Calcium ion dependence of myogenic renal plasma flow autoregulation: evidence from the isolated perfused rat kidney

Glucagon-like peptide-1 receptors in the kidney: impact on renal autoregulation

Glucagon-like peptide-1 (GLP-1) and strategies based on this blood sugar-reducing and appetite-suppressing hormone are used to treat obesity and type 2 diabetes. However, the GLP-1 receptor (GLP-1R) is also present in the kidney, where it influences renal function. The effect of GLP-1 on the kidney varies between humans and rodents. The effect of GLP-1 on kidney function also seems to vary depending on its concentration and the physiological or pathological state of the kidney. In studies with rodents or humans, acute infusion of pharmacological doses of GLP-1 stimulates natriuresis and diuresis. However, the effect on the renal vasculature is less clear. In rodents, GLP-1 infusion increases renal plasma flow and glomerular filtration rate, suggesting renal vasodilation. In humans, only a subset of the study participants exhibits increased renal plasma flow and glomerular filtration rate. Differential status of kidney function and changes in renal vascular resistance of the preglomerular arterioles may account for the different responses of the human study participants. Because renal function in patients with type 2 diabetes is already at risk or compromised, understanding the effects of GLP-1R activation on kidney function in these patients is particularly important. This review examines the distribution of GLP-1R in the kidney and the effects elicited by GLP-1 or GLP-1R agonists. By integrating results from acute and chronic studies in healthy individuals and patients with type 2 diabetes along with those from rodent studies, we provide insight into how GLP-1R activation affects renal function and autoregulation.

Effect of epithelial sodium channel blockade on the myogenic response of rat juxtamedullary afferent arterioles

The mechanotransduction mechanism underlying the myogenic response is poorly understood, but evidence implicates participation of epithelial sodium channel (ENaC)-like proteins. Therefore, the role of ENaC on the afferent arteriolar myogenic response was investigated in vitro using the blood-perfused juxtamedullary nephron technique. Papillectomy was used to isolate myogenic influences by eliminating tubuloglomerular feedback signals. Autoregulatory responses were assessed by manipulating perfusion pressure in 30-mm Hg steps. Under control conditions, arteriolar diameter increased by 15% from 13.0+/-1.3 to 14.7+/-1.2 microm (P<0.05) after reducing perfusion pressure from 100 to 70 mm Hg. Diameter decreased to 11.3+/-1.1 and 10.6+/-1.0 microm after increasing pressure to 130 and 160 mm Hg (88+/-1 and 81+/-2% of control diameter, P<0.05), respectively. Pressure-mediated autoregulatory responses were significantly inhibited by superfusion of 10 micromol/L amiloride (102+/-2, 97+/-4, and 94+/-3% of control diameter), or 10 micromol/L benzamil (106+/-5, 100+/-3, and 103+/-3% of control diameter), and when perfusing with blood containing 5 micromol/L amiloride (106+/-2, 97+/-4, and 97+/-4% of control diameter). Vasoconstrictor responses to 55 mmol/L KCl were preserved as diameters decreased by 67+/-4, 55+/-8, and 60+/-4% in afferent arterioles superfused with amiloride or benzamil, and perfused with amiloride, respectively. These responses were similar to responses obtained from control afferent arterioles (64+/-6%, P>0.05). Immunofluorescence revealed expression of the alpha, beta, and gamma subunits of ENaC in freshly isolated preglomerular microvascular smooth muscle cells. These results demonstrate that selective ENaC inhibitors attenuate afferent arteriolar myogenic responses and suggest that ENaC may function as mechanosensitive ion channels initiating pressure-dependent myogenic responses in rat juxtamedullary afferent arterioles.

Autoregulation of glomerular filtration rate in patients with type 2 diabetes during isradipine therapy

OBJECTIVE

Calcium-channel blockade impairs renal autoregulation in animals. Impaired renal autoregulation leads to transmission of the systemic blood pressure (BP) into the glomerulus, resulting in capillary hypertension. Information on the impact of calcium antagonist treatment on renal autoregulation in humans is lacking. This study examines the effect of isradipine treatment on the autoregulation of the glomerular filtration rate (GFR).

RESEARCH DESIGN AND METHODS

We performed a randomized double-blind crossover study with 5 mg o.d. isradipine retard and matching placebo in 16 hypertensive patients with type 2 diabetes. Each treatment arm lasted 4 weeks. On the last day of each treatment period, GFR (single-shot 51Cr-EDTA plasma clearance technique for 4 h) was measured twice between 8:00 A.M. and 5:00 P.M., first without clonidine and then after intravenous injection of 75 micro g clonidine. BP was measured every 10 min (Takeda TM2420; A&D, Tokyo).

RESULTS

Clonidine reduced mean arterial BP (MABP) by 15 +/- 1 vs. 11 +/- 1 mmHg (means +/- SE) during placebo and isradipine treatment, respectively (P < 0.05). GFR was reduced from 102 +/- 4 to 99 +/- 4 ml. min(-1). 1.73 m(-2) with placebo (P < 0.01) and from 106 +/- 5 to 98 +/- 5 ml. min(-1). 1.73 m(-2) during treatment with isradipine (P < 0.01). Mean difference (95% CI) between changes in GFR with placebo and isradipine was -4.6 ml. min(-1). 1.73 m(-2) (-10.0 to 0.6) (P = 0.08). Six patients had a reduction in GFR >13% (exceeding the normal limit of autoregulation) combined with a complete pressure-passive vasculature (defined as DeltaMABP% < or = DeltaGFR%) during isradipine treatment versus none during the placebo treatment (P < 0.05).

