The myometrial contractility during late pregnancy in dairy cows, in vitro

This study aimed to investigate the in vitro contractility of the myometrium and its relationship to the blood concentrations of estradiol-17β (E_2β), progesterone (P₄), 15-keto-13,14-dihydro-PGF_2α (PGFM) and ionised calcium (Ca²⁺) prior to tissue harvest in 12 healthy Holstein-Friesian cows in late pregnancy. Three circular (CM) and 3 longitudinal myometrial (LM) strips were dissected during a caesarean section and mounted in an organ bath containing modified Krebs solution (KS). The spontaneous contractility was recorded during five 30-min time periods (T1 to T5), after which the strips were exposed to increasing concentrations of oxytocin (OT; 10^-10-10^-7M), a natural PGF_2α-analogue (PGF; 10^-7-10^-4M) and KS (Cont) for four 30-min time periods (T6 to T9). The variables area under the curve (AUC), mean (MA) and maximal amplitude (maxA) were calculated for each T. The blood P₄, E_2ß, Ca²⁺ and PGFM values averaged 4.0±1.7ng/mL, 482.3±63.7 pg/mL, 0.8±0.3 mmol/L and 125.3±63.7pg/mL. The LM strips had greater AUC, MA, and maxA than CM, and OT caused greater AUC and MA in both muscle layers than PGF or control treatment (OT>PGF>Cont). Estradiol-17β correlated with AUC and MA of LM at T1 to T5 (r=0.69; P≤0.05). In conclusion, LM and CM strips have different contractile performance but show enhanced activity when stimulated with OT and less activity after PGF stimulation if compared with Cont. Blood concentrations of E_2β may be useful as an indicator of uterine contractile performance in late pregnant cattle.