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Facilitatory effect of Ca 2+ on the noradrenaline‐evoked cation current in rabbit portal vein smooth muscle cells
Author(s) -
Helliwell R. M.,
Large W. A.
Publication year - 1998
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.1469-7793.1998.731bd.x
Subject(s) - chemistry , conductance , time constant , calcium , analytical chemistry (journal) , biophysics , physics , biology , chromatography , organic chemistry , condensed matter physics , electrical engineering , engineering
1 The facilitatory effect of external calcium ions (Ca 2+ o ) on the α 1 ‐adrenoceptor‐activated non‐selective cation current (I cat ) was investigated in rabbit portal vein cells using noise and voltage‐jump relaxation analysis of the whole‐cell macroscopic current. 2 Micromolar concentrations of Ca 2+ o potentiated the peak amplitude of I cat at a holding potential ( V h ) of −50 mV. The effective [Ca 2+ ] o which produced a 50 % potentiation (EC 50 ) was 3 μ m . 3 From noise analysis the estimated single channel conductance (γ) was approximately 23 pS with [Ca 2+ ] o between 3 and 100 μ m , whereas in < 10 n m or 1 μ m Ca 2+ o γ was approximately 10 pS. 4 The spectral density function of I cat at negative potentials could be described by the sum of two Lorentzians in every [Ca 2+ ] o examined. The time constant of the lower frequency Lorentzian component (τ 1 ) was about 11 ms in < 10 n m Ca 2+ o and was about 45 ms in micromolar concentrations of Ca 2+ o (1–100 μ m ). In contrast, the time constant of the higher frequency component (τ 2 ) was similar in < 10 n m Ca 2+ o and 100 μ m Ca 2+ o (between 1 and 2 ms). 5 The lower frequency Lorentzian component was responsible for about half the total current variance in < 10 n m Ca 2+ o whereas in micromolar concentrations of Ca 2+ o it was responsible for most of the measured current variance. 6 In voltage‐jump experiments, on stepping the voltage from −50 to +50 mV the instantaneous current was followed by an exponential decline of I cat . Stepping back to −30 mV produced an exponential inward relaxation ( I relax,‐30 mV ) leading to an increase in the steady‐state amplitude of I cat in micromolar concentrations of Ca 2+ o , but this relaxation was not observed in < 10 n m Ca 2+ o . The relative amplitude of I relax,‐30 mV increased in an [Ca 2+ ] o ‐dependent manner (EC 50 was 2 μ m ) although the time constant of this relaxation (τ relax,‐30 mV ) remained unchanged (about 60 ms between 2 and 100 μ m Ca 2+ o ). 7 The data suggest that Ca 2+ o produces marked changes in the kinetics and single channel conductance of cation channels, which may account for the facilitatory effect of micromolar concentrations of Ca 2+ o on the peak amplitude of I cat .