Sodium-selective channels in membranes of rat macrophages

With the use of the patch-clamp technique, highly selective nonvoltage-gated sodium channels were found in the membrane of rat peritoneal macrophages. The inward single channel currents were measured in cell-attached and outside-out mode experiments at different holding membrane potentials within the range of -60 to +40 mV. The channels had a unitary conductance of 10.2 +/- 0.2 pS with 145 mM Na+ in the external solution at 23-24 degrees C. The results of ion-substitution experiments confirmed that this novel type of cation channel in macrophages is characterized by high selectivity for Na+ over K+ (as for Cs+, NH4+, Ca2+, Ba2+) ions, whose conduction through these sodium-permeable channels was not measurable. Lithium is the only other ion that is transported by this pathway; the unitary conductance was equal to 3.9 +/- 0.2 pS in the Li(+)-containing external solution. Single channel currents and conductance were found to be linearly dependent on the external sodium concentration. Sodium channels in macrophage membrane patches were not blocked by tetrodotoxin (0.01-1 microM). Single sodium currents were reversibly inhibited by the external application of amiloride (0.1-2 mM) and its derivative ethylisopropilamiloride (0.01-0.1 mM). The mechanism of channel block by amiloride and its analogue seems to be different.