Fig. 9. Schematic representations of possible triggers for peptide release, including that of CabTRP Ia, from Cancer midgut epithelial endocrine cells. In these schematics, intrinsic endocrine cells are colored red and epithelial cells are colored grey. At present, the cues triggering secretion of paracrines/hormones from the intrinsic endocrine cells of the crab midgut epithelium are unknown. However, two classes of epithelial endocrine cells (`open-type' and `closed-type') have been proposed, based on ultrastructural morphology (Endo and Nishiitsutsiji-Uwo, 1981; Fujita et al., 1988). (A) In open-type endocrine cells, the apical projections span the entirety of the epithelium, projecting into the gut lumen. It is proposed that these cells monitor the extracellular environment of the lumen, initiating (red arrows) or stopping secretion of hormones/paracrines when a threshold level of some chemical/ionic cue is achieved (shown here as a color gradient in the lumen). (B) In closed-type cells, the apical projections do not extend into the lumen. These cells are believed to be mechanosensory, monitoring changes in distension, which trigger (red arrows) or stop the release of hormonal/paracrine signaling agents. It should be noted that regardless of cell type, it is unclear how large the sphere of influence (pink oval in B) might be for a peptide released from gut epithelial endocrine cells. Likewise, it is not clear whether there is a directionality to release from these cells. Given that our study shows that circulating levels of CabTRP Ia are elevated in starved animals, and previous work has demonstrated a myotropic action for it on the musculature of the foregut (Messinger et al., 2005), we hypothesize that the TRP released from the midgut endocrine cells may play a crucial role in ensuring foregut muscle contraction in times of limited food availability. It should be noted that the endocrine cells in both panels of this schematic are highly stylized and should not be interpreted as representative of the morphology of epithelial endocrine cells in a general sense.