Isolated Cat Papillary Muscles Research Articles

Nonuniform contraction in the isolated cat papillary muscle.

Microspheres infused into the coronary microcirculation were used as markers to define segments within isolated cat papillary muscles. Video recording and analysis provided measurements of the variations of segment lengths as the muscles contracted at lengths of 76-100% Lmax. In all muscles, segments in the center region were found to shorten during muscle isometric contraction while those in the end regions lengthened. Central shortening was typically 10-15%. In the passive state, segment lengths varied directly with muscle length over a broad range characterized by low force. Segments in the center region, however, displayed an abrupt transition to high stiffness at a certain length while end regions continued to stretch. Force-length relationships obtained for the presumably healthy center segment are significantly different from those obtained for the whole muscle. These results suggest that there may be major difficulties with the interpretation of mechanical measurements on papillary muscles unless contractile inhomogeneity is eliminated or taken into account.

Separation of Pancreatic Cardiodepressant Activity into Peptide and Salt Activity

SummaryThe cardiodepressant activity of shock plasma has been ascribed to the presence of a circulating cardiodepressant factor of pancreatic origin. It has been suggested that the activity observed was due to the presence of excess salt in the assay system, rather than a specific pancreatic cardiodepressant peptide (PCP). We wish to report that we obtained cardiodepressant activity (CDA) from an incubated pancreatic homogenate (IPH) (1:6, w/v, in Krebs-Henseleit solution). Heat-precipita-ble proteins of the IPH were removed by heating (80°) and centrifugation, the clear supernatant was subjected to ultrafiltrations, and the fraction below a molecular weight of 1000 was applied to a Sephadex G-10 column. This material was separated into 40 fractions, and each fraction was lyophilized and reconstituted with 15 ml of Krebs-Henseleit buffer. The fractions were then assayed for Na+, K+, Cl-, and Ca2+ concentrations as well as CDA activity in the isolated cat papillary muscle (PM) assay system. The IPH was res...

On the relationship between action potential duration and tension in cat papillary muscle

Tension and action potentials have been measured simultaneously from isolated cat papillary muscles. Two groups of experiments are described. In the first group, the external conditions under which the muscle contracted were changed. Specifically, stimulation rate, extra-cellular [Ca++], extracellular [Na++] were altered, and adrenaline was added to the bathing fluid. A tendency for given levels of tension to be accompanied by action potentials of constant duration is demonstrated under some of these conditions. In the second group of experiments, tension and action potentials were recorded following some change in external conditions; specifically, after a long rest, after a change in muscle length, and after the muscle had been set up in the experimental apparatus (the 'running-in' period). In the period that followed each of these interventions, peak tension increased substantially over at least several minutes but all external conditions (for example, temperature, muscle length, stimulation rate, and composition of the bathing fluid) remained constant. In each of these three situations tension increased but in one case the action potential duration increased, in another it decreased, and in the third it was unchanged. It is concluded that change in action potential durations do not necessarily make an important contribution to the changes in tension of papillary muscles.

Length-dependent calcium inotropism in cat papillary muscle.

We studied changes in the relationship of isometric developed force to muscle length when extracellular calcium concentration was altered in isolated cat papillary muscles. In two series of experiments at temperatures of 30 degrees C and 32 degrees C, and rates of 12 and 30 beats/min, 21 muscles were exposed to calcium concentrations of 1.125, 2.25 and 4.5 mM. Muscle lengths were varied between 80% and 100% of the length at which maximum developed force occurred (Lmax). Peak developed force and the time from stimulus to peak were measured. The data indicate that force is not altered proportionately at all lengths when calcium concentration is changed. Rather, we found that a substantially greater modification of force occurs at short lengths than at long lengths. Similarly, the time to peak force increases with length at a rate which is more than 4 times greater at the low calcium concentration than it is at the high concentration. Small but consistent shifts of Lmax also are seen. We observed that Lmax is longer when inotropic changes reduce force and shorter when force is increased. These results indicate that the inotropic effect of extracellular calcium concentration changes is dependent on muscle length.

