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 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 4% 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 (_o1=0.8±0.4 ms), a double exponential distribution was obtained during AVP (_o1=0.8±0.5 ms, _o2=7.5±0.7 ms). The Ca²⁺ agonist BAYk8644 (1 M) changed the open time distribution similarly to AVP (_o1=1.0±0.5 ms, _o2=9±2.8 ms). With 1 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.