The effect of external divalent cations on spontaneous non-selective cation channel currents in rabbit portal vein myocytes

1. The effects of external divalent cations on spontaneous single non-selective cation channel currents were studied in outside-out patches from rabbit portal vein smooth muscle cells in K+-free conditions. 2. In an external medium containing 1.5 mm Ca2+ (Ca-o(2+)) the majority of spontaneous channel currents had a unitary conductance of 23 pS, reversal potential (V-r) of +10 mV and a low open probability (P-o) at negative patch potentials. Some channels opened to a lower conductance state of about 13 pS suggesting that the cation channels have two conductance states. Open time and burst duration distributions could both be described by two exponentials. with time constants of about of 1 ms and 7 ms for open times and 3 ms and 16 ms. for burst durations. 3. In 0 Ca-o(2+) the majority of spontaneous cation channels had a unitary conductance of 13 pS and 0 V-r was shifted to +4 mV. Moreover the longer open time and longer burst duration time constants were both reduced to approximately half the values in 1.5 mM Ca-o(2+). 4. Compared to 0 Ca-o(2+) the single channel currents in 3 muM and 100 muM Ca-o(2+) had a 5- to 6-fold 0 0 increase in P. which was accompanied by increases in both open times and burst durations. In 3 muM and 100 muM Ca-o(2+) the unitary conductance of the single channel currents was between 22 0 and 26 pS. 5. At positive membrane potentials the single channel currents had an increased P-o, compared to negative potentials which was associated with increased open times and burst durations but these values were similar in 3 muM, 100 muM and 1.5 mM Ca-o(2+). 6. In 1.5 mM Sr-o(2+) and 1.5 mM Ba-o(2+) channels opened to the higher conductance state, of about 0 0 22-25 pS and had a 3- to 7-fold greater P-o than in 0 Ca-o(2+). 7. In conclusion, external divalent cations have marked effects on the unitary conductance and kinetic behaviour of non-selective cation channels in rabbit portal vein smooth muscle cells.