Pulsatile Somatostatin Release from the Median Eminence of the Unanesthetized Rat and Its Relationship to Plasnia Growth Hormone Levels*

The technique of push-pull perfusion was combined with a sensitive somatostatin RIA to determine in vivo immunoreactive somatostatin (IRS) release from the median eminence (ME) of the freely behaving rat. IRS release was compared to plasma GH levels. Concentrations of IRS in ME perfusates indicated pulsatile release, with an interpeak interval of about an hour. In rats with normal plasma GH pulses and normal plasma PRL levels, IRS secretion ranged from 10-100 pg/15 min. In rats with suppressed GH secretion and high plasma PRL levels, IRS secretion was greater, ranging between 50-500 pg/15 min. A rise in the level of IRS in the ME perfusate often coincided with an increase in GH levels in plasma, suggesting that somatostatin may act by a short-loop negative feedback mechanism to regulate GH release. Gel chromatography of IRS from the perfusate suggested that somatostatin is released in more than one molecular weight form.