Effects Of Insulin-Like Growth Factor I On Regulating Postnatal Growth In Rats

PART ONE POSTNATAL GROWTH PATTERNS IN RATS【Objective】To identify postnatal growth patterns, especially linear growth patterns in rats via measuring body weight, body length and tail length from birth to 27 days of life in rats.【Subjects and Methods】Ten-litter neonatal SD (Sprague-Dawley) rats were included, eight rats of each litter were kept for the study. Body weight, body length and tail length were measured daily from birth to 27 days of age. Due to unexpected death observed, extra-nutrients were supplemented to lactating rats. Data used in analyses were adjusted according to the time at birth and exact measuring time points. All analyses or figures were performed or generated by using the SAS system v9.0.【Results】(1) After lactating rats receiving nutrient supplementation, no death happened, and growth was improved, especially body weight growth in young rats. (2) Body weight, body length and tail length in rats were significant difference between gender and among litters at birth. Males were heavier and longer than females. The differences were 0.335 g (t=-4.17,P=0.0002) and 0.114 cm (t=-3.03,P<0.005), respectively. For tail length, females were longer than males, on average, 0.043 cm (t=2.15,P<0.04). (3) Based on the growth curves produced using our continuous growth data from birth to 27 days of life in rats, obvious growth velocity increases were observed at different age points. Body weight, body length and tail length were increasing with age. Body weight growth velocity started to increase from 2 days of age and apparent increases were observed at the ages of 3, 15, 25, and 27 days. Body length growth velocity started to increase slowly from the age of 8 days, obvious increases at the ages of 17 and 25 days. Tail length growth velocity increase initiated from the second day of life, and remarkable increases at the ages of 3, 7, and 18 days. After 22 days of age, tail length velocity did not change much. (4) According to the characteristics of our data, paired t tests were employed to identify significant changes of growth velocities between two consecutive age points, i.e. velocity increases. At the age of 3 days, the growth velocities of body weight and tail length significantly increased (P<0.05), while body length growth velocity did not. Statistically significant growth velocity increases were identified at the ages of 25 and 27 days for body weight, and 18 days for tail length as well. For body length, significant increases of growth velocity occurred at the ages of 17 and 25 days.【Conclusions】(1) Body weight in rats is a sensitive index to nutrition. Tail length growth is not as much affected by nutrition as body weight and body length. (2) Postnatal growth patterns of body weight, body length and tail length in rats are different. The earliest growth velocity increase occurs in tail length. (3) The prepuberty of SD Rats may be at the age of 18 days according to tail length growth, and 25 days of age to body weight and body length. (4) The onset of childhood growth in rats mightoccur around 3 days of life. PART TWO EFFECTS OF INSULIN-LIKE GROWTH FACTOR I ON REHULATING THE ONSET OF CHILDHOOD IN RATS【Objective】To explore the roles of IGF-I on the onset of childhood growth in rats.【Subjects and Methods】Thirty litters were included. Eight newborn SD rats from each litter served as the study subjects. Body weight, body length and tail length were measured daily as part one. One rat was randomly selected from each litter and killed for collecting blood samples after finishing anthropometric measurements from birth to 7 days of age. The plasma IGF-I level and cycle of chondrocytes in rats were determined by methods of Enzyme Linked Immunosorbent Assay (ELISA) and flow cytometry, respectively. The heights of chondrocyte proliferative zone and hypertrophic zone in growth plate were measured using Simple PCI image analysis software. The concept for data adjustment was the same as part one if applicable. SAS system v9.0 was used for data analyses and presentations.【Results】(1) Body weight, body length and tail length were increasing during the study period, i.e. the first 7 days of life. Growth velocities were reaching the maxima at the age of 5 days for all three measurements. The velocity of body weight began to increase quickly at the age of 2 days. The velocity of Body length began to increase at the age of 3 days. For Tail length, the velocity began to increase slowly at the age of 2 days. (2) The changes of body weight growth velocity were statistically significant from 3 to 6 days of age; however, the magnitude of velocity change was negative at the age of 6 days, i.e. velocity significantly decreased. Significant body length growth velocity changes were observed at the ages of 4, 6 and 7 days, the value was positive at 4 days of age, but negative at 6 and 7 days of age. Tail length growth velocity was significantly increased at the ages of 3 and 5 days. (3) The plasma IGF-I levels were increasing as body measurements during the first week of life in rats. Increases at the ages of 3, 4, and 6 days were obvious. However, statistically significant increases were identified at the ages of 4 and 6 days only. (4) The relationship between IGF-I and tail length at birth was positively correlated (P<0.02). (5) During the first 7 days of life, IGF-I and body weight, body length, tail length were positively correlated in rats, and the correlation coefficients ranged from 0.34 to 0.40 (P<0.0001). IGF-I was only positively correlated to velocity of body weight (r=0.16, P=0.02). The changes of IGF-I and body weight, velocities of body weight, body length were positively correlated as well, correlation coefficients 0.15 (P=0.02), 0.16 (P=0.01), respectively. (6) Body weight, body length, and tail length were positively correlated. (7) The proportions of chondrocytes at G0-G1 stage in proximal tibia increased gradually, while the proportions of chondrocytes at S stage decreased with age during the 7-day study period. (8) The sizes of growth plate hypertrophic chondrocytes in proximal tibia obviously increased at the ages of 3, 4, 6 days. The height of growth plate hypertrophic zone increased gradually and paralleled to the changes of IGF-I. The height of growth plate proliferative zone was decreased gradually. (9) The height of growth plate hypertrophic zone was correlated with IGF-I (r=0.35, P=0.01), and significantly correlated with body sizes, correlation coefficients 0.73 to 0.81 (P<0.0001). (10) Various correlation patterns among length and weight of tibia, height and width of epiphysis in proximal tibia, body sizes, and heights of hypertrophic and proliferative zones were observed.【Conclusions】(1) The onset of childhood is verified to be around 3 days of life in rats. Robust evidence comes from plasma IGF-I secretion in rats as well. The onset of childhood in rats may be regulated by IGF-I. IGF-I might be one of the main factors regulating postnatal linear growth in rats. The switch of linear growth regulatory mechanism might occur at the age of 3 days as well. (2) It is not a valid measurement for assessing the growth of growth plate chondrocytes by cell cycle during early life of age due to under perichondrium growth dominating cartilage growth in epiphysis during this age period. (3) There are close relationships between the growth plate hypertrophic chondrocytes in proximal tibia and the growth of long bone, linear growth.