Comparative Activities of Growth Hormone and Luteinizing Hormone in the Direct Stimulation of Local Release of Progesterone from Microdialyzed Ovine Corpora Lutea In Vivo.

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.