DOMESTIC CATS SHOW EPISODIC VARIATION IN PLASMA-CONCENTRATIONS OF ADRENOCORTICOTROPIN, ALPHA-MELANOCYTE-STIMULATING HORMONE (ALPHA-MSH), CORTISOL AND THYROXINE WITH CIRCADIAN VARIATION IN PLASMA ALPHA-MSH CONCENTRATIONS

Blood samples were collected from 31 healthy domestic cats to characte rize possible episodic and/or circadian variation in plasma concentrat ions of adrenocorticotropin (ACTH), cortisol, thyroxine and alpha-mela nocyte-stimulating hormone (alpha-MSH). Samples were collected with mi nimal disturbance through indwelling jugular cannulae at two frequenci es: at 20-min intervals for 3 h for evaluation of episodic variation, and at 2-h intervals for 48 or 72 h to identify possible circadian cha nges. Episodic peaks in profiles of all hormones were found in the maj ority of cats. When data were compared across four bleed periods (each of 3 h duration), no differences were detected in average hormone con centrations or characteristics of episodic pulses. Correlation analyse s showed a significant (p < 0.001) relationship between concentrations of ACTH and cortisol (r = 0.44) when these hormones were measured in the same plasma sample. A weaker but significant correlation (r = 0.13 , p < 0.05) was also detected between concentrations of ACTH and alpha -MSH, suggesting that proopiomelanocortin peptide secretion from the p ars distalis and pars intermedia occurs at least on occasion in synchr ony. No differences in hormonal profiles were noted when comparing dat a across sexes. Data from the studies designed to evaluate circadian c hange (48 and 72-h bleed periods) indicated that, of the four hormones , only concentrations of alpha-MSH changed with a significant circadia n periodicity. A significant circadian component of period length 24-2 5 h was detected in 37% (seven of 19) of cats examined. Concentrations of alpha-MSH were greatest coincident with or shortly after the onset of darkness. These findings indicate that pituitary-adrenocortical ho rmones are secreted episodically in domestic cats and that, in contras t to the patterns shown by ACTH and cortisol, secretion of the pars in termedia product alpha-MSH occurs with a circadian rhythm in about one -third of cats.