Maximum Rate Of Tension Development Research Articles

Inorganic Phosphate Enhances Systolic and Diastolic Function in Excitable Papillary Muscle and Accelerates Myofilament Relaxation in Response to Lengthening

Inorganic phosphate (Pi) is an essential constituent of blood serum found in concentrations of roughly 1.1-1.4 mM. Cardiac myofilament force is reduced with increasing Pi due to its facilitating reversal of the force-producing myosin power stroke. This Pi-dependent reversal of the power stroke is observed to be further augmented by lengthening in demembranated myocardium. We hypothesized that relaxation in excitable myocardium is enhanced by increasing extracellular Pi concentrations and especially when the myocardium is lengthened during the relaxation phase of the force transient. We tested this hypothesis using excitable myocardium isolated from female Wistar rats aged 4-5 months. Rats were fully anesthetized, and their hearts removed. Left ventricular papillary muscles were attached to platinum clips, placed between a force transducer and a length motor at room temperature, and bathed in Krebs solution with 1.8 mM Ca2+ and varying Pi of 0, 1, 2 and 5 mM. Force transients were elicited by electrical stimulation at 1 Hz. Peak tension was statistically unchanged with Pi: 0.641±0.085 mN.mm-2 at 0 mM Pi and 0.749±0.153 mN.mm-2 at 5 mM Pi (mean±SEM, n=9, p=0.46 for repeated measures ANOVA). The maximum rate of tension development (+dP/dt max) normalized to peak tension was significantly enhanced by Pi: 11.0±0.3 s-1 at 0 mM Pi and 13.4±0.5 s-1 at 5 mM Pi (p<0.05). All temporal characteristics of the force transient were significantly shortened with increasing Pi. Time-to-peak was shortened from 162±6 ms at 0 mM Pi to 135±4 ms at 5 mM Pi (p<0.01), and time-to-50% recovery was shortened from 309±12 ms at 0 mM Pi to 258±9 ms at 5 mM Pi (p<0.01). A 1% lengthening stretch with varying duration of 10-200 ms was applied at time-to-50% recovery during the descending phase of the force transient. The recorded force response represented the full viscoelastic response of the activated muscle in late systole/early diastole. Matching lengthening stretches were also applied when the muscle was not stimulated, thus providing a control for the passive viscoelastic response. After subtracting the passive from the active force response, the resulting myofilament response demonstrated features of faster myofilament relaxation in response to the stretch. Temporal characteristics of the resulting myofilament response to stretch were significantly shortened due to the applied lengthening and with increasing Pi. For example, time-to-70% relaxation with 100 ms lengthening duration was shortened by 8.8±6.8 ms at 0 Pi, 19.6±4.8* ms at 1 mM Pi, 31.0±5.6* ms at 2 Pi, and 25.6±5.3* ms at 5 mM Pi (*p<0.01 compared to no change). Our findings suggest that increasing Pi availability at the myofilaments contributes directly to myofilament relaxation due to reversal of the myosin power stroke and especially as the myocardium transitions to relengthening in diastole.

Mechanisms underlying Li+ effects on the myocardium of vertebrates

The inotropic Li+ effects on the myocardium of frog Rana temporaria, as well as its influence on ion transport in the rat heart mitochondria (RHM), have been studied. The addition of Li+ to the normal Ringer solution (Li+-R) was found to attenuate the myocardium tension, decrease the maximal rate of tension development and its half-relaxation time. A comparison of the cardiac muscle contractile parameters in Li+-R with the effects of voltage-gated Ca2+ channel (Cav 1.2) inhibitors, verapamil and CdCl2, has shown that the negative inotropic effect of Na+ replacement by Li+ in the limited intracellular intermembrane (“fuzzy”) space is based on the block of Ca2+ influx into myoplasm via the reverse Ca2+/Na+ exchanger in the plasma membrane. This, in turn, prevents massive Ca2+-induced Ca2+ release to the myoplasm via RYR2-channels in the sarcoplasmic reticulum finally leading to the suppression of Ca2+-dependent myocardial contractions. The experiments on the Li+ effects on RHM have established that Li+ only slightly increases the passive permeability of the mitochondrial inner membrane for K+ and H+ and decreases the intensity of proton pumping from the matrix of energized mitochondria to the external medium. This may indicate no relationship between the oxidative processes in mitochondria and the decreased myocardium contractility under Na+ replacement in the medium by Li+.

