Only weak vasorelaxant properties of loop diuretics in isolated resistance arteries from man, rat and guinea pig

Loop Diuretics Inhibit Ischemia-Induced Intracellular Ca2+ Overload in Neurons via the Inhibition of Voltage-Gated Ca2+ and Na+ Channels

One consequence of ischemic stroke is disruption of intracellular ionic homeostasis. Intracellular overload of both Na⁺ and Ca²⁺ has been linked to neuronal death in this pathophysiological state. The etiology of ionic imbalances resulting from stroke-induced ischemia and acidosis includes the dysregulation of multiple plasma membrane transport proteins, such as increased activity of sodium-potassium-chloride cotransporter-1 (NKCC-1). Experiments using NKCC1 antagonists, bumetanide (BMN) and ethacrynic acid (EA), were carried out to determine if inhibition of this cotransporter affects Na⁺ and Ca²⁺ overload observed following in vitro ischemia-acidosis. Fluorometric Ca²⁺ and Na⁺ measurements were performed using cultured cortical neurons, and measurements of whole-cell membrane currents were used to determine target(s) of BMN and EA, other than the electroneutral NKCC-1. Both BMN and EA depressed ischemia-acidosis induced [Ca²⁺]_i overload without appreciably reducing [Na⁺]_i increases. Voltage-gated Ca²⁺ channels were inhibited by both BMN and EA with half-maximal inhibitory concentration (IC₅₀) values of 4 and 36 μM, respectively. Similarly, voltage-gated Na⁺ channels were blocked by BMN and EA with IC₅₀ values of 13 and 30 μM, respectively. However, neither BMN nor EA affected currents mediated by acid-sensing ion channels or ionotropic glutamatergic receptors, both of which are known to produce [Ca²⁺]_i overload following ischemia. Data suggest that loop diuretics effectively inhibit voltage-gated Ca²⁺ and Na⁺ channels at clinically relevant concentrations, and block of these channels by these compounds likely contributes to their clinical effects. Importantly, inhibition of these channels, and not NKCC1, by loop diuretics reduces [Ca²⁺]_i overload in neurons during ischemia-acidosis, and thus BMN and EA could potentially be used therapeutically to lessen injury following ischemic stroke.

The Effects of Bumetanide on Human Umbilical Artery Contractions

The authors investigate the effect of bumetan ide, an inhibitor of NKCC1 and a loop diuretic, on the tone of human umbilical artery (HUA). Rings of HUA (n = 35) from vaginal deliveries were suspended for isometric tension recordings in organ baths. Cumulative concentration-response curves to serotonin, histamine, and KCl were performed in the absence (control) or in the presence of bumetanide. The relaxant effect of bumetanide was also evaluated in serotonin- and histamine-induced contractions. Bumetanide inhibited HUA tone in serotonin- and histamine-induced contractions with significant changes in the potency (pD(2)) and maximum contractile response (E(max)) values. However, only pD( 2) values for KCl-induced contraction significantly changed in the presence of bumetanide. Bumetanide caused concentration-dependent and sustained relaxations in serotonin-induced contraction; however, there was refractoriness in histamine-induced contraction. These findings raise the possibility that NKCC1 may play a role in the regulation of the umbilical artery tone.

Unexpected extra-renal effects of loop diuretics in the preterm neonate

The loop diuretics furosemide and bumetanide are commonly used in neonatal intensive care units (NICUs). Furosemide, because of its actions on the ubiquitous Na(+) -K(+) -2Cl(-) isoform cotransporter and its promotion of prostanoid production and release, also has non-diuretic effects on vascular smooth muscle, airways, the ductus arteriosus and theoretically the gastrointestinal tract. Loop diuretics also affect the central nervous system through modulation of the GABA-A chloride channel.

CONCLUSION

  The loop diuretics have a variety of biological effects that are potentially harmful as well as beneficial. Care should be taken with the use of these agents because the range of their effects may be broader than the single action sought by the prescribing physician.

Vasodilatory action of loop diuretics: a plethysmography study of endothelial function in forearm arteries and dorsal hand veins in hypertensive patients and controls

INTRODUCTION

Intravenous administration of loop diuretics induces venodilation before the diuretic response. We investigated whether furosemide and torasemide exert a dilatory effect on arteries and veins mediated by endothelial release of nitric oxide.

METHODS

We performed intermittent venous occlusion plethysmography to study forearm blood flow and dorsal hand-vein distension in response to furosemide and torasemide infusion in hypertensive patients and healthy controls.

