Transport, uptake, and metabolism of blood-borne vasopressin by the blood-brain barrier

Transport, binding, and metabolism of phenylalanyl-3,4,5-³H(N)]arginine vasopressin (AVP) by the blood-brain barrier (BBB) was studied in adult guinea-pigs by means of a novel vascular brain perfusion (VBP)/capillary depletion technique and HPLC. A time-dependent, progressive brain uptake of ³H-radioactivity was measured over the 10 min period of VBP both in brain homogenates and in brain tissue depleted of cerebral microvessels. The unidirectional blood-to-brain transport constant, K_IN, estimated by multiple-time tissue uptake analysis of the homogenate and postcapillary supernatant, indicated that the BBB transfer rat ffor ³H]AVP (K_IN = 2.37±0.25 μl min⁻¹ per gram brain homogenate) was almost 10 times higher than for simultaneously perfused ¹⁴C]sucrose, a cerebrosvascular space marker. In contrast to homogenate and postcapillary supernatant, the ³H]radioactivity determined in the vascular pellet after dextran density centrifugation of the brain homogenate was very low and only somewhat higher than for ¹⁴C]sucrose. HPLC analysis of the perfused brain tissue revealed time-dependent degradation of the blood-borne neuropeptide. The percentage of intact ³H]AVP as determined in the postcapillary supernatant progressively declined during brain perfusion, from 49% at 1 min to 11.9% at 10 min. The major detectable labeled metabolite was ³H]phenylalanine, the labeled amino acid residue of ³H]AVP. The aminopeptidase inhibitor bestatin (0.5 mM), perfused simultaneously with ³H]AVP by the VBP technique, did not alter tissue uptake of ³H]AVP, indicating that there was no significant hydrolysis of peptide by the luminal BBB surface. The results suggest that rapid in vivo metabolism of AVP occurs after BBB transport in the brain parenchyma with no evidence of significant capillary sequestration, or degradation of AVP by the BBB.