Sperm capacitation in the domestic cat (Felis catus) and leopard cat (Felis bengalensis) as studied with a salt‐stored zona pellucida penetration assay

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Abstract
The ability of domestic cat or leopard cat spermatozoa to penetrate zonae pellucidae (ZP) of salt‐stored, domestic cat oocytes was examined as an assay for sperm capacitation. Ovarian oocytes were recovered after ovariectomy and matured in vitro for 18–36 h. Following removal of cumulus cells, the oocytes were used fresh, or stored (4°C, 0.5‐24 weeks) in a HEPES‐buffered hypertonic salt solution. Electroejaculated, washed sperm (2‐4 × 106 sperm/ml) were preincubated for 1.0 h (38°C, 5% CO2 in air) and then co‐incubated (2 × 105 sperm/ml) with fresh or stored oocytes for 6.0 h. Gametes were incubated in a protein‐free, modified Tyrode's solution (TLP‐PVA) or in the same medium containing 4.0 mg/ml bovine serum albumin (BSA; TALP‐PVA). Treatments were compared for percentage ZP penetration (defined as sperm heads reaching more than halfway through the ZP) as an index of sperm capacitation. In both the domestic cat and leopard cat, there was no difference (P > 0.05) in sperm penetration of fresh ZP (domestic cat, 42.5 ± 5.4%; leopard cat, 38.6 ± 2.8%) or stored ZP (domestic cat, 32.4 ± 4.2%; leopard cat, 27.6 ± 2.3%). Sperm incubated in protein‐free medium (TLP‐PVA) were less capable (P<0.05) of ZP penetration (domestic cat, 14.6 ± 5.9%; leopard cat, 7.9 ± 3.0%) than sperm incubated in medium TALP‐PVA containing BSA (domestic cat, 60.3 ± 5.9%; leopard cat, 58.4 ± 3.0%). These data indicate that (1) albumin facilitates capacitation and ZP penetrating ability of cat spermatozoa; (2) domestic cat ZP appear to lack a block to heterospecific penetration by “foreign” (leopard cat) sperm; and (3) penetration of stored domestic cat ZP can be used as an index of sperm capacitation in the domestic cat and the leopard cat.