David Winlaw

Selective Inhibition Of Nitric Oxide Production During Cardiac Allograft Rejection Causes A Small Increase In Graft Survival

Nitric oxide production is increased in allograft rejection and may have both beneficial and deleterious effects on graft function and survival. In animal models, conventional immunosuppressive agents have been shown to decrease nitric oxide production. The aim of our study was to determine what effect augmentation and selective inhibition of nitric oxide production may have on graft survival by using the model of heterotopic cardiac transplantation in the rat. L-Arginine, the naturally occurring substrate for nitric oxide production, was administered subcutaneously at 200 mg/kg/day. L-NG-monomethyl-L-arginine (L-NMMA) is a selective inhibitor of nitric oxide synthase and was administered at 500 mg/kg/day to allograft recipients from the day of operation. Endogenous nitric oxide production was quantified by analysis of urinary nitrate excretion, and time to rejection was determined by graft palpation. L-Arginine did not significantly alter urinary nitrate excretion by iso- or allografts, suggesting that nitric oxide production is not a substrate-limited process in this model. Graft survival in this group was unchanged. L-NMMA produced a small increase in graft survival from 5.1 +/- 0.1 to 6.3 +/- 0.3 days compared with control allografts (P = 0.001) and abolished the rise in urinary nitrate excretion seen with control allografts. Lower doses of L-NMMA produced dose-related decrements in urinary nitrate excretion, but did not alter graft survival. We found that allograft rejection can proceed to graft loss despite complete inhibition of the increase in nitric oxide production that occurs during untreated rejection. The small increase in graft survival suggests that nitric oxide plays a minor role as a cytotoxic effector molecule in this model of acute rejection.

Urinary nitrate excretion is a noninvasive indicator of acute cardiac allograft rejection and nitric oxide production in the rat

Cytokine induction of calcium-independent nitric oxide synthase is associated with production of large amounts of nitric oxide (NO). NO is a free radical that is rapidly degraded to nitrite and nitrate. Measurement of plasma and urinary nitrate is an indirect marker of NO production and previous studies have demonstrated that plasma nitrate rises with allograft rejection. The purpose of this study was to examine the temporal relationship between the rise in urinary nitrate excretion and the onset of graft rejection, and to determine the effect of conventional immunosuppression on nitrate excretion. The heterotropic model of cardiac transplantation in the rat was used, with Brown-Norway to Lewis allografts and Lewis to Lewis isograft controls. Twenty-four-hour urine specimens were collected before and after transplantation. Urinary nitrate excretion was measured by gas chromatography/mass spectrometry. Each group was treated with (1) no immunosuppression, (2) dexamethasone (3 mg/kg), or (3) CsA (10 mg/kg) on days 0, 1, and 2. Time to rejection for untreated allografts was 5.1 +/- 0.1 days, extending to 8.4 +/- 0.5 and 9.6 +/- 0.4 days with dexamethasone and CsA treatment, respectively. There was a significant rise in nitrate excretion on days 4, 7, and 9 for control, dexamethasone-treated, and CsA-treated allografts, respectively, preceding evidence of rejection. Untreated allograft rejection was associated with a peak in nitrate excretion 8 times that of basal excretion by isografts. Treatment of the allografts with dexamethasone and CsA significantly attenuated peak nitrate excretion compared with untreated allografts with a only a 2- to 3-fold rise preceding rejection. Results indicate that allograft rejection is associated with a dramatic increase in peak urinary nitrate excretion that is attenuated by standard immunosuppressive therapy. An increase in nitrate excretion precedes evidence of graft rejection, and may serve as a noninvasive marker of graft rejection.

Mycoplasma hominis infection in heart and lung transplantation

Culture-negative wound infection and mediastinitis secondary to Mycoplasma hominis have been reported after cardiothoracic surgery but no case cluster has ever been described. We report 4 cases of infection in 3 cardiac and 1 bilateral sequential lung transplant recipient over 3 weeks of hospitalization. Successful treatment was achieved with early aggressive surgical intervention and combination antibiotics of clindamycin, doxycycline and/or ciprofloxacin. This cluster raises the question of nosocomial transmission of infection and supports a recommendation for single-room isolation and universal precautions for infected individuals.

The role of nitric oxide in heart failure. Potential for pharmacological intervention.

SummaryThere is now considerable evidence that nitric oxide (NO) production and action are abnormal in patients with heart failure. Spontaneous NO release from the vascular endothelium is preserved or enhanced in patients with heart failure and this may help to maintain tissue perfusion by blunting the vasoconstriction induced by various neurohumoral factors. On the other hand, endothelial NO release in response to various stimuli including exercise appears to be diminished and this may contribute to the impaired exercise capacity of patients with heart failure.It is now apparent that NO produced within the heart plays an important role in the modulation of cardiac contractility under physiological conditions. In patients with heart failure, however, increased myocardial NO production in response to cytokines such as tumour necrosis factor-α may contribute to reduced contractility and myocyte injury.Our understanding of the role of NO in the control of vascular tone has provided an explanation for the efficacy of nitrovasodilators in heart failure and has stimulated novel approaches to augmenting endogenous vascular NO production. There is also evidence that ACE inhibitors act to restore normal endothelial function in patients with heart failure.Increased NO production within the heart, particularly that produced via the pro-inflammatory inducible NO synthase, may be detrimental. It remains to be determined whether selective inhibition of inducible NO synthase can favourably modify the course of this lethal condition.

The Role of Nitric Oxide Production during Cardiac Allograft Rejection

The role of nitric oxide (NO) production in acute allograft rejection is not known. In normal human vasculature, nitric oxide synthase is constitutively expressed within endothelial cells (cNOS), and small quantities of NO produced by this pathway mediate vasorelaxation via a cyclic GMP-dependent mechanism. During cardiac allograft rejection, NO may also be produced by the inducible isoform of nitric oxide synthase (iNOS), present in endothelial cells, smooth muscle cells and cardiac myocytes. Unlike cNOS activity, which is an integral part of cardiovascular homeostasis, iNOS activity is a component of the inflammatory response and may have both beneficial and deleterious effects during rejection. Early reports suggested that acute cardiac allograft rejection was associated with an increase in nitric oxide production. The general aim of this study was to document the way in which nitric oxide production is affected by allograft rejection, and to determine the significance of these changes. Enhanced nitric oxide synthesis may have a graft protective effect by inhibiting clonal expansion of immune cells targeting alloantigen. However, immunosuppressive agents have been shown to block NO production, raising the possibility that NO may modulate graft function and/or survival. The following hypotheses were tested; that acute cardiac allograft rejection is associated with a rise in NO production and an increase in iNOS expression, that conventional immunosuppressive agents reduce NO production by limiting iNOS expression, that an increase in substrate availability for nitric oxide synthase would increase NO production and decrease graft survival, and that L-NMMA, a competitive inhibitor of both constitutive and inducible isoforms would prolong graft survival.

Increased Nitric Oxide Production in Heart Failure and Correlation with Functional Severity

The role of nitric oxide (NO) in heart failure is unknown. Studies of isolated blood vessels from patients with heart failure demonstrate impaired NO-dependent relaxation to acetylcholine. On the other hand, endomyocardial biopsy specimens from patients with dilated cardiomyopathy show increased activity of the inducible isoform of nitric oxide synthase (iNOS). Plasma levels of TNF-α, an activator of iNOS, are increased in heart failure, and TNF-α has been shown to have negative inotropic effects in hamster papillary muscle through an NO-dependent mechanism.