Cannulation and continuous cross-sectional area measurement of small blood vessels

Myography of isolated blood vessels: Considerations for experimental design and combination with supplementary techniques

The study of the mechanisms of regulation of vascular tone is an urgent task of modern science, since diseases of the cardiovascular system remain the main cause of reduction in the quality of life and mortality of the population. Myography (isometric and isobaric) of isolated blood vessels is one of the most physiologically relevant approaches to study the function of cells in the vessel wall. On the one hand, cell-cell interactions as well as mechanical stretch of the vessel wall remain preserved in myography studies, in contrast to studies on isolated cells, e.g., cell culture. On the other hand, in vitro studies in isolated vessels allow control of numerous parameters that are difficult to control in vivo. The aim of this review was to 1) discuss the specifics of experimental design and interpretation of data obtained by myography and 2) highlight the importance of the combined use of myography with various complementary techniques necessary for a deep understanding of vascular physiology.

VasoTracker, a Low-Cost and Open Source Pressure Myograph System for Vascular Physiology

Pressure myography, one of the most commonly used techniques in vascular research, measures the diameter of isolated, pressurized arteries to assess the functional activity of smooth muscle and endothelial cells. Despite the widespread adoption of this technique for assessing vascular function, there are only a small number of commercial systems and these are expensive. Here, we introduce a complete, open source pressure myograph system and analysis software, VasoTracker, that can be set-up for approximately 10% of the cost of commercial alternatives. We report on the development of VasoTracker and demonstrate its ability to assess various components of vascular reactivity. A unique feature of the VasoTracker platform is the publicly accessible website (http://www.vasotracker.com/) that documents how to assemble and use this affordable, adaptable, and expandable pressure myograph.

Mechanics of Vascular Smooth Muscle

Vascular smooth muscle (VSM; see Table 1 for a list of abbreviations) is a heterogeneous biomaterial comprised of cells and extracellular matrix. By surrounding tubes of endothelial cells, VSM forms a regulated network, the vasculature, through which oxygenated blood supplies specialized organs, permitting the development of large multicellular organisms. VSM cells, the engine of the vasculature, house a set of regulated nanomotors that permit rapid stress-development, sustained stress-maintenance and vessel constriction. Viscoelastic materials within, surrounding and attached to VSM cells, comprised largely of polymeric proteins with complex mechanical characteristics, assist the engine with countering loads imposed by the heart pump, and with control of relengthening after constriction. The complexity of this smart material can be reduced by classical mechanical studies combined with circuit modeling using spring and dashpot elements. Evaluation of the mechanical characteristics of VSM requires a more complete understanding of the mechanics and regulation of its biochemical parts, and ultimately, an understanding of how these parts work together to form the machinery of the vascular tree. Current molecular studies provide detailed mechanical data about single polymeric molecules, revealing viscoelasticity and plasticity at the protein domain level, the unique biological slip-catch bond, and a regulated two-step actomyosin power stroke. At the tissue level, new insight into acutely dynamic stress-strain behavior reveals smooth muscle to exhibit adaptive plasticity. At its core, physiology aims to describe the complex interactions of molecular systems, clarifying structure-function relationships and regulation of biological machines. The intent of this review is to provide a comprehensive presentation of one biomachine, VSM.

Vasomotion dynamics following calcium spiking depend on both cell signalling and limited constriction velocity in rat mesenteric small arteries

