Previous studies showed that fatty acid-induced CCK secretion is chain-length dependent with fatty acids shorter than dodecanoic acid (C12) having no effect on plasma CCK (McLaughlin et al. 1999). Chain-length dependence is also observed in the established CCK secreting enteroendocrine cell line, STC-1. Recently, we demonstrated that STC-1 cells respond to hydrophobic fatty acids as aggregates, since the cellular response to fatty acid filtrates was significantly decreased (Benson et al. 2002). In the light of this, we have investigated the structural components of the fatty acid molecule that are necessary for inducing CCK secretion.

Several fatty acid analogues were examined and compared to C12 in their ability to induce CCK secretion and [Ca²⁺]_i mobilisation in STC-1 cells. [Ca²⁺]_i was measured using Fura-2 and dual excitation ratiometric fluorescence microscopy. CCK secretion was measured by radioimmunoassay.

The fatty acid analogue 1,10-decanedicarboxylic acid, which replaces the terminal methyl group with a second carboxylic group, was unable to stimulate CCK secretion or a rise in [Ca²⁺]_i (1,10-decanedicarboxylic acid, 0.78 ± 0.08 pmol mg^-1; C12, 1.68 ± 0.23 pmol mg^-1; means ± S.E.M., P < 0.05, Student’s paired t test). This suggests that the amphipathic nature of fatty acids is necessary for their detection by STC-1 cells. The non-metabolisable fatty acid 2-bromododecanoic acid induced CCK secretion (2-bromododecanoic acid, 1.59 ± 0.15 pmol mg^-1; control, 0.22 ± 0.08 pmol mg^-1; means ± S.E.M., P < 0.05, Student’s paired t test) and a rise in [Ca²⁺]_i, suggesting that fatty acid metabolism is not necessary. Interestingly, more major changes to the fatty acid molecule (e.g. replacing each hydrogen by a fluorine) did not perturb the STC-1 cellular response. To investigate the possibility that insoluble fatty acid aggregates of a certain size were responsible for CCK secretion (rather than fatty acid monomers in solution), the fatty acid analogue emulsions were sized, using enhanced laser diffraction and polarization intensity differential scattering. Generally, there was a poor correlation between particle size and CCK secretion, since some hydrophilic fatty acid analogues which did not form particles, such as 1-undecanesulfonic acid and the fluorine-containing analogues, were able to induce CCK secretion.

These data add a cautionary note to our previous finding that fatty acid-induced CCK secretion is mediated by particles of a certain size independent of chemical composition. However, this study remains consistent with the hypothesis that the signal transduction pathway involved in fatty acid recognition is not mediated by a receptor that specifically responds to saturated carboxylic acids, since extensive modification of the fatty acid molecule still results in agents which are able to induce CCK secretion and calcium mobilisation in STC-1 cells.