Incorporation of radioactive precursors into the membrane and contents of the neurosecretory granules of the rat neurohypophysis as a method of studying their fate.

Rat neural lobes have been separated into subcellular fractions by differential centrifugation at various times after an intracisternal injection of [35S]cysteine or [3H]choline. Both isotopes led to a rise and fall in the radioactivity of neurosecretory granules (NSG) which paralleled that found previously for the neurohypophysial hormones and the neurophysins. While the radioactivity of the NSGs resulting from [35S]cysteine injection was predominantly associated with granular contents, [3H]choline injections led to a preferential labelling of the granular membrane. There was no indication of a sequential movement of radioactivity from the NSG-membrane fraction into the microsomal fraction (containing the so-called small vesicles) which might be expected if granular membrane were recaptured as small vesicles after release of secretory product by exocytosis. When release was stimulated in injected animals by giving them 2% NaCl solution to drink, 35S diappeared from the gland as expected, but 3H was retained and, moreover, appeared in the NSG-membrane fraction; results compatible with membrane conservation occurring by recapture of large vesicles. There was an indication that some of the neurophysin in the NSG was membrane-bound and that this too was retained after release of the granular contents.