Eckhart Büssemaker

Reduced Agonist-Induced Endothelium-Dependent Vasodilation in Uremia Is Attributable to an Impairment of Vascular Nitric Oxide

Current concepts for the explanation of endothelial dysfunction and accelerated atherosclerosis in uremia propose a reduced vascular bioavailability of nitric oxide (NO). The aim of the present study was to test the contributions of NO and NO/prostacyclin (PGI(2))-independent mechanisms to both baseline vascular tone and agonist-induced endothelium-dependent vasodilation in patients on hemodialysis (HD). In 10 HD patients and eight matched healthy control subjects, forearm blood flow (FBF) was measured at rest and during intrabrachial infusions of norepinephrine (NE; endothelium-independent vasoconstrictor, 60, 120, and 240 pmol/min) and N-monomethyl-L-arginine (blocker of NO synthases, 16 micromol/min). After inhibition of cyclo-oxygenase by ibuprofen (1200 mg orally), endothelium-dependent and -independent vasodilation was assessed by infusion of acetylcholine (ACh; 1, 5, 10, 50, 100, and 300 nmol/min) and sodium-nitroprusside (2.5, 5, and 10 microg/min). NO/PGI(2)-independent vasodilation was tested by equal infusions of ACh during NO clamp. N-monomethyl-L-arginine reduced resting FBF to a comparable degree in both groups. Vascular responses to ACh were reduced in HD (P = 0.003 versus control by ANOVA), whereas those to sodium nitroprusside were mainly at control level. Infusion of ACh during NO clamp caused a similar increment of FBF in both groups. NO-mediated vasodilation as calculated by the difference between ACh-induced responses without and with NO clamp was substantially impaired in HD (P < 0.001) compared with control. In HD patients, baseline NO-mediated arteriolar tone is at control level. This study provides first evidence that endothelial dysfunction of uremic patients as shown by reduced agonist-induced endothelium-dependent vasodilation is attributable to reduced stimulation of NO, whereas the NO/PGI(2)-resistant portion of ACh-mediated vasodilation is unaffected.

Baseline blood flow and bradykinin-induced vasodilator responses in the human forearm are insensitive to the cytochrome P450 2C9 (CYP2C9) inhibitor sulphaphenazole.

A substantial portion of the vasodilator response elicited by bradykinin in the human forearm is unaffected by the combined inhibition of nitric oxide (NO) synthases and cyclo-oxygenases. The cytochrome P450 (CYP) 2C9 inhibitor sulphaphenazole was recently identified as a potent inhibitor of NO- and prostacyclin (PGI2)-independent relaxation in porcine coronary arteries. The aim of the present study was to determine the effect of sulphaphenazole on basal and bradykinin-induced NO/PGI2-independent changes in the forearm blood flow (FBF) of healthy subjects. Eleven healthy male volunteers participated in this placebo-controlled study. Test agents were infused into the brachial artery and FBF was measured by bilateral venous occlusion plethysmography. Sulphaphenazole (0.02-2 mg/min) alone did not affect basal blood flow. Inhibition of the NO synthases by NG-monomethyl-L-arginine (L-NMMA; 4 micromol/min) and cyclo-oxygenases by ibuprofen (1200 mg, orally) reduced FBF to 48 +/- 7% in the absence and 50 +/- 8% in the presence of sulphaphenazole (2 mg/min; P=not significant). After pretreatment with L-NMMA (16 micromol/min) and ibuprofen (1200 mg, orally), sulphaphenazole (6 mg/min) did not substantially inhibit bradykinin-induced vasodilation. We conclude that CYP2C9-derived metabolites (i) are not involved in the regulation of baseline blood flow, and (ii) do not mediate bradykinin-induced NO/PGI2-independent vasorelaxation in the human forearm. However, determining the contribution of this enzyme to regulation of blood flow in pathological conditions associated with endothelial dysfunction requires further studies.

