Intracellular transport of proteins in active and resting secretory cells of the venom gland of Vipera palaestinae.

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Abstract
The intracellular transport of venom proteins was studied in active and resting venom glands of the snake V. palaestinae by EM radioautography after in intra-arterial injection of [3H]leucine. In the active gland, most of the label is initially (10 min) found over the RER [rough endoplasmic reticulum]. The relative grain density of the Golgi complex reaches its maximum by 30 min with concomitant increase in the labeling of the condensing vacuoles. A steep increase in radioactivity of the secretory granules is later observed. At 3 h, these granules, which comprise about 2% of the cell volume, contain 22% of the total grains. Their labeling declines the following hour and at the same time the radioactivity of the secreted venom is increased. In the active cell, venom proteins are transported via the Golgi apparatus into membrane-bounded granules which are the immediate source of the secreted venom. An alternative pathway which involves the RER cisternae as a storage compartment seems unlikely, since incorporated label does not accumulate in this compartment after prolonged postpulse intervals. The route of intracellular transport of proteins in the resting glands is similar to that of the active ones, but the rate of synthesis and transport is much slower. The present results and earlier data show that the increase in the rate of secretion after initiation of a new venom regeneration cycle is the result of accelerated rates of both synthesis and transport.