In isolated human crural veins studied in vitro pinacidil (0.038-380 microM) caused a concentration-related inhibition of noradrenaline, 18 microM (NA) and 127 mM K+-induced contractions. Pinacidil was more potent in inhibiting the NA-contraction than that induced by K+, whereas the reverse was seen for nifedipine. At the highest concentrations greater inhibitions of the NA-induced contractions could be obtained with pinacidil than with nifedipine. The inhibitory effect of pinacidil on the K+-induced contractions was eliminated during a 1 hr wash-out period. In contrast to this, the inhibitory effect of nifedipine could not be eliminated during 4 hrs repeated wash-out. Pinacidil was completely devoid of inhibitory effect on 45Ca net influx in rat aorta, whereas nifedipine caused a significant reduction of influx. The results indicate that both pinacidil and nifedipine are effective vasodilatators in human vessels. Pinacidil seems to be more effective in NA-induced contractions than does nifedipine. The mechanism of action of pinacidil cannot be attributed to an inhibitory effect on cellular calcium entry.
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