Regulation of systolic [Ca2+]i and cellular Ca2+ flux balance in rat ventricular myocytes by SR Ca2+, L-type Ca2+ current and diastolic [Ca2+]i.
The force-frequency response is an important physiological mechanism regulating cardiac output changes and is accompanied in vivo by beta-adrenergic stimulation. We sought to determine the role of sarcoplasmic reticulum (SR) Ca2+ content and L-type current (ICa-L) in the frequency response of the systolic Ca2+ transient alone and during beta-adrenergic stimulation. Experiments (on single rat ventricular myocytes) were designed to be as physiological as possible. Under current clamp stimulation SR Ca2+ content increased in line with stimulation frequency (1-8 Hz) but the systolic Ca2+ transient was maximal at 6 Hz. Under voltage clamp, increasing frequency decreased both systolic Ca2+ transient and ICa-L. Normalizing peak ICa-L by altering the test potential decreased the Ca2+ transient amplitude less than an equivalent reduction achieved through changes in frequency. This suggests that, in addition to SR Ca2+ content and ICa-L, another factor, possibly refractoriness of Ca2+ release from the SR contributes. Under current clamp, beta-adrenergic stimulation (isoprenaline, 30 nm) increased both the Ca2+ transient and the SR Ca2+ content and removed the dependence of both on frequency. In voltage clamp experiments, beta-adrenergic stimulation still increased SR Ca2+ content yet there was an inverse relation between frequency and Ca2+ transient amplitude and ICa-L. Diastolic [Ca2+]i increased with stimulation frequency and this contributed substantially (69.3 +/- 6% at 8 Hz) to the total Ca2+ efflux from the cell. We conclude that Ca2+ flux balance is maintained by the combination of increased efflux due to elevated diastolic [Ca2+]i and a decrease of influx on IC-L) on each pulse.
Dibb
K M
KM
Unit of Cardiac Physiology, University of Manchester, Core Technology Facility, 46 Grafton Street, Manchester M13 9NT, UK. katharine.dibb@manchester.ac.uk
Eisner
D A
DA
Trafford
A W
AW
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D003971:Diastole / Q000502:physiology; D006352:Heart Ventricles / Q000166:cytology; D009200:Myocardial Contraction / Q000502:physiology; D032383:Myocytes, Cardiac / Q000502:physiology*; D018408:Patch-Clamp Techniques; D051381:Rats; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism*; D013599:Systole / Q000502:physiology; D016276:Ventricular Function
D020746:Calcium Channels, L-Type; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D002118:Calcium