CONCLUSIONS

Isradipine impairs GFR autoregulation in a sizeable proportion of hypertensive type 2 diabetic patients.

Store-operated calcium influx inhibits renin secretion

On the basis of evidence that changes in the extracellular concentration of calcium effectively modulate renin secretion from renal juxtaglomerular cells, our study aimed to determine the effect of calcium influx activated by depletion of intracellular calcium stores on renin secretion. For this purpose we characterized the effects of the endoplasmatic Ca(2+)-ATPase inhibitors thapsigargin (300 nM) and cyclopiazonic acid (20 microM) on renin secretion from isolated perfused rat kidneys. We found that Ca(2+)-ATPase inhibition caused a potent inhibition of basal renin secretion as well as renin secretion activated by isoproterenol, bumetanide, and by a fall in the renal perfusion pressure. The inhibitory effect of Ca(2+)-ATPase inhibition on renin secretion was reversed within seconds by lowering of the extracellular calcium concentration into the submicromolar range but was not affected by lanthanum, gadolinium, flufenamic acid, or amlodipine. These data suggest that calcium influx triggered by release of calcium from internal stores is a powerful mechanism to inhibit renin secretion from juxtaglomerular cells. The store-triggered calcium influx pathway in juxtaglomerular cells is apparently not sensitive to classic blockers of the capacitative calcium entry pathway.

Class differences in the effects of calcium channel blockers in the rat remnant kidney model

BACKGROUND

Controversy persists as to the existence of class differences between calcium channel blockers (CCBs) in their ability to provide renoprotection and as to potential mechanisms involved.

METHODS

Rats with 5/6 renal ablation were left untreated or received diltiazem, verapamil, or felodipine after the first week, and the relationship between continuous radiotelemetrically measured blood pressure (BP) and glomerulosclerosis (GS) was assessed at seven weeks. Additionally, the effects of these CCBs on renal autoregulation and hypertrophy were examined at three weeks after renal ablation.

RESULTS

Although an excellent linear correlation was observed between the average BP levels and GS in all groups (r = 0.75 to 0.84, P < 0.01), significant protection was not achieved with any of the CCBs, but for different reasons. The antihypertensive effects of diltiazem were not sustained beyond the second week. Verapamil significantly reduced the average BP (144 +/- 4 mm Hg vs. 181 +/- 8 in untreated rats) but shifted the slope of the relationship between BP and GS (increase in percentage GS/mm Hg increase in average systolic BP) to the left (x intercept 121 vs. 144 mm Hg for untreated rats, P < 0.01) so that GS was not reduced. Felodipine also significantly reduced the average BP (144 +/- 3 mm Hg) and shifted the slope to the left (x intercept 123 mm Hg), but additionally made the slope steeper (2.3 +/- 0.5 vs. 0.82 +/- 0.2 in untreated rats). Because of these differing effects on the relationship between BP and GS, the rank order of GS for any given BP elevation was as follows: felodipine > verapamil > diltiazem = untreated. Felodipine, but not verapamil or diltiazem, caused additional impairment of the already impaired renal autoregulation in untreated rats, thereby explaining its adverse effects on GS. By contrast, the adverse effects of verapamil on GS were attributable to the greater amplitude of BP fluctuations that was observed in the verapamil-treated rats such that for any given average BP, these rats were exposed to greater peak pressures as compared with the other groups. None of the CCBs had a significant effect on glomerular hypertrophy.

CONCLUSIONS

These class differences between CCBs in their relative impact on systemic BP profiles, renal autoregulation, and glomerular pressure transmission may have clinically significant implications and may account for the variable glomeruloprotection that has been observed with these agents in both experimental models and in humans.

Modulation of vascular calcium channel activity in response to acute volume expansion in rats

The mechanisms of the increased resistance in hypertension are still unclear. Several studies have indicated that the potential-sensitive Ca2+ channels (PSC) are altered in arteries isolated from hypertensive patients or animals. An expansion of body fluid volume may trigger local autoregulatory responses or may induce the release of humoral factors, either of which could increase systemic vascular resistance and cause volume-dependent forms of hypertension. We tested the hypothesis that volume expansion per se may cause the alterations of PSC similar to those seen in hypertension. For this, we examined the alterations of PSC in aortas from volume-expanded rats with the use of dihydropyridine-type Ca2+ channel activator, BayK 8644, in parallel with the changes in endothelium-dependent relaxation. Volume expansion was produced by a rapid intravenous infusion of saline (10% of body weight) over 30 min in rats. At the end of infusion, rats were killed and aorta removed for in vitro measurement of isometric tension. Relaxation to acetylcholine (10(-7)-10(-5) mol/L, % relaxation to 10(-7) mmol/L norepinephrine contraction) was not significantly changed. In contrast, contractile response to BayK 8644 (10(-9)-10(-6) mol/L, % response to 50 mmol/L KCl) was significantly enhanced in rats with volume expansion (12 control rats: 11.6 +/- 4.9%; 18 volume-expanded rats: 40.9 +/- 10.4% at 10(-6) mol/L, p < 0.05). These findings suggest that acute volume expansion could induce a similar enhanced vascular Ca2+ channel activity to that seen in hypertension in rats.

