Background: Zidovudine (AZT) as used in the treatment of AIDS causes a mitochondrial myopathy characterized by enzymatic defects in the respiratory chain system, accumulation of lipid droplets, and carnitine deficiency. Human myotubes treated with AZT demonstrate abnormal mitochondria, accumulation of lipid, and increased lysosomes. Because L-carnitine plays a major role in the transport of long chain fatty acids across the inner mitochondrial membrane and facilitates the beta-oxidation of fatty acids, we examined whether L-carnitine can enhance the recovery of the affected myotubes after withdrawal of AZT and can improve the structural changes of the myotubes while AZT treatment continues.
Experimental design: Myotubes, prepared from human muscle biopsies, were exposed to 250 microM of AZT for 3 to 6 weeks. After 3 weeks of AZT treatment, the cultures were treated with L-carnitine or medium for 3 weeks, while AZT treatment was either withdrawn or continued for 3 more weeks. The cultures were evaluated with: (a) light microscopy; (b) immunocytochemistry, to count the number of myotubes stained with antibodies to Leu-19; (c) oil red O stain, to assess the lipid droplet accumulation; and (d) electron microscopy, to count all the organelles within representative sections of the myotubes, at x24,000, and to calculate the volumetric density (Vvi) of each organelle per unit volume of tissue.
Results: In the post-AZT-treated cultures, L-carnitine increased the number of Leu-19-positive myotubes from 3.83 +/- 1.23 to 23 +/- 1.5 per field, normalized their mitochondria, decreased the lipid droplets, and increased the Vvi of the myofibrils. In the cultures treated with 3 weeks of L-carnitine while AZT treatment continued for 3 more weeks, the number of myotubes increased from 3.3 +/- 0.74 to 6.87 +/- 1.35; the absolute number of the mitochondria increased from 1.65 +/- 0.35 to 9.02 +/- 1.11 and their Vvi from 3.67 +/- 0.83 to 6.57 +/- 0.78 (p < 0.05); the Vvi of the myofibrils increased from 2.50 +/- 0.52 to 5.37 +/- 0.76 (p < 0.05); and the Vvi of the lipid droplets decreased from 5.06 +/- 1.44 to 2.72 +/- 0.72 (p < 0.05). In the AZT-treated cultures that did not receive L-carnitine, the mitochondria demonstrated extensive vacuolation, abnormal cristae, and paracrystalline inclusions; in contrast, in the L-carnitine-treated cultures, the mitochondria had substantially improved in spite of continuation of AZT.
Conclusions: L-carnitine enhances the pace and degree of recovery of the AZT-associated destruction of human myotubes, restores and preserves the structure of mitochondria, mobilizes the endomyotubular fat, and allows the regeneration of myofibrils, even if AZT treatment continues. The findings may have potential clinical implications in improving the myotoxicity of AZT in patients with AIDS when the administration of AZT treatment must continue.