Structurally reduced distensibility of cardiovascular ‘low-pressure’ compartments in primary hypertension,as studied in spontaneously hypertensive rats (SHR)

Adult male spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) were compared to explore to what extent venous ‘unstressed’ volume, compliance and wall distensibility are structurally altered in primary hypertension. The perfused, maximally vasodilated hindquarters and the entire, completely relaxed cardiovascular system during cardiac arrest were used for comparisons of ‘initial’ volumes and pressure-volume characteristics of the respective low-pressure compartments. In both preparations SHR and WKY showed identical ‘unstressed’ venous volumes, computed by extrapolation to zero pressure from initial volumes and the nearly linear pressure-volume relationships, while venous compliance (Δ/ΔP) was in each case about 20% reduced in SHR. Consequently, the structurally determined wall distensibility of the low-pressure compartment, calculated as the square root of volume compliance/unstressed (or initial) volume, was significantly reduced in SHR; about 10%. Such venous ‘structural resetting’ has important hemodynamic consequences, not least because it reinforces increase of venous return and cardiac filling pressure in SHR, caused by given sympathetic activations. Evidently, not only resistance, cardiac and barostat functions but also the venous capacitance function are structurally reset early in primary hypertension, implying a redesign of the entire cardiovascular system to operate at a higher pressure equilibrium.