Premium
Ca 2+ regulation in guinea‐pig colonic smooth muscle: the role of the Na + ‐Ca 2+ exchanger and the sarcoplasmic reticulum
Author(s) -
Bradley Karen N.,
Flynn Elaine R. M.,
Muir Thomas C.,
McCarron John G.
Publication year - 2002
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 - 10.1113/jphysiol.2001.013039
Subject(s) - cyclopiazonic acid , ryanodine receptor , caffeine , chemistry , depolarization , thapsigargin , endoplasmic reticulum , calcium , biophysics , endocrinology , medicine , biochemistry , biology , organic chemistry
To study the contribution of the Na + ‐Ca 2+ exchanger to Ca 2+ regulation and its interaction with the sarcoplasmic reticulum (SR), changes in cytoplasmic Ca 2+ concentration ([Ca 2+ ] c ) were measured in single, voltage clamped, smooth muscle cells. Increases in [Ca 2+ ] c were evoked by either depolarisation (−70 mV to 0 mV) or by release from the SR by caffeine (10 m m ) or flash photolysis of caged Ins P 3 (Ins P 3 ). Depletion of the SR of Ca 2+ (verified by the absence of a response to caffeine and Ins P 3 ) by either ryanodine (50 μ m ), to open the ryanodine receptors (RyRs), or thapsigargin (500 n m ) or cyclopiazonic acid (CPA, 10 μ m ), to inhibit the SR Ca 2+ pumps, reduced neither the magnitude of the Ca 2+ transient nor the relationship between the influx of and the rise in [Ca 2+ ] c evoked by depolarisation. This suggested that Ca 2+ ‐induced Ca 2+ release (CICR) from the SR did not contribute significantly to the depolarisation‐evoked rise in [Ca 2+ ] c . However, although Ca 2+ was not released from it, the SR accumulated the ion following depolarisation since ryanodine and thapsigargin each slowed the rate of decline of the depolarisation‐evoked Ca 2+ transient. Indeed, the SR Ca 2+ content increased following depolarisation as assessed by the increased magnitude of the [Ca 2+ ] c levels evoked each by Ins P 3 and caffeine, relative to controls. The increased SR Ca 2+ content following depolarisation returned to control values in approximately 12 min via Na + ‐Ca 2+ exchanger activity. Thus inhibition of the Na + ‐Ca 2+ exchanger by removal of external Na + (by either lithium or choline substitution) prevented the increased SR Ca 2+ content from returning to control levels. On the other hand, the Na + ‐Ca 2+ exchanger did not appear to regulate bulk average Ca 2+ directly since the rates of decline in [Ca 2+ ] c , following either depolarisation or the release of Ca 2+ from the SR (by either Ins P 3 or caffeine), were neither voltage nor Na + dependent. Thus, no evidence for short term (seconds) control of [Ca 2+ ] c by the Na + ‐Ca 2+ exchanger was found. Together, the results suggest that despite the lack of CICR, the SR removes Ca 2+ from the cytosol after its elevation by depolarisation. This Ca 2+ is then removed from the SR to outside the cell by the Na + ‐Ca 2+ exchanger. However, the exchanger does not contribute significantly to the decline in bulk average [Ca 2+ ] c following transient elevations in the ion produced either by depolarisation or by release from the store.