Single fiber analyses of type IIA myosin heavy chain distribution in hyper- and hypothyroid soleus

The objectives of this study were to 1) examine the effect of hypo- and hyperthyroidism (triiodothyronine treatment) on the distribution of type IIA myosin heavy chain (MHC) in the soleus at the single fiber level and 2) correlate changes in the single fiber distribution of type IIA MHC with the maximal shortening velocity of whole skeletal muscle. The presence of the type IIA MHC in single fibers was determined using a monoclonal antibody reactive to the type IIA MHC and quantified with a Meridian ACAS 570 interactive laser cytometer. The findings of this study demonstrate that 1) hyperthyroidism significantly increases the relative number of muscle fibers that express type IIA MHC, 2) not all type I fibers are capable of expressing fast type IIA MHC under hyperthyroid conditions, and 3) there is a high correlation between maximal shortening velocity and the relative number of type IIA fibers. This latter observation suggests that the maximal shortening velocity of whole skeletal muscle may not be solely determined by its fastest fiber(s) but rather by the relative proportion of fibers expressing fast type IIA MHC.

Glycolipids and transmembrane signaling: antibodies to galactocerebroside cause an influx of calcium in oligodendrocytes.

This is the first study to provide evidence that one function for the surface glycolipid galactocerebroside (GalC) is participation in the opening of Ca2+ channels in oligodendroglia in culture. This glycolipid is a unique differentiation marker for myelin-producing cells; antibodies to GalC have been shown to markedly alter oligodendroglial morphology via disruption of microtubules (Dyer, C. A., and J. A. Benjamins. 1988. J. Neurosci. 8:4307-4318). This study demonstrates that extracellular EGTA blocks anti-GalC-induced disassembly of microtubules in oligodendroglial membrane sheets, demonstrating that an influx of extracellular Ca2+ mediates the cytoskeletal changes. The Ca2+ influx was examined directly by loading oligodendroglia with the fluorescent dye Indo-1 in defined medium, and measuring changes in Ca2+ in individual cells with a laser cytometer. Upon addition of anti-GalC IgG, a marked sustained increase in intracellular Ca2+ occurred in 80% of the oligodendroglia observed. EGTA blocked the increase, indicating the increase is due to an influx of extracellular Ca2+, and not due to release from intracellular stores. The effect is specific, since Ca2+ levels remain normal in oligodendroglia treated with nonimmune IgG; astrocytes do not respond to the anti-GalC. The Ca2+ response in oligodendrocytes is dependent on concentration of antibody and GalC on the oligodendroglial membrane surface. The Ca2+ influx is not mediated by voltage-sensitive Ca2+ channels: it is not blocked by cadmium, and depolarization with K+ does not mimic the response. The kinetics of the response suggest that second messenger-mediated opening of Ca2+ channels is involved.