Seasonal variations in hepatic and ovarian zinc concentrations during the annual reproductive cycle in female channel catfish ( Ictalurus punctatus)

Abstract

Seasonal variations in hepatic and ovarian zinc concentrations were studied during the reproductive cycle in female channel catfish ( Ictalurus punctatus). The gonadal somatic index (GSI) increased dramatically in April and peaked in June prior to spawning. Exogenous vitellogenesis was initiated in the fall as noted by increases in serum estradiol and testosterone and liver somatic index (LSI). Total hepatic zinc and zinc in the cytosolic high-molecular weight (HMW) and metallothionein-like (MT) fractions were elevated in March but decreased during rapid vitellogenic oocyte growth in April and May. Following spawning in July, total hepatic zinc and zinc in the HMW and MT-like fractions were again elevated. Total ovarian zinc and zinc associated with HMW and MT-like fractions increased with GSI, then decreased to a low after spawning. However, on a per g tissue basis, ovarian zinc concentrations in both cytosolic pools declined during rapid oocyte growth, indicating a different intracellular localization of the zinc binding proteins. Hepatic MT-like proteins exhibit UV absorption profiles similar to mammalian MT while the ovarian proteins appear to be different. Results give evidence for the homeostatic regulation of hepatic zinc by MT during exogenous vitellogenesis and a similar function for the ovarian MT-like protein during oocyte development.