Pancreatic insulin responses to exogenous glucose in Friesian heifers of low or high genetic merit for milk-fat yield

Abstract

Two studies were conducted to examine the effects of selection for milk-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 (HBI) 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 controlled 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 fig/kg live weight) were administered via indwelling jugular cannulas and were followed by serial blood sampling to measure responses. Plasma insulin concentrations immediately following the glucose challenge were significantly (P < 0-05) greater in HBI than in LBI heifers, peak insulin concentrations (above baseline) in the HBI group being approximately 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 the exogenous insulin challenge significantly reduced circulating glucose concentrations in all heifers, there was no difference between the lines in the magnitude of insulin-induced hypoglycaemia.The second study was designed to determine whether insulin responses to an exogenous glucose load were influenced by stage of the oestrous cycle in heifers of the two lines. In this study, eight yearling HBI heifers and eight 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 tivo occasions, the first being after 17 days of CIDR treatment (simulating the luteal phase of the oestrous cycle) and the second 46 h after withdrawal of the CIDRs (when the heifers were expected to be in oestrus). HBI heifers again showed significantly greater circulating insulin 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 milk-fat yield have an increased pancreatic response to exogenous glucose which is apparently independent of the stage of the oestrous cycle. This difference may provide a useful means of identifying heifers of superior genetic merit for milk-fat production prior to their first lactation.

Application of new technologies in sheep breeding

“…Blair et al (1990) and Baker et al (1990) suggested that metabolic or physiological phenotypes could be used as indirect predictors of genetic merit to significantly improve rates of genetic gain. However, despite significant research effort (see for example, Blair & Lee 1997;Xing et al 1993;Blair et al 2002), none of these types of measures have been applied in any livestock industry in New Zealand. Recently, Primegro™ have begun offering a test for plasma levels of insulin-like growth factor-1 (IGF-1) to Australian beef cattle and pig breeders (www.primegro.com.au/html/fr_primeg-ro_igf.htm).…”

Use of Plasma Metabolite Concentrations after Glucose Injection to Predict the Genetic Ability of Milk Production in Young Calves

“…The insulin responses to exogenous glucose in the animals from a high breeding index line were much stronger than those of the animals from a low breeding index line 24 . Mackenzie et al 7 reported that during the challenges of both glucose and insulin, the clearance of blood glucose was most rapid in the calves with a high breeding index than in those with a low breeding index.…”

“…The sensitivity of the peripheral tissues to insulin was higher in the calves with a high breeding index than in those with a low breeding index. Therefore, the insulin response to an exogenous glucose load can be used as a metabolic marker to measure the genetic merit of milk production 24 . Glucose showed a highly positive genetic correlation with milk yield.…”

“…Glucose and insulin are homeostatic regulators of the lipid metabolism in the adipose tissue 2 . Xing et al 24 reported that the insulin response to an exogenous glucose load could be used as a metabolic marker to predict the production performance. Fast tends to depress the amount of insulin released in response to the glucose load and results in a reduction of glucose tolerance 3 .…”

Relationship between plasma hormone and metabolite levels and breeding values of bulls1

“…X ing et al (1993) found that plasma insulin concentrations after glucose challenge were significantly greater in high‐index heifers than in low‐breeding‐index animals. L ukes et al (1989) investigated bGH, insulin, prolactin, FFA, and glucose plasma levels in Holstein cows of differing genetic merit after administration of either adrenaline or bGH releasing factor.…”