A CALCIUM-ACTIVATED AND NUCLEOTIDE-SENSITIVE NONSELECTIVE CATION CHANNEL IN M-1 MOUSE CORTICAL COLLECTING DUCT CELLS

We recently reported that M-1 mouse cortical collecting duct cells sho w nonselective cation (NSC) channel activity (Proc. Natl. Acad. Sci. U SA 89:10262-10266, 1992). In this study, we further characterize the M -l NSC channel using single-channel current recordings in excised insi de-out patches. The M-1 NSC channel does not discriminate between Na+, K+, Rb+, Cs+, and Li+. It has a linear I-V relation with a conductanc e of 22.7 +/- 0.5 pS (n = 78) at room temperature. The P-cation/P-anio n ratio is about 60 and there is no measurable conductance for NMDG, C a2+, Ba2+, and Mn2+. Cytoplasmic calcium activates the M-1 NSC channel at a threshold of 10(-6) M and depolarization increases channel activ ity (NP0). Cytoplasmic application of adenine nucleotides inhibits the M-1 NSC channel. At doses of 10(-4) M and 10(-3) M, ATP reduces NP0 b y 23% and 69%, respectively. Furthermore, since ADP (10(-3) M) reduces NP0 by 93%, the inhibitory effect of adenine nucleotides is not depen dent on the presence of a gamma-phosphoryl group and therefore does no t involve protein phosphorylation. The channel is not significantly af fected by 8-Br-cGMP (10(-4) M) Or by cGMP-dependent protein kinase (10 (-7) M) in the presence of 8-Br-cGMP (10(-5) M) and ATP (10(-4) M). Th e NSC channel is not sensitive to amiloride (10(-4) M cytoplasmic and/ or extracellular) but flufenamic acid (10(-4) M) produces a voltage-de pendent block, reducing NP0 by 35% at depolarizing voltages and by 80% at hyperpolarizing voltages. We conclude that the NSC channel of M-1 mouse cortical collecting duct cells belongs to an emerging family of calcium-activated and nucleotide-sensitive nonselective cation channel s. It does not contribute to amiloride-sensitive sodium absorption and is unlikely to be a major route for calcium entry. The channel is nor mally quiescent but may be activated under special physiological condi tions, e.g., during volume regulation.