The Na‐K‐ATPase is a target for an EDHF displaying characteristics similar to potassium ions in the porcine renal interlobar artery

The present study was performed to determine the characteristics of the endothelium‐derived hyperpolarizing factor (EDHF) that mediates the nitric oxide (NO)‐ and prostacyclin (PGI2)‐independent hyperpolarization and relaxation of porcine renal interlobar arteries. Bradykinin‐induced changes in isometric force or smooth muscle membrane potential were assessed in rings of porcine renal interlobar artery preconstricted with the thromboxane analogue U46619 in the continuous presence of Nω‐nitro‐L‐arginine and diclofenac to inhibit NO synthases and cyclo‐oxygenases. Inhibition of NO‐ and PGI2‐production induced a rightward shift in the concentration‐relaxation curve to bradykinin without affecting maximal relaxation. EDHF‐mediated relaxation was abolished by a depolarizing concentration of KCl (40 mM) as well as by a combination of charybdotoxin and apamin (each 100 nM), two inhibitors of calcium‐dependent K+ (K+Ca) channels. Charybdotoxin and apamin also reduced the bradykinin‐induced, EDHF‐mediated hyperpolarization of smooth muscle cells from 13.7±1.3 mV to 5.7±1.2 mV. In addition to the ubiquitous α1 subunit of the Na‐K‐ATPase, the interlobar artery expressed the γ subunit as well as the ouabain‐sensitive α2, α3 subunits. A low concentration of ouabain (100 nM) abolished the EDHF‐mediated relaxation and reduced the bradykinin‐induced hyperpolarization of smooth muscle cells (13.6±2.8 mV versus 5.20±1.39 mV in the absence and presence of ouabain). Chelation of K+, using cryptate 2.2.2., inhibited EDHF‐mediated relaxation, without affecting NO‐mediated responses. Elevating extracellular KCl (from 4 to 14 mM) elicited a transient, ouabain‐sensitive hyperpolarization and relaxation that was endothelium‐independent and insensitive to charybdotoxin and apamin. These results indicate that in the renal interlobar artery, EDHF‐mediated responses display the pharmacological characteristics of K+ ions released from endothelial K+Ca channels. Smooth muscle cell hyperpolarization and relaxation appear to be dependent on the activation of highly ouabain‐sensitive subunits of the Na‐K‐ATPase.

Kidney transplantation improves endothelium-dependent vasodilation in patients with endstage renal disease.

Kidney transplantation (Tx) improves the cardiovascular outcome of patients receiving hemodialysis (HD). Therefore, we asked whether Tx improves the endothelial dysfunction of HD patients. Eight patients were studied twice: (1) during HD and (2) after Tx. We also studied eight matched control subjects. We measured forearm blood flow by venous occlusion plethysmography. We administered intrabrachial infusions of three doses of norepinephrine, glycerol trinitrate, acetylcholine (ACH), and N-monomethyl-L-arginine. The response to ACH was reduced in HD patients compared with controls (P <0.001). The response to ACH in HD patients improved after Tx, and this change was significant for low-dose ACH (P <0.05 for dose one and two compared with HD). The response to glycerol trinitrate, which was reduced in HD patients compared with controls (P <0.01), remained unchanged after Tx. N-monomethyl-L-arginine and norepinephrine comparably reduced forearm blood flow in all groups. This is the first evidence showing an improvement of endothelial dysfunction in HD patients after Tx.

The kidney in cardiac failure: today’s perspective

An old German saying to describe the extremes of thoroughness compares this job symbolically (“etwas auf Herz and Nieren prufen”) with a meticulous examination of the heart and the kidney together. In this way, the saying tacitly implies that the heart and the kidney are at the center of things — but only if in cooperation. As we now know, the heart, the kidney, and the blood vessel organs constitute the essential limbs of the cardiovascular perfusion system, the functions of which are regulated in an organized manner. To the physician scientist, cardiac failure therefore should make for an interesting abnormality: it challenges the compensatory capabilities of the kidney. Given that the latter receives the highest blood perfusion rate amongst the parenchymal organs of the body — at least under normal conditions — and that it is endowed with an unparalleled array of vascular control mechanisms, it is clear that cardiac failure will be a telling condition as regards the biological function of the kidney — and its many fine details. The present contribution will broadly outline the conceptual framework of renal function in heart failure as it evolved over the last 150 years (Fig. 1); it will then mainly focus on the many aspects of recent and ongoing progress that has been made. The way(s) in which the kidney appears to be — relatively — protected from ischemic acute tubular necrosis in cardiac failure will also be addressed.