Governance of arteriolar oscillation by ryanodine receptors

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Hypoxia differently modulates gene expression of inositol 1,4,5-trisphosphate receptors in mouse kidney and HEK 293 cell line

Hypoxia is a state of insufficient oxygen supply of the tissue or cell. Kidney tissue is highly sensitive to oxygen deprivation and easily develops renal ischemic injury. Calcium transporters very sensitively react to oxygen deficiency. We investigated whether hypoxia affects the gene expression of intracellular calcium transporters in the intact kidney, and we compared the response to that of HEK 293 cells. Our results showed that, while in mouse kidney tissue hypoxia elevates mRNA for inositol 1,4,5-trisphosphate receptors (IP3R) type 1 (IP3R1) and ryanodine receptors (RyR) type 2 (RyR2), in culture of HEK 293 cells the gene expression of all IP3Rs decreased without affecting viability of the cells. RyR2 mRNA in HEK 293 cells was not significantly changed, but RyR1 gene expression was significantly increased by hypoxia. The different response of kidney tissue and HEK 293 cells to hypoxia could be due to unequal differentiation state of the cells in intact tissue and cultured embryonic cell line. The physiological relevance of this observation remains to be determined.

Vasomotion dynamics following calcium spiking depend on both cell signalling and limited constriction velocity in rat mesenteric small arteries

Vascular smooth muscle cell contraction depends on intracellular calcium. However, calcium-contraction coupling involves a complex array of intracellular processes. Quantitating the dynamical relation between calcium perturbations and resulting changes in tone may help identifying these processes. We hypothesized that in small arteries accurate quantitation can be achieved during rhythmic vasomotion, and questioned whether these dynamics depend on intracellular signalling or physical vasoconstriction. We studied calcium-constriction dynamics in cannulated and pressurized rat mesenteric small arteries (∼300 μm in diameter). Combined application of tetra-ethyl ammonium (TEA) and BayK8644 induced rhythmicity, consisting of regular and irregular calcium spiking and superposition of spikes. Calcium spikes induced delayed vasomotion cycles. Their dynamic relation could be fitted by a linear second-order model. The dirac impulse response of this model had an amplitude that was strongly reduced with increasing perfusion pressure between 17 and 98 mmHg, while time to peak and relaxation time were the largest at an intermediate pressure (57 mmHg: respectively 0.9 and 2.3 sec). To address to what extent these dynamics reside in intracellular signalling or vasoconstriction, we applied rhythmic increases in pressure counteracting the vasoconstriction. This revealed that calcium-activation coupling became faster when vasoconstriction was counteracted. During such compensation, a calcium impulse response remained that lasted 0.5 sec to peak activation, followed by a 1.0 sec relaxation time, attributable to signalling dynamics. In conclusion, this study demonstrates the feasibility of quantitating calcium-activation dynamics in vasomoting small arteries. These dynamics relate to both intracellular sig-nalling and actual vasoconstriction. Performing such analyses during pharmacological intervention and in genetic models provides a tool for unravelling calcium-contraction coupling in small arteries.

Effects on kidney filtration rate by agmatine requires activation of ryanodine channels for nitric oxide generation

Agmatine, decarboxylated arginine, is produced in the kidney and can increase nephron and kidney filtration rate via renal vasodilatation and increases in plasma flow. This increase in filtration rate after agmatine is prevented by administration of nitric oxide synthase (NOS) inhibitors. In endothelial cells, agmatine-stimulated nitrite production is accompanied by induction of cytosolic calcium. NOS activity requires calcium for activation; however, the source of this calcium remains unknown. Ryanodine receptor (RyR) calcium-activated calcium release channels are present in the kidney cortex, and we evaluated if RyR contributes to the agmatine response. Agmatine microperfused into Bowman's space reversibly increases nephron filtration rate (SNGFR) by approximately 30%. cADP-ribose (cADPR) regulates RyR channel activity. Concurrent infusion of agmatine with the cADPR blocker 8-bromo-cADPR (2 microM) prevents the increase in filtration rate. Furthermore, direct activation of the RyR channel with ryanodine at agonist concentrations (5 microM) increases SNGFR, and, like agmatine, this increase is prevented by administration of N(G)-monomethyl-l-arginine, a nonselective NOS blocker. We demonstrate that agmatine does not elicit ADPR cyclase activity in vascular smooth muscle membranes and does not directly affect RyR calcium channel responses using sea urchin egg homogenates. These results imply interplay between endothelial cell cADPR/RyR/Ca(2+)/NO and the cADPR/RyR/Ca(2+) pathways in vascular smooth muscle cells in arterioles in the regulation of kidney filtration rate. In conclusion, we show that agmatine-induced effects require activation of cADPR and RyR calcium release channels for NO generation, vasodilation, and increased filtration rate.

