Emerging issues in the cellular biology of the cardiovascular system

Current theory suggests that life began in a prebiologic era, progressed to a ribonucleic-deoxyribonucleic cellular era, and finally entered an era characterized by multicellular organisms. If this progression is correct, it is not surprising that, as medicine studies living organisms with increasing sophistication, factors that are initially discovered to have systemic effects are, in many instances, later determined to have paracrine, autocrine or even intracellular ("intracrine") effects. This schema is potentially of value in analyzing the pathogenesis of cardiovascular disease and, in particular, the development of the sequelae of hypertension. A case is made for the idea that the actions of common peptide and nonpeptide factors at local tissue levels can play an important role in the development of atherosclerosis and left ventricular hypertrophy. In making this case, the potential roles of insulin, angiotensin II and other vasoactive factors are considered. In addition, it is argued that some peptide and nonpeptide factors with cardiovascular impact may operate in the intracellular environment, thus broadening prospects for study and intervention. Finally, genomic alterations either spontaneously occurring or resulting from chronic stimulation or viral infection are considered and their potential role is discussed.

Does the renin-angiotensin-aldosterone system modify cardiac structure and function in essential hypertension?

To determine the impact of the renin-angiotensin-aldosterone system on left ventricular function and structure, 36 untreated patients with essential hypertension (WHO class I and II) were examined. Posterior wall thickness, relative wall thickness, and left ventricular mass were determined by M-mode echocardiography. Plasma renin activity, aldosterone, angiotensin I, and angiotensin II levels were measured by radioimmunoassay. Plasma renin activity was related to 24-hour urinary sodium excretion. Of all the endocrine parameters, only the angiotensin II level correlated with posterior wall thickness (r = 0.50, p less than 0.05) and relative wall thickness (r = 0.46, p less than 0.05). This relationship was confirmed by stepwise multiple regression analysis taking arterial pressure, obesity, and sodium excretion into account (p less than 0.05). Plasma renin activity but not the angiotensin II level correlated positively with the ejection fraction (r = 0.42, p less than 0.05) and velocity of circumferential fiber shortening (r = 0.57, p less than 0.01). Thus, angiotensin II emerged as a determinant of left ventricular structural adaptation in essential hypertension.

Cellular mechanisms of growth in cardiovascular tissue

Left ventricular hypertrophy is a complex cellular response to a variety of pathologic states. In recent years it has become clear that a variety of hormones are present in the heart and may participate in the genesis of left ventricular hypertrophy. Our group has demonstrated the synthesis of renin by cultured canine arterial smooth muscle cells and has recently demonstrated the presence of renin and angiotensin II in myocardial cell preparations. Angiotensin II concentration was (+/- standard error of the mean) 4.19 +/- 1.52 pg/10(6) cells. Evidence has been developed to suggest that these components of the renin-angiotensin system are modulatable. These data, taken together with studies from our laboratory and others, raise the distinct possibility that components of the renin-angiotensin system together with catecholamines play a role in the genesis of left ventricular hypertrophy. Recent evidence suggests that the myc proto-oncogene is activated in cells undergoing catecholamine-induced hypertrophy and our laboratory has detected activation of the sis proto-oncogene in at least 1 model of left ventricular hypertrophy. These findings raise the possibility that similar pathogenetic cellular mechanisms operate to produce myocardial hypertrophy, vascular hyperplasia and hypertrophy, and some forms of benign neoplasia. The possible significance of these findings and their relation to peptide hormones and growth factors are discussed.

The renin-angiotensin systems

The renin-angiotensin systems are important regulators of cardiovascular homeostasis and participate in a variety of pathological conditions. Recent advances have not only clarified the functioning of the systemic renin cascade but have also indicated the importance of the generation of angiotensin in tissues.

Evidence for the existence of renin in the heart

Renin and other components of the renin-angiotensin system have been reported to be present in many extrarenal tissues. We have detected reninlike activities in mouse and rat hearts. Cardiac renin has a pH optimum similar to that of renal renin and its activity is inhibited by antirenin antibody. This enzyme is synthesized in the heart, based on the following evidence: renin messenger RNA was detected in mouse heart by Northern blot hybridization and renin activity was found in isolated cardiac myocytes. Nifedipine lowered the intracellular renin activity of isolated cardiac myocytes in vitro. These results demonstrate the existence of a cardiac renin system that may play a local regulatory role on cardiac function.

Copper-rich nucleoprotein generated by micrococcal nuclease

Nuclei from calf thymus tissue digested with micrococcal nuclease under nonchelating conditions yielded soluble nucleoprotein enriched in copper. Following limited digestion, the ratio of μg Cu:mg DNA was inversely related either to percent solubility of chromatin or to levels of enzyme maintaining an enzyme:A 260 ratio of 0.059. The enzyme appeared to cleave preferentially regions of chromatin where copper is localized, releasing no additional metal upon further digestion. Moreover, the highest copper: DNA ratio was always associated with the least-digested sample.The distribution between copper and angiotensin II (AII) in chromatin fragments following slight nuclease digestion suggests a possible link between copper and nuclear AII binding. When nuclei are incubated with AII prior to digestion and dialysis, solubilized chromatin contained about three times more copper than buffer control. Metal profiles generated from gel (A-5 M) chromatography for these samples were distinctive: copper peaks appeared near or adjacent to linker DNA regions, and in the case of AII, coincided with fragments containing specific AII receptors; thus, there appears to be an enrichment of copper in these active nucleoprotein fragments.

