Intravascular pressure distribution and dimensional analysis of microvessels in hamsters with renovascular hypertension

Initial characterization of hamsters with spontaneous hypertension

The purpose of this study was to begin to characterize a new inbred strain of adult male hamsters with established spontaneous hypertension along with their genetically/age-matched normotensive controls. We found that mean arterial pressure was 162+/-3 mm Hg in hypertensive hamsters and 94+/-4 mm Hg in controls (mean+/-SEM; P<.05). Body weight was significantly lower in hypertensive hamsters relative to normotensive hamsters (P<.05). Hypertension was associated with a significant increase in heart weight, thickness of the left ventricular wall, and amplitude of the QRS complex in standard electrocardiographic leads I and aVR (P<.05). No gross or microscopic abnormalities were observed in other organs. Plasma renin activity and the number of circulating neutrophils were significantly increased in hypertensive hamsters relative to controls (P<.05). Serum concentrations of creatinine, blood urea nitrogen, sodium, potassium, and calcium as well as urinalysis were similar in both groups. Overall, these data suggest that the spontaneously hypertensive hamster could be a suitable model for the study of spontaneous hypertension.

Small artery resistance increases during the development of renal hypertension

Vascular pressures were measured in the principal (A1) arteriole and in upstream small arteries of the rat cremaster muscle to investigate vascular resistance changes associated with one-kidney, one clip Goldblatt hypertension. Pressure measurements were made at a proximal and distal site of each vessel using a servonull micropipette system. Vessel diameters were measured using video microscopy. A1 arteriole and external spermatic artery diameters were both decreased after 2 and 4 weeks of hypertension. Mean arterial pressure was elevated after 2 weeks of hypertension (106 +/- 4 mm Hg versus 140 +/- 5 mm Hg). Likewise, vascular pressures were elevated at every site: pudicepigastric artery (36%), external spermatic artery (47%), and A1 arteriole (38%). The pressure drop along the external spermatic artery was increased (87%) after 2 weeks of hypertension. Mean arterial pressure was further elevated from 2-4 weeks of hypertension (105 +/- 4 mm Hg versus 162 +/- 7 mm Hg) but only the proximal pudic-epigastric artery pressure was further elevated during this time from 2 to 4 weeks (131 +/- 5 mm Hg versus 147 +/- 7 mm Hg) of hypertension development. This was associated with an increased pressure drop (87%) along the artery compared with the situation at 2 weeks. These data indicate that small arteries upstream from the microcirculation contribute significantly to the increase in vascular resistance during hypertension. In addition, these data indicate that the increases in small artery resistance do not develop uniformly throughout all vessel branches.

Penetration of the systemic blood pressure into the microvasculature of rat skeletal muscle

A series of arterial micropressure measurements in different skeletal muscles of the Wistar-Kyoto and spontaneously hypertensive rat is presented. The micropunctures were carried out with minimal surgical intervention through small skin incisions and the micropressures were recorded simultaneously with femoral artery pressures. The measurement sites were located at the entry points into the muscles for the proximal and distal supply arteries and at the midpoint of the arteriolar arcade bridge which directly connects these two supply arteries in the center of the muscle parenchyma. In contrast to feed artery pressure values from exteriorized muscles, which in the past have been reported to be as low as 40 mm Hg, the current mean pressure values are substantially higher and in the range between 70 and 100 mm Hg, equivalent to 70 to 90% of the mean systemic pressure. Systolic and diastolic values exhibit comparable trends to the mean pressures and they are similar in muscles at different locations in the body. Although in spontaneously hypertensive rats the absolute pressures were significantly higher compared with their controls, the normalized pressures were virtually identical at the locations used in this study. These data indicate that the absolute pressure in the central arteries of spontaneously hypertensive animals is reduced to a greater degree than in Wistar-Kyoto rats, while in both strains the major pressure reduction in skeletal muscle still occurs in the microcirculation.

