Calcium Sensitivity and Agonist-Induced Calcium Sensitization in Small Arteries of Young and Adult Spontaneously Hypertensive Rats
Abstract The sensitivity of the myofilaments to Ca2+ is increased during agonist-induced contraction of vascular smooth muscle. Given the important contribution of vascular tone to the elevation of peripheral resistance observed in genetic hypertension, we have investigated whether alterations in myofilament Ca2+ sensitivity occur in small arteries from spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) controls during the developmental and established phases of hypertension. Segments of mesenteric, renal, and femoral artery with an average lumen diameter <300 μm from 5- or 20-week-old rats were mounted in a wire myograph. Morphological measurements were made and the vessels permeabilized with Staphylococcus aureus α-toxin. Dose-response curves to increasing concentrations of Ca2+ were obtained and the ability of 100 nmol/L endothelin-1 (ET-1) or 10 μmol/L norepinephrine (NE) in the presence of 10 μmol/L GTP to enhance tension in response to low Ca2+ (pCa6.7) was determined. Systolic, diastolic, and mean blood pressures were higher in SHR than in WKY at 5 and 20 weeks. The media thickness:lumen diameter ratio was increased in mesenteric and femoral arteries from SHR compared with WKY at 5 and 20 weeks. There was no difference in media thickness:lumen diameter ratio in renal arteries or between 5- and 20-week animals in any vascular bed. The pCa curves were not different in mesenteric, renal, or femoral arteries from hypertensive compared with normotensive rats or between age groups, except in femoral arteries at 20 weeks, which exhibited a greater sensitivity to Ca2+ in SHR. Tension developed in response to maximal Ca2+ (pCa5.0) was greater in permeabilized mesenteric arteries from SHR compared with WKY at 20 weeks of age only; media stress was again similar in both strains but increased in older animals compared with younger animals in mesenteric arteries from WKY. The submaximal contraction induced by pCa6.7 was greater in femoral and renal than mesenteric arteries. GTP (10 μmol/L) augmented the tension developed to pCa6.7 in mesenteric arteries at 5 and 20 weeks and in renal arteries at 20 weeks. Addition of 100 nmol/L ET-1 or 10 μmol/L NE in the continued presence of GTP markedly increased tension in mesenteric arteries at 5 and 20 weeks. In renal arteries, 10 μmol/L NE enhanced Ca2+ sensitivity in the presence of GTP in SHR at 5 and 20 weeks and WKY at 5 weeks. In femoral arteries, there was a tendency for ET-1 and NE to increase Ca2+ sensitivity, but this increase was significant in WKY at 20 weeks (ET-1) and SHR at 5 weeks (NE) only. We have demonstrated that the sensitivity of the myofilaments to Ca2+ and ET-1– or NE-induced Ca2+ sensitization is not different in permeabilized small mesenteric, renal, or femoral arteries from SHR compared with WKY controls. Only in SHR mesenteric arteries at 20 weeks of age was there evidence of increased active tension in response to maximal Ca2+, despite structural differences, consistent with increased muscle mass in femoral arteries from SHR. We conclude that it is unlikely that a ubiquitous abnormality of the sensitivity of the contractile apparatus to Ca2+ or agonist-induced Ca2+ sensitization in vascular smooth muscle underlies the elevated total peripheral resistance associated with hypertension.
