PANCREATIC INSULIN RESPONSES TO EXOGENOUS GLUCOSE IN FRIESIAN HEIFERSOF LOW OR HIGH GENETIC MERIT FOR MILK-FAT YIELD

Two studies were conducted to examine the effects of selection for mil k-fat yield on pancreatic responses to exogenous glucose, as measured by circulating insulin concentrations following an intravenous glucose load, in heifer calves. The first study involved 6-month-old Friesian heifers, eight each from the Massey University high breeding index (H BI) and low breeding index (LBI) lines. Average breeding indices (BI) for milk-fat yield of the calves based on ancestry BI were 136 (s.d. 2 ) and 112 (s.d. 2), respectively. Oestrous cycling of the heifers was controlled by intravaginal insertion of progesterone-impregnated contr olled internal drug-releasing devices (CIDRs). Challenges of glucose ( 0, 75, 150 and 300 mg/kg live weight) and insulin (0, 0.1, 1, and 10 m ug/kg live weight) were administered via indwelling jugular cannulas a nd were followed by serial blood sampling to measure responses. Plasma insulin concentrations immediately following the glucose challenge we re significantly (P < 0.05) greater in HBI than in LBI heifers, peak i nsulin concentrations (above baseline) in the HBI group being approxim ately twice those in the LBI group. Plasma glucose concentrations were marginally lower in HBI heifers, probably due to the secondary effect of the relatively greater hyperinsulinaemia in this group. Although t he exogenous insulin challenge significantly reduced circulating gluco se concentrations in all heifers, there was no difference between the lines in the magnitude of insulin-induced hypoglycaemia. The second st udy was designed to determine whether insulin responses to an exogenou s glucose load were influenced by stage of the oestrous cycle in heife rs of the two lines. In this study, eight yearling HBI heifers and eig ht LBI heifers (with BI of 135 (s.d. 4) and 111 (s.d. 1) respectively) were administered an intravenous glucose load (150 mg/kg live weight) on two occasions, the first being after 17 days of CIDR treatment (si mulating the luteal phase of the oestrous cycle) and the second 46 h a fter withdrawal of the CIDRs (when the heifers were expected to be in oestrus). HBI heifers again showed significantly greater circulating i nsulin concentrations than LBI heifers following the glucose challenge but the magnitude of this effect was not influenced by time relative to CIDR withdrawal. It is concluded that heifers selected for high mil k-fat yield have an increased pancreatic response to exogenous glucose which is apparently independent of the stage of the oestrous cycle. T his difference may provide a useful means of identifying heifers of su perior genetic,merit for milk-fat production prior to their first lact ation.