Relationship of growth and serum growth hormone concentration in the prepubertal labrador bitch

Body weights of purebred Labrador Retriever bitches were recorded from birth through 40 weeks of age. Blood samples were collected twice weekly between 6 and 26 weeks of age. Samples were analyzed for serum growth hormone concentrations using a validated growth hormone radioimmunoassay. From a mean birth weight of 0.46 kg, the animals reached their mean adult body weight (24.8 kg) by 40 weeks of age. Growth rate showed a sigmoid response curve with age. Growth rate was linear between 4 and 26 weeks having a correlation coefficient of 0.96, a slope of 0.98, and an intercept of -2.28. Concentrations of growth hormone during the same period exhibited a bimodal distribution. From 8.3 +/- 0.6 ng/ml at 7 weeks, mean growth hormone concentration steadily declined reaching 1.8 +/- 0.4 ng/ml at 12 weeks. There was a secondary increase to 9.7 +/- 0.6 ng/ml at 20 weeks and a subsequent decline to 6.4 +/- 0.9 ng/ml at 26 weeks. The mean percent growth increment (defined as percent weight increase per week over previous body weight) was calculated to study actual growth. The growth increment was 34% at 6 weeks, 14.8% at 12 weeks, 15.1% at 16 weeks, and 3.5% at 26 weeks of age. Once hourly blood samples for 6 hours were collected at 10 and 24 weeks of age from six animals to determine the episodic release pattern of growth hormone and evaluate possible stress from venipuncture. Growth hormone concentrations ranged from 2.0 to 23.0 ng/ml in individual animals, with at least one episodic release of growth hormone detected within this time. Venipuncture did not stimulate any rise in growth hormone.