Testis‐specific isoform of Na/K‐ ATP ase ( ATP 1A4) regulates sperm function and fertility in dairy bulls through potential mechanisms involving reactive oxygen species, calcium and actin polymerization

Traditional bull breeding soundness evaluation ( BBSE ) eliminates bulls that are grossly abnormal; however, bulls classified as satisfactory potential breeders still vary in field fertility, implying submicroscopic differences in sperm characteristics. The testis‐specific isoform of Na/K‐ ATP ase ( ATP 1A4) is involved in regulation of sperm motility and capacitation in bulls through well‐established enzyme activity and signaling functions. The objective was to determine ATP 1A4 content, activity and their relationship to post‐thaw sperm function and field fertility, using semen samples from low‐fertility ( LF ) and high‐fertility ( HF ) Holstein bulls ( n = 20 each) with known FERTSOL rates (measure of field fertility, based on non‐return rate). Frozen‐thawed sperm from HF bulls had increased ATP 1A4 content and activity compared to LF bulls. Furthermore, post‐thaw sperm from HF bulls had increased tyrosine phosphorylation, ROS , F‐actin content, and low intracellular calcium compared to LF bulls. Subsequent incubation of HF bull sperm with ouabain (a specific ligand of Na/K‐ ATP ase) further augmented the post‐thaw increase in tyrosine phosphorylation, ROS production, and F‐actin content, whereas the increase in intracellular calcium was still low compared to LF bull sperm. ATP 1A4 content and activity, ROS , F‐actin and calcium were significantly correlated with fertility. In conclusion, we inferred that ATP 1A4 content and activity differed among dairy bulls with satisfactory semen characteristics and that ATP 1A4 may regulate sperm function through mechanisms involving ROS , F‐actin and calcium in frozen‐thawed sperm of HF and LF dairy bulls.