Effect of glucose availability on pulsatile luteinizing hormone release inrams before and after castration

The hypothesis tested was that availability of glucose modulates the contro l of luteinizing hormone (LH) release. A second objective was to determine the role of testicular hormones in the control of pulsatile LH secretion du ring depressed blood glucose. Serial blood samples were collected at 15 min intervals for 8 h from intact pubertal Suffolk rams (n = 8; 7 months old) on consecutive days (Days 1, 2 and 3). Rams were castrated after sampling o n Day 3 and samples were collected 3 weeks later on consecutive days (Days 4, 5 and 6). Insulin (120 units, iv) was given at Hour 4 of each of the six days to lower blood glucose. On Days 1 and 4, no other treatments were giv en (Control). On Days 2 and 5, LH releasing hormone (LHRH; 5 ng/kg, iv) was given at Hours 5, 6 and 7 to assess the ability of the pituitary to releas e LH. On Days 3 and 6, N-methyl-D,L-aspartate (NMA; 5 mg/kg, iv) was given at Hours 5, 6 and 7 to assess the ability of the hypothalamus to release LH RH. Insulin reduced plasma glucose by 52% for at least 3 h (P < 0.001). Ins ulin reduced the mean LH concentration(P < 0.05) and tended to reduce the L H response area (P < 0.10) in castrated animals during the control period. LHRH increased LH pulse number (P < 0.001) in intact rams and increased mea n LH concentration (P < 0.01), LH pulse amplitude (P < 0.05) and LH respons e area (P < 0.01) in castrated animals compared to respective control perio ds. NMA increased mean LH concentration in intact rams (P < 0.0001) but did not affect mean LH in castrates. NMA increased LH pulse number in rams (P < 0.0001) but decreased number of pulses in castrates (P < 0.0001) compared to control periods. NMA increased LH pulse amplitude in both intact (P < 0 .001) and castrated animals (P < 0.05). In conclusion, these results suppor t the hypothesis that blood glucose concentrations influence the control of LN release in sheep. In addition, LH release in response to the LHRH secre tagogue, NMA, is positively influenced by testicular hormones. (C) 1999 Els evier Science B.V. All rights reserved.