Differential regulation of cytosolic calcium between afferent arteriol ar smooth muscle cells from mouse kidney

Using a combination of whole-cell patch-clamping and calcium microspectrofluorimetry we have examined the calcium regulation and calcium-activated electrical currents in smooth muscle cells along mouse renal afferent arterioles. In cells located at a distance up to 50microm from the glomerular vascular pole, guanosine 5'-[gamma-thio]triphosphate, (GTP[tau -S], 100micromol/l) and angiotensin II (1micromol/l) evoked internal calcium mobilization and oscillations of intracellular calcium concentration. Membrane depolarization up to +30mV did not increase cytosolic free calcium in these cells, which displayed calcium-activated chloride currents. In cells located more distant than 100microm from the glomerular vascular pole, GTP[tau -S] did not increase cytosolic calcium at negative membrane potentials. Depolarization of these cells to 0mV or positive membrane potentials increased intracellular calcium in a dihydropyridine-sensitive manner. These cells responded to angiotensin II with single calcium transients and also displayed calcium-activated chloride currents. These findings suggest that intracellular calcium is differentially regulated between afferent arteriolar smooth muscle cells from mouse kidney: in smooth muscle cells which are distant from the glomerular vascular pole, cytosolic calcium is increased primarily via calcium influx through potential-operated calcium channels, whilst in cells which are close to the vascular pole intracellular calcium is elevated predominantly via mobilization from internal stores. Both cell types are equipped with receptors for angiotensin II and possess calcium-activated chloride channels.

Deleterious effects of calcium channel blockade on pressure transmission and glomerular injury in rat remnant kidneys

Hypertensive mechanisms are postulated to play a major role in the progressive glomerulosclerosis (GS) after renal mass reduction. But, in contrast to converting enzyme inhibitors, BP reduction by calcium channel blockers, has not provided consistent protection. Radiotelemetric BP monitoring for 7 wk was used to compare nifedipine (N) and enalapril (E) in the rat approximately 5/6 renal ablation model. After the first week, rats received N, E, or no treatment (C). The overall averaged systolic BP in C (173 +/- 7 mmHg) was reduced by both E and N (P < 0.001), but E was more effective (113 +/- 2 vs. 134 +/- 3 mmHg, P < 0.01). GS was prevented by E (2 +/- 1 vs. 26 +/- 5% in C) but not by N (25 +/- 6%). GS correlated well with the overall averaged BP in individual animals of all groups, but the slope of the relationship was significantly steeper in N compared with C+E rats (P < 0.02), suggesting greater pressure transmission to the glomeruli and GS for any given BP. Since autoregulatory mechanisms provide the primary protection against pressure transmission, renal autoregulation was examined at 3 wk in additional rats. Autoregulation was impaired in C rats, was not additionally altered by E, but was completely abolished by N. These data demonstrate the importance of autoregulatory mechanisms in the pathogenesis of hypertensive injury and suggest that calcium channel blockers which adversely affect pressure transmission may not provide protection despite significant BP reduction.

Effect of elevated extracellular glucose concentrations on transmembrane calcium ion fluxes in cultured rat VSMC

Blood flow autoregulation is impaired in early diabetes mellitus, predisposing the renal microcirculation to injury. These hemodynamic changes have been strongly implicated in the development and progression of diabetic glomerulopathy. Blood flow autoregulation is predominantly a myogenic reflex which is strongly dependent on Ca2+ uptake by vascular smooth muscle cells (VSMC). Because impaired blood flow autoregulation may be responsive to glycemic control, the present study examined the effects of elevated extracellular glucose concentrations on basal, voltage sensitive and receptor operated Ca2+ uptake by VSMC. Confluent cultured rat VSMC were exposed to: (1) control medium (CM; 5 mM glucose); (2) high glucose medium (HGM; 10 to 30 mM glucose); or (3) osmotic control medium (OCM; glucose 5 mM + L-glucose 25 mM or mannitol 25 mM). A threshold glucose concentration of 15 mM markedly and maximally depressed basal Ca2+ uptake by VSMC (HGM 52% vs. CM). In addition, HGM significantly depressed voltage sensitive Ca2+ uptake by VSMC as determined by responses to BAY K 8644 (10(-7) M) or high extracellular [K+] (65 mM, HGM 50% vs. CM). HGM similarly depressed pressor hormone-stimulated Ca2+ uptake (AVP or Ang II 10(-7) M) by VSMC. The effects of HGM on Ca2+ uptake were time exposure dependent and reversible. Ca2+ uptake by VSMC in the presence of OCM did not differ from CM. Elevated extracellular glucose concentrations thus exert a direct and profound effect on basal, voltage sensitive and receptor operated Ca2+ uptake by VSMC. These observations may provide a biochemical basis for glucose-induced dysregulation of regional blood flow autoregulation in early diabetes mellitus.

