344
lationship between acute reduction in circulating potassium levels and digitalis-induced arrhythmias, and the only discovered work on chronic potassium depletion in laboratory animals did not show increased sensitivity to the action of digitalis.12 Therefore, on what grounds are we entitled to teach that diuretic-induced chronic hypokalaemia in patients makes them more likely to have arrhythmias when digitalis is being given concomitantly ? Some recent experimental work helps delineate the problem. Dogs anaesthetised with sodium pentobarbitone were infused with 3H-digoxin at 20 tJ.g. per minute until atrioventricular dissociation occurred. This was chosen in preference to ventricular tachycardia, for in the presence of low plasma-potassium concentrations ventricular tachycardia is difficult to diagnose. Routine electrocardiograms and also atrial electrograms were done during the infusion of digoxin to help in the diagnosis of the arrhythmias, and plasma and tissue were taken for analysis for tritium content ten minutes after the digoxin infusion had been stopped after the first appearance of atrioventricular dissociation lasting more than 3 seconds. The control series contained 8 dogs, an acutely hypokalaemic group (glucoseinsulin infusion) had 10 dogs in it, while the chronic hypokalxmic dogs (hydrochlorthiazide plus high sodium, no potassium synthetic diet) contained 6 dogs. Although complete analysis has not been accomplished, the average plasma-potassium concentrations in these groups when the digoxin infusion was started were 4-0,2-3, and 2-6 meq. The amount of digoxin infused to produce per litre. atrioventricular dissociation was 151, 144, and 145 tJ.g. per kg. Digitalis-induced ventricular irritability appears to be induced first in the left ventricle,21 and the left ventricular myocardial digoxin content in the normal and acutely hypokalaemic groups (chronic hypokalasmia not yet analysed) at atrioventricular dissociation averaged 630 and 679 ng. per g. wet
weight.
In these experiments no evidence has come to light to support the idea that hypokalaemia, either acute or chronic, sensitises the myocardium to digoxin. Of particular importance are the chronic hypokalaemia experiments where 100 mg. hydrochlorthiazide a day and a zero potassium intake produced an average negative potassium balance of 78 meq. over a period of about ten days. This design was employed to give the nearest practical approximation to the condition of a patient on chronic diuretic therapy. As there was no evidence that those animals were more sensitive to digoxin, perhaps our teaching should be less
pragmatic. Section of Cardiology, Pennsylvania Hospital, Philadelphia, Pennsylvania 19107, U.S.A.
P. F. BINNION.
MULTIPLE SCLEROSIS SIR,—The paper by Sir Charles Symonds (Jan. 18, p. 155) raises some interesting points in comparative pathology and biochemistry. It also points out how sound scientific observations can be largely ignored in the quest for fashionable explanations of disease phenomena. Certainly Sir Charles Symonds is right to question Acheson’s statement that any relation between swayback and multiple sclerosis can be discounted. The demyelinating disease swayback is not a simple manifestation of copper deficiency. Copper and vitamin-A metabolism are closely interrelated and a decrease in bloodcopper level diminishes the liver’s capacity to store the vitamin in preparation for its many metabolic roles.22 Among these is the activation of sulphate to 3’-phospho21. 22.
Kastor, J. A., Spear, J. F., Moore, E. N. ibid. 1972, 45, 952. Owen, E. C. Fd Cosmet. Toxicol. 1965, 3, 701.
adenosine-5’-phosphosulphate, an essential step in the of the sulphatides of the medullary sheath. synthesis Owen 23 has discussed mechanisms by which copper and/or vitamin-A deficiency may interfere with sulphate metabolism and myelin integrity. Irving and Richards 2’ studied the relationship between the level of vitamin A in the diet and the incidence of demyelinated fibres in the medulla of the rat brain. Demyelination occurred at levels none of which was so low that overt signs of vitamin deficiency were produced. However, some consider that any damage to the nervous system in hypovitaminosis A is caused by skeletal dysplasia.25 Since the optimum human requirement of vitamin A is not known with any certainty, certain members of the community may be deficient in it. In his classic Nutrition and Disease, Mellanby suggested that a vitamin-A-rich diet might be of value in treating the inflammatory stage which occurs before the glial overgrowth in multiple sclerosis and produced some clinical evidence to support thiS.26 This work has never been adequately followed up and it is only recently that attention has once again been focused on nutritional influences in multiple sclerosis. 27 Many factors may operate in the development of this disease and the nutritional status of the individual may control how he responds to some of these-e.g., viral infection-and should not be neglected. As Mellanby remarked forty years ago, it is important that "... no possibility is missed of adding knowledge to the eetiology or therapeutics of an otherwise intractable disease ". Metabolic Unit, University College Hospital, London WC1E 6JJ.
