Cardiovascular autonomic dysfunction in a novel rodent model of polycystickidney disease

Joanne L. Harrison a,1, Cara M. Hildreth b,1, Stephen M. Callahan a,Ann K. Goodchild b, Jacqueline K. Phillips a,b,!a Integrated Health Institute, Faculty of Health Sciences, Murdoch University, Perth WA, Australiab Australian School of Advanced Medicine, Macquarie University, Sydney NSW, Australia

a b s t r a c ta r t i c l e i n f o

Article history:Received 10 July 2009Received in revised form 14 September 2009Accepted 25 September 2009

Keywords:HypertensionSympatheticParasympatheticHeart rate variabilityBarore!ex function

Autonomic dysfunction, hypertension and cardiovascularmorbidity in end stage renal disease are critically linked,however there are limitedmodels available to investigate this relationship and develop clinical interventions. Thisstudy aimed to de"ne the relationship between hypertension and autonomic function in a new rodent model ofpolycystic kidney disease (PKD). Usingmeasures of heart rate and systolic blood pressure variability (HRV, SBPV),and time domain analysis of cardiac and sympathetic barore!ex function, we compared the Lewis PKD model(LPK) to a Lewis control. Systolic BP and SBPV were signi"cantly higher in LPK vs. Lewis (168±7 vs. 131±8mmHg, P!0.01, total power: 11±3.1 vs. 1.3±0.3 mm Hg/Hz2, P!0.05). LPK has a higher resting HR (437±17 vs.330±11 beats perminute [bpm],P!0.001) associatedwith reducedHRV (total power [1.7±0.3 vs. 4.6±1.1 ms/Hz2, P!0.01]). Atenolol decreased HR to a greater extent in the LPK (90±10 vs. 20±17 bpm, P!0.001) whilesubsequent methylatropine administration produced a greater increase in Lewis HR (24±9 vs. 66±9 bpm,P!0.01). No difference in intrinsic HR following both drugs existed. Cardiac barore!ex function was impairedin LPK vs. Lewis (0.6±0.4 vs. 1.2±0.2 bpm/mm Hg P!0.05, and 0.3±0.1 vs. 3.1±0.6 ms/mm Hg, P!0.001,respectively). The sympathetic barore!ex function curve was shifted upwards and towards the right in LPK(P!0.01). Sympathetic barore!ex gain was not altered. This data suggests that sympathetic hyperactivity andreduced vagal function underlies the hypertension and reduced cardiac barore!ex function in the LPK model.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Tonic and re!ex control of heart rate (HR) is dependent upon thebalance of vagal and sympathetic input and disruption of this balance,through a decrease in vagal tone and/or an increase in sympathetictone, leads to cardiac dysfunction (La Rovere et al., 1998). Increasedsympathetic activity is linked to the development of left ventricularhypertrophy, arterial remodeling and arrhythmias, independent ofblood pressure (BP) (Mancia et al., 1999). Autonomic function can beassessed through the analysis of HR and systolic BP variability (HRVand SBPV) (Stauss, 2007) as well as the determination of barore!exsensitivity (BRS). Increased SBPV, reduced HRV and attenuated BRSare re!ective of abnormal autonomic control of the heart andvasculature and are strong predictors of subsequent cardiovascularevents (La Rovere et al., 1998, 2001). In patients with renaldysfunction, cardiovascular disease is a signi"cant cause of morbidityand mortality (Ranpuria et al., 2008), and cardiac autonomic

dysfunction has been convincingly documented in patients withboth diabetic and non-diabetic renal disease (Cashion et al., 2000;Dursun et al., 2004).

