Organic and inorganic calcium blockers on voltage and receptor operated channels of resistance arteries of the rat.

The vasoconstrictor responses induced by noradrenaline (NA), adrenaline (AD), serotonin (ST) and vasopressin (VP) on the anterior mesenteric artery of the rat and its branches, were maintained, although somewhat reduced when perfused with a Ca free solution that depletes extracellular Ca. The vasoconstrictor responses were abolished when Lanthanum, EDTA, Verapamil or Nifedipine were added to the Ca free solution. These drugs are known to displace plasmalemmal bound Ca that triggers vasoconstriction when the agonists attach to the receptors thus blocking the vasoconstrictor responses. When the mesenteric arteries were perfused with a Ca containing solution, to block the vasoconstriction induced by the agonists the concentration of La3+, EDTA, Verapamil and Nifedipine must be raised. Thus these drugs appear to compete with extracellular ionic Ca for the membranal sites involved in the activity of the agonists. K induced vasoconstriction was abolished when extracellular Ca was depleted by a Ca free solution and with lower concentrations of the anticalcic drug than those used to cancel the effect of the agonists. Ca appears to be attached to voltage operated channels less firmly than to receptor operated channels. NA, AD, ST and VP showed different sensitivity to the blocking effect of the various anticalcic drugs. This is probably explained by small differences in the structure of the Ca channels operated by the agonists.