Dobutamine restores the reduced efficiency of energy transfer from total mechanical work to external mechanical work in stunned porcine myocardium

Myocardial contractility does not determine the haemodynamic response during dialysis

BACKGROUND

LV systolic dysfunction in dialysis patients has been implicated in the genesis of dialysis hypotension. End-systolic elastance (E(es)), a relatively load-independent parameter of myocardial contractility, was assessed by testing the acute left ventricular (LV) response to nitroglycerine (NTG) in hypotension-prone (HP) and hypotension-resistant (HR) patients.

METHODS

Routine measurement of ejection fraction (EF) was done before dialysis in 15 patients without significant valvular disease or symptoms of coronary heart disease. Continuous arterial pressure was measured by Finapres, with systolic blood pressure (SBP) as surrogate for LV end-systolic pressure. Simultaneously, LV area was measured using automated border detection. SBP and LV area data were combined online to create pressure-area loops in real time following intravenous NTG bolus. E(es) was determined offline by beat-to-beat analysis of consecutive pressure-area loops.

RESULTS

SBP, at baseline 168 mmHg (128-188 mmHg), decreased to 127 mmHg (79-161 mmHg). End-systolic LV area, at baseline 6 cm2 (1-12 cm2), decreased to 4 cm2 (1-10 cm2). E(es) in the HP group (11 mmHg cm(-2); 7-22 mmHg cm(-2)) was not different from E(es) in the HR group (9 mmHg cm(-2); 4-16 mmHg cm(-2)). EF was 61% (45-73%). There was no correlation between E(es) and EF.

CONCLUSIONS

In this population of dialysis patients without clinically manifest heart disease, the HP and HR groups had a similar E(es). Therefore, these two types of dialysis patients were not distinguished by a difference in myocardial contractility. The results of this study argue against a role for reduced myocardial contractility in the genesis of intradialytic hypotension.

Calcium sensitizer EMD 57033, but not the beta1-adrenoreceptor agonist dobutamine, increases mechanical efficiency in stunned myocardium

External work, efficiency of energy transfer (EET), and mechanical efficiency (defined as the ratio of external work over myocardial oxygen consumption (MVO2) are reduced in stunned myocardium. We therefore evaluated how inotropic stimulation by dobutamine and the calcium sensitizer EMD 57033 affected the regional stress-strain relationship as, from which contractility (E(es)), external work at the working point (EWwp), maximal external work (EWmax), EETwp (%), and EETmax are determined. Thirty minutes after regional stunning in 11 open chest pigs by two 10-min coronary occlusions separated by 10 min of reperfusion, dobutamine (0.5, 1, and 2 microg x kg(-1) x min(-1)) was infused, after an ample washout period followed by infusion of EMD 57033 (0.05, 0.1, 0.2 mg x kg(-1) x min(-1)). Stunning decreased E(es) (30%), EWwp (56%), EWmax (63%), EETwp (34%), EETmax (33%) and mechanical efficiency (55%), but MVO2 was unaffected. EWwp, EWmax, EETwp, and EETmax increased similarly with the two drugs, whereas MVO2 increased only after dobutamine. Consequently, mechanical efficiency increased linearly with contractility during EMD 57033 infusion but remained constant during infusion of dobutamine.

Dobutamine enhances both contractile function and energy reserves in hypoperfused canine right ventricle

Although the beta(1)-adrenergic agent dobutamine is used clinically to provide inotropic support to the failing myocardium, it could jeopardize the myocardium by depleting energy reserves. This investigation delineated the contractile and energetic effects of low versus high dobutamine doses in the hypoperfused right ventricular (RV) myocardium. The right coronary artery (RCA) of anesthetized dogs was cannulated for controlled perfusion with arterial blood, and regional RV contractile function was measured. RCA perfusion pressure was lowered from 100 mmHg baseline to 40 mmHg, and flow fell by 54%. At 15-min hypoperfusion, dobutamine was infused into the RCA at either 0.01 (low-dose dobutamine) or 0.06 microgram. kg(-1). min(-1) (high-dose dobutamine) for 15 min. Regional power (systolic segment shortening x isometric developed force x heart rate) stabilized at 63% of baseline during hypoperfusion. Low-dose dobutamine restored power to baseline but did not increase RV myocardial O(2) consumption (MVO(2)) and thus increased myocardial O(2) utilization efficiency (O(2)UE:power/MVO(2)). At 5 min, high-dose dobutamine enhancement of power was similar to that of low-dose dobutamine, but by 15 min, power and O(2)UE fell to untreated levels. Remarkably, low-dose dobutamine tripled cytosolic phosphorylation potential; in contrast, high-dose dobutamine lowered phosphorylation potential to 45% of the untreated value. Analyses of glucose uptake and glycolytic intermediates revealed sustained enhancement of glycolysis by low-dose dobutamine, but glycolysis became limited at glyceraldehyde 3-phosphate dehydrogenase during high-dose dobutamine treatment. In summary, low-dose dobutamine improved mechanical performance and efficiency of the hypoperfused RV myocardium while increasing myocardial energy reserves, but high-dose dobutamine failed to sustain improved function and depleted energy reserves. Dobutamine is capable of improving both contractile function and cellular energetics in the hypoperfused RV myocardium, but dosage should be carefully selected.

