Activation of bovine epididymal sperm respiration by caffeine: Its transient nature and relationship to the utilization of acetyl carnitine

Caffeine, which stimulates the motility of freshly extruded bovine epididymal spermatozoa, caused a large but transient increase in the respiratory activity of these cells incubated in a modified Ringer buffer without exogenously added substrate. In spermatozoa that were incubated without added substrate for 2 h at 30 °C or for 15 min at 37 °C, caffeine addition failed to increase respiratory activity even transiently. However, subsequent addition of pyruvate to these aged and caffeine-treated cells resulted in a rapid increase in the respiratory rate, nearly equal to that observed after caffeine addition to fresh cells or to cells stored at 4 °C. These observations indicate that the loss in metabolic response to caffeine is a result of the active metabolism of the spermatozoa.In freshly prepared sperm that were incubated without added substrate, the acetyl carnitine content declined and the free carnitine content of the sperm increased in amounts sufficient to account for the entire respiratory increment produced by caffeine addition. Respiratory stimulation by caffeine was sustained in the presence of those exogenously added substrates that are capable of entering the acetyl carnitine pool, such as acetate, pyruvate, l(+)-lactate, glucose, fructose or β-hydroxybutyrate. Tricarboxylic acid cycle intermediates were not effective.These observations clarify the relationship between the stimulatory effects of caffeine and the metabolic state of the spermatozoan and suggest the importance of the acetyl carnitine pool to the activation of sperm motility and oxidative metabolism.