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Divalent cation permeability and blockade of Ca 2+ ‐permeant non‐selective cation channels in rat adrenal zona glomerulosa cells
Author(s) -
Lotshaw David P.,
Sheehan Katherine A.
Publication year - 1999
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.1469-7793.1999.397ae.x
Subject(s) - divalent , zona glomerulosa , chemistry , permeability (electromagnetism) , biophysics , membrane , biochemistry , angiotensin ii , biology , receptor , organic chemistry
1 The effects of the divalent cations Ca 2+ , Mg 2+ and Ni 2+ on unitary Na + currents through receptor‐regulated non‐selective cation channels were studied in inside‐out and cell‐attached patches from rat adrenal zona glomerulosa cells. 2 External Ca 2+ caused a concentration‐dependent and voltage‐independent inhibition of inward Na + current, exhibiting an IC 50 of 1.4 m m . The channel was also Ca 2+ permeant and external Ca 2+ shifted the reversal potential as expected for a channel exhibiting a constant Ca 2+ :Na + permeability ratio near to 4. 3 External and internal 2 m m Mg 2+ caused voltage‐dependent inhibition of inward and outward Na + current, respectively. Modelling Mg 2+ as an impermeant fast open channel blocker indicated that external Mg 2+ blocked the pore at a single site exhibiting a zero voltage K d of 5.1 m m for Mg 2+ and located 19% of the distance through the transmembrane electric field from the external surface. Internal Mg 2+ blocked the pore at a second site exhibiting a K d of 1.7 m m for Mg 2+ and located 36% of the distance through the transmembrane electric field from the cytosolic surface. 4 External Ni 2+ caused a voltage‐ and concentration‐dependent slow blockade of inward Na + current. Modelling Ni 2+ as an impermeant slow open channel blocker indicated that Ni 2+ blocked the pore at a single site exhibiting a K d of 1.09 m m for Ni 2+ and located 13.7% of the distance through the transmembrane electric field from the external surface. 5 External 2 m m Mg 2+ increased the K d for external Ni 2+ binding to 1.27 m m , consistent with competition for a single binding site. Changing ionic strength did not substantially affect Ni 2+ blockade indicating the absence of surface potential under physiological ionic conditions. 6 It is concluded that at least two divalent cation binding sites, separated by a high free energy barrier (the selectivity filter), are located in the pore and contribute to Ca 2+ selectivity and permeability of the channel.