In vivo evidence that EMD 57033 restores myocardial responsiveness to intracoronary Ca(2+) in stunned myocardium

Despite ample in vitro evidence that myofilament Ca(2+)-responsiveness of stunned myocardium is decreased, in vivo data are inconclusive. Conversely, while Ca(2+)-sensitizing agents increase myofilament Ca(2+)-responsiveness in vitro, it has been questioned whether this also occurs in vivo. We therefore tested in open-chest anesthetized pigs whether EMD 57033 (the (+) enantiomer of 5-[1-(3,4-dimethoxybenzoyl)-1,2,3, 4-tetrahydro-6-quinolyl]-6-methyl-3,6-dihydro-2H-1,3, 4-thiadiazin-2-one) increases responsiveness to Ca(2+) of non-stunned myocardium and restores function of stunned myocardium by normalizing the responsiveness to Ca(2+). Studies were performed under beta-adrenoceptor blockade to minimize the contribution of the phosphodiesterase-III inhibitory actions of EMD 57033. Consecutive intracoronary Ca(2+) infusions were used to evaluate the contractile response (assessed by the left ventricular end-systolic elastance, E(es)) to added Ca(2+) of non-stunned myocardium and myocardium stunned by 15 min coronary artery occlusion and 30 min reperfusion. In non-stunned propranolol-treated myocardium, the Ca(2+) infusions doubled E(es) (baseline 6.9+/-0.9 mmHg mm(-2), n=8). Following Ca(2+)-washout, subsequent EMD 57033 infusion (0.1 mg kg(-1) min(-1), i.v.) tripled E(es) (P<0.05) and potentiated the Ca(2+)-induced increase in E(es) to 55.7+/-10.0 mmHg mm(-2) (P<0.05). Stunning (n=7) decreased E(es) to 5.3+/-0.6 mmHg mm(-2) (P>0.10) and attenuated the Ca(2+)-induced increase in E(es) (P<0.05). Subsequent infusion of EMD 57033 increased E(es) to 6.8+/-1.8 mmHg mm(-2) (P<0. 05) and restored responsiveness to added Ca(2+). These in vivo findings are consistent with the in vitro observations that myofilament Ca(2+)-responsiveness of stunned myocardium is reduced and that EMD 57033 increases contractility by enhancing myofilament Ca(2+)-responsiveness.

Efficiency of energy transfer, but not external work, is maximized in stunned myocardium

There is no evidence regarding the effect of stunning on maximization of regional myocardial external work (EW) or efficiency of energy transfer (EET) in relation to regional afterload (end-systolic stress, sigma(es)). To that end, we studied these relationships in both the left anterior descending coronary artery (LADCA) and left circumflex coronary artery regions in anesthetized, open-chest pigs before and after LADCA stunning. In normal myocardium, EET vs. sigma(es) was maximal at 75.4 (69.7-81.0)%, whereas EW vs. sigma(es) was submaximal at 12.0 (6.61-17.3) x 10(2) J/m(3). Increasing sigma(es) increased EW by 18 (10-27)%. Regional myocardial stunning decreased EET (27%) and EW (36%) and caused the myocardium to operate both at maximal EW (EW(max)) and at maximal EET (EET(max)). EET and EW became also more sensitive to changes in sigma(es). In the nonstunned region the situation remained unchanged. Combining the data from before and after stunning, both EW(max) and EET(max) displayed a positive relationship with contractility. In conclusion, the normal regional myocardium operated at maximal EET rather than at maximal EW. Therefore, additional EW could be recruited by increasing regional afterload. After myocardial stunning, the myocardium operated at both maximal EW and maximal EET, at the cost of increased afterload sensitivity. Contractility was a major determinant of this shift.

