THE EFFECTS OF DEXAMETHASONE ON TRANSCAPILLARY TRANSPORT IN EXPERIMENTAL BRAIN-TUMORS .2. CANINE BRAIN-TUMORS

We studied the effect of dexamethasone on transcapillary transport in ten Avian Sarcoma Virus (ASV)induced canine brain tumors, before and o ne week after administration of dexamethasone, 2.5 mg/kg/day. A comput ed tomographic (CT) method was used to measure regional values of K-1 (blood-to-tissue transfer constant), k(2) (tissue-to-blood efflux cons tant), and V-p (tissue plasma vascular space) of meglumine iothalamate (Conray-60(TM)); the values were reconstructed for each 0.8x0.8x5 mm volume element of the CT data. For all tumors considered together, the re was a decrease in the whole tumor K-1 value of meglumine iothalamat e from 26+/-2.2 (SE) before dexamethasone to 24+/-2.9 mu l/g/min after dexamethasone. V-p decreased from 7.2+/-0.7 to 6.7+/-0.9 ml/100 g, an d the size of the tumor extracellular space (V-e) decreased from 0.30 to 0.26 ml/g. These changes were not statistically significant. Howeve r, when each tumor was used as its own control, K-1 significantly decr eased after dexamethasone in four tumors, significantly increased in t wo and was unchanged in four. These results suggest that decreased blo od-to-tissue transport may be one mechanism underlying resolution of t umor associated cerebral edema in some brain tumors and that the effec ts of dexamethasone on blood-to-tissue transport in brain tumors are v ariable from one tumor to the next. Decreased 'permeability' may not b e the sole mechanism by which dexamethasone reduces tumor-associated c erebral edema.