LETTERS

Lornrnent on Erythrocyte Phospholipid Metabolism in Myotonic Muscular Dystrophy Jane Grey, PhD,’ Hillel Gitelman, MD,” and Allen D. Roses, M D t

The observations of Moore, Appel, and Plishker [ 3 ] indi- cate that calcium-dependent phosphatidic acid synthesis is not impaired in erythrocytes from patients with myotonic muscular dystrophy. These results conflict with earlier findings from our laboratory [2]. Communication with the authors prompted us to reevaluate the analytical techniques employed to measure phosphatidic acid production in red blood cells. In our original procedure, phosphatidic acid was determined after thin-layer chromatography by mea- suring the change in rhodamine G fluorescence of an extract of the phosphatidic acid zone. We now find that we are unable to measure phosphatidic acid quantitatively by this procedure even though our original data showed otherwise. Our current procedure for quantitating phos- phatidic acid involves measurement of total phosphorus in the phosphatidic acid zone after digestion with perchloric acid.

We have subsequently examined the formation of phos- phatidic acid in erythrocytes using incubating media con- taining 500 pmol of calcium per liter with 2 wmol of A23187 per liter to promote calcium entry into the intact cells. Normal erythrocytes show maximal production of phosphatidic acid with these incubation conditions. In parallel experiments, erythrocytes from individuals with myotonic muscular dystrophy have displayed accumula- tions of phosphatidic acid that are not significantly different from those observed in erythrocytes from normal individu- als. Thus, our current results indicate no impairment of calcium-dependent phosphatidic acid synthesis in erythro- cytes from individuals with myotonic muscular dystrophy and are now consistent with the observations of Moore and his associates. These results are also in agreement with the studies of Yamaoka et al, who have observed normal activities of polyphosphodiesterase and diacylglycerol ki- nase in erythrocyte ghosts prepared from individuals with myotonic dystrophy (Yamaoka LH, Vance JM, Roses AD: Myotonic muscular dystrophy: calcium-dependent phos- phatidate metabolism in the erythrocyte membrane. Sub- mitted for publication).

Two apparently discrepant observations provided the impetus for examining calcium-dependent phospholipid metabolism in erythrocytes from patients with myo- tonic muscular dystrophy. Their erythrocytes display an enhanced permeability to calcium [4], yet a diminished cal- cium-dependent potassium leak had been noted in eryth- rocytes after adenosine triphosphate (ATP) depletion pro- duced by exposure to iodoacetic acid and adenosine [I].

Our studies with ATP-replete cells demonstrate that the potassium leak has a normal sensitivity to calcium [2]. These results suggest that the diminished calcium sensitiv- ity seen previously [ I ] was related to some consequence

of ATP depletion, iodoacetate exposure, o r adenosine. It is not known how any of these factors might alter the calcium sensitivity of the potassium leak. Since alterations pro- duced by such factors need not bear any direct relationship to membrane function in ATP-replete cells, the paradoxical nature of the observations referred to here is more appar- ent than real.

I t should be cautioned with regard to the calcium- dependent phospholipid pathway that the full range of calcium activation has not been examined in intact cells. Consequently, we cannot exclude the possibility of an alteration in the activation of the calcium-dependent phospholipid pathway in individuals with myotonic mus- cular dystrophy. The normal amounts of phosphatidic acid in unstimulated erythrocytes and the normal activities of the enzymes required for calcium-dependent phospha- tidic acid synthesis make this possibility less likely.

‘University of North Carolina Chapel Hill, N C 27514 +Duke University Medical Center Durham. N C 27710

References 1. Appel SH, Roses AD: Membrane biochemical studies in

myotonic muscular dystrophy. In Boles L et al (eds), Mem- branes and Disease. New York, Raven, 1976, pp 183-195

2. Grey JE, Gitelman HJ, Roses AD: Myotonic muscular dys- trophy. Defective phospholipid metabolism in the erythrocyte plasma membrane. J Clin Invest 65:1478-1482, 1980

3. Moore RB, Appel SH, Plishker GA: Myotonic dystrophy: cal- cium-dependent phosphatidic acid synthesis in erythrocytes. Ann Neurol 10:491-493, 1981 (this issue)

4 . Plishker GA, Gitelman WJ, Appel SH: Myotonic muscular dystrophy: altered calcium transportation in erythrocytes. Sci- ence 200:323, 1978

Canine Ataxia Linda C . Cork, DVM, PhD

In the bibliography of our paper “Canine Inherited Ataxia” (Ann Neurol 9:492-498, 198 1), my colleagues and I were not able to include a reference which was then in press. The articie (de LaHunta A, Fenner WR, Indrieri RJ, Mellick FW, Gardner S, Bell JS: Hereditary cerebellar cortical abiotrophy in the Gordon setter. J Am Vet Med Assoc 177:538-541, 1980) briefly describes the cerebellar disease in Gordon setters and compares the disorder with other cerebellar degenerations that occur in different species of domestic animals. The article provides a helpful review of the various cerebellar degenerations in domestic animals.

TheJournal of the American Veterinary Medical Association has accepted for publication our second article describing, in detail, the clinical signs and progression of disease in Gordon setters within our colony.

Division of Comparative Medicine The Johns Hopkins University School o f Medicine Baltimore, M D 21205

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