Altered cremaster muscle hemodynamics due to disruption of the deferential feed vessels

Surgical preparation of the cremaster muscle for microvascular studies typically requires disruption of collateral vessels within the muscle and between the cremaster and the structures of the epididymis/ductus deferens. To study the effect of interrupting these vascular connections on cremaster hemodynamics, two modified preparations were examined in addition to the conventional open cremaster muscle preparation. One of these preparations enabled the measurement of feed vessel (1A) pressure and diameter with all cremaster vascular connections intact. The second preparation involved interruption of intramuscle collaterals to open the cremaster sac but with intact collateral pathways between the cremaster and deferential vessels. Intravascular pressures in the main cremasteric arteriole (1A) were similar in all three preparations with pressure in the 1A, expressed as a percentage of femoral artery pressure, varying between 53% in the intact preparation and 48% in the standard open preparation. These data support the existence of substantial upstream vascular resistance regardless of the extent of surgery. Selective occlusion of the branches of the deferential vessels significantly increased red cell velocity in the cremasteric 1A and major draining venule (1V) so that calculated blood flow increased by approximately 40% (P less than 0.01) in the 1A and 95% (P less than 0.01) in the 1V. Also, intravascular pressure fell significantly (P less than 0.01) in the 1A and increased in the 1V. Despite these compensatory changes total blood flow to the muscle was reduced by approximately 40% in the standard open preparation, compared to the preparation with the deferential feed pathway intact. Further studies where the 1A flow was transiently occluded indicated that the deferential pathway was capable of providing significant collateral blood flow to the muscle. Collectively these studies demonstrate that the surgical modifications of the cremaster vascular supply required for in vivo microscopy significantly alter normal hemodynamics within the vascular bed. The surgery does not, however, entirely explain the large pressure drop that exists upstream of the cremasteric 1A.

Angiotensin reactivity in the cheek pouch of the renovascular hypertensive hamster

Increased reactivity to vasoconstrictor agents and decreased arteriolar luminal diameter have been implicated in the maintenance of hypertension. The same hamster cheek pouch microvessels were tested for angiotensin I (Ang I) and angiotensin II (Ang II) reactivity before and 10 to 14 days after Grollman (two-kidney, one figure-8) or sham operation. Microvascular geometric parameters were measured before and after a maximal vasodilator dose of adenosine. Then maximal vasoconstrictions to Ang I or Ang II were measured: Ang I and Ang II were applied adjacent to arterioles (10(-2)-10(0) pmol) and venules (10(-1) pmol) in 10-microliter aliquots for 1 minute. Blood pressure (178 +/- 11/133 +/- 8 mm Hg) of renovascular hypertensive hamsters was elevated significantly over blood pressure of sham-operated hamsters (120 +/- 11/97 +/- 10 mm Hg). No change was observed in venular geometry or reactivity in renovascular hypertensive hamsters. Arteriolar luminal diameter, wall thickness, wall/lumen ratio, and wall area were not altered in hypertensive hamsters in the normal or vasodilated state; vasodilator capacity was the same in all groups. Conversion of Ang I to Ang II (response to Ang I divided by response to Ang II) for first-order and Third-order arterioles and third-order venules was 74 +/- 5, 79 +/- 3, and 72 +/- 6%, respectively, and was unaltered in renovascular hypertensive hamsters. Although vessel geometry was not altered, there was a significant shift to the left of the Ang I and Ang II dose-response curves of first-order and third-order arterioles, indicating increased sensitivity to these vasoconstrictors.(ABSTRACT TRUNCATED AT 250 WORDS)

A Mathematical Model of Countercurrent Exchange of Oxygen Between Paired Arterioles and Venules

A mathematical model is formulated for diffusive countercurrent exchange of oxygen between paired arterioles and venules. A closed form solution of the problem is obtained by linearizing the nonlinear oxyhemoglobin dissociation curve at the inlet P_O2 in the vessel. The closed form solution is compared with the corresponding numerical solution of the nonlinear problem. Under normal conditions, longitudinal gradients of venular P_O2 are found to be small. Examples are presented where the model predicts significant gradients of venular P_O2 when the blood flow rate in the venule is several times smaller than that in the arteriole.