Vascular smooth muscle cell contraction depends on intracellular calcium. However, calcium-contraction coupling involves a complex array of intracellular processes. Quantitating the dynamical relation between calcium perturbations and resulting changes in tone may help identifying these processes. We hypothesized that in small arteries accurate quantitation can be achieved during rhythmic vasomotion, and questioned whether these dynamics depend on intracellular signalling or physical vasoconstriction. We studied calcium-constriction dynamics in cannulated and pressurized rat mesenteric small arteries (∼300 μm in diameter). Combined application of tetra-ethyl ammonium (TEA) and BayK8644 induced rhythmicity, consisting of regular and irregular calcium spiking and superposition of spikes. Calcium spikes induced delayed vasomotion cycles. Their dynamic relation could be fitted by a linear second-order model. The dirac impulse response of this model had an amplitude that was strongly reduced with increasing perfusion pressure between 17 and 98 mmHg, while time to peak and relaxation time were the largest at an intermediate pressure (57 mmHg: respectively 0.9 and 2.3 sec). To address to what extent these dynamics reside in intracellular signalling or vasoconstriction, we applied rhythmic increases in pressure counteracting the vasoconstriction. This revealed that calcium-activation coupling became faster when vasoconstriction was counteracted. During such compensation, a calcium impulse response remained that lasted 0.5 sec to peak activation, followed by a 1.0 sec relaxation time, attributable to signalling dynamics. In conclusion, this study demonstrates the feasibility of quantitating calcium-activation dynamics in vasomoting small arteries. These dynamics relate to both intracellular sig-nalling and actual vasoconstriction. Performing such analyses during pharmacological intervention and in genetic models provides a tool for unravelling calcium-contraction coupling in small arteries.

Automated measurement of diameter and contraction waves of cannulated lymphatic microvessels

BACKGROUND

Studies of lymphatic function often employ collecting lymphatic vessels that exhibit large-amplitude, spontaneous contractions. Data from such preparations have been analyzed using cardiac pump analogies that require accurate determination of vascular dimensions, including external (OD) and internal (ID) diameters. These measurements would be facilitated by an accurate automated measurement system.

METHODS AND RESULTS

A computer-based diameter tracking system was developed specifically for lymphatic vessels, with advantages over previous automated systems. The system also permits continuous diameter tracking at two axial locations, enabling the measurement and analysis of contraction wave conduction. The method was validated using spontaneously contracting segments of rat thoracic duct which sometimes exhibited conducted contraction waves. In such preparations, conduction wave velocity was modulated by the axial flow rate and could be easily measured by the tracking system.

CONCLUSIONS

The method offers improvement and increased convenience over manual diameter measurements in lymphatic vessels, with little or no sacrifice in accuracy. It should be a useful tool for general studies of collecting lymphatic function as well as for the analysis of contraction wave conduction and coordination.

A New Biomechanical Perfusion System for ex vivo Study of Small Biological Intact Vessels

The vascular endothelium transduces physical stimuli within the circulation into physiological responses, which influence vascular remodelling and tissue homeostasis. Therefore, a new computerized biomechanical ex vivo perfusion system was developed, in which small intact vessels can be perfused under well-defined biomechanical forces. The system enables monitoring and regulation of vessel lumen diameter, shear stress, mean pressure, variable pulsatile pressure and flow profile, and diastolic reversal flow. Vessel lumen measuring technique is based on detection of the amount of flourescein over a vessel segment. A combination of flow resistances, on/off switches, and capacitances creates a wide range of pulsatile pressures and flow profiles. Accuracy of the diameter measurement was evaluated. The diameters of umbilical arteries were measured and compared with direct ultrasonographic measurement of the vessel diameter. As part of the validation the pulsatile pressure waveform was altered, e.g., in terms of pulse pressure, frequency, diastolic shape, and diastolic reversal flow. In a series of simulation experiments, the hemodynamic homeostasis functions of the system were successfully challenged by generating a wide range of vascular diameters in artificial and intact human vessels. We conclude that the system presented may serve as a methodological and technical platform when performing advanced hemodynamic stimulation protocols.

Relationship between structural remodeling and reactivity in pulmonary resistance arteries from hypertensive piglets

