3D imaging and morphometry of the heart capillary system in spontaneously hypertensive rats and normotensive controls.

Systemic arterial hypertension is a highly prevalent chronic disease associated with hypertensive cardiomyopathy. One important feature of this condition is remodelling of intramural small coronary arteries and arterioles. Here, we investigated the implications of this remodelling in the downstream vascular organization, in particular at the capillary level. We used Spontaneously Hypertensive Rats (SHR) exhibiting many features of the human hypertensive cardiomyopathy. We generated 3D high-resolution mesoscopic reconstructions of the entire network of SHR hearts combining gel-based fluorescent labelling of coronaries with a CLARITY-based tissue clearing protocol. We performed morphometric quantification of the capillary network over time to assess capillary diameter, linear density, and angular dispersion. In SHRs, we found significant remodelling of the capillary network density and dispersion. SHR capillary density is increased in both ventricles and at all ages, including before the onset of systemic hypertension. This result suggests that remodelling occurs independently from the onset of systemic hypertension and left ventricular hypertrophy. On the contrary, capillary angular dispersion increases with time in SHR. Consistently, our multicolor imaging underlined a strong correlation between vascular dispersion and cellular disarray. Together our data show that 3D high-resolution reconstruction of the capillary network can unveil anatomic signatures in both physiological and pathological cardiac conditions, thus offering a reliable method for integrated quantitative analyses.

Differences in striatal spiny neuron action potentials between the spontaneously hypertensive and Wistar-Kyoto rat strains.

The spontaneously hypertensive rat (SHR) and the Wistar-Kyoto (WKY) inbred rat strains display behavioral differences characterized by relative increases and decreases in levels of activity. Both strains have subsequently been utilized as animal models of hyperactive and hypoactive behavioral traits. The etiology of these behavioral characteristics is poorly understood, but may stem from alterations in the physiology of selected neural circuits or catecholamine systems. This study investigated the cellular properties of neurons from three genetically related strains: the SHR; WKY; and Wistar (WI). In vivo intracellular recordings were made under urethane anesthesia from spiny projection neurons in the striatum, a brain area involved in behavioral activation. Results obtained from 71 spiny projection neurons indicate that most cellular properties of these neurons were very similar across the three strains. However, the amplitude and half-duration of both spontaneously occurring and current-evoked action potentials were found to be significantly different between the SHR and WKY strains with neurons from the SHR firing action potentials of relatively greater amplitude and shorter duration. Action potential parameters measured from the WI rats were intermediate between the two other strains. These differences in action potentials between two behaviorally distinct strains may reflect altered functioning of particular membrane conductances.

Suppressed expression of cystathionine ß-synthase and smaller cerebellum in Wistar Kyoto rats.

We previously reported that Wistar Kyoto rats, an animal model of depression, have a characteristically abnormal serine metabolism in the brain, i.e., lower serine and cystathionine, which is a metabolite of serine, concentrations in the brain. To explore the mechanism underlying this abnormality, the expression of cystathionine ß-synthase and serine racemase, which are the enzymes involved in the serine metabolism, was investigated in the cerebellum and hippocampus of Wistar and Wistar Kyoto rats. Wistar Kyoto rats exhibited a significantly lower mRNA expression of cystathionine ß-synthase in the cerebellum in comparison with Wistar rats, while expression levels in the hippocampus did not differ between strains. Previous study indicated that the reduction of cystathionine ß-synthase in the brain induced cerebellar aplasia in mice. Therefore, the cerebellar size was compared between Wistar rats and Wistar Kyoto rats. Wistar Kyoto rats displayed a lower ratio of cerebellum weight to whole-brain weight compared with Wistar rats of the same generation or similar body weight, suggesting that Wistar Kyoto rats exhibit smaller cerebellum. These results suggest that the lower mRNA expression of cystathionine ß-synthase in the cerebellum and the smaller size of cerebellum may be related to the depression-like behavior in Wistar Kyoto rats.

Hepatic arteries in normotensive and spontaneously hypertensive rats. A morphometric study.