J. GODFREY HEATHCOTE.
AGGREGATION OF HUMAN PLATELETS BY PLATELET-ACTIVATING FACTOR SIR,—Platelet-activating factor (P.A.F.) is a basic phospholipid from human or rabbit basophils which aggregates rabbit platelets and releases their vasoactive amines 28; rabbit P.A.F. has been implicated in deposition of immune complexes in acute serum sickness.29 The possible role of P.A.F. in human immunological diseases raises the question of its target cell. We have demonstrated its action on human platelets. Human platelets were prepared by centrifugation and washing on a cushion of red blood-cells.3O Aggregation was measured in an aggregometer; the heights of the recorded curves were expressed as arbitrary units. After aggregation, platelets were centrifuged and the serotonin content of supernatants and pellets was measured.31 The percentage of serotonin released into the supernatants was calculated. Human P.A.F. used in these experiments was a phospholipidic fraction from a silica-gel thin-layer chromatography. Typical results are given in the accompanying table. P.A.F. induced a two-stage aggregation curve; the second stage was most probably related to endogenous adenosine diphosphate (A.D.P.) release by the platelets; it was suppressed by apyrase, an A.D.P. inhibitor. A.D.P. itself induced first-stage and second-stage aggregations of the same magnitude as those observed with P.A.F. Clotting of 23. 24. 25. 26. 27.
28. 29. 30. 31.
Owen, E. C. Wld Rev. Nutr. Dietet. 1965, 5, 184. Irving, J. T., Richards, M. B. Biochem. J. 1940, 34, 198. McIlwain, H., Bachelard, H. S. Biochemistry and the C.N.S.; p. 300. London, 1971. Mellanby, E. Nutrition and Disease. Edinburgh, 1934. Millar, J. H. D., et al. Br. med. J. 1973, i, 765. Benveniste, J. Nature, 1974, 241, 581. Benveniste, J., Henson, P. M., Cochrane, C. G. J. exp. Med. 1972, 136, 1356. Bang, N. V., Heidenreich, R. O., Trygstad, C. W. Ann. N. Y. Acad. Sci. 1972, 201, 280. Weissbach, H., Andrews, E., Seligson, H. T. Standard Methods of Clinical Chemistry; vol. IV, p. 197. New York, 1963.
lationship between acute reduction in circulating potassium levels and digitalis-induced arrhythmias, and the only discovered work on chronic potassium depletion in laboratory animals did not show increased sensitivity to the action of digitalis.12 Therefore, on what grounds are we entitled to teach that diuretic-induced chronic hypokalaemia in patients makes them more likely to have arrhythmias when digitalis is being given concomitantly ? Some recent experimental work helps delineate the problem. Dogs anaesthetised with sodium pentobarbitone were infused with 3H-digoxin at 20 tJ.g. per minute until atrioventricular dissociation occurred. This was chosen in preference to ventricular tachycardia, for in the presence of low plasma-potassium concentrations ventricular tachycardia is difficult to diagnose. Routine electrocardiograms and also atrial electrograms were done during the infusion of digoxin to help in the diagnosis of the arrhythmias, and plasma and tissue were taken for analysis for tritium content ten minutes after the digoxin infusion had been stopped after the first appearance of atrioventricular dissociation lasting more than 3 seconds. The control series contained 8 dogs, an acutely hypokalaemic group (glucoseinsulin infusion) had 10 dogs in it, while the chronic hypokalxmic dogs (hydrochlorthiazide plus high sodium, no potassium synthetic diet) contained 6 dogs. Although complete analysis has not been accomplished, the average plasma-potassium concentrations in these groups when the digoxin infusion was started were 4-0,2-3, and 2-6 meq. The amount of digoxin infused to produce per litre. atrioventricular dissociation was 151, 144, and 145 tJ.g. per kg. Digitalis-induced ventricular irritability appears to be induced first in the left ventricle,21 and the left ventricular myocardial digoxin content in the normal and acutely hypokalaemic groups (chronic hypokalasmia not yet analysed) at atrioventricular dissociation averaged 630 and 679 ng. per g. wet
weight.
In these experiments no evidence has come to light to support the idea that hypokalaemia, either acute or chronic, sensitises the myocardium to digoxin. Of particular importance are the chronic hypokalaemia experiments where 100 mg. hydrochlorthiazide a day and a zero potassium intake produced an average negative potassium balance of 78 meq. over a period of about ten days. This design was employed to give the nearest practical approximation to the condition of a patient on chronic diuretic therapy. As there was no evidence that those animals were more sensitive to digoxin, perhaps our teaching should be less
pragmatic. Section of Cardiology, Pennsylvania Hospital, Philadelphia, Pennsylvania 19107, U.S.A.
