In vivo ovarian uptake and processing of vitellogenin in the rainbow trout, Salmo gairdneri

Abstract

AbstractThe in vivo uptake and processing of vitellogenin (VTG) into developing oocytes of rainbow trout were studied. Oestrogenized trout injected with 32P and 3H‐leucine incorporated these labels into newly synthesized vitellogenin. This dual‐labelled VTG was purified from the serum and injected intraperitoneally into sexually maturing female trout. The 32P. 3H‐VTG was readily taken up from the peritoneal cavity into the blood, without degradation, and thus became available for sequestration by developing oocytes. Ovarian uptake of 32P. 3H‐VTG was very rapid. Eight h after injection, approximately half of the labelled VTG was in the blood, but this figure had fallen to 12% after 48 h, and to 0.2% after 2 weeks. This fall in blood 32P. 32H‐VTG level coincided with increases in the level of both radiolabels in the ovary. Eight hours post‐injection, a significant amount of 32P. 32H‐VTG was found in the ovary homogenate. Most of this material was intact, primarily representing VTG bound to the follicle surface. However, small amounts were contained within the follicles and probably represented newly sequestered VTG which had not yet been degraded into the yolk proteins, lipovitellin and phosvitin. Twenty‐four hours post‐injection, approximately 25% of the injected 32P. 3H‐VTG had been sequestered by the ovary. Although a small proportion of the labels were associated with intact VTG, the majority of the 3H and 32P eluted with lipovitellin and phosvitin, respectively, the yolk proteins derived from VTG. After 2 weeks, nearly 70% of the 32P. 3H‐VTG initially injected had been incorporated into the ovaries, all of which was present as labelled yolk proteins. The rate of uptake of VTG was highest in the fish with the largest follicles, where it approached 120 ng.mm2 follicle surface h−1. The results demonstrate that both the uptake and processing of VTG by trout oocytes are rapid features that are reflected in the rate of ovary growth seen during exogenous vitellogenesis.