1380 Ewo.2 l The effect of bufalin on Na+, K +-ATPase activity and signal transmission in rabbit pulmonary artery Oberfrank, F., Etia, M.E., Wibo, M., Godfraind, T. and Vizi, E.S. Institute of Experimental Medicine, Hungarian Academy of Sciences, H-1450 Budapest, POB 67, Hungary and Universitd Cathoiiquede Louvain, B-1200 Brussels, Belgium An unident~ed mammalian endogenous sodium-pump inhibitor has a putative substantial role in the regulation of cardiovascular function and in several pathophysiological processes, for example hypertension. Bufalin (Sfl, 20[22]- bufadienolide-3fi, 14-dio|), which is prepared from toad skin, has a similar stucture to that of an endogenous ouabain-like compound (OLC), recently prepared and purified from toad by Lichtstein et al. (1986). In this study the effect of bufalin on No,+, K+-ATPase activity and on signal transmission in rabbit pulmonary artery was investigated. Na +, K+-ATPase was prepared from rat brain and kidney using the method previo.sly described (Noel and Godfrained, 1984) with slight modification. The Na +, K+-ATPase activity was measured by continuously recording NADH oxidation by the method described by Godfraind and Tona Lutete (1979). The measurement oi' 3H- noradrenaline release from rabbit pulmonary artery and the contraction of the preparation was as described earlier by Vizi et al. (1984). The parameters of electrical field stimulation were 10 V/s, 2 Hz frequency, 1 ms duration, 360 shocks. Bufalin and ouabain inhibited Na +, K+-ATPase activity prepared from rat brain and kidney in a dose dependent manner. The concentrations which inhibitect the activity of brain enzyme to 50% of the maximal activity (15o) were 0.058 pM for bufalin and 0.27/tM for ouabain. In the case of kidney enzyme the Is0 values were 24.1/tM for bufalin and 97.1/~M for ouabain. Bufalin and ouabain enhanced the electrically evoked release of radioactivity from rabbit pulmonary artery preloaded with 3H-NA in a concentration dependent manner (EDs0 0.7/~M), but only slightly enhanced the resting release of 3H-NA. The isometrically recorded contraction of pulmonary arterial smooth muscle in response to stimulation or to administration of 1-NA (0.1 /~M) was also dose-dependently enhanced by bufalin (EDso 0.75 pM) even in the presence of bufalin prevented the contraction produced by stimulation or by administra- tion of 1-NA. Bufalin administered in a concentration exceeding 7.5 pM, however, caused constriction of the preparation resisting to alphal blockade. These results suggest that bufalin has a direct effect on neuronal elements, but, as well, on non-neuronal tissues, in this respect it is very similar to OLC and ouabain (Vizi et al., 1987; Oberff,'mk et al., 1987, 1988). References Godfraind, T. and Tona Lutete, 1979, Eur. J. Pharm. 60, 329. Lichtstein, D., Kachalsky, S., Deutsch, J., 1986, Life Sci. 38, 1261. Noel, F. and Godfraind, T., 1984, Biochem. Pharm. 33, 47. Oberfrank, F. et al., 1987, Neurosci. 22, $698. Oberfrank, F. et al., 1988, Pol. J. Pharmacol. Pharm. 40, 685. Vizi, E.S., TiSri~k,T. and Magyar, K., 1984, J. Neurochem. 42, 670. Vizi, E.S. et al., 1987, Neuropharm. 26, 1541.