Membrane potassium channels and human bladder tumor cells. I. Electrical properties

These experiments were conducted to determine the membrane K+ currents and channels in human urinary bladder (HTB-9) carcinoma cells in vitro. K+ currents and channel activity were assessed by the whole-cell voltage clamp and by either inside-out or outside-out patch clamp recordings. Cell depolarization resulted in activation of a Ca(2+)-dependent outward K+ current, 0.57 +/- 0.13 nS/pF at -70 mV holding potential and 3.10 +/- 0.15 nS/pF at 30 mV holding potential. Corresponding patch clamp measurements demonstrated a Ca(2+)-activated, voltage-dependent K+ channel (KCa) of 214 +/- 3.0 pS. Scorpion venom peptides, charybdotoxin (ChTx) and iberiotoxin (IbTx), inhibited both the activated current and the KCa activity. In addition, on-cell patch recordings demonstrated an inwardly rectifying K+ channel, 21 +/- 1 pS at positive transmembrane potential (Vm) and 145 +/- 13 pS at negative Vm. Glibenclamide (50 microM), Ba2+ (1 mM) and quinine (100 microM) each inhibited the corresponding nonactivated, basal whole-cell current. Moreover, glibenclamide inhibited K+ channels in inside/out patches in a dose-dependent manner, and the IC50 = 46 microM. The identity of this K+ channel with an ATP-sensitive K+ channel (KATP) was confirmed by its inhibition with ATP (2 mM) and by its activation with diazoxide (100 microM). We conclude that plasma membranes of HTB-9 cells contain the KCa and a lower conductance K+ channel with properties consistent with a sulfonylurea receptor-linked KATP.