Abstract
BackgroundSeveral studies indicate that hypertension causes major changes in the structure of the vessel wall by affecting the regulation of blood supply to the tissues. Recently, it has been observed that capillary blood flow is also considerably influenced by the structural arrangement of the microvascular networks that undergo rarefaction (reduction of the perfused vessel number). Therefore, this study aimed to assess the geometric arrangements of the pial arteriolar networks and the arteriolar rhythmic diameter changes in spontaneously hypertensive rats (SHRs).MethodsFluorescence microscopy was utilized to observe in vivo the pial microcirculation through a closed cranial window. Pial arterioles were classified according to Strahler’s method. The arteriolar rhythmic diameter changes were evaluated by a generalization short-time Fourier transform.ResultYoung SHRs showed four orders of vessels while the adult ones only three orders. The diameter, length, and branching number obeyed Horton’s law; therefore, the vessels were distributed in a fractal manner. Larger arterioles showed more asymmetrical branches than did the smaller ones in young SHRs, while in adult SHRs smaller vessels presented asymmetrical branchings. In adult SHRs, there was a significant reduction in the cross-sectional area compared with the young SHRs: this implies an increase in peripheral resistance. Young and adult age-matched normotensive rats did not show significant alterations in the geometric arteriolar arrangement with advancing age, both had four orders of arteriolar vessels, and the peripheral resistance did not change significantly. Conversely, the frequency components evaluated in arteriolar rhythmic diameter changes of young and adult SHRs showed significant differences because of a reduction in the frequency components related to endothelial activity detected in adult SHRs.ConclusionIn conclusion, hypertension progressively causes changes in the microarchitecture of the arteriolar networks with a smaller number of vessels and consequent reduced conductivity, characteristic of rarefaction. This was accompanied by a reduction in the formation and release of independent and dependent – endothelial nitric oxide components regulating arterial vasomotion.
Highlights
Cerebral microcirculation has unique features because it is vulnerable under physiopathological conditions, such as hypertension and aging (Hutchins et al, 1996; Levy et al, 2001; Bohlen, 2009)
Young spontaneously hypertensive rats (SHRs) showed four orders of vessels according to diameter, length, and branchings
The classification started from capillaries to which order 0 was assigned; subsequently, to the arteries, which give rise to the capillaries, order 1 was given, and larger orders were gradually given up to order 4, which has the largest vessels we found in our preparations (Table 1 and Figure 1A)
Summary
Cerebral microcirculation has unique features because it is vulnerable under physiopathological conditions, such as hypertension and aging (Hutchins et al, 1996; Levy et al, 2001; Bohlen, 2009). It has been suggested that microvascular networks are depleted of vessels (rarefaction) with consequent reduction of the blood supply to the tissues (Boegehold et al, 1991) This feature is critical because the transport of oxygen and nutrients to the cells is over a physical distance; the system of vessels must distribute blood effectively and efficiently, so that the work of transport is not too expensive in terms of energy or time (Prewitt et al, 1982; Sokolova et al, 1985; Baumbach and Heistad, 1989). This study aimed to assess the geometric arrangements of the pial arteriolar networks and the arteriolar rhythmic diameter changes in spontaneously hypertensive rats (SHRs)
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