Heterogeneous activation mechanisms in the renal microvasculature

Single-cell transcriptomic heterogeneity between conduit and resistance mesenteric arteries in rats

The endothelium contains morphologically similar cells throughout the vasculature, but individual cells along the length of a single vascular tree or in different regional circulations function dissimilarly. When observations made in large arteries are extrapolated to explain the function of endothelial cells (ECs) in the resistance vasculature, only a fraction of these observations are consistent between artery sizes. To what extent endothelial (EC) and vascular smooth muscle cells (VSMCs) from different arteriolar segments of the same tissue differ phenotypically at the single-cell level remains unknown. Therefore, single-cell RNA-seq (10x Genomics) was performed using a 10X Genomics Chromium system. Cells were enzymatically digested from large (>300 µm) and small (<150 µm) mesenteric arteries from nine adult male Sprague-Dawley rats, pooled to create six samples (3 rats/sample, 3 samples/group). After normalized integration, the dataset was scaled before unsupervised cell clustering and cluster visualization using UMAP plots. Differential gene expression analysis allowed us to infer the biological identity of different clusters. Our analysis revealed 630 and 641 differentially expressed genes (DEGs) between conduit and resistance arteries for ECs and VSMCs, respectively. Gene ontology analysis (GO-Biological Processes, GOBP) of scRNA-seq data discovered 562 and 270 pathways for ECs and VSMCs, respectively, that differed between large and small arteries. We identified eight and seven unique ECs and VSMCs subpopulations, respectively, with DEGs and pathways identified for each cluster. These results and this dataset allow the discovery and support of novel hypotheses needed to identify mechanisms that determine the phenotypic heterogeneity between conduit and resistance arteries.

P2 receptors in renal autoregulation

Autoregulation of renal blood flow and glomerular filtration rate is an essential function of the renal microcirculation. While the existence of this phenomenon has been known for many years, the exact mechanisms that underlie this regulatory system remain poorly understood. The work of many investigators has provided insights into many aspects of the autoregulatory mechanism, but many critical components remain elusive. This review is intended to update the reader on the role of P2 purinoceptors as a postulated mechanism responsible for renal autoregulatory resistance adjustments. It will summarize recent advances in normal function and it will touch on more recent ideas regarding autoregulatory insufficiency in hypertension and inflammation. Current thoughts on the nature of the mechanosensor responsible for myogenic behavior will be also be discussed as well as current thoughts on the mechanisms involved in ATP release to the extracellular fluid space.

Functional importance of T-type voltage-gated calcium channels in the cardiovascular and renal system: news from the world of knockout mice

Over the years, it has been discussed whether T-type calcium channels Cav3 play a role in the cardiovascular and renal system. T-type channels have been reported to play an important role in renal hemodynamics, contractility of resistance vessels, and pacemaker activity in the heart. However, the lack of highly specific blockers cast doubt on the conclusions. As new T-type channel antagonists are being designed, the roles of T-type channels in cardiovascular and renal pathology need to be elucidated before T-type blockers can be clinically useful. Two types of T-type channels, Cav3.1 and Cav3.2, are expressed in blood vessels, the kidney, and the heart. Studies with gene-deficient mice have provided a way to investigate the Cav3.1 and Cav3.2 channels and their role in the cardiovascular system. This review discusses the results from these knockout mice. Evaluation of the literature leads to the conclusion that Cav3.1 and Cav3.2 channels have important, but different, functions in mice. T-type Cav3.1 channels affect heart rate, whereas Cav3.2 channels are involved in cardiac hypertrophy. In the vascular system, Cav3.2 activation leads to dilation of blood vessels, whereas Cav3.1 channels are mainly suggested to affect constriction. The Cav3.1 channel is also involved in neointima formation following vascular damage. In the kidney, Cav3.1 regulates plasma flow and Cav3.2 plays a role setting glomerular filtration rate. In conclusion, Cav3.1 and Cav3.2 are new therapeutic targets in several cardiovascular pathologies, but the use of T-type blockers should be specifically directed to the disease and to the channel subtype.

