Importance of Cardiac, but Not Vascular, Hypertrophy in the Cardiac Baroreflex Deficit in Spontaneously Hypertensive and Stroke-Prone Rats GEOFFREY

A.

HEAD,

P~.D.,

NAOYOSHI

MINAMI,M.D., Prahran,

L/ictoria, Australia

In the present study, we examined whether antihypertensive treatment of young and adult hypertensive rats with the angiotensin-converting enzyme (ACE) inhibitor perindopril could restore the baroreflex vagal deficit and whether this was related to prevention of cardiac or vascular hypertrophy. Spontaneously hypertensive (SHR), stroke-prone spontaneously hypertensive (SHR-SP), and WistarKyoto (WKY) rats were untreated or treated with perindopril (3 mg/kg/day) in the drinking water from 4-9 and from 14-20 weeks of age. Steady-state sigmoidal mean arterial pressure (MAP&heart rate (HR) reflex curves were obtained in the conscious rats by the injection of pressor and depressor agents before and after atenolol (vagal component). Increased left ventricle .to bodyweight ratio (LV/BW) indicated cardiac hypertrophy. After ganglion blockade, the minimum MAP produced by nitroprusside and the maximum produced by methoxamine were used as indications of vascular hypertrophy. Perindopril treatment reduced cardiac and vascular hypertrophy to different extents in SHR and SHR-SP. The 4-9 and 14-20 week treatments reduced MAP and both minimum and maximum blood pressure of the SHR to the levels of the untreated WKY. However, only in the older animals was LV/BW restored. In the SHR-SP, early treatment had a much than on greater effect on vascular hypertrophy LV/BW. The reverse occurred for the 14-20 week animals. In untreated hypertensive animals the baroreflex curves were shifted to the right with reduced vagal HR range. Perindopril treatment shifted the baroreflex curves back towards the WKY curves. Vagal HR range was strongly correlated with the LV/BW, From the Baker Medical Research Institute, Prahran, Victoria, Australia. These studies were supported by a block institute grant from the National Health and Medical Research Council of Australia and by a grant from the Requests for reprints should be addressed to Geoffrey A. Head, Ph.D., Baker Medical Research Institute, Commercial Road, Prahran, Victoria, 3181, Austra-

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whereas vagal HR range was less well related to the level of vascular hypertrophy or blood pressure. These results suggest that antihypertensive treatment can restore cardiac baroreflex function and that it is related to the reduction in cardiac hypertrophy. Although the mechanism of this relationship remains to be elucidated, these findings suggest that cardiac vagal afferents may be important.

n both humans and experimental animals with Ibaroreflex chronic hypertensionthe changesto the cardiac consist of a shift to the right and a decrease in the slope of the mean arterial pressure (MAP)-heart rate (HR) relationship [l-4]. It has usually been measuredby the “ramp technique” of Smythe et a2 [5] in which the slopegives an indication of the gain or sensitivity of the reflex. However, the baroreflex MAP-HR relationship using the “steady state” technique of Korner et al has been shown to be sigmoidal with clear upper and lower plateausin humans[6], rabbit [7], and rat [a]. In hypertensionthe major deficit in the cardiacbaroreflex is a reduction in the vagal componentof the HR range [g-13]. Since this is common to several types of hypertension with different etiologies, it hasbeensuggestedthat the depressionin vagal HR range is not genetically determined but is a consequenceof hypertension[12]. We have shownthat in young spontaneouslyhypertensive rats (SHR) the vagal HR range is normal at 6 weeks when hypertension is evident, but that it is reduced some weeks later [9]. Cardiovascularhypertrophy is a major accompaniment of chronichypertension. Studies performed in SHR show that arterial baroreceptors themselveswere less responsiveto changesin pressure [14]. The reduction in arterial baroreceptorsensitivity, which may be related to vasculardistensibility, has generally beenconsideredto be the major determinant of the depressedgain of baroreflex HR control in hypertension. In addition, the central componentis alsoimpaired in SHR [15]. It is not at all clear to what extent the abnormal baroreflex

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functions are caused by cardiovascular hypertrophy in SHR. The present study examined whether antihypertensive treatment could prevent or reverse the baroreflex vagal deficit and whether this was related to prevention of cardiac or vascular hypertrophy. It has been shown that angiotensin-converting enzyme (ACE) inhibitors have a potent action not only to lower blood pressure, but also to prevent and reverse cardiovascular hypertrophy [16-191. In order to produce animals with a wide ranging combination of cardiac and vascular hypertrophy, both adult and young SHR, stroke-prone spontaneously hypertensive (SHR-SP), and Wistar-Kyoto (WKY) rats were treated for various periods with the ACE inhibitor perindopril [20]. We examined cardiac baroreflex function after cessation of treatment to eliminate any acute effects of the drug.

triggered from the arterial pulse. After connection of catheters, at least 1 hour was allowed before starting baroreflex measurements. MAP-HR curves were produced as described previously [8]. Intravenous injection of l-50 PL of methoxamine (2-100 pg/kg, Wellcome) and nitroprusside (l-50 pug/kg) were given to produce a series of graded steady-state changes in MAP (+5-50 mm Hg) in each animal. The doses were given to ensure that points were located on the upper and lower plateaus as well as the linear high-gain middle portion of the sigmoid curve. Responses mediated by efferent cardiac vagus nerve were assessed in the presence of atenolol (1 mg/kg intravenously). The steady-state changes in MAP and HR were fitted to a sigmoidal logistic equation as follows:

METHODS

where PI = lower HR plateau, P2 = HR range, P3 = a curvature coefficient, and P, = BPsO, i.e., the MAP at half the HR range [8]. P3 is also the normalized gain, as it indicates the gain of the curve that is independent of the HR range (i.e., when HR range is normalized to 100%). The average gain (e), or slope of the curve between the two inflection points, is a product of the range and the normalized gain and is given by G = ( -Pz x P&4.562. The upper plateau = PI + HR range (Pd.

