Propofol Attenuates Hydrogen Peroxide-induced Mechanical and Metabolic Derangements in the Isolated Rat Heart

The protocol of animal experiments in the current study was approved by the Asahikawa Medical College Committee on Animal Research. Male Sprague-Dawley rats (weighing 300–350 g, aged 9–10 weeks) were used. The animals were anesthetized with 50 mg *symbol* kg sup -1 intraperitoneal sodium pentobarbital. After thoracotomy, the heart and aortic arch were rapidly excised and immersed in a cold Krebs-Henseleit bicarbonate (KHB) buffer (4 degrees C). The composition of the KHB buffer was (in mM): NaCl 118, KCl 4.7, KH²PO⁴1.2, MgSO⁴1.2, CaCl²2.5, NaHCO³25, and glucose 11. The heart was perfused via retrograde cannulation of the aorta with the KHB buffer at a constant pressure of 80 cmH²O. The buffer was equilibrated with a gas mixture of 95% Oxygen sub 2 -5% CO², which produced a P^O2range of 500–550 mmHg, a P^CO2range of 36–42 mmHg, and a pH value of 7.38–7.46. Temperature was maintained at 37 degrees C throughout the experiment with a thermostatically controlled recirculating water bath. About 10 min after constant-pressure perfusion, the flow was switched to constant-flow perfusion (10 ml *symbol* min sup -1) using a microtube pump (Eyela MP-3, Tokyo-Rikakikai Instruments, Tokyo, Japan), and this flow was maintained at a constant rate throughout the experiment. Constant-flow perfusion was regulated to maintain a constant supply of oxygen, H²O², propofol, and Intralipid (Kabi Pharmacia, Uppsala, Sweden) to the heart throughout the experiment. The heart rate was kept constant by pacing the heart at 300 beats *symbol* min sup -1 throughout the study via stimuli (3F46, San-Ei Instruments, Tokyo, Japan) applied to the left ventricle (4 V, two time threshold; 2-ms duration).

For measurement of left ventricular pressure, a saline-filled polyethylene cannula connected to a pressure transducer was inserted into the left ventricular cavity via the left atrium. The end of the polyethylene cannula in the left ventricle was open to the left ventricular cavity. The left ventricular systolic pressure (LVSP), left ventricular end-diastolic pressure (LVEDP), and left ventricular developed pressure (LVDP) were employed as indices of myocardial mechanical function. The values of LVSP, LVEDP, and LVDP were determined from the left ventricular pressure curves recorded continuously during the course of the study. The coronary perfusion pressure (CPP) was measured by a pressure transducer connected to the aortic cannula.

The effects of propofol and/or vehicle solution on the mechanical function and energy metabolism in the H²O²-untreated hearts (normal heart experiments) and the H²O²-treated hearts (H²O²-treated heart experiments) were investigated. The normal and H sub 2 O²-treated heart groups were then divided into five groups: the KHB group, Intralipid group, 25-micro Meter propofol group, 50-micro Meter propofol group, and 50-micro Meter propofol/Intralipid group. In these groups, all hearts were initially allowed to stabilize for 20 min under the condition of constant-flow perfusion before the observation period. The experimental protocol during the observation period is illustrated in Figure 1.

H²O²-treated Heart Experiments. Each of the KHB, Intralipid, 25-micro Meter propofol, and 50-micro Meter propofol groups was divided into two subgroups. In the first subgroup, KHB buffer, Intralipid, or propofol was infused for 40 min to evaluate the mechanical function and energy metabolism. In the second subgroup, KHB buffer, Intralipid, or propofol was infused for 25 min to evaluate lipid peroxidation. In the 50-micro Meter propofol/Intralipid group, the hearts were infused with 50 micro Meter propofol for 30 min followed by Intralipid for 10 min to evaluate the mechanical function and energy metabolism. KHB buffer, Intralipid, or propofol was infused into the aortic cannula at a constant flow rate of 0.05 ml *symbol* min sup -1. Hydrogen peroxide was infused for 4 min into the aortic cannula at a constant flow rate of 0.1 ml *symbol* min -1 from 10 min after the start of infusion of KHB buffer, Intralipid, or propofol.