Disparate effects of calcium channel blockers on pressure dependence of renin secretion and flow in the isolated perfused rat kidney

Using the model of isolated perfused rat kidneys this study was performed to investigate whether or not voltage-operated calcium channels are essentially involved in the pressure control of renin secretion from the kidneys. At a perfusion pressure of 100 mm Hg (13.3 kPa) renin secretory rates were 4.2 +/- 0.7 (ng angiotensin I h-1) min-1 g-1. Stepwise reduction of renal perfusion pressure to 80, 60, and 40 mm Hg (10.6, 8.0, 5.3 kPa) resulted in an increase of renin release yielding a 30-fold stimulation at 40 mm Hg vs 100 mm Hg. Increasing the perfusion pressure above 100 mm Hg did not further significantly decrease renin secretion. The perfusate flow rate was also pressure-dependent. Flow rates increased linearly with pressure and reached a plateau at 100 mm Hg, which was maintained up to 160 mm Hg (21.3 kPa). The averaged flow rate at the plateau was 15.5 ml min-1 g-1. In the presence of the three different calcium antagonists nifedipine (5 microM), nitrendipine (3 microM) and verapamil (5 microM), myogenic autoregulation of flow was abolished as indicated by the rise of the pressure/flow curve between 40 and 160 mm Hg. At the same time, however, these calcium channel blockers did not alter the relationship between perfusion pressure and renin secretion. Moreover, the calcium channel agonist Bay K 8644 (5 microM) caused a strong and long-lasting vasoconstriction, without changing renin secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against a role of arachidonic acid metabolites in autoregulatory responses of the isolated perfused kidney of the rat

The role of arachidonic acid metabolites in renal autoregulatory responses to changes in pressure was examined in rat isolated perfused kidneys. We also studied the influence of diabetes, a condition associated with hyperfiltration and altered renal eicosanoid production, on autoregulatory responses. The perfused rat kidney demonstrated autoregulation of flow within a pressure range of 100-150 mm Hg, with no differences between diabetic and control rat kidneys. Nifedipine resulted in vasodilatation and loss of autoregulation. Inhibition of the cyclooxygenase pathway of arachidonic acid metabolism with indomethacin failed to alter autoregulatory capacity. Similarly, inhibition of lipoxygenase with BW755C or NDGA, or inhibition of cytochrome P450-dependent enzymes with NDGA, clotrimazole or 7-ethoxyresorufin were without effect on autoregulatory responses. In vivo treatment with stannous chloride to deplete renal cytochrome P450-dependent enzymes also failed to modify autoregulatory responses. These results argue against a role of arachidonic acid metabolites in autoregulation of perfusate flow in the isolated kidney.

Influence of verapamil on regional renal blood flow: a study using multichannel laser-Doppler flowmetry

In anaesthetized male Sprague-Dawley rats the influence of the calcium entry blocker verapamil (2.4 mg h-1 kg-1, i.v.) on renal superficial cortical (CO) and outer medullary (OM) blood flow was investigated with the aim of elucidating further the intrarenal heterogeneity in vasoreactivity. The blood flow of the two regions was monitored simultaneously with a laser-Doppler flowmeter, using fibre probes with an outer diameter of 0.75 mm. One probe was directed towards the cortex and a second probe was inserted through the cortex and positioned in the outer medulla, measuring the flow within a hemisphere with a depth of 0.5-1.0 mm. The OM probe position was verified by dissecting the kidneys after each experiment. Insertion of the OM probe did not affect whole-kidney glomerular filtration rate, renal plasma flow or electrolyte excretion. Thirty minutes of verapamil infusion increased OM blood flow by 26%, but did not change CO blood flow (-1%). In spite of the increase in OM blood flow, the urine osmolality remained unaltered. Sodium excretion increased by 39%, while potassium excretion was unchanged. Mean arterial blood pressure decreased by 13%. In conclusion, this study has further supported the suggestion that the vasoreactivity is higher in the juxtamedullary than in the superficial cortical vasculature. The heterogeneity of the response is most probably due to the previously documented pressure drop along the interlobular arteries, which will create different haemodynamic conditions for the juxtamedullary and superficial afferent arterioles.

The effect of two different calcium antagonists on the glomerular haemodynamics in the dog

Kidney function of beagles fed a constant amount of food containing 3 mmol sodium.kgbodywt-1.day-1, and anaesthetized with pentobarbitone was investigated by clearance and micropuncture techniques during an intrarenal infusion of saline or the calcium antagonists verapamil (VER, 4 micrograms.kgbodywt-1.min-1) or nifedipine (NIF, 0.3 microgram.kgbodywt-1.min-1). Neither drug changed the mean arterial pressure. Apart from the natriuresis and diuresis, which were significantly greater with NIF than with VER, the response to both drugs was similar. Increases in renal blood flow (RBF; 17% with VER, 20% with NIF), glomerular filtration rate (GFR; VER: 34%; NIF: 39%) and filtration fraction (VER: 12%; NIF: 14%) were observed; similar values were obtained at the single nephron level. Pressure in glomerular capillaries, measured directly after ablation of a thin layer of cortex corticis, was increased by 11% with VER and 10% with NIF; no changes in proximal tubular and peritubular capillary pressure were seen. The glomerular ultrafiltration coefficient (Kf) did not change with either drug. Total arteriolar resistance was decreased (VER: 20%; NIF: 15%) due to a decrease in afferent resistance (VER: 31%; NIF: 27%) with no corresponding change in efferent resistance. The cause of the lack of responsiveness of the efferent arteriole remains unclear. In conclusion, in acute experiments with intrarenal administration, both drugs increase RBF and GFR by a preferential afferent dilatory mechanism without any change in Kf.