Cardiovascular effects of long-term caffeine administration in aged rats

To evaluate the possible beneficial effect of long-term caffeine administration on cardiovascular system in aged rats. This study was carried out on 20 senile male albino rats, aged 18-20 months. Rats were divided into caffeine-treated group, which received 25 mg/kg caffeine by gavage daily for 6 weeks, and control group. All rats were subjected to the following in vivo measurements: body weight, arterial blood pressure, heart rate and ECG recording. In vitro measurements included studying isolated hearts in a Langendorff apparatus for their intrinsic properties as well as plasma levels of lipids, malondialdehyde and nitrate. Caffeine administration to aged rats significantly enhanced the baseline and maximum values achieved in response to isoproterenol infusion in isolated heart preparation for both maximum rate of tension development (dT/dt max) and time to peak tension (TPT). The responses of hearts isolated from caffeine group to the three doses of isoproterenol infusion also showed a significantly higher dT/dt max with shorter TPT and half relaxation time (1/2 RT) compared to controls. Plasma nitrate was significantly increased and ECG recording was nonsignificantly affected in caffeine group. The plasma levels of total cholesterol, LDL cholesterol as well as in vivo heart rate and systolic, diastolic and mean arterial blood pressures were all significantly increased associated with a significant decrease in HDL cholesterol in caffeine group. Long-term caffeine administration in aged rats increased cardiac inotropy, lusitropy and preserved NO level, which points to a promising potential favorable effect on cardiac intrinsic properties.

Urocortins Improve Dystrophic Skeletal Muscle Structure and Function through Both PKA- and Epac-Dependent Pathways

In Duchenne muscular dystrophy, the absence of dystrophin causes progressive muscle wasting and premature death. Excessive calcium influx is thought to initiate the pathogenic cascade, resulting in muscle cell death. Urocortins (Ucns) have protected muscle in several experimental paradigms. Herein, we demonstrate that daily s.c. injections of either Ucn 1 or Ucn 2 to 3-week-old dystrophic mdx(5Cv) mice for 2 weeks increased skeletal muscle mass and normalized plasma creatine kinase activity. Histological examination showed that Ucns remarkably reduced necrosis in the diaphragm and slow- and fast-twitch muscles. Ucns improved muscle resistance to mechanical stress provoked by repetitive tetanizations. Ucn 2 treatment resulted in faster kinetics of contraction and relaxation and a rightward shift of the force-frequency curve, suggesting improved calcium homeostasis. Ucn 2 decreased calcium influx into freshly isolated dystrophic muscles. Pharmacological manipulation demonstrated that the mechanism involved the corticotropin-releasing factor type 2 receptor, cAMP elevation, and activation of both protein kinase A and the cAMP-binding protein Epac. Moreover, both STIM1, the calcium sensor that initiates the assembly of store-operated channels, and the calcium-independent phospholipase A(2) that activates these channels were reduced in dystrophic muscle by Ucn 2. Altogether, our results demonstrate the high potency of Ucns for improving dystrophic muscle structure and function, suggesting that these peptides may be considered for treatment of Duchenne muscular dystrophy.

Food restriction impairs myocardial inotropic response to calcium and β-adrenergic stimulation in spontaneously hypertensive rats