In neonatal pulmonary hypertension, the pulmonary arteries fail to adapt to extrauterine life and remain thick walled. In a previous study on normal neonatal resistance arteries, perfusion myography and confocal microscopy showed that responses to agonist stimulation were related to wall structure. We hypothesized that in hypertensive resistance pulmonary arteries, an enhanced response to contractile and relaxant agonist stimulation would be associated with an increased wall thickness and abnormal postnatal cytoskeletal remodeling of smooth muscle cells (SMC). Pulmonary arteries (110-140 microm external diameter) from normal piglets and those exposed to chronic hypobaric hypoxia from birth or from 3 d of age were mounted on a perfusion myograph. Lumen diameter and SMC nuclear positions were tracked after addition of KCl, the thromboxane mimetic U46619, and bradykinin. After fixation in situ, SMC dimensions were measured using confocal and electron microscopy. In all hypertensive animals, wall thickness and SMC density were increased and SMC length/width ratio decreased. After hypoxic exposure for 3 d, arteries from animals exposed from birth showed a greater and faster contractile response than controls, but arteries from piglets first exposed at 3 d of age did not, though both showed similar structural appearance. Increase of exposure to 11 d elicited an enhanced response and further cytoskeletal remodeling. All vessels relaxed fully to bradykinin. SMC remodeling and reactivity appear to be influenced by the age at onset and the duration of the hypoxic insult.

Organoid culture of cannulated rat resistance arteries: effect of serum factors on vasoactivity and remodeling

We developed an organoid culture technique to study the mechanisms involved in arterial remodeling. Resistance arteries were isolated from rat cremaster muscle and mounted in a pressure myograph at 75 mmHg. Vessels were studied during a 4-day culture period in DMEM with either 2% albumin, 10% heat-inactivated FCS (HI-FCS) or 10% dialyzed HI-FCS (12 kDa cut off) added to the perfusate. The albumin group showed a gradual loss of endothelial function and integrity, whereas smooth muscle agonist and myogenic responses were retained. No remodeling was observed. Vessels cultured in the presence of serum showed a progressive constriction. Smooth muscle responses and substance P-induced endothelium-dependent dilation were maintained. An inward remodeling of 17 +/- 4% in the HI-FCS group and 26 +/- 3% in the dialyzed HI-FCS group was found, while media cross-sectional areas were unchanged. These data show that pressurized resistance arteries can be maintained in culture for several days and undergo eutrophic remodeling in vitro in the presence of high molecular weight serum factors.

Relation between outer and luminal diameter in cannulated arteries

Resistance in blood vessels is directly related to the inner (luminal) diameter (ID). However, ID can be difficult to measure during physiological experiments because of poor transillumination of thick-walled or tightly constricted vessels. We investigated whether the wall cross-sectional area (WCSA) in cannulated arteries is nearly constant, allowing IDs to be calculated from outer diameters (OD) using a single determination of WCSA. With the use of image analysis, OD and ID were directly measured using either transillumination or a fluorescent marker in the lumen. IDs from a variety of vessel types were calculated from WCSA at several reference pressures. Calculated IDs at all of the reference WCSA were within 5% (mean <1%) of the corresponding measured IDs in all vessel types studied, including vessels from heterozygote elastin knockout animals. This was true over a wide range of transmural pressures, during treatment with agonists, and before and after treatment with KCN. In conclusion, WCSA remains virtually constant in cannulated vessels, allowing accurate determination of ID from OD measurement under a variety of experimental conditions.

Myogenic activation and calcium sensitivity of cannulated rat mesenteric small arteries

Pressure-induced activation of vascular smooth muscle may involve electromechanical as well as nonelectromechanical coupling mechanisms. We compared calcium-tone relations of cannulated rat mesenteric small arteries during pressure-induced activation, depolarization (16 to 46 mmol/L K+), and alpha1-adrenergic stimulation (1 micromol/L phenylephrine). The intracellular calcium concentration was expressed as the fura-2 ratio, normalized to the maximal and minimal ratios. In order to compare activation levels at various pressures, tone was expressed as the ratio of active wall tension to the maximal active tension. The passive and maximal active pressure-diameter relations needed for the calculation of tone were determined in a separate set of experiments, using isometric loading of cannulated vessels. Pressure steps from 20 to 60 and then to 100 mm Hg caused a modest rise of calcium. Nifedipine (1 micromol/L) blocked both the calcium rise and the resulting myogenic responses. Electromechanical coupling could not fully account for the myogenic response: the calcium sensitivity, defined as the slope of the calcium-tone relation, was five times higher during pressure-induced activation compared with potassium stimulation and twice as high as the sensitivity during alpha1-adrenergic stimulation. We therefore conclude that the myogenic response involves a small but necessary rise in calcium due to influx through L-type calcium channels, as well as a nonelectromechanical coupling mechanism that greatly enhances the calcium sensitivity of the contractile machinery.

