Plasma calcitriol in chickens producing strong and weak egg shells and the response of hens to exogenous calcitriol.

Calcitriol (CAL) regulates intestinal calcium transport by inducing an increase in membrane phosphatidylcholine (PC) and calbindin. A positive correlation has been reported between shell gland PC and egg shell strength. Three experiments were conducted to test the hypothesis that the improvement in egg shell strength associated with increased PC is mediated via changes in the concentration of CAL. Sera from aged laying hens, identified as either strong (SES) or weak (WES) egg shell producers, were obtained from blood samples collected at 16 h after oviposition and assayed for CAL. Laying hens were injected (i/m) with 1.0 or 0.4 micrograms CAL/d for 28 d. Serum CAL in the WES hens was 70% of that in the SES hens (P less than .05). Hens producing extremely weak shells did not respond to 0.4 micrograms CAL/d for 10 d. Administration of CAL did not improve egg shell strength nor was serum CAL increased when WES hens were molted and shell gland PC increased. The results show that serum CAL is lower in aged hens producing weak egg shells but shell strength is not responsive to exogenous CAL and increased shell gland PC in WES hens after molting is not associated with higher serum CAL.