AJP - Regulatory, Integrative and Comparative Physiology, Vol 246, Issue 5 698-R704, Copyright © 1984 by American Physiological Society
Calcium homeostasis in cows, with special reference to parturient hypocalcemia
C. F. Ramberg Jr, E. K. Johnson, R. D. Fargo and D. S. Kronfeld
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.
