There are contradictory results regarding the pattern of seasonal variation of serum lipids. The aim of this study was to compare serum lipid levels in different seasons in participants of the Tehran Lipid and Glucose Study. This was a cross-sectional study among 2890 men and 4004 women 20-64 years old from the participants of Tehran Lipid and Glucose Study (TLGS) between 1999 and 2001. Mean values of serum lipids in different seasons were compared with Analysis of Covariance (ANCOVA) after adjustment for age, physical activity level, smoking, BMI and Waist-to-hip ratio. In men, there was a significant trend for change in the values of cholesterol, LDL-C and HDL-C in different seasons, with higher cholesterol and LDL-C values in winter than in summer (P < 0.05). In women, only the mean values of triglycerides were significantly different between different seasons with values lower in winter than in summer. There was a 26.2% relative increase in the prevalence of hypercholesterolemia (> or = 240 mg/dl) in winter than in summer in men. The corresponding increase in the prevalence of high LDL-C (> or = 160 mg/dl) was 26.7% and 24.9% in men and women, respectively (P < 0.05). The prevalence of high triglycerides (> or = _ 200mg/dl) in women significantly decreased (23.8%) in winter relative to summer (P < 0.001). This study showed that there is seasonal variability in serum lipid values and this variability is greater in men than women. The increase in the prevalence of high LDL in winter in both sexes must be considered in population screening and in the follow-up of hyperlipidemic patients.
Thyroxine (T4) enters the brain either directly across the blood-brain barrier (BBB) or indirectly via the choroid plexus (CP), which forms the blood-cerebrospinal fluid barrier (B-CSF-B). In this study, using isolated perfused CP of the sheep by single-circulation paired tracer and steady-state techniques, T4 transport mechanisms from blood into lateral ventricle CP has been characterized as the first step in the transfer across the B-CSF-B. After removal of sheep brain, the CPs were perfused with (125)I-T4 and (14)C-mannitol. Unlabeled T4 was applied during single tracer technique to assess the mode of maximum uptake (Umax) and the net uptake (Unet) on the blood side of the CP. On the other hand, in order to characterize T4 protein transporters, steady-state extraction of (125)I-T4 was measured in presence of different inhibitors such as probenecid, verapamil, BCH, or indomethacin. Increasing the concentration of unlabeled-T4 resulted in a significant reduction in Umax%, which was reflected by a complete inhibition of T4 uptake into CP. In fact, the obtained Unet% decreased as the concentration of unlabeled-T4 increased. The addition of probenecid caused a significant inhibition of T4 transport, in comparison to control, reflecting the presence of a carrier mediated process at the basolateral side of the CP and the involvement of multidrug resistance-associated proteins (MRPs: MRP1 and MRP4) and organic anion transporting polypeptides (Oatp1, Oatp2, and Oatp14). Moreover, verapamil, the P-glycoprotein (P-gp) substrate, resulted in ~34% decrease in the net extraction of T4, indicating that MDR1 contributes to T4 entry into CSF. Finally, inhibition in the net extraction of T4 caused by BCH or indomethacin suggests, respectively, a role for amino acid "L" system and MRP1/Oatp1 in mediating T4 transfer. The presence of a carrier-mediated transport mechanism for cellular uptake on the basolateral membrane of the CP, mainly P-gp and Oatp2, would account for the efficient T4 transport from blood to CSF. The current study highlights a carrier-mediated transport mechanism for T4 movement from blood to brain at the basolateral side of B-CSF-B/CP, as an alternative route to BBB.