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 t he administration of growth hormone-releasing hormone [GHRH(1-29)-NH2] according to a pre-determined protocol to four normal adult volunteer s. We used the Hybritech immunoradiometric assay to measure the immuno active GH concentrations. Bioactive GH concentrations were measured us ing the highly quantitative and precise eluted stain bioassay system ( ESTA). The high sample capacity of the ESTA bioassay permitted us to m onitor the bioactivities in closely timed sequential samples, and in f ar greater detail than has previously been possible. Two pulses of GHR H(1-29)-NH2 were administered intravenously to the four adult male vol unteers (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 mu g/kg) were tested. These were separated by specified time intervals (60 or 120 min). Resp onses in the four individuals were variable. However, although the imm uno- and bioactivities generally agreed well, there was a systematic a nd progressive increase in the bioactivity/immunoactivity (B/I) ratios as half of the response peaks were approached. After these peak conce ntrations, the B/I ratios subsequently returned to values that were cl ose to unity. The enhanced bioactivity of the peak samples from the tw o volunteers in whom the largest magnitudes of response were observed was found to be labile after long-term storage at -20 degrees C. We su ggest that the preferential rise in GH bioactivity, as opposed to immu noactivity, in response to GHRH(1-29)-NH2 was due to progressive chang es in the concentrations of isoforms of GH that are not detectable in the Hybritech immunoassay.