Abstract
Cation transport systems were investigated in mutant Madin‐Darby Canine Kidney (MDCK) cells resistant to the diuretic drug amiloride. The mutants were isolated previously as clones resistant to the cytotoxic effects of 3 × 10^−4^ M amiloride. Decreased amiloride transport by the Na^+^ channel was implicated as the basis of the resistance (Taub, '78). Consistent with this hypothesis, Na^+^ accumulation was lower in amiloride resistant cells than in normal sensitive MDCK cells. Kinetic studies indicated that Na^+^ uptake in MDCK cells occurs by a single ATP independent transport system—the Na^+^ channel.
In several amiloride‐resistant clones, including clone Am^r^2, the decreased Na^+^ uptake was associated with a decrease in both the K~m~ and V~max~for Na^+^ uptake by the Na^+^ channel. In Am^r^2 cells no significant alteration in the inhibitory effect of amiloride on Na^+^ uptake was observed. As the Na^+^ channel may actually be a general uptake system for monovalent cations (a number of cations inhibit Na^+^ uptake), the uptake of these inhibitory cations was examined in Am^r^2 cells. Both Ca^++^ and ouabain‐insensitive Rb^+^ uptake occurred at decreased rates in Am^r^2 cells as compared with normal MDCK cells. However, further uptake studies suggested that Na^+^, Ca^++^ and ouabain‐insensitive Rb^+^ uptake all occur by different systems. Thus several transport systems may be defective in Am^r^2 cells. Am^r^2 cells were also resistant to the inhibitory effects of amiloride on CO~2~ evolution from pyruvate. These observations indicate that alterations at a number of molecular sites may be associated with defective Na^+^ transport via the Na^+^ channel in amiloride‐resistant cells. Thus the amiloride‐resistant cells are potentially valuable in examining the interrelationships between Na^+^ transport and other cellular functions.