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Local Ca 2+ transients and distribution of BK channels and ryanodine receptors in smooth muscle cells of guinea‐pig vas deferens and urinary bladder
Author(s) -
Ohi Yoshiaki,
Yamamura Hisao,
Nagano Norihiro,
Ohya Susumu,
Muraki Katsuhiko,
Watanabe Minoru,
Imaizumi Yuji
Publication year - 2001
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.1469-7793.2001.t01-3-00313.x
Subject(s) - ryanodine receptor , chemistry , vas deferens , biophysics , bk channel , depolarization , patch clamp , myocyte , guinea pig , immunostaining , anatomy , membrane potential , endoplasmic reticulum , medicine , endocrinology , receptor , biology , biochemistry , immunohistochemistry
1 The relationship between Ca 2+ sparks spontaneously occurring at rest and local Ca 2+ transients elicited by depolarization was analysed using two‐dimensional confocal Ca 2+ images of single smooth muscle cells isolated from guinea‐pig vas deferens and urinary bladder. The current activation by these Ca 2+ events was also recorded simultaneously under whole‐cell voltage clamp. 2 Spontaneous transient outward currents (STOCs) and Ca 2+ sparks were simultaneously detected at ‐40 mV in approximately 50 % of myocytes of either type. Ca 2+ sparks and corresponding STOCs occurred repetitively in several discrete sites in the subplasmalemmal area. Large conductance Ca 2+ ‐dependent K + (BK) channel density in the plasmalemma near the Ca 2+ spark sites generating STOCs was calculated to be 21 channels μm −2 . 3 When myocytes were depolarized from ‐60 to 0 mV, several local Ca 2+ transients were elicited within 20 ms in exactly the same peripheral sites where sparks occurred at rest. The local Ca 2+ transients often lasted over 300 ms and spread into other areas. The appearance of local Ca 2+ transients occurred synchronously with the activation of Ca 2+ ‐dependent K + current (I K,Ca ). 4 Immunofluorescence staining of the BK channel α‐subunit (BKα) revealed a spot‐like pattern on the plasmalemma, in contrast to the uniform staining of voltage‐dependent Ca 2+ channel α1C subunits along the plasmalemma. Ryanodine receptor (RyR) immunostaining also suggested punctate localization predominantly in the periphery. Double staining of BKα and RyRs revealed spot‐like co‐localization on/beneath the plasmalemma. 5 Using pipettes of relatively low resistance, inside‐out patches that included both clustered BK channels at a density of over 20 channels μm −2 and functional Ca 2+ storage sites were obtained at a low probability of ≈5 %. The averaged BK channel density was 3‐4 channels μm −2 in both types of myocyte. 6 These results support the idea that a limited number of discrete sarcoplasmic reticulum (SR) fragments in the subplasmalemmal area play key roles in the control of BK channel activity in two ways: (i) by generating Ca 2+ sparks at rest to activate STOCs and (ii) by generating Ca 2+ transients presumably triggered by sparks during an action potential to activate a large I K,Ca and also induce a contraction. BK channels and RyRs may co‐localize densely at the junctional areas of plasmalemma and SR fragments, where Ca 2+ sparks occur to elicit STOCs.