ABSENCE OF ULTRADIAN RHYTHM OR DIURNAL VARIATION IN CIRCULATING SOMATOMEDIN-C (Sm-C) IN RATS

Abstract

We studied the variation in plasma Sm-C in unrestrained, cannulated rats. Six rats were sampled every 15 min for 6 hr, and 5 rats were sampled every 2 hr for 36 hr. Plasma was assayed for Sm-C (RIA) before and after acid-ethanol extraction (AE), and for growth hormone (GH). AE increased the immunoreactivity of Sm-C in heparinized rat plasma and in freshly-collected serum 3-fold AE had no effect on serum stored at −20° C. >4 wks, which had comparable potency to AE serum and AE plasma. In rats sampled every 15 min, GH showed high amplitude, light entrained pulses every 3.3 ± 0.15 (SEM) hr. By visual inspection, there were no pulses of Sm-C. We further analyzed the data with 2 computer algorithms designed to detect hormone pulses. Of the 5 Sm-C pulses identified by both programs in unextracted plasma, only 1 was also identified in the corresponding AE plasma. When both pulse detection methods were applied to the data from both unextracted and AE plasma from all 6 rats, no pulses were identified in 10 of the 24 series of measurements. In 19 identical dilutions of a single serum sample, one program identified 2 “pulses,” and the other, none. In rats sampled every 2 hr for 36 hr, there was no episodic fluctuation of Sm-C. Thus, there is little evidence for episodic release of Sm-C into blood, either as pulsations entrained to light or to feeding cycles. Sm-C measurements made on single blood samples from normal animals reliably reflect Sm-C concentrations over the course of 24 hr.