Vasodilatory effect of pulsatile pressure on coronary resistance vessels

Intramyocardial pressure becomes high in systole and decreases in diastole. Therefore, the transmural pressure of the intramyocardial vessels is pulsatile, resulting in the cyclic distension of these vessels. However, the effect of pulsatility on the behavior of the coronary resistance vessels has not been evaluated. To assess the influence of pulsatile pressure on the behavior of the coronary arterioles, we measured the luminal cross-sectional area (CSA) of coronary arterioles under cyclically changing transmural pressure. Isolated porcine coronary arterioles (internal diameter, 100 to 150 microns) were cannulated with two micropipettes and pressurized with square waves (1 Hz) through both pipettes so as not to induce flow-dependent vasodilation. During the presence (active, induced by acetylcholine; n = 7) or absence (passive, abolished by bradykinin; n = 7) of vascular tone, the CSA was measured under the following conditions: (1) The amplitude of the pressure pulse was changed at a fixed mean pressure. (2) The mean pressure was changed at a fixed pressure pulse. With increasing pulse pressure, the mean CSA at steady state increased under active conditions, whereas it decreased under passive conditions (P < .0001). This vasodilatory effect of pulse pressure remained present after endothelial denudation (P < .0001; n = 6 vessels with basal tone, n = 9 vessels with U46619-induced tone). The mean steady state CSA under passive conditions increased with the mean pressure (P < .05), whereas under active conditions it remained constant in the range of mean pressures between 50 and 100 mm Hg, reflecting myogenic responsiveness. These results indicate that an increase in amplitude of the pressure pulse dilates coronary arterioles. The vasodilating effect of the pulsation may compensate partly for the extra compressing effect of cardiac contraction on the intramyocardial vessels.

Differences in sensitivity of rat mesenteric small arteries to agonists when studied as ring preparations or as cannulated preparations

1. Pharmacological experiments on vascular tissue are normally performed on isometric ring or strip preparations. The aim of this study was to compare the isometric characteristics with the characteristics obtained if vessels were examined under the more physiologically appropriate isobaric condition. 2. Rat mesenteric small arteries were mounted either on two steel wires for isometric force measurement (wire-myograph) or cannulated for measurement of the internal diameter under isobaric conditions (pressure-myograph). 3. The passive pressure-diameter characteristics of the small arteries were similar on the wire- and pressure-myograph (using the Laplace relation to convert wall tension-internal circumference data from the wire-myograph to effective pressure-diameter characteristics). 4. In cumulative concentration-response experiments with noradrenaline and phenylephrine, the threshold concentration was 8-10 times lower, and the EC50-concentration was 4-5 times lower, in the pressure myograph compared to the wire-myograph. Thus vessels were not only more sensitive on the pressure myograph, but the slopes of the concentration-response curves were less steep. Similar experiments with vasopressin also showed this difference in the threshold-concentration and slope, but EC50 concentrations were similar. 5. Cumulative concentration-response experiments with K+ showed no difference either in EC50 or in slope on the wire- and pressure-myographs. 6. On the wire-myograph, some vessels were stretched longitudinally (to mimic the longitudinal stretch which had to be used in the pressure-myograph to avoid buckling). Such stretch did not affect the passive characteristics. 7. The differences between the EC50 determined on the wire- and pressure-myographs as regards noradrenaline and phenylephrine were eliminated when neuronal noradrenaline uptake was inhibited by denervation. However, the slope of the concentration-response curves on the wire-myograph was not affected by denervation.8. When vessels were exposed to cocaine (3 MicroM) the noradrenaline concentration-response curves were the same on the wire- and pressure-myographs as regards both EC50 and slope.9. On the wire-myograph, the calcium antagonist, methoxyverapamil, (D600) reduced the maximal contractile effect of noradrenaline by 50%, but on the pressure-myograph D600 did not affect the maximal response.10. The present results show that results obtained from vascular tissue under isometric conditions may differ substantially from the characteristics which would be obtained under isobaric conditions.

