Membrane cholesterol alters gallbladder muscle contractility in prairie dogs

The cause and effect relationship between membrane cholesterol and gallbladder muscle contractility was examined by altering membrane cholesterol to phospholipid mole ratio using cholesterol-rich or cholesterol-free liposomes. Gallbladder single muscle cells, from prairie dogs that were fed either a regular or high-cholesterol (1.2%) diet, were isolated enzymatically with collagenase. Plasma membranes of gallbladder muscle were purified in sucrose gradient. Cholesterol was measured using the cholesterol oxidase method. Phospholipids were measured with the method of G.R. Bartlett (J. Biol. Chem. 234: 466-468, 1959). The results of this experiment are 1) after high-cholesterol feeding, cholesterol contents and cholesterol/ phospholipid mole ratio in plasma membranes of gallbladder muscle increased 90%, and muscle cell contraction in response to cholecystokinin octapeptide decreased 58%; 2) similar changes were observed when normal gallbladder muscle cells were incubated with cholesterol-rich liposomes for 2 h; and 3) the changes induced either in vivo or in vitro were reversed when muscle cells were subsequently incubated with cholesterol-free liposomes for 2-6 h. We conclude that gallbladder muscle may incorporate excess cholesterol into its plasma membrane when exposed to a cholesterol-rich environment, that excess membrane cholesterol impairs muscle contractility, and that these changes appear to be reversible.