The media thickness (m), luminal radius (r) and m/r ratio were determined in the hepatic arterial trunk and in intra-hepatic arterial branches as was the number of arteries per cm2 sectioned liver tissue in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto controls (WKY). The cross-sectional vessel parameters were calculated for a standardized condition, in which the internal elastic membrane is smooth and circular. Both intra-hepatic arterial branches and the hepatic arterial trunk showed significantly higher m/r ratios in SHR than in WKY controls. The luminal radius of the hepatic arterial trunk was larger in SHR than in WKY (P less than 0.05). The number of arteries per cm2 sectioned liver tissue was greater in SHR (P less than 0.05). It is suggested that the consequences of the increased m/r ratio in hepatic arteries of SHR are counteracted to some extent by an increased vascularization, but that during hypovolaemia and compensatory vasoconstriction, a greater decrease in hepatic arterial blood flow occurs in SHR than in WKY.

Evidence for differences in cultured left ventricular fibroblast populations isolated from spontaneously hypertensive and Wistar-Kyoto rats.

OBJECTIVE: To compare fibroblast populations derived from spontaneously hypertensive rats (SHRLJ) and normotensive Wistar-Kyoto rats (WKYLJ) for angiotensin II receptor binding, gene expression of the AT1 receptor and angiotensinogen, hormone responsiveness and phenotypic changes. METHODS: Fibroblasts were isolated by either collagenase B or collagenase P and grown to confluency in the presence of 10% fetal bovine serum. Angiotensin II receptor binding was assessed under both serum and serum-free conditions. Hormonal treatment of cells was conducted in a serum-free background. The concentrations of AT1 receptor and angiotensinogen messenger RNA (mRNA) were determined by liquid hybridization. Phenotypic changes in fibroblast populations were analysed by visualization of lipid-containing vacuoles (oil red O stain) or of alpha-smooth muscle actin-containing fibres (immunostain). RESULTS: SHRLJ collagenase-B cells grew more slowly and had nearly twofold fewer angiotensin II receptors than WKYLJ cells as measured by both radioligand binding and AT1 mRNA content (SHRLJ 1.34 +/- 0.05 versus WKYLJ 5.94 +/- 0.41 pg mRNA per microgram total RNA) but contained significantly more angiotensinogen mRNA (SHRLJ 147 +/- 12 versus WKYLJ 98 +/- 8 fg mRNA per microgram total RNA). Collagenase-P cells from the two strains exhibited similar binding and growth properties. Collagenase-B fibroblasts also exhibited greater responses to exogenous steroids, including a greater shift towards an adipocyte phenotype, than collagenase-P cells. Exogenous angiotensin II promoted transformation towards a myofibroblast cell type, especially in collagenase-P SHRLJ cells. CONCLUSION: Our results indicate that subsets of fibroblasts that differ in growth rate, angiotensin II receptor binding, AT1 and angiotensinogen mRNA levels, structure and steroid responsiveness may be isolated from the left ventricle. The potential importance of these altered phenotypes to cardiac remodelling and hypertrophy warrants further examination.

Deficits in spatial learning and nicotinic-acetylcholine receptors in older, spontaneously hypertensive rats.

Spontaneously hypertensive rats are often used as models of attention deficit hyperactivity disorder and to investigate the effects of hypertension on cognitive function. Along with the wide variety of cardiovascular anomalies, these animals as young adults also exhibit deficits in memory and attention and central nicotinic-acetylcholine receptor sites. These findings may have particular significance since nicotinic receptors appear to be involved in the regulation of cerebral circulation and mnemonic function. Furthermore, a lack of high affinity nicotinic receptors (in knockout mice) has also been shown to accelerate both the structural and cognitive degeneration associated with age, findings that may be especially relevant to age-related memory disorders such as Alzheimer's Disease where large deficits in nicotinic receptors are observed. Since spontaneously hypertensive rats appear to be both memory-impaired and deficient in nicotinic receptors at a young age (compared to the non-hypertensive phenotype, Wistar-Kyoto rats), we were interested to learn if these conditions were exacerbated in older animals with particular interest in specific nicotinic receptor subtypes in memory areas of the brain. Spatial learning was assessed in 15-month-old subjects of each phenotype (i.e. hypertensive and non-hypertensive) using a two-phase water maze paradigm, and nicotinic receptors were measured via autoradiography with [125I]-alpha-bungarotoxin and [3H]-epibatidine. In the water maze, both groups learned to locate a hidden platform as indicated by progressively shorter latencies across training days, however, Wistar-Kyoto rats were more efficient in both phases. While the number of both bungarotoxin and epibatidine binding sites was lower in the hypertensive rats across several brain regions, in the case of epibatidine binding, the magnitude of the difference and the number of areas affected was generally greater and included areas important for spatial learning (e.g. frontal and entorhinal cortex). In a direct comparison between 3-month-old and 15-month-old rats of each phenotype, epibatidine sites were markedly reduced by age (i.e. by greater than 50% in some cases) across multiple brain regions in both groups, although Wistar-Kyoto rats appeared to be more substantially affected by age. These data further support the use of the spontaneously hypertensive rat as model for studying learning-impairment and reduced central nicotinic receptors and also indicate that these characteristics persist and (in the case of high affinity nicotinic receptor cites) worsen with age.