Intracellular free Ca2+ concentration is a major determinant of smooth muscle tone, and the level of this ion is tightly regulated by a number of Ca2+-mobilizing and Ca2+-homeostatic mechanisms.1 During agonist-induced contraction, intracellular Ca2+ levels rise through two mechanisms: release from intracellular stores and influx across the plasma membrane.2 There is evidence that NE-stimulated Ca2+ influx is considerably higher in SHR than in WKY, which may contribute to the enhanced sensitivity to this agonist in mesenteric arteries.1 However, in addition to increasing intracellular Ca2+, vasoconstrictor hormones sensitize the contractile myofilaments to Ca2+, and an abnormality in this mechanism could lead to enhanced vasoconstriction. Agonist-induced Ca2+ sensitization appears to be a G protein–mediated effect3 and involves downstream effectors such as myosin light chain phosphatase,4 protein kinase C,5 and tyrosine kinases.6 7 Recently Kanagy and Webb8 demonstrated that carotid artery strips from hypertensive rats had a greater responsiveness than strips from normotensive rats to mastoparan, a peptide that activates G proteins directly, suggesting that there may be altered G protein–mediated signaling in hypertension. Furthermore, Satoh et al9 reported augmented 5-hydroxytryptamine– and GTPγS-induced Ca2+ sensitization in coronary arteries from SHRSP, again implicating altered intracellular signaling pathways. Also, it is possible that myofilament Ca2+ sensitivity is greater in arterial smooth muscle from hypertensive animals, although the evidence is conflicting. Increased Ca2+ sensitivity has been demonstrated in saponin-skinned aortic and portal vein tissue from SHR compared with WKY,10 and Bian and Bukoski11 showed that after Ca2+ depletion, mesenteric arteries from SHR developed greater force to increasing concentrations of Ca2+ in the presence of 10 μmol/L NE than vessels from normotensive strains. In contrast, no alteration in the sensitivity of myofilaments to Ca2+ was detected in skinned rat tail artery rings12 or coronary arteries9 from the SHRSP compared with vessels from normotensive control animals. A possible explanation for these conflicting reports may be the diversity of vascular beds, size of vessel, and agonists studied.
Contraction mediated directly by Ca2+ and agonist-induced Ca2+ sensitization may be investigated in small arteries permeabilized with α-toxin. In this preparation, small pores are formed in the plasma membrane that allow the levels of ions such as Ca2+ to be controlled but leave receptors coupled to their signaling systems.3 13 Using this preparation, we have investigated whether the sensitivity of the myofilaments to Ca2+ is altered in the early and established phases of hypertension in small arteries from the mesenteric, renal, and femoral vascular beds in hypertensive compared with normotensive rats. We have examined also whether there is altered NE- or ET-1–induced Ca2+ sensitization in these vessels at two stages of the hypertensive process and whether structural changes of the arterial wall influence these responses.
For this study, male SHR and their control strain WKY were used. They were obtained from Charles River Ltd (Margate, Kent, UK). All procedures performed were carried out by trained personnel in accordance with institutional guidelines and the UK Animals (Scientific Procedures) Act of 1986.
Blood Pressure Measurement
Direct blood pressures were measured as previously described.14 Briefly, at 5 or 20 weeks of age, femoral artery cannulas were implanted under anesthesia (0.26 mg fentanyl citrate, 8.25 mg fluanisone, and 4.1 mg midazolam per kilogram body weight), and the proximal tip of the catheter was advanced into the abdominal aorta to permit direct measurement of aortic blood pressure. The distal region of the catheter was exteriorized between the scapulae, flushed with saline containing 100 U/mL heparin, and blocked with a stainless steel spigot. Analgesia was provided by 0.3 mg buprenorphine per kilogram body weight. Twenty-four hours after catheter implantation, unrestrained, conscious blood pressure recordings were made with a Viggo-Spectramed TXX-R pressure transducer connected to a chart recorder. Mean aortic blood pressure was calculated as diastolic blood pressure plus one third pulse pressure.
Small Artery Structure
On the day of study, rats were stunned by a blow to the head and killed by cervical dislocation, in accordance with Institutional and American Veterinary Medical Association guidelines on euthanasia.15 Femoral small arteries (third-order branch vessels originating distal to the femoral artery were used in 20-week rats, but due to size limitations, second-order vessels were used in 5-week rats), mesenteric small arteries (third-order branches of the superior mesenteric artery), and renal small arteries (first-order branches of the main renal artery) were dissected out and cleaned of extraneous fat and connective tissue. Segments (approximately 2 mm long) were mounted as ring preparations in an isometric myograph16 and warmed to 37°C.
