GREATER TENSION IS DEVELOPED AT THE SAME LEVEL OF CYTOSOLIC Ca²⁺ CONCENTRATION IN THE PRESENCE OF CLONIDINE, AN ADRENERGIC PARTIAL AGONIST, THAN IN THE PRESENCE OF NOREPINEPHRINE

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Fura 2-loaded thoracic aorta strips from rabbits were used. Norepinephrine and clonidine induced an increase in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and in muscle tension in a concentration-dependent manner. A positive correlation between ([Ca²⁺]_i) and tension development due to norepinephrine and clonidine was noted in muscle strips both untreated and treated with phenoxybenzamine (PBZ). As the total receptor population was reduced by the PBZ treatment, norepinephrine acted as a partial agonist in the treated strips. The slope of a regression line for norepinephrine in the untreated muscle strips was significantly smaller than the slopes for the same drug in PBZ-treated strips and for clonidine, suggesting that norepinephrine in the PBZ-treated strips and clonidine induced a greater tension at same [Ca²⁺]₁ than did norepinephrine in the untreated strips.