Connexins 37 and 40 transduce purinergic signals mediating renal autoregulation

Our previous data indicated that various subtypes of connexin (Cx) were expressed in the juxtaglomerular apparatus. Experiments were performed to characterize the effects on renal autoregulation of specific mimetic peptides that inhibit these Cx subtypes in Wistar-Kyoto rats. Intrarenal infusion of (Cx37,43)GAP27 increased autoregulatory index of renal plasma flow (0.06 +/- 0.05 to 0.47 +/- 0.06, n = 6, P < 0.05) and glomerular filtration rate (GFR; 0.01 +/- 0.07 to 0.49 +/- 0.07, P < 0.05). The additional administration of 8-cyclopentyl- 1,3-dipropylxanthine (CPX) produced a further elevation of autoregulatory index of RPF (0.86 +/- 0.07, P < 0.05) and GFR (0.88 +/- 0.09, P < 0.05), compared with (Cx37,43)GAP27 alone. However, the addition of pyridoxal-phosphate-6-azophenyl-2,4-disulfonic acid (PPADS) to (Cx37,43)GAP27 did not. Combined treatment with CPX and PPADS markedly worsened autoregulatory index of RPF (0.04 +/- 0.10 to 0.81 +/- 0.06, n = 6 P < 0.01) and GFR (0.05 +/- 0.08 to 0.79 +/- 0.05, P < 0.01). (Cx40)GAP27 induced similar changes to (Cx37,43)GAP27. Renal autoregulation was preserved in the presence of (Cx43)GAP26. Our results indicate that the inhibition of gap junction impaired renal autoregulation. Furthermore, the present data provide evidence that both adenosine and purinergic receptors contribute to glomerular autoregulation. Finally, our findings suggest that gap junctions, at least in part, transduce purinergic signals mediating renal autoregulation.

Calcium oscillations induced by ATP in human umbilical cord smooth muscle cells

Arterial smooth muscle cells exhibit vasomotion, related to oscillations in intracellular Ca(2+) concentration, but the origin and function of these has not yet been fully determined. We measured intracellular Ca(2+) using conventional fluorescent methods in primary cultured, human umbilical cord artery smooth muscle cells (HUCASMC). Spontaneous oscillations in Ca(2+) was found in only 1% of all cells but exogenous, micromolar concentrations of ATP could induce Ca(2+) oscillations in 70% of cells with the most common pattern being one of regular amplitude and frequency with a return to basal levels between each peak. The P2Y agonist, UTP, but not the P2X agonist alphabeta-methylene ATP, could also induce Ca(2+) oscillations. Once induced, these oscillations could not be blocked by G-protein, PLC, VGCC or TRP channel antagonists applied individually, but could be prevented when antagonists were applied together. In the presence of EGTA, micromolar concentrations of ATP induced an elevation in intracellular Ca(2+) but did not induce Ca(2+) oscillations. The oscillation frequency induced by ATP was affected by bath Ca(2+) concentration. Taken together, these data suggest that external Ca(2+) entry maintains the Ca(2+) oscillation induced by activation of P2Y receptors. Once induced, multiple mechanisms are involved to maintain the oscillation and the oscillation frequency is determined by the speed of Ca(2+) refilling. Chronic hypoxia enhanced the Ca(2+) response and altered the oscillation frequency. We suggest that these oscillations may play a role in the maintenance of umbilical blood flow during situations in which GPCR are activated.