The myograph was designed with a small Perspex window situated directly below the mounting heads so that when it is mounted on the stage of a light microscope (Leitz, UK Ltd), the artery wall can be imaged by using a water-immersion objective (×40, Zeiss Ltd). A calibrated micrometer eyepiece (×8, Zeiss Ltd) was used to measure the thickness of the media. Media CSA (equivalent to media volume per unit length) was calculated from the media thickness and internal circumference. The resting tension–internal circumference relation was determined for each segment of artery as described previously.16 From this value, the internal circumference (L100) that the artery would have if relaxed and under a transmural pressure of 100 mm Hg (13.3 kPa) was calculated. The corresponding internal diameter (l100) was calculated as L100/π. Each artery was then set to the normalized effective internal diameter (l0), where l0=0.9 l100.16 Assuming constant media volume, the media thickness could be calculated for l0.
After the vessels were incubated in relaxing solution for 15 minutes, they were permeabilized according to the method of Kitazawa et al.3 Briefly, a 10-μL droplet of pCa6.7 solution containing 1250 units of Staphylococcus aureus α-toxin and 10 μmol/L A23187, to deplete the sarcoplasmic reticulum of Ca2+, was placed onto each vessel segment. After tension development had reached a plateau (a period of 15 to 20 minutes), the vessels were equilibrated in relaxing solution and bubbled with 100% O2. All experiments were carried out at room temperature.
After permeabilization, each artery was stimulated by a series of solutions of increasing Ca2+ concentration. A pCa curve was obtained by normalizing the tension for each submaximal Ca2+ contraction with respect to maximal tension and plotting as a function of pCa. The contractile response to the maximum Ca2+ concentration was calculated as active tension (mN/mm), defined as force response divided by twice vessel segment length and active media stress (mN/mm2), defined as the active tension divided by media thickness at l0, which has been described previously.17
The Ca2+-sensitizing effects of (1) 10 μmol/L GTP, (2) 10 μmol/L GTP+100 nmol/L ET-1, and (3) 10 μmol/L GTP+10 μmol/L NE at pCa6.7 were determined. After each addition, the contraction elicited by that particular compound was allowed to reach a plateau before the next was added. Initial dose-response curves showed that 100 nmol/L ET-1 and 10 μmol/L NE produced the maximum contractile response to these agonists.
Vessels were dissected out and held in the myograph in physiological saline solution of the following composition (in mmol/L): NaCl 119, KCl 4.7, CaCl2 2.5, NaHCO3 25, MgSO4 1.17, KH2PO4 1.18, K2EDTA 0.026, and glucose 5.5, which was bubbled with 95% O2/5% CO2 to give a pH of 7.4 at 37°C. The composition of solutions involved in permeabilization were as follows. Relaxing solution consisted of (in mmol/L) PIPES 30, sodium creatine phosphate 10, Na2ATP 5.16, magnesium methane sulfonate 7.31, potassium methane sulfonate 74.1, K2EGTA 1; pH was adjusted to 7.1 with KOH. In the α-toxin or activating solutions, 10 mmol/L EGTA was used, and a specified amount of calcium methane sulfonate was added to give the desired concentration of free Ca2+ ions.3 18
Staphylococcus aureus α-toxin was obtained from GIBCO-BRL. A23187, GTP, NE, and ET-1 were obtained from Sigma Chemical Company. A23187 was dissolved in DMSO (final concentration <1%), and GTP, NE, and ET-1 were dissolved in distilled water.
