Modulation of sarcoplasmic reticulum function by Na+/K+ pump inhibitors with different toxicity: digoxin and PST2744 [(E,Z)-3-((2-aminoethoxy)imino)androstane-6,17-dione hydrochloride].
OBJECTIVE
To gain some insight on the lesser arrhythmogenic properties of PST2744 [(E,Z)-3-((2-aminoethoxy)imino)androstane-6,17-dione hydrochloride] compared with digoxin, we compared modulation of intracellular Ca2+ dynamics by the two agents.


METHODS
SERCA (sarcoplasmic reticulum Ca2+-ATPase) activity and Ca2+ leak rate were measured in sarcoplasmic reticulum (SR) vesicles from guinea pig ventricles. Membrane current, intracellular Ca2+, and twitch amplitude were evaluated in guinea pig ventricular myocytes with or without blockade of the Na+/Ca2+ exchanger.


RESULTS
In SR vesicles, PST2744 (30-300 nM), but not digoxin, increased SERCA activity; digoxin only (> or =0.1 nM) increased SR Ca2+ leak. In myocytes with blocked Na+/Ca2+ exchanger, Ca2+ reloading of caffeine-depleted SR was enhanced by PST2744 and slightly inhibited by digoxin. In myocytes with functioning Na+/Ca2+ exchanger, both agents increased diastolic Ca2+, SR Ca2+ content, the gain of Ca2+-induced Ca2+ release, the rate of cytosolic Ca2+ decay, twitch amplitude, and relaxation rate. Consistent with the observations in SR vesicles, the effects on SR Ca2+ content and Ca2+ decay rate were significantly larger for PST2744 than for digoxin.


CONCLUSIONS
In isolated SR vesicles, PST2744 and digoxin directly affected SR function in opposite ways; this could be reproduced in myocytes during Na+/Ca2+ exchanger blockade. Under physiological conditions (functioning Na+/Ca2+ exchanger), the two agents affected Ca2+ dynamics in the same direction, as expected by their Na+/K+ pump inhibition; however, differential SR modulation was still expressed by quantitative differences. Thus, the more favorable inotropy-to-toxicity ratio previously described for PST2744 appears to be associated with direct SERCA stimulation and/or lack of enhancement of Ca2+ leak.
Rocchetti
Marcella
M
Dipartimento di Biotecnologie e Bioscienze, Università degli Studi Milano-Bicocca, P.zza della Scienza 2, 20126 Milano, Italy.
Besana
Alessandra
A
Mostacciuolo
Gaspare
G
Micheletti
Rosella
R
Ferrari
Patrizia
P
Sarkozi
Sandor
S
Szegedi
Csaba
C
Jona
Istvan
I
Zaza
Antonio
A
D000818:Animals; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004077:Digoxin / Q000494:pharmacology* / Q000633:toxicity; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology* / Q000633:toxicity; D005043:Etiocholanolone / Q000031:analogs & derivatives* / Q000494:pharmacology* / Q000633:toxicity; D006168:Guinea Pigs; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009200:Myocardial Contraction / Q000187:drug effects; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000201:enzymology*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors*
D004791:Enzyme Inhibitors; D004077:Digoxin; D005043:Etiocholanolone; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium; C468128:Istaroxime