Membrane physical properties do not explain increased cyclic AMP production in hepatocytes from rats fed menhaden oil

To study the effect of altering plasma membrane fatty acid composition on t he glucagon signal transduction pathway, cAMP accumulation was measured in hepatocytes from rats fed diets containing either menhaden oil (MO) or coco nut oil (CO). Hepatocytes from MO-fed animals produced significantly more c AMP in response to glucagon and forskolin compared to CO-fed animals. Gluca gon receptor number and affinity were similar in MO- and CO-fed rats. Liver plasma membranes from MO-fed animals were enriched in long-chain n-3 fatty acids and contained significantly lower amounts of saturated C-10-C-16 and 18:1n-9 than CO-fed animals. Membrane physical properties were examined us ing both Fourier transform infrared spectroscopy (FTIR) and the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH). FTIR analysis revealed that bel ow 34 degrees G, CO membranes were more ordered than MO membranes. However, as assay temperature approached 37 degrees C, MO and CO membranes became s imilarly ordered. DPH polarization values indicated no differences in membr ane order at 37 degrees C, whereas membrane order was decreased in CO-fed a nimals at 25 degrees C. These data indicate the importance of assay tempera ture in assessing the influence of membrane physical properties on the acti vity of signal transduction pathways. Whereas increased signal transduction activity has been correlated to reduced membrane order in MO-fed animals, these data indicate that at physiological temperatures membrane order did n ot vary between groups. Enhanced cAMP accumulation in response to forskolin indicates that adenylate cyclase activity or content may be elevated in MO - vs. CO-fed rats. Enhanced adenylate cyclase activity may result, in part, from changes in specific fatty acids in hepatocyte plasma membranes withou t demonstrable changes in membrane physical properties.