Biological activity of an angiotensin II--ferritin conjugate on rabbit aortic smooth muscle cells

Lack of involvement of endothelin-1 in angiotensin II-induced contraction of the isolated rat tail artery

1. The contribution of endothelin-1 (ET-1) to angiotensin II (Ang II)-mediated contraction of the isolated rat tail artery was assessed with measurements of tension, and cytosolic calcium ([Ca(2+)](i)). The distribution of the AT(1) receptor was studied with RT - PCR and immunohistochemistry. 2. Ang II induced an endothelium-independent contraction (pEC(50) 7.95+/-0.06 and E(max): 0.46 g+/-0.05 with endothelium vs 7.81+/-0.02 and 0.41 g+/-0.07 without endothelium; P>0.05). Ang II (0.003 - 0.3 microM)-induced a non-sustained contraction of endothelium-intact preparations which was not antagonized by BQ-123 (1 microM), but was inhibited by losartan (10 nM). In addition, the maximal contraction induced by ET-1 (0.1 microM) could be further increased by the addition of 0.1 microM Ang II. 3. Ang II (0.001 - 0.3 microM) elevated [Ca(2+)](i) in single vascular smooth muscle cells (VSMCs) in a dose-dependent manner (pEC(50) 9.12+/-0.26) and the Ang II-induced increases in [Ca(2+)](i) were not affected by a Ca(2+)-free solution, but were abolished by pretreatment with caffeine (5 mM). Ang II did not increase [Ca(2+)](i) in endothelial cells. ET-1 (0.1 microM) increased [Ca(2+)](i) in single VSMCs in a normal Ca(2+) containing physiological saline solution (PSS), but not in a Ca(2+)-free solution. 4. Ang II-induced contraction was insensitive to inhibition by nifedipine (0.1 microM), an antagonist of L-type voltage-gated Ca(2+) channels, and SK&F96365 (10 microM), which blocks non-selective cation channels, whereas that to ET-1 was inhibited by SK&F69365. 5. RT - PCR data indicate the expression of AT(1A) and AT(1B) on both VSMCs and endothelial cells, but immunohistochemical evidence illustrates that the AT(1) is located primarily on VSMCs. 6. These results indicate that endothelium-derived ET-1 is not involved in the Ang II-mediated vasoconstriction of the rat tail artery and that Ang II- and ET-1-mediated VSM contractions utilize distinct pathways.

Development of a new fluorescent angiotensin II probe

Angiotensin II (ANG II) was conjugated to polystyrene Latex fluorescent microspheres (0.5 or 0.05 micron diam) with carbodiimide. Biological activity of the ANG II-conjugated microspheres (ANG II microspheres) was assessed in dispersed hepatocytes. The 0.05 micron ANG II microspheres inhibited glucagon-stimulated adenosine 3',5'-cyclic monophosphate accumulation and stimulated phosphorylase activity in hepatocytes, whereas the 0.5 micron ANG II microspheres only stimulated phosphorylase activity. The biological activity of the ANG II microspheres was caused by the conjugated ANG II and not by "free" ANG II associated with the spheres or by trypsin-like activity of hepatocytes causing release of ANG II from the microspheres. Binding of 0.05 micron ANG II to hepatocytes was readily observed with fluorescence microscopy. Little, if any, binding was observed with microspheres without conjugated ANG II. This fluorescent preparation of ANG II may have great utility in the study of receptor behavior after binding in individual cells.

Interaction of angiotensin II with functional smooth muscle cells in culture

In this study we report on the characterization of a highly enriched population of cultured vascular smooth muscle cells (SMC) prepared from collagenase-treated medial layer explant outgrowths of rabbit aortae. Studies done on cells from first passage explant outgrowths showed that the cells retain the fine structural features of vascular SMC in situ, can be immunostained with anti-smooth muscle myosin IgG, and bind [125I]angiotensin II (ANG II) in a specific and saturable manner with an apparent Kd of 1 nM. Addition of ANG II (0.1 microM) to the cultures causes obvious shape changes and retraction of cell processes. Electron microscopic autoradiography of cells labeled with [125I]ANG II show that the initial site of interaction of ANG II with the SMC is the plasma membrane. The distribution of ANG II receptors among cells in the population was studied using light microscopic autoradiography. The autoradiographical grain density varied among cells in the population ranging from cells that were heavily labeled to those that possessed virtually no label. These data imply that the expression of ANG II receptors may be limited to a certain progeny within the cell population or is a function of their stage within the cell cycle.

Angiotensin II rapidly increases phosphatidate-phosphoinositide synthesis and phosphoinositide hydrolysis and mobilizes intracellular calcium in cultured arterial muscle cells

Smooth muscle cells were cultured from rat thoracic aorta and labeled to a stable specific activity with 45Ca2+, myo-[2-3H]inositol, or 32Pi. The efflux of 45Ca2+ was monitored over 10-sec intervals. Angiotensin II (AII) increased the amount of 45Ca2+ lost by 5-fold in the first 10-sec interval after the addition of AII and by 10-fold in the second 10-sec interval. AII-stimulated 45Ca2+ release was blocked by the angiotensin antagonist [1-sarcosine, 8-leucine]AII and by La3+. The removal of external Ca2+ had no effect on AII-stimulated 45Ca2+ release. Depolarization with high external K+ only slightly increased 45Ca2+ efflux and had no effect on AII-induced 45Ca2+ release. AII had no effect on the initial rate of 45Ca2+ influx. These results indicate that the rapid 45Ca2+ efflux evoked by AII is probably due to the release of 45Ca2+ sequestered intracellularly rather than to an increase in the Ca2+ permeability of the plasma membrane. AII provoked rapid increases in the levels of phosphatidic acid and phosphoinositides in the cells. These increases in phospholipids were associated with increases in phospholipase C-generated inositol phosphates (tri-, di-, and mono-). It appears that AII simultaneously increases phosphoinositide hydrolysis and synthesis in vascular smooth muscle, and both phospholipid effects may contribute to inositol triphosphate generation, which was sufficiently rapid to have a role in intracellular Ca2+ mobilization.