Recognition of peptide orientation: studies with angiotensin II in the guinea-pig

Antipeptide sera of defined subspecificities were obtained when angiotensin II was unidirectionally conjugated to its carrier protein. Antisera with predominantly amino terminal specificity were produced when guinea-pigs were immunized with angiotensin II conjugated via its carboxy terminus to thyroglobulin. When angiotensin II was coupled via its amino terminus, carboxy terminal-specific antisera were obtained. The same conjugates failed to provide antisera with the corresponding specificities when rabbits were used instead of guinea-pigs. This work demonstrates that the peptide-protein coupling strategy may affect the specificity of the resulting antiserum.

Angiotensin II receptors in chromatin

When isolated rat hepatic nuclei or bovine thymus nuclei were incubated with 125I-angiotensin II (ANG II) in the presence or absence of cold hormone, displaceable binding was consistently detected in micrococcal nuclease generated fragments Nanomolar concentrations of ANG II produced detectable displacement. Little or no specific binding was found when nuclei were first digested and then treated with hormone, suggesting that ANG II solubilizes its own receptor. The binding moiety was partially purified by DNP gel electrophoresis. These studies indicate the existence in chromatin of high affinity receptors for ANG II, and further suggest that hormone binding to these receptors produces conformational changes in chromatin similar to those seen during enhanced transcriptional activity. Thus, the present studies suggest the existence of functional intracellular renin-angiotensin systems.

Cellular biology of the renin-angiotensin systems

The renin-angiotensin system is a major determinant of arterial pressure and intravascular volume in human beings. Recent evidence, however, suggests that renin can be synthesized at local tissue sites and that these local renin-angiotensin systems subserve important physiologic functions. In addition, it appears that there exist intracellular renin-angiotensin "systems" capable of generating angiotensin II intracellularly.

Angiotensin II receptors in chromatin fragments generated by micrococcal nuclease

Rat liver nuclei were digested with micrococcal nuclease following incubation with 125I-angiotensin II (AII) or with 125I-AII and excess unlabeled hormone. Chromatin enriched in 125I was solubilized after 3 min and was applied to a BIO-GEL A-5 M column. Labeled hormone was 40-60% displaceable by unlabeled hormone, in nucleoprotein eluting with a V/Vo near 1.9, indicating that these solubilized chromatin fragments contained specific receptors for AII. Furthermore, a discrete AII binding nucleoprotein particle was resolved on DNP gel electrophoresis. Additionally, binding to specific AII nuclear receptors appeared to bring about changes in chromatin structure consistent with the induction of transcriptional activity.

Nuclear-hormone mediated changes in chromatin solubility

Rat liver nuclei were incubated with either thyroid hormone or angiotensin (AII) at varying concentrations or with buffer (control) prior to digestion with micrococcal nuclease. Concentrations of hormones greater than 10(-10)M were effective in increasing the solubilization of chromatin with physiological levels (10(-9)M) of AII showing an approximate 2.4 fold increase over control. Nuclei were also isolated from animals treated in-vivo with either AII or buffer (control) and chromatin solubility was increased in the AII treated nuclei even prior to the addition of exogenous nuclease, presumably from the action of endogenous nucleases. The data suggest that hormone-induced increases in solubility are a reflection of structural changes in chromatin which enhance the accessibility of DNA to endonuclease attack.

Specific nuclear binding of angiotensin II by rat liver and spleen nuclei

1. Nuclei were isolated from rat liver and spleen by a standard technique which preserved the nuclear membrane. Electron microscopy, DNA/protein ratios and 5′-nucleotidase determinations confirmed the purity of the preparation.

2. Both liver and spleen nuclei specifically bound 125I-labelled angiotensin II (ANG II) with high affinity (Kd ≅ 1.0 nmol/l). Saralasin ANG III and ANG II competed with tracer for ligand whereas ANG I did so less effectively and neurotensin not at all.

3. These results suggest the presence of specific nuclear intracellular ANG II receptors and raise important questions regarding the mechanism of action of this peptide.

