A number of in vivo studies indicate that alteration in dietary fat intake results in adaptation of exocrine pancreatic secretion. At the cellular level, the mechanism of this adaptation remains largely unknown. Dietary fats influence the lipid composition of plasma membranes in several tissues, including pancreas (Martinez et al. 2002), and this may, in turn, induce functional changes. This study was designed to clarify the effects of feeding different oils on cholecystokinin octapeptide (CCK-8)-evoked amylase release and Ca2+ mobilization in isolated rat pancreatic acinar cells.
Male weaning Wistar rats were fed ad libitum with one of two semi-purified diets differing only in the type of fat added (10 % wt): virgin olive oil (group O) and sunflower oil (group S). After 8 weeks, rats were humanely killed and the pancreas removed. A fraction was used for plasma membrane isolation and analysis of fatty acid composition by gas-liquid chromatography. The rest of the gland served to provide acini by collagenase digestion. Amylase release was determined by incubating the cells with CCK-8. The activity released into the medium was quantified by the Phadebas blue starch method (Ceska et al. 1969) and expressed as a percentage of the cell total activity at the beginning. [Ca2+]i was measured fluorimetrically in a suspension of fura-2 AM-loaded cells by established methods.
The lipid composition of acinar cell membranes reflected the type of dietary fat given. Membranes of olive oil-fed animals displayed a level of total monounsaturated fatty acids of 47.36 ± 2.37 (percentage of total fatty acids, mean ± S.E.M., n = 11), significantly higher (P < 0.05, Student’s unpaired t test) than did the sunflower oil-fed group (29.72 ± 1.41, n = 15). Accordingly, a significantly higher (P < 0.05, Student’s t test) percentage of total polyunsaturated fatty acids was found in membranes of group S rats (29.94 ± 2.25, n = 15) than in those of group O (11.26 ± 1.73, n = 11). No differences in the saturated fatty acid level were observed among the groups. Adaptation to different fats resulted in significant differences (P < 0.05, Student’s t test) in basal amylase release (group O: 5.96 ± 0.60 %, n = 9); group S: 12.20 ± 0.48 %, n = 13). The increase above basal evoked by CCK-8 10-10, 10-9 and 10-8 M in group O was 15.32 ± 1.39 % (n = 23), 14.22 ± 1.20 % (n = 23) and 12.50 ± 0.32 % (n = 15), respectively. These responses were significantly higher (P < 0.05, Student’s t test) than those observed in group S (9.61 ± 0.51 %, n = 35; 8.02 ± 0.44 %, n = 37 and 6.90 ± 0.52 %, n = 22 for CCK-8 10-10, 10-9 and 10-8 M, respectively). Dietary fat had no influence on basal [Ca2+]i. However, the rise in [Ca2+]i evoked by all three concentrations of CCK-8 was significantly enhanced (P < 0.05, Student’s t test) in group O compared with group S. CCK-8 10-10, 10-9 and 10-8 M resulted in group O in a peak response of 556.49 ± 30.24 nM (n = 4), 677.44 ± 40.28 nM (n = 5) and 601.18 ± 48.79 nM (n = 4), respectively, whereas values of 306.07 ± 12.07 nM (n = 7), 404.70 ± 12.20 nM (n = 5) and 455.29 ± 10.45 nM (n = 6) were reached in group S in response to the same concentrations of the secretagogue.
The results suggest that dietary fat can modulate not only the composition of membrane lipids of rat pancreatic acinar cells but also the secretory activity and signal transduction evoked by CCK-8. How these facts are linked and whether more complex mechanisms are involved deserve further investigation.