Heterogeneity of pericyte populations in equine skeletal muscle and dermal microvessels: a quantitative study.

The objective of this ultrastructural investigation was to determine if populations of pericytes in equine dermal and skeletal muscle capillaries increase in a head-to-foot direction, as has been reported in human skeletal muscles. Samples of equine microvessels were obtained from the longissimus dorsi skeletal muscle 150 cm. from the ground, from the dermis above this muscle, from the extensor carpi radiali muscle at 55 cm. from the ground, from the dermis adjacent to that muscle, and from dermis 15 cm. from the ground, just above the hoof wall. Tissues were processed for transmission electron microscopy. Electron micrographs were analyzed with a digitizing tablet and computer, to determine the ratios of endothelial cell outer circumference and pericyte inner lengths. Pericytes were separated into two classes; those closest to the endothelial cells were defined as covering capillaries. Those separated from endothelial cells by another layer of pericytes were termed enveloping pericytes. There was much greater coverage and envelopment of dermal capillaries (85% and 135%) than skeletal muscle capillaries (27% and 31%). Regression analysis of the pericyte coverage and envelopment of dermal capillaries revealed a significant increase in pericytes toward the ground. Similarly, the two skeletal muscle tissues differed significantly in their pericyte coverage and envelopment (25/27% at 150 cm., 31/35 at 55 cm.). The data indicate that, as in humans, capillary pericytes are not homogeneously distributed within the same tissues, but are more numerous closer to the ground. Differences in pericyte populations could affect studies of microvessel function.

Limitations of monastral blue as a vascular label: rapid rate of clearance is age-dependent, and interactions with anesthetics depress arterial blood pressure in rats.

Monastral blue (MB) has been described as an inexpensive, nontoxic vascular label. Discrepancies as to its rate of removal from circulation and physiological side effects prompted this study in which retention time of MB in the vascular system and effects of MB upon arterial blood pressure with different anesthetics (halothane, isoflurane, and pentobarbital) were measured in rats. Arterial pressure was monitored during intravenous infusion of MB with or without Evans blue, an albumin label. Localized areas of leakage were created by injecting 30 microL of 10(-4) M histamine into abdominal dermis at -2, 0, 5, 7, 10, and 15 minutes from infusion of MB. Mean arterial pressure decreased by 25-30% after MB infusion when halothane or isoflurane was used, but not with pentobarbital. Sites which leaked at 10 and 15 minutes did not usefully label with MB, although Evans blue-labelled albumin appeared in the interstitium. Younger, lighter rats (125-200 vs. 200-250 gm) retained MB longer in circulation, and had a shorter duration of MB-induced hypotension. Spectrophotometric analysis of rat serum showed rapid elimination of MB from the vascular system, with a half-life of 3.5 +/- 1.9 minutes. While MB remains a useful vascular label, its rapid removal from the circulation and its hypotensive effect must be recognized.