Evaluation of growth hormone (GH) responses to pulsed GH-releasing hormone administration using the MTT-ESTA bioassay

We compared the immunoactivity of human growth hormone (hGH) with its bioactivity after stimulation of hGH release into the circulation by the administration of growth hormone-releasing hormone [GHRH(1-29)-NH2] according to a pre-determined protocol to four normal adult volunteers. We used the Hybritech immunoradiometric assay to measure the immunoactive GH concentrations. Bioactive GH concentrations were measured using the highly quantitative and precise eluted stain bioassay system (ESTA). The high sample capacity of the ESTA bioassay permitted us to monitor the bioactivities in closely timed sequential samples, and in far greater detail than has previously been possible. Two pulses of GHRH(1-29)-NH2 were administered intravenously to the four adult male volunteers (aged 24-37 years) on a weekly basis over a 4-week period. Two different doses of GHRH(1-29)-NH2 (0.1 and 1.0 micrograms/kg) were tested. These were separated by specified time intervals (60 or 120 min). Responses in the four individuals were variable. However, although the immuno- and bioactivities generally agreed well, there was a systematic and progressive increase in the bioactivity/immunoactivity (B/I) ratios as half of the response peaks were approached. After these peak concentrations, the B/I ratios subsequently returned to values that were close to unity. The enhanced bioactivity of the peak samples from the two volunteers in whom the largest magnitudes of response were observed was found to be labile after long-term storage at -20 degrees C. We suggest that the preferential rise in GH bioactivity, as opposed to immunoactivity, in response to GHRH(1-29)-NH2 was due to progressive changes in the concentrations of isoforms of GH that are not detectable in the Hybritech immunoassay.