Interrelation between Somatostatin Output from the Mediobasal Hypothalamus and Plasma Growth Hormone Levels in Conscious Rats: Effects of Glucagon Administration*

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Abstract
The interrelationship between SRIF [somatostatin] output from the mediobasal hypothalamus [MBH] and plasma GH [growth hormone] levels was studied in conscious male rats using the push-pull perfusion technique in combination with repeated blood samplings. The MBH was perfused with artificial CSF at the rate of 30 .mu.l/min, and blood samples were collected every 20 min from 1000-1700 h. In control animals, which received injection of acidified saline as 1100 h into the lateral ventricle, 2 large episodes of spontaneous GH secretion occurred regularly at around 1200 and 1540 h, and troughs occurred around 1400 h. In contrast, SRIF levels from MBH perfusate fluctuated at random, ranging from 10-116 pg/ml, with a mean value of 39.2 pg/ml. Mean SRIF levels at 1200 and 1540 h (43.4 .+-. 9.0 and 24.4 .+-. 4.2 pg/ml, respectively; n = 8) were not different from those at 1400 h (39.9 .+-. 12.2 pg/ml). When glucagon (10 .mu.g/rat) was injected at 1100 h, plasma GH levels decreased and remained low until 1600 h, whereas perfusate SRIF levels were elevated and remained high for the period. In these animals, the mean plasma GH levels during 1120-1540 h were lower than those in control rats [17.2 .+-. 2.4 ng/ml (n = 9) vs. 143.4 .+-. 17.5 ng/ml (n = 8); P < 0.01]. In contrast, the mean SRIF levels in glucagon-treated rats were higher than those in controls [112.5 .+-. 15.9 pg/ml (n = 9) vs. control 40.1 .+-. 4.3 pg/ml (n = 8); P < 0.01]. SRIF apparently plays a role in tonic inhibition of GH release in response to the intracerebroventricular injection of glucagon in conscious rats, although SRIF plays, if any, a minor role in regulating episodic GH secretion.