Adenosine A1 receptor-dependent and independent pathways in modulating renal vascular responses to angiotensin II

AIM

Renal afferent arterioles are the effector site for autoregulation of glomerular perfusion and filtration. There is synergistic interaction between angiotensin II (ANG II) and adenosine (Ado) in regulating arteriolar contraction; however, the mechanisms are not clear. In this context, this study investigated the contribution of A1 receptor-dependent and independent signalling mechanisms.

METHODS

Isolated perfused afferent arterioles from transgenic mice (A1 (+/+) and A1 (-/-) ) were used for vascular reactivity studies. Cultured vascular smooth muscle cells (VSMC) were used for phosphorylation studies of signalling proteins that induce arteriolar contraction.

RESULTS

Maximal arteriolar contraction to ANG II was attenuated in A1 (-/-) (22%) compared with A1 (+/+) (40%). Simultaneous incubation with low-dose ado (10(-8)  mol L(-1) ) enhanced ANG II-induced contraction in A1 (+/+) (58%), but also in A1 (-/-) (42%). An ado transporter inhibitor (NBTI) abolished this synergistic effect in A1 (-/-) , but not in wild-type mice. Incubation with Ado + ANG II increased p38 phosphorylation in aortic VSMC from both genotypes, but treatment with NBTI only blocked phosphorylation in A1 (-/-) . Combination of ANG II + Ado also increased MLC phosphorylation in A1 (+/+) but not significantly in A1 (-/-) , and NBTI had no effects. In agreement, Ado + ANG II-induced phosphorylation of p38 and MLC in rat pre-glomerular VSMC was not affected by NBTI. However, during pharmacological inhibition of the A1 receptor simultaneous treatment with NBTI reduced phosphorylation of both p38 and MLC to control levels.

CONCLUSION

Interaction between ANG II and Ado in VSMC normally involves A1 receptor signalling, but this can be compensated by receptor independent actions that phosphorylate p38 MAPK and MLC.

Differential effect of T-type voltage-gated Ca2+ channel disruption on renal plasma flow and glomerular filtration rate in vivo

Voltage-gated Ca(2+) (Cav) channels play an essential role in the regulation of renal blood flow and glomerular filtration rate (GFR). Because T-type Cav channels are differentially expressed in pre- and postglomerular vessels, it was hypothesized that they impact renal blood flow and GFR differentially. The question was addressed with the use of two T-type Cav knockout (Cav3.1(-/-) and Cav3.2(-/-)) mouse strains. Continuous recordings of blood pressure and heart rate, para-aminohippurate clearance (renal plasma flow), and inulin clearance (GFR) were performed in conscious, chronically catheterized, wild-type (WT) and Cav3.1(-/-) and Cav3.2(-/-) mice. The contractility of afferent and efferent arterioles was determined in isolated perfused blood vessels. Efferent arterioles from Cav3.2(-/-) mice constricted significantly more in response to a depolarization compared with WT mice. GFR was increased in Cav3.2(-/-) mice with no significant changes in renal plasma flow, heart rate, and blood pressure. Cav3.1(-/-) mice had a higher renal plasma flow compared with WT mice, whereas GFR was indistinguishable from WT mice. No difference in the concentration response to K(+) was observed in isolated afferent and efferent arterioles from Cav3.1(-/-) mice compared with WT mice. Heart rate was significantly lower in Cav3.1(-/-) mice compared with WT mice with no difference in blood pressure. T-type antagonists significantly inhibited the constriction of human intrarenal arteries in response to a small depolarization. In conclusion, Cav3.2 channels support dilatation of efferent arterioles and affect GFR, whereas Cav3.1 channels in vivo contribute to renal vascular resistance. It is suggested that endothelial and nerve localization of Cav3.2 and Cav3.1, respectively, may account for the observed effects.

