Identification of different sodium compartments from smooth muscle cells, fibroblasts and endothelial cells, in arteries and tissue culture

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Abstract
The 22Na efflux curve from the rat tail artery, at 35.degree. C, was analyzed as the sum of 3 distinct components, from 0-90 min of washout. After an initial diffusional component 2 late exponential components, Be-kBt and Ce-kCt have the values: B = 3.03 .+-. 0.15 mmol/kg wet wt and C = 0.56 .+-. 0.04; kB = 0.145 .+-. 0.005/min and kC = 0.015 .+-. 0.007. To identify the cellular origin of different compartments the 22Na efflux curve from the rat tail artery was compared with curves from whole rabbit aortal strips, rabbit aortal medial or adventitial strips; and primary cultures from rabbit aorta medial smooth muscle cells, cultures of a non-fusing muscle cell line (BC3H1), fibroblasts and endothelial cells. The cellular compartments from which the different exponential components of the efflux from the whole arteries orginate were identified. Fibroblasts, endothelial cultures and adventitial strips exchange 22Na slowly with exponential constants resembling kC. Their efflux rate constants are: fibroblast cultures 0.010 .+-. 0.002/min, endothelial cells 0.015 .+-. 0.003/min and adventitial 0.019 .+-. 0.007/min. Smooth muscle cells are responsible for the intermediate component Be-kBt, but they present a slow component, indistinguishable from the slow exponential component from other types of cells in the artery. The rate constants for muscle cells are: rabbit aortic media kB = 0.25 .+-. 0.09/min and kC = 0.013 .+-. 0.013 .+-. 0.004/min; medial cultures kB = 0.202 .+-. 0.005 min/ and kC1 = 0.020 .+-. 0.003/min; and BC3H1 cell culture kB = 0.205 .+-. 0.083/min and kC = 0.016 .+-. 0.003/min. The efflux from compartment B of smooth muscle cells is inhibited by ouabain and in the absence of extracellular K+. The efflux from compartment C is inhibited by ouabain and not by the suppression of extracellular K+. Na+ in smooth muscle cells may be distributed in 2 intracellular compartments: Na+ freely dissolved in the sarcoplasm, exchanging with the kinetics of compartment B and a 2nd cellular compartment contained in the sarcoplasmic reticulum exchanging with the kinetics of compartment C. This model of Na+ distribution, suggests that the estimated sarcoplasmic concentration of Na+ is 9.6 mM, compatible with measurements in skeletal and heart muscle. The Na+ concentration in the sarcoplasmic reticulum would be 4-10 times higher than in the cytoplasm. To increase the accuracy of calculations the interdiffusion and back-diffusion of Na+ between compartments must be considered.