Normal Heart Experiments. Each of the KHB, Intralipid, 25-micro Meter propofol, and 50-micro Meter propofol groups was divided into three subgroups. In the first subgroup, KHB buffer, Intralipid, or propofol was infused for 40 min to evaluate the mechanical function and energy metabolism. In the second subgroup, KHB buffer, Intralipid, or propofol was infused for 25 min to evaluate lipid peroxidation. In the third subgroup, KHB buffer, Intralipid, or propofol was infused for 10 min to evaluate energy metabolism. In the 50-micro Meter propofol/Intralipid group, the hearts were infused with 50 micro Meter propofol for 30 min followed by Intralipid for 10 min to evaluate mechanical function and energy metabolism. The experimental conditions and protocol were almost the same as those in the H²O²-treated heart experiments, except that distilled deionized water was infused instead of H²O².

In each experiment, LVSP, LVEDP, LVDP, and CPP were continuously recorded during a 40-min observation period. To measure the tissue concentrations of adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), creatine phosphate (CrP), the hearts were frozen immediately before H²O²infusion (the third subgroup in the normal heart experiment) or at the end of the observation period for 40 min (the first subgroup in the H²O²-treated and normal heart experiments) with freezing clamps previously chilled in liquid nitrogen. Other hearts were frozen 11 min after the cessation of H²O²infusion (the second subgroup in the H²O²-treated and normal heart experiments) for measurements of the tissue concentrations of MDA.

The frozen myocardial samples were stored in liquid nitrogen (at -173 degrees C) until the biochemical analysis was performed. Each sample was pulverized in liquid nitrogen with a mortar and pestle. To measure the tissue water content and dry weight of the tissue, a part of the pulverized tissue powder was weighed and allowed to stand in a warm oven overnight. The remainder of the powder was used for determination of the tissue concentrations of ATP, ADP, AMP, CrP, and MDA.

ATP, ADP, AMP, and CrP were extracted from the samples with 6% perchloric acid. The tissue extract was centrifuged at 10,000g for 10 min at 4 degrees C. The supernatant was neutralized with 70% potassium hydroxide and centrifuged at 10,000g for 10 min at 4 degrees C again. The resultant solution was used for determinations of ATP, ADP, AMP, and CrP. These metabolites were measured according to the standard enzymatic procedures. [12]Values were expressed as micromoles per gram of dry weight.

Malondialdehyde, a product of lipid peroxidation, was measured using high-performance liquid chromatography instead of using thiobarbituric acid because the latter method may measure not only MDA but also other substances. [13]High-performance liquid chromatographic procedure was performed according to the method described by Koller and Bergmann. [14]Malondialdehyde was extracted from the pulverized tissue sample with 10 mM phosphate buffer (pH 8.0), and the tissue extract was centrifuged at 10,000g for 15 min at 4 degrees C. The supernatant was filtered through a Millipore filter (UFC3LCC, Nihon Millipore Kogyo, Yonezawa, Japan), at 2,000g for 10 min. The filtered solution (20 micro liter) was injected into the high-performance liquid chromatography system (LC-9A, Shimadzu, Kyoto, Japan), equipped with a guard column (G-ODS 0.4 x 1 cm, Shimadzu) and an analytic column (CLC-ODS 0.46 x 25 cm, Shimadzu). Malondialdehyde was separated with 15% acetonitrile (high-performance liquid chromatography grade; Nacalai tesque, Kyoto, Japan) containing 50 mM myristyltrimethylammonium bromide (Aldrich Chemical, Milwaukee, WI) and 1.0 mM Na²HPO⁴adjusted to pH 6.8, at a flow rate of 1.0 ml *symbol* min sup -1. The effluent was monitored at 267 nm using a spectrophotometric detector (SPD-2AS, Shimadzu). The quantitative analysis was performed by comparison with standard curves. Malondialdehyde standards were prepared by acid hydrolysis of malondialdehyde bis(dimethyl acetal)(Aldrich Chemical, Milwaukee, WI). An additional experiment was performed to determine whether propofol itself interferes with MDA measurement. The mixture of various concentrations of MDA standards and 50 micro Meter propofol in the 10-mM phosphate buffer (pH 8.0) was incubated for 30 min. The concentration of MDA in the buffer in the presence or absence of propofol was measured by high-performance liquid chromatography according to the method described earlier.

