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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.

A series of arterial micropressure measurements in different skeletal muscles of the Wistar-Kyoto and spontaneously hypertensive rat is presented. The micropunctures were carried out with minimal surgical intervention through small skin incisions and the micropressures were recorded simultaneously with femoral artery pressures. The measurement sites were located at the entry points into the muscles for the proximal and distal supply arteries and at the midpoint of the arteriolar arcade bridge which directly connects these two supply arteries in the center of the muscle parenchyma. In contrast to feed artery pressure values from exteriorized muscles, which in the past have been reported to be as low as 40 mm Hg, the current mean pressure values are substantially higher and in the range between 70 and 100 mm Hg, equivalent to 70 to 90% of the mean systemic pressure. Systolic and diastolic values exhibit comparable trends to the mean pressures and they are similar in muscles at different locations in the body. Although in spontaneously hypertensive rats the absolute pressures were significantly higher compared with their controls, the normalized pressures were virtually identical at the locations used in this study. These data indicate that the absolute pressure in the central arteries of spontaneously hypertensive animals is reduced to a greater degree than in Wistar-Kyoto rats, while in both strains the major pressure reduction in skeletal muscle still occurs in the microcirculation.

A quantitative analysis of blood flow dynamics in skeletal muscle requires a detailed picture of the microvascular network. This report presents an analysis of the arteriolar network structure in the spinotrapezius muscle of the rat. The microvasculature is visualized by injection of a carbon suspension and recorded in the form of photomicrographs with a complete reconstruction of the microvasculature on transparent overlays. The spinotrapezius muscle has several major feeding arterioles which supply blood into an extensive meshwork of interconnecting or arcading arterioles spanning the entire muscle. The connections from the arcade arterioles to the capillaries are provided by transverse arterioles, which branch from the arcades at regular intervals. Each transverse arteriole forms a single asymmetric dichotomous tree and within each muscle there is a wide range in the size of transverse arterioles. A new branching schema is proposed to describe the arteriolar network. A set of network parameters is derived and typical values of these parameters in the spinotrapezius muscle of the rat are provided.

The dual-window photometric analyzer is currently a widely used instrument to estimate volumetric flow rates in microvessels. This instrument provides an analog voltage that is proportional to cell velocity, but the question arises how this voltage is related to the mean velocity in the blood vessel. It requires a separate investigation and in this report a calibration of an improved version of a dual-window photometric analyzer for volumetric flow rate measurements in microvessels as large as 470 microns in diameter is presented. The relationship between correlator velocity and blood mean velocity is presented in a closed empirical form. Within a 5% error of the mean it is found to be dependent on tube diameter but independent of hematocrit (between 3 and 40%). A rotating transparent disc plated with dried red blood cells was used to test the dynamic response of the instrument for frequencies up to 10 Hz. The amplitude reduction was less than 10% up to 3 Hz. Thereafter it fell off at 6 db/octave.

A reconstruction of the capillary network in the rat spinotrapezius muscle was carried out using carbon-filled specimens. A new branching schema for the capillaries, consisting of individual capillary bundles, is proposed. Each bundle is provided with blood through transverse arterioles and drained by collecting venules. A capillary bundle may be divided into capillary bundle elements consisting of a transverse arteriole, a collecting venule, and the intermediate capillary network. A set of seven independent network parameters are proposed and measurements are provided for two rat strains: Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). A comparison of network parameters indicates a multifaceted difference between the strains. In SHR the number of capillary cross-connections is reduced, individual capillary lengths are greater, and vessel diameters are slightly larger.

A quantitative comparison of the anatomical arrangement of arterioles in the skeletal muscle of mature (16 to 20-week-old) Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) is provided. In both species several feeding arterioles supply blood to a network of arterioles covering the entire muscle, designated as arcade arterioles. The connections from the arcade arterioles to the capillary network are provided by the transverse arterioles. Comparison of the spinotrapezius muscle of the WKY and the SHR shows several types of rearrangement of the network. In both species there is a wide distribution of vessel size in the arcade and transverse arterioles. The length of the arcade arterioles per unit muscle volume is higher in the SHR, forming a denser network. There are almost twice as many transverse arteriolar trees per unit tissue volume in hypertensive animals although on the average each transverse tree has shorter branches. No evidence for significant anatomical rarefaction was found among arcade and transverse arterioles. Arcade arterioles following maximal dilation were found to be 25% narrower in the SHR, whereas for transverse arterioles no differences in diameter could be detected under these vasodilated conditions.

A new mechanism of lymph formation associated with pulsation and vasomotion of arterioles is proposed in skeletal muscle. To provide evidence for this proposal, an investigation of the lymphatic vessels in the spinotrapezius muscle of the rat was undertaken. The lymphatics were visualized by microinjection of a contrast medium (Evans blue albumin), and histological sections of the muscle were prepared and examined with the light and the electron microscope. The studies indicated that lymphatics ranging in size from 20 to 200 micron in this skeletal muscle have no smooth muscle but only a thin lining of endothelium. All lymphatics lie in immediate proximity to the arcading and transverse arterioles and occasional muscular venules, and appeared to be wrapped around them. It was further observed that when the arterioles were dilated, the contiguous lymphatics were partially or completely collapsed, whereas lymphatics around contracted arterioles were wide open. No significant deformation of the adjacent skeletal muscle cells was observed. These results suggest that the contraction of vascular smooth muscle, as well as pulsatile elastic distension of arterioles, may cause periodic changes in lymphatic volume and thereby serve as peristaltic mechanisms for lymph transport. Vascular smooth muscle may thus influence not only blood flow in the microcirculation, but also water transport in the interstitium and into the lymphatics.