Plasma concentration of immunoreactive proopiomelanocortin peptides and cortisol in clinically normal cats

Abstract

We measured immunoreactive (IR) plasma concentrations of the proopiomelanocortin (POMC)-derived peptides (adrenocorticotropic hormone [ACTH]; β-endorphin/β-lipotropin [βEND/βLPH]; and α-melanocyte stimulating hormone [αMSH]) and of cortisol in 100 clinically normal cats. Median plasma concentration of IR-ACTH was 2.7 pmol/L (range, ≤ 1.1 to 22 pmol/L), of βEND/βLPH was 28 pmol/L (range, 3.8 to 130 pmol/L), of αMSH was 36 pmol/L (range, ≤ 3.6 to 200 pmol/L), and of cortisol was 35 nmol/L (range, 5 to 140 nmol/L). Plasma concentrations of IR-ACTH, αMSH, and βEND/βLPH were at or below the assay sensitivity in 34, 3, and 0% of the cats, respectively. We did not detect a correlation between plasma concentrations of IR-ACTH and βEND/βLPH (r = 0.23) or between plasma concentrations of IR-ACTH and αMSH (r = 0.19). However, there was a significant (P < 0.001) correlation between plasma concentrations of IR-βEND/βLPH and αMSH (r = 0.81). There was not a significant correlation between plasma concentration of cortisol and plasma concentration of any of the IR-POMC peptides. High plasma concentrations of IR-αMSH and βEND, POMC peptides secreted predominantly by melanotrophs in other species, indicate that clinically normal cats have an actively secreting pars intermedia. Although the βEND/βLPH assay used in this study measures the pars distalis-derived peptide β-LPH, as well as βEND itself, over 95% of the IR-βEND/βLPH activity in feline plasma containing high concentrations of αmISH, but low concentrations of IR-ACTH, was found to coelute with human βEND on gel filtration chromatography. In contrast to the high plasma concentrations of IR-αMSH and μEND/βLPH, many cats had low to undetectable concentrations of IR-ACTH, a peptide secreted predominantly by pars distalis corticotrophs. The pattern of plasma POMC peptide concentrations found in cats is similar to that reported in rats, but is markedly different from that reported in dogs, in which the secretion of pars intermedia POMC peptides is normally low.