Abstract
1) Uptake and localization of 45Ca were investigated in the isolated rat neurohypophysis by means of electron microscope autoradiography. The gland was incubated in 45Ca-Locke's solution in which 1/10-1/20 of CaCl2 was replaced by 45CaCl2. The emulsion film was developed by a special method to obtain extremely fine silver grains and the number of grains struck by β-particles from 45Ca was counted in photographs of various parts of the tissue.
2) In the gland incubated in 56 mM K-45Ca-Locke's solution or stimulated by electrical pulses at 20Hz for 5min, 20-40 grains/mm2 were found in the fibers, whereas only 4-6 grains/mm2 were found in the control.
3) In 23 fibers depolarized by excess K (678 grains) and 18 fibers stimulated electrically, 50-60% of the grains were found on the granule membrane or within it. Less than 5% of the grains were found on the nerve membrane and in the mitochondria.
4) The conclusion is that 45Ca uptake of neurosecretory nerve fibers is increased by depolarization and that more than half the 45Ca entering a fiber moves to the membrane of the neurosecretory granules or into the granules themselves.
