ABSTRACT
At different stages of the annual reproductive cycle of Catla catla, a major Indian carp, specific binding of gonadotropic hormone to the plasma membrane receptors was demonstrated. Maximum specific binding of [125I] Catla gonadotropic hormone was obtained at 30°C and pH 7·5 during 2 h of incubation. Catla gonadotropic hormone binding was saturable with high affinity. Competitive inhibition experiment showed that binding site was specifically occupied by piscine gonadotropic hormone, Catla gonadotropic hormone and murrel gonadotropic hormone, human chorionic gonadotropin was a weak competitor while bovine thyroid stimulating hormone, bovine prolactin and ovine follicle stimulating hormone had no effect. Scatchard analysis of Catla gonadotropic hormone binding to the plasma membrane preparation from the carp oocytes of different reproductive stages showed that the range of dissociation constant (Kd) varied from 0·78 to 0·97 × 10-10 M. However, maximum binding capacity (B-max) varied remarkably between the different stages of reproductive cycle, it was 6·11 ± 0·36 fmol/mg protein in the preparatory stage which increased to about three-fold in prespawning stage of reproductive cycle (17·0 ± 0·29 fmol/mg protein) and spawning (18·7 ± 0·17 fmol/mg protein) and lowest in postspawning stage of reproductive cycle (5·28 ± 0·28 fmol/mg protein). Fluctuation in the number of gonadotropic hormone binding site at different stages of annual reproductive cycle was found to be coincided well with the pattern of ovarian steroidogenesis in response to Catla gonadotropic hormone as determined by the formation of progesterone from pregnenolone.