. In anesthetized dogs, the cardiovascular effects produced by enoximone (0.1 to 1 mg/kg) were not accompanied by significant alterations in myocardial oxygen consumption. Cardiac function was improved by enoximone in anesthetized dogs given myocardial depressant amounts of propranolol. Studies in vivo and in vitro have indicated that the actions of enoximone are direct and hot mediated by stimulation of adrenergic receptors, histaminic receptors, cholinerqic receptors, Ca ++-adenosine triphosphatase, Mg ++-adenosine triphosphatase, adenyl cyclase or inhibition of Na +, K +-adenosine triphosphatase. However, enoximone reversed the depressant effects of verapamil in the dog heart-lung preparation; this suggests that its action resulted in the activation of slow calcium channels. Enoximone was found to be potent and highly selective inhibitor of a high affinity cyclic adenosine monophosphate-phosphodiesterase type IV-phosphodiesterase from dog heart, whereas standard inhibitors (e.g., 3-isobutyl-1-methylxanthine and papaverine) inhibit all 3 cardiac phosphodiesterases. Further, enoximone produced an increase in cyclic adenosine monophosphate, but not cyclic guanosine monophosphate, in the isolated, blood perfused dog papillary muscle during the peak inotropic effect. Specific inhibition of type IV phosphodiesterase appears to have a role in the inotropic and vasodilator actions of enoximone. These properties suggest a useful role for enoximone in the therapy of patients with congestive heart failure.

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.

Calcium effects on contractility and heat production in mammalian myocardium.

The effects of changing external Ca concentration ([Ca]o) on contractile parameters and heat production were investigated in the interventricular rabbit septa and the dog papillary muscle. Double reciprocal plots of tension development as a function of [Ca]o yielded half-maximal activation values of 1.04 +/- 0.17 and 2.8 +/- 0.7 mM Ca for the septum and papillary muscle, respectively. Resting heat rate was similar in both preparations, 1.9 +/- 0.08 mW . g-1 for the septum and 1.7 +/- 0.07 mW . g-1 for the papillary muscle, and it was not altered by changes in [Ca]o. Active heat production (Ha) normalized per unit of force developed (19 +/- 1.3 microJ . mN-1 . g-1) for the septum and the dimensionless ratio Ha/(To . lo), (0.30 +/- 0.02) for the papillary muscle, where To is the isometric tension and lo, the muscle length, remained unaltered with changes in [Ca]o. Total heat production per beat normalized per unit of force developed (Ht/T) for the septum and the ratio Ht/(To . lo) for the papillary muscle decreased hyperbolically with [Ca]o. Therefore, as a result of the unaltered economy of the contractile system and the unchanged resting heat rate, muscle economy improves as [Ca]o approaches physiological levels. Further increase in [Ca]o, over the physiological levels, can only slightly improve muscle economy.

Comparative study on the cardiac actions of bovine parathyroid hormone (1–34)

The cardiac stimulatory effects of bovine parathyroid hormone (1–34) [bPTH-(1–34)] were studied on isolated atria and ventricular strips from cobra snakes ( Naja naja), ducklings ( Anas platyrhynchos), pigeons ( Columba livia), Japanese quails ( Coturnix coturnix japonica), bPTH-(1–34) caused positive chronotropism. This suggests that the chronotropic effect of PTH is ubiquitous among the terrestrial vertebrates. Only the atria of cobra snakes and ducklings gave positive inotropic responses to PTH. No ventricular preparations showed significant response to bPTH-(1–34). This suggests the absence of cardiac stimulatory PTH receptors in the ventricles. Dog papillary muscle, however, showed slight but significant responses to bPTH-(1–34). The reason for this discrepancy between the responses of palillary muscles and the ventricular strips to PTH is unknown.

167 TOXIC MYOCARDIAL DEPRESSION

We reported previously negative inotropic effects of serum from children with bacterial sepsis on an isolated dog papillary muscle preparation (Ped Res 18:1132, 1984). We have now studied sterile filtrate of a broth culture of Staphylococcus aureus (SA) from an infant with sepsis and depressed myocardial function. We suspended dog left ventricular papillary muscles in 50 ml oxygenated buffer and allowed 30 min of stabilization at a stimulated rate of 60/min. We measured maximum force generated (Fmax, mg) and rate of change of force (dF/dt, gm/sec) for muscles which were 1 mm2 in cross-sectional area and at 25 mg resting tension. Measurements were made before (C) and 10 minutes after addition of solutions listed below to the were as follows: Results suggest a non-dialyzable (12-14,000 heat labile, pH labile bacterial product depressed contractile performance of dog papillary muscles isolated and stimulated in a muscle bath.