Pressure-induced vasoconstriction of renal microvessels in normotensive and hypertensive rats. Studies in the isolated perfused hydronephrotic kidney

The capacity of small arteries to respond to increased intravascular pressure may be altered in hypertension. In the kidney, hypertension is associated with a compensatory shift in the autoregulatory response to pressure. To directly determine the effects of established hypertension on the renal microvascular response to changes of perfusion pressure, we evaluated pressure-induced vasoconstriction in hydronephrotic kidneys isolated from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Vessel diameters of interlobular arteries (ILAs) and afferent and efferent arterioles were determined by computer-assisted videomicroscopy during alterations in renal arterial pressure (RAP) from 80 to 180 mm Hg. Increased RAP induced a pressure-dependent vasoconstriction in preglomerular vessels (afferent arterioles and ILAs), but not in postglomerular vessels (efferent arterioles). The calcium antagonist nifedipine prevented pressure-induced afferent arteriolar vasoconstriction with a similar half-maximal inhibitory concentration (IC50) (WKY, 63 +/- 27 vs. SHR, 60 +/- 32 nM). The pressure-activation curves for ILAs in SHR and WKY were similar. In contrast, the pressure-activation curve for afferent arterioles in SHR kidneys exhibited a rightward shift, which was observed at every segment of the afferent arteriole (i.e., near ILA, at midportion, and near glomerulus). These findings demonstrate that the ILA and the afferent arteriole both possess the ability to constrict in response to increased pressure, whereas this property is lacking in the efferent arteriole. Hypertension was associated with a compensatory shift in the pressure response of the afferent arteriole, such that higher RAPs were required to elicit vasoconstriction in this vessel.

Intrarenal mechanisms that regulate sodium excretion in relationship to changes in blood pressure

Because pressure-related natriuresis may be central to the regulatory role of the kidney on blood pressure, it is important to understand the relationship of humoral systems involved in the control of renal hemodynamics and tubular function. The preglomerular endothelial synthesis of prostaglandin I2 and endothelium-derived relaxing factor seem to modulate autoregulatory control by the afferent arterioles and the release of renin by the juxtaglomerular apparatus. The release of renin is followed by an increase in angiotensin II in the renal interstitium, which is responsible for adjusting the vascular tone of the efferent arterioles and vasa recta and for stimulating proximal tubular reabsorption of sodium. Variations in medullary circulation induced by angiotensin II could alter medullary interstitial pressure and the medullary production of prostaglandins E2 and I2 and, ultimately, could modulate sodium reabsorption in the medullary thick ascending limbs and the collecting ducts.

Renal vasoconstriction by the endothelial cell-derived peptide endothelin in spontaneously hypertensive rats

The effects of endothelin on systemic and renal hemodynamics in anesthetized spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats were examined. Endothelin (500 ng i.v. +1,000 ng/hr per 300-g rat) elevated mean blood pressure by 13% (p less than 0.02) and decreased renal blood flow by 71% and glomerular filtration rate by 66% (both p less than 0.01), resulting in a 430% (p less than 0.05) increase in renal vascular resistance (RVR) in SHR. This rise in blood pressure was associated with a significant increase in hematocrit (+8%), but a decrease in urinary sodium excretion (-51%). This dose of endothelin reduced cardiac output by 40% (p less than 0.001) and brought about a 96% (p less than 0.01) rise in systemic vascular resistance (SVR). However, the SVR increase was significantly smaller than the RVR increase. These changes in systemic and renal hemodynamics were observed in a dose-dependent manner, and the degrees of change did not differ between the two strains. Additional infusion of atrial natriuretic peptide (0.33 microgram/kg/min) into SHR completely reversed the changes in blood pressure and renal hemodynamics caused by endothelin, resulting in pronounced natriuresis (+760%). The renal vascular casting study revealed that endothelin mainly constricted the arcuate and interlobular arteries, as well as afferent arterioles. These results suggest that endothelin may be involved in blood pressure and body fluid volume regulation through systemic and renal vasoconstriction.

Acute effect of Russell's viper (Vipera russelli siamensis) venom on renal hemodynamics and autoregulation of blood flow in dogs

Renal hemodynamics and autoregulation of blood flow were investigated following intravenous injection of Russell's viper venom (0.1 mg/kg) in dogs anesthetized with sodium pentobarbital. After venom injection, the glomerular filtration rate fell significantly throughout the experimental period of three hr. Urine flow rate and renal blood flow also decreased and the filtered load of electrolytes declined significantly. The fractional excretion of sodium, potassium and phosphorus increased following venom administration. These data suggest that the venom may depress both glomerular and tubular functions. The renal autoregulation of blood flow was maintained during the experimental reduction of renal arterial pressure. We conclude that the ability of renal vasculature to autoregulate renal blood flow is not inhibited by Russell's viper venom, even though renal function is depressed.