The Pharmacology of a New Hypoglycaemic Agent N‐[4‐(2‐(2,3‐dihydrobenzo(b)furan‐7‐carboxamid o)‐ethyl) ‐benzenesulphonyl]‐N′‐cyclohexylurea (NOVO CS 476). III. General Pharmacological Studies

The new sulphonylurea CS 476 was tested for positive iontropic effect on the isolated cat papillary muscle. Unlike tolbutamide CS 476 had no positive inotropic effect. CS 476 had no adverse effect on blood pressure in dogs, cats and rats nor on the electrocardiogram of dogs in doses up to ten times the therapeutic dose. Unlike chlorpromide CS 476 did not potentiate the effect of exogenous antidiuretic hormone in hydrated dogs. In therapeutic concentrations the drug had no effect on smooth muscle preparations. 1,000-10,000 times therapeutic concentrations had a papaverine-like effect - similar to therapeutic concentrations of tolbutamide.

Relationships between resting tension and mechanical properties of papillary muscle

The influence on mechanical properties of changes in resting tension over a range from 0.5 to 4.5 g was investigated in 12 isolated cat papillary muscles. At each resting tension, the muscles contracted isometrically with the exception of an externally applied sinusoidal stretch of 0.5% Lmax (deltaL) and 20 Hz. Stiffness (deltaT/deltaL) was determined from deltaL and the peak amplitudes (deltaT) of the individual cycles from the sinusoidal component of tension. Assuming that the muscle and experimental apparatus behaved as a linear second-order mechanical system, it was possible to divide stiffness into its elastic and viscous components. During rest, total stiffness and its components were linearly related to tension. During contraction, stiffness and its elastic component were linearly related to tension. Furthermore, increasing resting tension increased the intercept and decreased the slope of this linear stiffness-tension relationship. The relationship between viscous stiffness and tension during contraction is more complex in that it is a direct relationship at low resting tensions, but an inverse one at high resting tensions.

Effects of sodium nitroprusside and nitroglycerin on tension prolongation of cat papillary muscle during recovery from hypoxia.

Because of recent studies suggesting that vasodilators affect ventricular compliance, we studied the effect of sodium nitroprusside and nitroglycerin on the mechanical performance of 21 isolated cat papillary muscles. The muscles were stimulated isometrically at 36 beats/min. Sixteen of the muscles were made hypoxic (95% N2, 5% CO2) for 50 minutes and then reoxygenated. Sodium nitroprusside (10(-5) M) added to four of these muscles prior to hypoxia substantially diminished the tension prolongation (both the time to peak tension, TTP, and time for tension to fall to 1/2 its peak value, RT 1/2) that characterizes recovery from hypoxia. TTP and RT 1/2 measured 2 minutes after reoxygenation were 300 +/- 20 msec and 528 +/- 26 msec for the control muscles compared to 208 +/- 13 msec and 248 +/- 22 msec for the muscles pretreated with nitroprusside. Nitroprusside had no effect on the fall and recovery of peak developed force or on the rise and fall of resting force. Furthermore, nitroprusside had no effect on the above parameters in nonhypoxic muscles. We also found that nitroprusside in concentrations of 10(-7) M and nitroglycerin in concentrations of 10(-5) M had little or no effect on tension prolongation. The results of the study indicate that nitroprusside is capable of blocking the tension prolongation that occurs during recovery from hypoxia and may prevent the incomplete myocardial relaxation thought to characterize this phenomenon. Since nitroglycerin had no effect on tension prolongation, it is possible that other factors also may be important in the apparent increase in left ventricular compliance associated with administration of vasodilators to patients.