THE EFFECTS OF BACTERIAL SEPSIS ON MYOCARDIAL FUNCTION

We studied the effects of serum from children with bacterial sepsis and poor cardiac function on isolated myocardial muscle performance. We measured the force (Fmax -mg) and dF/dt (gm/sec) of isolated dog papillary muscle contraction. Control serum was obtained from 3 uninfected subjects and a normotensive child with S. aureus bacteremia. Our 4 study subjects had hypotension and bacteremia due to either H. influenza, N. meningitidis, Yersinia pestis or D. pneumoniae. All study subjects required mechanical ventilation and inotropic support - mean LV % fractional shortening = 21%(nl=28-40%), mean Doppler cardiac output = 3.2 L/min/M2 (nl=2.5-4.5) and mean systemic vascular resistance = 11.6 units (nl=18-28). Baseline Fmax and dF/dt were compared to values measured 10 minutes after the addition of 0.3-1.0 ml of serum to the 50ml muscle bath. Serum from study subjects caused significant reductions in Fmax and dF/dt. We conclude that children with bacteremia and depressed myocardial function have a serum myocardial depressant factor which may also reduce vascular tone. Definition of this factor may provide new directions for the early management of hemodynamic distress in these critically ill subjects.

Regulation of delayed afterdepolarizations and aftercontractions in dog ventricular muscle fibres

Delayed afterdepolarizations have recently been recognized as their important contributions to the genesis of arrhythmias, [3, 6]. This paper deals with the possible causes of the delayed afterdepolarizations and aftercontractions in low K+, high Ca2+ solutions. Simultaneous recordings of membrane potential and tension were employed in dog papillary muscles. The dependence of the amplitude of the delayed afterdepolarizations and the aftercontractions on stimulus frequency and duration of stimulus trains was examined. In addition, effects of isoproterenol, verapamil, caffeine and procaine on both activities were examined. Our results show that conditions causing Ca2+ -overload in the cell provided the development or the augmentation of them. Further, caffeine eliminated oscillatory fluctuations in both activities followed by a transient increase after the treatment and procaine reversibly abolished them. These results support the hypothesis that the delayed afterdepolarizations are caused by a cyclic release of Ca2+ from the sarcoplasmic reticulum under the conditions of Ca2+-overload.

Activity-dependent changes of slow inward current in ventricular heart muscle.

1. The relationships between membrane voltage, contractile force and slow inward current were studied in cat and dog papillary muscles or trabeculae employing the double sucrose gap voltage clamp technique. The experiments were performed at 30 degrees C and the preparations were stimulated at a frequency of 0.5 Hz. 2. The known relationships between steady state contractile force, slow inward current and membrane voltage were confirmed. 3. Under non-steady state conditions the slow inward current decreases during ascending and increases during descending contraction staircases when the clamp steps of the test train exceed about 60 mV from resting level. Depolarization clamp steps below 60 mV produce parallel changes of the slow inward current and contractile force. Those clamp conditions which increase the contractile force shift the threshold of Isi and of contraction towards more negative values. 4. During ascending staircases an increasing background outward current was regularly observed together with diminishing slow inward current. 5. The reported current transients agree with the changes of action potential configuration during mechanical transients: the prolongation of plateau during descending staircases corresponds to an increase, and the shortening of action potential during late repolarization corresponds to a decrease of slow inward current in the respective voltage ranges. 6. The slow inward current was tentatively separated into two components. The main component is inversely proportional to contractile force and it exhibits the well known current-voltage relationship for this current. The other one is directly proportional to contractile force and may be related to a regenerative response of reticular membranes.

Effects of selective coronary hypotension on regional myocardial blood flow of dog papillary muscle.