Role of physical properties of resistance vessel wall in regulating peripheral blood flow--II. Structural and mechanical behavior

In order to examine the structural and mechanical properties of the vessel wall resistance when subjected to autoregulatory flow control, a mechanical model for the vascular wall was derived from a mathematical model. The mechanical model was an analogue model which connected in series the Maxwell model (elastic modulus: K3) with the parallel elements of Hill's model (elastic modules: K2) and Hooke's elastic model (elastic modulus: K1); it was also mathematically equivalent to the Spring model (see part I). The structural and mechanical properties of the resistance vessel wall were characterized by the three elastic moduli (K1, alpha K2 and K3) [mmHg]. The parameter alpha was a modification factor of the elastic modulus K2 given by the myogenic mechanism. After a numerical analysis of the experimental data given by the mechanical model, we confirmed that the arterial pressure range for autoregulatory flow controls shifted to the upper region with an increase of the elastic modulus K1 and the flow regulation was reduced.

Comparison of the effects of calcium antagonists and converting enzyme inhibitors on renal function under normal and hypertensive conditions

Calcium antagonists decrease the ability of the kidney to autoregulate renal blood flow (RBF) and glomerular filtration rate (GFR). Therefore, when afferent renovascular resistance is elevated, as in essential hypertension, there is a resultant increase in RBF and GFR with the administration of calcium antagonists. These agents also induce a marked natriuresis because of direct tubular action through unknown mechanisms. The natriuresis can be dissociated from renal and systemic hemodynamic actions, indicating that the decreased sodium reabsorption could override other compensatory mechanisms explaining the absence of sodium retention during the treatment. The renal effects of converting enzyme inhibitors (CEIs) can be explained by the reduction of intrarenal formation in angiotensin II. Because the activation of the renin-angiotensin system is mainly responsible for inducing sodium retention during a decrease in systemic blood pressure, CEIs could have a protecting effect without disturbing other homeostatic mechanisms. CEIs decrease efferent glomerular resistance, reducing capillary pressure and thereby reducing GFR. This effect is not translated in sodium retention because the reduction of GFR is mild during captopril administration in kidneys with normal or increased renal perfusion pressure. At low renal perfusion pressure, the reduced glomerular afferent vasoconstriction can compromise GFR, leading to renal insufficiency. Although these situations are not likely to be encountered during the treatment of uncomplicated essential hypertension, in severe hypertension with hypertrophy of pre-glomerular vessels, glomerular perfusion may decrease. Combination therapy of calcium antagonists and CEIs has been reported to be an effective treatment of severe hypertension. Currently, little information is available on the manner in which renal function is affected by simultaneous administration of both drugs.

Effect of 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8), an inhibitor of intracellular Ca2+ release, on autoregulation of renal blood flow in the dog

To examine whether Ca2+ release from intracellular Ca2+ store sites contributes to autoregulation of renal blood flow, experiments were performed on perfused kidneys of anesthetized dogs. Control observations showed excellent autoregulation of renal blood flow over the perfusion pressure range of 120-200 mm Hg. This autoregulatory response was not influenced by the intra-arterial infusion of 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8, 1.0 mg/min), an inhibitor of intracellular Ca2+ release. However, TMB-8 (0.3 and 1.0 mg/min i.a.) suppressed the renal vasoconstriction induced by intra-arterial injection of noradrenaline (0.5-2.0 micrograms). On the other hand, TMB-8 (0.3 and 1.0 mg/min) had no effect on the renal vasoconstriction induced by the Ca channel activator, BAY K 8644 (0.5-2.0 micrograms). These results show that TMB-8 has no effect on renal vasoconstriction induced by the activation of voltage-dependent Ca channels, and does not influence autoregulation of renal blood flow. Thus, Ca2+ release from intracellular stores does not appear to contribute the processes of autoregulation of renal blood flow.

The calcium entry blocker verapamil increases red cell flux in the vasa recta of the exposed renal papilla

To investigate the vasoreactivity of the vessels of the juxtamedullary vasculature, measurements were made of the red blood cell flux (Qrbc) in the vasa recta of the exposed renal papilla of anaesthetized rats by means of fluorescently labelled erythrocytes. The flux was measured before and after intravenous administration of the calcium entry blocker verapamil, which was infused in two different doses. A control group receiving vehicle was studied in parallel. The lower dose of verapamil (0.6 mg h-1 kg-1 body wt) increased Qrbc by 30 +/- 6% (n = 9, P less than 0.05, mean +/- 1 SEM) and reduced the systemic blood pressure by 6% (P less than 0.05). The osmolality of urine samples from the papillary tip remained statistically unchanged (1199 +/- 60 vs. 1186 +/- 102 mosM kg-1 H2O). In the animals receiving the higher dose of verapamil (2.4 mg h-1 kg-1 body wt), Qrbc increased by 39 +/- 7% (n = 10, P less than 0.05) and the blood pressure was reduced by 9% (P less than 0.05), while urine osmolality was unchanged (1104 +/- 110 vs. 1264 +/- 123 mosM kg-1 H2O). In the time control group there was no statistically significant change in Qrbc (+6 +/- 8%, n = 10), blood pressure or urine osmolality (1006 +/- 61 vs. 1091 +/- 70 mosM kg-1 H2O). The verapamil-induced increase in Qrbc is probably mainly mediated by a relaxation of the afferent arteriole of the juxtamedullary nephrons. The results confirm our previous indication of a high vasoreactivity in the vessels of the juxtamedullary vasculature.(ABSTRACT TRUNCATED AT 250 WORDS)

