Postsynaptic alpha adrenoceptors on vascular smooth muscle.
A heterogeneous population of alpha adrenoceptors mediates vasoconstriction in the canine saphenous vein (CSV). Studies with isolated strips of venous smooth muscle incubated with selective alpha-adrenoceptor agonists and antagonists revealed that both alpha 1 and alpha 2 adrenoceptors exist independently in this tissue and both subtypes mediate a contractile response. Measurement of contractile responses in reduced or zero external calcium conditions indicates that stimulation of alpha 1 adrenoceptors induces contractions by influx of extracellular calcium and release of calcium from internal stores. In contrast, 45Ca uptake studies suggest that activation of the postsynaptic alpha 2 adrenoceptor produces vasoconstriction dependent only on influx of extracellular calcium. The influx of calcium produced by the selective alpha 2-adrenoceptor agonist BHT-920 is inhibited by calcium entry blockers. Measurements of transmembrane potentials from smooth muscle cells of the CSV suggest that alpha 1-adrenoceptor activation produces depolarization and contraction (electromechanical coupling) whereas alpha 2-adrenoceptor stimulation does not result in concentration-dependent depolarization of the smooth muscle cells (pharmacomechanical coupling).
D000818:Animals; D001381:Azepines / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D004285:Dogs; D004594:Electrophysiology; D009131:Muscle, Smooth, Vascular / Q000502:physiology*; D009638:Norepinephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D012501:Saphenous Vein; D013569:Synapses / Q000502:physiology*; D013997:Time Factors; D014661:Vasoconstriction
D001381:Azepines; D002121:Calcium Channel Blockers; D011942:Receptors, Adrenergic, alpha; C024763:talipexole; D002118:Calcium; D009638:Norepinephrine; D011224:Prazosin