Microvascular changes in the heart during chronic arterial hypertension

Changes of myocardial microvascular permeability in chronic renovascular arterial hypertension were studied. Hypertension was induced in dogs utilizing a one-kidney, one-clip Goldblatt model. Systemic arterial pressure, coronary sinus pressure, systemic venous pressure, myocardial lymph flow rate, myocardial interstitial fluid pressure, and the lymph-to-plasma protein concentration ratio for total plasma proteins and for beta-lipoprotein (CL/CP) were determined in control animals and 4-6 weeks following the Goldblatt procedure in hypertensive animals. Control values for the normotensive animals were 123 +/- 17 mm Hg, 7.3 +/- 1.3 mm Hg, 2.5 +/- 2.1 mm Hg, 3.1 +/- 2.1 ml/hr, 14.9 +/- 3.1 mm Hg, 0.82, and 0.33, respectively, while control values for the chronically hypertensive dogs were 160 +/- 20 mm Hg, 7.8 +/- 1.9 mm Hg, 2.9 +/- 2.5 mm Hg, 10.5 +/- 2.5 ml/hr, 24.8 +/- 3.7 mm Hg, 0.87, and 0.31, respectively. Under control conditions, myocardial lymph flow rate was significantly higher in the hypertensive group while no difference could be demonstrated in CL/CP between the two groups. This is indicative of either a change in myocardial microvascular permeability or an increase in microvascular exchange surface area. Coronary sinus pressure was elevated in both groups in order to increase transmicrovascular fluid flux and determine the filtration-independent reflection coefficient (sigma) for each group. Sigma is a surface area-independent indicator of microvascular permeability when determined for specific protein molecules at high transmicrovascular fluid fluxes.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of sequentially branching macro- and microvessels during reactive hyperemia in skeletal muscle

Small artery and microvascular responses during reactive hyperemia were compared to determine which resistance-bearing vessels played a role in controlling blood flow and resistance for the cremaster skeletal muscle. Using an intravital video microscopy system, measurements of microvessel pressure, flow velocity, and diameter were obtained from cremaster muscles in anesthetized rats. These were compared with measurements of diameter that were obtained from the small arteries feeding the cremaster muscle. After a 60-sec occlusion of the sacral aorta, total cremaster blood flow increased approximately 28% and calculated microvascular resistance for the cremaster muscle fell 50%. During the period of occlusion, diameters of small arteries (159-292 micron) decreased despite the presence of smooth muscle tone. Likewise, the diameters of large arterioles (65-117 micron) decreased whereas small arterioles (16-30 micron) dilated. The decrease in diameter of the small arteries and large arterioles was accompanied by a significant fall in intravascular pressure, suggesting that the behavior of these vessels was largely passive. Immediately following the release of occlusion, small arteries and large arterioles returned to their control diameters while small arterioles remained in a dilated state for approximately 2 min. These results indicate that for the cremaster muscle, vascular responses vary along the length of the arterial tree during reactive hyperemia, small but not large arterioles are primarily responsible for the decrease in network resistance and subsequent hyperemia following occlusion, and the small feeder arteries did not dilate during reactive hyperemia but instead acted to set a limit on the decrease in network resistance and the increase in blood flow.

Arteriolar reactivity to pressure stimuli in hamsters with renal hypertension

The responses to alterations in extravascular pressure were studied in five orders of arterioles in the cheek pouch of normotensive and renal hypertensive hamsters. Renal hypertension was induced by bilateral compression of both kidneys using figure-of-eight ligatures. Ten to 16 days later, hamsters were anesthetized with pentobarbital (6.0 mg/100 g body weight) and a Plexiglas chamber was positioned in the cheek pouch. Chamber pressure, or extravascular pressure, was increased and decreased by +/- 10, 20, and 40 mm Hg, and arteriolar diameters were monitored continuously. The responses at -20 mm Hg and the slope of the linear portion of the chamber pressure-diameter curve (arteriolar gains) were compared between groups for each branching order of arteriole. Arteriolar responses at one chamber pressure and the arteriolar gains were enhanced in third and fourth order arterioles of the renal hypertensive group compared with the normotensive group, and the responses of these small arterioles were greater than those of larger arterioles in both groups. Control diameters of second and third order arterioles were significantly smaller in the renal hypertensive group, while the diameters after adenosine were not different. These results suggest that the enhanced responses of small arterioles in the renal hypertensive group were not related to structural alterations but may be related to an increased reactivity of smooth muscles in these small arterioles to volume expansion, thus a pressure stimulus.

