E. Page et al., FLUID-PHASE ENDOCYTOSIS BY IN-SITU CARDIAC MYOCYTES OF RAT ATRIA, The American journal of physiology, 265(4), 1993, pp. 30000986-30000996
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.