Mechanism responsible for "down regulation" during successive .BETA.-agonist administration in rat hearts.

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Abstract
The role of phospholipase (PLase) on the mechanism of the so called "down regulation", which decreases the number of .beta.-adrenergic receptors (.beta.-AR) of rat cardiac membranes during successive .beta.-agonist administration, was investigated. In vivo study: The rats were divided into 3 groups: (1) the control group, untreated; (2) the isoproterenol (ISP) one day group, ISP (10 mg/kg) was subcutaneously injected once; and (3) the ISP 6 days group, ISP (10 mg/kg) was injected once a day for 6 successive days. Binding assays using [3H]dihydroalprenolol and analysis of the content and the composition of fatty acids in phospholipids in cardiac membranes were conducted. The endogenous PLase activity in heart homogenate and tissue ATP levels were also determined. In the 6 days group, the equilibrium dissociation constant of .beta.-AR was not affected; however, the number of .beta.-AR decreased singificantly when compared with the control group. Phospholipid content also decreased in parallel with the decrease in the number of .beta.-AR and the composition of fatty acids was altered. However, in the one day group, there was no significant changes in either the number of .beta.-AR or the phospholipid content. The PLase activity of heart homogenate increased significantly, and the tissue ATP levels decreased in both the one day and the 6 days group. In vitro study: Cardiac membranes prepared from intact rats were incubated with 0.03 and 0.1 unit of PLase A2. PLase A2 reduced the number of .beta.-AR dose-dependently. These results suggested that the degradation of membrane phospholipids caused by persistent activation of endogenous PLase is closely related to the decrease in number of .beta.-AR during successive administration of .beta.-agonist.