Although long-term food restriction (FR) has been shown to induce cardiac remodeling and dysfunction, there are few data on the effects of FR on pressure-overloaded hearts. The aim of this study was to examine the effects of FR on cardiac muscle performance during inotropic stimulation in the myocardium of spontaneously hypertensive rats (SHRs). Male 60-day-old SHRs were subjected to FR for 90 days. Food-restricted animals received 50% of the ad libitum amount of food consumed by the control group. Myocardial function was studied in isolated left ventricular papillary muscle under isometric contraction in basal condition (1.25 mmol/L extracellular Ca(2+) concentration) and after 3 inotropic maneuvers: (1) at postrest contraction of 30 seconds, (2) at extracellular Ca(2+) concentration of 5.2 mmol/L, and (3) after beta-adrenergic stimulation with 10(-6) mol/L isoproterenol. At basal condition, time from peak tension to 50% relaxation was greater in the food-restricted group (P < .05). Inotropic stimulation with postrest contraction and isoproterenol promoted a significant lower increase of developed tension, maximum rate of tension development, and maximum rate of tension decline in the food-restricted compared to the control group. The elevation of extracellular Ca(2+) concentration induced a lower increase of developed tension, maximum rate of tension development, and time from peak tension to 50% relaxation in the food-restricted than in the control group. In conclusion, long-term FR promotes impairment of myocardial inotropic response to calcium and beta-adrenergic stimulation in SHRs.

Cardiac inotropic and hypertrophic effects of Nigella sativa supplementation in rats

The intrinsic cardiac responses to 2-months Nigella sativa (black seed) oral supplementation (800 mg/kg) in rats were investigated. The isolated hearts were perfused in a Langendorff preparation to demonstrate the effects of N. sativa on the baseline inotropic and chronotropic functions and the myocardial flow rate. N. sativa supplementation induced moderate global (homogenous) cardiac hypertrophy, evident by significant increases in left ventricular and whole heart weights as well as the relative heart weight/body weight ratio. The isolated perfused hearts of Nigella-treated rats showed enhanced levels of baseline peak tension, maximum rate of tension development, heart rate and myocardial flow rate. Moreover, there was a significant increase in the tension developed per gram left ventricular weight. The present study provides the first evidence of a physiological cardiac hypertrophy in rats induced by long term N. sativa supplementation.

Myocardial remodeling and dysfunction are induced by chronic food restriction in spontaneously hypertensive rats

Several studies have shown alterations in hearts from animals subjected to food restriction (FR). However, few experiments in hearts evaluating pressure overload have been reported. We examined the effects of chronic FR on myocardial function and morphology in spontaneously hypertensive rats (SHR). Sixty-day-old SHR were fed a control (C) or a restricted diet (daily intake reduced to 50% of amount of food consumed by the control group) for 90 days. Myocardial performance was studied in isolated left ventricular (LV) papillary muscle. Food restriction decreased body weight and LV weight; LV weight/body-weight ratio was lower in the food-restricted group (SHR-C, 2.84 ± 0.21 mg/g; SHR-FR, 2.56 ± 0.24 mg/g; P < .05). Food restriction did not change arterial systolic blood pressure. Myocyte surface area was lower in the food-restricted group ( P < .01). Food restriction induced myocardial ultrastructural alterations including reduced sarcoplasm content, reduced and disorganized myofilaments, disorganized Z line, dilated sarcoplasmic reticulum, and deep infoldings of plasma membrane. Myocardial hydroxyproline concentration was increased in the restricted rats. Peak developed tension ( P < .05) and maximum rate of tension development ( P < .01) were decreased in the SHR-FR group. In conclusion, myocardium of SHR subjected to chronic FR presents attenuation of hypertrophy development, ultrastructural changes, increased collagen content, and systolic dysfunction.

Improved systolic ventricular function with normal myocardial mechanics in compensated cardiac hypertrophy.

There is still controversy about the relation between changes in myocardial contractile function and global left ventricular (LV) performance during stable concentric hypertrophy. To clarify this, we analyzed LV function in vivo and myocardial mechanics in vitro in rats with pressure overload-induced cardiac hypertrophy. Male Wistar rats (70 g) underwent ascending aortic stenosis for 8 weeks (group AAS, n = 9). LV performance was assessed by transthoracic echocardiography under anesthesia. Myocardial function was studied in isolated papillary muscle preparations during isometric contraction. The data were compared with age- and sex-matched sham-operated rats (group C, n = 9). LV weight-to-body weight ratio (C: 2.13 +/- 0.14 mg/g; AAS: 3.24 +/- 0.44 mg/g), LV relative wall thickness (C: 0.18 +/- 0.02; AAS: 0.33 +/- 0.09), and LV fractional shortening (C: 54 +/- 5%; AAS: 70 +/- 8%) were increased in group AAS (P < 0.05). Echocardiographic analysis also indicated a significant association (r = 0.74; P < 0.001) between the percent fractional shortening index and LV relative wall thickness. The performance of AAS isolated muscle revealed that active tension (C: 6.6 +/- 1.7 g/mm2; AAS: 6.5 +/- 1.5 g/mm2) and maximum rate of tension development (C: 69 +/- 21 g/mm2/s; AAS: 69 +/- 18 g/mm2/s) were not significantly different from group C (P > 0.05). In conclusion, compensated pressure-overload myocardial hypertrophy is associated with preserved myocardial function and increased ventricular performance. The improved LV function might be due to the ventricular remodeling characterized by an increased relative wall thickness.

