Effect of heat preconditioning on the uptake and permeability of R123 in brain microvessel endothelial cells during mild heat treatment.

The purpose of this study was to assess the effect of mild heat and heat preconditioning on the uptake and permeability of a P-glycoprotein (P-gp) substrate, rhodamine 123 (R123), in a cell culture model of the blood-brain barrier (BBB). An immediate goal was to determine whether prior mild heat treatment could render brain microvessel endothelial cells more resistant to future heat stress and affect BBB drug permeation by future ultrasound-induced mild heat (USMH) treatment. To address this issue, the expression level of two proteins, P-gp and heat shock protein 70 (Hsp70), and their effects on uptake of R123 and permeability of R123 and [14C]-sucrose in combination with mild heat and P-gp modulator PSC833 during and after mild heat treatment in heat-preconditioned and heat-unconditioned bovine brain microvessel endothelial cell (BBMEC) monolayers were studied. Mild heat caused a significant increase in BBB permeability of R123 and [14C]-sucrose when compared with control and PSC833. Exposure of BBMECs to heat preconditioning caused a slight but insignificant decrease in cellular uptake of R123 both during and immediately after mild heat treatment. Heat preconditioning also caused a slight but insignificant decrease in permeability of R123 and [14C]-sucrose in BBMEC monolayers during mild heat treatment. Because exposure of BBMEC monolayers to mild heat did not affect P-gp expression but slightly affected Hsp70 expression, a heat preconditioning that results in a reinforcement of the BBB other than increased expression of P-gp is suggested. However, heat preconditioning is not sufficient to override the permeation-enhancing effects of mild heat because mild heat caused a significant increase in R123 uptake and permeability of R123 and [14C]-sucrose in both heat-preconditioned and heat-unconditioned cells. Because Hsp70 is known to play a major role in cellular repair and protective mechanisms, our results would imply a relative benign nature of mild heat treatment. Because heating produced by ultrasonic waves can be controlled and localized to a small volume within the tissue, the present results also suggest that USMH could play a pivotal role in the treatment of brain tumors and other brain-related diseases.