Preserved atrial natriuretic peptide secretory function after cardiac transplantation

Preserved Atrial Natriuretic Peptide Secretory Function After Cardiac Transplantation Randall C. Starling, MD, Thomas M. O’Dorisio, MD, William 6. Malarkey, MD, Kevin D. Murray, MD, P. David Myerowitz, MD, and Robert J. Cody, MD

The purpose of this investigation was to determine whether atrial natriuretic peptide (ANP) secretory function,is preserved after cardiac transplantation. Thirteen hemodynamieally stable outpatients performed supine exercise on a bicycle an average of 7 months after orthotopic cardiac transplantation. Right atrial pressure increased 2.29fold (6 f 1 to 13 f 2 mm Hg) and pulmonary (11 f 1 to 23 f 7 artery wedge pressure P&fold mm Hg) with exercise in the transplant recipients. Resting venous ANP level (114 f 19 Pg/ml) and peak exercise venous level (373 f 61 pg/ml) was elevated in transplant recipients (p
trial natriuretic peptide (ANP) plasma levels increasein responseto a variety of physiologic stimuli in normal personsand in patients with cardiovascular disease.1-7Plasma levels of ANP are elevated in patients with congestive heart failure and there is a correlation between ANP and intracardiac hemodynamicsincluding right atrial, pulmonary artery and pulmonary artery wedge pressures.*-” Magovern et alI2 observedthat recipients of orthotopic heart transplants have elevated basal ANP levels compared with patients with coronary disease.Venous ANP levels increase in cardiac transplant recipients in responseto dynamic exercise;however, previous studies have not examined the relation between hemodynamics and ANP release.13J4Marked increasesin pulmonary artery wedge pressure occur with exercise in cardiac transplant recipients with normal baselinehemodynamits with treated cyclosporine.15,16Since pulmonary artery wedge pressurehas been shown to correlate with ANP levels, and pulmonary artery wedge pressureincreasessignificantly with exercisein cardiac transplant recipients, we hypothesized that exercise would be an excellent provocative physiologic stimulus (increased atria1 pressureand wall tension). This investigation determined whether the responsivenessof ANP secretion to exercisewas preservedand establisheda relation between hemodynamics and ANP release in cardiac transplant recipients.

A

METHODS

From the Divisions of Cardiology, Endocrinology and Cardiothoracic Surgery, Departments of Internal Medicine and Surgery, Ohio State University, Columbus, Ohio. This study was supported in part by a Grant-in-Aid from the Central Ohio Heart Association, Columbus, Ohio, and by General Clinical Research Center Grant RR-34 and National Institutes of Health Grant CA-16058 from the Core Peptide Laboratory, Bethesda, Maryland. Manuscript received December 20, 1990; revised manuscript received and accepted March 25, 1991. Address for reprints: Randall C. Starling, MD, 538A Scaife Hall, University of Pittsburgh, Pittsburgh, Pennsylvania 15213.

Orthotopic cardiac transplantation was performed using standard surgical techniques.l7 Stable outpatient cardiac transplant recipients were enrolled at the time of a surveillance endomyocardial biopsy. Demographics of the study population are listed in Table I. All cardiac transplant recipients were receiving cyclosporine, azathioprine and prednisone immunosuppressionand performed supine bicycle exercisein the nonsedatedpostabsorptive state. All medications except for immunosuppressantswere witheld for 12 hours before the study. Symptom-limited supine bicycle exercise was performed in 3-minute stagesbeginning at a work load of 25 W, with increasing increments of 25 W every 3 minutes as tolerated. Four or more endomyocardial specimens were obtained for histologic analysis and graded using the Billingham criteria.t7 A triple-lumen ATRIAL NATRIURETIC PEPTIDE AFTER TRANSPLANT

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thermodilution pulmonary artery catheter was inserted to measure resting and peak exerciseright-sided heart pressures. Thermodilution cardiac outputs were obtained in triplicate with 12 months before the study, Exercise was performed on a Quinton model 884 Uniwork upright bicycle ergometer beginning at a work load of 33 W and increasing by 33 W every 3 minutes until the subject could not continue any further due to exhaustion. Blood samplesfor ANP assaywere obtained from a peripheral vein at rest and every 8 minutes during the exercise period. The control subjects achieved a maximal heart rate >160 beats/min with exercise,and the time required to reach exhaustion was 21 f 1 minutes. Intracardiac pressureswere not obtained in the control population. Venous ANP levels (superior vena cava or peripheral vein) were used for comparisonsbetweenthe control and study population. The protocol was approved by the Human SubjectsReview Committee and all subjectsgave informed consent. The ANP levels were determined by a modification of the a-human ANP radioimmunoassaykit produced by Peninsula Laboratories. The sensitivity of the assay in our laboratory is 20 pg/ml of plasma. The intra- and interassay coefficients of variation for this radioimmunoassaywere 4 and 22%, respectively.The values were calculated using control-spiked samplesof plasma previously stripped with charcoal and assayedat 3 separate known concentrations on the standard curve. Unextracted charcoal-treated plasma was added to the assay curve and all plasma samples were assayedwith nonspecific (e.g., no ANP antibody added) tubes to correct for further nonspecific protein binding. Mdistics: Measurements are expressed as mean values,with the standard error of the mean as the index of dispersion.The unpaired Student’s t test was used to compare the resting ANP levels in the normal and transplant patient groups. Analysis of variance was performed on the ANP levels obtained from different intravascular sample sites. Rest and peak exercise ANP levels among matched patient groups were compared using the paired Student’s t test. Simple linear regression was used to determine the relation between the ANP level and hemodynamic measures.Results were consideredsignificant at p <0.05. 238

