Abstract
Cyclic AMP, which maintains the vertebrate oocyte in prophase arrest under physiological conditions, exhibits specific and saturable binding to the cytoplasmic face of the prophase-arrested Rana pipiens oocyte membrane. Scatchard type analyses of [ 3H]cAMP binding to isolated plasma membranes indicate a single class of binding sites with a K d = 19.3 ± 7.0 nM at cAMP concentrations below 10 −6 M and additional low affinity site(s) and/or non-specific binding at concentrations above 10 −6 M. Photoaffinity labeling of prophase oocyte plasma membranes with [ 32P]-8-N 3cAMP demonstrates cAMP/cGMP-displacable binding of 8-N 3[ 32P]cAMP to a 100–110 kDa peptide doublet. Plasma membrane fluidity was monitored by electron spin resonance in isolated plasma-vitelline membranes using a 5-doxyl stearic acid probe. Exogenous dibutryl cAMP (dbcAMP) produces an increase in membrane fluidity within minutes and blocks and/or reverses the progesterone-induced decrease in plasma membrane fluidity. The dbcAMP concentration that produced half-maximal fluidity increase (10 μM) corresponds to the half-maximal inhibiting dose of dbcAMP for progesterone induction of meiosis. Cholera toxin, which elevates intracellular cAMP and blocks meiosis, also increases membrane fluidity and inhibits progesterone-induced decrease in membrane fluidity. Elevated levels of intracellular cAMP thus appear to maintain meiotic arrest by binding to specific plasma membrane site(s) and maintaining the plasma membrane in a relatively fluid state. The progesterone-induced fall in intracellular cAMP first reported in Rana thus appears to be responsible for the progesterone-induced increase in membrane fluidity and further suggests that the change in membrane order is essential for the resumption of the meiotic divisions.
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