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Effects of lysophosphoglycerides on cardiac arrhythmias
Life Sciences, Vol. 32, pp. 1325-1330 Printed in the U.S.A.
Pergamon Press
EFFECTS OF LYSOPHOSPHOGLYCERIDES ON CARDIAC ARRHYTHMIAS Ricky Y.K. Man a, Turnly Wong b and Patrick C. Choy b Departments of Physiology a and Biochemistry b Faculty of Medicine, University of Manitoba Winnipeg, Manitoba, Canada R3E OW3 (Received in final form December 6, 1982) Summary The accumulation of lysophosphoglycerides has been implicated as an important biochemical factor for cardiac arrhythmias. Recently, we demonstrated that lysophosphatidylcholine caused cardaic arrhythmias in the isolated hamster heart. In this study, the arrhythmogenic nature of various lysophosphoglycerides with respect to acyl chain lengths and base groups were assessed. We demonstrated that all naturally occurring lysolipids tested were arrhythmogenic at 0.05-0.10 mM. Arrhythmias were also observed with Triton X-100 or sodium laurylsulfate at 0.05 0.i0 mM. Our data suggests that no correlation exists between the arrhythmogenic nature of the lysolipids and their critical micelle concentrations. We postulate that arrhythmias are produced by the detergent effect of lysophosphoglycerides. The occurrence of cardiac arrhythmias subsequent to onset of myocardial ischemia has been well-documented (1,2). Although several biochemical and physiological changes were observed during ischemia, the exact cause for disturbances in cardiac rhythm after onset of ischemia remains unknown. In the last few years, lysophosphoglycerides have been implicated as an important biochemical factor for cardiac arrhythmia associated with ischemia. Elevated levels of lysophosphatidylcholine were observed in ischemic areas of the heart (3,4,5,6). At present, it is not known whether the elevated levels of intracellular lysophospholipids during ischemia may have any direct effects in the production of cardiac arrhthmia. However, when lysophosphatidylcholine was added to the superfusate, depression of action potential and increase in automaticity were observed in cardiac tissues (7,8). Based on these electrophysiological parameters, the arrhythmogenic nature of lysophosphatidylcholine was postulated. Recently, we demonstrated the direct relationship of palmitoyl-lysophosphatidylcholine to cardiac arrhythmias in the isolated perfused heart (9). Since lysophosphatidylcholine is present in the cardiac tissue in multiple molecular forms with different acyl groups (I0), it is of interest to study the arrhythmogenicity of these individual lysophosphatidylcholine to cardiacarrhythmias. Moreover, the effects of other naturally occurring lysophosphoglycerides to arrhythmias were not known. In this communication, we report the differential effects of lysophosphatidylcholines with various acyl groups to arrhythmias in the isolated hamster heart. The arrhythmogenic nature of other lysophosphoglycerides was also assessed. 0024-3205/83/121325-06503.00/0 Copyright (c) 1983 Pergamon Press Ltd.
1326
Lysolipids Cause Cardiac Arrhythmias
Vol. 32, No.
