Calcium homeostasis in cows, with special reference to parturient hypocalcemia.

Calcium homeostasis is described in terms of controlled, disturbing, and controlling signals. Plasma calcium concentration [( Ca]p) and bone calcium mass (Mb) are the two main controlled signals. Disturbing signals include calcium intake (Vi) and clearance of plasma calcium to feces ( Clf ), bone (Clo+), placenta (Clp), and milk ( Clm ). Clo+ is a disturbing signal for [Ca]p and a controlling signal for Mb. Homeostasis is maintained by input control. Fractional renal calcium reabsorption (alpha r) decreases when calcium entry into plasma is excessive, but usually calcium entry rate (VT) responds effectively to small changes in [Ca]p associated with changes in disturbing signals. With increases in Vi, both fractional calcium absorption (alpha d) and fraction of bone calcium removed (lambda b) decrease to maintain VT at an appropriate level. As Clo+ declines with advancing age, [Ca]p rises slightly but VT decreases: a 1% change in [Ca]p would lead to a 13% change in VT. In heavily lactating cows, this response of VT may be enhanced. Parturient hypocalcemia may be the result of delayed negative feedback responses of lambda b (1 wk) and alpha d (1 day). Positive feedback between [Ca]p and gut motility may lead to transient suppression of VT during hypocalcemia, despite high plasma parathyroid hormone and 1,25-dihydroxyvitamin D3 concentrations. Parturient paresis is an example of a disordered accelerative response in a complex control system amenable to a variety of physiologically rational management procedures.