Biphasic redistribution of muscarinic receptor and the altered receptor phosphorylation and gene transcription are underlying mechanisms in the rat heart during sepsis

Objective: The purpose of this study was to investigate intracellular redis tribution of muscarinic cholinergic receptor (m2AChR) and the roles of rece ptor phosphorylation and gene transcription as underlying mechanisms in the rat heart during different phases of sepsis. Methods: Sepsis was induced b y cecal ligation and puncture (CLP). The density of m2AChR in the sarcolemm al and Light vesicle fractions was studied using [H-3]-quinuclidinyl benzil ate ([H-3]-QNB). Phosphorylation of m2AChR was studied by labeling of the m yocardial ATP pool by perfusing isolated hearts with [P-32]H3PO4 followed b y identification of the phosphorylated m2AChR with SDS-PAGE. The steady-sta te level of m2AChR mRNA was determined by RT-PCR and Southern blot analysis . Results: Septic rat hearts exhibit an initial hypercardiodynamic (9 h aft er CLP, early sepsis) and a subsequent hypocardiodynamic (18 h after CLP, l ate sepsis) state. During early sepsis, the B-max for [H-3]-QNB binding was increased in sarcolemma (+69%) but decreased in light vesicles (-22%), whe reas during late sepsis, the B-max was decreased in sarcolemma (-20%) but i ncreased in light vesicles (+32%). The sum of B-max for sarcolemmal and lig ht vesicle fractions was increased during early sepsis (+43%) but decreased during late sepsis (-14%). The phosphorylation of m2AChR was decreased dur ing early sepsis (-73%) but increased during late sepsis (+36% to +90%). Th e m2AChR mRNA abundance was increased during early sepsis (+52%) but decrea sed during late sepsis (-28%). Conclusions: The m2AChR in the rat heart was externalized from light vesicles to sarcolemma (overexpression) during ear ly sepsis but internalized from surface membranes to intracellular sites (u nderexpression) during late sepsis. Furthermore, changes in the receptor ph osphorylation and gene transcription are responsible for the biphasic redis tribution and the altered expression of m2AChR in the rat heart during the progression of sepsis. (C) 2000 Elsevier Science B.V. All rights reserved.