Efficacy of octa- and heptapeptide antagonists of angiotensin II as inhibitors of angiotensin III binding in the rat adrenal glomerulosa

An evolving story of angiotensin-II-forming pathways in rodents and humans

Lessons learned from the characterization of the biological roles of Ang-(1-7) [angiotensin-(1-7)] in opposing the vasoconstrictor, proliferative and prothrombotic actions of AngII (angiotensin II) created an underpinning for a more comprehensive exploration of the multiple pathways by which the RAS (renin-angiotensin system) of blood and tissues regulates homoeostasis and its altered state in disease processes. The present review summarizes the progress that has been made in the novel exploration of intermediate shorter forms of angiotensinogen through the characterization of the expression and functions of the dodecapeptide Ang-(1-12) [angiotensin-(1-12)] in the cardiac production of AngII. The studies reveal significant differences in humans compared with rodents regarding the enzymatic pathway by which Ang-(1-12) undergoes metabolism. Highlights of the research include the demonstration of chymase-directed formation of AngII from Ang-(1-12) in human left atrial myocytes and left ventricular tissue, the presence of robust expression of Ang-(1-12) and chymase in the atrial appendage of subjects with resistant atrial fibrillation, and the preliminary observation of significantly higher Ang-(1-12) expression in human left atrial appendages.

A unified model for T cell antigen recognition and thymic selection of the T cell repertoire

Positive selection of T cells during thymic differentiation predisposes mature T cells to recognize glycoproteins encoded by "self" alleles of the major histocompatibility complex (MHC) as "restricting elements" for antigen presentation. Yet, negative selection also occurs during thymic differentiation resulting in the clonal deletion of T cells reactive with "self" MHC glycoproteins. Thymic processes of positive and negative selection represent a paradox because the "altered self" view of T cell receptor (TcR) recognition indicates that the same type of TcR-MHC glycoprotein binding interaction mediates both positive and negative selection of the T cell repertoire. Most contemporary models of thymic selection attempt to explain these paradoxical observations by quantitative differences of affinity. That is, TcR interactions with MHC ligands that are of medium affinity lead to positive selection whereas those that are of high affinity lead to negative selection. The purpose of this manuscript is to provide an alternative model of thymic selection based on the efficacy (the ability of a ligand to catalyze receptor mediated biological activity) of TcR-MHC ligand interactions. The "efficacy" model predicts that among those thymocytes exhibiting affinity for self MHC ligands, some clones bind self MHC ligands without efficacy whereas others bind these ligands with efficacy. Immature T cells that bind MHC ligands without efficacy do not undergo TcR mediated activation and thereby escape clonal deletion. Instead, these T cells compete for growth--promoting sites on thymic antigen presenting cells (APC) based upon their clonotypic TcR affinity for self MHC ligands. These T cells experience positive thymic selection and eventually dominate a repertoire of mature T cells predisposed to exhibit non-efficacious binding to "self" MHC ligands. In contrast, immature T cells that exhibit efficacious binding to self MHC ligands are deleted from the T cell repertoire during thymic maturation. By this mechanism, the mature T cell repertoire is selected so that clonotypic T cells are predisposed to bind the very sites on MHC glycoproteins responsible for antigen presentation without risk of autoimmunity. Given the clonotypic diversity of the mature repertoire, complexes of foreign peptides and self MHC glycoproteins would be recognized by clones of the appropriate specificity as highly efficacious ligands. In summary, the "efficacy" model is entirely consistent with the "altered self" concept of T cell antigen recognition and readily accounts for both positive and negative selection of the T cell repertoire.

Identification of angiotensin II receptor subtypes

We have demonstrated the existence of two distinct subtypes of the angiotensin II receptor in the rat adrenal gland using radioligand binding and tissue section autoradiography. The identification of the subtypes was made possible by the discovery of two structurally dissimilar, nonpeptide compounds, DuP 753 and EXP655, that show reciprocal selectivity for the two subtypes. In the rat adrenal cortex, DuP 753 inhibited 80% of the total AII binding with an IC50 value on the sensitive sites of 2 x 10(-8) M, while EXP655 displaced only 20%. In the rat adrenal medulla, EXP655 gave 90% inhibition of AII binding with an IC50 value of 3.0 x 10(-8) M, while DuP 753 was essentially inactive. The combination of the two compounds completely inhibited AII binding in both tissues.

Angiotensin II and glucocorticoid release: direct effect at the adrenal level and modulation of the adrenocorticotropin-induced glucocorticoid release

Angiotensin II (A II) stimulates adrenal glomerulosa cells releasing mineralocorticoids; however, little is known about the A II effect on glucocorticoids output. The present study has been designed in order to see if A II could modify in vitro spontaneous and ACTH-induced corticosterone (B) release from both fasciculata-reticularis enriched and total adrenal cells. The results indicate that A II at 10(-12), 10(-11), 10(-10) and 10(-6) M concentrations did not modify basal B production and A II 10(-9), 10(-8) and 10(-7) M decreased basal B production from total adrenal cells. Whereas A II (10(-10)-10(-6) M) stimulated B release from fasciculata-reticularis enriched cells. On the one hand, 10(-8) M A II significantly decreased ACTH-elicited B release from total adrenal cells; effect completely abolished by saralasin (SAR, 10(-8) M), a specific A II receptor blocker. On the other hand, 10(-8) M A II did not modify ACTH-induced B release from fasciculata-reticularis enriched cells. Finally, 10(-10) to 10(-6) M A II and 22 pM ACTH stimulated aldosterone output from total adrenal cells; while, fasciculata-reticularis enriched cells did not secrete any measurable amount of aldosterone under basal condition and after incubation with A II. These data further suggest a regulatory role of A II in the release of glucocorticoids from the adrenal gland.

