Direct inotropic action of WB-4101 and prazosin in the guinea pig papillary muscle. Comparison with HOE-234, the new activator of ATP-sensitive K+ channels.

1. The alpha-1a adrenoceptor antagonist WB-4101, the alpha-1 adrenoceptor antagonist prazosin and the new activator of ATP-sensitive K+ channels HOE-234 were examined. The force of contraction and the rate of rise of contraction force were measured. 2. WB-4101 and HOE-234 were found to produce negative inotropic action in a dose dependent manner. This effect was attenuated by glibenclamide. On the other hand prazosin increased slightly the force of contraction and the rate of rise of contraction force in the same tissue. 3. The direct inotropic effects of above mentioned substances are compared and discussed.