We have hypothesized that the
cholesterol synthesis inhibitor,
U18666A, induces nuclear
cataracts in the rat by restricting the
sterol content of the lens plasma membrane and, therefore, disrupting the structure of gap junctions. In order to directly examine this hypothesis, we isolated total plasma membrane and plasma membrane enriched in gap junctions from the cortical and nuclear regions of
lenses from control and U18666A-treated rats. The
protein,
phospholipid and
sterol compositions of the membrane fractions were determined and compared.
U18666A treatment resulted in decreased
sterol concentrations of both membrane fractions isolated from both the cortical and nuclear regions. The
sterol content of total plasma membrane from the cortex and from the nucleus was decreased by 57% and 36% respectively. The
sterol content of the gap junctional membrane (membrane domain enriched in gap junctions) from the cortex and from the nucleus was decreased by 71% and 43% respectively. The observation of a selective decrease in the total
sterol content of the cortical gap junctional membrane was reinforced by finding a 50% decrease in the
sterol/
phospholipid molar ratio of this fraction. The corresponding decrease in the
sterol/
phospholipid ratio of cortical total plasma membrane was only 22%. The
sterol/
phospholipid ratio of nuclear total plasma membrane was slightly increased (16%), and the
sterol/
phospholipid ratio of nuclear gap junctional membrane was decreased by only 8%. These data suggest to us that inhibition of
cholesterol synthesis in the rat lens by
U18666A results in a specific restriction of
cholesterol availability for the synthesis of gap junctional membrane.(ABSTRACT TRUNCATED AT 250 WORDS)