Data Calculation and Statistical Analysis
Each contraction was measured from the baseline and calculated as tension and media stress. All results were expressed as mean±SEM. Individual group sizes are indicated in the text, tables, and figure legends. Repeated measures ANOVA was used to determine differences between SHR and WKY pCa dose-response curves. ANOVA plus the least significant difference test was used to assess differences in blood pressure and in mesenteric and renal arteries. Because femoral arteries from the two age groups were taken from different locations, differences between strains and pCa50 values (the pCa required to elicit half-maximal tension) were calculated using Student’s t test. All other comparisons were by Student’s t test. P<.05 was considered statistically significant.
Systolic blood pressure, diastolic blood pressure, and mean blood pressure were greater at 5 and 20 weeks in SHR than in WKY (P<.05, Table 1⇓). There was an increase in blood pressure in both strains of rat with age (P<.05, Table 1⇓).
Values of lumen diameter, media thickness, media CSA, and media thickness:lumen diameter are shown in Table 2⇓ and Fig 1⇓. Mesenteric and femoral arteries from SHR clearly showed structural differences compared with WKY controls at both 5 and 20 weeks of age. The only difference detected in the renal arteries from SHR was an increase in media thickness at 20 weeks of age. Within-strain age-related differences in morphology of mesenteric and renal vessels were observed also (Table 2⇓, Fig 1⇓). These comparisons could not be made for femoral arteries, as they were different order vessels at 5 and 20 weeks.
There was no significant difference in the active tension or media stress developed in response to pCa5.0 (maximal Ca2+) between SHR and WKY controls in any of the vessels studied, except for mesenteric arteries from 20-week-old SHR, which developed approximately 25% greater tension than those from 20-week-old WKY (Table 3⇓, Fig 1⇑). Fig 1⇑ clearly shows that the greater media:lumen ratio in femoral and mesenteric arteries from the hypertensive animals did not result in enhanced active tension development to pCa5.0 compared with vessels from age-matched normotensive animals. However, age-dependent changes were observed, such that 20-week animals developed approximately 60% greater tension to pCa5.0 than 5-week animals in mesenteric arteries (Table 3⇓). Media stress was increased also in older animals compared with younger animals in mesenteric arteries, but only in the normotensive animals (Table 3⇓). There was no difference in maximum tension or media stress in response to pCa5.0 between 5- and 20-week animals in renal arteries (Table 3⇓).
Comparison Between Vascular Beds
The tension developed to maximal Ca2+ (pCa5.0) was greater in femoral arteries than mesenteric arteries from both hypertensive and normotensive rats at 5 and 20 weeks of age and from renal arteries at 5 weeks only (Table 3⇑). These data correlate with the markedly greater media:lumen ratio of femoral arteries compared with mesenteric and renal vessels (Fig 1⇑).
There was a greater sensitivity to Ca2+ in femoral arteries from SHR than WKY at 20 weeks: pCa50 values were 6.35 and 6.07 for SHR and WKY, respectively; P<.05 (Fig 2⇓).
There was no difference in Ca2+ sensitivity of tension development at either 5 or 20 weeks in the remaining vessels studied between SHR and WKY controls. No age-dependent changes in Ca2+ sensitivity were detected in any of the vessels studied.
Agonist-Induced Ca2+ Sensitization
In the second stage of this study, we investigated whether NE and ET-1 in the presence of GTP were able to increase the sensitivity of the myofilaments to Ca2+ and whether these responses were altered in hypertension. An illustration of the protocol used and the sensitization response induced by 100 nmol/L ET-1 in a mesenteric artery from a 5-week WKY is shown in Fig 3⇓.
GTP (10 μmol/L) slightly enhanced the submaximal contraction induced by pCa6.7 (Fig 4⇓). The addition of 100 nmol/L ET-1, in the continued presence of GTP, markedly augmented tension at constant low Ca2+. A similar increase in tension in the presence of GTP and low constant Ca2+ was observed with 10 μmol/L NE. There was no difference in the Ca2+ sensitization induced by ET-1 and NE between vessels from SHR and WKY controls at either 5 or 20 weeks of age.
