Age‐ and pressure‐dependent changes of systemic resistance vessels concerning the relationships between geometric design, wall distensibility, vascular reactivity and smooth muscle sensitivity

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Abstract
The resistance vascular function in normotension, and its alterations in primary hypertension and ordinary aging, was analysed concerning the interactions between (1) geometric vascular design, (2) wall distensibility, (3) transmural pressure and (4) smooth muscle activity. Paired hindquarter perfusions were used, comparing hemodynamic resistance characteristics in young, adult and old normotensive (WKY) and spontaneously hypertensive rats (SHR). From the experimental data pressure‐resistance diagrams were constructed, which quantitatively interrelate the four factors in all SHR‐WKY groups. The diagrams show, over a wide pressure range, how altered smooth muscle activity as well as structural adaptation affect resistance vessel distensibility: likewise how distensibility considerably interferes with “active” resistance adjustments. They also show how markedly the range of active resistance responses during ordinary constant‐pressure perfusion is affected whenever pressure is reset to new levels. Finally, the diagrams illustrate the fundamental hemodynamic difference between hypertensive and normotensive resistance vessels, mainly because of factors 1 and 2, which render the hypertensive vessels stronger, stiffer and hyperreactive. Thereby they are capable of a widened response range despite the higher pressure level, which is further accentuated on acute pressure normalization. In contrast, ordinary aging alters resistance vascular behaviour only little, smooth muscle contractility and sensitivity remaining almost unchanged. However, vascular reactivity is moderately enhanced at lower pressures, presumably a geometric consequence of age‐dependent, intimal‐interstitial endowment.