COMPARISON OF BALLISTIC CONTRACTIONS PERFORMED WITH AND WITHOUT PRE-ACTIVATION

The motor command of a movement performed with maximal velocity (ballistic contraction) is preprogrammed and does not rely on sensory feedback mechanisms. PURPOSE This study examines whether the conditions during which ballistic contractions are performed induce acute adjustments in neural activation. METHODS To investigate the effects of submaximal sustained muscular activity on the characteristics of a subsequent ballistic contraction, 7 subjects performed rapid contractions, as fast as possible, of the ankle dorsiflexors under two different conditions: with and without muscular pre-activation. In the latter case, the subjects sustained a contraction at 20–25% of the maximal voluntary contraction (MVC) during 3–4s prior to the ballistic contraction. Isometric force and surface electromyogram (EMG) of the ankle dorsiflexors were simultaneously recorded. The behaviour and the discharge rate of motor units from the tibialis anterior were also recorded with fine wires electrode. RESULTS When ballistic contractions were performed without pre-activation, the maximal rate of tension development was greater (9.4 ± 1.7 vs 8.7 ± 1.8 Nm/s, p < 0.05) and the time to reach 50% of the integrated surface EMG activity was shorter (63.2 ± 7.4 vs 94.4 ± 6.0 ms, p < 0.05) than with pre-activation. In the former condition, the instantaneous motor unit discharge rate at the beginning of the ballistic contraction was also greater (115 ± 5.8 Hz vs 89.8 ± 3.8 Hz, p < 0.05; mean of the first three interspike intervals). Moreover, the activation pattern of these units showed opposite behaviours under the two experimental conditions. CONCLUSION A sustained pre-activation, prior to a ballistic contraction, reduces the instantaneous discharge rate of motor units and thus the maximal rate of muscle tension development. However, the negative effect of the pre-activation was abolished in the trials that showed a brief silent period in EMG activity at the transition between sustained and ballistic actions.

Importance of creatine kinase activity for functional recovery of myocardium after ischemia-reperfusion challenge.

To define the relation between the phosphoryl transfer via creatine kinase and the ability to recover from an ischemia-reperfusion challenge, the authors chemically inhibited creatine kinase activity with iodoacetamide (IAm) and then measured myocardial recovery after 2, 10, or 30 min of global ischemia followed by 30 min of reperfusion in the isolated, arterially perfused interventricular septa of the rabbit heart. During normoxia, IAm (0.5 M perfused for 15 min) did not by itself modify developed tension, maximal rate of tension development, or resting tension. In ischemia, IAm pretreatment increased the rate of developed tension loss and highly diminished developed tension recovery after reperfusion for all the ischemia periods tested. Moreover, IAm significantly enhanced the maximal increase in the resting tension induced by 10 or 30 min of ischemia plus reperfusion. Lactate dehydrogenase activity in reperfusion was also significantly increased over untreated septa. On the basis of the present results, the authors suggest that the aggravating effects exhibited by IAm on the ischemic myocardium are compatible with its creatine kinase inhibition properties and that creatine kinase activity is essential for full recovery from an ischemia-reperfusion challenge.

Effect of time of day on force variation in a human muscle.

