Atrial natriuretic peptide induces an acrosome reaction in giant panda spermatozoa and enhances their penetration of salt-stored porcine oocytes

Atrial natriuretic peptide (ANP) is a vasodilator peptide primarily produced in the heart. Locally synthesized ANP has been found in reproductive tissues of various mammals and humans, and plays an important role in rat oocyte maturation and human sperm function. The objective of the present study was to determine the effects of ANP on the function (acrosome reaction and zona penetration) of giant panda spermatozoa. In fresh and frozen-thawed spermatozoa that had been preincubated for 2.5h, treatment with ANP (for 60 min) significantly increased the proportion of acrosome-reacted spermatozoa; maximal response (an acrosome reaction in 18.3 and 21.8% of fresh and frozen-thawed spermatozoa, respectively) was detected at 1 nM ANP. Treatment with C-ANP-(4-23), an analogue of ANP and specific binder to natriuretic peptide receptors-C (NPRC), had no significant effect on the acrosome reaction. However, the cyclic guanosine 5'-monophosphate (cGMP)-dependent protein kinase (PKG) inhibitor KT5823 completely abolished the effect of ANP on acrosome reaction. The effects of ANP, caffeine and heparin on frozen-thawed sperm function were studied by insemination of porcine salt-stored oocytes in a modified Tris-buffered medium (mTBM). The presence of ANP, caffeine or heparin in the insemination medium resulted in a higher proportion (P < 0.05) of oocytes with spermatozoa in the zona and perivitelline space (PVS), and a higher average number of spermatozoa/oocyte (P < 0.05) in the zona and PVS. However, in the absence of ANP, caffeine and heparin, there were no oocytes with a spermatozoon in the PVS. There were no differences among ANP, caffeine or heparin treatments for the proportion of oocytes penetrated or average number of spermatozoa/oocyte in the zona and PVS. In conclusion, we inferred that ANP induced the acrosome reaction of preincubated giant panda spermatozoa by a PKG pathway. Furthermore, ANP enhanced the penetrability of porcine salt-stored oocytes by frozen-thawed giant panda spermatozoa.