Oscar H.L. Bing

Intracellular calcium transients in myocardium from spontaneously hypertensive rats during the transition to heart failure.

To investigate the mechanism of impaired myocardial function after long-term pressure overload, we studied cardiac muscle mechanical contraction and intracellular calcium transients using the bioluminescent indicator aequorin. Left ventricular papillary muscle preparations were examined from three groups of rats: 1) aging spontaneously hypertensive rats (SHR) with clinical and pathological evidence suggesting heart failure (SHR-F group), 2) age-matched SHRs with no evidence of heart failure (SHR-NF group), and 3) age-matched normotensive Wistar-Kyoto rats (WKY group). Isometric force development was depressed in both SHR groups relative to the WKY group. Resting [Ca2+]i was lower in the SHR-F group, and the time to peak [Ca2+]i was prolonged in this group. The relative increases in peak [Ca2+]i with the inotropic interventions of increased [Ca2+]o and the addition of isoproterenol were similar among groups. Although inotropy increased in all groups with increased [Ca2+]o, after isoproterenol, inotropy increased only in the WKY group. Thus, in SHR myocardium, [Ca2+]i increased after isoproterenol, but inotropy failed to increase. Myosin isozymes were shifted toward the V3 isoform in both SHR groups; the V3 isoform was virtually 100% in papillary muscles from the SHR-F group. These changes may reflect events directly contributing to the development of heart failure or represent adaptive changes to chronic pressure overload and heart failure.

Effects of treppe and calcium on intracellular calcium and function in the failing heart from the spontaneously hypertensive rat.

We studied functional and intracellular calcium responses to treppe and extracellular calcium in spontaneously hypertensive rat (SHR) hearts during the transition from compensated pressure overload to failure. Intracellular calcium was measured using aequorin, a bioluminescent Ca2+ indicator. Experiments were performed with intact, isovolumically contracting, buffer-perfused hearts from three rat groups: (1) aging SHR with evidence of heart failure (SHR-F), (2) age-matched SHR with no evidence of heart failure (SHR-NF), and (3) age-matched normotensive Wistar-Kyoto (WKY) rats. In each experiment, left ventricular pressure and intracellular calcium transients were simultaneously recorded. Hearts were studied at 30 degrees C and paced at a rate of 1.6 Hz while being perfused with oxygenated Krebs-Henseleit solution (95% O2/5% CO2) at 100 mm Hg. At the baseline state, peak systolic pressure was greatest in the SHR-NF group and lowest in the SHR-F group. Peak and resting [Ca2+]i were not significantly different among groups; however, the calcium transient was prolonged in the SHR-NF and SHR-F groups. With increasing perfusate [Ca2+]o from 0.5 to 3.0 mmol/L, the relative increases in peak [Ca2+]i and peak systolic pressure were similar among groups. When stimulation rate was increased from 1.6 to 2.0, 2.4, 2.8, and 3.2 Hz, peak [Ca2+]i, peak systolic pressure, and +/- dP/dt fell in SHR-F hearts. Peak systolic pressure decreased in the SHR-NF group at rates above 2.4 Hz but did not decline in the WKY group. Peak [Ca2+]i increased in the WKY and SHR-NF groups with increasing heart rates. Peak systolic pressure did not fall significantly in the WKY group at any heart rate. Elevation of diastolic [Ca2+]i and/or calcium transient and pressure alternans were present in 8 of 13 SHR-F hearts at the highest stimulation rate, findings that were absent in both the WKY and SHR-NF hearts. We conclude the following: (1) Under baseline conditions, depressed contractile function of failing myocardium cannot be explained by decreased peak [Ca2+]i, (2) relative increases in [Ca2+]i and inotropy with increasing [Ca2+]o are proportional among groups; and (3) although peak systolic [Ca2+]i and inotropy are maintained with increasing stimulation rate in the WKY and SHR-NF groups, peak systolic [Ca2+]i and pressure decrease in parallel in the SHR-F heart with increasing stimulation rate, suggesting that impaired calcium cycling may contribute to compromised pump function in the SHR-F heart.

Impaired myocardial performance and response to calcium in experimental alcoholic cardiomyopathy.

