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(Hypertension. 1995;25:124-131.)
© 1995 American Heart Association, Inc.

Articles

Hypertrophy and Hyperplasia of Smooth Muscle Cells of Small Intramyocardial Arteries in Spontaneously Hypertensive Rats

Kerstin Amann; Hassan Gharehbaghi; Sybille Stephan; Gerhard Mall

From the Department of Pathology, University of Heidelberg (K.A., H.G.), and the Department of Pathology, Städtische Kliniken Darmstadt (S.S., G.M.) (Germany).

Correspondence to Prof Dr Gerhard Mall, Pathologisches Institut der Städtischen, Kliniken Darmstadt Grafenstr 9, D-64283 Darmstadt, FRG.

	Abstract

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Abstract Hearts of stroke-prone spontaneously hypertensive rats (SHR) were investigated by means of stereology and were compared with those of normotensive Wistar-Kyoto controls. At the age of 9 months, hypertensive rats showed cardiac hypertrophy, marked myocardial fibrosis, activation of nonvascular interstitium, focal myocytial degeneration, reduction of capillarization, and microarteriopathy of small intramyocardial arteries. Stereologically, a significant increase in the total left ventricular arterial wall volume (+180% versus controls) was found in SHR hearts. By using new stereological techniques, the orientator and the nucleator, we investigated whether this significant increase in total left ventricular arterial wall volume was due to hyperplasia of smooth muscle cells in addition to the process of vascular smooth muscle cell hypertrophy that is common in SHR. Additionally, the nuclear size and ratio of cell volume to nuclear volume were determined using another new stereological technique, the selector. The stereological data indicate a significant increase in mean cell and nuclear volumes as well as in the total number of left ventricular arterial smooth muscle cells of SHR. Additionally, the total length of intramyocardial arteries was also significantly increased in hypertensive rats. The volume and number of arterial smooth muscle cells per arterial length were significantly (P<.001 and P<.05, respectively) higher in SHR than in normotensive controls. Thus, we conclude that hypertrophy and hyperplasia of smooth muscle cells are involved in intramyocardial arterial growth processes in hypertensive heart remodeling. Hyperplasia may be responsible for the realization of physiological growth processes, eg, length increase in the intramyocardial arterial tree, whereas hypertrophy predominantly causes the remarkable increase in the total wall volume of left ventricular intramyocardial arteries in stroke-prone SHR.

Key Words: hypertrophy • hyperplasia • muscle, smooth • arteries • rats, inbred SHR

	Introduction

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Myocardial growth processes are well-known consequences of arterial hypertension.¹ Whereas myocytes develop cellular hypertrophy and fibroblasts induce interstitial myocardial fibrosis, the capillaries show inadequate growth compared with growth of myocytes, thus leading to a decrease in myocardial capillary supply. Additionally, structural alterations of small intramyocardial arteries are considered to be an important factor leading to disturbances of myocardial microcirculation in hypertensive patients² and laboratory animals.³ Thickening of the arterial wall and changes in the wall-lumen ratio were the overall findings in quantitative and qualitative morphology.^{4 5 6} Intramyocardial arterial growth processes in hypertension, however, have not yet been investigated on the cellular level because unbiased stereological methods have not been available. Up to this time, it has not been known whether the thickening of the intramyocardial arterial tree is related to hyperplasia, to hypertrophy of vascular smooth muscle cells (SMCs), or to a combination of both. In the aorta of spontaneously hypertensive rats (SHR) and of rats with renovascular hypertension, Owens and Schwartz^{7 8} found evidence for hypertrophy of SMCs that was associated with hyperploidy of nuclei, whereas Mulvany et al,⁹ using the unbiased dissector method, found hyperplasia of SMCs in the mesenterial arteries of prehypertensive SHR. This finding was later confirmed by Inoue et al.¹⁰ Therefore, it is tempting to speculate that the reaction pattern of the arterial wall in hypertension depends on both the experimental model and the type of artery examined.^{11 12 13 14 15 16}

In the present investigation, we compared growth parameters of small intramyocardial arteries of stroke-prone SHR (SHRSP) with those of normotensive Wistar-Kyoto (WKY) controls. By means of extrapolation of measurements on two-dimensional histological sections into the three-dimensional space, we determined the arterial length, arterial wall volume, and, for the first time, mean volume and number of intramyocardial arterial SMCs as well as the mean volume of cell nuclei. We used recently developed stereological techniques (the orientator, the nucleator, and the selector) for the unbiased determination of mean cell and nuclear volumes as well as the number of cells from histological sections.

