Effects of gonadotropin releasing hormone analogue, human chorionic gonadotropin and gonadal steroids on milt volume in the New Zealand snapper, Pagrus auratus (Sparidae)

Abstract

Sexually mature male snapper ( Pagrus auratus) were captured from the Hauraki Gulf in northeastern New Zealand by long-lining, and treated with exogenous hormones within 2 days of return to the laboratory. Plasma and milt samples were taken up to 24 h post-injection (p.i.) and plasma concentrations of 17 α-hydroxyprogesterone (17P), 17 α,20 β-dihydroxy-4-pregnen-3-one (17,20 βP), testosterone (T) and 11-ketotestosterone (11KT), and milt volume were determined. Gonadotropin releasing hormone analogue (100 μg·kg −1) stimulated increases in milt volume at 4 and 24 h p.i., accompanied by significant increases in plasma T and 11 KT at 24 h p.i. Injection of human chorionic gonadotropin (1000 U·kg −1) also resulted in an increase in milt volume, and plasma levels of T only. Treatment with 17P and 17,20 βP (0.5 mg kg −1) both stimulated increases in milt volume by 24 h p.i., whereas treatment with 17 α,20 α-dihydroxy-4-pregnen-3-one, T and 11 KT did not. Injection of 17P resulted in only a small elevation in plasma 17P (2 h p.i.), but an increase in plasma 17,20 βP that was larger than that resulting from injection with 17,20 βP. Taken together, the results indicate that the increase in milt volume is at least partially gonadotropin (GtH)-dependent, and possibly mediated by 17,20 βP. The failure to detect increases in plasma 17,20 βP in response to treatments designed to increase GtH may be explained by the rapid metabolism and clearance of 17,20 βP from the plasma.