Mechanisms of 1,25(OH)2D3-induced rapid changes of membrane potential in proximal tubule: role of Ca2+-dependent K+ channels.

Eleven different secosteroids or steroids (10(-10) to 10(-8) M) were acutely and reversibly introduced in solutions delivered to the lumen of single proximal tubules of the amphibian Necturus kidney while recording basolateral cell membrane potential Vm. Seven of these molecules (1,25(OH)2D3, 25(OH)D3, 24,25(OH)2D3, 5,6-trans-25(OH)D3, 19-diol-cholesterol, estradiol and testosterone) resulted in changes of Vm (delta Vm) occurring in a few seconds, the largest delta Vm being observed with 1,25(OH)2D3, +6.5 +/- 0.75 mV (n = 19); these seven (seco)steroids, but not the four inactive sterols (vitamin D3, cholesterol, 1 alpha D3 and aldosterone) possess a hydroxyl group on at least one carbon of the C17 to C25 lateral chain of the sterol ring. The delta Vm effect was present in Na+-free or Cl-free media, but it was abolished in HCO3-free media. Depolarization of cell membrane potential by addition of glucose, 11 mM, in luminal perfusion fluid abolished the 1,25(OH)2D3-evoked delta Vm effect, suggesting dependence of the latter on the absolute value of membrane potential. Barium, a blocking agent of K+ conductances, suppressed the 1,25(OH)2D3-evoked delta Vm effect, even when the proper effects of barium of cell membrane potential were canceled by current clamp. Pretreatment with quinine, a putative blocker of Ca2+-dependent K+ channels also abolished the 1,25(OH)2D3-evoked depolarization. Such observations are consistent with the presence of Ca2+-dependent K+ channels at the apical cell membrane of the proximal tubule, these channels being inactivated by 1,25(OH)2D3 and probably by other (seco)steroids.