Exploring endocrine GH pattern in mice using rank plot analysis and random blood samples

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Abstract
GH plays important pleiotropic roles in development, growth, metabolism, and aging of vertebrate species. Mouse mutants with altered GH signaling have been increasingly instrumental in studying somatotropic pathophysiology. However, the pulsatile characteristics of GH secretion are difficult to study in mice because catheterization is cumbersome and long-term serial sampling is limited by small body size and blood volume. We therefore developed an approach routinely applicable to mice, which detects endogenous, physiological GH pattern from randomly obtained spot samples. We determined individual hormone concentration in large groups of mice, ranked the data by magnitude, and statistically analyzed the resulting profiles. This revealed that the nadir-to-peak distribution of plasma GH concentration in mice was similar to other mammals, and that nycthemeral and sex differences existed as well. We found handling stress to be a potent immediate downregulator of circulating GH. We showed that samples need to be taken within seconds to reflect true endogenous levels, unaffected by stress. GH receptor/Janus kinase 2/signal transducer and activator of transcription 5 activation measured in the liver correlated strongly with plasma GH levels, but peak concentrations did not further increase the pathway activation. We applied this rank plot analysis to the GH-deficient and long-lived brain-specific IGF-1 receptor knockout (bIGF1RKO+/−) mouse mutant and found a high proportion of low GH concentrations, indicative of extended trough periods and rare peaks. Taken together, we showed that rank plot analysis is a useful method that allows straightforward studies of circadian endogenous GH levels in mice.