Inhibitory effect of frusemide on sympathetic vasoconstrictor responses: dependence on a renal hormone and the vascular endothelium

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

Besides their diuretic action, loop diuretics may induce a rapid vasodilator effect that contribute to their short-term therapeutic properties. We examined the effects of furosemide (10(-6)-10(-3) mol l(-1)) in comparison with bumetanide (10(-6)-10(-4) mol l(-1)) on isolated resistance arteries from rat and guinea pig mesentery and human subcutaneous fat, and investigated the mechanism of the acute direct vasorelaxant action on an isometric microvascular myograph. Both loop diuretics induced concentration-dependent relaxation of resistance vessels irrespective of membrane potential. The maximal effect of furosemide was greatest in rat and least in human arteries. Both diuretics caused a rightward shift in the concentration-response curve to extracellular Ca(2+). Incubation with indomethacin (2 x 10(-5) mol l(-1)) or mechanical removal of the endothelium did not inhibit the loop diuretic-induced relaxation. At high concentrations (10(-4)-10(-3) mol l(-1)) loop diuretics exert only weak direct relaxant effects on isolated human subcutaneous resistance arteries compared to the vasorelaxant effects in rat and guinea pig mesenteric vessels.

Renal and haemodynamic effects of nitric oxide blockade in a Wistar assay rat during high pressure cross-circulation of an isolated denervated kidney

Blockade of NO synthesis with N-omega-nitro-L-arginine (L-NNA) inhibits the vasodepressor response seen in intact Wistar assay rats in which isolated kidneys perfused via an extracorporeal circuit are perfused at high pressure. This study explores the renal and haemodynamic changes associated with this inhibition. Isolated kidneys (IK) were perfused at high pressure (175 mmHg) by a pump in series with intact Wistar assay rats in which blood pressure (BP), haemodynamics and renal function were studied. Nitric oxide (NO) synthesis was blocked by L-NNA (2.5 mg kg-1) in 13 experiments (175NO) while 14 control experiments (175C) were performed. IK was perfused at 90 mmHg in seven experiments (90C). The BP drop in the 175C assay rat was blocked by L-NNA in 175NO (P < 0.01). However, when the blockade was reversed with L-arginine infusion (20 mg kg-1 min-1) BP declined also in 175NO. Effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) fell dramatically after L-NNA in both the assay rat and in IK despite a high perfusion pressure. The marked increase in filtration fraction (FF) after L-NNA suggests a dominating postglomerular vasoconstriction. The natriuretic response in IK to 175 mmHg was also markedly blunted by L-NNA. We conclude that NO blockade inhibits the renomedullary depressor mechanism probably by restricting renal blood flow, and also blunts the pressure induced natriuretic response as a result of a reduced sodium filtration. Finally, the autoregulation of whole kidney blood flow seems to be more efficient although set at a higher level of vasoconstriction.

Effect of loop diuretics on cholinergic neurotransmission in human airways in vitro

BACKGROUND

Frusemide can inhibit various indirectly acting bronchoconstrictor stimuli in asthmatic patients. Both frusemide and bumetanide also modulate airway neurotransmission in some species but there are no data on the effect of loop diuretics on neurotransmission in man. An in vitro study was performed in human airways to investigate the possible neuromodulatory action of two loop diuretics, frusemide and bumetanide, and to elucidate whether a cyclooxygenase inhibitor such as indomethacin could modulate the effect of frusemide. The effect of acetazolamide, a carbonic anhydrase inhibitor, was also investigated.

METHODS

Electrical field stimulation (EFS; 40 V, 0.5 ms, 0.5-32 Hz for 15 seconds) in human airways with or without epithelium was used to induce a cholinergic contraction (n = 5 in all experiments). Indomethacin was present throughout. After obtaining a control frequency-response curve, different concentrations of diuretic were added to the organ bath and another frequency-response curve was constructed. To determine whether the effect of the diuretic was prejunctional or postjunctional a cumulative concentration-response curve to exogenous acetylcholine (Ach, 0.3 mumol/l to 10 mmol/l) was constructed in the presence of a diuretic (frusemide 1 mmol/l or bumetanide 0.1 mmol/l) or its vehicle. In some experiments indomethacin was omitted from the organ bath to investigate the possible involvement of cyclooxygenase products.

RESULTS

Both frusemide (10 mumol/l to 1 mmol/l) and bumetanide (1 mumol/l to 0.1 mmol/l) produced a concentration-dependent inhibition of the EFS-induced cholinergic contraction in human airways in vitro but only in epithelium denuded tissues. Frusemide (1 mmol/l) produced a maximum inhibition of 46.3% (SE 9.9%) at 0.5 Hz and bumetanide (0.1 mmol/l 39.6 (6.2)% at 0.5 Hz. Without indomethacin in the organ bath the frusemide-induced inhibition was enhanced at 4, 8, and 16 Hz, but bumetanide-induced inhibition was not enhanced at any frequency when indomethacin was omitted. Frusemide (1 mmol/l) and bumetanide (0.1 mmol/l) had no effect on the cumulative concentration-response curve to exogenous Ach (0.3 mumol/l to 10 mmol/l). Acetazolamide (100 mumol/l) had no effect on the EFS-induced cholinergic contraction in tissues with or without epithelium.

