Michael E. Bizeau

Menhaden oil feeding increases lipolysis without changing plasma membrane order in isolated rat adipocytes

Abstract Alterations in dietary fatty acid composition can influence the regulation of metabolism in adipose tissue. To study the effect of dietary fatty acid composition on lipolysis we fed rats diets containing menhaden oil (MO) or coconut oil (CO) for 4 weeks. After 4 weeks on the diets body weight was similar in both groups although epididymal, retroperitoneal and mesenteric fat pad masses were significantly elevated (∼30%) in rats fed the CO diet. Epinephrine-stimulated lipolysis was elevated at concentrations >1M in epididymal adipocytes isolated from the MO group. The addition of insulin (10nM) to 1M epinephrine decreased lipolysis to a similar level in both groups. Plasma membrane fatty acid composition was altered to reflect the respective diets. Membranes from the CO fed group were enriched in saturated fatty acids (C10–16) and 18:1n9 and devoid of long chain n3 fatty acids compared to the MO fed group. To examine the effect of altered plasma membrane fatty acid composition on membrane physical properties we used Fourier transform infrared spectroscopy (FTIR). Membrane order was determined in adipocyte plasma membranes from −20°C to +58°C. Peak CH 2 symmetric and asymmetric stretching frequencies indicated that below 31°C membrane order is increased in CO fed animals. When temperature was increased above 31°C, there were no differences in membrane order between groups. The results of this study suggest that membrane order is not responsible for the lower rate of lipolysis in animals fed coconut oil when both variables are measured at 37°C.

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 the glucagon signal transduction pathway, cAMP accumulation was measured in hepatocytes from rats fed diets containing either menhaden oil (MO) or coconut oil (CO). Hepatocytes from MO-fed animals produced significantly more cAMP in response to glucagon and forskolin compared to CO-fed animals. Glucagon 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 C10−C16 and 18∶1n−9 than CO-fed animals. Membrane physical properties were examined using both Fourier transform infrared spectroscopy (FTIR) and the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH). FTIR analysis revealed that below 34°C, CO membranes were more ordered than MO membranes. However, as assay temperature approached 37°C, MO and CO membranes became similarly ordered. DPH polarization values indicated no differences in membrane order at 37°C, whereas membrane order was decreased in CO-fed animals at 25°C. These data indicate the importance of assay temperature in assessing the influence of membrane physical properties on the activity 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 not 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 without demonstrable changes in membrane physical properties.