PLASMA-CONCENTRATIONS OF IMMUNOREACTIVE PROOPIOMELANOCORTIN PEPTIDES AND CORTISOL IN CLINICALLY NORMAL CATS

We measured immunoreactive (IR) plasma concentrations of the proopiome lanocortin (POMC)-derived peptides (adrenocorticotropic hormone [ACTH] ; beta-endorphin/beta-lipotropin [beta END/beta LPH]; and alpha-melano cyte stimulating hormone [alpha MSH]) and of cortisol in 100 clinicall y normal cats. Median plasma concentration of IR-ACTH was 2.7 pmol/L ( range, less than or equal to 1.1 to 22 pmol/L), of beta END/beta LPH w as 28 pmol/L (range, 3.8 to 130 pmol/L), of alpha MSH was 36 pmol/L (r ange, less than or equal to 3.6 to 200 pmol/L), and of cortisol was 35 nmol/L (range, 5 to 140 nmol/L). Plasma concentrations of IR-ACTH, al pha MSH, and beta END/beta LPH were at or below the assay sensitivity in 34, 3, and 0% of the cats, respectively. We did not detect a correl ation between plasma concentrations of IR-ACTH and beta END/beta LPH ( r = 0.23) or between plasma concentrations of IR-ACTH and alpha MSH (r = 0.19). However, there was a significant (P < 0.001) correlation bet ween plasma concentrations of IR-beta END/beta LPH and alpha MSH (r = 0.81). There was not a significant correlation between plasma concentr ation of cortisol and plasma concentration of any of the IR-POMC pepti des. High plasma concentrations of IR-alpha MSH and beta END, POMC pep tides secreted predominantly by melanotrophs in other species, indicat e that clinically normal cats have an actively secreting pars intermed ia. Although the beta END/beta LPH assay used in this study measures t he pars distalis-derived peptide beta-LPH, as well as beta END itself, over 95% of the IR beta-END/beta LPH activity in feline plasma contai ning high concentrations of alpha MSH, but low concentrations of IR-AC TH, was found to coelute with human beta END on gel filtration chromat ography. In contrast to the high plasma concentrations of IR-alpha MSH and beta END/beta LPH, many cats had low to undetectable concentratio ns of IR-ACTH, a peptide secreted predominantly by pars distalis corti cotrophs. The pattern of plasma POMC peptide concentrations found in c ats is similar to that reported in rats, but is markedly different fro m that reported in dogs, in which the secretion of pars intermedia POM C peptides is normally low.