The microvasculature in skeletal muscle. VI. Adrenergic innervation of arterioles in normotensive and spontaneously hypertensive rats.

A microanatomical study of the adrenergic nerve plexus on the arterioles in the spinotrapezius muscle of normotensive and spontaneously hypertensive rats was carried out. The spinotrapezius muscle was selected since its microvasculature has been reconstructed in previous studies of this series. A modified glyoxylic acid amine densification technique was used to visualize the major portion of the microvascular nerve plexus. The nerve plexus density was quantified in the form of fiber length per unit area of vascular smooth muscle media. The adrenergic innervation was found to be limited to the arterial/arteriolar side of the microcirculation and positioned in close vicinity to vascular smooth muscle, in line with previous reports. Substantial variations of the nerve plexus density could be detected along the arterioles. Arcade arterioles show a significant reduction of the adrenergic innervation compared to that of the thoracodorsal supply artery. There was a significant elevation of the nerve plexus density at the origin of the transverse arterioles at the arcade arterioles, a site that in the past has been shown to exhibit the highest microvascular tonus in all arterioles of this organ. Distal to this site, transverse arterioles exhibit a progressive reduction of adrenergic plexus density toward their capillary endings, in line with the termination of vascular smooth muscle in these small branches. Sporadic fiber extensions were encountered leading from some of the transverse arterioles into the capillary network per se, but no regular innervation was detected in capillaries or in venules. These results suggest that the transverse arterioles may play a central role in nervous control of blood flow to the capillaries of muscle. Compared with the Wistar and Wistar-Kyoto strain, the spontaneously hypertensive rats exhibit qualitatively a similar pattern, but show quantitatively a significantly higher plexus density in the thoracodorsal artery and the arcade arterioles, a factor that may contribute to the elevated arteriolar tone.