Asymmetric distribution of the chemotactic peptide receptor on polymorphonuclear leukocytes

The distribution of chemotactic peptide receptors on polymorphonuclear leukocytes (PMNs) was visualized using tritiated chemotactic peptide, N-formylmethionyl-leucylphenylalanine, coupled to hemocyanin (HY-FMLP). This probe was biologically active and the number of HY-FMLP molecules bound to the cell in a saturable manner corresponded closely to the number of peptide receptors characterized for rabbit peritoneal polymorphonuclear leukocytes (Sullivan, S. J., and S. H. Zigmond, 1980, J. Cell Biol., 85:703-711). Cells exhibiting locomotion have a polar morphology easily recognized in the scanning electron microscope. HY-FMLP bound to these cells was asymmetrically distributed with the highest density of HY-FMLP bound to the midregion of the cell. There were very few particles bound to the tail regions. The binding to the leading ruffles was variable but usually less than to the midregion. Addition of high concentrations of uncoupled FMLP eliminated HY-FMLP binding, confirming that the hemocyanin observed was a marker for the saturable chemotactic peptide receptor. The asymmetry in receptor distribution was seen on cells that had been stimulated by low concentrations of either FMLP or another chemotactic factor, leukotriene B4. Thus, peptide binding to the receptor was not required for the development of the asymmetric distribution. The low density of receptors in the tail region of the cell was consistent with the decreased responsiveness of the tail to chemotactic stimulation (Zigmond, S. H., H. I. Levitsky, and B. J. Kreel, 1981, J. Cell Biol., 89:585-592). The receptor asymmetry may contribute to the polar behavior exhibited by polymorphonuclear leukocytes and would be expected to quantitatively modify the directional information available to a cell in a gradient of chemotactic peptide.

Angiotensin receptors and the control of Na+ and K+ transport in cultured aortic smooth muscle and brain microvessel cells

We examined the effect of angiotensin on Na+ and K+ transport by cultures of aortic smooth muscle and brain microvessel cells. Angiotensin II (AII) and angiotensin III (AIII) stimulated net Na+ uptake, which was assayed in the presence of ouabain to block Na+ efflux via the Na+-K+ pump. The combination of saturating concentrations of AII and AIII produced no greater stimulation of net Na+ uptake than either angiotensin by itself. AII also stimulated ouabain-sensitive 86Rb+ uptake by cultured aortic smooth muscle and brain microvessel cells. AII was not as effective as monensin, a Na+ ionophore, in stimulating the Na+-K+ pump. In the presence of monensin, AII had no effect on ouabain-sensitive 86Rb+ uptake. These data are consistent with previous observations suggesting that AII stimulates the Na+-K+ pump by supplying it with more of its rate-limiting substrate, Na+. 125I-AII bound tightly to cultured aortic muscle cells. Approximately 5 nM unlabelled AII half-maximally inhibited 125I-AII binding. Incubation of intact cultures with 1.0 microM AII produced a time-dependent decrease in specific 125I-AII binding. This slow loss of AII binding was prevented by methylamine, a known inhibitor of receptor clustering and internalization. We conclude (1) that angiotensin increases the Na+ permeability of smooth muscle cells in culture, thereby stimulating the Na+-K+ pump; (2) that AII binds to a specific receptor on the cell surface; and (3) that the AII-receptor complex may be internalized by a methylamine-inhibited pathway.

Analysis of angiotensin-stimulated sodium transport in cultured smooth muscle cells from rat aorta

Angiotensin peptides (AI, AII, AIII) increased the rate of Na+ accumulation by smooth muscle cells (SMC) cultured from rat aorta. The stimulatory effect of AII on Na+ uptake was observed when Na+ exodus via the Na+/K+ pump was blocked either by ouabain or by the removal of extracellular K+. AII was at least ten times more potent than AIII and about 100 times more potent than AI in stimulating Na+ uptake. Saralasin had little effect on Na+ uptake by itself but almost completely blocked the increase caused by AII. The stimulation of net Na+ entry by AI, but not AII, was prevented by protease inhibitors. The stimulation of Na+ uptake was almost completely blocked by amiloride. Tetrodotoxin, which prevented veratridine from increasing Na+ uptake, had no effect on the response to AII. Angiotensin increased the rate of ouabain-sensitive 86Rb+ uptake (Na+/K+ pump activity) but had no effect on ouabain-sensitive ATPase activity in frozen-thawed SMC or in microsomal membranes isolated from cultured SMC. The stimulation of ouabain-sensitive 86Rb+ uptake by AII was blocked by saralasin. Omitting Na+ from the external medium prevented AII from increasing 86Rb+ uptake. AII had no effect on cell volume or cyclic AMP levels in the cultured SMC. These results suggest that angiotensin peptides activate an amiloride-sensitive Na+ transporter which supplies the Na+/K+ pump with more Na+, its rate-limiting substrate.