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RESULTS Demographics, baseline and peak exercise hemodynamic findings are listed in Table I. All patients exerciseduntil exhaustion and the mean work load was 44 f 6 W. Endomyocardial biopsy specimens demonstrated that 12 of the 13 patients did not have rejection that required treatment. One patient had mild rejection and was treated. Twelve of the 13 patients had no fibrosis or focal areas considered to represent old biopsy sites, and 1 patient had patchy interstitial fibrosis. Demographic

Resting transplant

and hemodynamic

data (Table I):

atrial natriuretic peptide levels in cardiac recipients (Figure 1): The mean resting

ANP levels were superior vena cava 114 f 19 pg/ml, right atrium 165 f 33 pg/ml, right ventricle 130 f 20 pg/ml and pulmonary artery 134 f 20 pg/ml. An upward trend in ANP was seen from the superior vena cava to the right atrium. However, no significant difference in the levels were present when analyzed with analysis of variance. The baseline venous level (114 f 19) was significantly elevated in transplant recipients (p
ure 2). A 3.3-fold (I 14 to 373 pg/ml) increase in the ANP level from resting to exercise was observed in transplant recipients. In the control group a 4.4-fold (21-92 pg/ml) increase in the venous ANP level with exercise was demonstrated. In the cardiac transplant recipients, the right atria1 mean pressureincreased2.2fold (6 f 1 to 13 f 2 mm Hg) from baseline to peak exercise,while the pulmonary artery wedge pressureincreased2. l-fold (11 f 1 to 23 f 7 mm Hg). Hemodynamic correlations peptide levels: Simple linear

with

atrial

natriuretic

regressionwas performed to determine the relation between hemodynamics and ANP levels. Levels during rest and exercise in the 13 transplant recipients were analyzed with the respective hemodynamics, and the correlations were as follows: superior vena cava ANP and mean right atria1 pressure, r = 0.48 p = 0.01; superior vena cava ANP and mean pulmonary artery wedge pressure, r = 0.48 p = 0.01; right atria1 ANP and mean right atria1 pressure, r = 0.49 p = 0.01; right atria1 ANP and mean puhnonary artery wedge pressure, r = 0.51 p = 0.008; right ventricular ANP and mean right atria1 pressure, r = 0.56 p = 0.003; right ventricular ANP and mean pulmonary artery wedge pressure,r = 0.54 p = 0.004.

marked and consistent increase in ANP levels during exercise at all 4 sampling sites compared with levels during rest in cardiac transplant recipients. The exercise values were superior vena cava 373 f 61 pg/ml, right atrium 472 f 59 pg/ml, right ventricle 422 f 60 pg/ml and pulmonary artery 412 f 60 pg/ml. Analysis of variance demonstrated no significant intracardiac variability in the ANP levels sampled at peak exercise. Exercise venous ANP level (373 f 61) was significantly elevated in transplant recipients (p <.OOl) compared with levels in the control group (92 f 14) (Figp<.oo1

600 ,

F 8

f

500 E 2

200

3

q

REST

Ei

EJEtXXE

400 n=13 300

* P< ,001

200

q q

100

REST EXERCISE

0 WC

0 TRANSPLANT

RA SAMPLE

FM

PA

SITE

FIGURE 1. Resting and peak exercise at&l natriuretic peptlde (ANP) levels in cardii transplant recipients. There is a slgniinl lncroaee in the level from each sample sRe when comparing resting and exercise dues (p
FlGURE 2. Resting and peak exercise venous at&l natriuretic peptlds (ANP) levels are shown in normal subjscts (CONTROL) end cardiac transplant recipients (TRANSPLANT). Up per right inset, pulmonary artery wedge (PAW) and right atrial (RA) pressures at rest and duing peak exercise from transplant patients are shown. Peak exercise level in transplant group (373 pg/ml) was signiicantly higher than in control group (92 pg/ml, p
ATRIAL NATRIURETIC PEPTIDE AFTER TRANSPLANT

