Muscle afferent activity modulates bioassayable growth hormone in human plasma

Immunoassayable and bioassayable growth hormone responses to vibration-indu ced activation of muscle spindle afferents of the soleus (Sol) or tibialis anterior (TA) muscles were studied in 10 men. Subjects were supine while a 10-min vibration stimulus (100 Hz; 1.5-mm amplitude) was applied to the mus cle, with each of the muscles tested on separate days. Blood samples were c ollected before, during, immediately after, and after 5 and 10 min of vibra tion. Plasma growth hormone concentrations were determined by radioimmunoas say (IGH) for all sampling periods and by bioassay (BGH; measurement of tib ial epiphysial cartilage growth in hypophysectomized rats) for samples obta ined before and immediately after vibration. Plasma IGH concentrations were similar at all time points during the Sol or TA experiments. After 10 min of muscle vibration, mean plasma BGH was elevated 94% [1,216 +/- 148 (SD) t o 2,362 +/- 487 mu g/l; P = 0.0001] for TA and decreased 22% (1,358 +/- 155 to 1,058 +/- 311 mu g/l; P = 0.09) for Sol. These data demonstrate that ac tivation of TA muscle spindle afferents increases circulating BGH but not I GH. The absence of a similar vibration-induced BGH response for the Sol ind icates a differential regulation of BGH release by these two predominantly slow muscles, perhaps related to their respective flexor and extensor funct ions. These data indicate that a muscle afferent-pituitary axis modulates t he release of BGH, but not IGH, from the pituitary in humans and that this axis is muscle specific, similar to that observed in rats.