The following drugs were used: propofol (2,6-diisopropylphenol, Tokyo Kasei Kogyo, Tokyo, Japan), 10% Intralipid (Kabi Pharmacia, Uppsala, Sweden), sodium pentobarbital (Nacalai tesque, Kyoto, Japan), and H²O²(Santoku Chemical, Tokyo, Japan). The 10% Intralipid is a solvent of commercially available propofol, and contains 10% soy bean oil, 1.2% egg lecithin, and 2.25% glycerol. Propofol was dissolved in Intralipid and further diluted with distilled deionized water. The concentration of Intralipid in the drug solution was 2%. H²O²was diluted with distilled deionized water. Propofol or Intralipid was infused into the inflow tube connected to the side arm of the aortic cannula at a flow rate of 0.05 ml *symbol* min sup -1 using an infusion pump. Hydrogen peroxide solution was also infused to the inflow tube at a flow rate of 0.1 ml *symbol* min sup -1 using another infusion pump. The final concentration of propofol in the perfusate (KHB buffer perfused at a flow rate of 10 ml *symbol* min sup -1 plus propofol solution infused at a flow rate of 0.5 ml *symbol* min sup -1) was 25 or 50 micro Meter. The concentration of propofol changed from 25 to 24.8 micro Meter or from 50 to 49.5 micro Meter when H²O²was added to the perfusate, because total flow rate of perfusate increased from 10.05 to 10.15 ml *symbol* min sup -1 during H²O²infusion, but these small changes in the concentration of propofol did not affect left ventricular pressure or CPP. The final concentration of H²O²in the perfusate including infusion solution was 600 micro Meter. The reagents and enzymes used for biochemical analysis were purchased from Sigma Chemical Co. (St. Louis, MO).

All values were expressed as mean+/-SEM (standard error of the mean). Statistical analysis of the results was performed with one-way analysis of variance. This was followed by Fisher's protected least-significant difference for multiple comparisons. A P value of less than 0.05 was considered statistically significant.

Experiments were performed to examine how propofol affects myocardial mechanical function and energy metabolism in normal hearts (hearts not treated with H²O²) during 40 min of perfusion with KHB, Intralipid, 25 micro Meter propofol, or 50 micro Meter propofol (Table 1and Table 2). In the KHB and Intralipid groups (n = 8 for each), myocardial mechanical function and energy metabolism did not change significantly throughout the experimental period. Propofol significantly decreased LVSP, LVDP, and CPP in a dose-dependent fashion, whereas it did not affect LVEDP and energy metabolism. In the 50-micro Meter propofol/Intralipid group, LVSP, LVDP, and CPP decreased significantly during the period of infusion with 50 micro Meter propofol but returned to the initial level (0 min value) within 10 min after the cessation of propofol infusion.

Changes in left ventricular pressure induced by H²O²are shown in Figure 2. Before starting the H²O²infusion, there were no significant differences in mechanical function between the normal and H²O²-treated heart groups. In both KHB (n = 8) and Intralipid (n = 8) groups, H²O²induced a temporary but severe decrease in LVSP (Figure 2(A and B)). Left ventricular end-diastolic pressure also increased markedly after the H²O²infusion, and the increase that was accompanied by elevation of the LVSP to the pre-H sub 2 O²level, lasted throughout the observation period. Consequently, LVDP decreased significantly after the H²O²infusion and did not recover to the pre-H²O²level by the end of the experiment (Figure 3). These results indicate that H²O²produced a long-lasting mechanical dysfunction, although LVSP returned to the control level within 15 min of the cessation of H²O²infusion.

