Effects of Ca Antagonists on the Norepinephrine Release and Contractile Responses of Isolated Canine Saphenous Veins to High KCl

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Effects of verapamil, diltiazem and nicardipine on tritium overflow and contraction evoked by 40 mM KCl were evaluated using canine saphenous vein strips preloaded with [³H]norepinephrine. Phentolamine, 10^-6 M, almost completely inhibited the contraction induced by KCl, while it significantly enhanced the evoked tritium overflow. These responses to KCl were dependent on external Ca²⁺. All Ca antagonists tested significantly increased the spontaneous tritium overflow in a concentration-dependent manner without any changes in basal tension. Verapamil at 10^-6 M significantly inhibited the contraction with no significant effect on the evoked overflow; and at concentrations above 10^-5 M, it inhibited the contraction much more strongly than the evoked tritium overflow. Diltiazem and nicardipine at concentrations above 3×10^-6 M significantly inhibited both tritium overflow and contraction evoked by KCl. A significant correlation between inhibitions of both responses to KCl by the three Ca antagonists was observed, although the y-intercept and slope of the regression line for verapamil obtained by plotting the inhibition of the KCl-evoked contraction as a function of the inhibition of the evoked tritium overflow were greater than those for the other two antagonists. The inhibitory effects of verapamil and diltiazem on the tritium overflow and contraction evoked by KCl were not related to their local anesthetic activities. Neither the increase in the spontaneous tritium overflow nor inhibitions of the evoked tritium overflow and contraction by nicardipine were related to its phosphodiesterase inhibiting activity. These results suggest that diltiazem and nicardipine may inhibit the KCl-evoked contraction mainly by inhibiting Ca²⁺-dependent transmitter release from the nerve endings, while verapamil may inhibit it by acting on the postsynaptic sites and at the relatively higher concentrations used, by further inhibition of transmitter release.