Effects of induced subclinical hypocalcemia in early-lactation Holstein cows without milking during infusion on parathyroid hormone and serotonin concentrations.

The transition to lactation demands a substantial amount of calcium (Ca) to support colostrum and milk production. Extensive research has been focused on elucidating the interplay between the traditional Ca-parathyroid hormone-vitamin D axis and mammary-derived factors, such as serotonin (5-HT) and parathyroid-hormone-like hormone (PTHLH), in regulating Ca metabolism during the transition period. Here, we investigate the impact of induced subclinical hypocalcemia (SCH) on 5-HT and parathyroid hormone (PTH) concentrations in early-lactation dairy cows under conditions of 24-h milk stasis. Twelve multiparous Holstein cows in early lactation received either continuous intravenous infusion of saline solution or 5% ethylene glycol tetraacetic acid (EGTA) to maintain blood ionized calcium (iCa) below 1 mM (n = 6/treatment). Blood samples were collected hourly during infusion and 4, 8, 12, 24, and 48 h post-infusion. Urine samples were collected every 4 h during infusion and at 12, 24, and 48 h post-infusion, and milk samples were collected daily from 2 d pre-infusion to 4 d post-infusion. Infusion of EGTA resulted in decreased blood iCa during the infusion period, with iCa concentrations rebounding 24 h post-infusion. No significant treatment effects were observed on 5-HT and PTH blood concentrations. These findings underscore the importance of considering physiological distinctions in studies of Ca metabolism during the transition period.