The aim of the present study was to investigate the mechanisms regulating endothelin-1 (ET-1) secretion in rat thyroid FRTL-5 cells. ET-1 was found to be secreted after stimulation with adenosine and ATP. The release of ET-1 was sensitive to pertussis toxin, indicating a role of G-proteins in the stimulus-secretion coupling. The stimulation evoked by ATP or adenosine was inhibited by the P,-receptor antagonist 8-cyclopentyl-l,3-dipropylxanthine (DPCPX), and in the presence of adenosine deaminase the adenosine-and ATP-mediated ET-1 secretion was abolished. These evidences suggest a role of a P,-adenosine receptor in the secretion of ET-1. Increasing cyclic AMP with forskolin decreased the adenosine-mediated secretion. In addition, the intracellular calcium chelator BAPTA or inhibition of calcium entry with Ni2+ prevented the response. Protein kinase C (PKC) is also partly involved in ET-1 secretion in FRTL-5 cells. Activation of PKC with the phorbol ester phorbol 12-myristate 13-acetate (PMA) Stimulated the secretion of ET-1 in a timeand dose-dependent manner. Furthermore, downregulation of PKC decreased the secretion of ET-1 stimulated by adenosine. In conclusion, ET-1 secretion in FRTL-5 cells is stimulated via a pertussis toxin-sensitive P,-receptor pathway which is modulated by several signal transduction mechanisms including CAMP, Ca2+, and PKC. o 1996 Wiley-Liss, Inc.