Afferent input from rat slow skeletal muscle inhibits bioassayable growth hormone release.

The release of a bioassayable form of growth hormone (BGH), distinct from growth hormone as measured by immunoassay (IGH), from the rat pituitary into the blood is differentially regulated by afferent input from fast and slow skeletal muscles. Specifically, activation of low-threshold fast muscle afferents for 15 min increased plasma BGH by 217 and 295% and decreased pituitary BGH by 68 and 45% in male and female rats, respectively. In contrast, activation of slow muscle afferents inhibited BGH release, decreasing plasma BGH by approximately 60% and increasing pituitary BGH by 30-50% in male rats. Female rats from which food had been withheld for approximately 12 h had elevated basal plasma BGH levels, which then were decreased by 81% after slow muscle nerve stimulation. Plasma IGH concentrations were unchanged after any nerve stimulation condition. These results demonstrate that regulation of BGH release can be differentially mediated through low-threshold afferent inputs from fast or slow skeletal muscle. Furthermore, the results indicate that BGH responses are independent of gender or feeding status.