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.