ARGININE-VASOPRESSIN INDUCES MODE-2 GATING IN L-TYPE CA2+ CHANNELS (SMOOTH-MUSCLE CELLS OF THE URINARY-BLADDER OF THE GUINEA-PIG)

The effect of arginine-vasopressin (AVP, 0.1-mu-M) on elementary Ca2+ channel currents (L-type) was studied in cell-attached patches with 10 mM BaCl2 as the charge carrier. At a constant potential of -30 mV, bath applied AVP increased the channel openness (NP(o)) by a factor of 4.7 +/- 3.0 (mean +/- SD, n = 9), the effect resulted from an increase in the frequency of opening (factor 2.5 +/- 0.8) and from a longer mean open time. Under control, openings longer than 5 ms contributed only 40% of the total, however, with the application of AVP this contribution increased to 29%. Under control, the open times were distributed along a single exponential tau(o1) = 0.8 +/- 0.4 ms), a double exponential distribution was obtained during AVP (tau(o1) = 0.8 +/- 0.5 ms, tau(o2) = 7.5 +/- 0.7 ms). The Ca2+ agonist BAYk8644 (1-mu-M) changed the open time distribution similarly to AVP tau(o1) = 1.0 +/- 0.5 ms, tau(o2) = 9 +/- 2.8 ms). With 1-mu-M BAYk8644 in the bath, AVP did not significantly increase the relative contribution of long openings, however, AVP increased the frequency of openings by a factor of 2.0 +/- 1 (n = 6). The results are compatible with the idea that AVP can change the gating of L-type Ca2+, channels from mode 1 to mode 2.