Cardiovascular profile of the calcium sensitizer EMD 57033 in open-chest anaesthetized pigs with regionally stunned myocardium

1. Ca(2+) sensitizers enhance systolic function, but may impair relaxation in vitro; these effects may differ in stunned and normal myocardium. We therefore studied the effect of EMD 57033 on systolic and diastolic function of normal and stunned porcine myocardium in vivo. 2. Myocardial stunning by 15 min coronary occlusion and 30 min reperfusion abolished systolic shortening (SS) (baseline 13+/-1%) and decreased end-systolic elastance (E(es)) from 67+/-7 to 47+/-5 mmHg mm(-1) (both P<0.05). Maximum rate of fall of myocardial elastance (dE/dt(min)) decreased from -850+/-100 to -320+/-30 mmHg mm(-1) s(-1), while the time constant tau(e) of the decay of elastance increased from 58+/-3 to 68+/-6 ms (both P<0.05). End-diastolic elastance (E(ed)) was unchanged although the zero pressure intercept (L(0,ed)) had increased. 3. In the stunned region, EMD 57033 (0.2 mg kg(-1) min(-1) for 60 min, i.v., n=7) increased SS to 19+/-2%, E(es) to 287+/-40 mmHg mm(-1), dE/dt(min) to -3630+/-640 mmHg mm(-1) s(-1) and decreased tau(e) to 50+/-3 ms, while E(ed) remained unchanged. In the normal region, 4. EMD 57033 increased SS from 14+/-2 to 18+/-3%, E(es) from 59+/-4 to 263+/-23 mmHg mm(-1), dE/dt(min) from -480+/-70 to -2280+/-700 mmHg mm(-1) s(-1) and decreased tau(e) from 91+/-12 to 61+/-3 ms (all P<0.05), while E(ed) remained unchanged. These responses were minimally affected by adrenoceptor blockade (n=7). Vehicle (n=7) had no effect on either region. EMD 57033 increased cardiac output (up to 27+/-8%) and LVdP/dt(max) (86+/-19%). Mean aortic pressure decreased (19+/-7%) due to systemic vasodilation that was not amenable to blockade of adrenoceptors or NO synthesis. 5. In conclusion, EMD 57033 restored systolic and diastolic function of stunned myocardium, and produced similar improvements in systolic and diastolic function in normal myocardium.

Cardiovascular effects of the novel Ca2+-sensitiser EMD 57033 in pigs at rest and during treadmill exercise

1. To date no study has described the cardiovascular effects of increased myofilament Ca2+ responsiveness in awake animals both under resting conditions and during treadmill exercise. In the present study we therefore investigated the systemic, pulmonary and coronary haemodynamic actions of the Ca2+ sensitizer EMD 57033 in 16 chronically instrumented awake pigs at rest and during treadmill exercise, and compared these to the haemodynamic actions of the Ca2+ sensitizer/phosphodiesterase inhibitor pimobendan. 2. Under resting conditions EMD 57033 (0.2, 0.4 and 0.8 mg kg(-1) min(-1), i.v.) produced dose-dependent increases in LVdP/dt(max) (up to 65+/-17% (mean+/-s.e.mean), P < or = 0.05) and stroke volume (up to 20+/-3%, P < or = 0.05), with an increase in heart rate only after the highest dose (22+/-5%, P < or = 0.05), while mean aortic blood pressure and LVdP/dt(min) were not altered. EMD 57033 had also no effect on pulmonary vascular resistance, but produced dose-dependent decreases in systemic vascular resistance (32+/-4%, P < or = 0.05), and coronary vascular resistance (44+/-2%, P < or = 0.05). These effects were essentially unchanged when animals were pretreated with non-selective beta-adrenoceptor blockade, indicating that phosphodiesterase inhibition did not contribute to the positive inotropic actions of EMD 57033. 3. During exercise at 2, 3, and 4 km h(-1), the positive inotropic actions of EMD 57033 gradually waned at higher levels of exercise. This may have been caused by the exercise-induced increase in beta-adrenergic activity, because after pretreatment with propranolol the positive inotropic actions of EMD 57033 were preserved at all levels of exercise. In contrast, the positive inotropic and chronotropic effects of pimobendan were amplified during exercise, but were abolished (at rest) or markedly attenuated (during exercise) after pretreatment with propranolol. 4. The responses to EMD 57033 during exercise after combined alpha- and beta-adrenergic receptor blockade were not different from those after beta-adrenergic receptor blockade alone, indicating that the positive inotropic actions of EMD 57033 were not mediated via or did not depend on intact alpha-adrenergic receptor activity. 5. In conclusion, EMD 57033 increases left ventricular myocardial contractility in awake pigs. During exercise this effect is partially offset by the increased beta-adrenergic activity, with no effect of alpha-adrenergic activity, suggesting that EMD 57033 may be most effective in patients with severe loss of beta-adrenergic responsiveness.