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DISCUSSION The cardiac transplant recipients exerciseduntil exhaustion and developedsignificant increasesin the right atria1 and pulmonary artery wedge pressures.Simultaneously, a marked increase in the peak exercisevenous ANP levels (3.3-fold) was observed.This is similar to the increase reported with exercise in normal subjects (n = 4) by Raine et al8 (3.1-fold [33 to 102 pg/ml]) and the increase observedin our control subjects (4.4fold [21 to 92 pg/ml]). Despite the elevated resting ANP levels in cardiac transplant recipients, an increase in ANP levels can occur when a provocative stimulus that increasesright atria1 and pulmonary artery wedge pressuresis applied. We chose to sample ANP from multiple cardiac chambers because previous studies have shown the best hemodynamic correlations with right ventricular ANP levels.19A relation has been demonstrated in cardiac transplant recipients between intracardiac pressureand ANP levels. We cannot conclude whether superior vena cava, right atria1 or right ventricular ANP samplescorrelate best with hemodynamics becauseof the lack of significant variability in samplesfrom these sites. Potential factors related to elevated resting atrial natriuretic peptide levels in cardiac transplant recipientw The cardiac transplant recipients studied were hy-

pertensive (mean arterial pressure 115 f 3 mm Hg); however, hemodynamics at rest were essentially normal. A relation between resting ANP levels and blood pressure was not found. Therefore no direct etiologic relation was observedbetween ANP levels, blood pressure or systemic vascular resistance.ANP levels are elevated in subjects with essential hypertension, but the levels previously reported are lower than the resting ANP levels observed in our transplant recipients.20-22 Impairment of ventricular relaxation may contribute to ANP secretionby increasing left atria1 afterload. The transplanted heart is noncompliant,15J6 and perhaps the striking increase in pulmonary artery wedge pressure with exercise in transplant patients (with normal systolic function) indicates an abnormality of diastolic function. These factors may contribute to impairment of ventricular relaxation, which may basally stimulate ANP release. Renal function (glomerular filtration rate) and age are related to ANP levels.23No correlation between ANP levels and serum creatinine or age was demonstrated in the transplant recipients. All patients were receiving cyclosporine, which promotes renal tubular secretion of creatinine; thus, the serum creatinine and creatinine clearance underestimate the true glomerular filtration rate.24Cyc1OS porine impairs natriuresis in renal transplant recipients25;thus “cyclosporine related”

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sodium retention may serve as a stimulus for ANP release. The measurement of ANP levels in orthotopic cardiac transplant recipients not receiving cyclosporine would help to clarify these issues. Anatomically, the explanation may be a function of the ubiquitous biatrial enlargement and attendant wall stretch that occurs when the donor recipient anastomosis is created surgically at the time of transplantation. Roman et a126recently reported that marked left atria1 dilatation (distension) is a potent stimulus to ANP release. Although right atria1 and pulmonary artery wedge pressuresat rest remain normal, mechanically induced atria1 enlargement and stretch may serve as a chronic stimulus (increasedwall tension) to ANP secretion. Role of cardiac denervation in the control of atrial natriuretic peptide release: Atria1 pressureand perhaps

more importantly atria1 stretch or wall tension may independently govern ANP release. The involvement of the sympathetic nervous system in the release and action of ANP on target organs is poorly understood. Goetz et a127showed that dogs completely devoid of cardiac innervation can increasetheir circulating ANP >400% with atria1distention. Furthermore, despite a 4fold increasein ANP levels, no associatednatriuresis or diuresis was noted. Goetz et al concluded that cardiac innervation was not necessaryfor releaseof ANP, but the renal responseappeared dependent on cardiac innervation. This investigation has not determined what contribution to the net ANP level is made by the donor versus the recipient atria. This is a crucial issue because the donor atria are surgically denervatedand documentation of ANP release by donor atria would indicate that secretoryfunction is preserveddespite sympathetic denervation. Implantation of the total artificial heart leavesthe innervated native atria intact. Schwab et a128 reported ANP levels of 60 and 39 pg/ml in 2 humans implanted with a total artificial heart. Our data in 1 patient who received a total artificial heart is similar.29 In this person, serial measuresof venous ANP were obtained. Immediately before surgery the venous ANP level was 310 pg/ml, the level decreasedto 45 pg/ml 24 hours after surgery and <20 pg/ml 5 days after implantation. The levels measuredin these personsare much lower than the levels observed after orthotopic transplantation and implies that the denervated donor atria make a contribution to the net ANP level in cardiac transplant recipients. Study limitations: The control population was younger, entirely female and exercisedupright. Our control subjects(“normals”) were in fact trained athletes with presumably altered cardiovascular physiology; however,

JULY 15. 1991

the ANP exercise responseclosely approximated the only previously reported values in normal subjects.8The marked absolute differences (in ANP levels) between the control subjects and cardiac transplant recipients documents the striking elevation in ANP levels in cardiac transplant recipients at rest and during exercise. The control subjects were sampled upright and the transplant patients, supine. Differences in loading conditions and subsequently atria1 pressure between upright and supine posture may have partially contributed to the observeddifferences in ANP levels between the control and study population.30Becauseof limited patient numbers and lack of rejection episodes,we were unable to determine if AN? levelsincreasewith moderate or severerejection. Atria1 natriuretic peptide levels probably are not a useful marker of mild allograft rejection, which usually is present without associatedhemodynamic compromise. We sincerely thank Margaret Wooding-Scott, RN, Gail Schaeffer-McCall, RN, and J.R. Warner for their technical and secretarial assistance. Acknowledgment:

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