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No evidence for calcium electrogenic exchanger in frog semicircular canal hair cells
Author(s) -
Martini M.,
Rossi M. L.,
Farinelli F.,
Moriondo A.,
Mammano F.,
Rispoli G.
Publication year - 2002
Publication title -
european journal of neuroscience
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.346
H-Index - 206
eISSN - 1460-9568
pISSN - 0953-816X
DOI - 10.1046/j.1460-9568.2002.02234.x
Subject(s) - biophysics , chemistry , bapta , calcium , patch clamp , voltage clamp , intracellular , membrane potential , analytical chemistry (journal) , biochemistry , chromatography , biology , receptor , organic chemistry
We investigated the possibility that, in hair cells mechanically isolated from frog semicircular canals, Ca 2+ extrusion occurs via a Na + : Ca 2+ (cardiac type) or a Na + : Ca 2+ ,K + (retinal type) exchanger. Cells concurrently imaged during whole‐cell patch‐clamp recordings using the Ca 2+ sensitive fluorescent dye Oregon Green 488 BAPTA‐1 (100 µ m ) showed no voltage dependence of Ca 2+ clearance dynamics following a Ca 2+ load through voltage‐gated Ca 2+ channels. Reverse exchange was probed in hair cells dialyzed with a Ca 2+ ‐ and K + ‐free solution, containing a Na + concentration that saturates the exchanger, after zeroing the contribution to the whole‐cell current from Ca 2+ and K + conductances. In these conditions, no reverse exchange current was detected upon switching from a Ca 2+ ‐free external solution to a solution containing concentrations of Ca 2+ alone, or Ca 2+ + K + that saturated the exchanger. By contrast, the same experimental protocol elicited peak exchange currents exceeding 100 pA in gecko rod photoreceptors, used as positive controls. In both cell types, we also probed the forward mode of the exchanger by rapidly increasing the intracellular Ca 2+ concentration using flash photolysis of two novel caged Ca 2+ complexes, calcium 2,2′‐{[1‐(2‐nitrophenyl)ethane‐1,2‐diyl]bis(oxy)}bis(acetate) and calcium 2,2′‐{[1‐(4,5‐dimethoxy‐2‐nitrophenyl)ethane‐1,2‐diyl]bis(oxy)} bis(acetate), in the presence of internal K + and external Na + . No currents were evoked by UV‐triggered Ca 2+ jumps in hair cells, whereas exchanger conformational currents up to 400 pA, followed by saturating forward exchange currents up to 40 pA, were recorded in rod photoreceptors subjected to the same experimental conditions. We conclude that no functional electrogenic exchanger is present in this hair cell population, which leaves the abundant plasma membrane Ca 2+ ‐ATPases as the primary contributors to Ca 2+ extrusion.