AJP - Cell Physiology, Vol 235, Issue 3 90-C96, Copyright © 1978 by American Physiological Society
Aldosterone receptor occupancy and sodium transport in the urinary bladder of Bufo marinus
N. Farman, M. Kusch and I. S. Edelman
The concentration dependence of binding of [3H]aldosterone to cytoplasmic
and nuclear receptors was evaluated in urinary bladder epithelial cells of
Colombian toads. One class of specific sites (sensitive to displacement by
excess aldosterone) was detected in the cytosol. However, two classes of
specific nuclear [3H]aldosterone binding sites were evident. In the
nucleus, high-affinity (Kd = 2.7 X 10(-9) M), low-capacity (N = 15 X
10(-14) mol/mg DNA) sites (type I) were completely saturated at
approximately 4 X 10(-8) M aldosterone, a concentration which gave a
maximal increase in short-circuit current (SCC). Occupancy of low-affinity
(Kd = 4.6 X 10(-7) M), high-capacity sites (N = 150 X 10(-14) mol/mg DNA)
(type II) occurred at higher steroid concentrations and did not correlate
with further increase in SCC. Binding parameters of Colombian and Dominican
variants of Bufo marinus were compared. In both variants, the SCC increase
elicited by aldosterone correlated with accumulation of type I complexes in
the nucleus, but the relationship was markedly nonlinear. Various
alternatives were considered as the basis for a curvilinear dependence of
the increment in Na+ transport on abundance of nuclear type I complexes.
