Spontaneous and agonist‐induced 86Rb efflux from rabbit aortic smooth muscle cells in culture: A comparison with fresh tissue

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Abstract
Potassium efflux was measured in rabbit aortic strips and smooth muscle cells cultured from them by monitoring the release of isotope from preparations preloaded with 86Rb. The basal rate of 86Rb efflux from rabbit subcultured aortic smooth muscle cells was eightfold higher than from freshly isolated strips, but calculations of reuptake of isotope in the tissue indicated that the measured rate constant for efflux from aortic strips underestimated the true rate by about fourfold. The rate constant for efflux from freshly dispersed cells was less than half that of subcultured cells and remained unchanged for 5 days in culture. It then rose and by around day 10 had reached the value for subcultured cells. The increase in efflux coincided with the onset of cell division. The increased rate of efflux was accompanied by an increased rate of uptake so that the internal potassium content of the cells remained constant. Heparin decreased the efflux of 86Rb from subcultured cells to that of freshly isolated cells concomitant with a reduction in the rate of proliferation. The onset of cell division and increased basal efflux of potassium was associated with a loss of responsiveness to noradrenaline and histamine as assessed by monitoring 86Rb efflux, although depolarising solutions of potassium chloride were still able to elicit a response. Responsiveness to noradrenaline had histamine could be restored by the addition of heparin. These results suggest that the lack of responsiveness of subcultured cells is not due to irreversible dedifferentiation but to a reversible loss in proliferating cells of receptors for vasoactive agents or of a coupling mechanism between receptor occupation and ion gating.