BICARBONATE-DEPENDENT LIPID ORDERING AND PROTEIN AGGREGATION ARE PARTOF THE NONGENOMIC ACTION OF PROGESTERONE ON CAPACITATED SPERMATOZOA

The nongenomic action of progesterone (P) on capacitated sperm in medi ating acrosomal exocytosis operates through transmembrane signal trans duction involving increased intracellular calcium ions and modulation of protein kinases and phospholipid metabolism through a second messen ger pathway. Conflicting views exist regarding the nature of the nonge nomic receptor of P. It is thought to be a cell-surface receptor havin g the properties of a calcium channel, chloride channel, bicarbonate/c hloride exchanger, and gamma aminobutyric acid type A (GABA(A)). In th is study, we tried to understand the role of bicarbonate and/or P in i nducing membrane perturbations in capacitated and acrosome-reacting sp ermatozoa. We also attempted to characterize the membrane responses in P-stimulated and bicuculline-poisoned sperm. The presence of a high l evel of bicarbonate in the medium favored high rotational mobility of lipids and proteins in capacitated sperm. However, the capacitated spe rm exhibited low lipid ordering and unaltered protein ordering in resp onse to the high bicarbonate concentration. P and bicuculline appeared to bind to the same or similar binding sites, probably a GABA, recept or, eliciting similar motional perturbations in sperm membranes. It ap pears that both P and bicuculline can induce receptor aggregation and lipid ordering in sperm membranes, which alter the capability of sperm to bind zona pellucida, and that the P action on sperm takes effect t hrough a bicarbonate-dependent signal transduction mechanism.