ACTIVE AND PASSIVE ARTERIOLAR REGULATION IN SPONTANEOUSLY HYPERTENSIVE RATS

This study determined to what extent active and passive wall tensions increase in in vivo intestinal arterioles of 13- to 15-week-old and 25 - to 27-week-old spontaneously hypertensive rats (SHR) to maintain nor mal or smaller arteriolar diameters during microvascular hypertension. Acetylcholine and nitroprusside were used to determine whether vascul ar muscle relaxation to endothelium-derived relaxing factor or cyclic GMP is impaired. Large arterioles of hypertensive rats have passive te nsion-circumference relations that are steeper and shifted to the left compared with those of age-matched controls; passive resistance to di stension limits vasodilation in hypertensive rats except at their natu rally elevated arteriolar pressure. Passive tension contributes approx imately 30% of the total resting tension in arterioles of hypertensive and normotensive rats because a greater passive tension occurs at the 20% to 25% constricted resting diameter in hypertensive rats. Absolut e and relative changes in the diameter of SHR arterioles during acetyl choline and nitroprusside application were equal to or greater than th ose in Wistar-Kyoto rats. However, reduction in active tension was sup pressed in older SHR and remained approximately 50% higher than that f ound in older Wistar-Kyoto rats during drug application. Vasoconstrict ion and increased passive resistance to distension of the arteriolar w all diminish the active tension required to maintain normal or smaller resting diameters against microvascular hypertension. However, the el evated microvascular pressure in hypertensive rats is required to allo w near-normal dilation to compensate for their increased passive resis tance to stretch and decreased ability to relax active tension through cyclic GMP mechanisms.