Smooth muscle calcium and endothelium-derived relaxing factor in the abnormal vascular responses of acute renal failure

Abnormal renovascular reactivity, characterized by paradoxical vasoconstriction to a reduction in renal perfusion pressure (RPP) in the autoregulatory range, increased sensitivity to renal nerve stimulation (RNS), and loss of vasodilatation to acetylcholine have all been demonstrated in ischemic acute renal failure (ARF). To determine if ischemic injury alters vascular contractility by increasing smooth muscle cell calcium or calcium influx, the renal blood flow (RBF) response to reductions in RPP within the autoregulatory range and to RNS were tested before and after a 90-min intrarenal infusion of verapamil or diltiazem in 7-d ischemic ARF rats. Both calcium entry blockers, verapamil and diltiazem, blocked the aberrant vasoconstrictor response to a reduction in RPP and RNS (both P less than 0.001). In a second series of experiments the potential role of an ischemia-induced endothelial injury and of the absence of endothelium-derived relaxing factor (EDRF) production were examined to explain the lack of vasodilatation to acetylcholine. Acetylcholine, bradykinin (a second EDRF-dependent vasodilator), or prostacyclin, an EDRF-independent vasodilator, was infused intrarenally for 90 min, and RBF responses to a reduction in RPP and RNS were tested in 7-d ischemic ARF rats. Neither acetylcholine nor bradykinin caused vasodilatation or altered the slope of the relationship between RBF and RPP. By contrast, prostacyclin increased RBF (P less than 0.001), but did not change the vascular response to changes in RPP. It was concluded that the abnormal pressor sensitivity to a reduction in RPP and RNS was due to changes in renovascular smooth muscle cell calcium activity that could be blocked by calcium entry blockers. A lack of response to EDRF-dependent vasodilators, as a result of ischemic endothelial injury, may contribute to the increased pressor sensitivity of the renal vessels.

Mechanisms underlying pressure-related natriuresis: the role of the renin-angiotensin and prostaglandin systems. State of the art lecture

It has long been known that increments in renal perfusion pressure can induce an elevation of urine sodium excretion without changing renal blood flow or glomerular filtration rate. The mechanism underlying this pressure-related natriuresis remains undefined, although the interest in its elucidation has been stimulated by the notion that it may constitute the central phenomenon through which the kidney regulates blood volume and, thereby, blood pressure. Recently, the use of novel experimental techniques has disclosed some important clues about changes in renal hemodynamics that, along with changes in renal humoral regulators, allow us to visualize a possible sequence of events responsible for pressure-related natriuresis. According to this hypothesis, the autoregulatory responses responsible for maintaining glomerular filtration rate are elicited in preglomerular vasculature by changes in renal perfusion pressure. These myogenic responses are coupled through Ca2+ entry in juxtaglomerular cells with inversely related changes in the release of renin and, consequently, with the amount of angiotensin II generated in renal interstitium. The release of renin from juxtaglomerular cells is modulated by the synthesis of prostaglandin I2 from the adjacent endothelial cells. Interstitial angiotensin II could influence sodium tubular reabsorption directly by stimulating sodium transport in proximal renal tubules and indirectly by altering medullary blood flow and, thereby, medullary interstitial pressure. In the renal medulla, the effects of interstitial pressure on sodium reabsorption can be amplified by the release of prostaglandin E2 from interstitial cells. A deficient regulation of this relationship could result in a shift of the pressure-natriuresis curve, leading to hypertension.

Calcium antagonists and the renal hemodynamic response to vasoconstrictors

These findings demonstrate that calcium antagonists reverse renal vasoconstriction in a variety of settings. The ability of calcium antagonists to augment GFR of the vasoconstricted kidney is striking and has also been demonstrated in a number of in vivo settings. These observations and others raise the possibility that calcium antagonists have potential utility in the treatment of a number of disorders characterized by renal ischemia and consequent renal insufficiency. Further studies to evaluate this possibility are required. The unique effects of calcium antagonists on GFR reflect a regional heterogeneity within the renal microcirculation and a preferential action of calcium antagonists on the afferent arteriole. Final resolution of the pharmacological basis for the renal hemodynamic actions of calcium antagonists will require a more complete understanding of the divergent activating mechanisms within the renal microcirculation.