12, 1983
Methods Syrian Golden hamsters (90-120 g) were sacrificed by decapitation and the hearts were rapidly removed and placed in ice-cold Krebs-Henseleit buffer saturated with 95%02 - 5% CO 2. The heart was cannulated via the aorta as described by Langendorff (ii) and was perfused at a pressure of 90 - 120 mm Hg with a constant flow rate of 3-4 ml/min. The temperature of the perfusate was kept at 37 ° + 0.5°C. The detailed methodology of isolated heart perfusion has been reporte~ elsewhere (12). Electrocardiac recording was obtained by attaching one electrode on the aorta cannula and the other in the solution bathing the apex of the heart. The placement of electrodes in this manner provided assessment of the atrial and ventricular activities simultaneously. The signals were amplified and recorded by a Gould Brush 2400 paper recorder or a Sanborn i00 recorder. No significant changes were observed in the electrocardiac recording from 5 - 60 min of perfusion using normal oxygenated Krebs-Henseleit solution. This indicates that our model is suitable for assessment of the effects of the different compounds in the perfusate. Synthetic lysophosphatidylcholines with various acyl chain length (C12 C18) were obtained from Sigma Chemical Co. l-Hexanoyl-lysophosphatidylcholine was prepared by enzymatic hydrolysis of dihexanoylphosphatidylcholine (13). The purity of the lysophosphatidylcholines was assessed by thin layer chromatography and the homogeneicity of the acyl groups was determined by gas-liquid chromatography. Other lysophosphoglycerides were also obtained from Sigma. Triton X-IO0 and sodium laurylsulfate were obtained from British Drug House. All solutions were freshly prepared before the commencement of the experiment. The lysolipids were dissolved in Krebs-Henseleit buffer and the solutions were sonicated for 5-15 min to obtain a homogeneous solution. These solutions were used immediately for perfusion. Results Assessment of Arrhythmias - The hamster heart was stabilized with Krebs-Henseleit buffer during the first 15 min of perfusion. After the stabilization period the heart was perfused with Krebs-Henseleit buffer containing the potential arrhythmogen. The arrhythmogenic effect of the material was assessed at the end of a 5 min perfusion period. An arbitrary scale of 0, I and 2 was used as follows: 0 indicates no arrhythmia was observed throughout the perfusion (Fig. IA~; ~ indicates occurrence of an arrhythmia in forms of single and multiple premature ventricular contraction (Fig. IB); 2 indicates ventricular fibrillation and subsequent cessation of mechanical ~ctivity (Fig. IC). The observed arrhythmia in Fig. IB and IC were not reversed when the heart is reperfused with normal Krebs-Henseleit buffer for an additional 2 min. Effects of acyl groups of lysophosphatidylcholine on cardiac arrhythmiasThe effects of lysophosphatidylcholine with various acyl groups (C6-C18) on arrhythmia in the hamster hearts were assessed. The hearts were perfused in Krebs-Henseleit buffer containing 0.01 - 0.i0 mM lysohosphatidylcholine, and arrhythmia was assessed as described in the previous section. The results presented in Table I are the mean values (degree of arrhythmogenicity) of three separate assessments. No arrhythmias were observed when the hearts were perfused with hexanoyl-lysophosphatidylcholine, at 0.01-0.I0 rmM, but arrhythmias were inevitably observed with all other lysophosphatidylcholine
Vol. 32, No. 12, 1983
Lysolipids Cause Cardiac Arrhythmias
1327
(C12-C18) at 0.05 - 0.i0 mM. Both palmitoyl- and steroyl-lysophosphatidylcholine were the most potent lysolipids for producing arrhythmia and their arrhythmogenic nature did not seem to be dependent on the degree of saturation in the acyl moiety, since the same degree of arrhythmogenicity was obtained with oleoyl-lysophosphatidylcholine. Figure I Electrocardiac assessment of cardiac rhythm in the isolated hamster heart. The assessment of cardiac arrhythmia is described in text. roe horizontal bar represents i second. The vertical bar represents 5mV.
B
~
C
~
TABLE I Effects of acyl group of lysophatidylcholine on cardiac arrhythmia in hamster heart Lysophosphatidylcholine
Conc (mM)
C6:0
0.01 0.05 0.i0
0 0 0
C12:0
0.3 1.3 1.7
C14:0
C16:0
C18:0
C18:1
0.3 1.7 .
1.3 2.0 .
0.7 2.0
1.0 2.0
.
.
Lysophosphatidylcholine containing the acyl group as indicated was used for arrhythmogenic evaluation. The production of cardiac arrhythmia was evaluated as indicated in "Results". Each value reported was the degree of arrhythmogenicity of three separate experiments.
