Caffeine-activated large-conductance plasma membrane cation channels in cardiac myocytes: characteristics and significance.
Caffeine-activated, large-conductance, nonselective cation channels (LCCs) have been found in the plasma membrane of isolated cardiac myocytes in several species. However, little is known about the effects of opening these channels. To examine such effects and to further understand the caffeine-activation mechanism, we carried out studies using whole-cell patch-clamp techniques with freshly isolated cardiac myocytes from rats and mice. Unlike previous studies, thapsigargin was used so that both the effect of opening LCCs and the action of caffeine were independent of Ca(2+) release from intracellular stores. These Ca(2+)-permeable LCCs were found in a majority of the cells from atria and ventricles, with a conductance of approximately 370 pS in rat atria. Caffeine and all its direct metabolic products (theophylline, theobromine, and paraxanthine) activated the channel, while isocaffeine did not. Although they share some similarities with ryanodine receptors (RyRs, the openings of which give rise to Ca(2+) sparks), LCCs also showed some different characteristics. With simultaneous Ca(2+) imaging and current recording, the localized fluorescence increase due to Ca(2+) entry through a single opening of an LCC (SCCaFT) was detected. When membrane potential, instead of current, was recorded, SCCaFT-like fluorescence transients (indicating single LCC openings) were found to accompany membrane depolarizations. To our knowledge, this is the first report directly linking membrane potential changes to a single opening of an ion channel. Moreover, these events in cardiac cells suggest a possible additional mechanism by which caffeine and theophylline contribute to the generation of cardiac arrhythmias.
Zhang
Yu-An
YA
Department of Physiology, University of Massachusetts Medical School, Worcester, MA 01655, USA.
Tuft
Richard A
RA
Lifshitz
Lawrence M
LM
Fogarty
Kevin E
KE
Singer
Joshua J
JJ
Zou
Hui
H
D000818:Animals; D001145:Arrhythmias, Cardiac / Q000139:chemically induced / Q000378:metabolism; D002110:Caffeine / Q000009:adverse effects / Q000494:pharmacology*; D002120:Calcium Channel Agonists / Q000009:adverse effects / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects* / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects*; D002462:Cell Membrane / Q000187:drug effects* / Q000378:metabolism; D003408:Cresols / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006325:Heart Atria / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D006352:Heart Ventricles / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects*; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism; D018408:Patch-Clamp Techniques; D051381:Rats; D012430:Ruthenium Red; D012433:Ryanodine / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000187:drug effects / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism; D013748:Tetracaine / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology
D002120:Calcium Channel Agonists; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D003408:Cresols; D004791:Enzyme Inhibitors; D019837:Ryanodine Receptor Calcium Release Channel; D013748:Tetracaine; D012430:Ruthenium Red; D012433:Ryanodine; C006984:chlorocresol; D002110:Caffeine; D019284:Thapsigargin; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases