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Influence of CA 2+ on K + efflux during regulatory volume decrease in cultured astrocytes
Author(s) -
Quesada Octavio,
Ordaz Benito,
MoralesMulia Sandra,
PasantesMorales Herminia
Publication year - 1999
Publication title -
journal of neuroscience research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.72
H-Index - 160
eISSN - 1097-4547
pISSN - 0360-4012
DOI - 10.1002/(sici)1097-4547(19990801)57:3<350::aid-jnr7>3.0.co;2-z
Subject(s) - efflux , nitrendipine , verapamil , charybdotoxin , egta , channel blocker , apamin , chemistry , medicine , calcium , extracellular , endocrinology , dids , endoplasmic reticulum , potassium , potassium channel , biophysics , biology , biochemistry , organic chemistry , membrane
The calcium (Ca 2+ ) dependence of potassium (K + ) efflux activated by hyposmolarity in cultured cerebellar astrocytes was investigated, measuring in parallel experiments 86 Rb release and changes in cytosolic Ca 2+ ([Ca 2+ ] i ). Hyposmotic (50%) medium increased [Ca 2+ ] i from 117 to 386 nM, with contributions of extracellular Ca 2+ and Ca 2+ from the endoplasmic reticulum. Hyposmotic medium increased 86 Rb efflux rate from 0.015 min −1 to a maximal of 0.049 min −1 and a net release of 30%. This osmosensitive efflux was inhibited by Ba 2+ (0.028 min −1 ), quinidine (0.024 min −1 ), and charybdotoxin (0.040 min −1 ), but was unaffected by TEA, 4‐AP, or apamin. Removal of external Ca 2+ from the hyposmotic medium increased 86 Rb efflux to a maximal rate constant of 0.056 min −1 and a net release of 38% and caused a delay of inactivation. These changes were due to the overlaping of an efflux activated by Ca 2+ removal in isosmotic medium. This isosmotic 86 Rb efflux was unaffected by TEA or 4‐AP, reduced by verapamil, and abolished by Ba 2+ , nitrendipine, and Mg 2+ . With the swelling‐induced [Ca 2+ ] i rise suppressed by ethyleneglycoltetraacetic acid‐acetoxy‐methyl ester (EGTA‐AM), hyposmotic 86 Rb was 30% reduced. The Ca 2+ entry blockers Cd 2+ , Ni 2+ , La 3+ , and Gd 3+ did not affect 86 Rb efflux. A 40% decrease observed with verapamil and nitrendipine was found unrelated to Ca 2+ , because these agents did not affect the [Ca 2+ ] i rise and the inhibition persisted in the absence of external Ca 2+ . The phospholipase C blocker U‐73122 did not affect [Ca 2+ ] i nor 86 Rb efflux. Blockers of Ca 2+ /calmodulin W7 and KN‐93 decreased 86 Rb efflux to the same extent as EGTA‐AM. Ionomycin markedly potentiated 86 Rb release in hyposmotic conditions only when [Ca 2+ ] i was raised to about 1 μM, suggesting the implication of maxi‐K + channels at this [Ca 2+ ] i threshold, which nonetheless, was not attained during hyposmotic swelling. It is concluded that 86 Rb efflux in cerebellar astrocytes is largely (70%) Ca 2+ ‐independent and the Ca 2+ ‐dependent fraction is sustained essentially by Ca 2+ released from the endoplasmic reticulum and mediated by a mechanism involving Ca 2+ /calmodulin. J. Neurosci. Res. 57:350–358, 1999. © 1999 Wiley‐Liss, Inc.