AJP - Endocrinology and Metabolism, Vol 242, Issue 5 287-E291, Copyright © 1982 by American Physiological Society
Parathyroid, renal, and skeletal responses to induced hypocalcemia in the dog
J. Fox and H. Heath 3rd
This study was designed to determine 1) whether the plasma immunoreactive
parathyroid hormone (IPTH) response to acutely attained, constant (8 h)
hypocalcemia is biphasic, and 2) if so, how kidney and bone respond to
these changing plasma IPTH levels. We initiated constant hypocalcemia
(decrement of Ca, 1.7 mg/dl) in six conscious dogs using the "calcium
clamp" technique. Plasma IPTH concentrations increased maximally (fivefold)
within 15 min and then decreased gradually over 1 h to a constant, but
still elevated level (3.2-fold increase). Urinary excretion of phosphate
and hydroxyproline increased more slowly, reaching plateaus at 1.75 h (76%
increase) and 5.5 h (70% increase), respectively. The EGTA infusion rate
required to maintain constant hypocalcemia was virtually constant (85 +/- 9
mumol.kg-1.h-1) after 20 min and corresponded to skeletal release of about
80 mg Ca.kg-1.day-1. The contribution of the kidney in conserving filtered
calcium was relatively minor (2.0 +/- 0.5 mumol.kg-1.h-1). These data
demonstrate that the parathyroid response to acute, constant hypocalcemia
is biphasic and is temporally divergent from the uniphasic phosphaturic and
hydroxyprolinuric responses. The ensuing increased skeletal release of
calcium is very high and is maximal within minutes.