Role of wall tension in the vasoconstrictor response of cannulated rat mesenteric small arteries

1. We have studied the influence of mechanical loading conditions on the responses of cannulated rat mesenteric small arteries to noradrenaline, vasopressin and potassium. 2. The cross-sectional area (CSA) of vessels was continuously monitored. Isometric loading (CSA-controlled conditions) or isobaric loading (pressure-controlled conditions) was achieved by feedback adjustment of the distending pressure. 3. Noradrenaline (0.3 microM) and vasopressin (0.05 u l-1) induced myogenic responsiveness, resulting in a constant or declining CSA with increasing pressure. Potassium (32 mM) induced weak myogenic responsiveness. 4. At a constant pressure of 60 cmH2O, noradrenaline and vasopressin concentration-response curves were graded, the concentration-response curves of individual vessels being extended over two to three decades. Sensitivity to the vasoconstrictors, expressed as pD2 values (-log10 EC50), averaged 6.45 +/- 0.18 log M and 1.27 +/- 0.20 log u l-1 for the noradrenaline and vasopressin concentration-response curves respectively. The isobaric pD2 for K+ was 1.54 +/- 0.07 log M. 5. During CSA-controlled conditions, noradrenaline and vasopressin induced all-or-none responses to stretch. Potassium induced graded responses to stretch. 6. During CSA-controlled conditions, noradrenaline and vasopressin concentration-response curves also showed all-or-none behaviour. Almost the full response occurred through only a doubling of the concentration. pD2 values were 6.88 +/- 0.38 log M (noradrenaline) and 1.87 +/- 0.43 log u l-1 (vasopressin). Isometric vessels were significantly more sensitive to noradrenaline and vasopressin than isobaric vessels. Isometric K+ curves were gradual. pD2 was 1.54 +/- 0.07 log M, a value not different from the isobaric value. 7. These findings can be explained by assuming that agonist sensitivity is wall tension dependent, such that sensitivity increases with increasing wall tension. This concept accounts for partial regulation of wall tension during pressure-controlled conditions, as well as instability due to a positive feedback loop of active tension development and tension-induced sensitization during CSA-controlled conditions.

Influence of pressure alterations on tone and vasomotion of isolated mesenteric small arteries of the rat

1. Myogenic responses may account for control of organ blood flow. The study of these responses without interference from the organ requires an isolation technique for vessels which contribute significantly to flow resistance. This study reports on experiments on isolated small mesenteric arteries. 2. Distal rat mesenteric arcade arteries and first-order branches (diameter range 145-365 microns, mean 293 microns) were manually dissected and cannulated using a double-barrelled micro-cannula. Luminal cross-sectional area of these vessels was continuously monitored by means of a fluorescence technique. 3. Nine out of eighteen vessels developed basal tone at 80 mmHg distending pressure, resulting in a 45.2 +/- 5.1% (mean +/- S.E.M) decrease of cross-sectional area. Tone was induced in the other vessels by 0.3-1 microM-noradrenaline, resulting in a 59.5 +/- 7.1% decrease in cross-sectional area. 4. In vessels with either spontaneous or induced tone, stepwise changes of pressure resulted in passive effects, followed by myogenic responses. 5. Steady-state pressure-cross-sectional area relations of vessels with basal tone showed a significant negative slope (-0.5% mmHg-1), while pressure-cross-sectional area relations of vessels with induced tone were essentially flat between 40 and 120 mmHg. 6. Five vessels with basal tone and eight vessels with induced tone developed vasomotion at 80 mmHg. Frequencies of spontaneous and induced vasomotion were 14 (range 4-31) and 21 (9-25) cycles min-1 respectively. Amplitudes were 5 (1-10) and 8 (3-17)% of the passive cross-sectional area. In both groups, frequency was positively, and amplitude negatively correlated with pressure. 7. These data show that myogenic responses are induced by wall stress, rather than by distension of the vascular wall. Basal tone is not a prerequisite for the appearance of myogenic responses.