AXOPLASMIC TRANSPORT OF PROTEINS IN VITRO IN PRIMARY AFFERENT NEURONS OF FROG SPINAL CORD: EFFECT OF Ca²⁺-FREE INCUBATION CONDITIONS

Abstract

—The effects of Ca2+-free incubation medium on in vitro axoplasmic transport of proteins were studied in the central and peripheral branches of primary afferent spinal neurons of frog. Following exposure of dorsal root ganglia to [3H]leucine, the amount of radioactive protein transported along the axons during a subsequent 19 h period was decreased by approximately 60 per cent in preparations incubated in Ca2+-free, 1 mm-EGTA medium compared to those in normal medium. In similar Ca2+-free conditions the endogenous calcium levels were decreased to one-fourth the levels found following incubation in normal medium. Neither raising EGTA concentrations to 10 mm nor incubation in Ca2+-free medium prior to the [3H]leucine pulse were found to decrease the amount of transported protein in Ca2+-free medium by more than 70 per cent. The decrement in the amount of transported proteins did not appear to be due to an effect of Ca2+-free medium upon either the uptake of [3H]leucine into ganglion cells or upon the incorporation of radioactive amino acid into protein. The data are interpreted to suggest (i) that‘loading' of proteins onto the transport system is inhibited during Ca2+-free incubation and (ii) that the apparent transport of radioactive proteins during Ca2+-free incubation conditions might reflect proximo-distal movement of either microtubular protein or some other protein components of the transport system. It is proposed that calcium ions might function as reversible bonds between the transport system and‘transported' proteins.