Biological activity of catecholamines covalently linked to synthetic polymers: proof of immobilized drug theory.

l-Isoproterenol was covalently coupled via an azo linkage to soluble copolypeptides of molecular weight 1500 and 10,000 containing an aromatic amine. The polymeric azo-isoproterenol derivatives were purified by gel chromatography which reduced contamination by the parent isoproterenol to undetectable levels (i.e., less than 0.01%) and by 6-aminoisoproterenol to less than 0.4%. Both polymeric isoproterenol derivatives were found to elicit positive chronotropic responses in isolated perfused guinea pig hearts. The mean effective doses (ED50S) for the 1500 and 10,000 molecular weight derivatives were within 1.3 and 2.0 orders of magnitude, respectively, of the ED50 of l-isoproterenol. The responses cannot be attributed to free isoproterenol because this drug could not be detected in our preparations. Neither can the observed biological activity be attributed to 6-aminoisoproterenol, since this compound's dose-response is shifted 3 orders of magnitude to the right of l-isoproterenol and 1-2 orders of magnitude to the right of the polymeric derivatives. Inotropic response decay times in isolated cat papillary muscles following washouts indicate that the polymer-bound drug does not diffuse into the muscle tissues. We feel that our findings demonstrate that under controlled conditions the catecholamines can retain biological activity while covalently bound to a polymeric support.

Inactivation of contraction as a determinant of the length-active tension relation in heart muscle of the cat.

In isometrically contracting isolated cat papillary muscles, the mechanism determining the reduction in active tension (P) at lengths (L) below the optimal length (Lmax) for maximal tension development (Pmax) was studied. Increasing [Ca2+] from 1.25 to 5 mM, paired electrical stimulation and caffeine augmented P-O/max by 77,61 and 23% respectively, as well as the tension developed at any given length on the ascending limb of the length-active tension relation. To see whether below Lmax the degree of activation is dependent on length, these length-active tension relations were expressed as P/Pmax relative to L/Lmax where Pmax and Lmax in each inotropic state was 100%. These normalized length-active tension relations were not altered by different [Ca2+]o, but shifted upward and to the left by caffeine and paired stimulation. The effects of elevating [Ca2+] on the length-active tension relation suggests that restoring forces and fixed internal loads are not an important factor in decreasing active tension at short lengths. The effects of caffeine and paired stimulation support the view that in cardiac muscle inactivitation of contraction is a major factor determining the fall in active tension at decreasing length.

Absence of a cardiodynamic action of glucocorticoids in acute myocardial ischemia

Dexamethasone and methylprednisolone in concentrations equivalent to systemic doses of 6–30 mg/kg were given to normal and ischemic isolated perfused cat hearts and to isolated cat papillary muscles. Neither glucocorticoid exerted an inotropic, anti-arrhythmic, or coronary vasodilator action over the 1–2 hr observation period. Furthermore, neither dexamethasone nor methylprednisolone exerted significant effects on heart rate or on myocardial oxygen consumption in perfused cat hearts. These data do not substantiate the hypothesis that pharmacologic doses of glucocorticoids which protect the ischemic myocardium act via a cardiodynamic or hemodynamic action.

Myocardial actions of prostaglandins in isolated cat cardiac tissue

The inotropic responses to prostaglandins (PG) A 1, E 1, E 2 and F 2α were studied in isolated cat myocardial tissue. PGA 1 and F 2α exhibited no significant inotropic effects, whereas, PGE 2 and PGE 1 produced negative inotropic effects at concentrations of 2.8 × 10 −7 and 2.8 × 10 −6 M in isolated cat papillary muscles. In isolated perfused cat hearts, PGE 1 (2.8 × 10 −6M) produced a negative inotropic effect along with a significant increase in coronary flow. As flow declined, the negative inotropic effect became more severe. PGE 1 at 2.8 × 10 −9 M produced a sustained increase in coronary flow and oxygen consumption with no inotropic effect. PGE 2 and F 2α did not exert significant changes in coronary flow or contractile force. Thus prostaglandins do not appear to exert significant positive inotropic effects at physiologic or at generally accepted pharmacologic concentrations in isolated cat heart preparations. At extremely high concentrations, prostaglandins E 1 and E 2 exert a negative inotropic effect; however, this would not explain the protective effect of these prostaglandins in circulatory shock.