Before the start of H²O²infusion, propofol induced a dose-dependent decrease in LVSP. In the presence of propofol, H²O sub 2 was infused for 4 min. Hydrogen peroxide produced a temporary and severe decrease in LVSP in both the 25- and 50-micro Meter propofol groups (n = 8 for each group;Figure 2(C and D)). Increases in LVEDP and LVSP after the end of H²O²infusion, however, were significantly attenuated by propofol when compared with the KHB and the Intralipid groups. After 40 min of infusion, there was no significant difference in LVSP between the KHB and the 25-micro Meter propofol groups (P > 0.05), whereas LVSP in the 50-micro Meter propofol group was significantly lower than that in the KHB group (P < 0.05). Consequently, LVDP in the 25-micro Meter propofol group was significantly higher than that in the 50-micro Meter propofol and KHB groups (Figure 3).

There is a possibility that the depressed recovery of LVSP and LVDP in the 50-micro Meter propofol group is due to the cardiodepression induced by propofol itself. For this purpose, in the H²O²-treated heart experiments, we perfused the heart with 50 micro Meter propofol for 30 min and then with Intralipid for 10 min (50-micro Meter propofol/Intralipid group, n = 8), and compared the recovery of mechanical function between when the 50 micro Meter propofol infusion was performed during the whole course of the experiment (40 min) and when it was performed for 30 min and stopped. Changes in LVSP, LVEDP, and LVEDP during propofol infusion in the 50-micro Meter propofol/Intralipid group were similar to those in the 50-micro Meter propofol group. LVSP and LVDP increased greatly after the cessation of propofol infusion; these values at the end of experiment in the 50-micro Meter propofol/Intralipid group were significantly higher than those in the 50-micro Meter propofol group and similar to those in the 25-micro Meter propofol group (Figure 2and Figure 3). These results indicate that the cessation of propofol infusion does not reduce the recovery of mechanical function, or rather, it improves mechanical function (LVSP and LVDP) that has been suppressed by propofol.

(Figure 4) shows changes in CPP throughout the experimental period. Hydrogen peroxide produced a transient decrease followed by a long-lasting increase in CPP in each of the five groups. Before the H²O²infusion, there was a statistically significant decrease in CPP in both the 25- and 50-micro Meter propofol groups when compared with the KHB and the Intralipid groups. Increase in CPP after H²O²infusion was significantly attenuated in either the 25-micro Meter propofol, the 50-micro Meter propofol, or the 50-micro Meter propofol/Intralipid group.

(Figure 5) and Figure 6show the tissue concentrations of ATP, ADP, AMP, and CrP in the five groups 10 and 40 min after KHB buffer, Intralipid, or propofol infusions. There were no significant differences in the tissue concentrations of energy metabolites at 10 min (immediately before the H²O²infusion) among the KHB, Intralipid, or propofol groups. The tissue concentrations of ATP, ADP, and CrP at the end of the experiment in the propofol 25- and 50-micro Meter groups were higher than in the KHB and the Intralipid groups (40 min in Figure 5and Figure 6). Decreases in ATP and ADP induced by H²O²were significantly attenuated by propofol. In contrast, CrP did not change after H²O²infusion in the 25-micro Meter propofol group, whereas it increased in the 50-micro Meter propofol group. The tissue concentrations of ATP and CrP at the end of experiment in the 50-micro Meter propofol/Intralipid group were higher than those in the KHB and the Intralipid groups, but lower than those in the 50-micro Meter propofol group (40 min in Figure 5and Figure 6). The ratio of wet/dry tissue weight calculated for quantitative analysis of energy metabolites was not different among all groups in the both normal and H²O²-treated heart experiments.

