Michael J. Peach

Angiotensin II induces hypertrophy, not hyperplasia, of cultured rat aortic smooth muscle cells.

We have explored the hypothesis that contractile agonists are important regulators of smooth muscle cell growth by examining the effects of one potent contractile agonist, angiotensin II (AII), on both cell proliferation and cellular hypertrophy. AII neither stimulated proliferation of cells made quiescent in a defined serum-free media nor augmented cell proliferation induced by serum or platelet-derived growth factor. However, AII did induce cellular hypertrophy of postconfluent quiescent cultures following 4 days of treatment, increasing smooth muscle cell protein content by 20% as compared with vehicle-treated controls. AII-induced hypertrophy was maximal at 1 μM, had an ED50 of 5 nM, and was blocked by the specific AII receptor antagonist Sar1, Ile8 AII. The cellular hypertrophy was due to an increase in protein synthesis, which was elevated within 6–9 hours following AII treatment, while no changes in protein degradation were apparent. AII was even more effective in inducing hypertrophy of subconfluent cultures, causing a 38% increase in protein content after 4 days of treatment (1 μM) and showing a maximal response at concentrations as low as 0.1 nM. Interestingly, in subconfluent cultures, AII treatment (1 μM, 4 days) was associated with a 50% increase in the fraction of cells with 4C DNA content with the virtual absence of cells in S-phase of the cell cycle, consistent with either arrest of cells in the G2 phase of the cell cycle or development of tetraploidy. These studies show that AII is a potent hypertrophic agent but has no detectable mitogenic activity in cultured rat aortic smooth muscle cells and describe an in vitro model that should be extremely valuable in exploring the cellular controls of smooth muscle cell hypertrophy.

Role of endothelial cell cytoskeleton in control of endothelial permeability.

Increased permeability of the pulmonary microvasculature is felt to cause acute noncardiogenic lung edema, and histological studies of edematous lungs show gaps between apparently healthy endothelial cells. To determine whether alterations in endothelial cell cytoskeletons would alter endothelial permeability, we exposed monolayers of pulmonary artery endothelial cells grown on micropore filters to cytochalasin B or D. Cytochalasin exposed monolayers demonstrated a 2- to 3-fold increase in endothelial permeability that was readily reversible by washing the monolayers free of cytochalasins. Parallel phase contrast and fluorescence microscopy demonstrated retraction of cell cytoplasm and disruption of bundles of microfilaments in cytochalasin exposed cells. These changes also were readily reversed after washing the cells free of cytochalasins. To test the relevance of these findings to an in situ microvasculature, we added cytochalasin B to the perfusate of isolated rabbit lungs and observed that cytochalasin B caused a high permeability lung edema. These studies suggest that endothelial cell cytoskeletons may be important determinants of endothelial permeability.

Role of the intrarenal renin-angiotensin system in the control of renal function.

IN THIS review article, we will assess the evidence for the presence and the mechanism of the intrarenal renin-angiotensin system acting as a local hormonal system in the control of renal function. First, we will review the evidence for the presence of a renin-angiotensin system in the kidney. Second, using results of our own and other studies, we will develop the concept that the intrarenal renin-angiotensin system acts as a local hormonal system in the control of renal function. Third, we will discuss experimental observations on the intrarenal effects of angiotensin II. Fourth, we will evaluate the evidence for a direct renal tubular action of angiotensin II. Fifth, we will assess the role of the intrarenal renin-angiotensin system in renal autoregulation. Finally, we will speculate on the function of the intrarenal renin-angiotensin system in the regulation of fluid and electrolyte homeostasis in general.

Endothelium-dependent relaxation and cyclic GMP accumulation in rabbit pulmonary artery are selectively impaired by moderate hypoxia.