The effect of time of day on the neural activation and contractile properties of the human adductor pollicis muscle was investigated in 13 healthy subjects. Two different times of day were chosen, corresponding to the minimum (7 h) and maximum (18 h) levels of strength. The force produced was compared with the associated electromyographic (EMG) activity during voluntary and electrically induced contractions in order to determine whether peripheral or central mechanisms play a dominant role in diurnal force fluctuation. The results indicated that the force produced during a maximum voluntary contraction (MVC) was significantly higher (+8.9%) in the evening than the morning. Since the increase in force of the MVC and the tetanic contraction (100 Hz) were similar, it is suggested that peripheral mechanisms are responsible for diurnal fluctuations in force. This conclusion is supported by the observation that central activation, tested by the interpolated twitch method during an MVC, did not change, and that the EMG was less per unit force in the evening. In addition to the increase in maximum twitch and tetanus force, significant changes in muscle contractile kinetics were also observed. The maximum rate of tension development and the relaxation of the twitch and tetanus increased in the evening, and the twitch contraction time (CT) and the time to half-relaxation (TR(1/2)) were reduced. Because the mean range of variation in skin temperature (2. 6 degrees C) observed over the course of the day was very low, this change cannot entirely explain those observed in muscle contractile properties.

The rate of force generation by the myocardium is not influenced by afterload.

The influence of afterload on the rate of force generation by the myocardium was investigated using two types of preparations: the in situ dog heart (dP/dt) and isolated papillary muscle of rats (dT/dt). Thirteen anesthetized, mechanically ventilated and thoracotomized dogs were submitted to pharmacological autonomic blockade (3.0 mg/kg oxprenolol plus 0.5 mg/kg atropine). A reservoir connected to the left atrium permitted the control of left ventricular end-diastolic pressure (LVEDP). A mechanical constriction of the descending thoracic aorta allowed to increase the systolic pressure in two steps of 20 mmHg (conditions H1 and H2) above control values (condition C). After arterial pressure elevations (systolic pressure C: 119 +/- 8.1; H1: 142 +/- 7.9; H2: 166 +/- 7.7 mmHg; P < 0.01), there were no significant differences in heart rate (C: 125 +/- 13.9; H1: 125 +/- 13.5; H2: 123 +/- 14.1 bpm; P > 0.05) or LVEDP (C: 6.2 +/- 2.48; H1: 6.3 +/- 2.43; H2: 6.1 +/- 2.51 mmHg; P > 0.05). The values of dP/dt did not change after each elevation of arterial pressure (C: 3,068 +/- 1,057; H1: 3,112 +/- 996; H2: 3,086 +/- 980 mmHg/s; P > 0.05). In isolated rat papillary muscle, an afterload corresponding to 50% and 75% of the maximal developed tension did not alter the values of the maximum rate of tension development (100%: 78 +/- 13; 75%: 80 +/- 13; 50%: 79 +/- 11 g mm-2 s-1, P > 0.05). The results show that the rise in afterload per se does not cause changes in dP/dt or dT/dt.

Reduced cardiac contractile responsiveness to isoproterenol in obese rabbits.

Although obesity is characterized by increased sympathetic nervous system activity, there is often a paradoxical reduction in cardiovascular end-organ response to sympathetic stimulation. Mechanisms involved in reduced sympathetic responsiveness in obesity have not been well characterized. Therefore, we determined cardiac contractile responsiveness to beta-stimulation in the obese rabbit model using both isolated heart (IH) and isolated papillary muscle (IPM) preparations. Female New Zealand White rabbits were fed control (IH: n=9; IPM: n=6) or 10% fat diets (IH: n=9; IPM: n=7) for 12 weeks. Contractile responsiveness in the IH was determined using a modified Langendorff preparation to evaluate the dose-response relationship between isoproterenol and 1) peak developed pressure/g of left ventricular wet weight and 2) maximal rate of pressure development (+dP/dt/P). Contractile responsiveness in the IPM was determined using right ventricular papillary muscles to evaluate the dose-response relationship between isoproterenol and (1) peak developed tension (T)/mm2 cross-sectional area (CSA) and (2) maximal rate of tension development (dT/dt/CSA). In the IH, baseline and maximum developed pressure/g were reduced in obese rabbits by 37% and 31%, respectively (P< or =.05). In the IPM, baseline and maximum T/CSA responses were reduced in obese rabbits by 59% and 33%, respectively (P< or =.05). Potency of isoproterenol as reflected by the EC50 did not differ between lean and obese animals in either preparation. These results demonstrate that left ventricular contractility in obesity is reduced at baseline and in response to stimulation with isoproterenol and suggest that decreased responsiveness to beta-stimulation may be a factor in the obesity-related systolic dysfunction.