Parenchymal microvascular systems and cerebral atrophy in spontaneously hypertensive rats.

Spontaneously hypertensive rats (SHR) are hypertensive, hyperactive, and hydrocephalic; furthermore SHR have smaller brain volume and weight than age-matched, normotensive Wistar-Kyoto rats (WKY). At 6-7 months of age, local cerebral glucose is sizably lower in SHR than WKY. The hypothesis that these several abnormalities of SHR lead to variations in cerebral microvascular bed morphology was tested in 6-7-month-old SHR and WKY by quantitating various parameters of small, intermediate, and large parenchymal microvessels (grouped by luminal diameter) in 21 brain areas. Within each rat strain, the microvascular bed properties such as vessel profile frequency (density) varied considerably among the 21 brain areas. In opposition to the hypothesis, mean luminal diameter as well as profile frequency, surface area, and luminal volume of the microvascular beds per unit tissue mass were virtually identical in each brain area of SHR and WKY for the three groups of microvessels. These findings coupled with the reports of less tissue per structure but similar density of neurons throughout the brain of SHR and WKY indicate that there are fewer neurons and less vascular tissue per brain structure in 6-7-month-old SHR than WKY; in addition, they suggest a linkage between the size of parenchymal microvascular beds and the surrounding nervous tissue.

Glutamate-immunoreactive neurons of the central amygdaloid nucleus projecting to the subretrofacial nucleus of SHR and WKY rats: a double-labeling study.

Glutamate immunoreactivity was found in 9%, in the case of Wistar Kyoto rats (WKY), and 14%, in the case of spontaneously hypertensive rats (SHR), of neurons located in the medial division of the central amygdaloid nucleus (CeM) projecting ipsilaterally to the subretrofacial nucleus (SRF) in the rostral ventrolateral medulla using a double-labeling technique in combination with glutamate immunocytochemistry. The results indicate that possibly-glutamatergic neurons located in the CeM project to the SRF, in which vasomotor neurons are present, suggesting involvement of the CeM in blood pressure regulation. No significant difference was found between the distribution of labeled CeM neurons in SHR and WKY rats.

Spontaneous hyperplasia of the ventral lobe of the prostate in aging genetically hypertensive rats.

Recent studies have shown that the prostatic autonomic innervation takes part in its homeostasis and growth. Other works showed that spontaneously hypertensive rats (SHR) show excessive sympathetic activity, accompanied by lower urinary tract symptoms, increased growth capacity of prostatic stromal cells, and increased levels of androgens and their receptors. Furthermore, young SHR were reported to present incipient stages of benign prostatic hyperplasia (BPH). The aim of the present study was to examine whether this strain indeed develops spontaneous BPH with age, and can thus serve as a genuine natural model for this disorder. For this purpose, ventral lobes of prostates of one-year-old, male SHR and their normotensive counterparts, Wistar Kyoto (WKY) rats, were examined histopathologically, and the degree of hyperplasia was evaluated according to a score-chart protocol (histoscore). SHR exhibited severe adenomatous spontaneous BPH, characterized by piling-up of epithelial cells, with papillary formations, accompanied by a mild increase in the amount of fibrocytes and smooth muscle cells in the stroma. This was reflected by histoscore values of 38 +/-2. Thickening of prostatic arterioles also was noted, as well as mild chronic inflammatory exudate. WKY rats did not show any of these features of BPH despite their age (histoscore 17 +/- 3, significantly different from that of SHR). We conclude that SHR can serve as a rodent model for the spontaneous development of BPH with age, most probably due to the excessive neuroendocrine activity characteristic of this rat strain.

The transverse tubular system of the hypertrophic myocardium: morphology and morphometry in spontaneous hypertensive rats (SHR).