Comparative evaluation of the specificity and sensitivity of isometric indices of contractility.

It is generally held that a useful mechanical index of contractility is one that is unchanged by changes in initial muscle length, but that changes in appropriate direction and magnitude with a change in contractile state. A number of isometric indices were measured in vitro in isolated cat papillary muscles to quantitate their relative dependence on initial muscle length and contractile state. Results indicate that no one isometric index completely fulfills the definition of an ideal index of contractile state. In particular, indices that change considerably with alterations in contractile state (e.g., maximum dF/dt) are also moderately dependent on preload. Conversely, indices that are moderately insensitive to changes in contractile state (e.g., time-to-peak force) are also insensitive to changes in contractile state. Changes in extrapolated maximum velocity and peak VCE were similar and were less sensitive to changes in contractile state than maximum dF/dt. The most sensitive in vitro index of contractility appears to be maximum dF/dt at a constant preload.

Fragmentation of the Actin Filaments by Acute Pressure Overload to the Right Ventricle of the Conscious Dog.

We have postulated in a study of the isolated cat papillary muscle that the disparity between muscle length and sarcomere length may be the result of myofibrillar fragmentation. In contrast, Saphir and Karsner demonstrated in the rabbit myocardium subjected to an acute pressure overload, that segmentation or separation of intercalated discs occurred, and not myofibrillar fragmentation.In order to determine whether the myocardial fragmentation produced in the in vitro study has pathophysiological significance in the intact myocardium, tissues were studied from the right ventricle of the conscious dog subjected to an acute pressure overload.In order to produce the overload, we utilized a specially designed triple lumen pulmonary artery balloon distension catheter. This catheter can measure simultaneous pulmonary arterial and right ventricular pressures. The catheter was inserted through the jugular vein and wedged under flouroscopic control in a branch of the pulmonary artery. The balloon was situated in the main pulmonary artery.

Length-tension diagram and force-velocity relations of mammalian cardiac muscle under steady-state conditions.

Length-tension diagrams and force-velocity relations of isolated cat papillary muscles were measured under strictly stationary conditions: any analysis of isometric mechanograms was carried out only in stable isometric beats; each measurement in beats under any condition different from the isometric one was determined in the very first contraction after a period of stabilization under isometric conditions. The length-tension diagrams show a clear distinction between isometric and isotonic length-tension curves. Force-velocity curves on the basis of afterloaded isotonic, as well as of damped quick-release contractions, could be satisfactorily approximated by a polynomial of the 5th order. Vmax estimated on the basis of afterloaded and of damped quick-releases in the course of isometric and isotonic contractions, was significantly dependent on preload. This is also valid in a range of muscle length which is regarded as physiological.

Dobutamine: development of a new catecholamine to selectively increase cardiac contractility.

