Numerous studies show that altered dietary fat intake evokes adaptive changes in the cell membrane lipid composition. The membrane content of monounsaturated and polyunsaturated fatty acids (MUFA and PUFA, respectively) has been shown to reflect dietary fatty acid profile (Escudero et al. 1998). The observed changes in the amount of saturated fatty acids (SFA) in cell membranes after elevated SFA intake suggest that the cell attempts to maintain membrane fluidity through parallel increases in MUFA levels (Burns et al. 1983). It has been demonstrated that inclusion of cholesterol in the diet alters membrane cholesterol content as well as its fluidity (Muriana et al. 1992). Moreover, dietary cholesterol seems to affect the adaptive pattern of membranes to dietary fatty acids, triggering homeostatic control mechanisms to counteract membrane rigidity (Gatto et al. 2001).
The aim of this study was to investigate in the rabbit pancreas whether the adaptive pattern to dietary fat is interfered with by the presence of cholesterol in the diet.
Ten New Zealand rabbits weighing 3Ð3.5 kg were divided into two groups and fed for 50 days with either a standard chow (Panlab, Barcelona; control group, n = 6) or an atherogenic diet (95 % standard chow, 3.5 % lard and 1.5 % cholesterol, w/w; atherogenic group, n = 4). Prior to experimentation the animals were anaesthetized with sodium pentothal (30 mg kg-1, I.V.) and humanely killed. The pancreas was removed in order to measure total cholesterol by an enzymatic-colorimetric method and fatty acid composition of plasma membrane by gas-liquid chromatography.
Total cholesterol content, expressed as mg (g pancreas)-1, showed significantly higher values (P < 0.05, one-way ANOVA) in the atherogenic group (0.18 ± 0.05; mean ± S.E.M.; n = 4) compared with the animals fed the control diet (0.02 ± 0.01; n = 6). Compared with the control group, the intake of the atherogenic diet for 50 days induced a significant (P < 0.05) decrease in the percentage of total SFA (44.04 ± 0.81 vs. 47.54 ± 1.77) and a significant (P < 0.05) increase of total MUFA (37.51 ± 0.27 vs. 34.47 ± 0.68), whereas total PUFA (18.46 ± 0.86 vs. 17.99 ± 1.96) and unsaturation index (1.72 ± 0.07 vs. 1.56 ± 0.13) were similar.
These results show that the pancreatic membranes of the atherogenic group do not reflect dietary fatty acid profile. The absence of adaptation may be associated with either (or both) of these two factors: cholesterol content and total fat content in the diet. Concerning cholesterol content, an increase in this parameter causes membranes to become more rigid and the cell tends to counteract this fact through increasing the unsaturated fatty acid content. The presence of cholesterol in the diet seems to interfere with the adaptation process of membranes by a homeostatic control to keep membrane fluidity constant.
All procedures accord with current UK legislation.