The effect of chronic consumption of ethanol upon myocardial function has been in dispute. The mechanical performance of isolated left ventricular papillary muscles was studied in a group of eight male rats which had been on a diet in which ethanol provided 36% of total calories for 5 weeks. This group was compared to a pairfed control group of eight rats. Muscles were studied in oxygenated Krebs-Henseleit solution at 28°C, while stretched to Lmax and stimulated at 12/min to contract isometrically. Bath calcium concentrations were varied from 1.25 to 7.5 mm/l. Heart size and chamber weights did not differ between the groups. Peak developed tension, maximum rates of rise and fall of tension, and time to half relaxation were significantly lower in the ethanol group, whereas resting tension and time to peak tension did not differ in the two groups. All parameters except time to half relaxation exhibited a significant dependence on concentration of calcium in the bath. However, the degrees of augmentation of peak developed tension and maximum rates of rise and fall of tension with increase in calcium in the bath from 1.25 to 2.5 mm/l were significantly lower in the ethanol group. In addition to impairment of function with chronic alcohol administration, altered sensitivity to calcium was demonstrated. Thus, isolated muscle studies have defined a defect in defined a defect in myocardial performance not apparent from studies of hemodynamics in the intact animal.

Effect of ethanol on contraction and relaxation of isolated rat ventricular muscle

Abstract The effects of ethanol on myocardial mechanical properties were evaluated in isolated rat ventricular muscle preparations. Ethanol reduced peak developed tension by 6.3% at a concentration of 100 mg 100 ml , 11.8% at 200 mg 100 ml and 27.2% at 400 mg 100 ml . A similar degree of depression was observed in d T d t . Time to peak tension was slightly shortened, and isometric relaxation rate was increased at a high concentration of ethanol. The resting tension did not change significantly. Despite biochemical data by others suggesting an alcohol induced delay in calcium sequestration by cardiac microsomes, ethanol at concentrations found in human intoxication depressed the systolic performance of isolated muscle preparations but did not prolong relaxation or alter resting tension.

Oxygen cost of stress development in hypertrophied and failing hearts from the spontaneously hypertensive rat.

Left ventricular isovolumic stress development and metabolic parameters were studied in 18-24-month-old spontaneously hypertensive rats (SHRs) and age-matched Wistar-Kyoto (WKY) rat controls using the isolated, isovolumic (balloon in left ventricle) buffer-perfused rat heart preparation. After WKY rats and all SHRs were compared, SHRs were divided into two groups: those animals with (SHR-F) and without (SHR-NF) evidence of heart failure. Hearts were perfused at 100 mm Hg using a constant pressure system at a temperature of 37 degrees C. In the baseline state, peak systolic pressure was greatest in the SHR-NF group and lowest in the SHR-F group. Peak midwall stress was greatest in the WKY group and, again, lowest in the SHR-F group. Oxygen consumption was lowest in the SHR-F group. When the oxygen cost of stress development was estimated by normalizing myocardial oxygen consumption by peak developed midwall stress, values were lowest in the WKY, greater in the SHR-NF, and greatest in the SHR-F group. Lactate production did not occur in the baseline state in any of the groups. Functional and metabolic responses to graded hypoxia, induced by changing the gas mixture of the perfusate from 95% to 50%, 25%, and 0% oxygen at perfusion pressures of 100 and 130 mm Hg, were studied. Increasing perfusion pressure generally resulted in small increases in peak systolic pressure and myocardial oxygen consumption but did not substantially reverse the contractile or metabolic deficit present in the SHR-F group.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiologic loading of isolated heart muscle

Summary An electromechanical system with computer control has been developed which permits the simulation of physiologic loading for isolated cardiac muscle. In contrast to conventional loading techniques, where relaxation occurs at the same load as shortening, the new system allows isometric relaxation at peak shortening prior to isotonic relaxation at reduced load.

Prolongation of tension on reoxygenation following myocardial hypoxia: A possible role for mitochondria in muscle relaxation ☆

