Effects of [Ca2+]i, SR Ca2+ load, and rest on Ca2+ spark frequency in ventricular myocytes.
In heart, spontaneous local increases in cytosolic Ca2+ concentration ([Ca2+]i) called "Ca2+ sparks" may be fundamental events underlying both excitation-contraction coupling and resting Ca2+ leak from the sarcoplasmic reticulum (SR). In this study, resting Ca2+ sparks were analyzed in rabbit and rat ventricular myocytes with laser scanning confocal microscopy and the fluorescent Ca2+ indicator fluo 3. During the first 20 s of rest after regular electrical stimulation, both the frequency of Ca2+ sparks and SR Ca2+ content gradually decreased in rabbit. When rabbit SR Ca2+ content was decreased by reduction of stimulation rate. the initial resting spark frequency was also decreased, even though resting [Ca2+]i was unchanged. The rest-dependent decrease in spark frequency in rabbit cells was prevented by inhibition of Na+/Ca2+ exchange (which also prevents SR Ca2+ depletion during rest). These results suggest that elevation of SR Ca2+ content can increase Ca2+ spark frequency. In contrast to rabbit cells, 20 s of rest produced a gradual increase in spark frequency in rat cells, although SR Ca2+ content was constant and Ca2+ influx was completely prevented. This indicates that there is a time-dependent increase in spark probability during rest that is independent of [Ca2+]i or SR Ca2+. This effect was also apparent in rabbit cells when SR Ca2+ depletion was prevented by blocking Na+/Ca2+ exchange. Stimulation of Ca2+ extrusion via Na+/Ca2+ exchange in the rat (by Ca2+-free superfusion, which slowly depletes SR Ca2+ content) converted the normal rest-dependent increase in spark frequency to a decrease. The amplitude of individual Ca2+ sparks increased with increasing SR Ca2+ content. In the Ca2+-overloaded state, fusion of sparks or long-lasting localized increases of [Ca2+]i were observed with increased spark frequency. We conclude that the resting frequency of Ca2+ sparks can be independently affected by changes in SR Ca2+ content, [Ca2+]i, or rest period. The latter may reflect recovery of the SR Ca2+ release channels from inactivation or adaptation.
Satoh
H
H
Department of Physiology, Loyola University Chicago, Maywood, Illinois 60153, USA.
Blatter
L A
LA
Bers
D M
DM
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism*; D004558:Electric Stimulation; D006352:Heart Ventricles; D009200:Myocardial Contraction; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009994:Osmolar Concentration; D011817:Rabbits; D051381:Rats; D017207:Rats, Sprague-Dawley; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D002118:Calcium