AJP - Cell Physiology, Vol 235, Issue 3 82-C89, Copyright © 1978 by American Physiological Society
Binding of aldosterone to cytoplasmic and nuclear receptors of the urinary bladder epithelium of Bufo marinus
M. Kusch, N. Farman and I. S. Edelman
Binding of aldosterone to cytoplasmic and nuclear sites in urinary bladder
epithelia of Bufo marinus (Dominican variant) is saturable and dependent
upon steroid concentration. Scatchard analysis of specific cytoplasmic
binding yielded a maximal binding capacity (N) of 14.5 X 10(-14) mol/mg
protein and an apparent equilibrium dissociation constant (Kd) of 1.4 X
10(-8) M. Since Scatchard analysis of specific nuclear binding was complex,
this binding was resolved by a computer-generated cirve-fitting technique
which analyzed total aldosterone bound. Nuclear binding was resolved into
three sites: a nonsaturable site that was linearly dependent upon
aldosterone concentration, and two saturable sites (types I and II). Type I
sites had relatively low capacity for aldosterone (N = 31 +/- 1 X 10(-14)
mol/mg DNA) and high affinity (Kd = 2.5 +/- 0.5 X 10(-9 M); tffininty (Kd =
8.6 +/- 1.7 X 10(-7) M). Competition for [3H]aldosterone binding by
dexamethasone, corticosterone, cortisol, progesterone, testosterone, and 17
beta-estradiol demonstrated that type I nuclear sites have higher affinity
for aldosterone than for other steroids. The findings are consistent with
the inference that the type I site is the mineralocorticoid receptor.