P. F. BINNION.
MULTIPLE SCLEROSIS SIR,—The paper by Sir Charles Symonds (Jan. 18, p. 155) raises some interesting points in comparative pathology and biochemistry. It also points out how sound scientific observations can be largely ignored in the quest for fashionable explanations of disease phenomena. Certainly Sir Charles Symonds is right to question Acheson’s statement that any relation between swayback and multiple sclerosis can be discounted. The demyelinating disease swayback is not a simple manifestation of copper deficiency. Copper and vitamin-A metabolism are closely interrelated and a decrease in bloodcopper level diminishes the liver’s capacity to store the vitamin in preparation for its many metabolic roles.22 Among these is the activation of sulphate to 3’-phospho21. 22.
Kastor, J. A., Spear, J. F., Moore, E. N. ibid. 1972, 45, 952. Owen, E. C. Fd Cosmet. Toxicol. 1965, 3, 701.
adenosine-5’-phosphosulphate, an essential step in the of the sulphatides of the medullary sheath. synthesis Owen 23 has discussed mechanisms by which copper and/or vitamin-A deficiency may interfere with sulphate metabolism and myelin integrity. Irving and Richards 2’ studied the relationship between the level of vitamin A in the diet and the incidence of demyelinated fibres in the medulla of the rat brain. Demyelination occurred at levels none of which was so low that overt signs of vitamin deficiency were produced. However, some consider that any damage to the nervous system in hypovitaminosis A is caused by skeletal dysplasia.25 Since the optimum human requirement of vitamin A is not known with any certainty, certain members of the community may be deficient in it. In his classic Nutrition and Disease, Mellanby suggested that a vitamin-A-rich diet might be of value in treating the inflammatory stage which occurs before the glial overgrowth in multiple sclerosis and produced some clinical evidence to support thiS.26 This work has never been adequately followed up and it is only recently that attention has once again been focused on nutritional influences in multiple sclerosis. 27 Many factors may operate in the development of this disease and the nutritional status of the individual may control how he responds to some of these-e.g., viral infection-and should not be neglected. As Mellanby remarked forty years ago, it is important that "... no possibility is missed of adding knowledge to the eetiology or therapeutics of an otherwise intractable disease ". Metabolic Unit, University College Hospital, London WC1E 6JJ.
J. GODFREY HEATHCOTE.
AGGREGATION OF HUMAN PLATELETS BY PLATELET-ACTIVATING FACTOR SIR,—Platelet-activating factor (P.A.F.) is a basic phospholipid from human or rabbit basophils which aggregates rabbit platelets and releases their vasoactive amines 28; rabbit P.A.F. has been implicated in deposition of immune complexes in acute serum sickness.29 The possible role of P.A.F. in human immunological diseases raises the question of its target cell. We have demonstrated its action on human platelets. Human platelets were prepared by centrifugation and washing on a cushion of red blood-cells.3O Aggregation was measured in an aggregometer; the heights of the recorded curves were expressed as arbitrary units. After aggregation, platelets were centrifuged and the serotonin content of supernatants and pellets was measured.31 The percentage of serotonin released into the supernatants was calculated. Human P.A.F. used in these experiments was a phospholipidic fraction from a silica-gel thin-layer chromatography. Typical results are given in the accompanying table. P.A.F. induced a two-stage aggregation curve; the second stage was most probably related to endogenous adenosine diphosphate (A.D.P.) release by the platelets; it was suppressed by apyrase, an A.D.P. inhibitor. A.D.P. itself induced first-stage and second-stage aggregations of the same magnitude as those observed with P.A.F. Clotting of 23. 24. 25. 26. 27.
28. 29. 30. 31.
Owen, E. C. Wld Rev. Nutr. Dietet. 1965, 5, 184. Irving, J. T., Richards, M. B. Biochem. J. 1940, 34, 198. McIlwain, H., Bachelard, H. S. Biochemistry and the C.N.S.; p. 300. London, 1971. Mellanby, E. Nutrition and Disease. Edinburgh, 1934. Millar, J. H. D., et al. Br. med. J. 1973, i, 765. Benveniste, J. Nature, 1974, 241, 581. Benveniste, J., Henson, P. M., Cochrane, C. G. J. exp. Med. 1972, 136, 1356. Bang, N. V., Heidenreich, R. O., Trygstad, C. W. Ann. N. Y. Acad. Sci. 1972, 201, 280. Weissbach, H., Andrews, E., Seligson, H. T. Standard Methods of Clinical Chemistry; vol. IV, p. 197. New York, 1963.
