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" PKA, protein kinase A. b PI, phosphatidylinositol pathway.

c The increase of cytoplasmic calcium concentration may be important in the actions of PI stimulation. d Inhibitory pathway of PKA.

4. PRF and PIF

For some time, prolactin release has been known to occur experimentally by the effects of TRH. The negative effector has been considered to be dopamine. However, other work has generated peptides with these activities. Thus, PRF activity appears to be present primarily, if not exclusively, in the intermediate lobe and it is present in cultured posterior pituitary cells. Both PRF and PIF activities can be extracted from bovine neurointermediate lobes. PRF is not oxytocin and is active in vitro during dopamine blockade with domper-idone and in vivo in the presence of dopaminergic tone. PRF appears to be a peptide of molecular weight less than 5000 that rapidly stimulates release of PRL in a dose-dependent and specific manner during dopaminergic inhibition. Oxytocin, arginine vasopressin, and ^-endorphin do not contribute significantly to PRF activation in extracts of posterior pituitary.

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