We reported previously on a modified Golgi stain that, in conjunction with high voltage electron microscope stereoscopy, gives striking views of the elaborate network of the transverse tubular system (T system) in rat myocardium. In this report we used the same techniques to study three-dimensional arrangements of the T system in the left ventricular myocardium of spontaneous hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). High voltage electron microscope stereoscopy revealed distinctive morphological characteristics of the T system, such as undulating running, short dead-end branches, and labyrinth-like tubular aggregates in the hypertrophic myocardium of SHR. Quantitative analysis of the SHR T system indicated a surface area greater than that of WKY. These findings may support the hypothesis that making an additional T system membrane will compensate for the smaller surface-to-volume ratio. However, the normal regulatory mechanism required to maintain the surface-to-volume ratio does not function properly in SHR, resulting in morphological abnormalities and functional disturbances of the myocardium.

Microcirculatory changes in cremaster muscle during early spontaneous hypertension in the rat.

The microcirculatory control of flow was studied in the cremaster muscle of 5- to 6-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) using intravital microscopic techniques. Arterioles were classified according to a functional branching order from A1-A4 vessels. Capillary architecture was measured in a separate series of experiments. No significant differences were found in the WKY or SHR vessel diameters, with the exception of third-order arterioles, which were slightly, but significantly, larger in SHR. However, the number of A3 and A4 vessels as well as the capillary density was significantly lower in SHR than in WKY. The differences were even more pronounced during maximal vasodilation induced by local application of 10(-3) mol/l adenosine. The data show that rarefaction of small blood vessels rather than decreased arteriolar diameter is an early characteristic of spontaneous hypertension in the rat.

Morphometric analysis of the phrenic nerve in male and female Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR).

Ventilatory differences between rat strains and genders have been described but the morphology of the phrenic nerve has not been investigated in spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. A descriptive and morphometric study of the phrenic nerves of male (N = 8) and female (N = 9) SHR, and male (N = 5) and female (N = 6) WKY is presented. After arterial pressure and heart rate recordings, the phrenic nerves of 20-week-old animals were prepared for epoxy resin embedding and light microscopy. Morphometric analysis performed with the aid of computer software that took into consideration the fascicle area and diameter, as well as myelinated fiber profile and Schwann cell nucleus number per area. Phrenic nerves were generally larger in males than in females on both strains but larger in WKY compared to SHR for both genders. Myelinated fiber numbers (male SHR = 228 ± 13; female SHR = 258 ± 4; male WKY = 382 ± 23; female WKY = 442 ± 11 for proximal right segments) and density (N/mm²; male SHR = 7048 ± 537; female SHR = 10355 ± 359; male WKY = 9457 ± 1437; female WKY = 14351 ± 1448) for proximal right segments) were significantly larger in females of both groups and remarkably larger in WKY than SHR for both genders. Strain and gender differences in phrenic nerve myelinated fiber number are described for the first time in this experimental model of hypertension, indicating the need for thorough functional studies of this nerve in male and female SHR.

Can Wistar rats be used as the normotensive controls for nerve morphometry investigations in spontaneously hypertensive rats (SHR)?

PURPOSE: We compared the sural nerve morphology among Wistar (WR), Wistar-Kyoto (WKY) and Spontaneously hypertensive (SHR) rats, including the nerve fascicles and myelinated fibers morphometry. METHODS: Age matched (20 weeks) female WR (N=6), WKY (N=6) and SHR (N=7) had their right and left sural nerves removed, embedded in epoxy resin, and observed by light microscopy. Morphometric analysis was performed with the aid of computer software. RESULTS: Despite presenting the same age, WR were heavier than WKY and SHR, as were SHR compared to WKY. Systolic arterial pressure was higher in SHR compared to WR, but no differences between SHR and WKY or WR and WKY were observed. The sural nerves were morphometrically symmetric between proximal and distal segments on the same side and between sides in all strains with no differences in the myelinated fiber number. Schwann cell number and density were smaller in SHR and G ratio was larger in SHR, indicating that SHR have thinner myelinated fibers. CONCLUSION: Sural nerve morphology is similar between WKY and WR, allowing the use of WR as the SHR controls in morphological investigations involving peripheral neuropathies.