Polycystic kidney disease (PKD) is the predominant genetic cause ofend stage renal disease. In humans, there are two hereditary forms ofPKD: adult onset autosomal dominant (ADPKD) and autosomalrecessive (ARPKD), an important cause of early childhood nephropathy(Guay-Woodford, 2003; Zerres et al., 2003). In patients withADPKD, 60% develop hypertension prior to renal function impairment(Gabow et al., 1990). In ARPKD, hypertension occurs in up to 80% ofaffected children, with nearly all who survive the neonatal periodrequiring anti-hypertensive treatment (Capisonda et al., 2003; Sweeneyand Avner, 2006).With a high incidence of left ventricular hypertrophy,PKD has been described as a hypertensive heart disease (Bardaji et al.,2001). Several mechanisms have been proposed regarding thepathogenesis of hypertension in PKD including volume overloadassociated with an abnormal pressure-natriuresis response, activationof the renin–angiotensin system associated with renal cyst formation,induction of local tissue ischaemia and increased sympathetic nervoussystem (SNS) activity (Augustyniak et al., 2002; Fall and Prisant, 2005;Locatelli et al., 2003; Valvo et al., 1985). Recently, evidence has beenpresented to support an important link between sympathetic hyperac-tivity and cardiovascular morbidity in PKD patients (Cerasola et al.,

Autonomic Neuroscience: Basic and Clinical 152 (2010) 60–66

! Corresponding author. Australian School of Advanced Medicine, MacquarieUniversity, Sydney NSW 2109 Australia. Tel.: +61 2 98504000; fax: +61 2 98504010.

E-mail address: jacqueline.phillips@asam.mq.edu.au (J.K. Phillips).1 Authors contributed equally.

1566-0702/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.autneu.2009.09.019

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1998; Klein et al., 2001; Neumann et al., 2002). Muscle sympatheticnerve activity is increased in hypertensive PKD patients, regardless ofrenal function (Klein et al., 2001) and in a recent study using a ratmodelof ADPKD, renal denervation indicated that the SNS contributes to thehypertension, which in turn affects disease progression (Gattone et al.,2008).

There is limited data assessing cardiac autonomic function inpatients with PKD, (Johansson et al., 2005; Polak et al., 2004; Solderset al., 1985), and while strongly suggestive of autonomic dysfunction,it is restricted to small numbers, often in later disease when dialysis isrequired. Therefore, there is an important need for the establishmentof animal models to facilitate comprehensive examination of theunderlying mechanisms and investigate potential drug therapies tobreak the tie between renal disease and cardiovascular morbidity. Wehave recently established a rodent model of ARPKD in Lewis rats[Lewis polycystic kidney (LPK)], and shown that cysts form in thekidney from 3 weeks of age, preceding hypertension at 6 weeks(Phillips et al., 2007). Ganglionic blockade signi"cantly reduced meanBP in LPK animals compared to Lewis control animals (52% vs. 4%),whereas plasma-renin activity and angiotensin II were reduced in theestablished disease phase, suggesting a critical role for the SNS in thepathogenesis of the cardiovascular features of PKD (Phillips et al.,2007). In the current study we therefore aimed to de"ne therelationship between hypertension and autonomic function in theLPK model. This would additionally serve to de"ne the LPK as avaluable model of PKD for cardiovascular research. We achieved thisthrough measurements of HR, HRV, BP, SBPV and determination ofboth cardiac and sympathetic nerve barore!ex function.

2. Materials and methods

2.1. Animals

All experimentswere approved by the animal ethics committees ofMurdoch University and Macquarie University and conducted inaccordance with the Australian Code of Practice for the Care and Useof Animals for Scienti"c Purposes. Male LPK and Lewis controls (12–13 weeks old) were used. PKDwas con"rmed by the presence of renalcysts in all LPK. Animal numbers are stated in separate protocols.

2.2. Surgical procedures

Animals were anaesthetised with ethylcarbamate (urethane 1.3 g/kg i.p.). Depth of anaesthesia was assessed regularly using re!exresponses to tactile (corneal stroking) and noxious (hind paw pinch)stimuli. Additional doses of urethane (0.13 g/kg iv) were adminis-tered as required. The right femoral artery and vein were cannulatedto record arterial BP and administration of drugs, respectively. Rectaltemperature was monitored and maintained, using a thermostaticallycontrolled electric blanket and infrared heating source, at 37 °C. In asubgroup of animals (Protocol 2), the left greater splanchnic nervewas exposed, isolated, cut distally to eliminate afferent transmissionand maintained in paraf"n oil. Nerve activity was recorded usingbipolar silver wire electrodes, ampli"ed and "ltered between 10 and1000 Hz using a CWE bioampli"er and acquired at 2 kHz using either aCED 1401 plus and SPIKE 2 software (CED, Cambridge, UK) or aPowerlab data recorder (ADInstruments, Sydney, Australia) and ChartPro Software (v 5.5.6 ADInstruments). BP was acquired at 500 Hz.