To examine whether the beneficial effects of propofol on the H²O²-induced mechanical dysfunction and metabolic changes correlate with reduction of lipid peroxidation, the tissue concentration of MDA was measured 25 min after the start of infusion (i.e., 11 min after the end of H²O²infusion;Figure 7). The reason that a 25-min interval was chosen is that H²O²produced the greatest increase in LVEDP in the KHB and the Intralipid groups around that time (Figure 2). In the KHB and Intralipid groups, H²O²produced a significant increase in the MDA concentration by approximately 5 and 7 times their control values without H²O², respectively. In contrast, the tissue concentration of MDA in both 25- and 50-micro Meter propofol groups did not increase even after H²O²infusion. Propofol (25 and 50 micro Meter) also decreased the tissue MDA concentration in the normal heart (Figure 7). Nevertheless, propofol did not interfere with measurement of MDA (data not shown). These results suggest that propofol prevents the H²O²-induced lipid peroxidation.

Many studies have addressed the actions of substances that protect the myocardium from reactive oxygen-induced damage. For example, nifedipine and propranolol inhibit the lipid peroxidation of the sarcolemmal membrane induced by oxidative stress. [15]Catalase, lidocaine, [7]and agonists for adenosine A¹receptor [16]attenuate the mechanical dysfunction and metabolic changes induced by H sub 2 O²in the isolated perfused heart. In the current study, we examined the effect of propofol on mechanical dysfunction, metabolic changes, and lipid peroxidation induced by H²O²in the isolated perfused rat heart.

The reasons we used H²O²as a source of reactive oxygen metabolite can be summarized as follows:(1) H²O²and its metabolite, the hydroxyl radical, are known to be important in the pathogenesis of myocardial damage induced by ischemia-reperfusion [17]; and (2) H²O²penetrates the cell membrane, reaching intracellular sites, [18]and may therefore induce severe damage to the myocardial cell. The current study demonstrated that H²O²produced mechanical dysfunction (as evidenced by an increase in LVEDP and a decrease in LVDP), an increase in coronary resistance (as evidenced by an increase in CPP), and metabolic changes (as evidenced by a decrease in the tissue ATP and CrP concentrations). These alterations induced by H²O²were attenuated by 25 micro Meter and 50 micro Meter propofol, suggesting that propofol has a beneficial effect on the H²O²-induced mechanical dysfunction and metabolic derangements.

We found that the recovery of LVDP after H²O²infusion was better in the 25-micro Meter propofol group than that in the 50-micro Meter propofol group. However, high-energy phosphate after the H sub 2 O²infusion was better preserved in the 50-micro Meter propofol group than that in the 25-micro Meter propofol group. This is probably because propofol itself induced dose-dependent decrease in mechanical function, [19–21]which may preserve energy in the H²O²-treated hearts. In fact, the cessation of infusion with propofol in the 50-micro Meter propofol/Intralipid group greatly increased LVSP and LVDP, and the tissue concentrations of ATP and CrP in the 50-micro Meter propofol/Intralipid group were lower than those in the 50-micro Meter propofol group.

According to biochemical studies, the hydroxyl radical generated from H²O²by the Fenton reaction easily removes a hydrogen atom from a methylene carbon of an unsaturated fatty acid of membrane phospholipids. [3]Peroxidation of lipids then would inactivate membrane-associated enzymatic proteins, such as Na sup +-K sup +-adenosinetriphosphatase [22]and Ca²+-adenosinetriphosphatase, [23]and increase membrane permeability. [3]These biochemical and physicochemical alterations of the cell membrane may be responsible for mechanical and metabolic derangements.

To test the hypothesis that the beneficial effects of propofol on the H²O²-induced mechanical dysfunction and metabolic changes correlate with reduction of lipid peroxidation induced by H²O², we measured the tissue MDA concentration 11 min after the H²O²infusion, when H²O²produced the greatest increase in LVEDP in the KHB or Intralipid groups. In the KHB and the Intralipid groups, H²O²increased the tissue concentration of MDA significantly. The increase in the tissue MDA concentration in the Intralipid group was greater than that in the KHB group. There may be some interaction between H²O²and Intralipid, because Intralipid contains egg lecithin (phosphatidylcholine). Nevertheless, both 25 and 50 micro Meter propofol completely suppressed the H²O²-induced increase in tissue MDA. Our findings suggest that the beneficial effects of propofol on H²O²-induced myocardial damage correlate with inhibition of lipid peroxidation. According to reports on propofol's antioxidant effect, [9–11]it is assumed that propofol behaves in the same way as phenol-based antioxidants, which scavenge free radical species (e.g., hydroxyl radical and lipid peroxyl radical) by a process of hydrogen abstraction and thereby themselves become a less reactive phenoxyl radical.

