The role of receptor phosphorylation on the redistribution of -adrenergic receptors (-ARs) in rat hearts during different phases of sepsis was investigated. Sepsis was induced by cecal ligation and puncture (CLP). Changes in the distribution of -ARs in the sarcolemmal and light vesicle fractions were studied using ()-[4,6-propyl-³H]dihydroalprenolol ([³H]DHA). Phosphorylation of -ARs was studied by perfusing hearts with [³²P]H₃PO₄ followed by identification of the phosphorylated -ARs with immunoprecipitation using anti-₁-AR antibody. The results show that septic rat hearts exhibit an initial hypercardiodynamic (9 h after CLP; early sepsis) and a subsequent hypocardiodynamic (18 h after CLP; late sepsis) state. [³H]DHA binding studies show that, during early sepsis, the maximum binding capacity (B_max) was increased by 26% in sarcolemma but was decreased by 30% in light vesicles, whereas, during late sepsis, the B_max was decreased by 39% in sarcolemma but increased by 31% in light vesicles. These data indicate that -ARs in the rat heart were externalized from light vesicles to sarcolemma during early sepsis but were internalized from surface membranes to intracellular sites during late sepsis. The immunoprecipitation studies reveal that the externalization of -ARs during early sepsis was coupled with a concomitant decrease (28.5 to 30.6%, P < 0.01) in the receptor phosphorylation, whereas the internalization of -ARs during late sepsis was accompanied by a simultaneous increase (30.3 to 33.8%, P < 0.01) in the receptor phosphorylation. Because the phosphorylation/dephosphorylation of ₁-ARs regulate their functional coupling and may reflect their subcellular distribution, it is suggested that the increase in receptor phosphorylation seen in late sepsis leads to the receptor internalization observed in late sepsis; similarly, externalization of (dephosphorylated) receptors in early sepsis may give rise to the apparent decrease in sarcolemmal receptor phosphorylation observed during this interval.