Effects of 17 beta-estradiol and carp gonadotropin on vitellogenesis in normal and hypophysectomized European silver female eel (Anguilla anguilla L.) employing a homologous radioimmunoassay for vitellogenin.

The European silver eels are at the early stages of vitellogenesis before the marine reproductive migration. Vitellogenesis was induced by 17 beta-estradiol (E2) alone and by a purified carp gonadotropin (cGTH). We studied and compared their effects on plasma vitellogenin (Vg) levels and ovarian yolk contents in female normal (N) and hypophysectomized (H) eels for both treatments. To this purpose an homologous radioimmunoassay (RIA) was established. Eel Vg was purified to homogeneity on 0.1% SDS-Electrophoresis. Native Vg has a molecular weight of 340 +/- 15 kilodalton (kDa) and was partially separated into subunits. The RIA was established with a sensitivity of 1.1 ng and was specific for eel Vg. In control (N and H) silver eels, plasma Vg levels were 0.04 +/- 0.02 microgram/ml and unchanged throughout the experiment. Similarly, yolk was indetectable in control ovarian extracts. E2 treatment increased plasma Vg levels proportionally with time to 783.4 +/- 130.7 micrograms/ml in N eels. The same profile was seen in H eels but terminally the mean value was 36.7 times lower than in N eels (P less than 0.01). Yolk at 0.005 microgram/g in N eels was indetectable in H eels. cGTH treatment gave a biphasic kinetic change: plasma Vg increased within 12 days, peaked at 93.6 +/- 13.0 micrograms/ml at 20 to 24 days, and stabilized to decrease at 40.2 +/- 7.5 micrograms/ml. The gonadosomatic index (GSI) increased alongside the yolk content (980.4 +/- 153.1 micrograms Vg/g). The kinetic profile for H eels was different: a peak was not apparent, rather there was a delayed increase, and at 67 days levels were still 8.23 times lower than in N eels (P less than 0.01). The GSI increased as the yolk content to 202.7 +/- 64.8 micrograms Vg/g ovary showing an ovarian incorporation of Vg in H eels.