Specificity of tetraethylammonium and quinine for three K channels in insulin-secreting cells.

The effects of tetraethylammonium (TEA) and quinine on Ca-activated [K(Ca)], ATP-sensitive [K(ATP)]K channels and delayed-rectifier K current [K(dr)] have been studied in cultured insulin-secreting HIT cells using the patch-clamp technique. K(Ca) and K(ATP) channels were identified in excised, outside/out patches using physiological solutions and had unitary conductances of 60.8 +/- 1.3 pS (n = 31) and 15.4 +/- 0.3 pS (n = 40), respectively. Macroscopic K(dr) current (peak current = 607 +/- 100 pA at +50 mV, n = 14) were recorded in the presence of 100 microM cadmium and 0.5 microM tetrodotoxin. Tetraethylammonium (TEA) blocked all three channel types but was more effective on K(Ca) channels (EC50 = 0.15 mM) than on K(ATP) channels (EC50 = 15 mM) or K(dr) currents (EC50 = 3 mM). Quinine also blocked all three currents but was less effective on K(Ca) channels (EC50 = 0.3 mM) while equally effective against K(ATP) channels and K(dr) currents (EC50 = 0.025 mM). TEA blocked K(Ca) and K(ATP) channels by reducing their single-channel conductances and decreasing the probability of K(ATP) channel opening. Quinine blocked K(Ca) channels by reducing the single-channel conductance, but blocked K(ATP) channels by reducing the probability of channel opening. Reinterpretation of previous microelectrode studies in light of these findings suggest that, (i) only K(ATP) channels are active in low glucose, (ii) both K(Ca) and K(dr) channels may assist Ca-spike repolarization, and (iii) K(Ca) channels play no role in forming the burst pattern of Ca spiking in the B cell.