GTP (10 μmol/L) augmented the tension developed to submaximal Ca2+ (pCa6.7) in arteries from both SHR and WKY at 20 weeks of age only (Fig 5⇓). ET-1 did not induce a further increase in tension compared with GTP alone in either strain or at either age. In contrast, NE in the presence of GTP increased tension at constant low Ca2+ in renal arteries from SHR at 5 and 20 weeks and from WKY at 5 weeks.
GTP (10 μmol/L) did not augment the tension developed to submaximal Ca2+ (pCa6.7) significantly (Fig 6⇓). There was a tendency for tension to increase in the presence of ET-1 and GTP, although this increase was significant in arteries from WKY at 20 weeks of age only. Similarly increased Ca2+ sensitivity in response to NE was slight, and significant changes were observed in vessels from SHR at 5 weeks of age only.
To investigate whether the sensitivity of the myofilaments to Ca2+ was altered in small arteries in hypertension, we studied α-toxin–permeabilized vessels in vitro taken from the mesenteric, renal, and femoral vascular beds from SHR and WKY controls during the onset and established phases of hypertension. At 5 weeks of age, blood pressure was elevated in SHR compared with age-matched WKY controls, and structural differences were already present in the mesenteric and femoral arteries. Furthermore, the structural modifications that occurred varied between the individual beds, enabling us to investigate the contractile response to a directly acting stimulus, ie, Ca2+, in conjunction with four different structural adaptations: (1) remodeling, which involved reduced lumen diameter and increased media:lumen ratio, with no accompanying change in media CSA, in mesenteric arteries at 5 weeks in SHR; (2) growth, which was demonstrated by increased wall thickness and no change in lumen diameter in femoral arteries at 5 and 20 weeks in SHR; (3) remodeling and growth in mesenteric arteries at 20 weeks in SHR (remodeling and growth indices were calculated as in Reference 1919 ; data not shown); and (4) no significant media:lumen ratio difference in renal arteries at 5 and 20 weeks of age in SHR compared with WKY. These structural alterations are in good agreement with previous studies in mesenteric14 17 20 21 and femoral22 23 arteries, confirming that the contributions of remodeling and growth to the structural modifications that occur in the vascular wall of small arteries in SHR vary according to the vascular bed studied.19 We did not detect any difference in either the maximum contractile response or sensitivity to Ca2+ in any of the arteries studied at 5 weeks of age between normotensive and hypertensive animals, despite elevated blood pressure and increased media:lumen ratio in femoral and mesenteric arteries. These data demonstrate that increased myofilament sensitivity to Ca2+ is unlikely to be involved in the developing stage of hypertension in this animal model. Later, when hypertension was established, there was an increase in the tension developed to maximum Ca2+ in mesenteric arteries from SHR. However, this effect was lost when the structural differences were accounted for, suggesting that the increased tension was due to increased media thickness. A similar response has been demonstrated in intact small mesenteric arteries from 20- to 24-week-old SHR constricted with high K+.17 Our data suggest that marked (>50%) changes in media:lumen ratio are required to observe significant increases in active tension in response to a direct activator of contraction, because mesenteric vessels from 5-week and femoral vessels from 5- and 20-week animals exhibited 35% to 40% increases in media:lumen ratio without enhancement of the tension developed to maximal Ca2+ (see Fig 1⇑). Or it may be that a combination of remodeling and growth, seen only in mesenteric arteries from 20-week-old SHR, which may result in a more efficient arrangement of muscle cells around the lumen, is required for this response. When dose-response curves were constructed, the sensitivity to Ca2+ was not altered in permeabilized mesenteric arteries from SHR, indicating that this was not the explanation for the increased maximal tension in response to Ca2+ in these arteries. In fact, only femoral arteries from 20-week-old hypertensive rats exhibited an increase in Ca2+ sensitivity compared with normotensive animals. No previous studies have investigated Ca2+ sensitivity in permeabilized femoral arteries in hypertension. However, in larger intact femoral arteries from the SHRSP, no difference in myofilament Ca2+ sensitivity was observed when membrane depolarization was achieved with K+.24 These differences may reflect the size of femoral artery used or the different protocols. Certainly, in permeabilized preparations, no difference in Ca2+ sensitivity of the contractile apparatus has been found in coronary9 or tail12 arteries of the SHRSP or in intact small arteries from the mesenteric20 25 26 bed of SHR. In contrast, Soloviev and Bershtein10 demonstrated increased Ca2+ sensitivity in permeabilized aorta and portal vein smooth muscle from SHR compared with WKY, suggesting differences between large conduit and small resistance arteries.
