Effects of cryo-injury on progesterone receptor(s) of canine spermatozoa and its response to progesterone.

The integrity of sperm progesterone (P4) receptor(s) and its response to steroid stimulation might be crucial for the maintenance of sperm fertilizing ability after cryopreservation. The aim of the current investigation was to study the effect of cryo-procedures on canine sperm P4 receptor(s). In addition, alteration of P4 receptor(s) at the molecular level and their functional integrity following cryo-procedures was evaluated. Fresh and frozen-thawed semen samples (n=6 same dogs) after capacitation were treated with 10 microg/mL P4 to induce the acrosome reaction (AR, FITC-PNA staining). Parallel samples were treated with 50% canine seminal plasma (SP) prior to AR induction with P4. The percentages of AR in capacitated fresh and frozen-thawed semen samples after treatment with P4 were 31.0+/-6.7 and 21.6+/-4.1% (P<0.05), respectively. The percentage of AR in fresh and frozen-thawed semen samples pretreated with SP and incubated with P4 was; 11.5+/-4.8 and 16.5+/-2.0% (P<0.05), respectively. The incidence of the spontaneous AR (P>0.05) in fresh and frozen-thawed semen samples at the onset (5.5+/-2.2 and 6.1+/-1.8%; respectively) and after a 2h (9.6+/-5.1 and 10.4+/-2.7%; respectively) capacitation, avoiding P4 stimulation, were not different. The percentage of progesterone-BSA-FITC staining over the acrosomal region was 18.3+/-10.3% in fresh semen, 36.0+/-11.9% in capacitated (P<0.05) and less than 5% in SP treated spermatozoa. This staining was barely visible in frozen-thawed spermatozoa regardless of capacitation status. In western blot analysis, mAb C262 recognized two bands (54 and 65 kDa). Digitonin treated fresh and frozen-thawed spermatozoa, labeled with [3H]-progesterone, revealed that the P4 binding capacity decreased from 6.0+/-4.4 in fresh to 3.0+/-2.1 nM in frozen-thawed spermatozoa. In nearly all samples tested (except one) 65 kDa protein band decreased significantly after freeze-thaw procedures while the 54kDa protein was increased. These results indicate that the reduced incidence of AR in response to P4 in frozen spermatozoa is possibly due to the conformational changes of P4 receptor(s) and/or reduced P4 receptor density derived from freezing injury.