9-AAP, a fluorescent beta-adrenergic antagonist, enters the hamster sperm acrosome in a manner inconsistent with binding to beta-adrenergic receptors.

We have previously shown that the hamster sperm acrosome reaction, an event essential for fertilization, is stimulated by beta-adrenergic agonists and inhibited by beta-adrenergic antagonists. In the present report, we describe attempts to use (+/-)-9-aminoacridylpropranolol (9-AAP), a fluorescent derivative of a potent beta-adrenergic antagonist, to microscopically detect beta-adrenergic receptors on cauda epididymal hamster spermatozoa in vitro. 9-AAP binding to washed hamster spermatozoa was localized primarily in the acrosomal region, but we were unable to consistently displace the 9-AAP with the biologically active (-)-stereoisomers of several beta-adrenertic agonists and antagonists. Such displacement is necessary in order to separate binding to receptors from "nonspecific binding." Thus we did not detect sperm beta-adrenertic receptors by this method. Failure to detect the receptors with 9-AAP may be due to their presence in numbers too low for detection by this compound or to the masking of the receptors in these uncapacitated sperm. However, we could displace 9-AAP with either 5.0 mM NH4Cl or 1.2 microgram/ml nigericin, compounds capable of discharging pH gradients across cell membranes. These compounds have also been previously reported to displace the fluorescent portion of 9-AAP, 9-aminoacridine from the acrosome by such mechanism. The present results suggest that 9-AAP fluorescence does not always represent binding to beta-adrenergic receptors or "nonspecific binding," but may also represent the concentration of 9-AAP in acidic compartments within a cell.