Plasma progesterone concentrations in the mid-luteal phase are dependent on luteal size, but independent of luteal blood flow and gene expression in lactating dairy cows

The objective of the present study was to investigate if plasma progesterone (pP₄) concentrations are dependent on luteal size, blood flow, or gene expression in luteal tissue. To induce cycles with high and low pP₄ concentrations, respectively, 20 lactating dairy cows received either a single treatment with 25mg prostaglandin F₂α (PGF₂α) on Day 4 Hour 12 (PG1; n=8), or two treatments (25mg PGF₂α each) on Day 4 Hours 0 and 12 (PG2; n=12) of the estrous cycle (Day 1, Hour 0=ovulation). In four cows, ovulation occurred between 4 and 6d after the second PGF₂α treatment; these cows and one lame cow were excluded from the study. In the 15 remaining cows with physiological interovulatory intervals, pP₄, area (LTA) and volume (LTV) of luteal tissue, as well as absolute (LBF) and relative (rLBF) luteal blood flow were determined on Day 9, and relative luteal P₄ (rLP₄) as well as luteal mRNA expression of important receptors, angiogenic, vasoactive, and steroidogenic factors were quantified on Day 11 (±1) during two successive estrous cycles. Furthermore, rLP₄ was multiplied by LTV to produce a semiquantitative assessment of absolute luteal P₄ (LP₄). There was no effect (P>0.05) of treatment (one or two PGF₂α treatments), neither on pP₄ concentrations nor on any other parameter in the present study. Nevertheless, there was a lower LP₄ (P=0.01), LTA (P=0.03), and LTV (P=0.02), as well as tendencies of lower pP₄ (P=0.06) and LBF (P=0.09) at first compared with second diestrus. Plasma P₄ was related with LP₄ (r=0.43, P=0.04), LTA (r=0.65, P=0.0001), and LTV (r=0.43, P=0.02), but not with rLBF (r=−0.18, P=0.34). Furthermore, there was no significant correlation between gene expression of important steroidogenic factors and P₄ concentrations in luteal tissue. Results indicate that plasma P₄ concentrations in the mid-luteal phase were dependent on luteal size, but independent of blood flow and gene expression per luteal tissue unit.