The mechanism for tension prolongation during reoxygenation following myocardial hypoxia was investigated. It was found that prior addition of isoproterenol, reserpine, quinidine, lidocaine and insulin or a change in pH, temperature, stimulation rate, preload or duration of hypoxia did not qualitatively alter the appearance of the phenomenon. On reoxygenating hypoxic preparations in the presence of 5, 10 or 20% oxygen, tension prolongation was clearly present when little increase in isometric tension was evident. Inhibition of glycolysis by iodoacetate did not alter the appearance of the phenomenon during reoxygenation. Three respiratory inhibitors, antimycin A, rotenone and cyanide, at concentrations which did not prevent an increase in isometric tension during recovery from myocardial hypoxia, all completely prevented the appearance of tension prolongation. Two uncouplers of oxidative phosphorylation, 2–4 dinitrophenol and carbonyl cyanide m-chlorophenyl hydrazone at concentrations large enough to lead to mechanical deterioration despite the presence of oxygen, failed to prevent the appearance of tension prolongation upon reoxygenation. It is concluded that myocardial respiratory activity, independent of ability to generate high energy phosphate, appears capable of altering the duration of mechanical events during reoxygenation of hypoxic heart muscle.

Lactate dehydrogenase and isoenzyme changes in rats with experimental thiamine deficiency

Heat and liver lactate dehydrogenase (LDH) and isoenzyme distributions were studied in rats with experimental thiamine deficiency. No change in total heart LDH activity was observed on days 24 and 40 of a thiamine deficient diet. At the time of symptoms (average 53 days on diet), total heart muscle LDH was slightly decreased in thiamine deficient rats (248 +/- 12 U/g tissue, Mn +/- SE) compared to ad lib fed controls (293 +/- 12, p less than 0.05), but did not differ from pair fed controls (273 +/- 15). Heart muscle LDH isoenzyme distribution showed a relative increase in the H4 band and a decrease in the H2M2 and HM3 bands in thiamine deficiency on day 24. This pattern became more promient by day 40. Calculated total M-LDH activity of heart muscle in thiamine deficient rats was significantly decreased at day 40 (44 +/- 3) compared to pair fed controls (57 +/- 5, p less than 0.05) and ad lib fed controls (85 +/- 7 p less than 0.001). Further changes were not observed in the symptomatic stage. Total H-LDH in heart muscle did not change during the period of study, while total liver LDH activity was reduced in thiamine deficient rats at the symptomatic stage (137 +/- 15) compared with pair fed controls (193 +/- 12, p less than 0.05) and ad lib fed controls (407 +/- 50, p less than 0.01). Liver isoenzyme electrophoresis showed an almost 100% distribution to the M4 band. The present investigation demonstrates a decrease in cardiac muscle M-LDH in experimental thiamine deficiency. Differences in LDH isoenzyme distribution in chronic hypoxia and thiamine deficiency are discussed.

Contractile performance of rat myocardium after chronic tobacco smoke inhalation.

The mechanical performance of isolated left ventricular muscle preparations from rats exposed to smoke from Kentucky Reference cigarettes was examined for possible chronic effects. Eight rats were subjected to smoke for periods of 10 min/hr for 5 hr/day for 180 days. Nineteen additional rats served as either sham-smoked controls or weight-matched, food-deprived controls. Rats exposed to tobacco smoke had a significant diminution in body and left ventricular weight compared to sham-smoked controls. When compared to food-deprived rats, no differences in weights were observed. Contraction mechanics were measured for each muscle at the peak of its length tension curve. No significant difference in cardiac muscle performance was found in rats exposed to tobacco smoke when compared to control animals with respect to contractile performance under oxygenated conditions, muscle performance during 60 min of hypoxia or subsequent reoxygenation, or sensitivity of mechanical performance to isoproterenol. Thus, chronic cigarette smoke exposure did not alter the intrinsic mechanical performance of isolated rat ventricular muscle.

Inhibition of hypoxic myocardial contracture by cobalt in the rat

We studied the relationship between the slow inward current, tissue ATP content, and the development of hypoxic myocardial contracture in the rat. Resting tension and active isometric tension were measured using isolated left ventricular papillary muscle preparations. Action potentials and membrane currents were studied using a single sucrose gap voltage clamp technique. The slow inward current (isi) was partially inhibited by verapamil and completely inhibited by cobalt but was not reduced by ryanodine. Active tension was reduced by all three drugs. Non-stimulation, verapamil, and ryanodine did not prevent contracture development with hypoxia, but contracture was markedly reduced by pre-treatment with cobalt. Despite contrasting effects on contracture, both non-stimulation and cobalt partially prevented ATP depletion with hypoxia, suggesting that contracture is not directly related to total muscle ATP content. Cobalt appears to block hypoxic contracture via a mechanism other than simple blockade of isi, inhibition of contraction, or preservation of tissue ATP content.