It should be noted, however, that propofol did not attenuate completely the H²O²-induced mechanical dysfunction (an increase in LVEDP and a decrease in LVDP) and metabolic derangements (a decrease in the tissue ATP and CrP concentrations), in spite of the complete inhibition of lipid peroxidation. Recently, Kong et al. [24]reported that prevention of lipid peroxidation does not prevent myocardial contractile dysfunction induced by reactive oxygen in isolated rabbit hearts. Therefore, factors other than lipid peroxidation may play a important role in myocardial damage induced by reactive oxygen. In fact, oxidation of sulfhydryl groups in protein or nonprotein has been demonstrated to contribute to myocardial damage induced by reactive oxygen. [25]These biochemical alterations, which are not attributed to lipid peroxidation of the cell membrane, may account for the incomplete action of propofol on H²O²-induced mechanical and metabolic derangements.

Propofol inhibited preferentially the H²O²-induced increase in LVEDP rather than the H²O²-induced decrease in LVDP. The beneficial action of propofol on the increase in LVEDP may be due to inhibition of intracellular Calcium²+ accumulation, because myocardial contracture induced by H²O²is, at least in part, mediated by an increase in intracellular Calcium²+. [26]Cook and Housmans [19]suggested that propofol inhibits the slow inward L-type Calcium²+ current across the sarcolemma. Although the beneficial role of nisoldipine and flunarazine against H²O²-induced mechanical dysfunction has been demonstrated,* there is no direct evidence to show that the beneficial effect of propofol to attenuate the H²O²-induced mechanical dysfunction and metabolic derangements is caused by inhibition of the Calcium²+ current. Further studies are needed to determine the detailed mechanism of the protective action of propofol on the H²O²-induced changes.

Peak plasma concentration of propofol after an intravenous induction dose increases to as much as 44 micro Meter, [27]whereas stable plasma concentrations of approximately 10–20 micro Meter are observed during a maintenance period. [28]Furthermore, more than 90% of propofol is bound to plasma protein. [29]The concentration range tested in this study therefore is considerably greater than that likely to be encountered during clinical practice. However, even trace plasma concentrations of propofol may play a crucial role in protecting the cell membrane against oxidant stress as well as vitamin E. [9,30]It may also be that propofol that is bound to proteins or that which is present in membranes may still be able to perform its antioxidant role in these settings, so that free drug concentration is less important than in receptor or partitioning studies.

In conclusion, the results of the current study demonstrate that propofol attenuates both mechanical dysfunction and metabolic derangements induced by exogenous H²O². Furthermore, an increase in the tissue concentration of MDA induced by H²O²was completely suppressed by either 25 or 50 micro Meter propofol after the H sub 2 O²infusion. The results also suggest that the beneficial effects of propofol correlate with inhibition of H²O²-induced lipid peroxidation.

The authors thank Professor A. Namiki, M.D., Ph.D., Chairman, Department of Anesthesiology, Sapporo Medical University School of Medicine, and Professor Y. Abiko, M.D., Ph.D., Chairman, Department of Pharmacology, Asahikawa Medical College, for the direction of this research and review of this manuscript; H. Tsuchida, M.D., Assistant Professor of Anesthesiology, Sapporo Medical University School of Medicine, for comments and suggestions concerning this manuscript; T. Ichimiya, M.D., Asahikawa City General Hospital, for assistance; T. Yokoyama, for technical assistance; and A. Rikiyama, for secretarial assistance.

*Britnell SL, Nayler WG: The protective effect of calcium antagonists against oxidative injury (abstract). J Mol Cell Cardiol 25:1, 1993.