Inotropic and electrophysiological effects of 8-substituted cyclic AMP analogues on guinea-pig papillary muscle

The inotropic potencies of 8-substituted cyclic AMP analogues, applied as sodium salts and in form of benzyl esters, were determined in isolated guinea-pig papillary muscles contracting isometrically at a frequency of 0.2 Hz. Half-maximally effective concentrations, EC₅₀, for the positive inotropic effect of 8-substituted cyclic AMP (sodium salt) increased in the order 8-(4-chloro-phenyl)thiocyclic AMP, 8-tertiary-butyl-thio-cyclic AMP, 8-benzylseleno-cyclic AMP, 8-benzyl-thio-cyclic AMP, 8-methylthio-cyclic AMP, 8-bromo-cyclic AMP. Neutralization of the phosphate hydroxyl residue of 8-substituted cyclic AMP by a benzyl group yielded cyclic AMP benzyl esters (cAMPO-Bn) which were 30 to 100 times more potent than the respective cyclic AMP salts. Cyclic AMP derivatives with a 8-(4-chloro-phenyl)thio- or a 8-tertiary butyl-thio substituent showed comparatively high inotropic potencies. The intrinsic activity was uniformely the same for all 8-substituted cyclic AMP derivatives and equalled that of isoprenaline. As measured by octanol/water partitioning (log P), the increase in lipophilicity of 8-substituted cyclic AMP by esterification with a benzyl group was 7000-fold for 8-bromo-cyclic AMP, 5000-fold for 8-methyl-thio-cyclic AMP, and approximately 1000-fold for the other derivatives. Within the series of benzyl esters, differences in lipophilicity were small. The positive inotropic effect of 8-substituted cyclic AMP analogues was accompanied by a shortening of contraction duration, mainly due to an abbreviation of relaxation time. In contrast to the effects of isoprenaline, 80% of the shortening induced by cyclic AMP analogues occurred already at concentrations which barely increased force of contraction. 8-substituted cyclic AMP analogues produced, in a concentration-dependent fashion, an elevation of the plateau hight of the normal action potential, and increased \(\dot V_{\max } \) , overshoot and duration of slow action potentials elicited at 24 mmol/l K⁺. The study indicates that 8-substituted cyclic AMP analogues produce their positive inotropic effects probably exclusively via stimulation of cyclic AMP-dependent protein kinase. The increased inotropic potency of the benzyl esters as compared to that of 8-substituted cyclic AMP can be explained by their high lipophilicity which enables these compounds to cross the cell membrane more readily. Since 8-substituted cyclic AMP analogues are not degraded by phosphodiesterase, the comparatively high inotropic potency of cyclic AMP analogues with the substituents 8(4-chloro-phenyl)thio and 8-tertiary-butyl-thio was probably due to an enhanced activation of protein kinase.