Serum progesterone concentration, volume, and apoptosis of corpora lutea in early, middle and late diestrus in the bitch.

The aim was to study number, volume, apoptosis of corpora lutea (CL), and serum P4 concentrations in early, middle, and late diestrus of dogs. Thirty-six bitches were ovari-hysterectomized (OVX): Early Diestrus (Group [G]1; OVX 20 days after end of estrus [DEH]); Mid-diestrus (GII; OVX between 21 and 40 days after DEH), and Late-diestrus (GIII; OVX between 41 and 60 days after DEH). Before OVX a blood sample was collected to quantify P4. After OVX, the number of CL (NCL) was recorded, CL measured using both ultrasonography (US) and caliper (CAL), and the volume (mm3) was calculated. Based on abundances of caspase-3, apoptotic luteal cells were detected. Bitches in early-diestrus had greater P4 concentrations than bitches in mid- and late-diestrus (23.52 ±â€¯3.78 and 10.86 ±â€¯3.88 ng/mL; P < 0.05). The NCL, cumulative USCLV, and CALCLV were similar among diestrus stages (P > 0.30). Bitches with CL (≥5) had twice the serum P4 concentrations as bitches with CL1-2 and CL3-4 (22.71 ±â€¯3.70 and 10.97 ±â€¯4.06 ng/mL; P < 0.05). There were correlations between P4 concentrations with USCLV, CALCLV, and NCL (r = 0.64, r = 0.67, r = 0.44; P < 0.0001). When serum P4 concentrations were adjusted for stages of diestrus, however, there were only correlations during early diestrus. The percentage of apoptotic cells was greater in GIII compared with GI and GII (13.75 ±â€¯2.26 % compared with 4.5 ±â€¯0.68 % and 4.6 ±â€¯1.5 %, respectively; P < 0.05). As days of diestrus increased, number of apoptotic cells increased, and serum P4 concentrations decreased.