In preliminary experiments, Lewis animals (n=4), received sequen-tial doses of atenolol (0.01, 0.03, 0.1, 0.3, 1, 3 and10mg/kg iv), in order todetermine themaximally effective dose for use in Protocol 1. Changes inHR in response to each dose were determined. The dose of 1 mg/kg waschosen for subsequentexperiments, as higher dosesof atenolol (3 mg/kgand 10 mg/kg iv) did not produce any further change in HR (298±7 vs.292±6 vs. 288±4 bpm, 1 mg/kg vs. 3 mg/kg vs. 10 mg/kg, respectively,P>0.05, n=4).

2.3. Experimental protocols

2.3.1. Basal parameters, HRV and SBPVData for basal cardiovascular parameters [BP, HR and spontaneous

barore!ex sensitivity (sBRS)], HRV and SBPV was acquired from 12LPK and 8 Lewis over a 5 min period following an initial rest period of20 min and prior to any pharmacological intervention. HRV and SBPVwere acquired from an 80 s segment during this period where BP andpulse interval were stationary. Each animal was then subjected to oneor both of the following protocols:

Protocol 1Sympathetic contribution to restingHRwas determined bymeasuringthe acute changes in HR in response to atenolol (1 mg/kg iv). After theHR response had reached a plateau following atenolol administration(minimum 15min), methylatropine (2 mg/kg iv) was then delivered.The subsequent acute change evoked indicated the level of cardiacvagal tone and the "nal resting HR attained after both atenolol andmethylatropine indicated the intrinsic pacemaker properties of theheart (i.e. intrinsic HR).Protocol 2Cardiac and sympathetic baroreceptor re!exes were stimulatedusing ramp changes in BP with phenylephrine (PE) and sodiumnitroprusside (SNP) (each 10 µg/kg iv).

2.4. Data analysis

All data were analysed off-line using Spike 2 (version 6) andGraphPad software.

2.4.1. HRV and SBPVPulse interval (for HRV) and systolic BP (for SBPV) were derived

from the BP waveform and uniformly resampled at 10 Hz. Powerspectra were computed using fast Fourier transformation and viewedin a Hanning window (size 256) with a frequency resolution of0.04 Hz. Total, low frequency (LF, 0.25–0.75 Hz) and high frequency(HF, 1–3 Hz) powers were calculated.

2.4.2. Spontaneous BRSsBRS was calculated using the sequence method from the pulse

interval and the AP waveform (Hildreth et al., 2008; Padley et al.,2005). The algorithm used seeks out patterns of three consecutivelengthening or shortening pulse interval associated with an increaseor decrease in MAP of at least 0.5 mmHg, respectively (Hildreth et al.,2008). The linear regression of all sequences "tting these criteria wasdetermined and sBRS was calculated as the average regression of allsequences.

2.4.3. BRS derived from induced changes in BP

2.4.3.1. HR barore!ex gain. Linear analysis was used to assess there!ex change in HR in response to induced changes in BP with PEand SNP over a de"ned change in MAP (40 mm Hg) as describedpreviously (Padley et al., 2005). Separate values for PE and SNP werereported.

2.4.3.2. Sympathetic nerve activity barore!ex gain. Splanchnic sym-pathetic nerve activity (SNA) was recti"ed and averaged (timeconstant 1 s). SNA was normalised by setting the level of SNAfollowing euthanasia to 0% and resting SNA to 100%. Baroreceptorre!ex function curves were generated using the active phases of PEand SNP induced changes in AP and SNA. Nonlinear regressionanalysis was used to "t normalised SNA values vs. MAP to a four-parameter sigmoid logistic function curve (GraphPad Prism v5):y=P1+[(P2"P1)/(1+10{P3" x}P4))], where y is SNA, x is MAP, P1 is

61J.L. Harrison et al. / Autonomic Neuroscience: Basic and Clinical 152 (2010) 60–66

the bottom plateau, P2 is the top plateau, P3 is MAP halfway betweentop and bottom plateau, P4 is the steepness of the curve. Curves withR2 values less than 0.95 were not included in the data set. The "rstderivative of the logistic function curve was used to calculate maximalgain.