This study is designed to investigate whether a selective hypotension of the left circumflex coronary artery (LCCA) causes selectively increased ischemia in the posterior papillary muscle (PPM-L) of the dog. Regional myocardial flow was measured by Rb86 clearance technique. In control animals, the distribution of blood flow between subendocardium and subepicardium, between the left and right ventricles and the septum, and between the free wall and the papillary muscle was studied. In normal situation, the subendocardial to subepicardial flow ratio of the left and right ventricle was approximately unity, however, in the papillary muscle the subendocardial perfusion was less than that of the central core. In selective LCCA hypotension by partial obstruction, the subendo/central core flow ratio of the PPM-L increased in contrast to a marked decrease of subendo/subepi flow ratio of the left ventricular free wall. The ratio of blood flow in the ischemic PPM-L and/or posterior wall of the left ventricle to that in the normally perfused anterior wall of the left ventricle dropped profoundly when LCCA is sufficiently narrowed to cause a pressure drop in the distal portion of the vessel. The flow drop in the PPM-L was greater than that in the other part of the subendocardium. These results demonstrated a selective impairment of blood flow to the posterior papillary muscle after partial obstruction of the left circumflex artery which may have direct clinical relevance.

EFFECTS OF BUFETOLOL AND PROPRANOLOL ON ACTIVE AND PASSIVE MEMBRANE PROPERTIES OF DOG PAPILLARY MUSCLE

Effects of bufetolol and propranolol, adrenergic β-receptor blocking and anti-arrhythmic drugs, on active and passive membrane properties of the dog papillary muscle were investigated with microelectrode and sucrose-gap methods. Bufetolol (10−5 to 10−4 g/ml) and propranolol (10−5 g/ml) significantly decreased the maximum rate of rise of the action potential. The maximum responsive frequency to driving stimulus was decreased in the presence of bufetolol (3×10−5 g/ml) and propranolol (10−5 g/ml), whereas the effective refractory period was not affected. The critical threshold potential was shifted to more positive potential in the presence of the drugs. The passive membrane property, the space constant (λ), the time constant (τ) and the current-voltage relations of the muscle membrane were not significantly altered by the drugs. It is concluded that bufetolol and propranolol suppress the excitability of the muscle membrane and this action may be ascribed to the decrease in the sodium conductance (gNa) and to the rise of gNa onset potential without alteration in the passive membrane property.

The Inotropic and Chronotropic Responses of the Guinea Pig and Dog Myocardium to Isoprenaline and Salbutamol

The relative effects of isoprenaline and salbutamol on the inotropic and chronotropic responses of the denervated myocardium of the chloralose anesthetized dog and of the isolated guinea pig atrium, and the inotropic response of the isolated dog papillary muscle were studied. Both the in vivo dog heart and the in vitro guinea pig atrium displayed a similar relative response pattern to isoprenaline and salbutamol with regard to their inotropic and chronotropic responses. However, a comparison of the relative inotropic responses of the dog heart in vivo and in vitro showed that in vitro, salbutamol has a much lower affinity and efficacy for the adrenergic receptors than isoprenaline.

Papillary muscle shortening in the intact dog; a cinderadiographic study of tranquilized dogs in the upright position.

Shortening of the anterior papillary muscle of the left ventricle was demonstrated in six intact, tranquilized dogs. Two small metal markers that had been surgically implanted 3-50 months earlier were cineradiographically photographed during approximately ten sequential cardiac cycles in each of two orthogonal positions. Distances between markers were plotted for successive frames. The resulting curves were used to obtain maximum velocities of papillary muscle shortening and lengthening: 1.08 plus or minus 0.29 muscle lengths/sec and 1.39 plus or minus 0.48 muscle lengths/sec, respectively. From the two orthogonal planes, the average maximum spatial distance and the average minimum spatial distance between the markers were calculated. The mean percent shortening of 22.8 plus or minus 6.5% was surprisingly large: it approximated the distance from the foot to the peak of the ascending limb of the myocardial length-tension curve derived from isolated muscle studies. Mechanical studies on isolated papillary muscle have consistently shown reduced shortening with increasing loads. Since the in vivo dog papillary muscle has been reported to be under considerable tension during systole, there appears to be some contradiction between the degree of shortening found in the present study and the shortening observed in isolated papillary muscle studies.

