Evidence for a role for cellular alkalinization in the cyclic adenosine 3',5'-monophosphate-mediated initiation of motility in bovine caput spermatozoa

Abstract

Bicarbonate ion, the local anesthetics procaine and dibucaine, and the ionophores monensin and nigericin have been shown to markedly increase the ability of agents that elevate cyclic adenosine monophosphate (cAMP) levels to initiate motility in bovine caput spermatozoa. A number of other weak bases, including theophylline, D-600 and dipyridamole, elevate cAMP levels maximally in caput sperm at low levels but induce motility only at high levels. These compounds thus appear to have a dual role in the initiation of motility, i.e., they elevate both cAMP levels and internal pH. Confirmation of this view was provided by the demonstration that bicarbonate ion and procaine permit initiation of motility by theophylline, D-600 and dipyridamole at markedly reduced levels. Also, forskolin (a neutral adenylate cyclase activator) elevates cyclic AMP levels in caput sperm but initiates motility only in the presence of bicarbonate or procaine, and the membrane-permeant cAMP analogue 8-bromo-cAMP is capable of inducing motility only in the presence of bicarbonate. Thus, motility in caput sperm is induced only under conditions that elevate both intracellular cAMP and pH, whereas caudal sperm motility is stimulated by an elevation of either cAMP or pH. These data suggest that the epididymal development of motility requires a maturational increase in internal pH. This suggestion was confirmed by direct measurement of the internal pH of caput and caudal sperm; the internal pH of the former was found to be 5.84 +/- 0.1 and the latter 6.27 +/- 0.05.