Comparative plasma lipid response of pullets and laying hens to estradiol and progesterone

The effects of estradiol and progesterone treatment on plasma hormone and lipid concentrations were measured in laying hens and sexually immature pullets. Pullets and hens were divided into three groups and injected with estradiol (1 mg/kg bw), progesterone (4 mg/kg bw), or vehicle (propylene glycol) once each day for 14 days. Blood samples were collected before treatment and 24 hr after the 7th and 14th treatments. Plasma progesterone (P), estradiol (E2), triglycerides (TG), cholesterol (C), and phospholipids (PL) were measured. E2 treatments elevated hen plasma TG 7.2X, PL 5.1X, and C 7.2X; and pullet plasma TG 6.8X, PL 3.7X, and C 2.5X. However, because hen plasma was initially mildly hyperlipidemic, the E2-treated hens developed severe hyperlipidemia, but egg production was unaffected. Progesterone treatments of pullets had little or no effect on plasma lipids, but progesterone treatment of hens significantly reduced initial plasma TG and PL and also reduced egg production. No substantial differences were found in circulating E2 or P in hormone-treated hens and pullets, which indicated no extreme differences in plasma hormone clearance rates. These results indicate that long-term rather than short-term hepatic priming may account for observed differences in layer and pullet response to estradiol treatment.