Anatomic and hemodynamic characteristics of the blood vessels feeding the cremaster skeletal muscle in the rat

The anatomic arrangement, pressure distribution, and resting vascular tone of the feed arteries located upstream from the rat cremaster microcirculation were determined to characterize the sites of the vascular resistance in this macrovessel segment of the cremaster circulation. The cremaster microcirculation and its feeding arteries were studied using an intravital video microscopy system. Vascular diameters and pressures were measured with an image shearing monitor and servo-null micropipet system, respectively. The central arteriole of the cremaster muscle was found to be a distal segment of the external spermatic artery which branched from the pudic-epigastric artery that in turn arose from the common iliac artery. Together the length of these vessels, from the aorta to the cremaster muscle, was 37 mm and they accounted for 42% of the total pressure drop across the cremaster vascular network. The largest pressure drop (31 mm Hg) upstream from the cremaster occurred across the external spermatic artery which was also the longest (17.7 mm) feed vessel. Topical application of adenosine (1 X 10(-3) M) significantly dilated the pudic-epigastric artery and the external spermatic artery, indicating that these vessels had significant tone. In summary, our data indicate that the large fraction of network vascular resistance located in the feed vessels upstream from the cremaster is the result of both architectural features and vascular tone.

Communication between feed arteries and microvessels in hamster striated muscle: segmental vascular responses are functionally coordinated

Pressures in the primary arterioles of the cremaster muscle are reported to be approximately 50% of systemic, indicating that arterial resistance proximal to microvessels is high and may limit maximal blood flow. With no change in arterial resistance, increases in perfusion normally associated with muscle work either could not occur or would require increments in systemic pressure far greater than those actually observed in vivo. Therefore, we hypothesized that the small arteries feeding the muscle may participate in the hyperemic response. To test this hypothesis, male golden hamsters (n = 31, 118 g) were anesthetized (pentobarbital, 70 mg/kg i.p.), and the right cremaster was opened to expose its feed arteries, which originated from the iliac artery. Preparations were superfused and maintained at 35 +/- 1 degree C. Feed arteries had substantial tone, as shown by the fact that topical acetylcholine, applied at supramaximal concentration, dilated these vessels from 115 +/- 8 microns at rest to 158 +/- 9 microns (mean +/- SE; n = 38 vessels; p less than 0.01), corresponding to an estimated 4.4-fold increase in conductance. Stimulation of the sectioned motor nerve (8 Hz, 30 seconds) induced striated muscle contraction and increased feed vessel diameter from 93 +/- 5 microns to 116 +/- 5 microns (n = 14; p less than 0.01), consistent with a 2.6-fold increase in conductance. A 5-minute occlusion of the iliac artery resulted in feed artery dilation of similar magnitude. Supramaximal doses of acetylcholine applied topically to the distal portions of the cremaster resulted in striated muscle contraction and a dilation that propagated upstream to increase feed artery diameter by 25%.(ABSTRACT TRUNCATED AT 250 WORDS)

The microvasculature in skeletal muscle. II. Arteriolar network anatomy in normotensive and spontaneously hypertensive rats

A quantitative comparison of the anatomical arrangement of arterioles in the skeletal muscle of mature (16 to 20-week-old) Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) is provided. In both species several feeding arterioles supply blood to a network of arterioles covering the entire muscle, designated as arcade arterioles. The connections from the arcade arterioles to the capillary network are provided by the transverse arterioles. Comparison of the spinotrapezius muscle of the WKY and the SHR shows several types of rearrangement of the network. In both species there is a wide distribution of vessel size in the arcade and transverse arterioles. The length of the arcade arterioles per unit muscle volume is higher in the SHR, forming a denser network. There are almost twice as many transverse arteriolar trees per unit tissue volume in hypertensive animals although on the average each transverse tree has shorter branches. No evidence for significant anatomical rarefaction was found among arcade and transverse arterioles. Arcade arterioles following maximal dilation were found to be 25% narrower in the SHR, whereas for transverse arterioles no differences in diameter could be detected under these vasodilated conditions.