Functional and pharmacological consequences of the distribution of voltage-gated calcium channels in the renal blood vessels

Calcium channel blockers are widely used to treat hypertension because they inhibit voltage-gated calcium channels that mediate transmembrane calcium influx in, for example, vascular smooth muscle and cardiomyocytes. The calcium channel family consists of several subfamilies, of which the L-type is usually associated with vascular contractility. However, the L-, T- and P-/Q-types of calcium channels are present in the renal vasculature and are differentially involved in controlling vascular contractility, thereby contributing to regulation of kidney function and blood pressure. In the preglomerular vascular bed, all the three channel families are present. However, the T-type channel is the only channel in cortical efferent arterioles which is in contrast to the juxtamedullary efferent arteriole, and that leads to diverse functional effects of L- and T-type channel inhibition. Furthermore, by different mechanisms, T-type channels may contribute to both constriction and dilation of the arterioles. Finally, P-/Q-type channels are involved in the regulation of human intrarenal arterial contractility. The calcium blockers used in the clinic affect not only L-type but also P-/Q- and T-type channels. Therefore, the distinct effect obtained by inhibiting a given subtype or set of channels under experimental settings should be considered when choosing a calcium blocker for treatment. T-type channels seem to be crucial for regulating the GFR and the filtration fraction. Use of blockers is expected to lead to preferential efferent vasodilation, reduction of glomerular pressure and proteinuria. Therefore, renovascular T-type channels might provide novel therapeutic targets, and may have superior renoprotective effects compared to conventional calcium blockers.

Antialbuminuric advantage of cilnidipine compared with L-type calcium channel blockers in type 2 diabetic patients with normoalbuminuria and microalbuminuria

We evaluated the antialbuminuric advantage of cilnidipine, an N/L-type calcium channel blocker (CCB), compared with L-type CCBs in diabetic patients with normoalbuminuria and microalbuminuria. The study was a multicenter, non-randomized crossover trial. Participants were 90 type 2 diabetic patients exhibiting either normo- or microalbuminuria, and undergoing CCB treatment for ≥6 months prior to study entry. The CCB at the time of entry was continued for the first 6 months (Period 1). Treatment was subsequently switched from cilnidipine to an L-type CCB, or vice versa, for the second 6-month observation period (Period 2). During Period 1, the L-type CCB group showed a significant increase of urinary albumin excretion (UAE) over time, while the cilnidipine group showed no significant elevation. During Period 2, switching of the treatment from the L-type CCB to cilnidipine resulted in significant reduction of the UAE, whereas switching from cilnidipine to the L-type CCB resulted in no significant change in the UAE. This study demonstrated that the antialbuminuric effect of Cilnidipine, but not the L-type CCBs, was sustained even in patients treated for a long time. In addition, the antialbuminuric effect can be anticipated after switching from an L-type CCB to cilnidipine, but not vice versa.

Calcium and chloride channel activation by angiotensin II-AT1 receptors in preglomerular vascular smooth muscle cells

The pathways responsible for the rapid and sustained increases in [Ca(2+)](i) following activation of ANG II receptors (AT(1)) in renal vascular smooth muscle cells were evaluated using fluorescence microscopy. Resting intracellular calcium concentration [Ca(2+)](i) averaged 75 +/- 9 nM. The response to ANG II (100 nM) was characterized by a rapid initial increase of [Ca(2+)](i) by 74 +/- 6 nM (n = 35) followed by a decrease to a sustained level of 12 +/- 2 nM above baseline. The average time from peak to 50% reduction from the peak value (50% time point) was 32 +/- 4 s. AT(1) receptor blockade with 1 microM candesartan (n = 5) prevented the responses to ANG II. In nominally calcium-free conditions (n = 8), the peak increase in [Ca(2+)](i) averaged 42 +/- 7 nM but the sustained phase was absent and the 50% time point was reduced to 11 +/- 4 s. L-type calcium channel blockade with diltiazem reduced the peak [Ca(2+)](i) to 24 +/- 8 nM and the sustained level to 4 +/- 2 nM (n = 10). In cells preincubated in low Cl(-) (3.0 mM), the peak response to ANG II was suppressed as was the sustained response. Blockade of chloride channels with DIDS eliminated both the peak and sustained responses (n = 11); chloride channel blockade with DPC (n = 17) suppressed the peak increase in [Ca(2+)](i) to 18 +/- 5 and also prevented the sustained response. IP3 receptor blockade by 10 microM TMB-8 (n = 6) reduced the peak to 22 +/- 8 and prevented the sustained response. Exposure to 10 microM TMB-8 in the presence of Ca(2+)-free medium prevented the ANG II response (n = 9). In the presence of 100 microM DPC and 10 microM TMB-8 (n = 7), the ANG II response was also prevented. Thus the rapid initial increase in [Ca(2+)](i) is due not only to release from intracellular stores, but also to Ca(2+) influx from the extracellular fluid. Although Ca(2+) entry via L-type calcium channels is responsible for the major portion of the sustained response, other entry pathways participate. The finding that chloride channel blockers markedly attenuate both rapid and sustained responses indicates that chloride channel activation contributes to, rather than being the consequence of, the initial rapid response.

