Dynamics and pathways of 125I-labeled albumin (RISA) outflow from brain to deep cervical lymph have been studied in anesthetized rabbits between 4 and 25 h after microinjection of 1 microliter RISA into the internal capsule or midbrain. Lymph from the jugular lymph trunks was collected for periods of 2-11 h. RISA was cleared from brain with half-times of disappearance from internal capsule and midbrain of 18.2 and 11.9 h, respectively. RISA was distributed in high concentration to subarachnoid arteries that supplied the tissue injection site; this was consistent with RISA drainage from brain via perivascular spaces. Outflow through lymph rose to a maximum value 15-20 h after tracer injection. Mean recovery of RISA from lymph over the 25-h collection period accounted for 22% of total loss from internal capsule and 18% from midbrain. This result compares with mean recoveries from caudate nucleus and cerebrospinal fluid of 47% and 30%, respectively [M.W.B. Bradbury, H.F. Cserr, and R.J. Westrop, Am. J. Physiol. 240 (Renal Fluid Electrolyte Physiol. 9): F329-F336, 1981]. These are minimal estimates of total outflow to lymph because of the 15- to 20-h delay in RISA passage from brain to lymph.

SUMMARY1. Regulation of brain extracellular and intracellular water content, regarded as volume, and electrolytes in response to 90 min of hypernatremia has been studied in the cerebral cortex of rats under urethane anaesthetic.2. Total tissue electrolytes and water were partitioned between extracellular and intracellular compartments based on measurements made in two series of experiments. In one, tissue samples were collected and analysed for total water, Na+, K+ and Cl-. In the other, tissue extracellular volume fraction, [Na+] and [K+] were measured in situ using ion-selective microelectrodes.3. Osmotically induced water loss from cerebral cortex was less than that predicted for ideal osmotic behaviour, revealing a degree of volume regulation, and this regulation was associated with net tissue uptake of Na+, Cl-and K+.4. Total water content was 3'77 g H20 (g dry weight)-' in control cortex and this decreased by 7 % after 30 min of hypernatremia and then remained relatively stable at this value. Control extracellular water content, based on an extracellular volume fraction of 0-18, was 0-88 g H20 (g dry weight)-'. Control intracellular water content, estimated as the difference between total and extracellular water contents, was 2-89 g H20 (g dry weight)-'. After 30 min of hypernatremia, extracellular water content decreased by an average of 27 % but intracellular water did not change. This indicates selective regulation of cell volume. By 90 min the extracellular water content had decreased by 47 % and the loss in extracellular water content appeared to be accompanied by a roughly equivalent increase in intracellular water content. The intracellular volume increase, however, was not statistically significant. The tortuosity of the extracellular space averaged 1-57 and increased to 1-65 during the hypernatremia.

Regulation of brain water and electrolytes during acute hyperosmolal states has been studied in anesthetized rats. Rats were injected intravenously or intraperitoneally with hypertonic NaCl, mannitol, or sucrose (hyperosmolal series) or with isotonic NaCl (isosmolal controls). Terminal plasma osmolality varied from 290 to 385 mosmol/kg and the experimental duration from 15 to 120 min. Osmotically induced water loss from brain tissue for the different protocols was only 26-78% of that predicted for ideal osmotic behavior, revealing a degree of tissue volume regulation, and the brain gained Na, Cl, and K. This gain was sufficient to account quantitatively for tissue volume regulation at 120 min of hypernatremia but not at shorter times or during mannitol- or sucrose-induced hyperosmolality. Water loss and electrolyte uptake occur simultaneously, over 30 min, which limits the degree of brain shrinkage. Results of this analysis of the time course and magnitude of tissue electrolyte gain during acute hyperosmolality form the basis for the following two studies of the volume regulatory influx of electrolyte from plasma and CSF, respectively.