Frequency modulation of renal myogenic autoregulation by perfusion pressure

Recent studies of renal autoregulation have shown modulation of the faster myogenic mechanism by the slower tubuloglomerular feedback and that the modulation can be detected in the dynamics of the myogenic mechanism. Conceptual and empirical considerations suggest that perfusion pressure may modulate the myogenic mechanism, although this has not been tested to date. Here we present data showing that the myogenic operating frequency, assessed by transfer-function analysis, varied directly as a function of perfusion pressure in the hydronephrotic kidney perfused in vitro over the range from 80 to 140 mmHg. A similar result was obtained in intact kidneys in vivo when renal perfusion pressure was altered by systemic injection of NG-nitro-l-arginine methyl ester (l-NAME). When perfusion pressure was not allowed to increase, l-NAME did not affect the myogenic operating frequency despite equivalent reduction of renal vascular conductance. Blood-flow dynamics were assessed in the superior mesenteric artery before and after l-NAME. In this vascular bed, the operating frequency of the myogenic mechanism was not affected by perfusion pressure. Thus the operating frequency of the renal myogenic mechanism is modulated by perfusion pressure independently of tubuloglomerular feedback, and the data suggest some degree of renal specificity of this response.

SPONTANEOUS OSCILLATORY CONTRACTIONS IN AORTAS OF RATS WITH ARTERIAL PRESSURE LABILITY CAUSED BY SINOAORTIC DENERVATION

1. The spontaneous variation of blood pressure is defined as arterial pressure lability. Sinoaortic denervation (SAD) is characterized by arterial pressure lability without sustained hypertension. 2. The phenomenon of spontaneous oscillatory contractions (SOCs) occurs more frequently in the vascular beds of hypertensive animals. In large arteries, such as the aorta, SOCs occur only occasionally or they can be initiated by application of chemical stimuli. 3. In the present study, we investigated whether the arterial pressure lability evoked by SAD could be related to the emergence of SOCs in the aorta of rats submitted to SAD compared with sham-operated rats (SO). Three days after surgery (SAD or SO), aortic rings were placed in an organ chamber and the incidence (percentage of rats presenting SOCs), frequency (number of SOCs in 10 min) and amplitude (mN) of SOCs were measured. The participation of external Ca(2+) and K(+) channels in the maintenance of SOCs was also verified. 4. The incidence and frequency of SOCs were higher in endothelium-denuded aortas from SAD rats (82% and 38 +/- 4 SOCs/10 min, respectively) than in aortas from SO rats (40% and 14 +/- 2 SOCs/10 min, respectively). In aortas from SAD rats, verapamil (0.2 micromol/L), pinacidil (0.3 micromol/L) and tetraethylammonium (TEA; 5 mmol/L) totally inhibited SOCs, whereas increasing the CaCl(2) concentration to 2.0 and 2.5 mmol/L increased the frequency of SOCs. Interestingly, increasing the concentration of CaCl(2) to 3.5 mmol/L inhibited these contractions in aortas from SAD rats. 5. These results show that although SAD rats did not become hypertensive, their aortas were capable of initiating SOCs without the application of any chemical stimuli. The SOCs seem to be dependent on Ca(2+) influx sensitive to verapamil and also involve K(+) channels sensitive to pinacidil and TEA.

Dynamics of a three-variable nonlinear model of vasomotion: comparison of theory and experiment

The effects of pharmacological interventions that modulate Ca(2+) homeodynamics and membrane potential in rat isolated cerebral vessels during vasomotion (i.e., rhythmic fluctuations in arterial diameter) were simulated by a third-order system of nonlinear differential equations. Independent control variables employed in the model were [Ca(2+)] in the cytosol, [Ca(2+)] in intracellular stores, and smooth muscle membrane potential. Interactions between ryanodine- and inositol 1,4,5-trisphosphate-sensitive intracellular Ca(2+) stores and transmembrane ion fluxes via K(+) channels, Cl(-) channels, and voltage-operated Ca(2+) channels were studied by comparing simulations of oscillatory behavior with experimental measurements of membrane potential, intracellular free [Ca(2+)] and vessel diameter during a range of pharmacological interventions. The main conclusion of the study is that a general model of vasomotion that predicts experimental data can be constructed by a low-order system that incorporates nonlinear interactions between dynamical control variables.

