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Department of Pharmacology, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
1. The effects of endothelin (ET) on the Ca2+ channel current in smooth muscle cells of the guinea-pig portal vein were investigated using the patch-clamp technique with whole-cell and cell-attached configurations. 2. ET augmented the macroscopic Ba2+ current in a dose-dependent manner; this effect was inhibited by nifedipine or Cd2+. Augmentation of the inward current by ET did not depend on the amplitude of the depolarizing pulse. Further, when the membrane potential was held at -60 mV, ET increased the amplitude of the Ba2+ inward current measured at the peak and end of the depolarizing pulse to the same extent. 3. By contrast, when the membrane potential was held at -80 mV, depolarizing pulses to potentials more negative than 0 mV produced greater augmentation of the inward current than did those more positive than 0 mV. Moreover, when a depolarizing pulse to below 0 mV was applied, ET increased the peak amplitude of the inward current more than the amplitude measured at the end of pulse. 4. Using the patch-clamp technique with cell-attached configuration, two types of unitary Ba2+ current with conductances of 22 and 12 pS were obtained in 50 mM-Ba2+ solution. Nifedipine inhibited both types of unitary channel current, but the sensitivity of the 22 pS Ca2+ channel to nifedipine was 20-fold higher than the 12 pS Ca2+ channel. 5. Bath application of ET prolonged the mean open time, reduced the number of sweeps in which no Ca2+ channel was opened ('blank' sweep), and increased the number of channel openings evoked by each depolarizing pulse without changes of conductance. As a consequence, ET increased the open probability of both channels. 6. Augmentation of the 12 pS channels by ET was seen only in the early phase of a depolarizing pulse (57 ms from the onset of 170 ms pulse), while augmentation of the 22 pS channels was seen during the entire period of a depolarizing pulse. 7. When ET was added to the pipette solution, the activity of both Ca2+ channels was increased. However, this effect was less frequently observed than when ET was applied in the bath. 8. These results suggest that ET augments both the nifedipine-sensitive and resistant Ca2+ channels in the smooth muscle cell membrane of the guinea-pig portal vein, but in different ways. Presumably, ET acts indirectly on the voltage-dependent Ca2+ channel.
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