Evidence for separate receptors for insulin and insulin-like growth factor-I in choroid plexus of rat brain by quantitative autoradiography.
Binding of insulin and insulin-like growth factor-I (IGF-I) to the choroid plexus was quantitatively characterized using autoradiography and computer densitometry. Slide-mounted brain slices were incubated in 0.1 nM [125I]-insulin or [125I]-[Thr59]IGF-I. To determine specificity of the binding sites, the labeled peptides were mixed with unlabeled analogues. Autoradiography was done with LKB Ultrofilm and analyzed with a computer image analysis system and program for densitometry. Results showed that binding was time and temperature dependent and reversible. Binding of the iodinated insulin and IGF-I was inhibited by unlabeled peptides in a dose-dependent manner. The rank order of potency of these peptides in competing for the choroid plexus iodoinsulin binding sites was: chicken insulin greater than porcine insulin greater than desoctapeptide insulin greater than IGF-I. IGF-I was more potent than porcine insulin in competing for the choroid plexus iodolGF-I binding sites. Somatostatin was ineffective. Non-linear regression analysis revealed the presence of high- (Kd 1.3 +/- 0.2 nM) and low-affinity (Kd 36 +/- 1.4 nM) binding sites for insulin and a single high-affinity binding site (Kd 3.1 +/- 0.3 nM) for IGF-I in the choroid plexus. There were approximately 50 times more binding sites (Bmax) for IGF-I than for insulin high-affinity sites, whereas the number of low-affinity sites for insulin was about equal to the number of IGF-I high-affinity sites. The results of these binding studies with iodinated insulin and [Thr59]IGF-I support the conclusion that the rat choroid plexus has separate high-affinity receptors for insulin and IGF-I, and that the IGF-I receptors outnumber the insulin receptors.
Davidson
D A
DA
Veterans Affairs Medical Center, Division of Endocrinology/Metabolism, Seattle, Washington 98108.
Bohannon
N J
NJ
Corp
E S
ES
Lattemann
D P
DP
Woods
S C
SC
Porte
D
D
Dorsa
D M
DM
Baskin
D G
DG
D000818:Animals; D001345:Autoradiography; D001667:Binding, Competitive; D002831:Choroid Plexus / Q000166:cytology / Q000378:metabolism / Q000648:ultrastructure*; D003720:Densitometry; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011972:Receptor, Insulin / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017451:Receptors, Somatomedin; D013696:Temperature; D013997:Time Factors
D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D011972:Receptor, Insulin