Distributions of microvascular pressure in skeletal muscle of one-kidney, one clip, two-kidney, one clip, and deoxycorticosterone-salt hypertensive rats

Studies were performed on the cremaster skeletal muscle in rats to investigate the microvascular changes that are associated with established one-kidney, one clip (1K1C) and two-kidney, one clip (2K1C) Goldblatt hypertension and with deoxycorticosterone (DOC)-salt hypertension. Rats were anesthetized with urethane and chloralose; and cremaster muscles with intact circulation and innervation were suspended in a controlled Krebs bath. Microvascular pressures and vessel diameters were measured at three consecutive arteriolar (A) and venular (V) branch levels. Arteriolar diameters (means +/- SEM) in normotensive (NT) rats were 119 +/- 7, 86 +/- 5, and 31 +/- 3 micron respectively for 1A, 2A, and 3A arterioles; and venule diameters were 218 +/- 12, 141 +/- 15, and 53 +/- 7 micron respectively for 1V, 2V, and 3V venules. As compared to NT rats, there was a selective decrease in lumen size (percent reduction from control) for 1A and 2A (23% to 38%) in 1K1C and 2K1C rats and for 1A, 2A, and 3A (42% to 44%) in DOC rats. Venule diameters were not significantly different between normotensive and hypertensive animals at any branch level. Femoral artery pressures were significantly elevated (greater than or equal to 43%) in all three forms of hypertension; however, this increase in pressure was not proportionally transmitted throughout the microcirculation. This was evidenced by normal pressure in 3A arterioles and in all venules for 1K1C and 2K1C rats and by normal pressures in 3V and larger venules for DOC rats. Our findings indicate that elevated arterial pressure in chronic renal hypertension is not transmitted uniformly across all microvascular segments.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic neurons: are they present on microvessels in cheek pouches of hamsters?

Catecholaminergic innervation was investigated in the cheek pouch, mesentery, and cremaster muscle of the hamster. These tissues were removed from normal hamsters after anesthetization with sodium pentobarbital. They were prepared immediately for visualization of catecholamines by using a glyoxylic acid fluorescent technique. Visualization was accomplished via a Zeiss fluorescent microscope with a 470-nm barrier and a 420-nm excitation filter. No fluorescence was observed in or around the microvessels of the cheek pouch; however, intense fluorescence was demonstrated around and along microvessels in the mesentery and cremaster muscle. Thus, within the limitations of this technique, this study demonstrates that local catecholamines, either as a component of adrenergic neurons or a part of the local tissue milieu, are nondetectable in the microvessels within the cheek pouch as compared to mesenteric and muscle tissue of the hamster.

Evidence for a vascular sensitizing factor in plasma of saline-loaded dogs

This study investigates whether plasma extracts previously demonstrated to have natriuretic and antinatriferic activity have effects on vascular reactivity of rat cremaster arterioles. Plasma from hydropenic and saline-loaded dogs was subjected to Diafiltration, and eluted on a strong cation exchange column (SCX). The effects of intraarterial injections of various column fractions on constrictor responses to repeated injections of 33.3 ng of norepinephrine (NE) were used to indicate changes in vascular responsiveness in third order cremaster arterioles. SCX fraction I (void volume) from saline-loaded dogs (FI-S) caused an increase in constrictor response to NE of 101%. Increased vascular responsiveness peaked at 40 minutes and remained significantly elevated (p less than 0.05) for 130 minutes. Fraction I from plasma of hydropenic dogs (FI-H) and fraction III from plasma of saline-loaded dogs (FIII-S) did not increase vascular responsiveness to NE. FI-S shifted the dose response curves for NE, arginine vasopressin, and angiotensin II parallel and to the left relative to control by a factor of 3.05-, 2.95-, and 5.63-fold, respectively, at the 50% constrictor dose. Systemic injections of FI-S, but not FI-H, caused a 10 mm Hg rise in blood pressure at 50 minutes, and blood pressure was significantly elevated for 30 to 90 minutes after injection (p less than 0.01). These data demonstrate a vascular-sensitizing factor in FI-S. The factor appears in the same chromatographic fraction previously demonstrated to contain natriuretic, antinatriferic, and digoxin-like activity. The correlation of these activities with salt loading suggests they are due to the same substance, which may be the putative natriuretic hormone.