Voltage-gated Ca2+ entry and ryanodine receptor Ca2+-induced Ca2+ release in preglomerular arterioles

We have previously shown that in afferent arterioles, angiotensin II (ANG II) involves activation of the inositol trisphosphate receptor (IP(3)R), activation of adenine diphosphoribose (ADPR) cyclase, and amplification of the initial IP(3)R-stimulated release of cytosolic Ca(2+) ([Ca(2+)](i)) from the sarcoplasmic reticulum (SR) (Fellner SK, Arendshorst WJ. Am J Physiol Renal Physiol 288: F785-F791, 2004). The response of the ryanodine receptor (RyR) to local increases in [Ca(2+)](i) is defined as calcium-induced calcium release (CICR). To investigate whether Ca(2+) entry via voltage-gated channels (VGCC) can stimulate CICR, we treated fura 2-loaded, freshly isolated afferent arterioles with KCl (40 mM; high KCl). In control arterioles, peak [Ca(2+)](i) increased by 165 +/- 10 nM. Locking the RyR in the closed position with ryanodine (100 microM) inhibited the [Ca(2+)](i) response by 59% (P < 0.01). 8-Br cADPR, a specific blocker of the ability of cyclic ADPR (cADPR) to sensitize the RyR to Ca(2+), caused a 43% inhibition. We suggest that the lower inhibition by 8-Br cADPR (P = 0.02, ryanodine vs. 8-Br cADPR) represents endogenously active ADPR cyclase. Depletion of SR Ca(2+) stores by inhibiting the SR Ca(2+)-ATPase with cyclopiazonic acid or thapsigargin blocked the [Ca(2+)](i) responses to KCl by 51% (P not significant vs. ryanodine or 8-Br cADPR). These data suggest that about half of the increase in [Ca(2+)](i) induced by high KCl is accomplished by activation of CICR through the ability of entered Ca(2+) to expose the RyR to high local concentrations of Ca(2+) and that endogenous cADPR contributes to the process.

New cardioprotective agent K201 is natriuretic and glomerular filtration rate enhancing

BACKGROUND

K201 (JTV519) is a newly developed 1,4-benzothiazepine drug with antiarrhythmic and cardioprotective properties. It functions via stabilization of the ryanodine receptor-calcium release channel in the heart (RyR2). This receptor has been identified in the kidney, and in vitro studies suggest a role in the control of renal hemodynamics. To date, the in vivo function of this receptor is undefined. We hypothesized that this new drug, which is being developed for the treatment of heart failure for its myocardial actions, also possesses renal hemodynamic enhancing and excretory properties. We also used immunohistochemistry to identify RyR2 in the normal canine kidney.

METHODS AND RESULTS

We investigated the renal actions of K201 during intrarenal infusion in normal anesthetized dogs. K201 was infused after baseline measurements at 2 doses (0.1 and 0.5 mg.kg(-1).min(-1)). Immunohistochemistry was used to identify RyR2 presence in the kidney not exposed to K201. K201 was potently natriuretic and diuretic, with glomerular filtration rate- and renal blood flow-enhancing actions. The excretory responses to K201 administration were associated with decreases in distal tubular reabsorption of sodium despite a mild decrease in mean arterial pressure, which returned to baseline levels after K201 discontinuation. Immunohistochemistry of the normal canine kidney revealed the presence of RyR2 in the medullary collecting duct cells.

CONCLUSIONS

We report for the first time that the newly developed cardioprotective drug K201 possesses natriuretic, diuretic, glomerular filtration rate-enhancing, and vasodilating properties that go beyond myocardial actions and may support its therapeutic role in treatment of heart failure.

Exogenous 5'-nucleotidase improves glomerular autoregulation in Thy-1 nephritic rats