The relationship between endogenous hyperprolactinaemia and plasma aldosterone

It has been suggested that prolactin is a regulator of aldosterone secretion. In order to test this hypothesis, we measured prolactin, thyrotrophin and aldosterone by radioimmunoassay and plasma renin activity by the radioimmunoassay of angiotensin I in eight normal women before and after the intravenous injection of 200 microgram of thyrotrophin releasing hormone (TRH). Prolactin increased from 4.1 +/- 1.1 ng/ml (mean +/- SE) to a peak of 27.4 +/- 3.8 (P less than 0.005) at 15 min following TRH. Plasma renin activity was not different from control levels (1.0 +/- 0.2 ng/ml/h) during the first hour following the administration of TRH, nor did the plasma aldosterone concentration differ significantly from the control levels (39 +/- 7 pg/ml) during this period. However, with upright posture, an increase in aldosterone (from 31 +/- 3 pg/ml at 1 h to 68 +/- 9 at 2 h, P less than 0.005) and in plasma renin activity (from 0.9 +/- 0.2 ng/ml/h at 1 h to 2.0 +/- 0.5 at 2 h, P less than 0.05) was noted, demonstrating a normal capacity to secrete aldosterone in these subjects. Similarly, no change in aldosterone was seen in nine patients with primary hypothyroidism given TRH, despite the fact that the increase in prolactin was greater than normal. Chronic hyperprolactinaemia was not associated with hyperaldosteronism in six patients with pituitary tumour. These data demonstrate that acutely or chronically elevated serum prolactin levels do not result in increased plasma aldosterone levels in humans.

Metabolic clearance and secretion rates of subunits of human thyrotropin

Metabolic clearance rates (MCR) of the alpha and beta subunits of human thyrotropin (hTSH-alpha and hTSH-beta) were determined by a constant infusion to equilibrium method. In 15 normal individuals (six men, six premenopausal women, and three post-menopausal women), the mean MCR of hTSH-alpha (68 ml/min per m2) was significantly faster than that of hTSH-beta (48 ml/min per m2) was significantly faster than that of hTSH-beta (48 ml/min per m2); both were two to three times more rapid than the previously determined MCR of hTSH. In patients with primary hypothyroidism, MCR were significantly slower with a mean value of 55 ml/min per m2 for hTSH-alpha and 37 ml/min per m2 for hTSH-beta. However, MCR of subunits were not significantly faster than normal in hyperthyroid patients. Serum concentrations of alpha subunits and hTSH-beta were measured by radioimmunoassay, and secretion rates of alpha and hTSH-beta from the pituitary were calculated using hTSH-alpha and hTSH-beta MCR, respectively. In the normal individuals, alpha secretion rates averaged 91 mug/day per m2, greater than those previously determined for hTSH and human follicle-stimulating hormone. Alpha secretion rates were significantly elevated in the normal postmenopausal women (211 mug/day per m2) and in the premenopausal hypothyroid women (202 mug/day per m2); they were also elevated in the postmenopausal hypothyroid women (277 mug/day per m2). Alpha secretion rates were significantly decreased in the premenopausal hyperthyroid women (66 mug/day per m2). Usually, the secretion rates of hTSH-beta could not be calculated in normal individuals, and the rates in hyperthyroid patients could never be calculated because serum hTSH-beta was not detected. Six normals had detectable hTSH-beta secretion rates (17 mug/day per m2); hTSH-beta secretion rates were significantly increased in patients with primary hypothyroidism (28 mug/day per m2). Although we had previously demonstrated a 50-fold increase in hTSH secretion rates in primary hypothyroidism, there was only a 2-fold increase in alpha and hTSH-beta secretion rates. Thus, increased subunit synthesis appears to be utilized predominantly for production of complete hTSH.

The effect of glucocorticoid administration on human pituitary secretion of thyrotropin and prolactin

In order to determine the mechanism by which glucocorticosteroids decrease the serum concentration of thyrotropin (TSH), we studied eight normal subjects before and after they received 16 mg of dexamethasone daily for 2 1/2 days. Serum levels of TSH and prolactin (PRL) were measured in the basal state and in response to the intravenous administration of 200 mug thyrotropin-releasing hormone (TRH); T4, free T4 (fT4), T3, and free T3 (fT3) were measured before TRH injection. Metabolic clearance rates of TSH corrected for body surface area (MCR-TSH/m2) were determined by the method of constant infusion to equilibrium; the production rates of TSH (PR-TSH/m2) were calculated. Dexamethasone produced a decrease in basal TSH from 2.2 to 0.8 muU/ml (P less than 0.02), a statistically insignificant elevation in MCR-TSH/m2 from 25.8 to 34.1 ml/min/m2, and a decrease in PR-TSH/m2 from 79 to 30 mU/day/m2 (P less than 0.01). Peak TSH response to TRH decreased from 16.4 to 5.8 muU/ml (P less than 0.005), as did TSH reserve from 1.58 to 0.54 mU - min/ml (P less than 0.005). Repetitive TRH testing alone did not account for these changes. Basal PRL, peak PRL after TRH, and PRL reserve did not change significantly after dexamethasone administration. Although Basal T4 and fT4 did not change significantly, dexamethasone did decrease T3 from 106 to 61 ng/dl (P less than 0.001) and fT3 from 174 to 76 pg/dl (P less than 0.05). Dexamethasone produced similar changes in patients with various thyroid disorders. In addition, when plasma cortisol was lowered by metyrapone administration in 25 euthyroid patients, the serum TSH concentration rose from 1.6 to 3.1 muU/ml (P less than 0.001). These data indicate that dexamethasone a) suppresses TSH secretion without increasing fT3 and fT4 and b) blunts the TSH, but not the PRL response, to TRH. Hence, one effect of the administration of dexamethasone in high dose is a direct suppression of pituitary TSH secretion. Furthermore, physiologic levesl of circulating cortisol also have a suppressive effect on serum TSH.