Using stereological methods in vivo, we have investigated the rarefaction of arterioles and capillaries in male spontaneously hypertensive rats (SHR) and the Wistar-Kyoto controls (WKY) at 6-8, 12-14, and 16-18 wk of age. Under chloralose-urethan anesthesia, the gracilis muscle was isolated for microscopic observation. Vessel length and surface area per unit volume of tissue (density) were determined during three consecutive states: innervation, denervation, and vasodilation with nitroprusside. Arteriolar wall-to-lumen ratio was measured after vasodilation. At 6-8 wk capillary density was reduced in the SHR. At 12-14 wk there was a reduction of arteriole and capillary density under innervated and denervated conditions but not after vasodilation (a state of functional rarefaction). At 16-18 wk there was a reduction of arteriolar and capillary density under all three conditions (a state of anatomical rarefaction). At 12-14 and 16-18 wk there was an elevated level of arteriolar vasoconstriction in the SHR that was masked in any one state by the closure of the smaller arterioles. Arteriolar wall-to-lumen ratio was not elevated in the SHR at any time. Arteriolar closure was not reversed by acute denervation.

We have investigated the neural and local vascular effects on vessel length and surface area per unit volume in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) by quantitative stereology. Fourteen SHR and 14 KWY (70-130 g) were anesthetized with chloralose-urethan, and the cremaster muscle was exposed for microscopic observation. The large arterioles entering the muscle were termed the first order, and consecutive branches were termed second-, third-, and fourth-order arterioles. The data were collected in three consecutive states: innervated, denervated, and vasodilated with nitroprusside. The third- and fourth-order arteriole and capillary lengths per unit volume in the SHR were less than those of the WKY in all three states. The vessel surface area per unit volume was also reduced in the SHR. Denervation and sodium nitroprusside (Nipride) resulted in larger percent increases in vessel length and surface area in the SHR than in the WKY. We conclude that in the SHR cremaster muscle there are fewer terminal arterioles and capillaries anatomically present, and, under resting conditions, a greater percentage but similar absolute number of them are closed to flow.