The effects of cyclic nucleotide phosphodiesterase inhibitors on ejaculated porcine spermatozoan metabolism

Abstract

The effects of various cyclic nucleotide phosphodiesterase inhibitors and cyclic nucleotide analogs on porcine ejaculated sperm metabolism and fertility rate were studied. The phosphodiesterase inhibitors and cyclic nucleotide analogs stimulated cell motility and respiration with pyruvate as substrate, but not in the presence of endogenous substrate. Caffeine was studied in greater detail and shown to stimulate respiration with fructose or pyruvate as substrate but not in the presence of acetate or citrate. Although motility was stimulated within a few seconds after caffeine addition, respiration did not approach maximal rates until 2 to 4 min. Concentrations of ATP were reduced within 1 min by caffeine either with endogenous substrate or with fructose as substrate. Adenosine 3',5' monophosphate (cyclic AMP) concentrations were increased within 30 sec in sperm using fructose as substrate, but were not significantly increased in cells oxidizing endogenous substrate until 10 min after caffeine addition. Cyclic AMP concentrations were significantly increased by various concentrations of caffeine (0.25 to 5.0 mM). Bongkrekic acid (BKA), a mitochondrial adenine nucleotide translocase inhibitor, inhibited sperm respiration and reduced sperm motility to a nonprogressive state in the presence of glucose. Porcine sperm has a very low rate of glycolysis compared to bovine spermatozoa. BKA completely blocked motility in the presence of endogenous substrate or with acetate and pyruvate as substrate. Glucose utilization was not significantly altered by caffeine in the presence or absence of BKA, although lactate accumulation from glucose, in the absence of BKA, was markedly reduced by caffeine. Pyruvate utilization was reduced by caffeine, but this was associated with a marked inhibition of lactate accumulation from pyruvate. These experiments suggest that cyclic AMP may act as a regulator of sperm cell motility, with resultant secondary effects on cell metabolism.