Two human adenocarcinoma cell lines, HCA-7 and Col 29 [l] secrete anions in response both to agents that increase intracellular cyclic AMP and to those which increase Ca2+[2, 3]. Calcitonin gene-related peptide (CGRP) is known to cause a secretory response in rat colonic epithelia, although both the nature of the signal transduction pathway and the pharmacology of the receptor are uncertain [4]. Most actions of CGRP are mediated by cyclic AMP, but there are reports of increased inositol phosphate levels, and that some CGRP responses are blocked by pertussis toxin (implying coupling to Gi/Go)[5, 6, 7]. In this study we have examined the signals connecting CGRP receptor activation to anion secretion in Col 29 and HCA-7 cells. plates for measurement of cyclic AMP [5] or on collagen coated millipore filters (exposed area of 0.2 cm2) for measurement of short circuit current (SCC) under voltage clamp conditions [2, 3]. All measurements were made on confluent cells 10-15 days after seeding on respective surfaces. In both cell lines rat aCGRP increased SCC (maximum 3pA/0.2 cm2 HCA-7; 1.6 pA10. 2cm2 Col 29). HCA-7 cells were more sensitive to CGRP than Col 29 (EC50 0.63 nM HCA-7; 4.4 nM Col 29). Rat aCGRP also increased cyclic AMP levels in both cell lines, but it was more potent in HCA-7 cells than Col 29 (EC50 1nM v 10 nM). The time course of cyclic AMP generation was examined in both cell lines, and compared with the SCC response to CGRP and forskolin, an activator of adenylate cyclase. In preparations of passage number less than 25, the cyclic AMP and SCC responses were similar in both cell lines and are illustrated for Col 29 (figl). The cyclic AMP response was rapid, but returned to near-base line within an hour. At higher passage numbers the cyclic AMP response was sustained (figla). The SCC response to both CGRP and forskolin was rapid and sustained (fig lb, c). The duration of the CGRP SCC response changes little with passage number (data not shown). activates anion secretion in the two cell lines by increasing cyclic AMP. CGRP was more potent in HCA-7 cells both at increasing SCC and cyclic AMP levels. The rate of increase of cyclic AMP matches that of SCC and CGRP responses were mimicked by forskolin. The cyclic AMP response in cells of low passage number was not sustained, unlike the SCC. The dose response