AJP - Renal Physiology, Vol 242, Issue 5 457-F462, Copyright © 1982 by American Physiological Society
Regulation of PTH receptor-adenylate cyclase system of canine kidney: influence of Mn2+ on effects of Ca2+, PTH, and GTP
E. Bellorin-Font, J. Tamayo and K. J. Martin
Metal ions play important roles in the regulation of the activation of
adenylate cyclase. Previous studies have suggested that an important site
of action of metal ions is at or closely related to the nucleotide
regulatory protein. The present studies examine the nature of the
regulation of enzyme activity by divalent cations and the influence of Mn2+
on hormone binding and stimulation of adenylate cyclase. Studies were
performed in canine renal cortical membranes. Substitution of Mg2+ by Mn2+
was associated with a progressive decline in the ability of GTP or PTH to
stimulate adenylate cyclase activity. Mn2+ did not alter specific binding
of an iodinated PTH analogue. However, in spite of the loss of guanine
nucleotide stimulation of enzyme activity, the effects of guanine
nucleotide on PTH binding were not altered in the presence of Mn2+.
Substitution of Mg2+ by Mn2+ abolished the inhibitory effect of Ca2+ on
basal adenylate cyclase activity. Similarly, the effects of GTP or PTH to
enhance the inhibitory effects of Ca2+ on enzyme activity were abolished in
the presence of Mn2+. Since Mg2+ and Ca2+ compete for a common allosteric
site and Mn2+ abolished the effects of these cations, it would appear that
Mn2+ also competes for the binding site of Mg2+ and Ca2+. The present
studies demonstrating that Mn2+ does not affect hormone binding or the
actions of guanine nucleotides on hormone binding yet totally eliminates
the effect of GTP on enzyme activity indicate that the effect of Mn2+
occurs at the level of the interactions of the nucleotide regulatory
component with the catalytic unit. In addition, these data suggest that
there are two functionally distinct sites of guanine nucleotides with
different ionic requirements.
