Annual reproductive phase-related profile of sex steroids and their carrier, SHBG, in the Indian major carp, Labeo rohita

This study reports annual cyclic changes in the plasma levels of sex hormone binding globulin (SHBG), sex steroids (E2, estradiol; T, testosterone; P, progesterone) and protein (PRT) in correlation with reproductive phases in both sexes of an Indian major carp, Labeo rohita. Plasma SHBG, E2, T and PRT levels in females and SHBG and T levels in males showed steady increases (p < 0.005) in parallel with the gonadosomatic index (GSI) escalation in the preparatory and prespawning phases attaining peak levels before spawning. During the late spawning and early postspawning phases, E2, T and SHBG levels revealed an abrupt and significant descent coincident with the GSI in both the sexes. The plasma P levels in females, attaining measurable quantities only in prespawning phase, underwent a rapid increase followed by a quick decline to undetectable levels in the spawning phase. This occurrence of P for a short duration suggests a limited role in ovarian function. During the resting phase, the E2, T and SHBG levels in both sexes declined to minimum in conjunction with a decrease in the GSI, while PRT showed an increase. These data suggest that a significant increase in SHBG (p < 0.005) prior to spawning enhances its binding capacity with sex steroids in both sexes synchronizing the reproductive activity. The significant increase of PRT in females during early spawning reflects its role in oocyte function, while PRT and E2 in males may not significantly affect the testicular activity.