Comparison of the pituitary effects of the mammalian endothelins: vasoactive intestinal contractor (endothelin-beta, rat endothelin-2) is a potent inhibitor of prolactin secretion.

Direct pituitary effects of vasoactive intestinal contractor (VIC), which has been described recently to be the rat form of endothelin-2 (ET-2), were compared to those previously reported for rat ET-1, rat ET-3, and human ET-2. In static incubations of cultured dispersed anterior pituitary cells, the minimum effective dose of VIC necessary to inhibit PRL release after 1-h incubation was 1 pM, and the maximum effective dose was 1 nM. Similar inhibition was observed with human ET-2. The minimum effective inhibitory dose of ET-1 was also 1 pM; however, that of ET-3 was 0.1 nM. PRL release inhibition by VIC was not mediated via the D2-dopamine receptor and was not prevented by calcium channel blockade with 100 nM nifedipine. The inhibitory effect of VIC was not present in cells treated with 100 nM staurosporine, a dose that inhibits protein kinase-C activity. Time-course studies revealed a transient stimulation of PRL release with higher doses of VIC (10 and 100 nM), which occurred within the first 15 min of incubation and was unaffected by calcium channel blockade or inhibition of protein kinase-C activity. No stimulation of PRL release was observed with doses of VIC lower than 10 nM. Instead, we observed the maintenance of the inhibitory effect for 4 h of incubation. GH release was not significantly affected by doses of VIC ranging from 10(-13)-10(-7) M; however, the release of LH was slightly, yet significantly, stimulated by 10 and 100 nM VIC. This release was prevented by pretreatment with nifedipine, but unaffected by protein kinase-C inactivation. A physiological role for VIC (rat ET-2) in the control of lactotroph function is suggested by its effectiveness at picomolar doses and its long-lasting action.