Levitt model plotting D(x)/Do versus pore diameter for cAMP (○) and LY (•), where D(x) is the diffusion coefficient for a solute within the channel and Do is the equivalent within the cytoplasm. D(x)/Do is assumed to be approximated by the calculated flux ratios for cAMP/K⁺ and LY/K⁺. The continuous and dashed lines represent cAMP/K and LY/K⁺ ratios for Cx43 and Cx40, respectively.