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SUMMARY This study sought to determine if neural influences and/or alterations in arteriolar responses to oxygen could contribute to an elevated microvascular resistance in spontaneously hypertensive rats (SHR). Diameters of third-order arterioles (3A) and fourth-order arterioles (4A) were measured in the cremaster muscle of 12-to 15-week-old SHR and normotensive Wistar-Kyoto (WKY) controls anesthetized with pentobarbital. The preparation was suffused with physiological salt solution (PSS) equilibrated with various concentrations of oxygen (0% O 2 , 5% O 2 , or 10% O 2 ) with and without local neural blockade with Kh 7 g/ml tetrodotoxin (TTX). Total active tone was assessed with 1(H M adenosine. SHR 3A (but not 4A) exhibited a smaller resting diameter than WKY, and larger dilations in response to TTX and adenosine. When suffusion solution PO 2 was elevated in the presence or absence of TTX, SHR arterioles constricted more than did those of WKY, and SHR 4A exhibited a higher incidence of complete closure. Therefore, both neural influences and local vascular control mechanisms may contribute to an elevated microvascular resistance in SHR. (Hypertension 6: 530-535, 1984) KEY WORDS.• microcirculation • myogenic autoregulatory mechanisms metabolic autoregulatory mechanisms • tetrodotoxin • hypertension • oxygen • cremaster muscle

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