PROPERTIES OF STRETCH-ACTIVATED CHANNELS IN MYOCYTES FROM THE GUINEA-PIG URINARY-BLADDER

1. Stretch-activated channels (SACs) were analysed on patches attached to myocytes isolated from the guinea-pig urinary bladder. At 22-degre es-C application of - 2 to -4 kPa to the patch electrode induced SACs at a density of one to two per patch (3-5 MOMEGA electrodes). 2. With electrodes containing 145 mM K+, 20 mM TEA and 2 mM Mg2+, the single c hannel current followed a linear I-V curve with a slope conductance of 39 +/- 5 pS (mean +/- S.D.) and a reversal potential of 2 +/- 6 mV. S ubstitution of chloride by aspartate ions left both parameters unchang ed suggesting that the anions do not contribute to the currents. 3. Hy perpolarization from -30 to -80 mV did not open channels by itself but increased channel activity (NP(o); where N is the number of channels in the patch and P(o) is the probability of the channel being open) tw ofold. The hyperpolarization-induced increase in NP(o) can be attribut ed to a reduction of long closures. At positive patch potentials numer ous blank records strongly diminished NP(o). 4. Inward currents throug h SACs can be carried by a variety of cations. In the presence of 2 mM Mg2+, the respective channel conductance was 40 +/- 4 pS for 140 mm K + > 34 +/- 2 pS for 140 mM Na+ greater-than-or-equal-to 33 +/- 6 pS fo r 140 mm Cs+ > 19 +/- 2 pS for 110 mm Ba2+ > 17 +/- 2 pS for 110 mM Ca 2+. 5. Reduction of CaCl2 from 110 to 10 mM did not change the conduct ance but shifted the reversal potential from +7 to -7 mV; the reversal potentials suggest that SACs are slightly more permeable for Ca2+ tha n for K+. 6. In the absence of divalent cations, the conductance of K was 82 +/- 4 pS for inward but 45 pS for outward currents. Addition o f either 2 mM Ca2+ or 2 mM Mg2+ reduced the conductance for inward cur rents to 40 pS. 7. The change from 140 to 14 mM KCl plus 136 mm Tris-C l reduced the conductance from 82 to 56 pS whereas the reversal potent ial shifted only from -4 to -9 mV. When 20 mM K+ and 300 mM sucrose we re applied. the conductance fell to 39 pS and the reversal potential s hifted by -30 mV. The results suggest that Tris' can permeate through SACs when extracellular divalent cations are absent. 8. Channel openin gs were blocked by 200 or 20 muM Gd3+. Gd3+ (5 muM) reduced the appare nt single channel conductance and the mean lifetime of the open state. 9. We discuss the physiological importance of cell stretch; length ch anges due to filling of the urinary bladder could activate SACs and th ereby contraction firstly by Ca2+ influx through SACs which could incr ement cellular Ca2+ content directly or trigger Ca2+ release from intr acellular stores. In addition, Na+ influx through SACs depolarizes the membrane and activates Ca2+ influx through L-type Ca2+ channels.