1328
Lysolipids
Cause Cardiac Arrhythmias
Vol. 32, No. 12, 1983
Effects of various bases in lysophospho$1ycerides to cardiac arrhythmiasSince the arrhythmogenecity of lysophosphatidylcholine is dependent on the chain length of the acyl groups, it was of interest to study the relationship of other naturally occurring lysophosphoglycerides (with different base groups) to cardiac arrhythmias. Our results (Tables II) indicated that lysophosphatidylglycerol exhibited the same degree of arrhythmogenicity as did the naturally occurring lysophosphatidylcholine, which suggests that the net charge of the base group has no bearings on the arrhythmogenic nature of these lipids. Both lysophosphatidylethanolamine and lysophosphatidylserine appeared to be less arrhythmogenic than the former lysolipids. However, this may be due to the insoluble nature of both serine and ethanolamine containing lysolipids. Both lysophosphatidylethanolamine and lysophosphatidylserine were not soluble and were present in the perfusate as a homogeneous suspension after sonication. We were not able to obtain a clear solution when these lipids (0.01 - 0.I0 mM) were present in the Krebs-Henseleit buffer. TABLE II Effects of various Conc
lysophospho$1ycerides
on cardiac arrhythmia
LPC
LPE*
LPS*
LPG
0.3 1.7 2.0
"0 0 1.0
0 0 0.7
1.0 1.7 -
(mM) 0.01 0.05 0.i0
LPC: lysophosphatidylcholine (bovine liver); LPE: lysophosphatidyl-ethanolamine (bovine liver); LPS: lysophosphatidylglycerol (egg yolk). Each number represents the degree of arrhythmogenicity of three separate experiments. * A slightly cloudy solution was obtained when the lipid was present in the perfusate. Effects of detersent and other lysophospholipid metabolites to cardiac arrhythmia. - Although all naturally occuring lysophospholipids tested in this study are arrhythmogenic and the arrhythmogenicity of these lipids is not dependent on the base groups, the arrhythmogenicity of their immediate metabolites is not known. Since the lysophosphoglyeerides are believed to originate from the respective phospholipids, and the lysolipids may be further metabolized, the arrhythmogenicity of the immediate metabolites of the major lysophosphoglyceride in the heart were investigated. Both phosphatidylcholine and glycerophosphorylcholine, the immediate metabolites of lysophosphatidylcholine, were found to have no effect on cardiac arrhythmia (Table III), which suggests that the lysophosphoglycerides would lose their arrhythmogenic nature when they are reacylated or deacylated. Since lysophosphoglycerides are detergents which are cytolytic at high concentrations, the arrhythmogenicity of some common detergents was also investigated (Table III). Both sodium laurylsulfate and Triton X-IO0 were found to be highly arrhythmogenic when they were present at the same concentrations as the major lysophosphoglycerides.
Vol. 32, No. 12, 1983
Lysolipids Cause Cardiac Arrhythmias
1329
TABLE III Effects of detersents and other phospno$1ycerides arrhythmia Conc (n~M)
PC
GPC
0.01 0.05 0.I0
0 0 0
0 0 0
Triton X-100 0 2 -
on cardiac
sodium laurylsulfate 0 0.3 1.7
PC: phosphatidylcholine (bovine liver); GPC: glycerophosphorylcholine. Each number represents the degree of arrhythmogenicity of three separate experiments. Discussion Lysophosphoglycerides are ubiquitous in all mammalian hearts. Lysophosphatidylcholine, which is the major lysophosphoglycerides, constitutes 0.5 - 3.5% of total cardiac phospholipid (i0). These lysophospholipids are formed from the hydrolysis of cardiac phospholipids by the action of phospholipase A 2. The lysophospholipids are further metabolised by lysophospholipases, or reacylated back into phospholipids by the action of lysophosphoglyceride: acyl CoA acyltransferase (14). Under normal physiological conditions, the concentrations of lysophospholipids are rigidly