Contribution of asynchrony and nonuniformity to mechanical interaction in normal and stunned myocardium

In anesthetized pigs, we investigated whether asynchrony (delta T) and nonuniformity (regional differences) in contractility (delta E) could describe the interaction between normal and stunned myocardium. Mechanical interaction was evaluated by regional postsystolic work (PSW) before and after production of stunning by a 5-min occlusion of the left circumflex coronary artery [LCX (LCX stunning)] and a subsequent 10-min occlusion of the left anterior descending coronary artery [LAD (LAD stunning)]. delta T and delta E were intensified by intracoronary (LAD) infusions of dobutamine. From regional end-systolic pressure-segment length relationships, systolic segment shortening (SS), end-systolic elastance (E), external work (EW), and PSW were determined. LCX stunning decreased SSLCX from 14 +/- 2 (mean +/- SE, n = 9) to 10 +/- 2% and ELCX from 103 +/- 25 to 52 +/- 7 mmHg/mm, whereas the LAD region was unaffected. EWLCX decreased from 165 +/- 16 to 138 +/- 20 mmHg.mm, whereas PSWLCX increased from -4 +/- 6 to 8 +/- 3 mmHg.mm. Additional LAD stunning reduced SSLAD from 16 +/- 2 to 9 +/- 3% and ELAD from 79 +/- 10 to 31 +/- 6 mmHg/mm, without affecting SSLCX and ELCX. In the normal myocardium, PSWLAD increased and PSWLCX decreased, but, during local LAD dobutamine infusions after stunning, both PSWLCX and PSWLAD increased. In normal myocardium, the changes in PSWLCX could be described by delta T (65 +/- 11%) and delta E (37 +/- 15%). After stunning of the LAD area, the contribution of delta E increased to 55 +/- 14% at the expense of delta T (37 +/- 15%). Similar contributions of delta E (54 +/- 13%) and delta T (57 +/- 13%) were found when both the LCX and LAD distribution areas were stunned. In normal myocardium, both delta T and delta E modulate mechanical interaction, with the contribution of delta T exceeding that of delta E. In stunned myocardium, both factors contribute, but the contribution shifts in favor of delta E.

In vivo assessment of regional myocardial work in normal canine hearts using 3D tagged MRI

A non-invasive method for assessing regional myocardial work is presented. The method utilizes tagged magnetic resonance images (MRI) obtained from two sets of orthogonal planes to mark and reconstruct 24 small myocardial cuboids at end-diastole (ED) and end-systole (ES) in the in vivo left ventricle (LV). Regional myocardial work is assessed by calculating the area enclosed by the endocardial wall tension-area (T-A) loop of each studied cuboid. The method was applied to six normal canine hearts. In addition, a global myocardial work index was obtained from the corresponding estimated pressure-volume (P-V) loops. The average work index calculated using the T-A loop was 0.242 +/- 0.088 J/100gr/beat, in agreement with the average index obtained from the P-V loop: 0.296 +/- 0.089 J/100gr/beat. The two indices correlate linearly with a correlation coefficient of 0.82.

Stunned myocardium and the attenuation of stunning by calcium antagonists

Myocardial "stunning" is characterized by a reversible postischemic contractile dysfunction despite full restoration of blood flow. The underlying mechanisms are not clearly understood. Inadequate energy supply and impaired sympathetic neurotransmission may have been excluded. Potential mechanisms, which are not mutually exclusive, may include damage to membranes and enzymes by free radicals, an increase in free cytosolic calcium during ischemia and reperfusion, and a lower calcium sensitivity of myofibrils. The equally pronounced increases in regional contractility in normal and stunned myocardium during postextrasystolic potentiation and the infusion of calcium or the calcium-sensitizing agent AR-L-57, however, suggest an unchanged calcium sensitivity in reperfused myocardium. Pretreatment with calcium antagonists before ischemia attenuates myocardial stunning. This effect is probably related to a lessened myocardial calcium overload during early ischemia. The potential benefit of treatment with calcium antagonists after reperfusion is established remains controversial.

Myofibrillar Ca2+ sensitization predominantly enhances function and mechanical efficiency of stunned myocardium

BACKGROUND

Myocardial stunning is characterized not only by a decreased regional postischemic function but also by a relatively high oxygen consumption (ie, decreased mechanical efficiency). Several lines of evidence suggest that the underlying mechanism may involve a decreased sensitivity of the myofibrils to calcium, but in vivo evidence is lacking. We therefore evaluated this hypothesis in vivo using EMD 60263, a calcium-sensitizing agent, which is devoid of any phosphodiesterase-inhibiting properties.

