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Ca 2+ images and K + current during depolarization in smooth muscle cells of the guinea‐pig vas deferens and urinary bladder
Author(s) -
Imaizumi Yuji,
Torii Yuichi,
Ohi Yoshiaki,
Nagano Norihiro,
Atsuki Kaoru,
Yamamura Hisao,
Muraki Katsuhiko,
Watanabe Minoru,
Bolton Thomas B.
Publication year - 1998
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1998.705bj.x
Subject(s) - depolarization , cyclopiazonic acid , vas deferens , chemistry , current clamp , biophysics , membrane potential , contraction (grammar) , calcium , voltage clamp , anatomy , endoplasmic reticulum , endocrinology , biology , biochemistry , organic chemistry
1 Electrical events and intracellular calcium concentration ([Ca 2+ ]) imaged using fluo‐3 and laser scanning confocal microscopy were simultaneously monitored in single smooth muscle cells freshly isolated from guinea‐pig vas deferens or urinary bladder. 2 Images obtained every 8 ms, during stepping from ‐60 to 0 or +10 mV for 50 ms under voltage clamp, showed that a rise in [Ca 2+ ] could be detected within 20 ms of depolarization in five to twenty small (< 2 μm diameter) ‘hot spots’, over 95 % of which were located within 1.5 μm of the cell membrane. Depolarization at 30 s intervals activated hot spots at the same places. 3 Cd 2+ or verapamil abolished both hot spots and Ca 2+ ‐activated K + current ( I K,Ca ). Caffeine almost abolished hot spots and markedly reduced I K,Ca . Cyclopiazonic acid, which raised basal global [Ca 2+ ], decreased the rise in hot spot [Ca 2+ ] and I K,Ca amplitude during depolarization. These results suggest that Ca 2+ entry caused Ca 2+ ‐induced Ca 2+ release (CICR). 4 Under voltage clamp, hot spot [Ca 2+ ] closely paralleled the rise in I K,Ca and reached a peak within 20 ms of the start of depolarization, but the rise in global [Ca 2+ ] over the whole cell area was much slower. Step depolarization to potentials positive to ‐20 mV caused hot spots to grow in size and coalesce, leading to a rise in global [Ca 2+ ] and contraction. Ca 2+ hot spots also occurred during the up‐stroke of an evoked action potential under current clamp. 5 It is concluded that the entry of Ca 2+ in the early stages of an action potential evokes CICR from discrete subplasmalemma Ca 2+ storage sites and generates hot spots that spread to initiate a contraction. The activation of Ca 2+ ‐dependent K + channels in the plasmalemma over hot spots initiates I K,Ca and action potential repolarization.