Experiments were performed to characterize renal hemodynamics in Thy-1 nephritic rats. A monoclonal antibody against Thy-1 was intravenously injected to induce mesangiolysis in rats, and 2 days later renal hemodynamic responses to variations in blood pressure were determined. In the first series of experiments, autoregulation of renal plasma flow (RPF) or glomerular filtration rate (GFR) was impaired in nephritic rats. In response to a reduction in blood pressure (98 +/- 2 to 80 +/- 1 mmHg), both RPF (4.17 +/- 0.63 to 3.20 +/- 0.45 ml x min(-1) x g kidney wt(-1), P < 0.05, n = 6) and GFR (0.88 +/- 0.05 to 0.75 +/- 0.06 ml x min(-1).g kidney wt(-1), P < 0.05) were decreased in nephritic rats. Intravenous administration of furosemide and 30% albumin, both of which inhibit tubuloglomerular feedback, diminished renal autoregulation in control but not nephritic rats. In the second studies, the infusion of 5'-nucleotidase, an enzyme expressed on mesangial cells, into a renal artery ameliorated the magnitude of autoregulatory decrements in GFR in nephritic rats (-16 +/- 5 to -6 +/- 2%, P < 0.05, n = 6), but this enzyme failed to alter renal autoregulation in control rats. In the third studies, the effects of indomethacin were examined in nephritic rats. Inhibition of prostaglandin synthesis reduced RPF (4.07 +/- 0.30 to 1.54 +/- 0.22 ml x min(-1) x g kidney wt(-1), P < 0.05, n = 5) and GFR (1.03 +/- 0.18 to 0.69 +/- 0.13 ml x min(-1) x g kidney wt(-1), P < 0.05) in nephritic rats. However, cyclooxygenase inhibition failed to restore renal autoregulation in nephritic rats. Our results indicate that renal autoregulation is impaired in Thy-1 nephritis. Furthermore, the present data provide evidence that prostanoids contribute to maintain renal circulation in nephritic rats. Finally, our findings suggest that mesangial cells and/or 5'-nucleotidase plays an important role in mediating renal autoregulation.

Rhythmicity in arterial smooth muscle

Many arteries and arterioles exhibit rhythmical contractions which are synchronous over considerable distances. This vasomotion is likely to assist in tissue perfusion especially during periods of altered metabolism or perfusion pressure. While the mechanism underlying vascular rhythmicity has been investigated for many years, it has only been recently, with the advent of imaging techniques for visualizing intracellular calcium release, that significant advances have been made. These methods, when combined with mechanical and electrophysiological recordings, have demonstrated that the rhythm depends critically on calcium released from intracellular stores within the smooth muscle cells and on cell coupling via gap junctions to synchronize oscillations in calcium release amongst adjacent cells. While these factors are common to all vessels studied to date, the contribution of voltage-dependent channels and the endothelium varies amongst different vessels. The basic mechanism for rhythmical activity in arteries thus differs from its counterpart in non-vascular smooth muscle, where specific networks of pacemaker cells generate electrical potentials which drive activity within the otherwise quiescent muscle cells.

Angiotensin II Ca2+ signaling in rat afferent arterioles: stimulation of cyclic ADP ribose and IP3 pathways

ANG II induces a rise in cytosolic Ca(2+) ([Ca(2+)](i)) in vascular smooth muscle (VSM) cells via inositol trisphosphate receptor (IP(3)R) activation and release of Ca(2+) from the sarcoplasmic reticulum (SR). The Ca(2+) signal is augmented by calcium-induced calcium release (CICR) and by cyclic adeninediphosphate ribose (cADPR), which sensitizes the ryanodine-sensitive receptor (RyR) to Ca(2+) to further amplify CICR. cADPR is synthesized from beta-nicotinamide adenine dinucleotide (NAD(+)) by a membrane-bound bifunctional enzyme, ADPR cyclase. To investigate the possibility that ANG II activates the ADPR cyclase of afferent arterioles, we used inhibitors of the IP(3)R, RyR, and ADPR cyclase. Afferent arterioles were isolated from rat kidney with the magnetized microsphere and sieving technique and loaded with fura-2 to measure [Ca(2+)](i). In Ca(2+)-containing buffer, ANG II increased [Ca(2+)](i) by 125 +/- 10 nM. In the presence of the IP(3)R antagonists TMB-8 and 2-APB, the peak responses to ANG II were reduced by 74 and 81%, respectively. The specific antagonist of cADPR 8-Br ADPR and a high concentration of ryanodine (100 microM) inhibited the ANG II-induced increases in [Ca(2+)](i) by 75 and 69%, respectively. Nicotinamide and Zn(2+) are known inhibitors of the VSM ADPR cyclase. Nicotinamide diminished the [Ca(2+)](i) response to ANG II by 66%. In calcium-free buffer, Zn(2+) reduced the ANG II response by 68%. Simultaneous blockade of the IP(3) and cADPR pathways diminished the [Ca(2+)](i) response to ANG II by 83%. We conclude that ANG II initiates Ca(2+) mobilization from the SR in afferent arterioles via the classic IP(3)R pathway and that ANG II may lead to activation of the ADPR cyclase to form cADPR, which, via its action on the RyR, substantially augments the Ca(2+) response.