Identification of different sodium compartments from smooth muscle cells, fibroblasts and endothelial cells, in arteries and tissue culture.
1. The (22)Na efflux curve from the rat tail artery, at 35 degrees C, can be analysed as the sum of three distinct components, from 0 to 90 min of washout. After an initial diffusional component the two late exponential components Be(-kBt) and Ce(-kCt) have the following values: B = 3.03 +/- 0.15 m-mole/kg wet wt. and C = 0.56 +/- 0.04; k(B) = 0.145 +/- 0.005 min(-1) and k(C) = 0.015 +/- 0.007.2. In order to identify the cellular origin of the different compartments we compared the (22)Na efflux curve from the rat tail artery with the curves obtained 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 (BC(3)H1), fibroblasts and endothelial cells.3. It is possible to identify under these experimental conditions the cellular compartments from which the different exponential components of the efflux from the whole arteries originate. Fibroblasts and endothelial cultures, as well as adventitial strips exchange (22)Na slowly with exponential constants resembling k(C). Their efflux rate constants are: fibroblast cultures 0.010 +/- 0.002 min(-1), endothelial cells 0.015 +/- 0.003 min(-1) and adventitia 0.019 +/- 0.007 min(-1). Smooth muscle cells are exclusively responsible for the intermediate component Be(-kBt), but they present also a slow component, indistinguishible from the slow exponential component from the other types of cells in the artery. The rate constants for muscle cells are: rabbit aortic media k(B) 0.25 +/- 0.09 min(-1) and k(C) = 0.013 +/- 0.004 min(-1); medial cultures k(B) = 0.202 +/- 0.005 min(-1) and k(C1) = 0.020 +/- 0.003 min(-1); and BC(3)H1 cell culture k(B) = 0.205 +/- 0.083 min(-1) and k(C) = 0.016 +/- 0.003 min(-1).4. 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 only by ouabain but not by the suppression of extracellular K(+).5. We propose a distribution of Na(+) in smooth muscle cells in two intracellular compartments: (1) Na(+) freely dissolved in the sarcoplasm, exchanging with the kinetics of compartment B and (2) a second cellular compartment which could be contained in the sarcoplasmic reticulum exchanging with the kinetics of compartment C.6. On the basis of the previous model of Na(+) distribution, considering our values, and without any correction, the estimated sarcoplasmic concentration of Na(+) is 9.6 mM, compatible with the direct measurements obtained in skeletal and heart muscle. The Na(+) concentration in the sarcoplasmic reticulum would be 4-10 times higher than in the cytoplasm. In order to increase the accuracy of our calculations it would be necessary to account for the interdiffusion and back diffusion of Na(+) between compartments. It is not possible to attain this goal at the present time.