Dynamometric assessment of the plantarflexors in hemiparetic subjects: relations between muscular, gait and clinical parameters

The aims of this study were to investigate, in 16 subjects with hemiparesis, the plantarflexor muscle performance of the paretic side and to determine the level of the relationships between muscular parameters, clinical measures and gait performance. A Biodex dynamometric system was used to evaluate static and dynamic torques, power and maximal rate of tension development of the plantarflexor muscles. The clinical measures included the Fugl-Meyer assessment (FMA), the "Up &amp; Go" test and an evaluation of ankle muscle tone. Velocity, cadence, stride length and gait cycle duration were determined for each subject at both comfortable and maximal safe speeds using foot contacts and videographic data. Results indicated that dynamometric values produced by the hemiparetic subjects were reduced in comparison to those reported for healthy subjects. Their torque-angle curves had a curvilinear shape which indicated pronounced decrease of torque for plantarflexion efforts at the beginning of the movement. Torques produced at different velocities of testing did not demonstrate significant differences (MANOVAs: p &gt; 0.05) but power values were significantly different. Results also showed that all the selected muscular parameters (torque, power and maximal rate of tension development) were moderately to highly interrelated (0.65 &lt; r &lt; 0.94; p &lt; 0.01) suggesting that a common factor of muscular performance was assessed. Furthermore, the dynamometric data were significantly associated with some of the clinical measures (sensation and lower limb motor control scores of the FMA) but were not related to the gait variables (Pearson's r &lt; 0.45; p &gt; 0.05). This last finding suggests that the relationship between plantarflexor strength and the level of gait performance in adults with stroke is complex. The relationship may be influenced by other factors such as muscular compensations within and between limbs and motor control impairments.

Oxygen Dose-Response Curve of Cardiac Papillary Muscle From Fetal and Nonpregnant Adult Sheep Exposed to Long-Term, High-Altitude Hypoxemia

We tested the hypothesis that hearts of fetal and nonpregnant adult sheep exposed to long-term hypoxemia would be able to sustain higher contractile function during exposure to acute hypoxia than hearts from normoxic animals. Pregnant and nonpregnant sheep were exposed to high altitude (3820 m) for 100 days. Right and left ventricular papillary muscle strips were obtained from fetuses and nonpregnant adults, mounted in an isolated bath system, stimulated electrically and subjected to acute hypoxia in a dose response manner. Measurements were made of maximum tension production (Tmax), maximum rate of tension development (+dT/dtmax), maximum rate of relaxation (-dT/dtmax), time to peak tension, and duration of contraction. Results were compared to papillary muscle from a normoxic group of animals. Baseline values (95% O2 + 5% CO2 bubbled in the bath) of Tmax and +/- dt/dtmax for each ventricle were greater in adults than fetuses in both normoxic and long-term hypoxemic groups. During hypoxia (at 40 and 20% O2) Tmax and +/- dT/dtmax, were all maintained at significantly higher values in papillary muscle from long-term hypoxemic fetuses than in papillary muscle from normoxic fetuses. Duration of contraction and time to peak tension did not differ between the normoxic and hypoxemic groups. In both ventricles of the long-term hypoxemic adult, Tmax and +/- dT/dtmax, as well as duration and time to peak tension, were significantly higher than in normoxic adults, but only at the lowest level of hypoxia (20% O2). Contrary to the original hypothesis, heart muscle from both fetal and adult sheep that had been exposed to long-term hypoxemia could maintain contractile function better during acute hypoxia. The responsible mechanisms are not clearly understood.

Age-dependent Changes in the Effects of Endocardial Endothelium on the Contractile Characteristics of its Adjacent Myocardium in Rats

Cardiac contractility and the ability of myocardium to respond to various stimuli have been shown to be age-related. Endocardial endothelium has been shown to modulate the performance of its adjacent myocardium, and it has been proven that these changes are reversible by increasing extracellular calcium. This study was designed to evaluate whether the effects of endocardial endothelium and their modifications with changing extracellular calcium were age-dependent. Experiments were carried out on left ventricular papillary muscles isolated from male Wistar rats belonging to three different age groups: 7 weeks, 4 months, and 9 months old. Extracellular calcium concentration (0.7 to 5 mm) curves were conducted in papillary muscles with intact endocardial endothelium (+EE) and after its removal (−EE) with Triton X-100. At a low extracellular calcium concentration (0.7 mm), total tension (TT), maximum rate of tension development (+dT/dt), time to peak tension (TTPT), and time to half tension decline from maximum tension (RT) increased with increasing age (P<0.01). Increasing extracellular calcium concentration eliminated age-related differences in TT and RT did not affect TTPT and caused +dT/dtto increase to greater levels in 7-week-old as compared to 4-month-old myocardium. Endocardial endothelial removal (−EE) had a greater effect on contractile indices in younger rats. Increasing extracellular calcium concentration eliminated EE-mediated changes in contractile indices regardless of age. Thus, increasing extracellular calcium concentration had a greater effect in younger rats and after −EE. Increasing extracellular calcium from 0.7 to 2.5 mmincreased total tension in 7-week-olds, between 56% +EE and 131% −EE (P<0.01); in 4-month-olds, from 29% +EE to 57% −EE (P<0.01); and in 9-month-olds, from 13% +EE to 23% −EE (P=0.08). These results indicate that the contractile effects of endocardial endothelium are both age- and calcium-dependent, with responsiveness to endocardial endothelium decreasing with age. Myocardial contractile characteristics also vary with age and these differences are only partially reversible with increasing extracellular calcium concentration.

Exercise training improves lusitropy by isoproterenol in papillary muscles from aged rats.

Aging is associated with a decreased cardiac responsiveness to beta-adrenergic stimulation. We examined the effect of endurance exercise training of old Fischer 344 male rates on beta-adrenergic stimulation of the function of isolated left ventricular papillary muscle. Three groups were examined: sedentary mature (SM; 12-mo old), sedentary old (SO; 23-24 mo old), and exercised old (EO; 23-24 mo old) that were treadmill trained for 4-8 wk. The isometric contractile properties were studied at 0.2 Hz and 0.75 mM calcium. Without beta-adrenergic stimulation, there were no group differences for peak tension, maximum rate of tension development (+dP/dt), or maximum rate of tension dissipation (-dP/dt). The time to peak tension was longer (P < 0.05) for both EO and SO than for SM rats. Half relaxation time (RT1/2) was prolonged (P < 0.05) for SO compared with SM and EO (which did not differ). The three groups did not differ in the beta-adrenergic stimulation by isoproterenol of peak tension, -dP/dt, time to peak tension, or contraction duration. The inotropic response (+dP/dt) of SM was greater (P < 0.05) than that in SO or EO rats (which did not differ); however, the lusitropic response (RT1/2) was lesser (P < 0.05) in SO than in SM or EO rats (which did not differ). Thus exercise training of old rats improved the lusitropic response to isoproterenol without altering the age-associated impairment in inotropic response.

β-Adrenergic responsiveness of papillary muscles in the rat postinfarction model

Rats in which ligation of the left anterior descending (LAD) coronary artery did not result in an infarct visible to the naked eye have generally been called sham-operated controls and have been used to evaluate changes in the characteristics of the remaining viable myocardium of rats where myocardial infarction was present. Whether these sham-operated rats are equivalent to normal controls or whether they have been altered by the surgery is unknown. The purpose of this study was to evaluate potential differences in papillary muscle mechanical characteristics and responsiveness to beta-adrenergic stimulation in control, open-chest, sham-operated (aborted infarct), and infarct rats 4 weeks after surgery. Absence of significant myocardial damage was verified by morphologic examination in sham-operated hearts: myocardial infarction scar was 1.5 +/- 0.6% (mean +/- SE) of circumference, no infarct being transmural. Basal contractile indices were decreased in papillary muscles in infarct rats compared with the three other groups. The response in contractile indices of infarct muscles to isoproterenol was also less compared with the other three groups. However, although basal contractility was the same in muscles from sham-operated, open-chest, and controls, the responsiveness of sham-operated muscles to isoproterenol was less: change in total tension of 2.9 +/- 1.2% vs. 9.5 +/- 2.7% for controls and 9.3 +/- 2.4% for open-chest (p < 0.05). Percent change in maximum rate of tension development (+dT/dt) was also less in muscles from sham-operated versus control muscles (21 +/- 2 vs. 33 +/- 6%; p < 0.05). Twitch configuration changed similarly with isoproterenol in all four groups. Thus, although contractility is unaffected by sham operation, beta-adrenergic responsiveness of tension-generating indices is modified.

