The Na+/Ca2+ exchange blocker SEA0400 fails to enhance cytosolic Ca2+ transient and contractility in canine ventricular cardiomyocytes.
OBJECTIVE
This study was designed to evaluate the effects of the Na(+)/Ca(2+) exchange (NCX) inhibitor SEA0400 on Ca(2+) handling in isolated canine ventricular myocytes.


RESULTS
Intracellular Ca(2+) ([Ca(2+)](i)) transients, induced by either field stimulation or caffeine flush, were monitored using Ca(2+) indicator dyes. [Ca(2+)](i)-dependent modulation of the inhibitory effect of SEA0400 on NCX was characterized by the changes in Ni(2+)-sensitive current in voltage-clamped myocytes. Sarcoplasmic reticulum (SR) Ca(2+) release and uptake were studied in SR membrane vesicles. Gating properties of single-ryanodine receptors were analysed in lipid bilayers. Ca(2+) sensitivity of the contractile machinery was evaluated in chemically skinned myocytes. In myocytes paced at 1 Hz, neither diastolic [Ca(2+)](i) nor the amplitude of [Ca(2+)](i) transients was significantly altered by SEA0400 up to the concentration of 1 microM, which was shown to inhibit the exchange current. The blocking effect of SEA0400 on NCX decreased with increasing [Ca(2+)](i), and it was more pronounced in reverse than in forward mode operation at every [Ca(2+)](i) examined. The rate of decay of the caffeine-induced [Ca(2+)](i) transients was decreased significantly by 1 microM SEA0400; however, this effect was only a fraction of that observed with 10 mM NiCl(2). Neither SR Ca(2+) release and uptake nor cell shortening and Ca(2+) sensitivity of the contractile proteins were influenced by SEA0400.


CONCLUSIONS
The lack of any major SEA0400-induced shift in Ca(2+) transients or contractility of myocytes can well be explained by its limited inhibitory effect on NCX (further attenuated by elevated [Ca(2+)](i) levels) and a concomitant reduction in Ca(2+) influx due to the predominantly reverse mode blockade of NCX and suppression of L-type Ca(2+) current.
Birinyi
Péter
P
Department of Physiology, University of Debrecen, H-4012 Debrecen, PO Box 22, Hungary.
Tóth
András
A
Jóna
István
I
Acsai
Károly
K
Almássy
János
J
Nagy
Norbert
N
Prorok
János
J
Gherasim
Iuliana
I
Papp
Zoltán
Z
Hertelendi
Zita
Z
Szentandrássy
Norbert
N
Bányász
Tamás
T
Fülöp
Ferenc
F
Papp
Julius Gy
JG
Varró
András
A
Nánási
Péter P
PP
Magyar
János
J
D000814:Aniline Compounds / Q000494:pharmacology*; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects*; D002304:Cardiac Pacing, Artificial; D048429:Cell Size / Q000187:drug effects; D003600:Cytosol / Q000378:metabolism; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D005260:Female; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D008297:Male; D008564:Membrane Potentials; D009200:Myocardial Contraction / Q000187:drug effects*; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010647:Phenyl Ethers / Q000494:pharmacology*; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013997:Time Factors
D000814:Aniline Compounds; D020746:Calcium Channels, L-Type; D010647:Phenyl Ethers; D019837:Ryanodine Receptor Calcium Release Channel; C430918:SEA 0400; D019831:Sodium-Calcium Exchanger; D002110:Caffeine