controlled because they are cytolytic at high concentrations (15). The cytolytic effect is probably due to the detergent-like properties of the lysolipids. The elevated level of lysophospholipids found in ischemic tissues may result from alterations in one or a combination of the enzymes responsible for their metabolism. The majority of hamster cardiac lysophosphatidylcholine have saturated acyl groups consisting of either C16 or C18 moieties (Slater and Choy, unpublished results). In this study, we have demonstrated that both these naturally occuring molecular species of lysophosphatidylcholines are arrhythmogenic at identical concentrations in the perfusate. Another point that can be concluded from this study is that the arrhythmogenicity of lysophosphatidylcholines does not appear to be directly related to their critical micelle concentrations as suggested by other investigators (16). It has been shown that the critical micelle concentrations of C12 to C18 lysophosphatidylcholines range from 2 x i0 -I to 2 x 10 -4 mM (15). Since all lysophosphatidylcholines from C12 to C18 inevitably cause arrhythmias at 5 x 10 -2 mM, we feel that there is no direct correlation between the arrhythmogenicity of the lysophosphatidylcholines and their critical micelle concentrations. Hence, we postulate that cardiac arrhythmia is induced by these lysophospholipids via their detergent property. This postulation is based on the fact that detergents at similar concentrations in the perfusate are arrhythmogenic. Furthermore, the short chain lysophospholipid tested (C 6) with weaker detergent properties (15) is less arrhythmogenic. Although the physiological role of lysophosphoglycerides and their relationship to cardiac arrhythmia is still unclear, this study has demonstrated unambiguously that all naturally occurring cardiac lysophosphoglycerides tested are arrhythmogenic. It must be emphasized that this study is directed to test the arrhythmogenicity of different exogenous
1330
Lysolipids
Cause Cardiac Arrhythmias
Vol. 32, No. 12, 1983
lysolipids and hence, does not provide us with the information to evaluate the physiological role of these lysolipids produced during ischemia. At present, the concise molecular events on how arrhythmias are caused by these lysolipids are not known. From our previous study (17), it is clear that the lysolipids are transiently associated with the sarcolemma prior to their transport into the cells. We have also demonstrated that these lipids are not reacylated during transport across the sarcolemma (17). The incorporation of lysolipids into the sarcolemma was also observed by Gross et al. (18). Based on these studies, it is possible that the characteristics of the sarcolemma may be altered by the lysophospholipids during their brief association with the membrane. Changes of membrane fluidity by detergent-like agents may cause the observed depression of action potentials, which is a prerequisite for cardiac arrhythmia. Acknowledsement This work was supported by the Medical Research Council of Canada and Manitoba Heart Foundation. P.C.C. is a Canadian Heart Foundation Scholar. References i. 2. 3. 4. 5. 6. 7. 8. 9. i0.
ii 12. 13. 14. 15. 16. 17. 18.
V. ELHARRA and D.P. ZIPES, Am. J. Physiol. 233 H329-H345 (1977). D.H. SINGER, C.M. BAUMGARTEN and R.E. TEN EICK, Prog. Cardiovasc. Dis. 24 97-156 (1981). B.E. SOBEL, P.B. CORR, A.K. ROBINSON, R.A. GOLDSTEIN, F.X. WITOWSKI and M.S. KLEIN, J. Clin. Invest. 62 546-553 (1978). N.A. SHAIKH and E. DOWNAR, Circ. Res. 49 316-325 (1981). K.R. CHIEN, J.P. REEVES, L.M. BUJA, F. BONTE, R.W. PARKEY and J.T. WILLERSON, Circ. Res. 48 171-179 (1981). M. PELLETIER, R.Y.K. MAN and P.C. CHOY, Fed. Biol. Soc. 25 (1982). P.B. CORR, M.E. CAIN, F.X. WITKOWSKI, D.A. PRICE and B.E. SOBEL, Circ. Res. 44 822-832 (1979). P.B. CORR, D.W. SNYDER, M.E. CAIN, W.A. CRAFFORD Jr., R.W. GROSS and B.E. SOBEL, Circ. Res. 49 354-363 (1981). R.Y.K. MAN and P.C. CHOY, J. Molec. Cell. Cardiol. 