Ca2+-dependent K+ channels in the cortical collecting duct of rat

In the cortical collecting duct of the rat two Ca2+-dependent K+ channels have been described so far. In the luminal membrane a maxi K+ channel with a single channel conductance of 139 +/- 3 pS in excised membrane patches (n = 91) at 0 mV clamp voltage and asymmetrical KCI-concentrations in pipette and bath was found, while in the basolateral membrane an intermediate conductance K+ channel (85 +/- 1 pS, n = 53) and a smalt K+ channel (28 +/- 2 pS, n = 15) was described. All these K+ channels had similar pharmacological properties since all could be blocked by the K+ channel inhibitors Ba2+, TEA(+), and charybdotoxin. Verapamil, known as a L-type Ca2+ channel blocker, was also capable of inhibiting these K+ channels. While the maxi K+ channel from the luminal membrane was upregulated by intracellular Ca2+ (EC50: 5 mu M), the small and the intermediate K+ channel from the basolateral membrane were downregulated (IC50: 10 mu M). When the cytosolic Ca2+-activity was in the physiological range below 1 mu M the activity of the maxi K+ channel was low and regulated via intracellular pH and ATP. Furthermore, when CCD cells were strongly depolarized and under hypoosmotic stress, Ca2+ rose and activated this K+ channel, indicating that this channel is involved in volume regulation. Like the maxi K+ channel the intermediate conductance K+ channel from the basolateral membrane was also sensitive to intracellular changes of pH where acidic pH inhibited while alkaline pH activated this channel. But unlike the KC channels from the luminal membrane the K+ channel from the basolateral membrane is not regulated by ATP up to 5 mM. The activity of the K+ channels from the basolateral membrane decreased steadily after excision of the membrane. This decrease could be prevented by applying cGMP and MgATP to the bath and thus, activating a membrane-bound cGMP-dependent protein kinase (PKG). The activation of the PKG could be reversed by its specific inhibitor KT5823 (1 mu M). Due to the opposite regulation via intracellular Ca2+ and the involvement of different protein kinases a specific and independent regulation of K+ secretion and Na+ reabsorption is possible in the CCD of the rat.