Effects of Somatostatin and Urotensin II on Tilapia Pituitary Prolactin Release and Interactions between Somatostatin, Osmotic Pressure Ca++, and Adenosine 3′,5′-Monophosphate in Prolactin Release in Vitro *

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Abstract
Both somatostatin (SRIF) and urotensin II, a dodecapeptide from the teleost caudal neurosecretory system, inhibit PRL release from the organ-cultured rostral pars distalis of the tilapia, Sarotherodon mossambicus, in a dose-related manner. The inhibitory action of SRIF on PRL release was completely prevented by the presence of the calcium ionophore A 23187. PRL release was also blocked when Ca++ was excluded from the incubation medium, even in the presence of the ionophore. Both dibutyryl cAMP (dbcAMP) and the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine, alone or in combination, stimulated PRL release during incubation in high osmotic pressure medium. The effect of dbcAMP appeared to be dose related. Together, dbcAMP and 3-isobutyl-l-methylxanthine were also effective in preventing the inhibition of PRL release by SRIF. These results are consistent with the notion that Ca++, and possibly cAMP, may be important mediators of PRL secretion, and it is likely that SRIF may inhibit PRL release by blocking a Ca++-or cAMP-mediated mechanism.

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