	Methods

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Nine male SHRSP and six normotensive WKY rats (Department of Pharmacology, University of Heidelberg and Ivanovas Co) were studied at the age of 9 months. The animals were caged individually, received tap water ad libitum, and were fed a standard diet (Altromin C1000, Lage/Lippe).

For quantitative morphological investigations, the viscera were fixed by retrograde vascular perfusion via the abdominal aorta with rats under 10% chloralhydrate anesthesia at a pressure of 110 mm Hg as described elsewhere.¹⁷ After perfusion, the hearts were carefully excised, and right and left ventricle weights were determined with a precision balance. From the left ventricles, including the intraventricular septa, probes with independent uniform random orientation in space were prepared according to the orientator technique.¹⁸ Briefly, this technique generates isotropic orientation of histological sections in space (IUR) in a random systematic manner that increases analysis efficiency by reducing variance. In practice, the hearts were cut into 1-mm-thick transversal sections beginning at the apex using a razor blade. The slices were then put onto a grid with 100 grid points using random numbers. Two slices were selected for further preparation. The specimens were temporarily embedded in a circular cylinder consisting of agar (Merck Co). Two sets of four sections were generated by systematic cutting of the agar cylinder with a razor blade according to the orientator technique. Thus, thin tissue slices were produced and embedded in Epon-Araldite (Serva Co). Semithin sections (1-µm) were prepared and stained with methylene blue and basic fuchsin.¹⁷ In addition, the remaining parallel slices of the left ventricles were embedded in paraffin (Cambridge Instrument Co) and stained with silver and nuclear fast red as described elsewhere.¹⁹

Stereological Technique
Various definitions of arteries are in use today. Some investigators have classified arteries, arterioles, and capillaries according to their diameters.^{20 21} However, diameters depend on the arterial size as well as on the contractile state of vascular SMCs. Therefore, we preferred a qualitative definition of arteries for our study. Small intramyocardial arteries were identified according to Wiest et al¹⁹ as vessels having a complete layer of SMCs separated from the endothelium by a continuous basement membrane, ie, without any myoendothelial contact.²² A subclassification into arterioles, terminal arterioles, and precapillary sphincters was not made.

Total Length of Intramyocardial Arteries
Length density, L_V (length of one-dimensional structures per unit reference volume), was derived from counts on paraffin sections according to L_V=CxQ_A, with Q_A being the number of arterial sectional transects per unit sectional area and C the coefficient of correction. C was empirically determined as described elsewhere.¹⁹ Q_A of arteries was counted as a final magnification of 160:1 using a Zeiss eyepiece with an integrated square area of 0.468 mm² ("true" area corrected for tissue shrinkage after embedding in paraffin). Squares were superimposed on all histological sections (mean total sectional area, 500 mm²). An average of at least 500 arteries per animal were counted. The total length (L_art) of the arterial tree was determined as L_art (mm)=L_V (mm/mm³)xV (mm³). The left ventricular volume V (mm³) was derived from the left ventricular weight (LVW) divided by the specific weight of perfusion-fixed hearts (1.04 mg/mm³) according to V (mm³)=LVW/1.04 mg/mm³.

Arterial Wall Volume, Wall Thickness, and Wall-Lumen Ratio of Small Intramyocardial Arteries
For determination of the arterial wall volume (v_wall), eight orientator semithin sections per animal were analyzed at a final magnification of 400:1 using the semiautomatic image-analysis system Videoplan (Kontron Co). The ratio of sectional area of arterial walls per reference sectional area of the myocardium (A_A) was determined.

Since A_A (area per reference area)=V_V (volume per reference volume), the total volume of the left ventricular arterial wall (v_wall) could be derived from the total left ventricular volume (V).

Wall thickness and minimal lumen diameter of small intramyocardial arteries were measured on eight semithin orientator sections per animal at a final magnification of 1000:1 using oil immersion and a Videoplan computer. Wall thickness was calculated as the mean of two opposite measurements in the direction of the minimal diameter. This direction was chosen because the minimal lumen diameter is less affected by the sectioning angle than any other direction. Wall thickness is also minimal in the direction of the minimal lumen diameter; thus, planimetric measurements at this site are less affected by cutting artifacts. The wall-lumen ratio was calculated from wall thickness divided by the minimal lumen diameter.

