Two-step Ca2+ intracellular release underlies excitation-contraction coupling in mouse urinary bladder myocytes.
The relative contributions of Ca(2+)-induced Ca(2+) release (CICR) versus Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs) to excitation-contraction coupling has not been defined in most smooth muscle cells (SMCs). The present study was undertaken to address this issue in mouse urinary bladder (UB) smooth muscle cells (UBSMCs). Confocal Ca(2+) images were obtained under voltage- or current-clamp conditions. When UBSMCs were activated by a 30-ms depolarization to 0 mV, intracellular Ca(2+) concentration ([Ca(2+)](i)) increased in several small, discrete areas just beneath the cell membrane. These Ca(2+) "hot spots" then spread slowly through the myoplasm as Ca(2+) waves, which continued even after repolarization. Shorter depolarizations (5 ms) elicited only a few Ca(2+) sparks, which declined quickly. The number of Ca(2+) sparks, or hot spots, was closely related to the depolarization duration in the range of approximately 5-20 ms. There was an apparent threshold depolarization duration of approximately 10 ms within which to induce enough Ca(2+) transients to spread globally and then induce a contraction. Application of 100 microM ryanodine to the pipette solution did not change the resting [Ca(2+)](i) or the VDCC current, but it did abolish Ca(2+) hot spots elicited by depolarization. Application of 3 microM xestospongin C reduced ACh-induced Ca(2+) release but did not affect depolarization-induced Ca(2+) events. The addition of 100 microM ryanodine to tissue segments markedly reduced the amplitude of contractions triggered by direct electrical stimulation. In conclusion, global [Ca(2+)](i) rise triggered by a single action potential is not due mainly to Ca(2+) influx through VDCCs but is attributable to the subsequent two-step CICR.
Morimura
Kozo
K
Department of Molecular and Cellular Pharmacology, Graduate School of Pharmaceutical Sciences, Nagoya City University, Japan.
Ohi
Yoshiaki
Y
Yamamura
Hisao
H
Ohya
Susumu
S
Muraki
Katsuhiko
K
Imaizumi
Yuji
Y
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D020013:Calcium Signaling / Q000502:physiology; D002478:Cells, Cultured; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D007211:Indoles / Q000494:pharmacology; D047028:Macrocyclic Compounds; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018613:Microscopy, Confocal; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D032389:Myocytes, Smooth Muscle / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D010080:Oxazoles / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D012433:Ryanodine / Q000494:pharmacology; D001743:Urinary Bladder / Q000166:cytology*
D015220:Calcium Channels; D004791:Enzyme Inhibitors; D007211:Indoles; D047028:Macrocyclic Compounds; D010080:Oxazoles; C108644:xestospongin A; D012433:Ryanodine; D002118:Calcium; C000543:cyclopiazonic acid