2.5. Statistical analysis

All data are expressed as means±standard error of the mean(SEM) and analysed using Student's t-test. Grubb's test was used toidentify signi"cant outliers, subsequent animal numbers are stated inthe result following the removal of identi"ed outliers. P!0.05 wasconsidered signi"cant.

3. Results

3.1. LPK has increased systolic blood pressure and variability

Systolic BPwas signi"cantly higher in LPK comparedwith Lewis rats:168±7 vs. 131±8 mm Hg (P!0.01, 12 LPK and 8 Lewis, Fig. 1A),comparable to tail cuff systolic BP recordings from conscious animalspublished previously (Phillips et al., 2007). Mean arterial pressure wasalso elevated in LPK compared with Lewis 119±5 vs. 101±6 mm Hg(P!0.05). The elevation in SBP in the LPK was associated with anincrease in SBPV: total power 11±3.1 vs. 1.3±0.3 mm Hg/Hz2

(P!0.05, 11 LPK and 8 Lewis, Fig. 1B). Variability in both HF and LFfrequency bands, were elevated compared with Lewis (Fig. 1C and D,respectively).

3.2. LPK has a higher resting heart rate due to increased sympathetic anddecreased parasympathetic tone

LPK has an elevated resting HR: 437±17 vs. 330±11 bpm(P!0.001, 12 LPK and 8 Lewis, Fig. 2A) which is not associated withany differences in intrinsic HR as determined following combinedadministration of atenolol and methylatropine: 377±16 vs. 371±

6 bpm (7 LPK and 5 Lewis; Fig. 2B). Administration of atenolol (1 mg/kg iv) decreased HR to a much greater extent in the LPK comparedwith Lewis: 90±10 vs. 20±17 bpm (P!0.001, Fig. 2C). Subsequentadministration of methylatropine (2 mg/kg iv) produced a greaterincrease in HR in the Lewis: 66±9 vs. 24±9 bpm (P!0.01, Fig. 2D).

3.3. LPK has reduced heart rate variability

The differences in resting HR in the LPK were associated withsigni"cantly reduced HRV. Total power of HRV: 1.7±0.3 vs. 4.6±1.1 ms/Hz2 (P!0.01); HF power: 1.1±0.2 vs. 2.5±0.6 (P!0.05) andLF power: 0.1±0.02 vs. 1.2±0.4, were all reduced in LPK compared toLewis (P!0.01, 11 LPK and 8 Lewis, Fig. 3A–C).

3.4. LPK has impaired cardiac barore!ex but not sympatheticbarore!ex gain

Barore!ex function was assessed and compared between LPK andLewis in response to both induced (5 LPK and 6 Lewis) and autogenicchanges in BP (11 LPK and 8 Lewis). Pharmacologically inducedchanges in BP with PE and SNP evoked a re!ex decrease and increasein splanchnic SNA respectively (Fig. 4A). The sympathetic barore!exlogistic function curve of the LPK was shifted upwards and to the rightcompared with Lewis (P!0.001, Fig. 4B) re!ecting the higher restingBP of the LPK. Sensitivity of the splanchnic nerve to changes in BP(sympathetic barore!ex gain) did not differ between LPK and Lewis:"3±0.1 vs."3±0.4% SNA/mmHg (P>0.05, Fig. 4B). BRS, calculatedusing both the sequence method: 0.3±0.1 vs. 3.1±0.6 ms/mm Hg(P!0.001, Fig. 4C), and PE: 0.6±0.4 vs. 1.2±0.2 bpm/mm Hg(P!0.05, Fig. 4C), was reduced in the LPK. BRS calculated using SNPdid not differ between LPK and Lewis: 0.6±0.2 vs. 0.6±0.1 bpm/mmHg (P>0.05, Fig. 4C).

