Purification of two structurally and morphologically distinct populations of rat neurohypophysial secretory granules.

A method for the subcellular fractionation of the neurosecretory granules of the rat Neurol lobe has been developed which employs density gradients of sucrose and metrizamide which are isoosmotic at 360m osmolality (osmol/kg solvent) (0.30 M sucrose). This procedure produces two populations of granules: one with an isopycnic density of ρ ≈ 1.13 containing granules of dia. 172 ± 2 (S.E.M.) nm and a second with density of ρ ≈ 1.11 containing granules of dia. 197 ± 9nm. The two populations contain oxytocin, vasopressin and the neurophysins. The denser granules are insensitive to osmotic change while the less dense particles behave like perfect osmometers. When compared to the tissue homogenate these fractions were purified 4–5-fold in relation to the mitochondria and 7-fold in relation to lysosomes. Chromatography of the gradient fractions on CPG 10–3000 porous glass beads results in further purification.In tissue from the neurohypophysis fixed as a whole, granules located in the nerve terminals and in dilated axonal segments (swellings) measure 173 ± 2 nm and 188 ± 2 nm respectively. It is proposed that one of the populations of granules isolated on the iso-osmotic gradient is derived from the nerve terminals whereas the other is derived from the axonal swellings. We suggest that the second population is formed from the first by a yet unspecified process.