A developmental analysis of differences in the uptake of [123I]isopropyliodoamphetamine versus 99mTc-pertechnetate by the choroid plexus and brain.

The uptake of intravascular [123I]isopropyliodoamphetamine (IMP) and 99mTc-pertechnetate into choroid plexus (CP) and brain (frontal cortex) was studied by an indicator fractionation method applied to immature, ketamine-anesthetized Sprague-Dawley rats (1.5, 2, and 3 wk). Assessment of the rate and extent of uptake of these indicators provides functional information (eg blood flow; transport) about various regions of the developing CNS. IMP uptake by lateral ventricle CP was 1.15 ml/g/min in 1.5-wk-old infant rats and gradually increased to 3.9 mg/g/min by adulthood (7-8 wk) (P < 0.05); over the same postnatal period, 99mTc uptake went from 2.82 to 3.18 ml/g/min. IMP uptake by cortex was 0.39 and 0.99 ml/g/min in infants and adults, respectively (P < 0.05); however, 99mTc uptake by cortex was only 0.07 +/- 0.01 ml/g/min at all ages, reflecting early development of blood-brain barrier (BBB) to pertechnetate. Overall, our findings indicated a progressive increase with age in the rate of uptake of IMP by CP and brain; and that 99mTc penetration into CP was relatively constant and substantially greater than into cortex at all developmental stages. Thus the nature of uptake of IMP, relative to 99mTc, was markedly different at the blood-cerebrospinal fluid barrier (i.e., CP) vs. the blood-brain barrier.

Microdialysis analysis of effects of loop diuretics and acetazolamide on chloride transport from blood to CSF.

With the hypothesis that the NaCl cotransporter in mammalian choroid plexus (CP) has a role in CSF formation, we postulated that loop diuretic agents would curtail transport of Cl from blood to CSF. Microdialysis in the cisterna magna of Sprague-Dawley rats was used to assess the ability of furosemide and ethacrynic acid (i.e. loop agents that interfere directly with cotransport) to inhibit 36Cl transport from blood to CSF over a 3-h period. Cl uptake by CSF was quantified as % volume of distribution (Vd) of 36Cl, i.e. 100 x cpm/g CSF divided by cpm/ml plasma. Uptake curves of Vd vs. time were constructed for the various treatments; then, to compare drug effects, the curves were analyzed for: (i) the early slope of uptake (Kin), (ii) the steady-state value for Vd, and (iii) the area-under-curve (AUC). Assessment of the curve parameters collectively revealed that at 5 mg/kg, both furosemide (FUR) and ethacrynic acid (EA) reduced Cl penetration into CSF by one quarter; at 50 mg/kg, these loop agents decreased Cl uptake by about a third. On the other hand, 50 mg/kg of the carbonic anhydrase inhibitor, acetazolamide, reduced Cl uptake into CSF by 55-60%. Thus, NaCl cotransport inhibitors maximally reduced Cl transport in the rat by about 35%; this inhibition was less extensive than that brought about by acetazolamide, which interferes with CSF secretion by a different mechanism.

Maturational differences in acetazolamide-altered pH and HCO3 of choroid plexus, cerebrospinal fluid, and brain.

The carbonic anhydrase inhibitor acetazolamide is useful for analyzing ion transport, pH regulation, and fluid formation in developing central nervous system. We used the 14C-labeled dimethadione technique to measure alterations in steady-state pH, and to estimate the HCO3 concentration [HCO3], in choroid plexus (CP), cerebrospinal fluid (CSF), and cerebral cortex of 1- and 3-wk-old Sprague-Dawley rats treated with acetazolamide or probenecid. These drugs can suppress transport of HCO3 and other anions in some cells, consequently altering intracellular pH. In 1-wk-old infant rats whose CSF secretory process is incompletely developed, 1 h of acetazolamide treatment did not significantly change CP intracellular pH or [HCO3]. However, in 3-wk-old rats, in which the ability of CP to secrete ions and fluids is almost fully developed, acetazolamide caused marked increases in CP cell intracellular pH and [HCO3]. In contrast, acetazolamide-induced alkalinization was not observed in CSF or cerebral cortex of the 1- and 3-wk-old animals. The other test agent, probenecid (an inhibitor of anion transport but not of carbonic anhydrase), did not alter the pH of any region at any age investigated. Overall, the results are interpreted in light of developmental changes in carbonic anhydrase and previous findings from kinetic analyses of ion-translocating systems in CP. Acetazolamide may interfere with a CP apical membrane HCO3 extrusion mechanism not fully operational in infant rats.