Hypertension did not affect the Ca2+ sensitization induced by GTP, ET-1, or NE in α-toxin–permeabilized mesenteric arteries. This is an important observation, as increased sensitivity to NE20 and decreased vascular smooth muscle sensitivity to ET-127 have been reported in intact mesenteric small arteries from SHR compared with WKY controls. This observation suggests that the differences in reactivity observed in intact mesenteric arteries to these agonists are not caused by an abnormality of the processes regulating myofilament contractility but may be due to altered Ca2+ handling. Indeed, there is evidence that smooth muscle Ca2+ homeostasis is compromised in hypertensive animal models.28
Our results in permeabilized mesenteric arteries are in contrast to the reported enhanced GTPγS- and 5-hydroxytryptamine–induced Ca2+ sensitization in permeabilized coronary arteries from SHRSP.9 An explanation for this difference is not immediately apparent, although it most probably reflects differences in the mechanisms coupling agonists to contraction in smooth muscle and/or differences in the vascular beds studied. The coronary arteries used by Satoh et al9 were smaller (approximately 140 μm) and exhibited no morphological differences compared with WKY control vessels, although the blood pressure of the animals was similar to our study. However, in agreement with our results, there was no difference in the sensitivity to Ca2+ in these coronary arteries, suggesting that the abnormality lay upstream of the contractile proteins and their immediate regulatory proteins, myosin light chain kinase and phosphatase. Also, these authors reported a decrease in desensitization of contraction in response to Ca2+ in coronary arteries from SHRSP compared with WKY, suggesting that the balance between these two mechanisms may be disturbed in these vessels in hypertension,9 which could result in an apparent increase in agonist-induced Ca2+ sensitization.
Endothelin-1– and NE-induced Ca2+ sensitization was poor in α-toxin–permeabilized renal and femoral arteries compared with mesenteric arteries. This finding was surprising, as intact renal and femoral arteries constrict to NE and ET-1 in vitro.23 29 It is unlikely that this effect was due to differences in efficiency of α-toxin permeabilization, as the tension developed to maximal Ca2+ showed a positive relationship with the thickness of the arterial wall (see Fig 1⇑), such that tension developed by the thicker-walled femoral arteries was greater than that of the thinner-walled mesenteric arteries. Any inefficiency of permeabilization would be expected to occur in the thicker-walled vessels, due to restricted diffusion of α-toxin, and this occurrence is not supported by our data. Another explanation is that ET-1– and NE-induced contraction in femoral and renal vessels is predominantly dependent on Ca2+, whereas in mesenteric arteries, modulation of the Ca2+ sensitivity of the myofilaments by these agonists may play a more substantial role. Therefore, in addition to heterogeneity in the contribution of pharmacomechanical and electromechanical coupling to the response to agonists, dependent on the size of vessel studied,30 31 there may be heterogeneity between arteries from different vascular beds. These differences between arteries may underlie the unequal contribution of organ resistance to the elevated total peripheral resistance reported in renovascular and genetic hypertension.32 However, further studies are required to test this possibility.
In conclusion, we have shown that the sensitivity of the myofilaments to Ca2+ and agonist-induced Ca2+ sensitization are not different in small arteries from peripheral vascular beds in SHR compared with WKY, despite increased blood pressure and marked structural modifications. These results are in contrast to the enhanced 5-hydroxytryptamine–induced Ca2+ sensitivity in coronary arteries from SHRSP9 and do not support a role for increased vascular smooth muscle Ca2+ sensitivity or agonist-induced Ca2+ sensitization in the maintenance of elevated peripheral resistance in genetic hypertension.
Selected Abbreviations and Acronyms
|SHR||=||spontaneously hypertensive rat(s)|
|SHRSP||=||spontaneously hypertensive stroke-prone rats|
This study was funded by the British Heart Foundation. We are grateful to Stuart J. Bund for performing the blood pressure measurements and to Ashley S. Izzard for discussion of the manuscript.
- Received August 23, 1996.
- Revision received September 18, 1996.
- Accepted January 21, 1997.
Khalil RA, van Breemen C. Hypertension: Pathophysiology, Diagnosis, and Management, I. 2nd ed. New York, NY: Raven Press, Ltd; 1995:523-539.
Kitazawa T, Kobayashi S, Horiuti K, Somlyo AV, Somlyo AP. Receptor-coupled, permeabilized smooth muscle. J Biol Chem. 1989;264:5339-5342.
Gong MC, Fuglsang A, Alessi D, Kobayashi S, Cohen P, Somlyo AV, Somlyo AP. Arachidonic acid inhibits myosin light chain phosphatase and sensitizes smooth muscle to calcium. J Biol Chem. 1992;267:21492-21498.
Nishimura J, Moreland S, Ahn HY, Kawase T, Moreland RS, van Breemen C. Endothelin increases myofilament Ca2+ sensitivity in α-toxin–permeabilized rabbit mesenteric artery. Circ Res. 1992;71:951-959.
Bian K, Bukoski RD. Myofilament calcium sensitivity of normotensive and hypertensive resistance arteries. Hypertension. 1995;25:110-116.
Mrwa U, Guth K, Haist C, Troschka M, Herrman R, Wojciechowski R, Gagelmann M. Calcium requirement for activation of skinned vascular smooth muscle from spontaneously hypertensive (SHRSP) and normotensive control rats. Life Sci. 1985;38:191-196.
Mulvany MJ, Halpern W. Contractile properties of small arterial resistance vessels in spontaneously hypertensive and normotensive rats. Circ Res. 1977;41:19-26.
Mulvany MJ, Hansen PK, Aalkjaer C. Direct evidence that the greater contractility of resistance vessels in spontaneously hypertensive rats is associated with a narrowed lumen, a thickened media, and an increased number of smooth muscle cell layers. Circ Res. 1978;43:854-864.
Heagerty AM, Aalkjaer C, Bund SJ, Korsgaard N, Mulvany MJ. Small artery structure in hypertension: dual processes of remodelling and growth. Hypertension. 1993;21:391-397.
Mulvany MJ, Aalkjaer C, Christensen J. Changes in noradrenaline sensitivity and morphology of arterial resistance vessels during the development of high blood pressure in spontaneously hypertensive rats. Hypertension. 1980;2:664-671.
Bund SJ, West KP, Heagerty AM. Effects of protection from pressure on resistance artery morphology and reactivity in spontaneously hypertensive and Wistar-Kyoto rats. Circ Res. 1991;68:1230-1240.
Soltis EE, Bohr DF. Vascular reactivity in the spontaneously hypertensive stroke-prone rat: effect of antihypertensive treatment. Hypertension. 1987;9:492-497.
Moreland RS, Webb RC, Bohr DF. Vascular changes in DOCA hypertension: influence of a low-protein diet. Hypertension. 1982;4:99-107.
Dohi Y, Luscher TF. Endothelin in hypertensive resistance arteries. Hypertension. 1991;18:543-549.
Cauvin C, van Breemen C. Different Ca2+ channels along the arterial tree. J Cardiovasc Pharmacol. 1985;7(suppl 4):S4-S10.