We systematically modified isoproterenol's chemical structure to reduce chronotropic, arrhythmogenic, and vascular side effects. Experiments on dogs showed that the resulting drug, dobutamine, had an inotropic efficacy as great as that of epinephrine due to a direct action on beta1 cardiac receptors. However, unlike epinephrine, dobutamine's effect on alpha and beta2 vascular receptors was slight. At equivalent inotropic doses, dobutamine had less than a fourth of the chronotropic effect of isoproterenol. Desmethylimipramine (DMI), which blocks the sympathetic nerve fiber uptake mechanism, had no effect on dobutamine's actions. In contrast, DMI antagonized dopamine's inotropic effect, and marked chronotropic and pressor responses occurred when we used doses of dopamine large enough to elicit a direct inotropic effect. Dobutamine increased the contractility of isolated cat papillary muscles more but the automaticity less than did isoproterenol. In ischemic dog hearts, dobutamine lacked significant arrhythmic activity, whereas dopamine, norepinephrine, and isoproterenol caused severe ectopic activity. In dogs with experimentally induced low cardiac contractility, low cardiac output, and hypotension, dobutamine produced dose-related increases in cardiac contractility and output, restored arterial blood pressure, and reduced total peripheral resistance slightly. In contrast, isoproterenol failed to restore blood pressure, had only a meager effect on cardiac contractility and output, cuased extreme tachycardia, and lowered peripheral resistance more than did dobutamine. Norepinephrine, which did not increase cardiac contractility or output as much as dobutamine, excessively elevated peripheral resistance and arterial blood pressure.

Deactivation of contraction by quick releases in the isolated papillary muscle of the cat. Effects of lever damping, caffeine, and tetanization.

In the isolated cat papillary muscle, the time course of activation during isometric contraction was measured using instantaneous velocity of shortening after quick releases at constant contractile element length and controlled load. Activation rose and declined ahead of isometric force. This course of activation could be altered by the mode of the quick release. Undamped quick releases increasingly deactivated contraction with time. Deactivation was reduced by damping the rapid movement of the lever after the release and by increasing the afterloads to which the releases were made, thus lessening the extent of the releases. The curves of activation obtained from releases to different afterloads converged after the first half of the rising phase of the twitch. Deactivation from quick releases was eliminated by tetanizing the muscle. Hence, the instantaneous relation of force and velocity of shortening was not unique relative to time except when the muscle was tetanized. Deactivation was also reduced by lowering the temperature, by adding 10 m M caffeine, or by increasing the Ca 2+ concentration in the bathing medium in the presence of 10 m M caffeine. These results suggest that quick releases deactivate contraction by uncoupling crossbridges which can be reformed if the Ca 2+ available to activate contractile sites is not sequestered elsewhere.

Effects of dobutamine on myocardial mechanics.

In the intact circulation in man and dog dobutamine ( (±)-4-[2-[[3-(p-hydroxyphenyl)-1 -methylpropyl] amino]ethyl] pyrocatechol hydrochloride), a new synthetic catecholamine (Eli Lilly and Co., Indianapolis, Ind.) exerted marked positive inotropic effects (1–4). Since these effects were obtained with little change in heart rate and mean arterial pressure it has been suggested that dobutamine is potentially of clinical value in the enhancement of myocardial contractility and may have advantages over other frequently used catecholamines such as noradrenaline and isoproterenol (1–4). In view of the potential clinical value of dobutamine, and to exclude effects on the vascular system, this study was undertaken of characterize the direct effects of dobutamine on myocardial mechanics of the isolated cat papillary muscle.

Influence of previous mechanical events on the contractility of isolated cat papillary muscle.

1. The influence of previous mechanical events on myocardial contractility has been investigated in the cat papillary muscle preparation.2. When a muscle that had been producing a stable response under isometric conditions was allowed to shorten isotonically, its ability to do so increased in successive beats until it reached a steady level, which represented a potentiated state compared with that seen in the first isotonic beat and in the preceding stable isometric contractions.3. The increase in tension development in the first isometric beat after a period of isotonic beating was used as an index of the degree of potentiation. It was found to be well correlated with the changes in other parameters that could have been used for this purpose.4. The main determinants of the degree of potentiation produced by a period of isotonic beating were:(a) the amount by which the muscle shortened. This was inversely related to the force opposing shortening (i.e. the isotonic load);(b) the number of isotonic beats. There was some potentiation (about 10%) after a single isotonic beat, but the number of beats required for maximal potentiation (up to 25%) depended on the frequency of stimulation; about 8 beats were required at 24 min(-1).5. An isotonic release during the rise of tension in an isometric response was even more effective in potentiating the next isometric beat than an afterloaded contraction against the same load. Isotonic releases at later times had a diminishing influence on tension development in the next isometric beat.6. In the absence of stimulation, the potentiated state produced by a period of isotonic beating decayed with a half-time of about 50 sec. When the muscle was stimulated it disappeared sooner, and its rate of decay depended on the frequency of stimulation; at 24 min(-1) about 8 beats were required to restore contractility to its previous steady level.7. The characteristics of the decay of the potentiated state were closely similar to those of the potentiated states that can be produced by various electrical interventions, and the possibility that all of these might have the same underlying mechanism is discussed.8. Attention is drawn to the practical implications of this phenomenon in the design of experiments in which the muscle contracts under changing mechanical conditions.

