On the apparent absence of maxi-K+ channel in rat aortic myocyte

In cultured rat aortic myocytes, no high-conductance Ca(2+)-and voltage-dependent K+ channels (maxi-K+ channels) such as those found in the human and rabbit were observed. However, we have found that in freshly dissociated rat aortic myocytes, the activities of high-conductance Ca(2+)-and voltage-dependent channels were present and predominant. In cell-attached patches measured with pipettes filled with normal saline solution, their single channel conductances were 139.6 +/- 2.5 picosiemens (mean +/- SE). Under the 140-mM symmetrical K+ condition (both bath and pipette solutions contained 140 mM KCl), that measurement became 207.2 +/- 3.6 picosiemens and increased by about 50% with detaching of the membrane patch. The relative conductances of the channel to K+, NH4+, and Rb+ were 1:0.61:0.48. The conductance of these channels can readily be reduced by more than 90% by extracellularly applied 0.5 mM tetraethylammonium chloride. These characteristics show that they were maxi-K+ channels.