Enhanced Ca2+ current and decreased Ca2+ efflux restore sarcoplasmic reticulum Ca2+ content after depletion.
[Ca2+]i was measured using the fluorescent indicator indo 1 in voltage-clamped ferret and rat ventricular myocytes. The Ca2+ content of the sarcoplasmic reticulum (SR) was estimated from the integral of the Na(+)-Ca2+ exchange current activated by caffeine. Refilling of the SR after caffeine removal was enhanced by stimulation. As the systolic Ca2+ transient recovered, the integral of the L-type Ca2+ current decreased and that of the Na(+)-Ca2+ exchange tail current increased. For the early pulses, the gain of Ca2+ via the Ca2+ current is greater than the loss via the exchanger, and during steady state stimulation, the fluxes are equal. The difference in the integrals gives a measure of the net gain of cell Ca2+ with each pulse. When these are summed, the calculated gain of cell Ca2+ agrees well with the increase of SR Ca2+ produced by stimulation, as measured from the caffeine-evoked currents. There was a nonlinear relationship between SR Ca2+ content and the magnitude of the systolic Ca2+ transient such that at high SR Ca2+ content a given increase of content had a greater effect on the Ca2+ transient than did an increase at low SR content. In conclusion, the effects of systolic Ca2+ on the Ca2+ current and Na(+)-Ca2+ exchange current provide a means to regulate SR Ca2+ content and thence the systolic Ca2+ transient.
Trafford
A W
AW
Department of Veterinary Preclinical Sciences, University of Liverpool, UK.
Díaz
M E
ME
Negretti
N
N
Eisner
D A
DA
D000818:Animals; D002118:Calcium / Q000172:deficiency / Q000378:metabolism / Q000502:physiology*; D004553:Electric Conductivity; D004558:Electric Stimulation; D005289:Ferrets; D009206:Myocardium / Q000166:cytology; D018408:Patch-Clamp Techniques; D051381:Rats; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D002118:Calcium