Vol. 293, Issue 3, 887-895, June 2000

Single-Cell Recombinant Pharmacology: Bovine _1a-Adrenoceptors in Rat-1 Fibroblasts Release Intracellular Ca²⁺, Display Subtype-Characteristic Agonism and Antagonism, and Exhibit an Antagonist-Reversible Inverse Concentration-Response Phase¹

John Daniel Pediani, Janet Fraser MacKenzie, Robert Paul Heeley², Craig James Daly and John Christie McGrath

Autonomic Physiology Unit, Division of Neuroscience and Biomedical Systems, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, United Kingdom

Phe-activated Ca²⁺ signals recorded from single rat-1 fibroblasts stably expressing the bovine _1a-adrenoceptor (AR) were characterized and used to analyze functional agonist-antagonist interactions. The response to Phe was initiated by the mobilization of stored Ca²⁺ and subsequently sustained by receptor-regulated Ca²⁺ influx. The selective _1A-AR agonist (R)-A-61603 was 141-fold more potent as an agonist than Phe. This potency ratio was consistent with the pharmacology of the native _1A-ARs. Functional responses evoked by concentrations of Phe of more than 0.3 µM displayed fade, which could be explained by agonist-dependent depletion of Ca²⁺ stores. The antagonists tested did not conform to the predictions of the Schild equation for competitive antagonism as expected from the nonequilibrium nature of the response. The antagonist potency series WB4101  prazosin BMY7378, however, was consistent with _1A-ARs. Antagonism exhibited by WB4101 and prazosin was compatible with a model in which antagonists dissociate so slowly from the receptor that this is a major factor in their inhibition of the transient agonist-mediated response, leading to the appearance of insurmountable antagonism. A consequence of this phenomenon was that an inverse concentration-response relationship at high agonist concentrations was abolished by low concentrations of antagonists. Overall, the results indicate that quantitative pharmacology can be studied successfully in single cells even though equilibrium could not be achieved in the agonist-antagonist-response relationship in this particular cell phenotype. The study also showed a form of fade that could be readily explained.