Real-time observation of glomerular hemodynamic changes in diabetic rats: Effects of insulin ARB

BACKGROUND

The progression of diabetic nephropathy is closely related to disturbances in glomerular hemodynamics, such as glomerular hypertension and/or hyperperfusion. The aim of this study was to observe and to analyze glomerular hemodynamics in rats with diabetes mellitus (DM) in vivo using confocal laser scan microscopy (CLSM). We also examined the effects of candesartan cilexetil (TCV-116), a selective angiotensin II type 1 receptor blocker (ARB), on glomerular hemodynamics in DM.

METHODS

Munich-Wistar rats were divided into six groups: (1) four-day control; (2) four-day DM; (3) 28-day control; (4) 28-day DM; (5) DM treated with insulin; (6) DM treated with TCV-116. The kidney-to-body weight ratio, glomerular volume, and proteinuria were estimated. Glomerular hemodynamic changes were observed using CLSM and renal expression of endothelial nitric oxide synthase (eNOS), and neuronal nitric oxide synthase (nNOS) was evaluated by immunofluorescence.

RESULTS

The kidney-to-body weight ratio, glomerular volume, the diameters of afferent arterioles (AA) and efferent arterioles (EA), erythrocyte velocities within glomeruli, and volume flow in glomerular capillary loops in four-day DM were significantly higher than in control rats, and increases were even more pronounced in the 28-day DM. TCV-116 treatment ameliorated all these findings and significantly decreased proteinuria, but there was no effect on the blood glucose level. On the other hand, insulin treatment was followed by normalization of all these changes induced in DM. Enhanced renal expression of eNOS in DM was suppressed when treated with either TCV-116 or insulin, while expression of nNOS was unaltered among the four groups.

CONCLUSION

This imaging procedure allowed us to evaluate glomerular microcirculation in vivo, including the diameters of AA and EA, erythrocyte velocity, and volume flow. DM significantly induced glomerular hemodynamic alteration and renal hypertrophy. DM treated with either insulin or ARB ameliorated these changes. This study shows that progress in imaging technology promises to make major contributions to revealing the involvement of hemodynamic changes in glomerular diseases, aiding prognosis and the monitoring of therapeutic effects, as well.

Renal autoregulation in P2X1 knockout mice

Autoregulation of renal blood flow is an established physiological phenomenon, however the signalling mechanisms involved remain elusive. Autoregulatory adjustments in preglomerular resistance involve myogenic and tubuloglomerular feedback (TGF) influences. While there is general agreement on the participation of these two regulatory pathways, the signalling molecules and effector mechanisms have not been identified. Currently, there are two major hypotheses being considered to explain the mechanism by which TGF signals are transmitted from the macula densa to the afferent arteriole. The adenosine hypothesis proposes that the released adenosine triphosphate (ATP) is hydrolysed to adenosine and this product stimulates preglomerular vasoconstriction by activation of A(1) receptors on the afferent arteriole. Alternatively, the P2 receptor hypothesis postulates that ATP released from the macula densa directly stimulates afferent arteriolar vasoconstriction by activation of ATP-sensitive P2X(1) receptors. This hypothesis has emerged from the realization that P2X(1) receptors are heavily expressed along the preglomerular vasculature. Inactivation of P2X(1) receptors impairs autoregulatory responses while afferent arteriolar responses to A(1) adenosine receptor activation are retained. Autoregulatory behaviour is markedly attenuated in mice lacking P2X(1) receptors but responses to adenosine A(1) receptor activation remain intact. More recent experiments suggest that P2X(1) receptors play an essential role in TGF-dependent vasoconstriction of the afferent arteriole. Interruption of TGF-dependent influences on afferent arteriolar diameter, by papillectomy or furosemide treatment, significantly attenuated pressure-mediated afferent arteriolar vasoconstriction in wild-type mice but had no effect on the response in P2X(1) knockout mice. Collectively, these observations support an essential role for P2X(1) receptors in TGF-mediated afferent arteriolar vasoconstriction.

