Somatomedin C and other hormones, as well as blood metabolites, were measured during the dry period and during lactation in dairy cows, given different amounts of energy and protein, to study metabolic and endocrine adaptations. Somatomedin C, specifically measured by radioimmunoassay after separation from its binding protein, did not exhibit typical diurnal variations, in contrast to somatotropin and insulin, which increased particularly after concentrate intake. Somatomedin C markedly decreased at parturition and reached lowest values around the peak of lactation, while levels of somatotropin, nonesterified fatty acids and ketone bodies were high and those of glucose, insulin, thyroxine and triiodothyronine were low. Thereafter somatomedin C values slowly increased up to the 12th week of lactation and remained elevated. Low energy and protein balances were characterized by particularly low somatomedin C concentrations. An additional protein deficit at peak lactation, when cows were already provided with low amounts of energy, did not further decrease somatomedin C levels. However, when high amounts of energy were given in the form of starch or crystalline fat, somatomedin C increased. Overall, there was a positive correlation of somatomedin C primarily with energy, but also with protein balances and a negative correlation with milk yield. Conversely, somatotropin increased markedly after parturition and was positively correlated with milk production and negatively with protein and energy balances. Thus, somatomedin C levels were paradoxically low in the presence of high circulating somatotropin. Insulin most closely paralleled somatomedin C levels. Therefore the anabolic state of metabolism at the end of pregnancy was characterized by high somatomedin C and insulin and relatively low somatotropin, whereas the catabolic state of early lactation was characterized by high somatotropin, low somatomedin C, insulin and thyroid hormones.

Zusammenfassung Die Blutplasmakonzentration von Somatomedin C und anderen Hormonen bei Milchkühen um die Geburt beim Kalb und in der Milch Die Milchkuh ist während der Umstellung am Ende der Trächtigkeit auf die Laktation charakterisiert durch einen Abfall der Blutplasmakonzentrationen von Somatomedin C, Insulin, Trijodthyronin, Thyroxin, Glukose (mit einem leichten Anstieg bei der Geburt), Harnstoff und einem Anstieg von Somatotropin und Freien Fettsäuren. Die erste Kolostralmilch enthielt, verglichen mit dem Blutplasma, viel größere Mengen an Somatomedin C (vierfach), Insulin (zwanzigfach), und Trijodthyronin (fünffach). Innerhalb von zwei Tagen fielen die Konzentrationen dieser Hormone auf die Höhe der Blutplasmakonzentrationen ab. Somatotropin und Thyroxin waren meist nicht nachweisbar. In neugeborenen Kälbern waren die Konzentrationen von Somatomedin C, Glukose und Albumin tief, aber die von Trijodthyronin, Thyroxin und Freien Fettsäuren hoch. Die Gehalte von Somatomedin C stiegen an, währenddem diejenigen der Schilddrüsenhormone absanken. Unsere Untersuchungen zeigen deutliche endokrine und metabolische Änderungen, die mit dem Einsetzen der Laktation im Zusammenhang stehen. Hohe Gehalte von Somatomedin C und Insulin in der Milch sind möglicherweise die Konsequenz von aktivem Transport vom Blut in die Milch und/oder von einer Eigensynthese von Somatomedin C des Euters. Die Übertragung von Somatomedin C und Insulin von der Kuh auf das Kalb durch das Kolostrum könnte wichtig sein für die Initialisierung von Wachstumsprozessen.

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Abstract. Our objective was to measure relative amounts and distributions of serum IGF-I binding proteins and concentrations of hormones and metabolites in dairy cows during the dry period and during lactation, as well as in calves and in growing bulls. Concentrations of IGF-I were lower in cows during lactation than during the dry period. Concentrations of IGF-I, growth hormone, insulin, T3, T4 and glucose were higher in calves and bulls as compared with cows, whereas concentrations of non-esterified fatty acids, urea, protein and albumin were lower. Three different IGF-I binding protein fractions were found with apparent molecular weights of >200, 140–160 and 45–65 kD. Relative amounts of IGF-I binding proteins were similar, although great differences in hormones and metabolites were found in cows, bulls and calves. Accessible binding sites were higher in dairy cows than in calves and bulls, mainly owing to the fact that dairy cows had lower concentrations of IGF-I bound to these fractions. Additionally, we found significant negative correlations between total accessible binding sites and total IGF-I concentrations. There were significant differences in the distribution of binding proteins. In particular, there was a shift of binding ability from the 140–160 to the 45–65 kD binding sites from the end of pregnancy to early lactation. Amounts of IGF-I bound to proteins increased from lactating to dry cows and calves and were highest in bulls. IGF-I levels in the >200 kD fraction was lower in lactating than in dry cows, and lower in cows than in calves and bulls. IGF-I measured in the 140–160 kD fraction was lowest in lactating cows and comparable in dry cows, calves and bulls. Concentrations of IGF-I in the 45–65 kD fraction were lowest in cows and highest in bulls. In conclusion, there were marked differences in growing, pregnant and lactating cattle, particularly as concerns accessible protein binding and amounts of IGF-I bound to proteins.

Insulin-like growth factor I, other hormones and blood metabolites were measured in growing heifers before, during and after a 3-day period of normal feed intake and a corresponding period of reduced feed intake. In addition, 0.1 or 0.5 mg recombinant bovine GH/kg was injected daily for 5 days during normal or during and following reduced feed intake. During reduced feed intake blood concentrations of insulin-like growth factor I, insulin, T4, T3, glucose and urea-nitrogen decreased, whereas those of non-esterified fatty acids, albumin and protein increased (P< 0.05). GH, insulin-like growth factor I and insulin increased, whereas urea-nitrogen decreased in response to exogenous GH when heifers were adequately fed (P< 0.05). In contrast, insulin-like growth factor I did not change during GH injections while heifers received reduced amounts of feed. Therefore, during insufficient energy and (or) protein intake, characterized by low glucose, insulin and thyroid hormone levels and increased non-esterified fatty acid concentrations, insulin-like growth factor I concentrations and responses to GH administration were markedly reduced.