Divalent cations and fast axonal transport in chemically desheathed (triton X-treated) frog sciatic nerve

We have studied the ability of divalent cations to restore to normal axonal transport (AXT) which was inhibited by deprivation of Ca²⁺ and/or Mg²⁺ ions. The epi- and perineurium of the frog sciatic nerve were damaged by a 30-s wash in Triton X-100 containing frog Ringer's. This treatment did not affect either AXT or nerve levels of Ca²⁺ and Mg²⁺, but made the ions more easily extractable with a Ca²⁺- and Mg²⁺-free Ringer's solution (CMFR). Inhibition of AXT was achieved by incubating Triton X-100-treated nerves in CMFR + EGTA for 5 h, followed by an additional incubation for 12 h in CMFR or Ringer's devoid of only Ca²⁺ (CFR). These treatments reduced Ca²⁺ and Mg²⁺ contents by 77% and 38% respectively. Addition of Ca²⁺ (1.1 mM) during the 12-h period stimulated AXT, measured as accumulation of ³H-labelled components in front of a ligature, several fold. Mg²⁺ could not substitute for Ca²⁺ but potentiated the stimulating effect of Ca²⁺. Addition of other divalent cations did not affect AXT (Sr²⁺ and Ba²⁺) or potentiated the inhibition caused by Ca²⁺-deprived medium (Mn²⁺ and Co²⁺). ATP and creatine phosphate contents were similar in nerves incubated in Ca²⁺-deprived medium and in Ca²⁺-containing Ringer's. Thus, inhibition of AXT in the former situation was not due to a decreased availability of high energy phosphates. Two calcium antagonists, D-600 and nifedipin, which are potent smooth muscle relaxants, effectively blocked AXT. The present ressults suggest that Ca²⁺ is specifically required to maintain AXT and that an anlogy exists between Ca²⁺ regulation during smooth muscle contraction and AXT.