Contractile properties of small arterial resistance vessels in spontaneously hypertensive and normotensive rats.

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Abstract
The small arteries play an important functional role in establishing the increased peripheral resistance found in essential hypertension. The direct measurement of the intrinsic mechanical and contractile properties of 2 categories of small arterial resistance vessels in the mesenteric bed of 5 mo. old normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) was investigated. The vessels had mean internal diameters of 246 .mu.m and 153 .mu.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.degree. C in the presence of 5 mM Ca2+) at an internal circumference for which .DELTA.T was approximately maximal, where .DELTA.T = Tactive - Trelaxed. From the average values of .DELTA.T measured it was estimated (on the basis of Laplace''s equation) that the SHR vessel would be 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. The greater contractility found in the SHR vessels may be due to their having a greater smooth muscle cell content. These vessel measurements were 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.