The effect of hypoxia on endothelium-dependent and endothelium-independent vasodilation was studied in phenylephrine-precontracted, isolated rings of rabbit first-branch pulmonary artery. Concentration-dependent relaxation responses to the endothelium-dependent dilators methacholine, ATP, and the calcium ionophore (A23187) as well as to the endothelium-independent dilators sodium nitroprusside and isoproterenol were obtained before, during, and after exposure to hypoxia (PO2=42 ± l mm Hg) in the presence of indomethacin (2.8 ×K−5 M). This moderate degree of hypoxia inhibited (p<0.05) endothelium-dependent but not endotheliumindependent relaxation responses without producing irreversible vascular damage. In parallel experiments, cyclic GMP accumulation in pulmonary vascular rings in response to maximal doses of the above vasodilators was measured in the presence and absence of hypoxia. Cyclic GMP accumulation in response to endothelium-dependent dilators (methacholine, ATP, and A23187) was inhibited (p<0.05) by hypoxia while cyclic GMP accumulation in response to the endothelium-independent dilator sodium nitroprusside was not. When phenylephrine precontracted vessels were exposed to hypoxia in the absence of vasodilators, a small, transient increase in tension occurred, which was greater in endothelium-intact than hi endotheliumdenuded vessels (0.70 ± 0.12 vs. 0.09 ± 0.03 g, respectively;p<0.01). This increase in tension was reduced in the presence of hemoglobin (l×l0−6 M; p<0.01), methylene blue (l × l×−7 M; p<0.01), and hydroqulnone (l×l0−6 M;<0.01) in endothelium-intact but not in endotheliumdenuded rings. Hypoxia also reduced basal cyclic GMP content in endothelium-intact phenylephrine-precontracted rings (1.23 ± 0.22 vs. 0.79 ± 0.19 pmol/mg protein;p<0.05). These data suggest that the transient vasoconstriction induced by hypoxia in these large pulmonary arteries is due partially to the inhibition of basal EDRF production. The observed pharmacological responses imply that the site of hypoxia-induced inhibition of endothelium-dependent dilation is distal to receptor-mediated events in the endothelial cell and proximal to activation of guanylate cyclase In the vascular smooth muscle.

Location and regulation of rat angiotensinogen messenger RNA.

The presence of angiotensinogen messenger RNA (mRNA) was detected in rat vascular and adipose tissue, Angiotensinogen mRNA in rat aorta was localized in the adventitia and surrounding adipose tissue, and not in the vascular smooth muscle. Freshly dispersed and cultured endothelial and aortic smooth muscle cells did not contain detectable amounts of angiotensinogen mRNA. In addition to periaortic adipose tissue, angiotensinogen mRNA was present in other fat depots of both brown and white types. To examine regulation of angiotensinogen gene expression, Sprague-Dawley rats were treated with angiotensin converting enzyme inhibitor or underwent bilateral nephrectomy. Relative levels of angiotensinogen mRNA in brown adipose tissues increased dramatically by 48 hours after bilateral nephrectomy. However, only one source of brown adipose tissue showed increased angiotensinogen mRNA levels after animals were treated for 5 days with converting enzyme inhibitor. In addition, angiotensinogen was released into the medium from incubated adipose tissues with levels increasing over a 2-hour period. These results demonstrate that angiotensinogen is synthesized by adipose tissue in the rat and may play a role in the function of this tissue.

Platelet-derived growth factor-BB-induced suppression of smooth muscle cell differentiation

Previously, we demonstrated that treatment of postconfluent quiescent rat aortic smooth muscle cells (SMCs) with platelet-derived growth factor (PDGF)-BB dramatically reduced smooth muscle (SM) alpha-actin synthesis. In the present studies, we focused on the expression of two other SM-specific proteins, SM myosin heavy chain (SM-MHC) and SM alpha-tropomyosin (SM-alpha TM), to determine whether the actions of PDGF-BB were specific to SM alpha-actin or represented a global ability of PDGF-BB to inhibit expression of cell-specific proteins characteristic of differentiated SMCs. SM-MHC and SM-alpha TM expression were assessed by one- or two-dimensional gel electrophoretic analysis of proteins from cells labeled with [35S]methionine, as well as by Northern analysis of mRNA levels. Synthesis of both SM-specific proteins was decreased by 50-70% in PDGF-BB--treated cells as compared with cells treated with PDGF vehicle. Treatment of cells with 10% fetal bovine serum, which produced a mitogenic effect equivalent to that of PDGF-BB, decreased SM-MHC synthesis by 40% but increased SM-alpha TM synthesis. SM-MHC and SM-alpha TM mRNA expression was decreased by 80% at 24 hours in PDGF-BB--treated postconfluent SMCs, whereas treatment with 10% fetal bovine serum did not decrease the expression of SM-alpha TM mRNA but did inhibit SM-MHC mRNA expression by 36%. Consistent with the absence of detectable PDGF alpha-receptors on these cells, PDGF-AA had no effect on either mitogenesis or expression of SM-MHC or SM-alpha TM.(ABSTRACT TRUNCATED AT 250 WORDS)

Augmentation of hypoxic pulmonary vasoconstriction in the isolated perfused rat lung by in vitro antagonists of endothelium-dependent relaxation.