14 173-175 (1982). D.A. WHITE, Forms and Function of Phospholipids, G.B. Ansell, J.N. Hawthorne and R.M.C. Dawson eds. 441-482, Elsevier Scientific Publishing Co., Amsterdam (1973). O. LANGENDORFF, Pflugers Arch. 61 291-332 (1895). T.A. ZELINSKI, J.D. SAVARD, R.Y.K. MAN and P.C. CHOY J. Biol. Chem. 255 11423-11428 (1980). O. HAYAISHI, Methods in Enzymology ! 660-674 (1955). L.M.G. VAN GOLDE and S.G. VAN DEN BERGH, Lipid Metabolism in Mammals, F. Snyder, ed. Vol. I, 1-33, Plenum Press, New York (1977). H.U. WELTZIEN, Biochim. Biophys. Acta 559, 259-287 (1979). S.R. BERGMANN, T.B. FERGUSON and B.E. SOBEL, Am. J. Physiol. 240 H229-H237 (1981). J.D. SAVARD and P.C. CHOY, Biochim. Biophys. Acta 711 40-48 (1982). R.W. GROSS, P.B. CORR, B.I. LEE, J.E. SAFFITZ, W.A. CRAFFORD, JR., and B.E. SOBEL, Circ. Res. 51 27-36 (1982).
Pergamon Press
EFFECTS OF LYSOPHOSPHOGLYCERIDES ON CARDIAC ARRHYTHMIAS Ricky Y.K. Man a, Turnly Wong b and Patrick C. Choy b Departments of Physiology a and Biochemistry b Faculty of Medicine, University of Manitoba Winnipeg, Manitoba, Canada R3E OW3 (Received in final form December 6, 1982) Summary The accumulation of lysophosphoglycerides has been implicated as an important biochemical factor for cardiac arrhythmias. Recently, we demonstrated that lysophosphatidylcholine caused cardaic arrhythmias in the isolated hamster heart. In this study, the arrhythmogenic nature of various lysophosphoglycerides with respect to acyl chain lengths and base groups were assessed. We demonstrated that all naturally occurring lysolipids tested were arrhythmogenic at 0.05-0.10 mM. Arrhythmias were also observed with Triton X-100 or sodium laurylsulfate at 0.05 0.i0 mM. Our data suggests that no correlation exists between the arrhythmogenic nature of the lysolipids and their critical micelle concentrations. We postulate that arrhythmias are produced by the detergent effect of lysophosphoglycerides. The occurrence of cardiac arrhythmias subsequent to onset of myocardial ischemia has been well-documented (1,2). Although several biochemical and physiological changes were observed during ischemia, the exact cause for disturbances in cardiac rhythm after onset of ischemia remains unknown. In the last few years, lysophosphoglycerides have been implicated as an important biochemical factor for cardiac arrhythmia associated with ischemia. Elevated levels of lysophosphatidylcholine were observed in ischemic areas of the heart (3,4,5,6). At present, it is not known whether the elevated levels of intracellular lysophospholipids during ischemia may have any direct effects in the production of cardiac arrhthmia. However, when lysophosphatidylcholine was added to the superfusate, depression of action potential and increase in automaticity were observed in cardiac tissues (7,8). Based on these electrophysiological parameters, the arrhythmogenic nature of lysophosphatidylcholine was postulated. Recently, we demonstrated the direct relationship of palmitoyl-lysophosphatidylcholine to cardiac arrhythmias in the isolated perfused heart (9). Since lysophosphatidylcholine is present in the cardiac tissue in multiple molecular forms with different acyl groups (I0), it is of interest to study the arrhythmogenicity of these individual lysophosphatidylcholine to cardiacarrhythmias. Moreover, the effects of other naturally occurring lysophosphoglycerides to arrhythmias were not known. In this communication, we report the differential effects of lysophosphatidylcholines with various acyl groups to arrhythmias in the isolated hamster heart. The arrhythmogenic nature of other lysophosphoglycerides was also assessed. 0024-3205/83/121325-06503.00/0 Copyright (c) 1983 Pergamon Press Ltd.
