SUMMARYThe small arteries play an important functional role in establishing the increased peripheral resistance found in essential hypertension. This paper concerns the direct measurement of the intrinsic mechanical and contractile properites of two categories of small arterial resistance vessels in the mesenteric bed of 5-month-old normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The vessels had mean internal diameters of 246 fim and 153 /*m when relaxed at 100 mm Hg effective transmural pressure. Segments (1 mm) were mounted in such a way that internal circumference could be controlled and the circumferential wall tension (T) measured. After mounting, each vessel was maximally activated (by K* depolarization at 37°C in the presence of 5 mM Ca 2+ ) at an internal circumference for which AT was approximately maximal, where AT = T actlve -T relaxed . From the average values of AT measured we have estimated (on the basis of Laplace's equation) that the SHR vessel would average values of AT measured we have estimated (on the basis of Laplace's equation) that the SHR vessels would have been able to contract against 34% greater pressures than the WKY vessels (P < 0.001). Optical measurements of the dimensions of these vessels showed a 23% greater wall thickness in the SHR vessels (P < 0.02). There were no significant differences in the calculated active wall stresses of the SHR and WKY vessels; this suggests that the greater contractility found in the SHR vessels may be due to their having a greater smooth muscle cell content. These vessel measurements have been examined as well as the rats' blood pressures and heart to body weight ratios. The comparison points to the possibility that the disturbance to the cardiovascular regulatory system which results in hypertension produces similar cellular responses in both the myocardium and the peripheral vasculature.HUMAN essential hypertension is associated with an elevated blood pressure but a normal cardiac output.1 The total peripheral resistance is therefore elevated. The cause of this elevation is not clear but a genetic factor seems to be involved.2 -3 As a means to investigate genetic hypertension in detail, a strain of spontaneously hypertensive rats (SHR) has been developed by Okamoto and his colleagues 4 from a colony of Wistar-Kyoto rats (WKY). These SHR reach a stage of established hypertension by the age of 5 months. At this time their systolic blood pressure is about 50% higher than that of their WKY controls although, as in human essential hypertension, their cardiac output is normal. 5The vascular bed of these rats contains vessels with diameters ranging from 3 mm in the aorta to about 7 ^m in the capillaries. Although all these vessels must contribute to the resistance of the vascular bed to some extent, it is in general the smaller arterial resistance vessels which present the greatest resistance, and which are most involved in regulating blood flow and capillary pressure. Technical difficulties have hitherto prevented direct...
Myogenic properties of posterior cerebral arteries from normotensive and hypertensive rats were analyzed in vitro and quantified in terms of both pressure range limits and degree of myogenic activity. Spontaneously hypertensive rat (SHR) vessels were significantly narrower in a fully relaxed state, and both wall thickness and wall-to-radius ratios were increased. After equilibration in 1.6 mM calcium physiological saline solution a substantial tone developed which resulted in average diameter decreases of 34 and 37% in Wistar-Kyoto (WKY) and SHR, respectively; average lumen diameters were approximately 125 micron. Rapid changes in transmural pressure (delta P 10-25 mmHg/s) were applied and diameter responses measured continuously. Myogenic responses began 1-3 s after a change in transmural pressure, and arteries regained their initial diameters after a pressure step in about 2 min; a final, steady-state diameter was achieved in 4-5 min. Myogenic pressure ranges were 49-145 mmHg in WKY and 64-181 in SHR; when responses were segregated according to positive and negative pressure steps, more myogenic responses were observed at lower pressures for pressure step decreases when compared with pressure step increases. Thus myogenic ranges for increasing pressure steps were 71-151 (WKY) and 72-188 mmHg (SHR) and for decreasing steps 45-117 (WKY) and 57-148 mmHg (SHR). Myogenic responses in SHR were weaker than in WKY rats: the former maintained essentially a constant diameter over a wide range of pressures, whereas arteries from the latter decreased diameter with increasing pressures.(ABSTRACT TRUNCATED AT 250 WORDS)
The muscular resistance arteries of the mesentery and brain serve two different control functions in the cardiovascular system. The former are representative vessels of vascular beds that influence total peripheral resistance and blood pressure; the latter are a good model of vessels in beds that demonstrate blood flow autoregulation. Our purpose was to develop a versatile myographic system appropriate for the in vitro study of 75-250 micron diameter vessels and to explore different physiological properties of cerebral and mesenteric arteries. In this paper the system is described in detail, examples of its use in determining the dynamic responses of the vessels to electrical stimulation are provided, and certain measures indicative of the extent of myogenic behavior are characterized. Cylindrical artery segments about 3-mm long were dissected from Wistar-Kyoto rats and mounted in a chamber filled with physiological saline solution maintained at 37 degrees C. The same solution was perfused via a syringe into one end of the vessel through a microcannula. The other end was then occluded so that experiments could be made over a wide range of transmural pressures without flow. The vessel was viewed through a microscope coupled with a TV camera, and the video output signal of a selected scan line was processed by an electronic dimension analyzing system. This permitted simultaneous digital presentation and analog voltage outputs of the vessel wall thicknesses and lumen diameter. We further incorporated servo control of the syringe using a motor drive. In this way, vessel tests could be carried out at constant pressure or constant diameter, and vessel responses could be obtained following either pressure or diameter command signals. Using the methods presented in this study, small vessels can be maintained under conditions that approximate their in vivo state more closely than other in vitro techniques using ring segments on wires. We also find that the opto-electronic instrumentation is ideally suited for studying the dynamic vessel properties that underlie the control of vascular smooth muscle.
Cerebral blood flow is regulated by brain metabolism, and there is evidence to suggest that changes in extracellular potassium concentration are important in linking brain metabolic activity with blood supply. In this study, the effect of low concentrations of potassium on the spontaneous tone of resistance-sized isolated posterior cerebral arteries from Wistar-Kyoto rats was examined. At a transmural pressure of approximately 58 mmHg, the vessels developed spontaneous tone that was 69 +/- 2% of their fully relaxed diameter of 184 +/- 2 microns (n = 50). Introduction of potassium (less than 5 mM) after a 5-min period in potassium-free physiological saline solution resulted in transient dilations, which were not attenuated by barium or cesium but abolished by ouabain. However, potassium concentrations between 7 and 15 mM produced dilations that lacked a transient component and were sensitive to barium, cesium, and ouabain. Maintained dilations to 10 mM K+ persisted in tetrodotoxin, tetraethylammonium, and glibenclamide and after endothelium removal. These results suggest that potassium dilation of cerebral arteries has two independent components, the first of which may be caused by stimulation of the electrogenic sodium pump (0-5 mM K+), whereas the second (greater than 7 mM K+) results from activation of a ouabain-, barium-, and cesium-sensitive process. The latter process describes a means by which potassium may effect prolonged changes in cerebral blood flow.
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