Blood metabolite responses to catecholamine injections in heifers of high or low genetic merit for milkfat production

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Abstract
Responses to acute intravenous injections of adrenalin and noradrenalin were examined in two groups of heifers (4 heifers/group) differing in predicted genetic merit by a mean of 33 breeding index units. A split-plot design was used in which the heifers were paired, one high breeding index and one low breeding index, for main plot comparisons between breeding index groups. Subplot treatments (four doses: 0, 0.25, 0.5, and 1 µg/kg bodyweight of noradrenalin; and four times of challenge: 1000 hand 1500 h on two consecutive days) were arranged in two 4 × 4 Latin Squares and applied to the four heifer pairs. The design was repeated on Days 3 and 4 using a second catecholamine, adrenalin. The concentrations of glucose and non-esterified fatty acids were measured in plasma collected at intervals (− 15, − 1, 5, 10, 15,20, 30, 45, 60, and 120 min) from treatment. Samples following the highest dose of each catecholamine were also analysed for insulin and 3-hydroxybutyrate. Glucose response to adrenalin injection (1 µg/kg bodyweight) was significantly greater in the high breeding index (233 ± 22 mg.min/dl) than the low breeding index group (168 ± 10 mg.min/dl). No difference in plasma non-esterified fatty acids response to adrenalin was observed between the two breeding index groups. There were no significant differences in plasma glucose or non-esterified fatty acids responses to noradrenalin over the whole range of doses but at the highest dose glucose release in response to noradrenalin was greater in the high breeding index group (P < 0.05). Plasma insulin and 3-hydroxybutyrate responses to the challenges were variable and no significant differences were found between the breeding index groups. Glucose responses to adrenalin and noradrenalin challenges appear to offer a means of differentiating between yearling Friesian heifers of high and low genetic merit.