AJP - Renal Physiology, Vol 237, Issue 3 241-F246, Copyright © 1979 by American Physiological Society
Effect of fasting on tubular phosphorus reabsorption
N. Beck, S. K. Webster and H. J. Reineck
In the thyroparathyroidectomized (TPTX) rat, fasting increased urinary
phosphorus excretion by decreasing the tubular reabsorption of P1 (TRP) and
resulted in hypophosphatemia. The administration of either sucrose or
NaHCO3 prevented the metabolic acidosis associated with fasting and
decreased the phosphaturia, indicating that the phosphaturia in fasting is
in part due to metabolic acidosis. In rats on partial reduction of P1
intake selectively, the phosphaturic response to parathyroid hormone (PTH)
was completely suppressed. On the other hand, the fasting rat partially
retained the phosphaturic response to PTH, although dietary P1 intake was
totally absent. These findings suggest that the renal P1 wasting in fasting
may take place by dual mechanisms: a) the PTH-independent decrease in TRP,
and b) an inability to totally suppress the response to PTH. Cyclic AMP
generation in response to PTH, determined both in vivo and in vitro, was
not measurably altered in fasting. However, the phosphaturic response to
cyclic AMP was decreased in fasting, suggesting that the mechanism of
partial resistance to PTH is probably not at but after cyclic AMP
generation.