Morphometric analysis of the phrenic nerve in male and female Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR)

Ventilatory differences between rat strains and genders have been described but the morphology of the phrenic nerve has not been investigated in spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. A descriptive and morphometric study of the phrenic nerves of male (N = 8) and female (N = 9) SHR, and male (N = 5) and female (N = 6) WKY is presented. After arterial pressure and heart rate recordings, the phrenic nerves of 20-week-old animals were prepared for epoxy resin embedding and light microscopy. Morphometric analysis performed with the aid of computer software that took into consideration the fascicle area and diameter, as well as myelinated fiber profile and Schwann cell nucleus number per area. Phrenic nerves were generally larger in males than in females on both strains but larger in WKY compared to SHR for both genders. Myelinated fiber numbers (male SHR = 228 ± 13; female SHR = 258 ± 4; male WKY = 382 ± 23; female WKY = 442 ± 11 for proximal right segments) and density (N/mm²; male SHR = 7048 ± 537; female SHR = 10355 ± 359; male WKY = 9457 ± 1437; female WKY = 14351 ± 1448) for proximal right segments) were significantly larger in females of both groups and remarkably larger in WKY than SHR for both genders. Strain and gender differences in phrenic nerve myelinated fiber number are described for the first time in this experimental model of hypertension, indicating the need for thorough functional studies of this nerve in male and female SHR.

Can Wistar rats be used as the normotensive controls for nerve morphometry investigations in spontaneously hypertensive rats (SHR)? / Ratos Wistar podem ser utilizados como os controles normotensos dos ratos espontaneamente hipertensos (SHR) em investigações morfométricas de nervos periféricos?

PURPOSE: We compared the sural nerve morphology among Wistar (WR), Wistar-Kyoto (WKY) and Spontaneously hypertensive (SHR) rats, including the nerve fascicles and myelinated fibers morphometry. METHODS: Age matched (20 weeks) female WR (N=6), WKY (N=6) and SHR (N=7) had their right and left sural nerves removed, embedded in epoxy resin, and observed by light microscopy. Morphometric analysis was performed with the aid of computer software. RESULTS: Despite presenting the same age, WR were heavier than WKY and SHR, as were SHR compared to WKY. Systolic arterial pressure was higher in SHR compared to WR, but no differences between SHR and WKY or WR and WKY were observed. The sural nerves were morphometrically symmetric between proximal and distal segments on the same side and between sides in all strains with no differences in the myelinated fiber number. Schwann cell number and density were smaller in SHR and G ratio was larger in SHR, indicating that SHR have thinner myelinated fibers. CONCLUSION: Sural nerve morphology is similar between WKY and WR, allowing the use of WR as the SHR controls in morphological investigations involving peripheral neuropathies.

OBJETIVO: Comparar a morfologia do nervo sural em ratos Wistar (WR), Wistar Kyoto (WKY) e espontanemanete hipertensos (SHR), incluindo a morfometria dos fascículos e fibras mielínicas. MÉTODOS: Os nervos surais direito e esquerdo de WR (N=6), WKY (N=6) e SHR (N=7), com 20 semanas de idade foram removidos e preparados para inclusão em resina epóxi e microscopia de luz. A morfometria foi realizada com o auxílio de um programa de computador. RESULTADOS: Apesar de apresentarem a mesma idade, WR são mais pesados que os WKY e SHR. Ainda, SHR são mais pesados que os WKY. A pressão arterial sistólica foi significativamente maior nos SHR comparados aos WR, sem diferença entre WKY e SHR ou WR e WKY. Os nervos surais são morfometricamente simétricos entre segmentos proximal e distal e entre lados direto e esquerdo nas três diferentes linhagens, sem diferença no número de fibras mielínicas. O número e a densidade de células de Schwann foram menores e a razão G foi maior nos SHR, indicando a presença de fibras mielínicas com bainha mais fina. CONCLUSÃO: A morfologia do nervo sural é semelhante ente WR e WKY, permitindo o uso de WR como controles dos SHR nas investigações envolvendo neuropatias periféricas.

Corrosion cast demonstration of retinal vasculature of normal Wistar-Kyoto rats.

Plastic corrosion casts of the rat retinal vasculature were studied by scanning electron microscopy. This technique demonstrated the entire retinal vasculature of the rat. The retinal blood vessels supplying the rat's retina have a definite and fairly constant pattern. At the disk there are usually six main artery and vein branches that run symmetrically towards the periphery. Veins are wider and more tortuous. The characteristic arrangement of endothelial cell nuclear indentations clearly differentiates arteries from veins. Retinal arteries have side-arm and dichotomous branchings. The number of vessel branches is greater on the nasal side than on the temporal side of the retina. The vein-over-artery crossing phenomenon is more frequent than the artery-over-vein. Retinal capillaries appear tortuous and are arranged cylindrically in two layers. The superficial network of capillaries comes essentially from arterioles, while deep layer capillary networks are more regularly and densely arranged and are mainly connected with venules. The inner and the outer capillary networks have interconnections, vertical runs and short vascular bridges. Within the retina there are regional variations in capillary pattern and distribution. More regular, dense and rich capillary networks are observed in the peripheral area than at the posterior pole area. No arteriovenous shunts were seen. The study of such plastic casts makes possible a more accurate assessment of some aspects of vascular abnormalities. These findings will be helpful in further investigations of retinal vascular abnormalities.

