Enantiomeric dissection of the effects of the inotropic agent, EMD 53998, in single cardiac myocytes.
The effects of the thiadiazinone derivative, 5-[1-(3,4-dimethoxybenzoyl)-1,2,3,4-tetrahydrochinolin-6-yl]-6-met hyl-3,6- dihydro-2H-1,3,4-thiadiazin-2-on (EMD 53998), and of its (+)EMD 57033 and (-)EMD 57439 enantiomers, were tested on the contractile properties and cytosolic [Ca2+] ([Ca2+]i) transients of single intact guinea pig cardiac myocytes. Cells were loaded with the ester form of the fluorescent probe, indo-1, and bathed in a N-2-hydroxyethyl-piperazine-N'-2-ethanesulfonic acid-buffered solution at 25 degrees C (1 mM of CaCl2, 1 Hz stimulation rate). All three substances exerted a pronounced increase in twitch amplitude: the maximal effect of the racemate (380% of control value) was approximately the sum of the effects of its two enantiomers (186 and 236% of control value for the (+)- and (-)-enantiomer, respectively). The [Ca2+]i transient, measured as the 410-to-490 nm indo-1 fluorescence ratio transient after excitation, was increased by the racemate and its (-)-enantiomer (172 and 152% of control value, respectively), but was not increased by the (+)-enantiomer. The racemate and the (-)-enantiomer, but not the (+)-enantiomer, markedly reduced the contraction duration and [Ca2+]i transient duration. In unstimulated cells resting length was significantly reduced by the (+)-enantiomer, and this was accompanied by a decrease in indo-1 fluorescence; the (-)-enantiomer had no effect on either parameter. In the presence of 2,3 butanedione monoxime (BDM), which markedly reduces twitch amplitude by inhibiting cross-bridge mechanics, addition of the (+)-enantiomer restored the twitch contraction to above the pre-BDM level without augmenting the [Ca2+]i transient. In contrast, the (-)-enantiomer failed to reverse the BDM-induced contractile depression, even though it caused a significant increase of the [Ca2+]i transient. Thus, in intact cells the positive inotropic effect of EMD 53998 is due to specific properties of its enantiomers: the (-)-enantiomer has adenosine 3',5'-cyclic monophosphate-like effects (increase in amplitude and reduction of [Ca2+]i transient and contraction durations), whereas the (+)-enantiomer enhances the myofilament-Ca2+ interaction.
Gambassi
G
G
Laboratory of Cardiovascular Science, Gerontology Research Center, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224.
Capogrossi
M C
MC
Klockow
M
M
Lakatta
E G
EG
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002316:Cardiotonic Agents / Q000494:pharmacology*; D000242:Cyclic AMP / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005456:Fluorescent Dyes; D006168:Guinea Pigs; D006863:Hydrogen-Ion Concentration; D007211:Indoles; D009200:Myocardial Contraction / Q000187:drug effects*; D011804:Quinolines / Q000008:administration & dosage / Q000737:chemistry / Q000494:pharmacology*; D013237:Stereoisomerism; D013268:Stimulation, Chemical; D013329:Structure-Activity Relationship; D013829:Thiadiazines / Q000008:administration & dosage / Q000737:chemistry / Q000494:pharmacology*
D002316:Cardiotonic Agents; D005456:Fluorescent Dyes; D007211:Indoles; D011804:Quinolines; D013829:Thiadiazines; C070609:EMD 53998; D000242:Cyclic AMP; C048960:indo-1; D002118:Calcium