The effect of a low pH saline perfusate upon the integrity of the energy-depleted rat blood-brain barrier

Impact of capillary flow hydrodynamics on carrier-mediated transport of opioid derivatives at the blood-brain barrier, based on pH-dependent Michaelis-Menten and Crone-Renkin analyses

Most studies of blood-brain barrier (BBB) permeability and transport are conducted at a single pH, but more detailed information can be revealed by using multiple pH values. A pH-dependent biophysical model was applied to the mechanistic analysis of published pH-dependent BBB luminal uptake data from three opioid derivatives in rat: pentazocine (Suzuki et al., 2002a, 2002b), naloxone (Suzuki et al., 2010a), and oxycodone (Okura et al., 2008). Two types of data were processed: in situ brain perfusion (ISBP) and brain uptake index (BUI). The published perfusion data were converted to apparent luminal permeability values, P_app, and analyzed by the pCEL-X program (Yusof et al., 2014), using the pH-dependent Crone-Renkin equation (pH-CRE) to determine the impact of cerebrovascular flow on the Michaelis-Menten transport parameters (Avdeef and Sun, 2011). For oxycodone, the ISBP data had been measured at pH7.4 and 8.4. The present analysis indicates a 7-fold lower value of the cerebrovascular flow velocity, F_pf, than that expected in the original study. From the pyrilamine-inhibited data, the flow-corrected passive intrinsic permeability value was determined to be P₀=398×10^-6cm·s^-1. The uptake data indicate that the neutral form of oxycodone is affected by a transporter at pH8.4. The extent of the cation uptake was less certain from the available data. For pentazocine, the brain uptake by the BUI method had been measured at pH5.5, 6.5, and 7.4, in a concentration range 0.1-40mM. Under similar conditions, ISBP data were also available. The pH-CRE determined values of F_pf from both methods were nearly the same, and were smaller than the expected value in the original publication. The transport of the cationic pentazocine was not fully saturated at pH5.5 at 40mM. The transport of the neutral species at pH7.4 appeared to reach saturation at 40mM pentazocine concentration, but not at 12mM. In the case of naloxone, a pH-dependent Michaelis-Menten equation (pH-MME) analysis of the data indicated a smooth sigmoidal transition from a higher capacity uptake process affecting cationic naloxone (pH5.0-7.0) to a lower capacity uptake process affecting the neutral drug (pH8.0-8.5), with cross-over point near pH7.4. Evidently, measurements at multiple pH values can reveal important information about both cerebrovascular flow and BBB transport kinetics.

A new in situ brain perfusion flow correction method for lipophilic drugs based on the pH-dependent Crone-Renkin equation

PURPOSE

To determine the flow-corrected luminal permeability, P(c), of lipophilic drugs measured by the in situ brain perfusion method under circumstances where the traditional Crone-Renkin equation (CRE) method, using diazepam as a flow marker, often fails.

METHODS

The pH-dependent rate of brain penetration of five lipophilic drugs (amitriptyline, atomoxetine, imipramine, indomethacin, maprotiline, sertraline), as well as of atenolol and antipyrine, were measured in Sprague-Dawley rats. A new pH-dependent CRE was derived and applied to remove the hydrodynamic component of effective permeability, P(e), to produce P(c) values.

RESULTS

It was shown by the analysis of the in situ data in the pH 6.5-8.5 interval for the lipophilic bases that the average vascular flow F(pf) = 0.036 mL∙g(-1)∙s(-1), centered in a "flow-limit window" (FLW) bounded by P (e) (min)  = 170 and P (e) (max)  = 776 (10(-6) cm∙s(-1) units). It was shown that the traditional CRE is expected not to work for half of the molecules in the FLW and is expected to underestimate (up to 64-fold) the other half of the molecules.

CONCLUSION

The new pH-CRE flow correction method applied to lipophilic ionizable drugs, based on the pH partition hypothesis, can overcome the limitations of the traditional CRE.

The effect of bile salts on the permeability and ultrastructure of the perfused, energy-depleted, rat blood-brain barrier

The action of bile salts upon the rat blood-brain barrier (BBB) was assessed in the absence of energy-yielding metabolism. Brains were perfused in situ with a Ringer solution for 5 min followed by a 1 min perfusion containing either sodium deoxycholate (DOC), taurochenodeoxycholate (TCDC), or Ringer/DNP. The integrity of the BBB was then determined by perfusing with the radiotracer [14C]mannitol for 2.5 min. Alternatively, the brains were perfusion fixed for ultrastructural assessment. At 0.2 mM DOC, the BBB remained intact and the cerebral ultrastructure was similar to the controls. At 1 mM and above, disruption of the BBB became evident. At 2 mM, the cerebral cortex became severely vacuolated, with damaged endothelium and collapsed capillaries. With TCDC, BBB disruption occurred at 0.2 mM without any apparent ultrastructural damage to the microvasculature. Following 2 mM TCDC, similar, but less widespread, structural changes to the 2 mM DOC-perfused animals was apparent. Opening of the BBB occurred at a concentration lower than that required to cause lysis of either red blood cells or cultured cerebral endothelial cells. It is proposed that the effect of bile salts at concentrations of 1.5 mM and above is largely due to their lytic action as strong detergents on endothelial cell membranes, but that at lower concentrations a more subtle modification of the BBB occurs.

Assessment of drug disposition in the perfused rat brain by statistical moment analysis

Drug disposition in the brain was investigated by statistical moment analysis using an improved in situ brain perfusion technique. The right cerebral hemisphere of the rat was perfused in situ. The drug and inulin were injected into the right internal carotid artery as a rapid bolus and the venous outflow curve at the posterior facial vein was obtained. The infusion rate was adjusted to minimize the flow of perfusion fluid into the left hemisphere. The obtained disposition parameters were characteristics and considered to reflect the physicochemical properties of each drug. Antipyrine showed a small degree of initial uptake. Therefore, its apparent distribution volume (Vi) and apparent intrinsic clearance (CLint,i) were small. Diazepam showed large degrees of both influx and efflux and, thus, a large Vi. Water showed parameters intermediate between those of antipyrine and those of diazepam. Imipramine, desipramine, and propranolol showed a large CLint,i compared with those of the other drugs. The extraction ratio of propranolol significantly decreased with increasing concentrations of unlabeled propranolol in the perfusion fluid. These findings may be explained partly by the tissue binding of these drugs. In conclusion, the present method is useful for studying drug disposition in the brain.

Mechanisms of blood-brain barrier breakdown

The functional status of the blood-brain barrier (BBB) must be taken into account when designing and interpreting brain imaging techniques. The integrity of the BBB is affected in many diseases of the brain, with the potential involvement of a number of different but poorly understood cellular mechanisms. Factors known to disrupt the BBB experimentally include arachidonic acid and the eicosanoids, bradykinin, histamine and free radicals. These active compounds, released in pathological tissue, may alter cytosolic calcium levels and induce second messenger systems leading to an alteration in BBB permeability. Extravasation of plasma proteins may occur via disrupted tight junctions, stimulation of fluid-phase vesicular transport or the formation of transcellular pores or channels.