CONCLUSIONS

In human airways in vitro both frusemide and bumetanide produced a concentration-dependent inhibition of the EFS-induced cholinergic contraction. This inhibition is mediated through a prejunctional mechanism. Epithelium removal was necessary to achieve this effect. The mechanism of action of frusemide and bumetanide on airway nerves remains unclear: inhibition of the Na-K-Cl cotransporter is a possibility and, for frusemide, release of endogenous cyclooxygenase products may be involved. Carbonic anhydrase inhibition, on the other hand, is unlikely to be a factor.

Flunixin meglumine blocks frusemide-induced bronchodilation in horses with chronic obstructive pulmonary disease

Six horses that developed acute airway obstruction (heaves) when housed in a barn and fed poor-quality hay were studied. Airway obstruction was verified by a maximal change in pleural pressure during tidal breathing (delta Pplmax) of at least 15 cmH2O. Frusemide (1.0 mg/kg bwt) or an equivalent volume of vehicle was then administered intravenously (iv) and lung function was measured 15, 30, 60, 120, 180, 240 and 300 mins after drug administration. The effect of frusemide on lung function was also studied after treatment of horses with the cyclooxygenase inhibitor flunixin meglumine (1.1 mg/kg every 8 h for 2 days before the experiment). Frusemide significantly reduced the delta Pplmax beginning 15 mins after drug administration. This effect persisted for 5 h. The reduction in delta Pplmax was due partly to an increase in dynamic compliance and partly to a decrease in pulmonary resistance. Tidal volume and respiratory frequency were unaffected by frusemide. Vehicle had no effect on lung function. Flunixin meglumine abolished the effect of frusemide on airway calibre but did not prevent diuresis. These results indicate that the effect of frusemide on airways of horses with heaves persists for at least 5 h, is mediated through prostanoids, and is not a result of diuresis.

The inhibitory effect of furosemide on the contractile response of equine trachealis to cholinergic nerve stimulation

The effects of furosemide on the responses of equine trachealis muscle with and without epithelium to electrical field stimulation (EFS) and exogenous acetylcholine (ACh) were investigated in organ baths. Tissues were pretreated with guanethidine and the parameters used for EFS were those previously demonstrated to activate postganglionic cholinergic neurons. In tissues with intact epithelium, furosemide (100 microM) shifted the frequency-response curve to the right. In the preparations without epithelium, furosemide did not affect the response to EFS. Neither in epithelium-on nor in epithelium-off tissues was the ACh dose-response curve affected by the administration of furosemide. We conclude that furosemide (100 microM) decreases the equine tracheal smooth muscle responses to EFS through inhibition of cholinergic neurotransmission, and that this effect is dependent on epithelium.

Effects of furosemide on ponies with recurrent airway obstruction

The effect of aerosol and intravenous administrations of furosemide was examined in ponies with recurrent obstructive pulmonary disease, commonly called 'heaves'. This recurrent airway disease bears many similarities to human asthma. Six ponies with the disease (principal animals) were studied during clinical remission and during an acute attack of airway obstruction precipitated by stabling and feeding dusty hay. Six control animals were also studied. Furosemide (1.0 mg/kg) or vehicle was administered by aerosol in the first study, and intravenously in a second study. In principal ponies with airway obstruction, furosemide administration significantly decreased pulmonary resistance and increased dynamic compliance, but had no effect on PaO2 or PaCO2. The time course and magnitude of lung function changes were similar following aerosol or intravenous drug administration. Vehicle was without effect in both groups of animals at all time periods and furosemide had no effect in controls or in principals in clinical remission.

Mechanisms behind the relaxing effect of furosemide on the isolated rabbit ear artery

The effect of furosemide on isometric contraction and 86Rb uptake were studied in the isolated rabbit central ear artery (CEA). A concentration-dependent relaxing effect of furosemide (0.06 mM-1.0 mM) was found in vessel segments with intact endothelium. The maximal relaxation was 28.6 +/- 3.9% (10). The effect was not diminished in segments deprived of endothelium, and removal of endothelium itself caused no change of the force development to electrical field stimulation. The relaxing effect was time-dependent and stimulation-dependent and was not significantly affected by membrane depolarization induced by increasing external [K+] from 10 to 120 mM. The 86Rb uptake was inhibited by both furosemide and ouabain (8.0 +/- 0.5(8) and 5.3 +/- 0.5(8) versus 12.8 +/- 0.9(16) nmol (K+).mm-1.(10 min.)-1 in the furosemide (1.0 mM), ouabain (1.0 mM) and control groups, respectively) without interaction between the two drugs. The 86Rb uptake was not further inhibited by increasing the furosemide concentration from 0.12 mM to 1.0 mM. Our results suggest: firstly, the direct relaxing effect of furosemide on isolated vessel segments is endothelium-independent and secondly, the inhibition of the Na(+)-K(+)-Cl- cotransport and a possible consequent hyperpolarization of the membrane is unlikely to be the sole mechanism responsible for the vasorelaxant effect of furosemide. The demonstrated direct effect on vascular tone may be of clinical importance in situations with very high plasma concentrations of the drug or very low concentrations of serum albumin.