Variations in hepatic estradiol-17β receptor concentrations during the annual reproductive cycle of diploid and triploid female catfish, Heteropneustes fossilis (Bloch)

Concentrations of hepatic estradiol-17β (E2) receptors (ER) in cytosolic and nuclear fractions were evaluated in diploid and triploid female catfish Heteropneustes fossilis (Bloch) during four different reproductive periods of a complete reproductive cycle. Basal level of ER concentration was noted in the resting period of both diploids and triploids. Receptor level gradually elevated through the preparatory period and reached a peak in the pre-spawning period in both diploids and triploids. However, ER concentrations were overall reduced in triploid to that of diploid females. In a single point assay, in diploids, ER concentration showed about a 3-fold rise (p < 0.001) in the cytosolic and a 4-fold rise (p < 0.001) in the nuclear extracts from resting to the pre-spawning period. In triploids, only a 2-fold rise was observed both in cytosolic (p < 0.01) and nuclear (p < 0.05) ER concentration during the same span. Finally, a sudden fall of receptor level was observed in the spawning period in both the ploidy groups with a lower concentration in the triploids. The Kd value did not differ between the females of diploids (cytosolic 1.12 ± 0.21 nM and nuclear 6.9 ± 0.9 nM) and triploids (cytosolic-1.13 ± 0.17 nM, nuclear-6.8 ± 2 nM). However, Bmax of the diploid showed about double the value than triploid females both in the cytosolic (diploid-367.4 ± 33.24 pmol/mg protein, triploid-187.3 ± 13.20 pmol/mg protein, p < 0.001) and nuclear extracts (diploid-946 ± 66 pmol/mg DNA, triploid-558 ± 98 pmol/mg DNA, p < 0.01) of liver. Lower E2 binding capacity and lower amount of E2 receptors of triploid catfish liver with a stunted vitellogenic status could be one of the major causes for reduced gonadal development and sterility in female triploids.