Brain volume is regulated during acute hyperosmolal states based, in part, on the tissue gain of Na, Cl, and K [H. F. Cserr, M. DePasquale, and C.S. Patlak, Am. J. Physiol. 253 (Renal Fluid Electrolyte Physiol. 22): F522-F529, 1987]. This study evaluates the contribution of influx from plasma to the volume regulatory gain of electrolyte. Blood-to-brain transfer constants, K1, were measured as a function of plasma osmolality, over the range 300-385 mosmol/kg, for sodium (22Na) and potassium (42K or 86Rb) and as a marker for nonselective changes in permeability for [14C]mannitol. Osmolality was elevated by intraperitoneal injection of hypertonic NaCl, mannitol, or sucrose. K1 for Na, K, and mannitol each increased linearly with plasma osmolality. Increases in K1 for Na and mannitol were small and were proportional to their respective diffusion coefficients, consistent with the development of a small leak pathway. Increases in K1 for K were much larger, consistent with osmotic stimulation of a selective permeability pathway. Quantitative analysis of the results suggests that uptake from plasma accounts for most of the K gained by brain tissue in response to acute hyperosmolality but for only a small fraction of the Na. This provides indirect evidence for an additional source of Na, presumably from cerebrospinal fluid.

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Brain volume is regulated during acute hyperosmolality based, in part, on the tissue gain of Na, Cl, and K. This study evaluates the contribution of bulk flow of cerebrospinal fluid (CSF) into brain to the volume regulatory gain of electrolytes. Artificial CSF containing radiolabeled albumin and diethylenetriamine penta-acetic acid (DTPA) was perfused for 60 min through the ventricles and/or subarachnoid space of anesthetized rats, and tracer clearances from CSF to brain were measured as a function of plasma osmolality. Osmolality was elevated after 30 min of perfusion by intraperitoneal injection of hypertonic NaCl or mannitol. Albumin and DTPA clearances increased with osmolality at the same rate, despite a sevenfold difference in diffusion coefficient, consistent with osmotic stimulation of a bulk flow component of tracer influx into brain. The volume shift estimated on the basis of this data is 114 microliters CSF/g dry wt brain for a 60-mosmol/kg increase in osmolality. Results indicate that CSF is a major source of the volume regulatory gain of Na and Cl, but not of K.

QT interval prolongation of the electrocardiogram has been associated with the occurrence of life-threatening fatal ventricular arrhythmias. To understand the relationship between preclinical cardiac conduction assessment to clinical outcome, comparisons of free (unbound)-plasma drug concentrations and their associated effects in the conscious mongrel dog were made to the free plasma concentrations in humans reported to produce QT prolongation. E-4031 (an experimental class III antiarrhythmic), cisapride, terfenadine, terodiline, and verapamil all affect cardiac repolarization and can produce QT prolongation in humans. In the conscious dog, the QT interval was assessed on a beat-to-beat basis in relation to each preceding RR interval at concentrations approximating the same unbound human concentrations. E-4031, cisapride and terodiline statistically increased the QT RR1000 interval [the QT interval at a 60 beats/min (bpm) heart rate] 23, 8, and 9 ms, respectively, at concentrations 0.3 to 15.8 times their relevant clinical level. Increases were not observed for terfenadine or verapamil (p Ͼ 0.05 at all doses). Inspection of individual dog QT versus RR interval relationships showed clear QT interval responses specific to each treatment but not readily apparent when data are averaged at a heart rate of 60 bpm. For specific rectifier K ϩ current (IKr) blockers, robust effects on mean QT prolongation can be detected. However, for drugs that affect repolarization through multiple channels, the effect on the mean QT interval may be more difficult to detect. Inspection of the beat-to-beat QT-RR interval relationship in an individual animal can increase the sensitivity for more accurate clinical prediction.In the past four years, several drugs such as sertindole, cisapride, and terfenadine have been withdrawn from the marketplace due to the rare occurrence of the fatal ventricular arrhythmia, Torsades de Pointes (TdP). Retrospective evaluation of the European database on these compounds indicated an association of QT prolongation in patients on these medications (Haverkamp et al., 2000). As a result, the Committee for Proprietary Medicinal Products (1997) issued a Points to Consider document on the conduct of studies for the development of noncardiovascular drugs. This and further impending guidance from the International Committee for Harmonization have focused a great deal of attention on the preclinical assessments used to predict the liability for patient populations to experience QT prolongation. Despite the ongoing controversy concerning whether there is a definitive cause and effect relationship, QT prolongation has by default become a surrogate marker for risk of developing TdP.This study is one of a series of assessments from our laboratories examining the utility of a variety of both in vitro and in vivo preclinical assessments of drugs that are known to produce clinically detectable QT prolongation in humans. Although the magnitude of the QT interval prolongation and the risk associated with TdP are s...