Seasonal changes in plasma retinol-binding holoprotein concentration in Japanese quail (Coturnix coturnix japonica)

1. Seasonal changes in retinol-binding holoprotein (holoRBP) concentration in plasma of groups of male and female Japanese quail (Coturnix coturnix japonica) were examined over 18 months. 2. In Expts 1 and 2 the birds were maintained under natural lighting conditions and in Expt 3 under artificial-light photoperiods corresponding to the changing daylength at 56 degrees N latitude. All groups were at 18-20 degrees and received Superlayers' (Rank Hovis McDougall) pellet diet. 3. The mean plasma holoRBP concentration in all groups changed in an annual cycle with minimal values in September-October and maximal values in February-April, when daylength or light photoperiod increased to more than 10 h. 4. The group mean values in the female cycle change 2-3-fold from 50-100 microgram/ml in late summer to 220-280 microgram/ml in the spring, whereas in the male the range is only 1.3-1.5 times, from 140-170 to 180-250 microgram/ml. 5. In the female the rate of egg laying was maximal in April-May and lowest in November-December. 6. The spring increase in plasma holoRBP reflects the increased vitamin A requirement of birds for reproduction and it is presumably under hormonal control. The wider amplitude in the female cycle compared with the male probably arises from the additional demand for the transfer of vitamin A into the eggs and hence the need for a higher initial secretion rate from the female liver to meet it.