1326
Lysolipids Cause Cardiac Arrhythmias
Vol. 32, No.
12, 1983
Methods Syrian Golden hamsters (90-120 g) were sacrificed by decapitation and the hearts were rapidly removed and placed in ice-cold Krebs-Henseleit buffer saturated with 95%02 - 5% CO 2. The heart was cannulated via the aorta as described by Langendorff (ii) and was perfused at a pressure of 90 - 120 mm Hg with a constant flow rate of 3-4 ml/min. The temperature of the perfusate was kept at 37 ° + 0.5°C. The detailed methodology of isolated heart perfusion has been reporte~ elsewhere (12). Electrocardiac recording was obtained by attaching one electrode on the aorta cannula and the other in the solution bathing the apex of the heart. The placement of electrodes in this manner provided assessment of the atrial and ventricular activities simultaneously. The signals were amplified and recorded by a Gould Brush 2400 paper recorder or a Sanborn i00 recorder. No significant changes were observed in the electrocardiac recording from 5 - 60 min of perfusion using normal oxygenated Krebs-Henseleit solution. This indicates that our model is suitable for assessment of the effects of the different compounds in the perfusate. Synthetic lysophosphatidylcholines with various acyl chain length (C12 C18) were obtained from Sigma Chemical Co. l-Hexanoyl-lysophosphatidylcholine was prepared by enzymatic hydrolysis of dihexanoylphosphatidylcholine (13). The purity of the lysophosphatidylcholines was assessed by thin layer chromatography and the homogeneicity of the acyl groups was determined by gas-liquid chromatography. Other lysophosphoglycerides were also obtained from Sigma. Triton X-IO0 and sodium laurylsulfate were obtained from British Drug House. All solutions were freshly prepared before the commencement of the experiment. The lysolipids were dissolved in Krebs-Henseleit buffer and the solutions were sonicated for 5-15 min to obtain a homogeneous solution. These solutions were used immediately for perfusion. Results Assessment of Arrhythmias - The hamster heart was stabilized with Krebs-Henseleit buffer during the first 15 min of perfusion. After the stabilization period the heart was perfused with Krebs-Henseleit buffer containing the potential arrhythmogen. The arrhythmogenic effect of the material was assessed at the end of a 5 min perfusion period. An arbitrary scale of 0, I and 2 was used as follows: 0 indicates no arrhythmia was observed throughout the perfusion (Fig. IA~; ~ indicates occurrence of an arrhythmia in forms of single and multiple premature ventricular contraction (Fig. IB); 2 indicates ventricular fibrillation and subsequent cessation of mechanical ~ctivity (Fig. IC). The observed arrhythmia in Fig. IB and IC were not reversed when the heart is reperfused with normal Krebs-Henseleit buffer for an additional 2 min. Effects of acyl groups of lysophosphatidylcholine on cardiac arrhythmiasThe effects of lysophosphatidylcholine with various acyl groups (C6-C18) on arrhythmia in the hamster hearts were assessed. The hearts were perfused in Krebs-Henseleit buffer containing 0.01 - 0.i0 mM lysohosphatidylcholine, and arrhythmia was assessed as described in the previous section. The results presented in Table I are the mean values (degree of arrhythmogenicity) of three separate assessments. No arrhythmias were observed when the hearts were perfused with hexanoyl-lysophosphatidylcholine, at 0.01-0.I0 rmM, but arrhythmias were inevitably observed with all other lysophosphatidylcholine
Vol. 32, No. 12, 1983
Lysolipids Cause Cardiac Arrhythmias
1327
(C12-C18) at 0.05 - 0.i0 mM. Both palmitoyl- and steroyl-lysophosphatidylcholine were the most potent lysolipids for producing arrhythmia and their arrhythmogenic nature did not seem to be dependent on the degree of saturation in the acyl moiety, since the same degree of arrhythmogenicity was obtained with oleoyl-lysophosphatidylcholine. Figure I Electrocardiac assessment of cardiac rhythm in the isolated hamster heart. The assessment of cardiac arrhythmia is described in text. roe horizontal bar represents i second. The vertical bar represents 5mV.
