To investigate whether GH secretion in acromegaly is subject to regulatory control by the hypothalamic GH-releasing hormone (GHRH) we studied GH secretion in 22 patients with acromegaly. Parameters of pulsatile GH secretion were assessed using frequent blood sampling (every 20 or 10 min for 24 h). Acute GH responses to GHRH-44 (0.1, 0.33, and 1.0 micrograms/kg BW, iv) were measured, and GH secretion during therapy with the long-acting somatostatin analog SMS 201-995 (Sandoz) was assessed. The results were compared to those in normal volunteers. Spontaneous GH pulse frequency was greater in patients with acromegaly than in 6 control subjects (8.6 +/- 0.6 vs. 4.3 +/- 1.1 pulses/24 h), as estimated by the 20-min sampling frequency. The 10-min sampling frequency revealed 12.9 +/- 0.7 pulses/24 h in acromegalics. Spontaneous GH pulse amplitude and acute GH rises in response to GHRH did not differ between control and acromegalic subjects. A similar degree of nocturnal augmentation of GH secretion was observed in both groups, and it persisted during SMS 201-995 therapy in patients with acromegaly. These observations suggest that GH secretion in acromegaly remains under stimulatory control by GHRH, which may be released at an abnormally high rate.

Glutamine is synthesized in skeletal muscle, released to the circulation, and transported to other tissues, where it may provide important substrate for gluconeogenesis, ammoniagenesis, and energy-yielding pathways. With the ultimate goal of delineating the factors that control glutamine production and release by skeletal muscle, we have studied the regulation of two key enzymes, glutamine synthetase and glutaminase, in the L6 line of rat skeletal muscle cells grown in monolayer culture. The cultured myotubes were found to have glutamine synthetase and phosphate-dependent glutaminase activities. Glutamine synthetase activity was increased following incubation (1) in glutamine-free medium (threefold); (2) in medium containing high glutamic acid concentrations (fourfold); and (3) in medium supplemented with dexamethasone (threefold). In each case the increase in glutamine synthetase activity required several hours to reach a maximum and was prevented by cycloheximide, suggesting that the change occurred through increased enzyme biosynthesis. No substances tested were found to affect glutaminase activity. We conclude that glutamine synthetase in cultured skeletal muscle is responsive to substrate, product, and hormonal regulation.

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