Fig. 1. Blood pressure parameters and variability. (A) LPK has a higher resting systolicblood pressure (SBP, 12 LPK and 8 Lewis). SBP variability (SBPV, 11 LPK and 8 Lewis)was determined and total power (B); HF power (C) and LF power (D) components wereincreased in LPK compared to Lewis. Values are means±SEM. !P!0.05, !!P!0.01.

Fig. 2. Heart rate parameters. (A) LPK has a higher resting heart rate (HR, 12 LPK and8 Lewis). (B) Following combined administration of atenolol and methylatropine, HRdid not differ between LPK and Lewis, indicating identical intrinsic HRs (7 LPK and 5Lewis). Acute changes in HR in response to atenolol (1 mg/kg) (C) and the subsequentchange in HR in response to methylatropine (2 mg/kg) (D) represent the sympatheticand parasympathetic contribution to resting HR, respectively. LPK has greatersympathetic (C) and less parasympathetic (D) contribution to resting HR comparedto Lewis. Values are means±SEM. !!P!0.01, !!!P!0.001.

62 J.L. Harrison et al. / Autonomic Neuroscience: Basic and Clinical 152 (2010) 60–66

4. Discussion

We have demonstrated that both hypertension and tachycardiacan be attributed to abnormal autonomic control of both thevasculature and heart in the LPK model of PKD when compared toLewis control animals. Our analysis shows that SBPV is increased,highly indicative of an increase in sympathetic vasculature tone andHRV is reduced, suggestive of abnormal vagal control of the heart.Pharmacological blockade at the level of the heart demonstrated thattonic sympathetic input is increased whereas vagal input is reduced,accounting for the tachycardic state. In response to induced andautogenic changes in BP, we show that the cardiac but notsympathetic barore!ex is impaired in the LPK model. Thus we havedemonstrated cardiovascular autonomic dysfunction in PKD.

Our demonstration that SPBV, notably the LF band, is elevated inLPK animals is comparable to studies in both hypertensive humans(Mancia et al., 1983) and a number of rodent models including thespontaneously hypertensive rat (SHR) (Friberg et al., 1989), renalwrapped hypertensive rats (VanNess et al., 1999) and animals withboth 2K1C and 1K1C Goldblatt hypertension (Ponchon and Elghozi,1996; Souza et al., 2008). The LF band of SBPV is attributed to cyclicchanges in sympathetic vascular tone and increased LF is directlyassociated with elevated sympathetic activity (Pagani et al., 1997;Brown et al., 1994; Cerutti et al., 1994; Souza et al., 2008). Accordingly,the increase in SBPV LF seen in the LPK re!ects increased sympatheticvasculature tone that presumably underlies the hypertension. There isa strong relationship between increased SBPV and both the preva-lence and severity of end-organ damage (Mancia et al., 1994; Miao etal., 2006; Parati et al., 1987; Su and Miao, 2001) including leftventricular hypertrophy (Miao et al., 2006). Along these lines, wehave previously reported cardiac hypertrophy in the late diseasephase in the LPK model (Phillips et al., 2007).

Our data also reveals that the LPK animals have amarkedly elevatedresting HR due to both increased sympathetic and reduced vagal inputto the heart, demonstrated by cardiac sympathetic blockade (ateno-lol), which evoked greater increases in HR in the LPK, and bysubsequent cardiac vagal blockade (methylatropine), which evokedsmaller increases in HR when compared to the normotensive Lewiscontrols. Importantly, the resting tachycardia in the LPK is neurogenic,as intrinsic HR did not differ between the strains. This increase incardiac sympathetic activity further supports the "nding of elevatedSBPV, and that a generalised elevation in sympathetic activity is amajor contributor to the elevated BP. This is in alignment with Kleinet al. (2001), who showed that hypertension in PKD in humans ischaracterised by sympathetic hyperactivity, as determined by muscleSNA. These authors also demonstrated that resting HR and cardiacbarore!ex were similar between PKD patients and healthy controls.The discrepancy between our "ndings and the reported data fromKlein et al. (2001) can perhaps be explained by disease phase and priormedical interventions, as while the PKD patients at the time ofexperimentwere drug free, they had been treated previously andwerevolume controlled during the study. These factors could have asustained protective effect on the cardiovascular system in thesepatients.

