Abstract
Cardiac sarcoplasmic reticulum (SR) membrane contains several chloride (Cl-) channels. We have characterized a 116-pS Cl- channel (500 mM cis, 50 mM trans Cl-) in cardiac SR that is activated by protein kinase A-dependent phosphorylation. To understand its function further, we examined the permeation of various anions and adenine nucleotides using the planar lipid bilayer-vesicle fusion technique. This Cl- channel showed a high selectivity to anions and its permeability sequence was Br- > Cl- > I- > NO3- > F-. When all anions were replaced with ATP in the cis solution, channel activity persisted. The conductance was 78 pS with 200 mM ATP and 68 pS with 100 mM ATP. The reversal potentials were +63 mV and +41 mV in 200 mM ATP and in 100 mM ATP, respectively. With 100 mM ADP or AMP in the cis solution, channel activities were also observed. The conductances were 87 pS with 100 mM ADP and 115 pS with 100 mM AMP. The apparent adenine selectivity of this channel was ATP > ADP > AMP, assuming that they exist as divalent anions. These results suggest that the SR Cl- channel in cardiac cells may serve as a transporter for the movement of adenine nucleotides between cytosol and SR lumen.
Highlights
In cardiac as well as skeletal muscles, the SR1 plays a central role in excitation and contraction coupling [1]
Permeation of Various Anions—We have reported that a 116 pS ClϪ channel in cardiac sarcoplasmic reticulum (SR) is activated via PKA-dependent phosphorylation or in the presence of MgATP [5, 6]
Because SR membrane contained ClϪ channels, and ryanodine receptor Ca2ϩ release channels (RyRs) and Kϩ channels [2, 7], these channels were blocked by replacement of Kϩ with Csϩ and the application of 10 M ryanodine to the cis solution
Summary
In cardiac as well as skeletal muscles, the SR1 plays a central role in excitation and contraction coupling [1]. It has been suggested that the mitochondrial voltage-dependent anion channel (VDAC) [15] may conduct adenine nucleotides. It is not known, whether the 116-pS ClϪ channel in cardiac SR conducts ATP or not. We examined the anion permeability and whether ATP might permeate through this ClϪ channel or not. We found that this channel could conduct adenine nucleotides, ATP, ADP, and AMP. A preliminary report of this work was presented in abstract form [16]
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