Effects of Repeated Phase Shifts in Light-Dark Cycles on Lipid Metabolism in Stroke-Prone Spontaneously Hypertensive Rats

In the present paper, we employed the stroke-prone spontaneously hypertensive rat (SHRSP) as an animal model and the Kyo: Wistar rat (WKY) as a control, and studied on the effect of repeated phase shifts in light-dark cycles on lipid metabolism. First, we investigated diurnal rhythms of the lipid metabolism in SHRSP and WKY. In both strains, the activities of lipoprotein lipase (LPL) and hepatic microsomal cholesterol 7α-hydroxylase in the dark period were significantly higher than those in the light period. In addition, in SHRSP, the serum apoA-IV level in the dark period was also higher than that in the light period.Next, we repeated the phase shifts in light-dark cycles twice a week with elongation of the light period for 4 weeks. LPL activity in the light period increased in response to the repeated phase shifts in both strains. This might be a defensive reaction to maintain homeostasis in the lipid metabolism in addition to energy production.Moreover, we performed repeated phase shifts in rats fed a high-fat and high-cholesterol diet containing 0.1% propylthiouracil to elucidate the effect on the development of hypercholesterolemia. The repeated phase shifts increased the levels of atherogenic lipoproteins and the atherogenic index (apoB/apoA-I). In particular, the effect was more marked in SHRSP. This deleterious effect could be due to the overproduction of very low density lipoprotein (VLDL, β-VLDL) in the liver.