Multiple effects of calcium entry blockers on renal function in hypertension

Characterization of the renal effects of calcium entry blockers has not been easy because the inhibition of Ca2+ cellular influx alters several regulatory functions. The ability of calcium blockers to dilate renal vasculature and to increase glomerular filtration rate is largely determined by the preexisting vascular tone. However, the increments in sodium excretion could occur without alterations in renal hemodynamics. Calcium blockers could increase sodium excretion by inducing a redistribution of renal blood flow toward juxtamedullary nephrons, by inhibiting tubuloglomerular feedback responses, or by a direct action on the tubular transport of sodium. These effects are poorly understood at present. In vitro studies show that the blockade of calcium entry enhances renin secretion and decreases prostaglandin synthesis. This dissociation has not been found during long-term administration, which has been proved to be effective for the treatment of essential hypertension with normal maintenance of renal function. In this respect, there are reports indicating that calcium blockers are particularly effective in a subgroup of patients with essential hypertension who exhibit subtle but detectable alterations in calcium metabolism. Further studies are needed to determine whether this significant response to calcium blockers is due to correction of an early defect of calcium cellular kinetics that initiated the increase in blood pressure.

Inhibition by diltiazem of pressure-induced afferent vasoconstriction in the isolated perfused rat kidney

The renal hemodynamic response to calcium entry blockade depends on the neural, hormonal and physiologic determinants influencing basal renal vascular tone. The effects of perfusion pressure per se on the renal vascular response of the rat kidney to diltiazem were evaluated using normal kidneys and hydronephrotic kidneys perfused extracorporally. In isolated perfused normal kidneys, diltiazem did not alter perfusate flow or glomerular filtration rate (GFR) when administered at a perfusion pressure of 100 mm Hg. In contrast, when diltiazem was administered at a perfusion pressure of 150 mm Hg, the calcium antagonists caused a striking increase in GFR, which was accompanied by an increase in renal perfusate flow. In the isolated perfused hydronephrotic rat kidney, elevation of perfusion pressure was associated with an increase in renal vascular resistance and a reduction in afferent arteriolar diameter. Diltiazem abolished the pressure-induced constriction of afferent arterioles and caused an increase in renal perfusate flow in hydronephrotic kidneys perfused at pressures above 100 mm Hg. These findings suggest that in the setting of increased renal perfusion pressure, diltiazem's effects on GFR are mediated in part by an inhibition of pressure-induced constriction of the afferent arteriole.

Effects of calcium channel blockade on renal vascular resistance responses to changes in perfusion pressure and angiotensin-converting enzyme inhibition in dogs

We conducted these experiments to evaluate the selectivity of calcium channel blockade on the renal autoregulatory mechanism and on angiotensin II-mediated renal vasoconstriction. Experiments were performed in anesthetized dogs in which renal arterial pressure, renal blood flow, and glomerular filtration rate were measured at normal and reduced renal arterial pressure. At control arterial pressures, renal arterial infusions of verapamil increased renal blood flow and glomerular filtration rate significantly. The decreases in renal vascular resistance elicited with verapamil (n = 13) and nifedipine (n = 4) occurred only at renal arterial pressure levels within the normal autoregulatory range. Renal blood flow autoregulatory efficiency was markedly attenuated, and the pressure-flow relationship obtained during calcium channel blockade approached that of a passive system. Systemic infusions of an angiotensin-converting enzyme inhibitor (captopril) during continued verapamil infusion caused further vasodilation at all renal arterial pressure values, as evidenced by an increase in slope of 27% of the pressure-blood flow relationship. This response was reversed by angiotensin II infusions. This shift indicates a reduction in minimal vascular resistance elicited by captopril, not obtainable with verapamil alone, and sensitive to angiotensin II. The effects of verapamil and nifedipine on renal blood flow autoregulation suggest a specific effect at preglomerular sites of potential operated membrane calcium channels in the autoregulatory phenomenon. The additional vasodilation elicited with captopril and reversed by angiotensin II indicates the presence of an angiotensin-sensitive postglomerular resistance component which is not influenced by calcium entry blockers.

Effects of verapamil in the prevention of warm ischaemia induced acute renal failure in dogs

Acute renal failure in the immediate postoperative period remains a significant complication of renal transplantation. A major factor in the pathogenesis may be warm ischaemia (WI). Recent evidence implicates a calcium mediated mechanism as a final common pathway in certain models of acute renal failure. This study was undertaken to evaluate the effects of Verapamil, a calcium antagonist, in the prevention of warm ischaemia-induced acute renal failure following renal autotransplantation in the dog. Twenty-one mongrel dogs were randomly allocated to three groups. Group 1 (control, 8 dogs) received 20 ml normal saline before a standardized 60 min warm ischaemic insult to the left kidney. Group 2 (6 dogs) received Verapamil (0.3 mg/kg) by intravenous injection and Group 3 (7 dogs) received Verapamil (0.3 mg/kg) by intra-arterial injection into the left renal artery prior to the same ischaemic insult. The left kidney was heterotopically grafted to the right iliac fossa in the warm ischaemic period. Contralateral nephrectomy was performed. The dogs were followed up to 7 days after operation by serial creatinine estimation. Histological examination of some autografts was performed. Of the eight controls, six showed marked renal impairment (serum creatinine greater than 800, or death in renal failure). Three of the six dogs given intravenous Verapamil showed marked renal impairment. None of the seven dogs receiving intra-arterial Verapamil showed marked renal impairment (P = 0.013, chi 2 test). The mean rate of serum creatinine rise for each group was analysed by multivariate analyses of variance.(ABSTRACT TRUNCATED AT 250 WORDS)