PHOTOLYSIS OF THE NOVEL INOTROPES EMD-57033 AND EMD-57439 - EVIDENCE THAT CA2+ SENSITIZATION AND PHOSPHODIESTERASE INHIBITION DEPEND UPON THE SAME ENANTIOMERIC SITE

1 We studied the effects of flash photolysis on the novel enantiomeric cardiac inotropes EMD 57033 (a calcium sensitizer) and EMD 57439 (a p hosphodiesterase III inhibitor) in rat isolated ventricular trabeculae . 2 In skinned trabeculae, EMD 57439 had no effect on force production , consistent with lack of an active cyclic AMP system in this preparat ion. In contrast, EMD 57033 potentiated force at partial and maximal a ctivation. A single flash of near u.v. light given at partial activati on (30-70%) reduced force potentiation by 52.4 +/- 5.2%. No effect was produced by flashes in the presence of EMD 57439 or in the absence of either drug. 3 The time course of relaxation induced by EMD 57033 pho tolysis was indistinguishable from that obtained on deactivating the m uscle by rapidly lowering Ca2+ using photolysis of the caged chelator of calcium, diazo-2. 4 In intact, twitching trabeculae, EMD 57033 incr eased diastolic and peak force and slowed relaxation. These effects we re simultaneously reduced by a light flash. In these muscles EMD 57439 reduced force, without affecting the twitch time course. These effect s were also reduced by a light flash. 5 The u.v. absorbance spectra of EMD 57033 and EMD 57439 were identical. After photolysis optical dens ity decreased substantially and the peak shifted from 320 nm to 280 nm . 6 The proton n.m.r. spectra of these compounds were identical. The m ain change post-photolysis was a decrease in the proton signal associa ted with the enantiomeric carbon atom. 7 This novel manipulation of th e molecular structure of EMD 57033 and EMD 57439 within an experiment thus provides direct evidence linking calcium sensitization to a parti cular molecular structure. The three main effects of the sensitizer on the twitch were simultaneously abolished and may be mechanistically l inked. Flash photolysis may be a useful tool for further investigation s of the actions of these compounds. In particular, flash photolysis o f the sensitizer represents a novel method of rapidly deactivating car diac muscle.