Vasopressin, angiotensin and adrenergic receptors of rat liver Golgi fractions--molecular weight of the angiotensin-receptor irreversible complex after in vitro and in vivo labelling

Pharmacological characterization of inositol 1,4,5-trisphosphate binding sites: relation to Ca2+ release

Two subcellular fractions, one enriched in plasma membranes and the other in endoplasmic reticulum membranes, were obtained from WRK1 cells using a combination of differential centrifugations and Percoll gradient fractionation. Specific inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) binding sites were detected in these two preparations. Endoplasmic reticulum membranes exhibited a binding capacity which was about 5-fold higher than that of plasma membranes. Dose-dependent Ins(1,4,5)P3 binding was determined. Experimental data obtained with endoplasmic reticulum membranes could be adequately fitted with a two-site model (a high-affinity binding site with Kd and Bmax values of 0.7 +/- 0.15 nM and 12.9 +/- 5 fmol/mg protein and a low-affinity binding site with Kd and Bmax values of 44.2 +/- 14.6 nM and 143 +/- 43 fmol/mg protein). Both the high- and low-affinity binding sites were selective for Ins(1,4,5)P3. Besides Ins(1,4,5)P3, Ins(1,3,4,5)P4 also discriminated between the two populations of sites while heparin interacted with the high- and low-affinity binding sites with the same affinity. Ins(1,4,5)P3-induced calcium release from endoplasmic reticulum vesicles was determined by monitoring the calcium concentration in the extravesicular compartment with fura-2. Under experimental conditions where the degradation of Ins(1,4,5)P3 was reduced (incubation at 0 degrees C), a high-affinity Ins(1,4,5)P3-induced calcium release (apparent Kact around 20 nM) could be demonstrated. These results suggest that in WRK1 cells, the endoplasmic reticulum is a major site for Ins(1,4,5)P3 action and that the high-affinity binding sites located on the endoplasmic reticulum membranes may contribute to the physiological regulation of the cytosolic free calcium concentration.

Inhibition of angiotensinogen production by angiotensin II analogues in human hepatoma cell line

The aim of this study was to examine in Hep G2, a human hepatoma-derived cell line, the presence of angiotensin II (ANG II) receptors and the effect of ANG II and its analogues on angiotensinogen production. The presence of ANG II receptors was demonstrated using a long-acting ANG II analogue, 125I-labeled [Sar1]ANG II. A single class of specific binding sites was identified in these cells with a dissociation constant (Kd) of 2 nM. The number and affinity of these binding sites were not changed by [Sar1]ANG II treatment over 24 h. ANG II showed an inhibitory effect on angiotensinogen production. [Sar1]ANG II also exhibited a similar inhibitory effect as that of ANG II but to a greater extent and therefore was used throughout these studies. [Sar1]ANG II inhibited angiotensinogen production in a dose-dependent manner, exhibiting a half-maximal inhibitory concentration (IC50) of 2 nM. Other ANG II analogues showed similar effects on angiotensinogen production. In order of decreasing ability, they were [Sar1]ANG II greater than [Sar1-Ala8]ANG II greater than [Sar1-Val8]ANG II greater than [Sar1-Val5-(Br5)-Phe8]ANG II greater than [Sar1-Val5-DPhe8]ANG II. Results of these studies show that the Hep G2 cell possesses specific ANG II receptors and that [Sar1]ANG II induces a dose-dependent inhibition of angiotensinogen production in this system.

Purification of an angiotensin II binding protein by using antibodies to a peptide encoded by angiotensin II complementary RNA

We have generated a monospecific antibody to a synthetic peptide encoded by an RNA complementary to the mRNA for angiotensin II (AII) and determined whether this antibody recognizes the AII receptor. We demonstrate that the antibody competes specifically with 125I-labeled AII for the same binding site on rat adrenal membranes. Furthermore, we show that this antibody inhibits the secretion of aldosterone from cultured rat adrenal cells, suggesting that the antibody recognizes the biologically relevant AII receptor. Finally, we demonstrate that antibody to the complementary peptide can be used to immunoaffinity-purify a protein of Mr 66,000 that specifically binds radiolabeled AII.