The secretion and metabolic clearance rates of growth hormone, insulin and prolactin in high- and low-yielding cattle at four stages of lactation

Abstract

The basal plasma concentration, secretion rate (SR), distribution space (DS) and metabolic clearance rate (MCR) of GH, insulin and prolactin were measured in groups of 4 high- (HY) and 4 low-yielding (LY) cows at around 30, 90 and 150 days of lactation and after the cattle had been dried off. The kinetic parameters were determined using the single injection technique and measuring the disappearance of each hormone from the plasma by radioimmunoassay. Throughout lactation the DS for GH was higher in the HY cows (P<0.01) falling significantly in both groups when they were dry (HY, P<0.001; LY, P<0.01). There was no difference between the lactating group in the MCR of GH, but in both there was a significant (P<0.001) drop in the dry period which was accompanied by a fall in the SR (P<0.05). No differences were found in the DS and MCR of insulin between the groups during lactation but the average SR of insulin in the LY group was more than twice that in the HY group (P<0.05). Overall differences between stages of lactation were found in both groups for the SR (P<0.05), DS (P<0.01) and MCR (P<0.001) of insulin. None of the kinetic parameters differed for prolactin in the two groups. The DS (P<0.05) and MCR (P<0.001) of prolactin increased from day 30 to day 150 of lactation in both groups and then fell significantly (DS, P<0.05; MCR, P<0.01) when the animals were dry. In both groups the SR (P<0.01) and basal plasma concentration (P<0.01) of prolactin were higher in the dry period than at 30 and 90 days of lactation; probably due to the influence of season on the hormone.