Structure of the Arterial Wall (V_Vcells=v_cells/v_wall)
For determination of the volume fraction of cellular (V_Vcells) and extracellular matrix of SMCs in the wall of intramyocardial arteries, plastic sections were quantitatively investigated by point counting using a 100-point grid (Zeiss Co). The percentage of the extracellular volume fraction was determined on 0.5-µm semithin sections stained with methylene blue and basic fuchsin. Additionally, qualitative ultrastructural investigations were performed at various magnifications using a Zeiss EM 10 on several ultrathin sections per animal.

Mean Cell Volume and Mean Nuclear Volume of Arterial SMCs
The nucleator method^{23 24} demands that IUR sections be used for determination of the mean volume of arterial SMCs (v_cell). This allows an unbiased estimation of the mean volume from a three-dimensional random sample of nuclei that was gained by a series of eight equally distanced semithin histological sections. For practical reasons, a set of eight parallel section planes at an interval of approximately 1 µm was gained from one orientator tissue probe. The investigations were performed on eight orientator probes per animal with every orientator section cut into eight serial sections, resulting in a total of 64 sections per animal. The sections were analyzed at a final magnification of 1000:1 using oil immersion and a Videoplan computer.

For preparation of the set of eight parallel sections, the following criteria must be fulfilled: (1) The distance of two serial sections has to be big enough to gain a visible change in organ structure but must also be small enough to compare the serial structures with each other. (2) This implies for the present study that the distance between two serial sections has to be smaller than the smallest nuclear diameter in any direction to avoid loss of small nuclei.

For practical measurements, one has to choose an isotropic uniform random section plane, the so-called lookup plane. Afterwards, a parallel section plane, the so-called measure or reference plane (ie, a second histological section at a certain distance from the first one), was chosen on which the measurements were performed. Comparison of the serial sections was performed by using a set of two microscopes and a video camera for permanent visualization of the lookup plane and measurements on the reference plane.

A cell profile is sampled on the measure plane if its nucleus is visible, provided that it is not detectable on the lookup plane. Then, a grid of 100 test points (Zeiss Co) is projected onto the section, and whenever a sampled nucleus is hit by a test point, the length of a random cell intercept through this point is determined. The lengths between the cell boundaries and the grid point are determined as l+ and l-. These parameters were measured in every sampled cell, and the procedure was repeated three times per cell (with 30°, 90°, and 150° direction) giving l_1⁺, l_1^-, l_2⁺, l_2^-, l_3⁺, and l_3^- (see Fig 1).

View larger version (44K): [in this window] [in a new window]   Figure 1. Diagram shows nucleator technique. Sampling of cell nuclei and measurements of cell intercepts (l+, l-) within three directions (30°, 90°, and 150°). Note that nuclear intercepts are measured in the same way.

The mean intercepts for each direction l₁, l₂, and l₃ were determined as

The mean intercept length in the third power per cell (l_o³) was then determined by the equation l_o³=(l_1³+l_2³+l_3³). Afterwards, the cell volume (v_o) and the estimated mean cell volume in the numerical distribution (v_N) were determined according to the following equations

with n the number of sampled cells.

The mean nuclear volume (v_nucleus) was estimated by using the selector method.^{25 26} This method is appropriate for determination of the mean volume of particles without subcomponents. After three-dimensional sampling of nuclei as described above, the volume of each particle was estimated from point-sampled intercepts by using the same optical system as that for the nucleator. Again, three directions (30°, 90°, and 150°) were chosen, leading to the nuclear intercepts l₁, l₂, and l₃. The mean nuclear intercept (l_o) in the third power was determined according to the equation l_o³=l_1³+l_2³+l_3³. The mean nuclear volume (v_o) was derived from v_o=/3xl_o³. For n measurements, v_N was determined as v_N=/3(l_01³+l_02³+. . .+l_0n³)/n.

Number of Arterial SMCs
In addition to the volume of arterial SMCs, the number of SMCs (n_cell) of the arterial walls was derived from the total volume of nonendothelial wall cells (V_wall) and the mean volume of SMCs (v_N) following the equation n_cell=V_wall/v_N.

Volume of Intramyocardial Fibrosis and Volume Density of Intramyocardial Nonvascular Interstitium
The volume of intramyocardial fibrosis [V(fib)] was determined using Sirius red–stained 4-µm paraffin sections of the heart. The area fraction (A_A) of red-stained collagen was determined using an automatic image-analysis system (IBAS II, Kontron Co) on the basis of gray-value discrimination (for details, see Reference 5⁵ ).

