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The gonadal hormone inhibin regulates daily sperm production (DSP) indirectly through negative feedback control of FSH secretion and may also affect DSP via direct actions within the testis. Studies attempting to increase DSP through the immunization against inhibin have yielded equivocal results. The current study compared 2 inhibin antigens for effects on DSP and hormone secretion. Hampshire ram lambs (BW=42 ±2 kg; age = 113 ± 3 d) were assigned randomly to 3 groups: 1) control (n = 4); 2) α-peptide conjugate (PTC, n = 6); and 3) α-subunit (SUB, n = 6). Antigen PTC consisted of an α-inhibin, N-terminal, 25-amino acid peptide conjugated to ovalbumin. Antigen SUB was the complete inhibin α-subunit. Lambs were immunized on d 0 (June 19, 2006), 18, 38, and 63. Body weight was recorded on immunization days and scrotal circumference on d 63. Blood samples were collected on d 0, 7, 14, 18, 28, 35, 38, 49, 56, 63, and 70. Rams were slaughtered on d 71. Testes were weighed, and parenchyma was obtained for DSP determination. Plasma α-inhibin antibody titer and LH, FSH, and testosterone concentrations were measured. α-Inhibin antibody titer was first detectable on d 14 in both PTC- and SUB-immunized ram lambs and generally increased thereafter. Mean DSP per gram of testis (DSP/g) was increased (P < 0.01) 26% in PTC- and SUB-immunized ram lambs over that in control ram lambs. Total DSP per ram lamb and testes weight did not differ among the 3 treatment groups. Variation in DSP per ram lamb and testes weight were greater (P = 0.05) in PTC- and SUB-immunized ram lambs than in control ram lambs. Plasma FSH concentrations were similar in PTC- and SUB-immunized ram lambs. Immunization against either α-inhibin antigen did not alter LH, testosterone, BW, or scrotal circumference. Findings indicate that 1) the 2 α-inhibin antigens increase DSP/g to similar extents; 2) α-inhibin antibody may act at least in part through an intratesticular mechanism because DSP/g was increased in some animals without concomitant increases in FSH; and 3) immunization against α-inhibin may affect testes weight by actions independent of those that regulate DSP/g.
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