Glucose regulation of growth hormone secretion in lactating sheep.

Abstract

Exogenous growth hormone stimulates milk secretion in all breeds of dairy animal [ I ] Thus an understanding of the mechanisms which regulating growth hormone secretion is vital if endocrine manipulation of dairy animals to improve milk production and composition are to become viable techniques in agriculture. The regulation of growth hormone secretion during lactation appears to differ markedly from that prevailing in the non-pregnant, non-lactating animal Growth hormone concentrations are elevated despite the high circulating concentrations of non-esterified fatty acids, which normally inhibit growth hormone secretion [2] In addition the normal positive correlation between growth hormone and IGF-I concentrations is no longer apparent [3] The elevated growth hormone concentrations do not appear to stimulate IGF1 secretion to the normal extent. A similar abnormal regulation of growth hormone secretion is observed in insulin-dependent diabetes [4,5] Here, too, the high circulating concentrations of non-esterifed fatty acids do not inhibit growth hormone secretion and circulating IGF-1 concentrations are low in view of the high growth hormone levels. In the diabetic subject hyperglycaemia and/or hyperinsulinaemia has been shown to stimulate growth hormone secretion by an, as yet, unidentified mechanism [5]. A similar stimulation of growth hormone secretion by glucose and elevated insulin has also been described in the sheep but these were of limited duration [6] Data are reported here which show the effects of longer infusions of glucose on plasma concentrations of insulin and growth hormone in lactating sheep which have been fed normally or undernourished by 24h starvation. Sheep at about 20 days postpartum (peak lactation) were infused vici a jugular catheter with sterile glucose at a rate of 12pmol/kg/min for 6h in fed sheep and 5h in 24h starved animals. Blood samples were taken via a second jugular catheter at regular intervals before, during and after inhsion Heparinised blood was centrifuged immediately and plasma stored at -20°C before analysis for glucose [6]and growth hormone [7]. Some blood was left to clot at room temperature and serum collected and stored at -20 C before analysis for insulin [6]. Plasma glucose concentrations rose significantly (p<O 00 1) by about 1.5 to 2 mM during glucose infusion in both groups of animals and remained elevated for the duration of the infusion (Fig I ) . Serum insulin concentrations rose significantly during the early stages of the infusions (p<O.OI and p<O 05 for fed and starved animals respectively) but started to return to basal values after about 2h (Fig 1) Plasma growth hormone concentrations increased rapidly and significantly (p<O 0 1) during the early stages of the infusion but then declined to return to basal values by about 2h. This response was significantly greater (p<O.OI) in the 24h starved compared with the fed group. The data reported here demonstrate that glucose inhsion results in a stimulation of growth hormone secretion as has been shown previously [8] However, this growth hormone response to glucose is relatively short-lived and is not maintained for the duration of the inhsion In this respect it mirrors the changes in serum insulin concentrations rather than those of glucose itself Thus the increase in growth hormone secretion may be better attributed to hyperinsulinaemia rather than hyperglycaemia. It is also apparent that changing insulin concentrations alone are not the sole regulator. Other factors are at work as the response is greatly accentuated in the 24h starved group compared with those on a normal feeding regime even though the increase in insulin is much reduced in this group. Thus energy balance also has a major role to play in regulating growth 24