A significant fraction of calcium transients in intact guinea pig ventricular myocytes is mediated by Na(+)-Ca2+ exchange.
Ca2+ mobilization elicited by simulation with brief pulses of high K+ were monitored with confocal laser scanned microscopy in intact, guinea pig cardiac myocytes loaded with the calcium indicator fluo-3. Single wavelength ratioing of fluorescence images obtained after prolonged integration times revealed non-uniformities of intracellular Ca2+ changes across the cell, suggesting the presence of significant spatial Ca2+ gradients. Treatment with 20 microM ryanodine, an inhibitor of Ca2+ release from the SR, and 10 microM verapamil, a calcium channel blocker, reduced by 42% and 76% respectively the changes in [Ca2+]i elicited by membrane depolarization. The overall spatial distribution of [Ca2+]i changes appeared unchanged. Ca2+ transients recorded in the presence of verapamil and ryanodine (about 20% of the size of control responses), diminished in the presence of 50 microM 2-4 Dichlorbenzamil (DCB) or 5 mM nickel, two relatively specific inhibitors of the Na+/Ca2+ exchange mechanism. Conversely, when the reversal potential of the Na+/Ca2+ exchange was shifted to negative potentials by lowering [NA+]o or by increasing [Na+]i by treatment with 20 microM monensin, the amplitude of these Ca2+ transients increased. Ca2+ transients elicited by membrane depolarization and largely mediated by reverse operation of Na(+)-Ca2+ exchange could be recorded in the presence of ryanodine, verapamil and monensin. These finding suggest that in intact guinea pig cardiac cells, Ca2+ influx through the Na+/Ca2+ exchange mechanism activated by a membrane depolarization in the physiological range can be sufficient to play a significant role in excitation-contraction coupling.
Santi
C M
CM
Departamento de Neurociencias, Instituto de Fisiología Celular, UNAM, México City, México.
Conner
J A
JA
Hernández-Cruz
A
A
D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002478:Cells, Cultured; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects; D006352:Heart Ventricles; D008564:Membrane Potentials / Q000187:drug effects; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D008985:Monensin / Q000494:pharmacology; D009124:Muscle Proteins / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D009532:Nickel / Q000494:pharmacology; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D012433:Ryanodine / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D019831:Sodium-Calcium Exchanger; D014700:Verapamil / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D002352:Carrier Proteins; D009124:Muscle Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D019831:Sodium-Calcium Exchanger; D012433:Ryanodine; D010042:Ouabain; D000584:Amiloride; D009532:Nickel; C064515:2',4'-dichlorobenzamil amiloride; D008985:Monensin; D014700:Verapamil; D011188:Potassium; D002118:Calcium