1. Archive
2. Vol.51, No.4

Properties of a Ca²⁺-Activated Large Conductance K⁺ Channel with ATP Sensitivity in Human Renal Proximal Tubule Cells

Abstract: The properties of a native Ca²⁺-activated large conductance K⁺ channel (BK channel) present in the surface membrane of cultured human renal proximal tubule epithelial cells (RPTECs) were investigated by using the patch-clamp technique. The slope conductance of the BK channel was about 295 pS, and the channel was selective to K⁺ over Na⁺, with a selectivity ratio of about 12.2. The activity of the channel was almost maximally enhanced by 10⁻⁴ M or more Ca²⁺ in the cytoplasmic surface of the patch membrane and was markedly diminished by reducing the cytoplasmic Ca²⁺ to 10⁻⁶ M at the membrane potential of about 0 mV. The depolarization of the patch membrane also enhanced the channel activity, and hyperpolarization lowered it. K⁺ channel blockers, Ba²⁺ (0.1-1 mM), tetraethylammonium (1 mM), and charybdotoxin (100 nM), were effective for the suppression of channel activity. A significant feature of the K⁺ channel was that channel activity maintained by 10⁻⁵-10⁻⁴ M Ca²⁺ in inside-out patches was inhibited by the addition of ATP (1-10 mM) to the bath solution. ATPS, and a nonhydrolyzable ATP analogue, 5′-adenylylimidodiphosphate (AMP-PNP), also had inhibitory effects on channel activity. However, an inhibitor of ATP-sensitive K⁺ channels, glibenclamide (0.1 mM), induced no appreciable change in channel activity from both intra- and extracellular sides. These results suggest that besides the common natures of the BK channel family such as regulation by cytoplasmic Ca²⁺ and membrane potential, the BK channel in RPTECs is directly inhibited by intracellular ATP independent of phosphorylation processes and sulfonylurea receptor.

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