SUBSTRATE ACTIVATION OF MECHANOSENSITIVE, WHOLE-CELL CURRENTS IN RENAL PROXIMAL TUBULE

Isolated, polarized, proximal tubule cells of Rana pipiens were voltag e clamped and examined for both single-channel and whole cell currents . Barium-sensitive whole cell conductances were calculated from the di fference in slopes of the current-voltage relations before and after 5 mM external barium. In 11 voltage-clamped cells with high K in the pi pette (and cell), isosmotic addition of 40 mM glucose to the bathing s olution increased cell volume by 23 +/- 4% within 2-3 min and increase d barium-sensitive conductance by 40 +/- 10% from 0.5 to 0.7 nS (P < 0 .005, with each cell as its own control). Isosmotic addition of nonmet abolizable methyl-alpha-D-glucopyranoside, which enters with Na across the apical membrane, produced a similar increase in barium-sensitive conductance (30 +/- 13%). In contrast, 3-O-methyl-D-glucopyranose, whi ch is not cotransported with Na, did not alter either cell volume or b arium-sensitive conductance. Isosmotic addition of 40 mM phenylalanine (Phe) increased cell volume by 21 +/- 3% and increased barium-sensiti ve conductance by 36 +/- 19% from 1.1 to 1.5 nS (P < 0.005, with each cell as its own control; n = 8). All K channels observed at the basola teral membrane of these amphibian cells were found to be activated by pipette suction (stretch) and inhibited by 5 mM external barium (outsi de-out patches). Hence, stretch-activated (SA) K channels must be medi ating the macroscopic increase in whole cell K conductance (G(K)) afte r isosmotic addition of glucose and Phe. The process does not seem to involve changes in ATP, because Phe increased G(K) even more when cyto solic ATP was maintained at high levels (10(-4) M extracellular ouabai n and 5 mM intracellular ATP). It is also unlikely that changes in cel l pH or calcium mediate the increase in G(K), because the bulk composi tion of the cell is ''clamped'' by the pipette solution in these exper iments (1-mum tip patch pipettes). Consequently, the substrate-induced increase in G(K) probably arises from a swelling-associated deformati on of the submembrane cytoskeleton or a direct change in membrane tens ion. In either case, SA channels would play a physiological role in pr oximal tubule K homeostasis during sugar and amino acid reabsorption i n the proximal tubule of the kidney.