BASIC PROPERTIES AND POTENTIAL REGULATORS OF THE APICAL K+ CHANNEL INMACULA DENSA CELLS

These studies examine the properties of an apical potassium (K+) chann el in macula densa cells, a specialized group of cells involved in tub uloglomerular feedback signal transmission. To this end, individual gl omeruli with thick ascending limbs (TAL) and macula densa cells were d issected from rabbit kidney and the TAL covering macula densa cells wa s removed. Using patch clamp techniques, we found a high density (up t o 54 channels per patch) of K+ channels in the apical membrane of macu la densa cells. An inward conductance of 41.1 +/- 4.8 pS was obtained in cell-attached patches (patch pipette, 140 mM K+). In inside-out pat ches (patch pipette, 140 mM; bath, 5 mM K+), inward currents of 1.1 +/ - 0.1 pA (n = 11) were observed at 0 mV and single channel current rev ersed at a pipette potential of -84 mV giving a permeability ratio (P- K/P-Na) of over 100. In cell-attached patches, mean channel open proba bility (N.Po, where N is number of channels in the patch and Po is sin gle channel open probability) was unaffected by bumetanide, but was re duced from 11.3 +/- 2.7 to 1.6 +/- 1.3 (n = 5, p < 0.02) by removal of bath sodium (Na+). Simultaneous removal of bath Na+ and calcium (Ca2) prevented the Na+-induced decrease in N.Po indicating that the effec t of Na+ removal on N.Po was probably mediated by stimulation of Ca2entry. This interpretation was supported by studies where ionomycin, w hich directly increases intracellular Ca2+, produced a fall in N.Po fr om 17.8 +/- 4.0 to 5.9 +/- 4.1 (n = 7, p < 0.02). In inside-out patche s, the apical K+ channel was not sensitive to ATP but was directly blo cked by 2 mM Ca2+ and by lowering bath pH from 7.4 to 6.8. These studi es constitute the first single channel observations on macula densa ce lls and establish some of the characteristics and regulators of this a pical K+ channel. This channel is likely to be involved in macula dens a transepithelial Cl- transport and perhaps in the tubuloglomerular fe edback signaling process.