Bicarbonate stimulation of boar sperm motility via a protein kinase A-dependent pathway: between-cell and between-ejaculate differences are not due to deficiencies in protein kinase A activation.

Because poorly motile sperm samples can often be stimulated by treatments that increase intracellular levels of cyclic adenosine monophosphate (cAMP), it has been supposed that such samples are unable to maintain an adequate supply of the cyclic nucleotide with which to activate protein kinase A (PKA). To investigate this hypothesis, we incubated boar sperm samples with bicarbonate (a stimulator of adenylyl cyclase) and compared its effect with that of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole 3',5'-cyclic monophosphothioate (cBIMPS, a highly permeable and stable cAMP analog). Videomicroscopy assessment of motility was followed by computer analysis of the sperm tracks to produce motility descriptor values for many individual cells in each sample, whence "cluster" analysis of these data identified groups of spermatozoa that differed in motility characteristics. Bicarbonate stimulation of motility was characterized by an increase in the linearity (LIN) and progressive velocity of part of the sperm population only. The size of this "fast linear" subpopulation varied considerably between ejaculates. However, treatment with cBIMPS did not induce significantly more "fast linear" sperm than treatment with bicarbonate. In further experiments investigating the role of protein kinases in motility control, bicarbonate stimulation was greatly inhibited by H89 (a specific inhibitor of PKA), whereas GF109203X and lavendustin A (inhibitors of protein kinase C [PKC] and protein tyrosine kinase [PTK], respectively) had essentially no effect. While inclusion of the protein phosphatase inhibitor calyculin stimulated motility, it failed to increase the overall percentage of "fast linear sperm" induced by bicarbonate. We conclude that intersperm and interejaculate differences in boar sperm motility are not due to inadequacy in cAMP supply or to ineffective PKA activity.