B
~
C
~
TABLE I Effects of acyl group of lysophatidylcholine on cardiac arrhythmia in hamster heart Lysophosphatidylcholine
Conc (mM)
C6:0
0.01 0.05 0.i0
0 0 0
C12:0
0.3 1.3 1.7
C14:0
C16:0
C18:0
C18:1
0.3 1.7 .
1.3 2.0 .
0.7 2.0
1.0 2.0
.
.
Lysophosphatidylcholine containing the acyl group as indicated was used for arrhythmogenic evaluation. The production of cardiac arrhythmia was evaluated as indicated in "Results". Each value reported was the degree of arrhythmogenicity of three separate experiments.
1328
Lysolipids
Cause Cardiac Arrhythmias
Vol. 32, No. 12, 1983
Effects of various bases in lysophospho$1ycerides to cardiac arrhythmiasSince the arrhythmogenecity of lysophosphatidylcholine is dependent on the chain length of the acyl groups, it was of interest to study the relationship of other naturally occurring lysophosphoglycerides (with different base groups) to cardiac arrhythmias. Our results (Tables II) indicated that lysophosphatidylglycerol exhibited the same degree of arrhythmogenicity as did the naturally occurring lysophosphatidylcholine, which suggests that the net charge of the base group has no bearings on the arrhythmogenic nature of these lipids. Both lysophosphatidylethanolamine and lysophosphatidylserine appeared to be less arrhythmogenic than the former lysolipids. However, this may be due to the insoluble nature of both serine and ethanolamine containing lysolipids. Both lysophosphatidylethanolamine and lysophosphatidylserine were not soluble and were present in the perfusate as a homogeneous suspension after sonication. We were not able to obtain a clear solution when these lipids (0.01 - 0.I0 mM) were present in the Krebs-Henseleit buffer. TABLE II Effects of various Conc
lysophospho$1ycerides
on cardiac arrhythmia
LPC
LPE*
LPS*
LPG
0.3 1.7 2.0
"0 0 1.0
0 0 0.7
1.0 1.7 -
(mM) 0.01 0.05 0.i0
LPC: lysophosphatidylcholine (bovine liver); LPE: lysophosphatidyl-ethanolamine (bovine liver); LPS: lysophosphatidylglycerol (egg yolk). Each number represents the degree of arrhythmogenicity of three separate experiments. * A slightly cloudy solution was obtained when the lipid was present in the perfusate. Effects of detersent and other lysophospholipid metabolites to cardiac arrhythmia. - Although all naturally occuring lysophospholipids tested in this study are arrhythmogenic and the arrhythmogenicity of these lipids is not dependent on the base groups, the arrhythmogenicity of their immediate metabolites is not known. Since the lysophosphoglyeerides are believed to originate from the respective phospholipids, and the lysolipids may be further metabolized, the arrhythmogenicity of the immediate metabolites of the major lysophosphoglyceride in the heart were investigated. Both phosphatidylcholine and glycerophosphorylcholine, the immediate metabolites of lysophosphatidylcholine, were found to have no effect on cardiac arrhythmia (Table III), which suggests that the lysophosphoglycerides would lose their arrhythmogenic nature when they are reacylated or deacylated. Since lysophosphoglycerides are detergents which are cytolytic at high concentrations, the arrhythmogenicity of some common detergents was also investigated (Table III). Both sodium laurylsulfate and Triton X-IO0 were found to be highly arrhythmogenic when they were present at the same concentrations as the major lysophosphoglycerides.