Lead–cadmium interaction effect on the responsiveness of rat mesenteric vessels to norepinephrine and angiotensin II

The comparison of the reactivity to norepinephrine (NE) and angiotensin II (A II) of isolated mesenteric blood vessels obtained from rats simultaneously poisoned with lead and cadmium to those responses of rats treated singly with lead or cadmium was performed. Male Buffalo rats aged 6-8 weeks were administered intragastrically with lead (35 mg Pb/kg body wt.) and/or cadmium (5 mg Cd/body wt.), once a week for a period of 7 weeks. Control rats were given equimolar amounts of sodium acetate and/or sodium chloride. Changes in mesenteric vascular resistance due to NE and A II injections were measured ex vivo as an increase in perfusion pressure in vessels prepared by McGregor's method. The dose-response curve for NE (0.01-5.0 microg) determined for vessels of rats poisoned simultaneously with lead and cadmium was shifted to the left in comparison to controls (not poisoned rats), similarly to these determined for rats poisoned with lead or cadmium. ED(50) NE pointed out in the control group (0.83+/-0.5 microg) was significantly greater than in metal treated groups (0.44+/-0.09; 0.45+/-0.26 and 0.5+/-0.11 microg in lead, cadmium, lead and cadmium-treated rats, respectively). This study indicated a tachyphylaxis in responses of isolated mesenteric vessels to A II injected in increasing doses, and the weaker, in comparison to controls, response of vessels of rats poisoned with lead and/or cadmium to A II at a dose of 0.4 microg. The decreasing response to A II could result from changes in calcium ions transport through L-type channels in vascular smooth muscle cells, because verapamil (2.0 microM) inhibited the A II-induced vasoconstriction more weakly in rats poisoned with metals than in controls. Inhibitor of prostaglandins synthesis, ketoprofen (200 microg/ml per min.) attenuated the pressor effect of NE in blood vessels obtained from all rats, but this effect was less potent in arteries of cadmium poisoned rats. Ketoprofen also inhibited the vasoconstrictory action of A II in all groups, but this effect was lower in vessels of rats poisoned simultaneously with lead and cadmium. We suggest that the release of vasoactive prostaglandins as a consequence of endothelial angiotensin receptor stimulation changes more under the influence of metals administered to rats simultaneously than under the influence or lead or cadmium administered singly. Treatment with a nitric oxide synthase inhibitor (L-NOARG; 22 microg/ml per min.) potentiated a NE-induced pressor response in all groups. However, the increase in perfusion pressure was greater in rats poisoned with cadmium in comparison to controls. L-NOARG potentiated the A II induced vasoconstriction only in cadmium poisoned rats, also indicating a greater influence of nitric oxide in cadmium treated rat vasculature. Two-way ANOVA showed the existence of lead-cadmium interactions effects on the reactivity of rat isolated mesenteric vessels to NE, A II and papaverine.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Differential neural control of intrarenal blood flow

To test whether renal sympathetic nerve activity (RSNA) can differentially regulate blood flow in the renal medulla (MBF) and cortex (CBF) of pentobarbital sodium-anesthetized rabbits, we electrically stimulated the renal nerves while recording total renal blood flow (RBF), CBF, and MBF. Three stimulation sequences were applied 1) varying amplitude (0.5-8 V), 2) varying frequency (0.5-8 Hz), and 3) a modulated sinusoidal pattern of varying frequency (0. 04-0.72 Hz). Increasing amplitude or frequency of stimulation progressively decreased all flow variables. RBF and CBF responded similarly, but MBF responded less. For example, 0.5-V stimulation decreased CBF by 20 +/- 9%, but MBF fell by only 4 +/- 6%. The amplitude of oscillations in all flow variables was progressively reduced as the frequency of sinusoidal stimulation was increased. An increased amplitude of oscillation was observed at 0.12 and 0.32 Hz in MBF and to a lesser extent RBF, but not CBF. MBF therefore appears to be less sensitive than CBF to the magnitude of RSNA, but it is more able to respond to these higher frequencies of neural stimulation.

