CIRCADIAN AND CIRCANNUAL PLASMA SECRETORY PATTERNS OF GROWTH-HORMONE AND PROLACTIN IN FRESIAN HEIFERS - HORMONAL PROFILES AND SIGNAL ANALYSIS

The circadian and circannual secretory patterns of growth hormone (GH) and prolactin (PRE) in Fresian heifers were analysed to identify the significant pulses, the shape and the periodic components of the pulsa tile signal that may be relevant to intercellular communication. The p ossible influence of temperature and photoperiod over these hormonal s ecretions has also been investigated. In the bovine GH (bGH) secretory patterns the basal levels and number, magnitude and amplitude of sign ificant secretory peaks seem to be independent and not affected by sea sonal changes. All the bGH circadian patterns show two periodic compon ents with distinct frequencies. The bovine PRE (bPRL) circadian secret ion shows basal levels, number and magnitude of the secretory peaks gr eater in summer than in winter; the periodic components are only one i n winter, but two in summer. There is a significant increase of plasma bPRL in the May-July period and a correlation of the circannual hormo ne profile with both temperature and daylength.