The role of the endothelium in hypoxic constriction of the intact pulmonary vascular bed has not been clearly elucidated. To test for a possible role for endothelium-derived relaxing factor(s) (EDRF) in the hypoxic pressor response, isolated, whole blood-perfused rat lungs from male Sprague-Dawley rats treated with meclofenamate were prepared. Three protocols were performed, including: (a) normal saline (control); (b) the putative EDRF inhibitors, eicosatetraynoic acid (ETYA, 1 X 10(-4) M) or nordihydroguaiaretic acid (NDGA, 1 X 10(-4) M) versus vehicle DMSO; and (c) the putative EDRF inhibitor hydroquinone (HQ, 1 X 10(-4) M) versus vehicle ethyl alcohol (ETOH). The pulmonary pressor response to angiotensin II (Ang II, 0.25 micrograms) injections alternated with 6-min periods of hypoxic ventilation (3% O2, 5% CO2) was measured before and after the administration of saline, inhibitors, or vehicles. The administration of the EDRF inhibitors ETYA, NDGA, and HQ resulted in a marked accentuation of the hypoxic pressor response that was not seen in the controls (P less than 0.05). In separate experiments, lungs precontracted with norepinephrine (1 X 10(-6) M) were pretreated with edrophonium (1 X 10(-4) M) and then observed for endothelium-dependent vasodilator responses to acetylcholine at increasing doses (1 X 10(-7)-1 X 10(-4) M). Administration of ETYA, NDGA, or HQ abrogated the observed vasodilatation to acetylcholine, which was not seen with vehicles alone (P less than 0.01). These studies suggest an important role for the endothelium in pulmonary vascular responsiveness to alveolar hypoxia through possible release of a relaxing factor(s) that attenuates the degree of pulmonary arterial constriction.

Localization of angiotensinogen messenger RNA in rat aorta.

The distribution of angiotensinogen messenger (mRNA) was determined in the rat aorta. Other investigators have demonstrated the presence of angiotensinogen mRNA in whole rat aorta; however, its precise location in the blood vessel wall has not been defined. When various layers of the vessel wall were separated by dissection or cell dispersion, angiotensinogen mRNA levels were greatest in the periaortic adipose tissue. Angiotensinogen mRNA was present in very small levels in the adventitia, with no detectable levels in the muscle layer. In addition to periaortic adipose tissue, angiotensinogen mRNA was also present in the interscapular brown fat pad of the rat. The high levels of angiotensinogen mRNA in periaortic brown adipose tissue suggests that angiotensin may be synthesized there and responses may exist in this tissue or adjacent sympathetic nerve terminals.

The angiotensin II receptor and the actions of angiotensin II.

Angiotensin II (Ang II) is a potent effector peptide of the renin-angiotensin system that exerts a wide variety of physiological actions on the cardiovascular, renal, endocrine, and central and peripheral nervous systems. Angiotensin exerts its actions by binding to specific receptors in the plasma membrane of various tissues. Structure-activity relationship studies and competition-binding experiments have identified a potency series of angiotensin analogues. Such studies have demonstrated that target organs display different preferences for Ang II and homologues such as Ang III and des-[Phe8] angiotensin II. Similarly, agents that normally are considered to be pure receptor antagonists for a given response (tissue) are full agonists in other tissues. Indirect evidence obtained from the above studies have led to the speculation that there are multiple angiotensin receptor subtypes among various tissues as well as within single cell types. Multiple mechanisms of signal transduction have been demonstrated for angiotensin. For example, depending on the effector organ, angiotensin stimulates phosphoinositide turnover and release of internal calcium, modulates voltage-dependent calcium channels, directly activates calcium channels, and inhibits adenylate cyclase activity. Recently, the identification of selective, high-affinity peptide and nonpeptide antagonists has resulted in further characterization of angiotensin receptors into distinct subtypes. In addition, dithiothreitol, an agent that reduces disulfide bridges, has been a useful tool in the characterization of angiotensin receptors as the subtypes apparently are not affected equally by this agent. However, further work needs to be performed to characterize angiotensin receptors with respect to heterogeneity, structure, transducing mechanisms, and physiological function.

Molecular biology of angiotensinogen.

A ngiotensinogen is a moderately abundant / \ 55,000-60,000 Da serum glycoprotein that is -Z \ the precursor to the angiotensin peptides and is the only known naturally occurring renin substrate. It is synthesized by a variety of cells, most prominently hepatocytes, adipocytes, and astrocytes. Most angiotensinogen is extracellular (it is constitutively secreted); thus, there is apparently no way that an organism can orchestrate rapid changes in angiotensinogen concentration. The angiotensinogen gene is regulated by several hormones (e.g., glucocorticoid and estrogen), and angiotensinogen is an acute-phase protein. Angiotensinogen is a member of the serpin gene superfamily, but there is little reason to suppose that this protein is a serine protease inhibitor. The available evidence indicates that angiotensinogen functions solely as an extracellular reservoir of angiotensin peptides. In this article, we attempt a critical review of the literature on the biology of this protein, with an emphasis on recent molecular biological studies. We recommend other recent reviews, for example, those of Tewksbury or Campbell, for alternative views of this subject matter.