Since A_A(fib)=V_V(fib), the total volume of left ventricular intramyocardial fibrosis [V(fib)] could be derived from the total left ventricular volumexV_V(fib).

Volume density of intramyocardial nonvascular interstitium was determined on eight semithin sections per animal by point counting using a 100-point test grid (Zeiss) according to P_P=V_V (for details, see Reference 5⁵ ).

Statistics
The arithmetic means of the absolute stereological parameters were compared using Student's t test.²⁷ Data are expressed as mean±SD and mean±SEM (Table 2).

View this table: [in this window] [in a new window]   Table 2. Vascular Stereology

	Results

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Morphology of the Rat Heart
In SHRSP the degree of left ventricular hypertrophy, which was assessed by the relative left ventricular weight (ratio of left ventricular weight to body weight), amounted to +90%. Despite the higher body weight in WKY rats, the heart weight (+10%) and the absolute (+14%) and relative (+90%) left ventricular weights were significantly (P<.05) higher in SHRSP (Table 1). Interstitial myocardial fibrosis had developed in SHRSP [V(fib): 32±8 versus 17±4 mm³ in WKY rats], and myocardial scarring was occasionally detected. Stereologically, the wall thickness, wall-lumen ratio, total wall volume and length of left ventricular arteries, ratio of arterial wall volume to arterial length, and volume density of fibrosis and of nonvascular myocardial interstitium were significantly higher in 9-month-old SHRSP than in WKY rats (for details, see Reference 5⁵ ). The ultrastructure of myocytes was grossly normal in SHRSP, except for a focal degeneration of myocytes with atrophy and loss of myofibrils.

View this table: [in this window] [in a new window]   Table 1. Animal Data

In our study, 49 to 287 small intramyocardial arteries per animal (WKY, 97.2±49.4; SHRSP, 117.5±85.8) were investigated with the nucleator method. The lumen diameters ranged from 20 to 140 µm, with most ranging from 20 to 60 µm. The mean lumen diameter of intramyocardial arteries was comparable in both groups (34.6±1.50 and 33.4±1.47 µm, respectively), whereas wall thickness and the wall-lumen ratio were significantly (P<.01 and P<.001) increased in SHRSP. Concomitantly, total left ventricular arterial wall volume was considerably higher in SHRSP (+237% versus WKY, P<.001, Table 2 and Figs 2 and 3). The total length of intramyocardial arteries was slightly higher in SHRSP (+18%, P<.05). Thus, length increase of arteries corresponded to the increase in absolute left ventricular weight.

View larger version (158K): [in this window] [in a new window]   Figure 2. Photomicrographs show small intramyocardial artery of a normotensive rat (Wistar-Kyoto) with thin vessel wall (A) and small intramyocardial artery of a stroke-prone spontaneously hypertensive rat (B). A, Semithin section, original magnification x1100. B, Note increased arterial wall thickness with hypertrophy and hyperplasia of smooth muscle cells. Semithin section, original magnification x1360.

View larger version (165K): [in this window] [in a new window]   Figure 3. A, Photomicrograph shows ultrastructure of the intramyocardial arterial wall of a stroke-prone spontaneously hypertensive rat (SHRSP). Note small endothelial layer and thickened media with hypertrophied smooth muscle cells containing increased number of mitochondria and actin filaments. Ultrathin section, original magnification x11 500. B, Photomicrograph shows ultrastructure of the wall of a small intramyocardial artery of an SHRSP. The media is composed of hypertrophied smooth muscle cells that are closely connected. Note enlarged cell nuclei and increased cytoplasm. Ultrathin section, original magnification x16 000.

Fig 4 shows the distribution of intramyocardial arterial wall thickness and corresponding lumen diameters. This frequency histogram indicates that vessels with a smaller wall thickness are overrepresented in WKY rats, whereas in SHRSP, vessels with a larger wall thickness are more frequent. The figure further shows a smaller arterial lumen for any given wall thickness in SHRSP compared with WKY rats. The arterial wall thickening was caused by hypertrophy of vascular SMCs because the mean cell volumes of SMCs of arterial walls were nearly doubled in SHRSP (+81%, P<.01) compared with WKY rats. Significant differences between the two groups were also found for nuclear volume, whereas the ratio of nuclear volume to cellular volume was not significantly changed. This indicates that hypertrophy is caused by an increase in cell cytoplasm as well as in nuclear volume. The number of SMCs in arterial walls (+104%, P<.05) and the number of arterial SMCs per millimeter of arterial length were significantly increased in SHRSP (+29%, P<.05). As shown in Fig 3, the media of small intramyocardial arteries is largely composed of SMCs (>95%) that are closely connected. Therefore, the percentage of extracellular matrix is only approximately 4% (SHRSP, 6.5±1.8%; WKY, 2.3±1.2%) of the total wall volume as detected by our measurements. Consequently, for our stereological measurements, we regarded the arterial wall as completely composed of SMCs and divided the total arterial wall volume by the mean cell volume of SMCs to get the mean cell number.

