MEASUREMENT OF PLASMA-VOLUME IN RATS WITH USE OF FLUORESCENT-LABELED ALBUMIN MOLECULES

We describe a method for measuring plasma volume (PV) in small animals that allows small sample sizes but does not require the use of radioi sotopes and thus is a convenient approach for making repeated measurem ents. Texas Red covalently bound to albumin (TR-A) was used in a typic al indicator-dilution technique to measure PV. The relative fluorescen t intensity of TR-A is linear to its concentration (up to 0.15 mg/ml) at an excitation lambda of 590 nm and an emission lambda of 610 nm. Ca theters were inserted through the right jugular vein of anesthetized r ats and threaded into the vena cava. A 0.5-ml control blood sample was taken, a measure quantity of TR-A was injected, and the catheter was flushed with saline. A 0.5-ml postinjection sample was taken 5 min aft er TR-A injection. PV was calculated by comparing the difference betwe en the relative fluorescent intensity of control and postinjection pla sma samples to a standard. The PV of 22 rats [362 +/- 14 (SE) g] was 1 4.1 +/- 0.4 ml (39.6 +/- 0.9 ml/kg body wt) measured by the TR-A metho d and 12.8 +/- 0.4 ml (35.9 +/- 1.0 ml/kg body wt) measured by a stand ard radioiodinated albumin method. There was a strong correlation betw een PV measured by both methods in the same rat (r = 0.90, P < 0.01). Infusion experiments indicated that the TR-A method can detect acute c hanges in PV, and repeated measurements of PV made on a chronically in strumented rat demonstrated that the method can reliably measure PV on consecutive days.