P2 receptors in the kidney

Our understanding of the actions of extracellular ATP in controlling kidney function via stimulation of P2 receptors is still at an early stage. Recently, several groups, including our own, have begun to address this subject: in this brief review, we discuss some of these effects and speculate on likely function of extracellular nucleotides in the kidney.

Dissimilar mechanisms of Ca(2+) response to bradykinin in different types of juxtamedullary glomerular arterioles

Bradykinin (BK)-induced changes in intracellular calcium level ([Ca(2+)](i)) were studied on fura 2-loaded afferent (AA) and efferent glomerular arterioles (EA) microdissected from juxtamedullary renal cortex. A distinction was made between thin and muscular EA. In AA and both types of EA, BK increased [Ca(2+)](i) through activation of B(2) receptors located only on the endothelium. The responses were not affected by nifedipine (10(-6) M) and were smaller in a Ca(2+)-free medium, providing evidence that BK opens voltage-independent Ca(2+) channels and mobilizes intracellular Ca(2+). Thin EA differed from AA and muscular EA by a lower sensitivity to BK (EC(50) = 6.95 +/- 3.81 vs. 0.21 +/- 0.08 and 0.18 +/- 0.13 nM, respectively; P < 0.05), a higher maximal response (89 +/- 5 vs. 57 +/- 5 and 44 +/- 7 nM; P < 0.001), and a spontaneous return to basal Ca(2+) level, even in the presence of BK. Genistein (10(-4) M) and herbimycin A (25 x 10(-6) M), specific inhibitors of tyrosine kinases, inhibited the [Ca(2+)](i) responses exclusively in AA. Genistein reduced the peak and plateau phases of responses by 69 +/- 9 and 82 +/- 6%, respectively, in a medium with Ca(2+) and the peak by 48 +/- 9% in a Ca(2+)-free medium. Similar reductions were observed with herbimycin A. These results show that dissimilar signal transduction pathways are involved in BK effects on juxtamedullary arterioles and that a tyrosine kinase activity could participate in the regulation of BK effect on AA but not on EA.

20-HETE and the kidney: resolution of old problems and new beginnings

The protean properties of 20-hydroxyeicosatetraenoic acid (HETE), vasoactivity, mitogenicity, and modulation of transport in key nephron segments, serve as the basis for the essential roles of 20-HETE in the regulation of the renal circulation and electrolyte excretion and as a second messenger for endothelin-1 and mediator of selective renal effects of ANG II. Renal autoregulation and tubular glomerular feedback are mediated by 20-HETE through constriction of preglomerular arterioles, responses that are maintained by 20-HETE inhibition of calcium-activated potassium channels. 20-HETE modulates ion transport in the proximal tubules and the thick ascending limb by affecting the activities of Na+-K+-ATPase and the Na+-K+-2Cl- cotransporter, respectively. The range and diversity of activity of 20-HETE derives in large measure from COX-dependent transformation of 20-HETE to products affecting vasomotion and salt and water excretion. Nitric oxide (NO) exerts a negative modulatory effect on 20-HETE formation; inhibition of NO synthesis produces marked perturbation of renal function resulting from increased 20-HETE production. 20-HETE is an essential component of interactions involving several hormonal systems that have central roles in blood pressure homeostasis, including angiotensins, endothelins, NO, and cytokines. 20-HETE is the preeminent renal eicosanoid, overshadowing PGE2 and PGI2. This review is intended to provide evidence for the physiological roles for cytochrome P-450-derived eicosanoids, particularly 20-HETE, and seeks to extend this knowledge to a conceptual framework for overall cardiovascular function.