STIMULATION OF RAINBOW-TROUT GALLBLADDER CONTRACTION BY CIONIN, AN ANCESTRAL MEMBER OF THE CCK GASTRIN FAMILY/

Cionin-from the protochordate Ciona intestinalis-is a putative ancesto r of cholecystokinin (CCK) and gastrin. Being sulfated on tyrosine in positions 7 and 6 (from the C-terminus), characteristic for CCK and ga strin, respectively, cionin is a structural hybrid of the two peptides . The effects of cionin have previously been characterized in mammalia n systems. This study examined a phylogenetically ancient CCK receptor , the rainbow trout (Oncorhynchus mykiss) gallbladder receptor, utiliz ing cionin, sulfated and nonsulfated CCK and gastrin, and the receptor antagonists L-364,718 and L-365,260. The sulfated peptides induced co ncentration-dependent contractions of isolated strips of gallbladder w ith equal efficacy and similar potencies [ED(50):42 (cionin), 23 (CCK- 8-s), and 74 nM (gastrin-17-s)], significantly different from the nons ulfated forms [ED(50):1.7 (CCK-8-ns) and 1.9 mu M (gastrin-17-ns)], Te n micromolar L-364,718 and L-365,260 both weakly but significantly inh ibited cionin-CCK-8-s, and gastrin-17-s-induced contractions. L-365,26 0 shifted the concentration response curves 1 1/2 decades to the right and L-364,718 only 1/2 decade. The results confirm that the rainbow t rout gallbladder CCK receptor does not distinguish sulfated CCK from s ulfated gastrin as do modern CCK, receptors, but does distinguish sulf ated from nonsulfated forms of both. However, for optimal effect the r eceptor does not require a double-sulfated peptide like cionin as migh t be expected from the lack of selectively between CCK and gastrin. Fi nally, studies with antagonists known to be specific for either CCK or gastrin receptors in mammalian systems indicate that this ancient rec eptor behaves more like a mammalian CCK, receptor than as a CCK, recep tor. (C) 1995 Academic Press, Inc.