EFFECTS OF CAFFEINE ON TENSION DEVELOPMENT IN DOG PAPILLARY MUSCLE UNDER VOLTAGE CLAMP

1. Effects of caffeine on tension in the dog papillary muscle were investigated using a voltage clamp with the double sucrose-gap method.2. An S-shaped relationship between membrane potential and developed tension was obtained in Tyrode solution. An increase of the external calcium concentration produced an upward shift of the relationship and a decrease of calcium, a downward shift.3. Caffeine (10 mM/liter) reduced the peak tension at inside positive potentials in Tyrode solution, but scarcely changed the tension at inside negative potentials.4. An administration of caffeine retarded the relaxation phase at first, but the contraction phase was also gradually slowed.5. The magnitude of tension elicited by an action potential after repetitive voltage clamp pulses was dependent on the magnitude of tension during prior clamp pulses in Tyrode solution, but the dependency disappeared in caffeine-treated preparations.6. It was suggested that a tension development in the dog papillary muscle is largely regulated by the sarcoplasmic reticulum and that caffeine reduces the amounts of Ca taken up by and released from the reticulum.

Calcium kinetics in frog heart

The kinetics of calcium exchange were studied in the perfused frog ventricle and compared to the previously defined kinetic analysis of calcium obtained in the dog papillary muscle. In addition, the pattern of calcium exchange which follows perfusion of the frog ventricle with low sodium solutions has been studied and compared to that of the dog. After labeling with 45Ca, the muscles were washed out and the curves were analyzed graphically. The washout curve could be defined in terms of four exponential phases for calcium with the following rate constants: 0 = 2.34 ± 0.060 min −1, 1 = 0.687 ± 0.038 min −1, 2 = 0.176 ± 0.008 min −1, 3 = 0.037 ± 0.002 min −1. When the sodium concentration of the perfusate was lowered to 25.5 m m/L, there was an increase in contractile force, accompanied by an increase of 45Ca activity in phase 2. No significant changes in the rate constants of the phases occurred during this inotropic intervention. Phase 2 is proposed to represent a site of calcium storage directly related to maintenance of contractility. A similar kinetically-defined component was found in the dog papillary muscle but was representative of some 7 times as much calcium. The characteristics of 45Ca exchange in frog and dog ventricle are compared in the light of their ultrastructural differences.

Electron-dense material within sarcoplasmic reticulum apposed to transverse tubules and to the sarcolemma in dog papillary muscle fibers.

AbstractThe space between the sarcoplasmic reticulum (SR) apposed to the transverse (T) tubules and the sarcolemma in dog papillary muscle fibers is traversed by structures of varying densities. The distance between apposed membranes is about 70 Å at the level of densities and about 100 Å at the level of less dense areas that alternate with the densities. The apposed SR shows a marked increase in electrondense material when comparison is made with the SR not apposed. The electron‐dense material is located in a dense layer parallel to and about midway between the membranes of the apposed SR and in numerous projections from the inside surface of the membranes. With appropriate staining procedures the central dense layer and the projections are as thick as membranes and they frequently show paired densities separated by a distance about equal to the distance between the outer dense layers of trilaminar membranes. The paired densities in the central dense layer and in the projections indicate that the electron‐dense material within apposed SR has structure. The projections are frequently in register with scallops or serrations in the SR membranes and they also appear to contribute to a waviness in the central dense layer. The serrations and waviness are interpreted as evidence that the projections exert a holding force because they project from the SR membranes and connect with the central dense layer.

Evidence for a structural relationship between sarcoplasmic reticulum and Z lines in dog papillary muscle.

AbstractElectron micrographs of dog papillary muscle fibers show the sarco‐plasmic reticulum (SR) as an extensive three‐dimensional network of diverging and converging tubules that is continuous throughout each sarcomere and in successive sarcomeres. The network surrounds each myofibril and is continuous across the fiber. Tubules of the SR network are oriented transversely and longitudinally. Interconnections of transversely and longitudinally oriented SR tubules form circular pathways in the network. A structural relationship appears to exist between the Z line and longitudinally and transversely oriented tubules of the SR network at their points of apposition with the Z line. A rather constant space approximately 100 Å wide is seen between SR tubules apposed at the Z line in cross‐sections and in favorable longitudinal sections of fibrils. This space is traversed by electron‐opaque strands which appear to be connections between SR tubules and the Z line. Other lines of evidence support the view that Z lines and SR tubules are connected. One of these is the change of position of Z line SR tubules in relation to the change of position of Z lines in adjacent fibrils. Another line of evidence is the continuity of SR apposed at the Z line and SR connected to the sarcolemma.