Regulation of renin secretion by calcium and chemiosmotic forces: (patho) physiological considerations

Extracellular cAMP: The Past and Visiting the Future in cAMP-Enriched Extracellular Vesicles

It is recently discovered that the cyclic nucleotide, cyclic adenosine monophosphate (cAMP) can be enriched in the extracellular vesicles (EVs) isolated from endothelial cells. In the current perspective a historical context for the discovery of the extracellular cAMP is provided. The story of extracellular cAMP through investigations addressing the molecule's role in the adenosine pathway is followed, which is widespread in mammalian physiology. The adenosine pathway mediates normal physiological conditions such as renin release, phosphate transport, etc., and participates in pathological conditions such as bronchoconstriction of the airways. Furthermore, adenosine mediated biological pathways are regulated via the receptor mediated intracellular cAMP pathway in mammalian cells. It then speculates on the question of whether cAMP enriched EVs could bypass typical receptor mediated cell signaling and directly activate cAMP signaling cascade in target cells. Preliminary studies to suggest cAMP enriched EVs are provided, added to naïve endothelial cells, results in an increase in intracellular cAMP. An alternate mechanism is proposed, apart from the traditional adenosine pathway, that extracellular cAMP may exert its effects and put into perspective how it might consider circulating cAMP moving forward.

Classical Renin-Angiotensin system in kidney physiology

The renin-angiotensin system has powerful effects in control of the blood pressure and sodium homeostasis. These actions are coordinated through integrated actions in the kidney, cardiovascular system and the central nervous system. Along with its impact on blood pressure, the renin-angiotensin system also influences a range of processes from inflammation and immune responses to longevity. Here, we review the actions of the "classical" renin-angiotensin system, whereby the substrate protein angiotensinogen is processed in a two-step reaction by renin and angiotensin converting enzyme, resulting in the sequential generation of angiotensin I and angiotensin II, the major biologically active renin-angiotensin system peptide, which exerts its actions via type 1 and type 2 angiotensin receptors. In recent years, several new enzymes, peptides, and receptors related to the renin-angiotensin system have been identified, manifesting a complexity that was previously unappreciated. While the functions of these alternative pathways will be reviewed elsewhere in this journal, our focus here is on the physiological role of components of the "classical" renin-angiotensin system, with an emphasis on new developments and modern concepts.

Nonmodulation as the mechanism for salt sensitivity of blood pressure in individuals with hypertension and type 2 diabetes mellitus

CONTEXT

It is assumed that in individuals with type 2 diabetes mellitus (T2DM), blood pressure sensitivity to salt intake and the frequency of a low renin state are both increased compared with the nondiabetic population. However, studies supporting these assumptions may have been confounded by participant inclusion criteria, and study results may reflect target organ damage.

OBJECTIVE

The objective of this study was to examine in a cohort of T2DM 1) the frequency of salt sensitivity of blood pressure and 2) whether alterations of the renin-angiotensin-aldosterone system (RAAS) contribute to salt sensitivity in this population.

DESIGN, PATIENTS, AND METHODS

Within participants of the HyperPATH cohort, four groups were analyzed: 1) T2DM with hypertension (HTN), n=51; 2) T2DM without HTN, n=30; 3) HTN only, n=451; and 4) normotensive, n=209. Phenotype studies were conducted after participants completed two dietary phases: liberal sodium (200 mmol/d) and low sodium (10 mmol/d) for 7 d each. Participants were admitted overnight to a clinical research center after each diet, and supine measurements of the RAAS before and after a 60-min angiotensin II infusion (3 ng/kg·min) were obtained.

RESULTS

Multivariate regression analysis demonstrated that T2DM status (all individuals with T2DM vs. individuals without T2DM) was not associated with the change in mean arterial pressure between the low and liberal sodium diets after accounting for age, gender, body mass index, race, and baseline blood pressure (T2DM status, P=0.5). Furthermore, two intermediate phenotypes of altered RAAS, low renin, and nonmodulation (NMOD), were associated with salt-sensitive blood pressure but occurred at different frequencies in the T2DM-HTN and HTN groups (low renin, 12% T2DM-HTN vs. 29% HTN; NMOD, 41% T2DM-HTN vs. 27% HTN; P=0.01).

CONCLUSION

The frequency of NMOD in participants with T2DM was significantly higher compared with HTN, suggesting that the salt sensitivity often seen in T2DM is driven by NMOD.