Vol. 32, No. 12, 1983
Lysolipids Cause Cardiac Arrhythmias
1329
TABLE III Effects of detersents and other phospno$1ycerides arrhythmia Conc (n~M)
PC
GPC
0.01 0.05 0.I0
0 0 0
0 0 0
Triton X-100 0 2 -
on cardiac
sodium laurylsulfate 0 0.3 1.7
PC: phosphatidylcholine (bovine liver); GPC: glycerophosphorylcholine. Each number represents the degree of arrhythmogenicity of three separate experiments. Discussion Lysophosphoglycerides are ubiquitous in all mammalian hearts. Lysophosphatidylcholine, which is the major lysophosphoglycerides, constitutes 0.5 - 3.5% of total cardiac phospholipid (i0). These lysophospholipids are formed from the hydrolysis of cardiac phospholipids by the action of phospholipase A 2. The lysophospholipids are further metabolised by lysophospholipases, or reacylated back into phospholipids by the action of lysophosphoglyceride: acyl CoA acyltransferase (14). Under normal physiological conditions, the concentrations of lysophospholipids are rigidly controlled because they are cytolytic at high concentrations (15). The cytolytic effect is probably due to the detergent-like properties of the lysolipids. The elevated level of lysophospholipids found in ischemic tissues may result from alterations in one or a combination of the enzymes responsible for their metabolism. The majority of hamster cardiac lysophosphatidylcholine have saturated acyl groups consisting of either C16 or C18 moieties (Slater and Choy, unpublished results). In this study, we have demonstrated that both these naturally occuring molecular species of lysophosphatidylcholines are arrhythmogenic at identical concentrations in the perfusate. Another point that can be concluded from this study is that the arrhythmogenicity of lysophosphatidylcholines does not appear to be directly related to their critical micelle concentrations as suggested by other investigators (16). It has been shown that the critical micelle concentrations of C12 to C18 lysophosphatidylcholines range from 2 x i0 -I to 2 x 10 -4 mM (15). Since all lysophosphatidylcholines from C12 to C18 inevitably cause arrhythmias at 5 x 10 -2 mM, we feel that there is no direct correlation between the arrhythmogenicity of the lysophosphatidylcholines and their critical micelle concentrations. Hence, we postulate that cardiac arrhythmia is induced by these lysophospholipids via their detergent property. This postulation is based on the fact that detergents at similar concentrations in the perfusate are arrhythmogenic. Furthermore, the short chain lysophospholipid tested (C 6) with weaker detergent properties (15) is less arrhythmogenic. Although the physiological role of lysophosphoglycerides and their relationship to cardiac arrhythmia is still unclear, this study has demonstrated unambiguously that all naturally occurring cardiac lysophosphoglycerides tested are arrhythmogenic. It must be emphasized that this study is directed to test the arrhythmogenicity of different exogenous
1330
Lysolipids
Cause Cardiac Arrhythmias
Vol. 32, No. 12, 1983
lysolipids and hence, does not provide us with the information to evaluate the physiological role of these lysolipids produced during ischemia. At present, the concise molecular events on how arrhythmias are caused by these lysolipids are not known. From our previous study (17), it is clear that the lysolipids are transiently associated with the sarcolemma prior to their transport into the cells. We have also demonstrated that these lipids are not reacylated during transport across the sarcolemma (17). The incorporation of lysolipids into the sarcolemma was also observed by Gross et al. (18). Based on these studies, it is possible that the characteristics of the sarcolemma may be altered by the lysophospholipids during their brief association with the membrane. Changes of membrane fluidity by detergent-like agents may cause the observed depression of action potentials, which is a prerequisite for cardiac arrhythmia. Acknowledsement This work was supported by the Medical Research Council of Canada and Manitoba Heart Foundation. P.C.C. is a Canadian Heart Foundation Scholar. References i. 2. 3. 4. 5. 6. 7. 8. 9. i0.
ii 12. 13. 14. 15. 16. 17. 18.
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