Extracellular divalent cations block a cation non‐selective conductance unrelated to calcium channels in rat cardiac muscle

Abstract

The effect of removing extracellular divalent cations on resting potential (Vrest) and background conductance of rat cardiac muscle was studied. Vrest was measured with 3 M KCl‐filled microelectrodes in papillary muscles, or with a patch electrode in ventricular myocytes. Whole‐cell membrane currents were measured in myocytes using step or ramp voltage commands. In both muscles and single cells, decrease or removal of Ca2+o and Mg2+o caused a nifedipine‐resistant depolarization, which was reversed upon readmission of Ca2+o or Mg2+o (half‐maximal effect at 0.8 mM Ca2+o or 3 mM Mg2+o in muscles). In single myocytes, removal of Ca2+o and Mg2+o had no effect on the seal resistance in non‐ruptured cell‐attached recordings, but reversibly induced a current with a reversal potential (Vrev) of −8 ± 3.4 mV (with internal Cs+; mean. s.e.m., n= 23) during whole‐cell recordings. The current was insensitive to nifedipine (3–100 μM) or amiloride (1 mM). Vrev was insensitive to changes in the equilibrium potential for chloride ions (Ecl. The current induced in the absence of extracellular divalent cations was blocked in a concentration‐dependent manner by Ca2+o. (At −80 mV, the affinity constant Kca was 60 μM with a Hill coefficient of 0.9.) KCa was voltage dependent at positive but not negative potentials. Mg2+o, Ni2+o, Sr2+o, Ba2+o, Cd2+o and Gd3+o also blocked the current. In 0 mM Na+ (145 mM NMDG+), the inward component of the divalent cation‐sensitive current was decreased and Vrev shifted to more negative potentials. These results suggest that a novel conductance pathway, permeable to monovalent cations but not to OF and blocked by divalent cations, exists in ventricular myocytes.

Hypocretin/Orexin Innervation and Excitation of Identified Septohippocampal Cholinergic Neurons

“…Thus, BaCl 2 slightly reduced the Hcrt-induced inward current at Ϫ65 mV (control: Ϫ54 Ϯ 5 pA; BaCl 2 : Ϫ46 Ϯ 6.7 pA; not significant; n ϭ 5) but significantly enhanced it at potentials negative to the calculated E k value of Ϫ103 mV (at Ϫ125 mV; control: Ϫ40.7 Ϯ 10.5 pA; BaCl 2 : Ϫ87.8 Ϯ 16.8 pA; p ϭ 0.007; n ϭ 5). Thus, the effect of Ba 2ϩ was inconsistent with that of a nonselective cationic current because these currents are reduced by external Ba 2ϩ throughout the voltage range tested (Mubagwa et al, 1997). Furthermore, addition of Ba 2ϩ to the Tris ACSF reduced the Hcrt inward current in the entire voltage range tested (Fig.…”

The Mg2+ and Mg2+-Nucleotide-Regulated Channel-Kinase TRPM7

“…Because MagNuM activates under the same experimental conditions typically used to study the store-operated Ca 2+ current I CRAC (Hermosura et al 2002;Kozak et al 2002;Prakriya and Lewis 2002), and both I CRAC and MagNuM will carry large monovalent cation currents when using divalent-free extracellular solutions (Hermosura et al 2002), endogenous TRPM7 channels had in fact been recorded before Cahalan 1998, 1999;Fomina et al 2000), but were erroneously interpreted to represent CRAC channels (Kerschbaum and Cahalan 1999). Monovalent currents that are unmasked by removal of divalent cations from the extracellular solution and are possibly due to TRPM7 have also been observed previously in myocytes (Mubagwa et al 1997), aortic smooth muscle cells (Zakharov et al 1999), and hippocampal neurons (Xiong et al 1997). Based on the whole-cell current densities of approximately 2-10 pA/pF at +80 mV that one typically sees in a range of different cell types, and assuming a single-channel conductance of approximately 40 pS and open probabilities of approximately 90%, one would estimate about 10-100 channels per cell.…”

Permeability characteristics of monovalent cations in atrial myocytes of the rat heart

“…Physiol. 85.2 cations are known to block NSC current (Mubagwa et al 1997). It is also well known that the permeability of Ca¥ channels in the absence of external Ca¥ is altered and Ca¥ channels become permeable to monovalent cations (Matsuda, 1986).…”