Verapamil-induced blockade of voltage-activated K+ current in small-cell lung cancer cells.

Verapamil, Ca++ channel antagonist, has proven clinically useful in the reversal of multiple drug resistance, which is a major detriment to chemotherapy. Recently, verapamil alone has been shown to diminish proliferation in a variety of neoplastic cell lines. Using the patch-clamp technique, the action of verapamil on voltage-gated K+ channels in two cell lines of human small-cell carcinoma of the lung, NCI-H146 and NCI-H82, was investigated. With inward Na+ current suppressed, virtually all control cells exhibited a slowly inactivating outward current that was insensitive to alterations in the external Ca++ concentration. Externally applied verapamil enhanced the rate and extent of outward K+ current (IK) inactivation. Verapamil at a concentration of 20 microM diminished peak IK, evoked by a test pulse to +60 mV from a holding potential of -80 mV, from 1.38 +/- 0.11 nA (mean +/- S.E.M., n = 29 cells) to 0.56 +/- 0.13 nA (n = 11) and caused IK to decay to less than 20% of the peak current within 60 msec. After blocking IK and Na+ current, Ca++ current (ICa) was measured in the presence of 10 mM Ca++. The addition of 100 microM verapamil to the external bath resulted in a 53% reduction of H146 ICa. Peak ICa fell from 81 +/- 9 pA (n = 22) to 38 +/- 8 pA (n = 12). Examination of the whole-cell K+ current on single cells before and immediately after the addition of 100 microM verapamil clearly revealed that the drug had no effect on the initial activation phase of IK, suggesting that K+ channels first open before interacting with the drug.(ABSTRACT TRUNCATED AT 250 WORDS)