Reduced cardiac vagal activity in the LPK as shown by pharmaco-logical blockade is substantiated by our demonstration of reduced HRV.It is generally agreed that the HF component of HRV is stronglydependent upon respiratory sinus arrhythmia and re!ects parasympa-thetic tone, and conversely, that oscillations due to the baroreceptorre!ex contribute to the LF component, which is commonly accepted asan indicator of both sympathetic and vagal activity (Parati et al., 2006;Ranpuria et al., 2008; Stauss, 2003; Task Force, 1996). In our study, bothHF and LF components were signi"cantly less in the LPK animalscompared to their normotensive controls. This is consistentwith studiesin end stage renal disease patients, where a marked decrease in bothparameters has also been documented (Steinberg et al., 1998). In arecent study of end stage renal disease patients under dialysis, both LFand HF coef"cients were markedly depressed, however these para-meters were shown to improve/normalize following renal transplanta-tion (Rubinger et al., 2009). Moreover, in some conditions of raisedsympathetic activity, such as heart failure and exercise, LF isparadoxically reduced (Henze et al., 2008; Kingwell et al., 1994;Notarius and Floras, 2001; Task Force, 1996).

Our analysis of cardiac BRS in the LPK provides additional evidencethat both tonic and re!ex vagal input to the heart is altered in the PKDmodel. The barore!ex is able to buffer acute changes in BP byreciprocal modulation of HR (vagal component) and total peripheralresistance (sympathetic component). Cardiovascular diseases areoften accompanied by impaired barore!ex responses, and suchdysfunction cannot only contribute to increased end-organ damageand progression of the underlying disease, but also be predictive ofincreased cardiovascular risk (La Rovere et al., 2008; Shan et al., 1999).An inverse relationship exists between resting HR and both HRV andBRS (Hesse et al., 2007; Hildreth et al., 2008; Platisa and Gal, 2006),however the SNP induced increases in HR mediated by sympatheticwithdrawal in this study were not signi"cantly different in the LPK,suggesting that a vagal de"cit contributes to the reduced HRV andBRS. The increased SBPV observed in the LPK animals may re!ectdiminished barore!ex function as other models of hypertension, suchas the SHR and 1K1C Goldblatt model, exhibit similar alterations(Friberg et al., 1989; Murphy et al., 1991; Souza et al., 2008). As aclinical indicator, BRS signi"cantly predicts cardiac mortality inpatients with chronic renal failure (Johansson et al., 2005) and recentwork has shown that depressed BRS can be correlated to blood ureanitrogen (La Rovere et al., 2009).

Methodological considerations in the present study include thealgorithm used to determine sBRS. The sequence method applied isroutinely used in our laboratory (Hildreth et al., 2008; Padley et al.,

Fig. 3. Heart rate variability. LPK has reduced heart rate variability (HRV) compared toLewis. Total power (A) as well as high frequency (HF, B) and low frequency (LF, C)power spectral components of HR were reduced in LPK compared to Lewis. Values aremeans±SEM. !P!0.05, !!P!0.01, 11 LPK and 8 Lewis.

63J.L. Harrison et al. / Autonomic Neuroscience: Basic and Clinical 152 (2010) 60–66

2005) and uses criteria that detect changes in MAP. While algorithmsthat use changes in SBP are also used (Braga et al., 2008), theimportant "nding of this study is not the absolute values of BRS, butrather that BRS is reduced in the LPK. Furthermore, as described,induced changes in BPwith phenylephrine, which is re!ective of vagalchanges in HR (Guyenet et al., 1987) is also reduced in the LPK,con"rming the relationship we identi"ed using the sequence method.Recent comparative studies indicate that both the classical pharma-cological and sequence methods are reliable procedures for deter-mining BRS (Braga et al., 2008).

