Abstract
To assess the direct effects of growth hormone (GH) on the release of progesterone (P) in the microenvironment within the ovine corpus luteum (CL) in vivo, a microdialysis system (MDS) was implanted into the CL during the breeding season. In experiment 1 (single CL model), on Day 7 after estrus, three microcapillary dialysis membranes (cut off Mr=1000 kDa; transfer rate, 1% for P and 0.1% for LH and GH) were surgically implanted into the cyclic CL paralleled to each other with a distance of at least a 5 mm between capillaries (n=5 ewes). Each MDS line was continuously perfused with Ringer's solution during the experiment. Serial fractions were collected every 30 min for 35 h on Days 8-9 following a pre-perfusion period of 10-12 h. A 16-h perfusion with GH (10 μg/ml) induced an acute increase of P release to 300% of the baseline throughout the infusion (P<0.001). A 3-h perfusion with oxytocin (OT: 2 × 10-5 M) also stimulated the P release to approximately 200% of the baseline (P<0.01). When OT was infused following the GH infusion, no additional effect was observed. In experiment 2 (a multiple CL model), four ewes were treated for superovulation, and multiple CLs formed thereafter were implanted with the MDS (1 line/CL) on Day 7 after GnRH injection. Infusions with GH or LH on Days 9-10 at doses of 2.5, 5 or 10 μg/ml for 4 h twice with an 8-h interval similarly induced a clear increase of P release. The 2nd GH infusion had a greater stimulative effect than the 2nd LH infusion at all the doses tested (P<0.05). Furthermore, the 2nd infusion with GH at 5 and 10 μg/ml induced a higher increase of P release than did the 1st infusion (P<0.05), while LH, at any of the tested doses, stimulated P release to a similar extent during the 1st and 2nd infusions. The finding that GH, like LH, directly stimulates the local release of P within the CL in cyclic and superovulated ewes in vivo strongly suggest that GH, like LH, is an essential luteotropic pituitary hormone in ewes.
