Novel diazinone derivatives separate myofilament Ca2+ sensitization and phosphodiesterase III inhibitory effects in guinea pig myocardium.

Abstract

The inotropic state of the myocardium can be enhanced via an increase in cell Ca2+ loading or in myofilament responsiveness to Ca2+. Although different pharmacological agents combine these properties, no presently available drug acts predominantly as a myofilament sensitizer in situ. We have investigated the effects and the mechanism of action of novel diazinone derivatives, EMD 54622, EMD 53998, and EMD 54650 (developed by E. Merck, Darmstadt), on guinea pig myocardial preparations. Force- and ATPase-pCa relations in skinned fibers show differing potencies of these agents on myofilament sensitization: EMD 54622 greater than EMD 53998 much greater than EMD 54650. This is in contrast to their relative potencies to inhibit isolated myocardial phosphodiesterase III: EMD 54650 greater than EMD 53998 greater than EMD 54622. In isolated hearts studied at constant coronary flow, each of the three diazinone derivatives had a positive inotropic effect. In enzymatically dissociated left ventricular myocytes loaded with the Ca2+ probe indo-1, the positive inotropic effect of EMD 54622 occurred with no change in the amplitude of the cytosolic [Ca2+] (Cai) transient. In contrast, both EMD 53998 and EMD 54650 enhanced Cai transient and twitch contraction amplitudes. Length-indo-1 fluorescence relations were analyzed to determine the effects of the three substances on myofilament responsiveness to Ca2+. EMD 54622 enhanced and EMD 54650 had no effect on myofilament responsiveness to Ca2+. Less uniform results were obtained with EMD 53998 (in two of five cells the myofilament responsiveness to Ca2+ was increased, whereas in three other cells it was unaltered). Our results indicate that structural changes in the diazinone molecule shift the mechanism of action for the positive inotropic effect of the diazinone derivatives in the intact cell from a predominant myofilament sensitization (EMD 54622) to an enhancement in cell Ca2+ loading and an augmentation in the Cai transient (EMD 54650).

Cardiac profile of EGIS-9377, a novel cardiotonic agent as a Ca2+ sensitizer with bradycardiac activity

“…This is characterized by an increase in the apparent Ca 2+ sensitivity, as demonstrated by a decrease in the pCa 50 value, a decrease in the steepness of the pCa-tension relation, and an increase in maximum Ca 2+ -activated force. In addition, the positive inotropic effect of EGIS-9377 was associated with an increase in the relaxation time of isometric contraction, probably as a consequence of an increase in the duration of attachment of myosin cross-bridges, as reported for those of other Ca 2+ sensitizers (Lee and Allen 1991;Ventura et al 1992). The molecular mechanisms underlying the effect of EGIS-9377 at the level of the regulatory proteins which are responsible for these characteristics are as yet unknown and cannot be derived directly from the present results.…”

Changes in Intracellular Ca2+ Mobilization and Ca2+ Sensitization as Mechanisms of Action of Physiological Interventions and Inotropic Agents in Intact Myocardial Cells.

“…'11,216-221) EMD 53998 has both a Ca 21 sensitizing action and a PDE III-inhibitory action ,21S) and it has been shown that the former action is due to the (+) enantiomer (EMD 57033) and the latter to the (-) enantiomer (EMD 57439) (Table II).111,219,221) In skinned cardiac muscle fibers, the pCa-tension relationship was shifted to the left and maximal tension was increased in the presence of EMD 53998. 215,217,218) In intact myocardial cells, contractile force or shortening was increased without or with only a small increase 11,220,221) or with a decrease 216) in the amplitude of Ca 21 transients. The small increase in peak Ca 21 transients was due mainly to inhibition of PDE III and the subsequent accumulation of cyclic AMP indeed by the (-) enantiomer BM 57439.18,220,22') Neither the (+) enantiomer EMD 57033 nor the (-) enantiomer EMD 57439 affected binding of Ca 2+ ions to troponin C in skinned fibers of canine heart.")…”

Cardiovascular effects of the novel Ca2+‐sensitiser EMD 57033 in pigs at rest and during treadmill exercise

“…EMD 57439, a pure PDE III inhibitor) and (+) EMD 53998 (i.e. EMD 57033), a Ca 2+ sensitiser with minimal PDE III inhibitory actions in vitro (Ventura et al, 1992;Lues et al, 1993). Data on the cardiovascular actions of EMD 57033 in vivo are lacking.…”