Retinal proteins were labeled by intraocular injections of radioactive amino acids. Tissue slices of the superior colliculus (SC) were prepared 18–20 hr later, i.e., when the rapid phases of the axonal transport had reached the SC terminals. The effect of depolarizing pulses of high K and of Ca withdrawal on the secretion of radioactivity was studied in a perfusion system. The effluents were separated into a trichloroacetic acid (TCA) precipitable fraction and a TCA-soluble fraction. High K evoked a release of TCA-soluble radioactivity when 3Hglycine, 3Hleucine, or 3Hproline were used as protein precursors. Small changes occurred for TCA-precipitable fractions. The evoked release of radioactivity was Ca dependent and particularly prominent after labeling with 3Hglycine. Ca withdrawal increased the efflux of exogenous GABA, primary amines, and TCA-precipitable radioactivity but not of TCA-soluble radioactivity when normal media were used. The formation of TCA-soluble radioactivity was measured by incubating combined homogenates of SC and the lateral geniculate body (LGB), containing labeled proteins transported by the slow or rapid phase. The proteolytic activity was highly Ca dependent, for the rapidly transported proteins the half maximum was at ∼0.1 mM Ca. The formation of TCA-soluble radioactivity was inhibited byp-chloromercuriphenylsulfonic acid (PCMS). Other divalent cations could not substitute for Ca. The rate of formation of TCA-soluble radioactivity and the influence of Ca ions was smaller when proteins of the slow phase were used as substr
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