Ca-controlled, reversible structural transition in myelin

When excised goldfish spinal cord is kept in physiological saline at room temperature, the myelin multilayers swell. As studied by X-ray diffraction, the original repeating distance of 150 A (AS myelin) swells to 177 A (AL myelin); no intermediate distances are seen. At least 70% of the myelin can undergo this gradual conversion. Omitting glucose from the saline, or substituting 2-deoxy-D-glucose for the glucose, or adding NaCN to the saline all promote conversion. Cooling retards the rate but does not prevent conversion. Omitting Ca does prevent conversion, however, and substituting Mg, Sr, or Ba for Ca also does so. Moreover, agents that increase the rate of conversion in Ca-containing saline by up to 5 fold (NaCN, colchicine, A23187) fail to convert myelin in Ca-free saline. We then converted AL myelin back to AS by withdrawing Ca. After converting in NaCN-containing saline, up to 3/4 of the AL myelin recompacts in Ca-free saline; however, none of the myelin recompacts if NaCN is present in the Ca-free saline. Little or no recompaction occurs after conversion in saline without NaCN. Based on our results, we suggest that the oligodendrocytes may maintain AS myelin in vivo by pumping Ca out of the myelin sheath. The myelin in a human PNS nerve has been induced to undergo a similar cycle of swelling and recompaction.