RESULTS

Furosemide increased venodilation from 0.56 +/- 0.09 to 0.88 +/- 0.06 (P=0.000) in control subjects and from 0.49 +/- 0.10 to 0.75 +/- 0.12 (P=0.000) in hypertensive patients. Torasemide increased venodilation from 0.46 +/- 0.06 to 0.70 +/- 0.11 (P=0.007) in control subjects and from 0.48 +/- 0.09 to 0.67 +/- 0.12 (P = 0.03) in hypertensive patients. Co-infusion of the Nitric Oxide Synthase Inhibitor (L-NMMA)-blocked this venodilation, and the action was reversed with L-arginine. There were no significant changes in the arterial bed.

CONCLUSIONS

Furosemide and torasemide induce a similar dose-response curve venodilation, but they have no effect on the arterial bed. Hypertensive patients show a smaller venous endothelium-dependent response than healthy controls. The venodilation induced by both diuretics requires release of nitric oxide.

Effect of the Na-K-2Cl cotransporter NKCC1 on systemic blood pressure and smooth muscle tone

Studies in rat aorta have shown that the Na-K-2Cl cotransporter NKCC1 is activated by vasoconstrictors and inhibited by nitrovasodilators, contributes to smooth muscle tone in vitro, and is upregulated in hypertension. To determine the role of NKCC1 in systemic vascular resistance and hypertension, blood pressure was measured in rats before and after inhibition of NKCC1 with bumetanide. Intravenous infusion of bumetanide sufficient to yield a free plasma concentration above the IC(50) for NKCC1 produced an immediate drop in blood pressure of 5.2% (P < 0.001). The reduction was not prevented when the renal arteries were clamped, indicating that it was not due to a renal effect of bumetanide. Bumetanide did not alter blood pressure in NKCC1-null mice, demonstrating that it was acting specifically through NKCC1. In third-order mesenteric arteries, bumetanide-inhibitable efflux of (86)Rb was acutely stimulated 133% by phenylephrine, and bumetanide reduced the contractile response to phenylephrine, indicating that NKCC1 influences tone in resistance vessels. The hypotensive effect of bumetanide was proportionately greater in rats made hypertensive by a 7-day infusion of norepinephrine (12.7%, P < 0.001 vs. normotensive rats) but much less so when hypertension was produced by a fixed aortic coarctation (8.0%), again consistent with an effect of bumetanide on resistance vessels rather than other determinants of blood pressure. We conclude that NKCC1 influences blood pressure through effects on smooth muscle tone in resistance vessels and that this effect is augmented in hypertension.

Chronic nitric oxide blockade modulates renal Na–K–2Cl cotransporters

OBJECTIVE

The Na-K-2Cl cotransporter (NKCC2 isoform) of the thick ascending limb of Henle's loop (TAL) plays an important role in renal sodium handling, and the vascular isoform (NKCC1) participates in the response to vasoconstrictors. Both isoforms appear to be regulated by nitric oxide. This study aimed to analyze the effect of chronic nitric oxide deficiency on tubular and vascular Na-K-2Cl cotransporters in kidney and their potential role in the development of N-nitro-L-arginine-methyl ester (L-NAME) hypertension.

METHODS

Wistar rats were given L-NAME (vehicle, 10, 35 and 80 mg/100 ml drinking water) for 4 weeks. Blood pressure was measured by the tail-cuff method. NKCC2 activity was estimated as the bumetanide-sensitive Rb influx in fresh isolated TAL tubules. NKCC1-contractile function was estimated as the bumetanide-sensitive vasocontractile response to phenylephrine in isolated perfused kidneys. Acute effects of L-NAME and endothelium removal were also evaluated. NKCC2 and NKCC1 protein expression were assessed by western blot analysis.

RESULTS

Chronic L-NAME administration increased, in a dose-dependent manner, both blood pressure and NKCC2 activity, and these changes significantly correlated (r2 = 0.89, P < 0.01). NKCC1-contractile activity decreased with the highest dose of L-NAME (80 mg/100 ml drinking water group) but it was not affected by acute nitric oxide blockade or endothelium removal. This 80 mg group showed increased NKCC2 expression in the renal medulla and decreased NKCC1 expression in aorta.

CONCLUSIONS

Chronic nitric oxide deficiency stimulates tubular Na-K-2Cl cotransporter, suggesting that NKCC2 hyperactivity contributes to the inability to excrete sodium, and hence to the development of L-NAME hypertension. In contrast, L-NAME hypertension develops independently of vascular NKCC1-contractile activity.