Inhibitor effects of diltiazem, nicardipine, nifedipine and verapamil on the norepinephrine-induced contractions of the canine saphenous vein in calcium-free medium.

Contractile responses to norepinephrine (noradrenaline, NE 10(-5) M) in the canine saphenous vein (SV) are significantly, although slightly, reduced (14%) when induced in a physiological medium depleted of calcium for 1 hour (+ EGTA). In contrast, they are inhibited by about 75% after 24 hr in calcium free physiological saline solution (P.S.S.). ED50 of norepinephrine in 1-hr calcium-free medium and in normal Ca++ P.S.S. are 6 x 10(-7)M and 4.2 x 10(-7)M, respectively. Two blockers of extracellular calcium entry have also been cited as inhibitors of intracellular calcium pool refilling. At concentrations of 10(-6)M, 10(-5)M and 10(-4)M, diltiazem and nicardipine inhibit the norepinephrine-induced contractions (NIC) in a concentration-dependent manner. At 10(-4)M, the two calcium blockers inhibit the NIC by 70% and by 72% respectively in Ca++ free (+ EGTA) P.S.S. Nifedipine and verapamil only begin to significantly inhibit NIC in Ca++ free (+ EGTA) P.S.S. at concentrations equal to or greater than 10(-5)M. At 10(-4)M concentration, control inhibition in Ca++ free P.S.S. was observed as 60% and 49%, respectively. Contrary to the other 3 calcium antagonists tested, diltiazem antagonises NIC significantly less in calcium-containing medium (45%) than in calcium-free medium (72%). Procaine at a concentration of 10(-3)M, described as sufficient to totally inhibit calcium release from its intracellular storage sites, only inhibits NIC by 52% in calcium free (+ EGTA) P.S.S. These results are consistent with the following conclusion: i) in the canine saphenous vein (SV), NIC is mainly mediated by calcium mobilization from its intracellular storage sites; ii) the calcium antagonists tested here and procaine are unable to totally inhibit, even at high concentrations, the contractions induced via intracellular calcium release; this characteristic is nonsignificant for nifedipine and verapamil at low concentrations (10(-6)M). iii) verapamil and nifedipine, like diltiazem and nicardipine at high concentrations, may not only possess the characteristics of extracellular calcium entry blockers, but also that of partial antagonist of NIC via non specific mechanisms; iv) diltiazem may relax the vascular smooth muscle of SV, not only by the above two properties, but also through another mechanism yet unknown; v) partial persistence of NIC on the SV under conditions of short or long extracellular calcium depletion may be due to a mechanism of intracellular Ca++ recycling, the smooth muscle cell partially retaining its intracellular Ca++.