Differential effect of triiodothyronine and thyroxine on liposomes containing cholesterol: physiological speculations.
The effect of thyroid hormones on the steady-state fluorescence polarization and on the release of the liposomal content was analyzed in liposomes composed of egg phosphatidylcholine and egg phosphatidyl choline:cholesterol in different molar ratios. Depending on liposome cholesterol composition, a dual effect of triiodothyronine was found. The fluorescence polarization of 1,6 diphenyl 1,3,5 hexatriene or 1-(4-trimethylaminophenyl) 6 phenyl-1, 3, 5 hexatriene decreased by the addition of the hormone when cholesterol content was in the range from 0 to 30 moles %, while it increased with cholesterol from 30 to 50 moles %. In the release experiments, the effect of triiodothyronine was also biphasic; the leakage was the highest at 0% and 50% and the lowest at 30 moles % of cholesterol. On the contrary, thyroxine was without effect on liposomes containing cholesterol from 30 to 50 mol %. This fact correlated with a lower incorporation of thyroxine, compared with that of triiodothyronine in liposomes containing up to 30 moles % of cholesterol. The fact that the above differential incorporation of thyroid hormones was also observed at physiological concentration and that most of the mammalian membrane cells have more than 25 moles % of cholesterol have for physiological implications to the observations reported here.
Chehin
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RN
Departamento de Bioquímica de la Nutrición, Instituto Superior de Investigaciones Biológicas (CONICET-UNT), San Miguel de Tucumán, Argentina.
Rintoul
M R
MR
Morero
R D
RD
Farias
R N
RN
D002784:Cholesterol / Q000737:chemistry*; D005454:Fluorescence Polarization; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008081:Liposomes; D010713:Phosphatidylcholines / Q000737:chemistry; D013974:Thyroxine / Q000494:pharmacology* / Q000502:physiology; D014284:Triiodothyronine / Q000494:pharmacology* / Q000502:physiology
D008081:Liposomes; D010713:Phosphatidylcholines; D014284:Triiodothyronine; D002784:Cholesterol; D013974:Thyroxine