SEASONAL REGULATION OF PROLACTIN SECRETION IN HYPOPHYSEAL STALK TRANSECTED BEEF-CALVES

Seasonal regulation of prolactin secretion was investigated in crossbr ed beef heifer calves. Calves were randomly assigned to hypophyseal st alk transection (HST, n = 6) or sham-operation control (SOC, n = 6) gr oups and fitted 1 day before surgery with an indwelling external jugul ar catheter. Prolactin (PRL), growth hormone (GH), thyroid stimulating hormone (TSH), thyroxine (T-4), and ni-iodothyronine (T-3) in periphe ral serum were measured by radioimmunoassay in samples obtained before and after HST or SOC. During the first 8 days after HST, PRL concentr ations remained significantly greater than SOC, but then decreased in both HST and SOC calves to 4 +/- 2 (+/- SE) and 10 +/- 3 ng/ml, respec tively (P < 0.001). PRL remained low in both HST and SOC groups for th ree months after surgery. By four months, HST calves had lower basal P RL (5 +/- 1 ng/ml) than observed in SOC (40 +/- 4 ng/ml), and seasonal changes in PRL blood concentration also were attenuated by I-IST. Alt hough I-IST reduced PRL secretion, it did not abolish the effect of se asonal changes (P < 0.01); circulating PRL concentration increased six -fold by shifts in photoperiod and temperature from winter to summer i n these stalk-transected calves. The SOC group had higher serum GH dur ing the winter (3.8 +/- 0.8) than in July (1.3 +/- 0.03 ng/ml). The HS T group had the opposite profile of GH concentration, however, with co ncentrations being higher during May through July. Thyroid stimulating hormone secretion was partly sustained after stalk transection possib ly by negative feedback of reduced circulating thyroxine and tri-iodot hyronine. These results in both hypophyseal stalk-transected and sham- operated beef calves maintained in a natural environment strongly sugg est that hypothalamic regulation of PRL secretion by adenohypophyseal cells is extremely sensitive to seasonal changes throughout the year. Additionally, immediately after HST, PRL blood concentration remains s ignificantly greater than in SOC calves but eventually decreases to lo w blood concentration in HST calves, and unlike that seen after HST in primates. Regardless, basal PRL serum concentration responds to seaso nal changes, but a less distinct change in basal GH serum concentratio n in HST calves than seen in the SOC calves. (C) 1998 Elsevier Science B.V. All rights reserved.