In vitro fertilization in the sheep: Effect of elevated calcium concentration at insemination

Abstract

Oocytcs (n = 273), collected from superovulated ewes, were inseminated with in vitro capacitated spermatozoa from four rams [Crozet et al., 1987]. In each experiment, paral lel insemination was performed using aliquants from a single ejaculate in either our stan-dard fertilization medium DM-H-SS (a modification of Bracken's defined medium, buffered with HEPES and containing 20% v/v sheep serum) or in the same medium supplemented with calcium lactate (DM-H-SS + Ca). The measured total calcium con centrations were Ca⁺⁺_T = 2.74 mM in DM-H-SS medium and Ca⁺⁺_T = 8.74 mM in DM-H-SS + Ca; the ratio of free to total calcium in DM-H-SS was Ca⁺⁺_F/Ca⁺⁺_T = 0.85. Fertilization was assessed at 17 hours postinsemination. Variations in the ejaculates were observed for each of the four rams tested. When DM-H-SS + Ca was used, the percentages of fertilized (75% vs. 50%) and monospermic (58% vs. 41%) eggs were significantly enhanced compared with use of DM-H-SS. No improve ment was observed in control medium DM-H-SS + lac containing neutralized lactic acid. Supplementing the fertilization medium with calcium had no apparent effect on the inci dence of polyspermy. These experiments show that the fertilization rate achievable in vitro by individual ejaculates from various rams can be increased by raising the calcium concentration in the fertilization medium to a value much higher than that present in tuba! fluids from estrous ewes. Extended incubation in such a high calcium concentration is unnecessary for in vitro capacitation of ram spermatozoa.