Hemodynamic and metabolic responses to vasodilator therapy in acute myocardial infarction.

Hemodynamic effects of vasodilator therapy (phentolamine or nitroprusside) were studied in 38 patients with acute myocardial infarction (AMI). Cardiac metabolism was studied in 19 of the 38 patients. According to the initial level of left ventricular filling pressure (LVFP) and left ventricular stroke work index (SWI), patients were divided into three groups: Group I-nine pa- tients with LVFP 15 mm Hg or less; Group II-14 patients with LVFP > 15 mm Hg and SWI >20 g-m/m 2 ; Group III-15 patients with LVFP > 15 mm Hg and SWI < 20 g-m/ m 2 . In Group I most patients were clinically uncomplicated. In Group IL most patients had clinical left ventricular failure including one patient who had clinical features of cardiogenic shock. Group III patients all had severe left ventricular failure, with eight patients in clinical shock. In all groups LVFP, pulmonary artery pressure, right atrial pressure, and systemic and pulmonary vascular resistance decreased significantly with vasodilator therapy with only a slight to moderate decrease in arterial pressure. In Group I patients SVI decreased (-7%) together with an increase in heart rate. Significant improvement in left ventricular performance, however, was observed in Groups II and III as indicated by increased stroke volume index (SVI) and cardiac index (CI) and decreased LVFP. The increase in SVI and CI was of similar magnitude in both Group LI (SVI +18%, CI +24%) and Group III (SVI +28%, CI +29%) patients, a change suggesting that vasodilation thereby may be applicable and beneficial even in the presence of severe depression of cardiac performance. Improved left ventricular performance in group II and III patients was accompanied by a slight decrease in coronary blood flow, myocardial oxygen consumption, and transmyocardial oxygen extraction. There was no change in myocardial lactate metabolism in any group. In vitro studies in isolated cat papillary muscle preparations showed no direct positive inotropic effect of either phentolamine or nitroprusside. Thus, significant improvement in left ventricular performance occurs during vasodilator therapy in patients with AMI and elevated LVFP, even in the presence of severe depression of cardiac performance. Furthermore, this improvement is not accompanied by increased metabolic cost. Vasodilator therapy, therefore, may have an important role in the treatment of pump failure complicating myocardial infarction.

Inotropic effects of isoflurane on mechanics of contraction in isolated cat papillary muscles from normal and failing hearts.

The inotropic effect of isoflurane (Forane) was studied in papillary muscles from cats with normal hearts (NH) and those with experimentally produced congestive heart failure (CHF). Mean maximal velocity of shortening (Vmax) and mean maximal developed force (Fm) of CHF muscles were lower than in the normal heart. Isoflurane at the concentration equivalent to MAC in man reduced Vmax an average of 36 per cent in NH and 51 per cent in CHF muscles. Average percentage decreases in Fm were 40 in NH and 52 in CHF muscles. When changes in myocardial contractility of CHF muscles exposed to isoflurane at MAC were compared with the NH control values, reductions of Vmax and Fm were 75 and 74 per cent, respectively. The combined negative inotropic effects of isoflurane and CHF were more pronounced than that of isoflurane alone on the normal heart.