Scaling of transepithelial potential difference in the mammalian trachea

Tracheal epithelia of different mammalian species differ widely with regard to the relative rates of Na⁺ absorption and Cl⁻ secretion. However, the short circuit current, a measure of total ion transport, appears to be consistently greater in large than in small mammals. Thus, we hypothesized that the in vivo tracheal electrical potential difference (PD) would vary among species as a function of body mass (M). To test this hypothesis we measured PD in ten mammalian species that ranged 1000-fold in mass. The results in mV (mean ± SE, lumen negative) were: 11.4 ± 1.0 in mice; 11.6 ± 1.2 in gerbils; 12.9 ± 1.4 in rats; 19.3 ± 0.9 in guinea pigs; 27.2 ± 2.2 in ferrets; 23.0 ± 1.6 in cats; 27.0 ± 0.6 in rabbits; 32.5 ± 2.6 in dogs; 37.0 ± 1.9 in sheep; and 49.0 ± 3.3 in pigs. Log-log correlation analysis of mean PD (in mV) and M (in kg) yielded PD = 20.9 M^0.19 (r = 0.96, P < 0.001). Analysis of published short circuit current (SCC, in μA/cm²) data revealed a similar relationship: SCC = 38.2 M^0.21. Thus, the transepithelial electrical potential and active charge transport by the tracheal epithelium are allometric variables that may have direct physiological significance. These results raise questions regarding the importance of net osmotic solute and water transport across the tracheal epithelium.