Distribution of a novel binding site for angiotensins II and III in mouse tissues

Abstract

A novel binding site for angiotensins II and III that is unmasked by parachloromercuribenzoate has been reported in rat, mouse and human brains. Initial studies of this binding site indicate that it is not expressed in the adrenal, liver or kidney of the rat and mouse. To determine if this binding site occurs in other mouse tissues, 8 tissues were assayed for expression of this binding site by radioligand binding assay and compared with the expression of this binding site in the forebrain. Particulate fractions of homogenates of testis, epididymis, seminal vesicles, heart, spleen, pancreas, lung, skeletal muscle, and forebrain were incubated with (125)I-sarcosine(1), isoleucine(8) angiotensin II in the presence or absence of 0.3mM parachloromercuribenzoate plus 10microM losartan and 10microM PD123319 (to saturate AT(1) and AT(2) receptors). Specific (3microM angiotensin II displaceable) high affinity binding occurred in the testis>forebrain>epididymis>spleen>pancreas>lung when parachloromercuribenzoate was present. Binding could not be reliably observed in heart, skeletal muscle and seminal vesicles. High affinity binding of (125)I-sarcosine(1), isoleucine(8) angiotensin II was observed in the absence of parachloromercuribenzoate in the pancreas on occasion. This suggests that this novel angiotensin binding site may have a functional role in these tissues.

Fig. 1

Representative saturation binding analyses of specific ¹²⁵I-SI Ang II binding to the novel AngII/AngIII binding site in mouse brain (Panels A–E), testis (Panel A), epididymis (Panel B), spleen (Panel C), pancreas (Panel D), and lung (Panel E). Values for each panel are: Panel A, Brain (B_max = 173.4 fmol/mg protein; K_D = 1.32 nM) and Testis (B_max = 342.2 fmol/mg protein; K_D = 4.04 nM). Panel B, Brain (B_max = 101.6 fmol/mg protein; K_D = 1.56 nM) and Epididymis (B_max = 75.3 fmol/mg protein; K_D = 5.46 nM). Panel C, Brain (B_max = 247.1 fmol/mg protein; K_D = 2.21 nM) and Spleen (B_max = 79.7 fmol/mg protein; K_D = 3.34 nM). Panel D, Brain (B_max = 141.9 fmol/mg protein; K_D = 1.50 nM) and Pancreas (B_max = 73.5 fmol/mg protein; K_D = 1.60 nM). Panel E, Brain (B_max = 122.0 fmol/mg protein; K_D = 3.65 nM) and Lung (B_max = 54.3 fmol/mg protein; K_D = 2.93 nM).

Fig. 2

¹²⁵I-SI Ang II binding to the novel Ang II/Ang III binding site. Control (150 mM NaCl) B_max = 93.6 ± 15.1 fmol/mg protein and high ionic strength (400 mM NaCl) group B_max = 98.9 ± 12.1 fmol/mg protein (Panel A). Control K_D = 2.08 ± 0.41 nM and high salt treatment group K_D = 1.74 ± 0.24 nM (Panel B) n = 6.