Intra‐axonal transport and turnover of neurohypophysial hormones in the rat

Abstract

SUMMARY1. The specific radioactivities of isotopically pure oxytocin and vasopressin prepared from the neural lobe of the pituitary gland have been measured at various times after an intracisternal injection of [3H]tyrosine.2. Radioactive hormone began to appear in the gland 1-2 hr after the injection which suggests an intra-axonal transport velocity of 1-2 mm/hr.3. From 7 days onwards the specific radioactivity of each hormone declined exponentially with the same rate constant and a half-life of about 13 days.4. If the decline in radioactivity can be equated with the release of the hormones, the rates of secretion for the male rat in water balance are 18-7 m-u./day for oxytocin and 28-9 m-u./day for vasopressin.5. Calculation from the secretion rates gave steady-state plasma concentrations of about 3 lzu./ml. for each hormone.

Neurohormonal Control of Lactation and Milk Let-down in Dairy Animals

“…Norepinephrine decreases mammary blood flow (amount of oxytocin to the gland); this is an inhibition at the mammary tissue level (Jones and Pickering, 1972). Norepinephrine reduces myoepithelial cell contractile response to oxytocin; this is a direct inhibition at the myoepithelial cell level (Clarke et al, 1978).…”

The destination of the aged, nonreleasable neurohypophyseal peptides stored in the neural lobe is associated to the remodeling of the neurosecretory axon

“…Under steady-state conditions, only 5% of the total vasopressin and oxytocin content of the gland is released to the blood stream during a 24-h period (Jones and Pickering, 1972;Ledereis and Jayasena, 1970). There is morphological and physiological evidence that the enormous hormonal pool of the neural lobe is not homogeneous (Nordmann, 1977;Sachs et al, 1969).…”

“…According to Sachs et al (1969), 10-20% of the neural lobe hormonal pool is readily releasable as a response to an appropriate acute stimulus, such as hemorrhage and salt loading; these authors also showed that when the readily releasable pool has been discharged, the gland, if properly stimulated, may continue to release vasopressin and oxytocin, but at a much lower rate. However, under extreme experimental conditions, such as 10-14 days of salt loading or water deprivation, about 15% of the hormone is retained in the neural lobe (Barer and Lederis, 1966;Jones and Pickering, 1972). Thus, the large store of neurohypophyseal peptides has at least three different pools, namely, a readily releasable pool reaching the blood stream at a high rate and during a short period of time; a large pool (about 70% of total content) releasable to the blood stream at a low rate and for a long period of time; and a pool of hormones that is retained within the gland even under a potent and prolonged stimulation.…”

Population Dynamics in Vasopressin Cells

“…The stores are indeed drained rapidly by dehydration, but secretion is maintained because synthesis is elevated to meet the increased demand. However, to increase the supply of vesicles requires time: the osmotic stimulation must produce an upregulation not only of vasopressin synthesis, but also of all the machinery and components required to assemble and pack the hormone in vesicles, and even then it takes several hours for a vesicle to be transported from cell body to terminals [70]. It is not possible for short-term fluctuations in demand to be met swiftly by increased synthesis, so the stores must be large.…”