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Eighty 4-month-old calves of both sexes and of two selected lines differing by 70 kg in their predicted total yield of milk fat and protein were injected intravenously with three of four GH secretagogues: these were, per kg liveweight; (i) 0·2 µg human GH-releasing factor(1–29) (GRF), (ii) 0·2 µg TRH, (iii) a combination of (i) and (ii), and (iv) 0·1 g arginine hydrochloride. The response of GH was measured for 2 h following administration. Geometric mean concentration of the 5-, 10-, 15- and 20-min samples following GRF, TRH and their combination were 29·3, 19·5 and 156 µg/l compared with baseline means of 6·5, 10·0 and 12·6 µg/l respectively, and for arginine (in which the mean response included the 30-min instead of the 5-min sample) 14·6 µg/l compared with a baseline of 8·31 µg/l. The line selected for greater yield responded more to each secretagogue by 1·53-fold following GRF (P < 0·01), 1·34-fold following TRH (P < 0·05), 1·11-fold following the combination (P > 0·01) and 1·26-fold following arginine (P < 0·1). Females responded 2·3-fold more than males following GRF administration (P < 0·001), only 1·2-fold more following TRH (P > 0·1), but less (0·63-fold) than males when GRF was combined with TRH (P < 0·05). For all secretagogues the concentration of GH before administration was important in determining the size of response (P < 0·001). It was concluded that the increased release of GH following the administration of GRF and TRH was a direct result of selection for dairy merit and that increased yields during lactation may, in part, be mediated directly through pituitary responsiveness.

Journal of Endocrinology (1991) 128, 419–424

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