View larger version (21K): [in this window] [in a new window]   Figure 4. Bar graph shows distribution of wall thickness (bars) and lumen diameters (lines) of small intramyocardial arteries. The lines connect the means of the corresponding lumen diameters for a given class of wall thickness.

	Discussion

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This study presents the first complete quantitative characterization of intramyocardial arteries in experimental genetic hypertension. Using new stereological techniques (the orientator, the nucleator, and the selector), we established pathological alterations of intramyocardial arteries in SHRSP that are based on three-dimensional parameters independent of the degree of vasodilation. Arteriopathy is frequently indicated by the wall-lumen ratio of arteries, which is affected by both thickening of the vessel wall and the state of contraction. In addition to an increase in wall thickness and wall-lumen ratio of small intramyocardial arteries in SHRSP, we found an increase in left ventricular arterial wall volume (+237%) and arterial wall volume per unit length of arteries (+180%). These latter findings exclude arterial contraction or remodeling as a possible cause of wall thickening. The increase in total left ventricular arterial wall volume results from hypertrophy (+81%) and hyperplasia (+104%) of vascular SMCs.

Myocardial growth processes in hypertension may be adaptive to myocardial hypertrophy, ie, enlargement of myofibers, or may result from pressure increase itself or from hypertension-associated factors, such as hormones. Therefore, we recently studied the length increase of the intramyocardial arterial tree during physiological growth¹⁹ and found similar growth rates in WKY rats and SHRSP, indicating adaptive lengthening in SHRSP in good correlation with the increase in absolute left ventricular weight. Evidently, arterial length increase is caused by hyperplasia of arterial SMCs and not by hypertrophy. On the other hand, the number of arterial SMCs per unit arterial length was significantly increased in SHRSP and contributed to wall thickening.

The problem of vascular SMC hypertrophy versus hyperplasia in hypertension has been addressed so far by several investigators in many different ways, leading to controversial results. The model of hypertension, the animal model, the type and topography of arteries in the study, and the methodological approach therefore were critical parameters. For example, in slow-developing models of hypertension (SHR; two-kidney, one clip hypertension), the major growth response in large arteries (ie, the aorta) was found to be hypertrophy of SMCs rather than hyperplasia.^{7 8 13 28 29 30} The results are somewhat different if models of acute pressure overload (abdominal aortic stenosis) are investigated, in which predominantly hyperplasia was found.^{31 32} Therefore, it is tempting to speculate that the sort and degree of wall stress applied could be responsible for different types of cellular reactions.

However, major differences may exist between the SHR and two-kidney, one clip model, especially as far as the onset and extent of hypertension are concerned. On the other hand, the well-known biological variability in SHR and WKY rats may help to explain the controversial experimental findings.³³ In our study, we concentrated on long-standing hypertension in 9-month-old SHRSP compared with normotensive WKY controls. Although we know that there are important differences in the genetics of both rat strains and that the use of WKY rats as a control for SHR has been criticized,^{34 35 36 37} we decided to introduce WKY rats as a normotensive control group. We are aware of the fact that the comparison of these two strains may be somewhat problematic, so we would like to emphasize that the data obtained in our study should be referred only to the strains we used.

Several vascular territories in hypertension have been investigated using various techniques and different animal models. Mulvany et al⁹ found wall thickening due to hyperplasia but not to hypertrophy of SMCs in third-order mesenteric arteries of prehypertensive SHR, thus indicating a difference between WKY rats and prehypertensive SHR that is not related to hypertension itself. This finding was supported by Lee et al,³⁸ who demonstrated hyperplasia in mesenteric arteries of SHR. Additionally, Owens et al³⁹ found hyperplasia in small mesenteric arteries of SHR compared with WKY rats. In larger arteries, however, Owens and Schwartz⁷ found no hyperplasia but hypertrophy of vascular SMCs in hypertension.

