title: Measurements of Ca2+ entry and sarcoplasmic reticulum Ca2+ content during the cardiac cycle in guinea pig and rat ventricular myocytes.
abstract: This study investigates the contribution of Ca2+ entry via sarcolemmal (SL) Ca2+ channels to the Ca2+ transient and its relationship with sarcoplasmic reticulum (SR) Ca2+ content during steady-state contraction in guinea pig and rat ventricular myocytes. The action potential clamp technique was used to obtain physiologically relevant changes in membrane potential. A method is shown that allows calculation of Ca2+ entry through the SL Ca2+ channels by measuring Cd(2+)-sensitive current during the whole cardiac cycle. SR Ca2+ content was calculated from caffeine-induced transient inward current. In guinea pig cardiac myocytes stimulated at 0.5 Hz and 0.2 Hz, Ca2+ entry through SL Ca2+ channels during a cardiac cycle was approximately 30% and approximately 50%, respectively, of the SR Ca2+ content. In rat myocytes Ca2+ entry via SL Ca2+ channels at 0.5 Hz was approximately 3.5% of the SR Ca2+ content. In the presence of 500 nM thapsigargin Ca2+ entry via SL Ca2+ channels in guinea pig cardiac cells was 39% greater than in controls, suggesting a larger contribution of this mechanism to the Ca2+ transient when the SR is depleted of Ca2+. These results provide quantitative support to the understanding of the relationship between Ca2+ entry and the SR Ca2+ content and may help to explain differences in the Ca2+ handling observed in different species.
authors.affiliation: Imperial College School of Medicine, National Heart and Lung Institute, London, England. c.terracciano@ic.ac.uk
authors.forename: C M
authors.initials: CM
authors.lastname: Terracciano
authors.forename: K T
authors.initials: KT
authors.lastname: MacLeod
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology*; D002478:Cells, Cultured; D004553:Electric Conductivity; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D006352:Heart Ventricles; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism; D009544:Niflumic Acid / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D051381:Rats; D012519:Sarcoplasmic Reticulum / Q000378:metabolism* / Q000502:physiology; D019284:Thapsigargin / Q000494:pharmacology; D013997:Time Factors
chemical_list: D015220:Calcium Channels; D002104:Cadmium; D002110:Caffeine; D009544:Niflumic Acid; D019284:Thapsigargin; D002118:Calcium