METHODS AND RESULTS

We first established the effect of two consecutive doses of EMD 60263 (0.75 and 1.5 mg/kg i.v., n = 7), administered at 15-minute intervals, on segment length shortening (SLS), external work index (EW; the area inside the left ventricular pressure-segment length loop), myocardial oxygen consumption (MVO2), and mechanical efficiency (EW/MVO2) in anesthetized pigs with normal myocardium. After the highest dose of EMD 60263, SLS in the distribution area of the left anterior descending coronary artery (LADCA) increased from 13 +/- 1% at baseline to 17 +/- 1% (P < .05). However, EW, MVO2, and EW/MVO2 were not significantly affected (123 +/- 10%, 98 +/- 9%, and 85 +/- 13% of baseline, respectively). In 14 other anesthetized pigs, myocardial stunning was induced by two sequences of 10 minutes of LADCA occlusion and 30 minutes of myocardial reperfusion. After induction of stunning, the two doses of EMD 60263 (n = 7) or saline (3 and 6 mL, n = 7) were infused. In the distribution area of the LADCA, the stunning protocol caused decreases in SLS from 16 +/- 1% to 8 +/- 1% (P < .05) and in EW to 49 +/- 5% of baseline (P < .05), whereas MVO2 was only minimally affected (P > .05). Consequently, mechanical efficiency decreased to 59 +/- 8% of baseline (P < .05). Saline infusion did not affect any of these regional myocardial variables, but after administration of EMD 60263 SLS recovered dose-dependently to 15 +/- 2% after the highest dose of the drug. EW and mechanical efficiency also recovered dose-dependently to 89 +/- 4% (P < .05 versus stunning) and to 88 +/- 7% (NS versus baseline) of baseline, respectively. In the not-stunned segment, SLS increased from 15 +/- 2% (at baseline) to 18 +/- 2% (after the highest dose), and EW per beat was not changed significantly. An adrenergic mode of action of EMD 60263 was excluded by blocking the alpha- and beta-adrenergic receptors with phentolamine and propranolol, respectively, 15 minutes before administration of EMD 60263 (ie, 15 minutes into the second reperfusion period) in five additional experiments. In these experiments the EMD 60263-induced increases in SLS and EW were not attenuated. Because EMD 60263 decreased heart rate from 106 +/- 4 to 76 +/- 3 beats per minute (P < .05) in the animals with stunned myocardium, we performed five experiments with the specific negative chronotropic compound zatebradine (UL-FS 49, 0.1 to 0.5 mg/kg) to rule out bradycardia as a factor contributing to the effects of EMD 60263. These zatebradine doses lowered heart rate from 116 +/- 5 to 55 +/- 1 beats per minute (P < .05) but had no effect on SLS of stunned and not-stunned myocardium.

CONCLUSIONS

Calcium sensitization affects function and mechanical efficiency of stunned myocardium more profoundly than of not-stunned myocardium, lending support to the hypothesis that Ca2+ desensitization of the myofibrils is involved in myocardial stunning.

Intracoronary trimetazidine does not improve recovery of regional function in a porcine model of repeated ischemia

We evaluated the effect of trimetazidine (TMZ) on recovery of regional cardiac function in anesthetized open-chest pigs, subjected to fifteen 2-minute occlusions of the left anterior descending coronary artery, separated by 2 minutes of reperfusion and a 120-minute recovery period. Regional myocardial function was evaluated by sonomicrometry-derived segment lengthening and the area enclosed by the left ventricular pressure-segment length loop (external work, EW) in animals, which received either an intracoronary infusion of TMZ (33 micrograms/kg/min, n = 6) or saline (1 ml/min, n = 7), starting 15 minutes before the first occlusion and ending 2 minutes after the 15th occlusion. In addition, myocardial malondialdehyde production to evaluate oxygen free radical production, oxygen consumption, and the ATP, ADP, and AMP content, as well as the energy charge, were determined at regular time intervals. In control pigs the sequences of occlusion-reperfusion did not affect systemic hemodynamics, except for the LVdP/dtmax, which decreased by 11% during the interventions and did not recover during the following reperfusion period of 2 hours (78% of baseline, p < 0.05). Systolic segment length shortening and EW were increased at the end of the first occlusion-reperfusion cycle, decreased gradually during the remainder of the occlusion-reperfusion periods, and did not improve during the recovery period. Energy charge and myocardial blood flow were not impaired, but oxygen consumption was decreased during the recovery period. The malondialdehyde data did not provide evidence for production of oxygen free radicals.(ABSTRACT TRUNCATED AT 250 WORDS)