Age- and weight-matched male WKY, SHR, and SHR-SP, bred at the Baker Medical Research Institute (Melbourne), were obtained post-weaning (4 weeks) and housed in individual cages with controlled temperature (23-25”(Z) and lighting (12 hours light, 12 hours dark) and with food and water ad libitum. To prevent cardiac and vascular hypertrophy WKY, SHR, and SHR-SP were treated from 4-9 weeks of age with perindopril (perindoprild-g, n=4-7) or water (as control, n=4-8) and studied at 14 weeks. To reverse hypertrophy, the three strains were treated from 14-20 weeks and studied at 22 weeks (perindopril14-z,,, n=6-8). In each case perindopril(3.0 mglkglday) was added to the drinking water with concentration adjusted to the individual water intake of the rats on a weekly basis. One week before the experiment, rats were implanted with arterial and venous catheters under short-acting intraperitoneal anesthesia (methohexital, 40 mg/kg, pentobarbital, 30 mg/kg, atropine, 5 mg/kg) as described previously [8]. The Teflontipped vinyl arterial catheter was inserted into the lower abdominal aorta and held in place with cyanoacrylate adhesive. The double lumen vinyl venous catheter was passed down the right common jugular vein so that the tip lay in the superior vena cava. After the operation, aortic and jugular catheters were filled with heparin 100 U/mL in 0.9% NaCl and the ends passed subcutaneously to emerge at the back of the neck. The rats were studied conscious and in their home cages. The arterial catheter was connected to a Statham pressure transducer and blood pressure (BP) was recorded on a Grass Instruments polygraph. HR was determined from a period meter

HR = Pi + P,l[l

+ ePa(MAP-P4)]

Estimation of Cardiac and Vascular Hypertrophy The blood pressure was determined at maximum dilation with nitroprusside (BP,,,) and at maximum constriction with methoxamine (BP,,,), both after autonomic blockade with pentolinium (10 mg/kg intravenously) and atenolol (1 mg/kg intravenously) [21]. Differences in these parameters were used as an index of whole body vascular hypertrophy. MAP-log dose response curves to intravenous methoxamine and nitroprusside were fitted to the sigmoidal logistic equation: MAP = p1 + p,/[l

+ ePs(L%’--WI

where PI = leftmost plateau, Pz = range, P3 = a curvature coefficient, and P4 = EDso, i.e., the log dose at half the MAP range. Pi + P, = BP,i, for nitroprusside and BP,,, for methoxamine. Left ventricle weight to bodyweight ratio (LVI BW, mg/g) was determined at the end of the study. Statistics Values were expressed as mean + standard error of the mean (SEM) or mean difference + standard error of the difference. Analysis of all parameters,

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TABLE I Effect of Perindopril Treatment from 4-9 Weeks of Age on 14 Week WKY, SHR, and SHR-SP WKY

SHR.SP

SHR

UNT

PERIN

UNT

PERIN

UNT

PERIN

SEM

MAP(mmHg)

107

3:;

145

107*

168

ii*

5.5

LV/BW(mg/g) HR(beats/min)

2972.13

3282.48

3142.32

2892.94

BP,, (mm&I BP,, (mm&I

1;:

1:;

l;:

2;;

2.02 1C*

2.50* 1%

%3 ::i

WKY = Wistar-Kyoto; SHR = spontaneously hypertensive rat; SHRSP = stroke-prone spontaneously hypertensive rat; UNT = untreated; PERIN = perindopril treatment; MAP = mean arterial pressure; HR = heart rate; LV/BW = left ventricule mass-to-body weight ratio: BP,, = minimum blood pressure after ganglionic blockade and dilator; BP,, = maximum blood pressure after ganglionic blockade and constrictor; SEM = average standard error of the mean, calculated from the analysis of variance and based on the harmonic n = 6.2 animals per group. *Significant difference between treated and untreated animals at p ~0.05

TABLE II Effect of Perindopril Treatment from 14-20 Weeks of Age on 22 Week WKY, SHR and SHR-SP WKY

SHR

SHR-SP

UNT

PERIN

UNT

PENN

UNT

PERIN

SEM

MAP(mmHgl HR(beats/min) LV/BW(mg/g)

286 115 2.20

2:: 2.09

161 340

121* 302*

309 173 3.09

148* 307

ii ok33

BP,, (mmHA BP,,,lmmHd

1;;

1:;*

2:;

1831:

2:;

2:*

::i

2.72

2.25*

2.45*

See Table I for abbreviations. *Significant difference between treated and untreated animals at p CO.05.

including those estimated in eachanimal from the baroreflex and dose-responsecurves, was by oneway analysis of variance where the betweencolumn (group) sums of squares (SS) were subtracted from the total SS to give the residual SS. The latter was usedto calculatethe averagetreatment SEM (indicating variation between animals). Comparisonswere madeby a set of orthogonalpartitionings of the between-column SS, which extracted factors of strain and treatment (perindopril) from the between-columnSS. RESULTS Effect of Perindopril Treatment on Blood Pressure and Cardiovascular Hypettrophy.

BP of untreated SHR-SP was higher than that of SHR, which in turn was higher than that of untreated WKY at both 14 and 22 weeks of age (Tables I and II). Perindopr&, prevented the hypertension of SHR and SHR-SP (p ~0.05). Perindoprill~-aOreducedblood pressure of SHR closeto that of untreated WKY (+5%, p