Evidently, true three-dimensional estimators of absolute numbers and volumes are superior to other approaches. In 1985 Mulvany et al⁹ introduced a stereological method (the dissector) that allowed the unbiased three-dimensional estimation of the number of nuclei in arterial walls. Compared with the dissector, which can only give nuclei number and volume, our method, the nucleator, offers a clear advantage: possible estimation of true cell volume and number. Additionally, in contrast to the dissector method, our method does not demand knowledge of the exact section thickness, facilitating the analysis.

Two-dimensional methods for the investigation of cellular reactions (counting of the number of SMCs on serial sections) were established by Loud et al⁴⁰ but failed to contribute to solving the problem of hypertrophy and hyperplasia per se. Olivetti et al^{28 29} performed a morphometric analysis in the aorta of postnatal SHR and WKY rats and found hypertrophy and hyperplasia in young SHR. As in our study, hypertrophy (+68%) was found to be the predominant process underlying the aortic tissue response to early hypertension. Cellular hyperplasia (+21%) was found to occur later as some sort of consolidation of the adaptive process. Additionally, Owens and Schwartz^{7 8} as well as Owens and Reidy³¹ performed DNA measurements combined with direct measurements of cell number to further address the problem of hypertrophy and hyperplasia in vascular remodeling and found hyperplasia to be involved in hypertension.

Considering the finding of hyperplasia in mesenteric arteries of prehypertensive SHR, one may speculate that the higher level of arterial smooth muscle in SHR cells is genetically determined and not a consequence of hypertension.⁹ On the other hand, cellular hypertrophy in established hypertension was demonstrated to be related to or at least augmented by an increase in wall tension.²⁸ Some studies investigated the influence of antihypertensive treatment in renovascular and genetic hypertension^{14 15} and did not find any difference in cell number but a decrease in cell volume after treatment with angiotensin-converting enzyme inhibitors. These findings also support the hypothesis that hypertrophy rather than hyperplasia is the direct consequence of chronic pressure overload.

In contrast, several studies clearly showed vascular hypertrophy in prehypertensive SHR,^{38 41 42} thus documenting the fact that vascular SMC hypertrophy could be clearly separated from pressure changes. It is speculated that other factors, such as hormones, an increased responsiveness to growth factors (ie, epidermal growth factor⁴³ ), and an increased noradrenergic activity in SHR,⁴⁴ may play a role in pressure-independent vascular changes in SHR. In addition, Head⁴⁵ and Donohue et al⁴⁶ postulated a possible role for trophic factors (ie, nerve growth factor) in young SHR providing the setting for the development of hypertension. Morton et al⁴⁷ suggested a trophic influence of the renin-angiotensin system on the development of structural changes in SHR during the early onset of experimental hypertension. In contrast, cardiac hypertrophy in genetic hypertension (SHR) was found to be closely related to an increase in blood pressure.^{44 48} It is therefore tempting to speculate that structural alterations of small intramyocardial arteries in SHRSP may be influenced by both structural and hemodynamic factors.

Being the first three-dimensional analysis of intramyocardial arteries in SHRSP, our study has several limitations. As mentioned above, differences in the mean pressure in intramyocardial arteries, the thoracic aorta, and the mesenteric resistance vessels may induce different cellular reactions. The mean volumes of SMCs of intramyocardial arteries in this study are lower than the volumes reported for larger arteries.³⁰ This may be related to differences in the animal model as well as to differences in vascular territories, but effects of different methods cannot be formally excluded. The investigations performed so far indicate that both aspects must be taken into consideration.

In summary, left ventricular myocardial hypertrophy in SHRSP is associated with an adaptive compensatory length increase of intramyocardial arteries (+18%) that is caused by hyperplasia of arterial SMCs. In contrast, intramyocardial arterial wall thickening in SHRSP is the result of hypertrophy (+80%) and hyperplasia (+104%). Further stereological studies should be performed on intramyocardial arteries in prehypertensive SHRSP to answer the question of whether cellular hyperplasia is inborne or pressure induced.

	Acknowledgments

 
The skillful technical assistance of Zlata Antoni, Gudrun Gorsberg, Birgit Hilbert, Peter Rieger, Jutta Scheuerer, and Winfried Nottmeyer is gratefully acknowledged. The authors thank Harald Derks for excellent phototechnical support and Cornelia Nichols for carefully reviewing the manuscript.

Received March 25, 1994; first decision May 9, 1994; accepted September 8, 1994.

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