Calcium antagonists and alpha 2-adrenoceptors: possible role of extracellular calcium ions in alpha 2-adrenoceptor-mediated vasoconstriction.
A survey concerning the influence of calcium antagonists on the vasoconstriction induced by the selective stimulation of either postsynaptic alpha 1- or alpha 2-adrenoceptors in the resistance vessels of pithed rats and cats, as well as in ganglion-blocked rabbits, is presented. In every species studied, all calcium antagonists (organic: verapamil, D 600, diltiazem, nifedipine, nisoldipine, and a variety of new compounds; inorganic: Co2+, Ni2+, and Mn2+) significantly reduced the pressor response towards the stimulation of postsynaptic vascular alpha 2-adrenoceptors with B-HT 920 (and other agonists). However, the various calcium antagonists showed little influence on the vasoconstriction provoked by the excitation of vascular postsynaptic alpha 1-adrenoceptors. A correlation study revealed that there exists a close correlation between the depressant influence of the calcium-antagonistic drugs on the vasoconstriction due to alpha 2-adrenoceptor stimulation and the calcium-antagonistic potency in vitro. Accordingly, it is speculated that the vasoconstriction due to alpha 2-receptor stimulation requires the influx of calcium ions from the extracellular space across the membrane. This process is obviously sensitive to the influence of calcium-antagonistic drugs.
D000818:Animals; D001667:Binding, Competitive; D001794:Blood Pressure / Q000187:drug effects; D002118:Calcium / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D004347:Drug Interactions; D004492:Edetic Acid / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011941:Receptors, Adrenergic / Q000187:drug effects*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects* / Q000502:physiology; D014661:Vasoconstriction / Q000187:drug effects*
D002121:Calcium Channel Blockers; D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D004492:Edetic Acid; D011188:Potassium; D002118:Calcium