The origin of the impaired cardiac barore!ex de"ciency in LPK isunknown. Numerous variables have been forwarded to explaindepressed cardiac BRS in other systems, including those related tothe afferent arm (such as elevated BP and loss of vascular compliance),central processing (such as neural de"cits in the barore!ex arch withinkey autonomic central sites), and the efferent arm (such as decreasedcardiac cholinergic responsiveness) (Johansson et al., 2005; Monahan,2007; Rubinger et al., 2009; Studinger et al., 2006). There may also bea genetic component, as individuals with a family history ofhypertension have lower BRS than those with no family history,regardless of BP (Andresen et al., 1980; Sleight, 1997). Furtherinvestigation of the time course of development of cardiac barore!exdysfunction during PKD progression is required.

Conversely, sympathetic BRS in the LPK appears similar to thatseen in Lewis albeit set to a higher BP. Despite this initially surprising"nding, it should be noted that when constructing sympatheticbarore!ex function curves, to enable comparison between animals,SNA is normalised with 100% set as the resting level of SNA, thereby

constraining the function curves in both strains and potentiallybiasing the data (Burke and Head, 2003). Nonetheless, the sympa-thetic barore!ex function curves do provide further support forsympathetic overactivity in LPK as the curves were set to higher levelsof SNA.

Our data indicates that elevated sympathetic activity underlies thehypertension in LPK during the established disease phase. Potentialpathophysiological determinants are inappropriate renin secretion orincreased sympathetic drive stimulated by the renal afferents (Kleinet al., 2001; Phillips, 2005). We previously presented in the LPKmodelthat plasma-renin activity was suppressed in LPK rats at the age usedin this study (Phillips et al., 2007), suggesting therefore that renalafferents may be a key generator of sympathetic overactivity.

5. Conclusions

This is the "rst study in which HRV and SBPV have been correlatedwith the autonomic parameters of cardiac barore!ex and sympatheticbarore!ex gain in an animal model of PKD. This data will contributesigni"cantly to our understanding of not only this newly discoveredmodel, but also the pathophysiological link between renal disease andcardiovascular mortality, since altered HRV, SBPV and BRS areconsidered major markers of cardiovascular risk in a clinical setting(Souza et al., 2008; Fukuta et al., 2003; Ranpuria et al., 2008).

Traditional treatment of PKD targets BP control as the critical ratelimiter of disease progression (Harris and Rangan, 2005). Our datademonstrates the presence of sympathetic hyperactivity, com-poundedwith impaired parasympathetic tone and reduced barore!ex

Fig. 4. Barore!ex sensitivity. Heart rate (HR) but not sympathetic barore!ex sensitivity (BRS) is reduced in LPK compared to Lewis. (A) A representative trace of pharmacologicallyinduced changes in arterial pressure (AP) with phenylephrine (PE) and sodium nitroprusside (SNP) evoked re!ex decreases and increases in HR and sympathetic nerve activity(SNA) respectively. (B) Individual barore!ex curves for LPK and Lewis are shown in the upper panel and the average gain of these curves is shown in the lower panel. Although,sympathetic barore!ex curves were shifted upward in the LPK compared to Lewis (upper panel), maximal gain was similar between the strains. The difference in the barore!exfunction curves suggests that a higher SNA resets the curve to the higher blood pressure measured in LPK. (C) Re!ex changes in HR due to autogenic changes in AP, measured usingthe sequence method, were attenuated in LPK (n=11 LPK and 8 Lewis). Re!ex changes in HR due to PE, but not SNP, were less in LPK compared to Lewis (5 LPK and 6 Lewis). Valuesare means±SEM. !P!0.05, !!!P!0.001.

64 J.L. Harrison et al. / Autonomic Neuroscience: Basic and Clinical 152 (2010) 60–66

function, thus highlighting the complexity of PKD and the need todevelop multi faceted pharmacological interventions. Future thera-peutics should aim to not only target hypertension, but to also correctautonomic dysfunction.

Acknowledgements

We gratefully acknowledge the guidance regarding data analysiskindly provided by Dr. James Padley (Macquarie University).

This project was supported by the National Health and MedicalResearch Council, Australia (JKP: 380475).

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