Plasma osmotic variations during developmental stages of the Caspian Kutum oocyte (Rutilus frisii kutum)

In the present study, plasma osmotic variations including Na+, K+, Ca++ ions, osmolarity and total protein were analyzed during developmental stages of oocyte in kutum (Rutilus frisii kutum). Also, changes of oocyte diameter was investigated during these stages. Sampling of kutum was performed monthly from catches along southern Caspian Sea (Anzali coast) and rivers ending to the sea (because of anadromic migration of the fish) during reproductive period during October 2007 to May 2008. Blood samples were immediately collected from caudal vessel. The Flame photometer, Multi analyzer, and Osmometer sets were used to measure Na+, K+ ions, Ca++ and osmolarity respectively. Total protein was measured by Bradford method. Based on histological observations, the oocyte development divided into six stages: 1) primary oocyte growth, 2) perinucleolar, 3) cortical alveolus, 4) vitellogenesis, 5) maturation, 6) spent. The results indicated that the most decreasing changes of Na+, K+ and osmolarity occurred concurrent with hydration process in maturation stage. The minimum value of Na+ (12010.12 mEq/l), K+ (0.450.08 mEq/l) and osmolarity (238.927.72 milliosmol/l) corresponding with maturation stage were measured in which showed significant difference comparing with earlier stages of oocyte development. The changes of total protein and calcium showed high positive correlation (R=0.88) in which there was an ascending trend although without a significant difference (P>0.05) from cortical alveolus until primary maturation stage. In final maturation (15 April sampling) and spent stages the changes of total protein and calcium were significantly dominated by a descending trend (P<0.05). In general, due to presence of demersal eggs (benthophil) in Kutum, hydration process occurred at lower than marine teleost possessing pelagic eggs (pelagophil) in maturation stage. The vitellogenesis function was responsible for main increment of oocyte diameter during ovarian growth. In addition, analysis of data showed K+ and Na+ played a major and minor role respectively during hydration.