Cardiac beta-adrenoceptor-effector coupling in portal vein-stenosed rats.

Cardiac beta-adrenergic signal transduction was examined in chronic portal vein-stenosed rats. Basal tension and maximum rate of tension development were significantly depressed in left ventricular papillary muscles (0.21 +/- 0.03 N/cm2 and 8.2 +/- 1.7 N.s-1.cm-2, respectively) compared with sham-operated controls (0.51 +/- 0.05 N/cm2 and 19.9 +/- 4.4 N.s-1.cm-2, respectively). The positive inotropic response to isoproterenol was also attenuated. Adenosine 3',5'-cyclic monophosphate formation was decreased significantly when GTP (-41.9%), isoproterenol with GTP (-45.3%), or guanosine 5'-O-(3-thiotriphosphate) (-52.4%) was used to stimulate adenylyl cyclase, but not when Mn2+ or forskolin was used. Beta-Adrenoceptor density (sham operated 24.6 +/- 2.0 fmol/mg; portal vein stenosed 26.4 +/- 2.1 fmol/mg) and the apparent dissociation constant (sham operated 0.26 +/- 0.04 nM; portal vein stenosed 0.29 +/- 0.04 nM) were unaffected. Portal venous hypertension did not alter beta-adrenergic receptor affinity for isoproterenol. However, it was necessary for isoproterenol to occupy three times the number of receptors in papillary muscles from stenosed animals to produce an equal increase in force generation. These data suggest that although portal vein stenosis does not alter cardiac beta-adrenoceptor density or affinity for ligands, transduction of the signal between the receptor and adenylyl cyclase is adversely influenced and may be responsible for the diminished responsiveness of beta-adrenoceptors in the myocardium.

Enhanced Negative Inotropic Effect of an Adenosine A 1-Receptor Agonist in Rat Left Atria in Hypothyroidism

Left atria were isolated from rats made hypothyroid by adding propylthiouracil to their drinking water, such rats after saturating doses of thyroid hormones, and from control rats. Isoproterenol (ISO: 1 μM) increased the values of developed tension (DT), maximal rate of tension development (+dT/d t) and tension fall (-dT/d t). The effect was largest in hypothyroid and lowest in hyperthyroid atria. The adenosine A 1-receptor agonist N 6-(phenylisopropyl)-adenosine (PIA) has a powerful negative inotropic effect in ISO-stimulated atria. The effects of PIA on +dT/d t. -dT/d t and DT were enhanced in hypothyroidism. Adenosine receptor number was not decreased. The amount of total G i-like proteins was estimated by pertussis toxin labeling. The amounts of G i2 and G i3 were estimated in Western blots such antisera raised in rabbits against peptides corresponding to parts of their sequences, using purified recombinant α subunits as standards. The amounts of low and high molecular weight forms of G s were estimated by cholera toxin labeling. G i2, G i3 and pertussis toxin substrate concentrations were slightly lower in the hypothyroid animals, while the amounts of both forms of G s per mg of protein were only half of those in euthyroid rat atria. The levels of G i2 and G i3 were greatly elevated as compared to G s as membrane marker. These changes were reversed by treatment of the hypothyroid rats with thyroid hormones. In conclusion, the present results show an enhanced negative inotropic effect of an adenosine A 1-receptor agonist in rat left atria in hypothyroidism, reflected both in sensitivity and responsiveness, and suggest that this is not caused by a change in receptor number but by a relative in G i proteins.