Vasopressin release from individually superfused neurohypophyses decreases in aged rats

We studied the effect of aging on vasopressin (VP) release from isolated neurohypophyses (NH) individually superfused with synthetic medium TC 199 with Hanks' salts. The superfusion technique is described in detail. Male Fisher 344 rats 2 months (young, n = 17), 12 months (adult, n = 14) and 30 months (old, n = 17) of age were used. VP was measured by radioimmunoassay (RIA). The age-related VP release was analyzed as follows: initial traumatic release, basal release, VP release evoked by electrical stimulation (10-second trains every 20 s of matched biphasic stimuli, 0.2 ms width, 8 mA, 30 Hz), by chemical stimulation (56 mM K+) and total release for the entire superfusion period. When VP release was expressed per milligram NH, it was significantly lower under all conditions in the old rats than in the young ones. In the adult rats, traumatic, basal and total release values were similar to those of the young rats, whereas their responses to chemical and electrical stimulation were similar to those of old rats. Residual VP content expressed per whole NH was significantly higher in the old and adult rats, reflecting a larger glandular size, but when expressed per milligram NH tissue, it was low in the old rats. The percent of VP released during the entire superfusion period relative to the residual VP content was significantly lower in the old than in the young and adult rats. The magnitude of the maximal VP release exceeding the basal release, in response to electrical and chemical stimulation, was similar in young and old rats. However, in the adult rats it was significantly lower than in both the young and old following chemical stimulation. A significantly larger number of old and adult NHs exhibited a more delayed response to chemical, but not to electrical stimulation, than did the young NHs. These data demonstrate an age-related decrease in VP release in Fisher 344 rats. Since the traumatic and basal VP release in the adult rats is similar to the release in the young rats, while the stimulated release in the adult rats resembles the response of the old animals, the results suggest that an impairment of stimulated VP release occurs at an earlier stage of the aging process than does an impairment of spontaneous (traumatic and basal) release.