Distribution of beta-adrenoceptor blocking drugs into cerebrospinal fluid in rats.

The distribution into cerebrospinal fluid (CSF) of five 3H- or 14C-labelled beta-adrenoceptor blocking drugs given intravenously was studied in urethane-anaesthetized rats. The doses (mg/kg) were: alprenolol 5, metoprolol 1, oxprenolol 5, pindolol 0.5, propranolol 0.1 and 5. Within 15 min. a marked fraction of the given radioactivity appeared in CSF with all substances. During the follow-up period of 315 min., only propranolol, independently of dose, caused high CSF/plasma ratios (0.54-0.73) of radioactivity. The other four beta-adrenoceptor blocking drugs caused levels of CSF radioactivity which were 0.05-0.28 of that in plasma and 0.05-0.14 of the calculated levels of activity in the whole body. Since propranolol is bound to plasma proteins to a higher degree than the other drugs studied, the penetration of the unbound fraction of propranolol into rat CSF is even more pronounced than what would be expected on the basis of CSF/plasma ratio measured.