GENERAL AND COMPARATIVE ENDOCRINOLOGY 55, 25 l-259 (1984) Comparative Studies on the Effects of Cholecystokinins, Caerulein, Bombesin 6-14 Nonapeptide, and Physalaemin on Gastric Secretion in the Ascidian Styela clava’ MICHAEL C. THORNDYKE AND PETER J. R. BEVIS~ Department of Zoology, Bedford College, London, England Accepted September 20, 1983 The effects of cholecystokinins (CCK), caerulein, bombesin 6-14 nonapeptide (bombesin), and physalaemin on gastric secretion in Styela clava were measured using a perfusion technique. Varying concentrations of both CCK,, and CCK, produced a significant dose- dependent response. Dose for dose, CCK,, was more potent than CCK,, while the assay was unable to show any discrimination between sulphated and nonsulphated forms of CCK,. The specific CCK inhibitor BtzcGMP significantly reduced the response to both CCK,r and CCK,. Both caerulein and physalaemin were effective although with a considerably reduced response compared with CCK. Bombesin was the most potent of all secretagogues tested while glucagon was without effect on gastric secretion. It is suggested that the primitive prepancreatic zymogen cells in Styela possess a receptor or receptors with an ability to recognise those peptides which are also active on vertebrate pancreatic acinar cells. It is further suggested that while the results indicate a receptor system less sophisticated than that found in vertebrates, they also imply the presence of an endogenous polypeptide hor- mone or hormones with a sequence which might be expected to show similarities to more than one vertebrate gastrointestinal peptide. In recent years the role of secretagogues in the control of acinar cell zymogen secre- tion in the mammalian exocrine pancreas has been the subject of intense investiga- tion (Gardner and Jensen, 1980; Gardner et al., 1980; Jensen and Gardner, 1979; Jensen et al., 1978, 1981; Schulze and Stolze, 1980). Such studies have shown that the mammalian pancreatic acinar cell carries a number of specific receptors which fall nat- urally into at least six classes according to their ability to interact with various secre- tagogues and specific inhibitors. In this way receptor classes have been identified which bind peptides from the secretin, cholecys- tokinin (CCK), bombesin 6-14 nonapeptide ’ A preliminary report of part of this work was pre- sented at the Ninth International Symposium on Com- parative Endocrinology, Hong Kong, 1982. 2 Present address: Endocrine Unit, Royal Postgrad- uate Medical School, Ducane Road, London W12 OHS, England. (bombesin), and substance P families as well as those responsive to acetylcholine and cholera toxin (Gardner and Jensen, 1980; Jensen and Gardner, 1979; Jensen et al., 1978, 1981). Detailed investigation of the structural requirements for the action of CCK and related peptides on enzyme se- cretion in the mammalian pancreas high- light the importance of the sulphated tyro- sine at position 27 (position 7 from the C terminus). Thus, desulphation or a change in position of the sulphated tyrosine from 7 to 6 (as in gastrin) considerably reduces potency although not always efficacy (Gardner et al., 1980). Such subtle altera- tions in sequence allow for the sophisti- cated target specificity achieved in mam- mals where peptides of the CCK/gastrin complex are also involved in the control of gall bladder contraction and gastric acid se- cretion (Dockray, 1979, 1981). In lower vertebrates the role of peptides 251 0016-6480/84 $1.50 Copyright 0 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.