Ca2+ influx through carbachol-activated non-selective cation channels in guinea-pig gastric myocytes

摘要

class="enumerated" style="list-style-type:decimal">Ca²⁺ microfluorometry (100 μm K5 fura-2) and the voltage-clamp technique were combined to study the effect of carbachol (CCh, 50 μm) in inducing currents (ICCh) through non-selective cation channels (NSCCCh) and increments in global cytosolic Ca²⁺ concentration (Δ[Ca²⁺]c).In Na⁺-containing bath solution, ICCh fell from an initial phasic to a subsequent small (5 %) tonic component; Δ[Ca²⁺]c fell to zero. Tonic ICCh and [Ca²⁺]c became prominent after substitution of extracellular 140 mm Na⁺ by 140 mm Cs⁺. Tonic ICCh and Δ[Ca²⁺]c were insensitive to intracellular heparin (3 mg ml⁻¹) and ryanodine (4 μm), i.e. they did not depend on Ca²⁺ release from sarcoplasmic reticulum (SR).Single channel currents of NSCCCh could be resolved in whole-cell recordings. Substitution of Na⁺ by Cs⁺ increased NSCCCh activity by one order of magnitude and slope conductance from 22 to 30 pS. Extracellular quinidine (3 μm) reversibly blocked the NSCCCh activity.Both tonic ICCh and tonic Δ[Ca²⁺]c (a) followed a similar time course of activation, desensitization and facilitation, (b) were reversibly blocked by 3 μm quinidine, and (c) persisted upon block of SR Ca²⁺ release.A Ca²⁺ fractional current of tonic ICCh (f_Ca) of 0.009 was calculated by comparing the ratio Δ[Ca²⁺]_c (corrected for simultaneous Ca²⁺ redistribution) over I_CCh with depolarization-induced *Δ[Ca²⁺]_c (Δ[Ca²⁺]_c calculated from I_Ca induced by a 400 ms depolarization from −60 to 0 mV at 2 mm[Ca²⁺]_o, 145 mm[Cs⁺]_o) over I_Ca. f_Ca was 0.023 at [Ca²⁺]_o = 4 mm.With 110 mm extracellular CaCl₂ and 145 mm intracellular CsCl, I_CCh reversed at +19.5 mV suggesting a permeability ratio P_Ca/P_Cs of 2.8.We conclude that Ca²⁺ influx through NSC_CCh under physiological [Ca²⁺]_o could induce Δ[Ca²⁺]_c. The f_Ca was, however, much smaller than the one calculated from the reversal potential.