FLUID-PHASE ENDOCYTOSIS BY IN-SITU CARDIAC MYOCYTES OF RAT ATRIA

Fluid-phase endocytosis (FPE) associated with recycling of fused plasm alemma-secretory granules or membranes and/or membrane receptors by in situ cardiac myocytes was studied at 37-degrees-C in in vitro noncont racting adult rat atrial preparations. Measurements included 1 ) the v olume (Vs) of the compartment consisting of presumptive endocytotic v esicles and the endosomes or lysosomes transiently in continuity with them (S), which internalizes [C-14]-sucrose but is inaccessible to si multaneously measured [methoxy-H-3] inulin, 2) the kinetics of [C-14]s ucrose efflux from S, and 3) morphometry to quantify interstitial spa ce and non-heart muscle cells. Vs (0.39 +/- 0.04 ml/g1 dry atrium for unstretched atria at 37-degrees-C) was 1) variable over a 3.7-fold ra nge under various experimental conditions, 2) significantly increased by neomycin or by lowering the temperature to 18-degrees-C, and 3) sig nificantly decreased by alpha1-adrenergic stimulation. Analysis of suc rose efflux kinetics confirmed the presence of an intramyocytic sucros e-containing compartment. A smaller inulin-inaccessible sucrose space (S) was also present in right ventricle. Thus, during FPE, vesicles a nd endosomes initially containing high (extracellular) Ca2+ and Cl- co ncentrations continually enter, circulate within, and undergo exocytos is from myocardial cells.