Blood Pressure–Dependent Inhibition of Renin Secretion Requires A1 Adenosine Receptors

Renal perfusion pressure (RPP) regulates renin release with a reduction of RPP stimulating and an elevation inhibiting renin secretion. The precise sensing and effector mechanisms by which changes in arterial pressure are linked to the exocytosis of renin are not well-defined. The present experiments were designed to study the potential role of adenosine as a mediator of this renal baroreceptor mechanism. In isolated perfused mouse kidneys a stepwise reduction of RPP from 90 mm Hg to 65 and 40 mm Hg stimulated renin secretion rates (RSR) 1.4-fold and 3.6-fold, whereas stepwise elevations of RPP from 90 mm Hg to 115 and 140 mm Hg suppressed RSR to 64% or 40% of baseline. Inactivation of A1 adenosine receptors by either pharmacological blockade (DPCPX 1 micromol/L) or genetic deletion (A1AR(-/-) mice) did not modify the stimulation of renin release by a low RPP, but completely prevented the suppression of renin secretion by higher perfusion pressures. In vivo, the induction of arterial hypertension by either acute (single subcutaneous injection) or chronic (osmotic minipump for 72 hours) application of phenylephrine significantly reduced plasma renin concentration (PRC) in wild-type mice to approximately 40% of control, whereas it did not significantly affect PRC in A1AR(-/-) mice. Together these data demonstrate that A1 adenosine receptors are indispensable for the inhibition of renin secretion by an increase in blood pressure, suggesting that formation and action of adenosine is responsible for baroreceptor-mediated inhibition of renin release. In contrast, the stimulation of the renin system by a low blood pressure appears to follow different pathways.

Biomechanical coupling in renin-releasing cells

The renin-angiotensin system is a major regulatory system controlling extracellular fluid volume and blood pressure. The rate-limiting enzyme in this hormonal cascade is renin, which is synthesized and secreted into the circulation by renal juxtaglomerular (JG) cells. The renal baroreceptor is a key physiologic regulator of renin secretion, whereby a change in renal perfusion pressure is sensed by these cells and results in a change in renin release. However, the mechanism, direct or indirect, underlying pressure transduction is unknown. We studied the direct application of mechanical stretch to rat JG cells and human renin-expressing (CaLu-6) cells on the release of renin. JG cells released a low level of baseline renin, comprising < 5% of their total renin content. By contrast, renin secretion from CaLu-6 cells comprised approximately 30% of cellular stores, yet was also stimulated twofold by 10 microM forskolin (P </= 0.001). In JG cells, mechanical stretch inhibited basal renin release by 42% (P < 0.01) and forskolin-stimulated renin release by 25% (P < 0.05). In CaLu-6 cells, stretch inhibited basal- and forskolin-stimulated renin release by 30 and 26%, respectively (both P < 0.01). Northern blot analysis demonstrated a stretch-induced reduction in baseline renin mRNA accumulation of 26% (P < 0.05) in JG and 46% (P < 0.05) in CaLu-6 cells. The data demonstrate that mechanical stretch in renin-releasing cells inhibits basal and stimulated renin release accompanied by a decrease in renin mRNA accumulation. Further studies will be necessary to characterize the intracellular events mediating biomechanical coupling in renin-expressing cells and the relationship of this signaling pathway to the in vivo baroreceptor control of renin secretion.

Exocytosis of secretory granules in the juxtaglomerular granular cells of kidneys

BACKGROUND

There is little agreement as to the secretory process of renin granules in juxtaglomerular granular cells (JG cells) of kidneys, although a large number of studies of the regulation of renin secretion have been reported.

METHODS

The structural correlation between the stimuli and the secretory process was examined in mouse JG cells on renal cortical slices incubated with the beta-adrenergic agonist, isoproterenol; the loop diuretic, furocemide; the Ca2+ chelator, EGTA; and the actin filament-disrupting agent, cytochalasin B.

RESULTS AND CONCLUSIONS

Treatment with isoproterenol (10(-5)-10(-3) M) or furocemide (10(-3) M) in Ca(2+)-containing medium did not significantly affect the ultrastructure of JG cells. In slices incubated with isoproterenol or furocemide in the Ca(2+)-free medium, JG cells occasionally contained a few electron-lucent granules at the cell periphery in addition to the electron-dense mature granules observed in the control slices. On rare occasions, the JG cells displayed omega-shaped cavities with electron-lucent matrices, a feature similar to the contents of electron-lucent granules. Cytochalasin B markedly promoted the effects of these stimulants in Ca(2+)-free medium. These findings suggest that participation of actin filament disassembly in the exocytotic process of the mature granules in JG cells.