Wall thickness to lumen diameter ratios of arteries from SHR and WKY: comparison of pressurised and wire-mounted preparations.

Passive properties (diameter, wall-to-lumen ratio and axial length) of small mesenteric arteries from SHR and WKY rats were measured with the artery segments cannulated and pressurised, or mounted on wires in a myograph. The measurements were made with a range of distending pressures (or calculated equivalent distending pressures when wire-mounted) from 0 to 180 mm Hg. The axial length of artery segments increased with increasing distending pressure when cannulated, but not when wire-mounted. The axial extension was greater for arteries from WKY (up to 105%) than for arteries from SHR (up to 65%). The arteries from SHR had significantly smaller diameters and greater wall-to-lumen ratios than the arteries from WKY. However, the diameters calculated for the arteries when wire-mounted were less than the measured diameters, and the wall-to-lumen ratio was always greater when wire-mounted than when cannulated because of the underestimated diameter and the absence of axial extension. Wall-to-lumen ratios decreased with increased distending pressure; values at 180 mm Hg were only 18 and 25% of those at 0 mm Hg for WKY and SHR arteries, respectively. The large degree of variability of wall-to-lumen ratios obtained from the two different preparations and the large range of values that are obtained from a single artery at different distending pressures must call into question the validity of characterising vascular hypertrophy by any single estimation of this parameter.

Internal carotid and vertebral arteries of spontaneously hypertensive and normotensive rats. A morphometric study on extracranial, intraosseous and intracranial arterial segments.

Arteries from spontaneously hypertensive rats (SHR) and Wistar Kyoto controls (WKY) were investigated. Specified cross-sections were collected from the extra- and intracranial parts of the internal carotid arteries, from the carotid segment situated in the skull bone and from intra- and extracranial segments of the vertebral arteries. The internal radius, media thickness and ratio between media thickness and internal radius (m/r ratio) were calculated for a standardized condition implying a smooth and circular internal elastic membrane. The m/r ratio was significantly increased in all arterial segments of 7-month-old SHR compared to WKY. This was caused mainly by decrease of radius in the vertebral arteries and in the intracranial carotid segment. In the extracranial and intraosseous carotid segments the increase of m/r ratio in SHR was mainly due to an increase of media. Whereas the radius was the same in the intra-osseous and intracranial carotid segments of WKY, the m/r ratio was greater intracranially. The results suggest that the physical properties of supporting tissues may influence the normal arterial development and modify the influence of an increased arterial pressure on the vessel wall.

Regional differences in cardiac myocyte dimensions and number in Sprague-Dawley rats from different suppliers.

Previous experiments conducted in this laboratory have indicated that adult Sprague-Dawley rats obtained from Holtzman Company (H) have hearts that are significantly larger than those obtained from Charles River (CR). To investigate potential differences in cell size and number, isolated myocytes were prepared by retrograde coronary perfusion with collagenase. Cell volume (V) was measured with a Coulter Channelyzer, cell length (L) was measured directly, and cross-sectional area (CSA) was calculated from V/L. Cell number was calculated using data from isolated cells and whole-sectioned tissue. Although V for left and right ventricular myocytes was similar in CR and H rats, myocytes from CR tended to be shorter in L (P less than 0.001), but larger in CSA (N.S.) than myocytes obtained from H rats. In both H and CR, left ventricular V and CSA were greater (P less than 0.01) in endomyocardium than epimyocardium and middle myocardium. Values for V and CSA of right ventricular myocytes were less (P less than 0.01) than those from left ventricles in both H and CR. Hearts from H had 19% more myocytes than those from CR (P less than 0.05). Values for cellular dimensions from CR generally showed less variability than those from H. We conclude that significant regional differences in myocyte size are present in rats from both H and CR. Hearts from H rats have significantly more myocytes than those from weight-matched CR rats. Therefore, investigators should be aware that differences in heart weight and myocyte number can be found in rats of the same strain, but obtained from different suppliers.