Pulsatility of Immunoreactive Somatomedin-C in Chronically Cannulated Rats*

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Abstract
Pituitary GH [growth hormone] secretion is pulsatile in man and the rat, but evidence of pulsatility in the GH-dependent somatomedins (insulin-like growth factors) has not been described. Serum immunoreactive somatomedin-C periodicity was examined in 10 chronically cannulated unstressed rats. Blood samples were taken at 15 min intervals over 6 h, and serum rat GH and somatomedin-C measured by RIA [radioimmunoassay]. For somatomedin-C assay samples were first extracted into acid-ethanol to dissociate protein-bound peptide. Serum GH levels indicated episodic secretion, with a frequency of 2.85 .+-. 0.24 h; some secretory episodes were polyphasic. The mean frequency of all GH spikes reaching 400 ng/ml or greater was 1.99 .+-. 0.87 h. Somatomedin-C levels showed fluctuations over an average 2-fold concentration range, 0.60 .+-. 0.20 to 1.21 .+-. 0.29 U/ml (mean, 0.86 .+-. 0.18 U/ml), with peaks occurring 1-1.5 h after most GH secretory peaks. The somatomedin-C peak frequency was 1.93 .+-. 0.47 h. Summed GH values from 0-5 h were significantly correlated with summed somatomedin-C values from 1-6 h (r = 0.861, P = 0.0007), suggesting a 1 h lag between GH pulses and the following rise in somatomedin-C. Somatomedin-binding protein showed no regular fluctuations. Serum somatomedin-C levels in unstressed rats show periodicity which may be directly related to pulsatile GH secretion.