Quantitative in vitro autoradiographic characterization of [125I]angiotensin III binding sites in rat adrenal gland

A single class of high-affinity binding sites for [125I]angiotensin III and [125I]angiotensin II were found in rat adrenal medulla and zona glomerulosa by quantitative autoradiography. In the medulla, Kd were 1.46 and 1.16 nM, and Bmax 1700 and 1700 fmol/mg protein, for [125I]angiotensin II and [125I]angiotensin III, respectively. In the zona glomerulosa, Kd were 0.86 and 0.90 nM, and Bmax 790 and 560 fmol/mg protein, for [125I]angiotensin II and [125I]angiotensin III, respectively. Unlabeled angiotensin III and angiotensin II displaced [125I]angiotensin III with similar potency in both adrenal zona glomerulosa and medulla. Our findings suggest that angiotensin III and angiotensin II might share the same binding sites in adrenal gland and support the hypothesis of a role for angiotensin III in the adrenal medulla and zona glomerulosa.

Pressor responses to amastatin, bestatin and Plummer's inhibitors are suppressed by pretreatment with the angiotensin receptor antagonist sarthran

The aminopeptidase inhibitors, amastatin (AM) and bestatin (BE), and carboxypeptidase inhibitor Plummer's (PL) were applied intracerebroventricularly (ICV) in rats following pretreatment with the angiotensin receptor antagonist sarthran (Sar1,Thr8-AII) or artificial cerebrospinal fluid. Angiotensin II (AII) was also included as a comparison vasoactive peptide. Pressor responses were recorded at 30 min intervals for 90 min to ascertain the duration of the antagonistic effect of sarthran on subsequent injections of AM, BE, PL and AII. Sarthran was effective in suppressing pressor activity to AII- and PL-induced pressor activity until 60 min following pretreatment, and AM- and BE-induced pressor responses until 90 min following pretreatment. These data suggest that AM, BE and PL are having their pressor effects via the central angiotensinergic system and that the patterns of AM, BE, PL and AII recovery from the influence of a specific angiotensin receptor antagonist are similar. The results are consistent with the concept that these inhibitors may increase endogenously synthesized angiotensins which are associated with pressor responses.

The effects of the aminopeptidase inhibitors amastatin and bestatin on angiotensin-evoked neuronal activity in rat brain

During a recent comparison of iontophoretically applied angiotensin II (AII) and angiotensin III (AIII) in the paraventricular nucleus of the rat, we observed that the response latency for AIII was much shorter than that for AII. This suggested that AII may have to be converted to AIII before it becomes active. To test this hypothesis we performed 3 experiments. (1) We examined the effects of bestatin, an aminopeptidase B inhibitor, on the activity of applied AII and AIII. (2) Next, we monitored the effects of amastatin, a specific aminopeptidase A inhibitor, on the action of co-applied AII or AIII. (3) And, finally, we examined the response to the aminopeptidase-resistant analog Sar1-AII, both applied alone and in combination with AII or AIII. Bestatin, while having no activity of its own, dramatically enhanced the actions of both AII and AIII. Amastatin, on the other hand, had little effect on AII's action and diminished or totally blocked AII-dependent activity. Like bestatin, amastatin had no effect alone. Sar1-AII reduced spontaneous activity of angiotensin-sensitive neurons and inhibited the actions of AII and AIII in a reversible manner. The same cells were also blocked by the recognized angiotensin antagonist Sar1, Ile8-AII. In total these results strongly support the notion that AII must be converted to AIII in the brain before it is activated.

Stimulation by angiotensins I and II of ACTH release from goldfish pituitary cell columns

Dispersed, superfused goldfish anterior pituitary cell columns were stimulated with pulses of salmon angiotensin I (sAI), human angiotensin I (hAI), and human angiotensin II (hAII). Human AII stimulated the greatest release of ACTH. The dose-response curves for hAI and sAI were similar and revealed that hAI and sAI were about one-tenth as potent as hAII in stimulating ACTH release. In mammals, AI must be converted to AII in order to stimulate ACTH release. In goldfish, the angiotensin-converting enzyme inhibitor captopril, which inhibits the conversion of AI to AII, was not able to block sAI-stimulated ACTH release. This finding suggests that the angiotensin receptor of the goldfish corticotrope is less discriminating than that of the mammalian corticotrope and recognizes both AI and AII. This hypothesis was supported by the observation that sarcosine analogs of AII, which block AII-stimulated ACTH release in mammals, failed to block hAII-stimulated ACTH release in goldfish. Saralasin showed negligible, [Sar1,Thr8]-AII slight, and [Sar1, Ile8]-AII moderate, intrinsic ACTH-releasing activity. These findings suggest that the ACTH-releasing activity of angiotensin appeared early in the evolution of the vertebrate pituitary.