The quantal nature of calcium release to caffeine in single smooth muscle cells results from activation of the sarcoplasmic reticulum Ca(2+)-ATPase.
Calcium release from intracellular stores occurs in a graded manner in response to increasing concentrations of either inositol 1,4,5-trisphosphate or caffeine. To investigate the mechanism responsible for this quantal release phenomenon, [Ca2+] changes inside intracellular stores in isolated single smooth muscle cells were monitored with mag-fura 2. Following permeabilization with saponin or alpha-toxin the dye, loaded via its acetoxymethyl ester, was predominantly trapped in the sarcoplasmic reticulum (SR). Low caffeine concentrations in the absence of ATP induced only partial Ca2+ release; however, after inhibiting the calcium pump with thapsigargin the same stimulus released twice as much Ca2+. When the SR Ca(2+)-ATPase was rendered non-functional by depleting its "ATP pool," submaximal caffeine doses almost fully emptied the stores of Ca2+. We conclude that quantal release of Ca2+ in response to caffeine in these smooth muscle cells is largely due to the activity of the SR Ca(2+)-ATPase, which appears to return a portion of the released Ca2+ back to the SR, even in the absence of ATP. Apparently the SR Ca(2+)-ATPase is fueled by ATP, which is either compartmentalized or bound to the SR.
Steenbergen
J M
JM
Department of Physiology, University of Massachusetts Medical School, Worcester 01655, USA.
Fay
F S
FS
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002024:Bufo marinus; D002110:Caffeine / Q000008:administration & dosage*; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002451:Cell Compartmentation / Q000187:drug effects; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D003902:Detergents / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects; D009124:Muscle Proteins / Q000187:drug effects; D009130:Muscle, Smooth; D019837:Ryanodine Receptor Calcium Release Channel; D012503:Saponins / Q000737:chemistry; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
D015220:Calcium Channels; D003902:Detergents; D004791:Enzyme Inhibitors; D009124:Muscle Proteins; D019837:Ryanodine Receptor Calcium Release Channel; D012503:Saponins; D013729:Terpenes; D002110:Caffeine; D019284:Thapsigargin; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium