Carotid artery injection technique: bounds for bolus mixing by plasma and by brain.

Estimation of Michaelis-Menten kinetic parameters (Km, Vmax) of blood-brain barrier (BBB) transport processes with the carotid artery single injection technique assumes that mixing of the bolus with unlabeled substrate either from (a) circulating plasma or (b) amino acid efflux from brain, is minimal. The maximum extent to which the bolus could mix by these two sources is quantified in the present studies by measuring 14C-phenylalanine extraction in pentobarbital-anesthetized and conscious rats after the addition of 0-80% rat serum to the arterial injection solution. An upper bound (+/- SE) of bolus mixing due to mixing from both sources, expressed in terms of percentage of rat plasma, is 8.8 +/- 1.9 and 7.0 +/- 2.1% for the anesthetized and conscious rat, respectively. The estimated contribution to bolus mixing due to amino acid efflux from brain is 3.3 and 2.1% for the anesthetized and conscious rat, respectively. Based on these estimates, the upper bound for bolus mixing with circulating rat plasma is only 5.5 and 4.9%, respectively, for the anesthetized and conscious catheterized rat. Thus, any bolus mixing after rapid carotid injection is relatively small and is comparable to the mixing effects observed with the carotid artery infusion technique. Mixing effects on the order of 5% are shown to have no significant effect on the estimation of kinetic parameters of BBB nutrient transport, except for neutral and basic amino acid transport, which are characterized by very low Km values relative to the usual amino acid plasma concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)