Plasma somatostatin level during natural and interrupted nocturnal sleep in man

The plasma levels of somatostatin (SRIF), growth hormone (GH) and thyrotrophin (TSH) in 4 normal men were measured during nocturnal sleep on 6 nights. In the waking state, the fluctuation of plasma SRIF level was within +/- 2.1 pg/ml (SD) of the mean value for each individual (range of mean value, 10.7 pg/ml-15.4 pg/ml). A marked episodic surge of GH release (peak value, 24.4 +/- 6.3 ng/ml (mean +/- SD] was noted in the initial period of slow wave sleep (SWS) in all subjects, but no significant negative correlation was observed between SRIF and GH in the plasma during either increase or decrease in the GH surge. There was no relation between the distribution of the plasma SRIF concentration and the stage of sleep, including the period before sleep, judged by electroencephalography (EEG). A small, but significant, increase of plasma TSH (10.0 muU/ml) independent of the SWS-related GH burst was detected in 1 of 4 subjects, but in that period the plasma SRIF level did not change. Even when the GH surge was suppressed by interruption of SWS, the plasma levels of SRIF and TSH did not show any significant change. From these findings, it is concluded that SRIF in the peripheral plasma does not fluctuate during the sleeping period as in the waking state and that, whatever its source, it is not related to the plasma GH or TSH level in the systemic circulation.