Evidence for two cellular sodium compartments in suspensions of smooth muscle cells

Unidirectional transmembrane fluxes of 24Na were examined in suspensions of enzymatically dispersed toad stomach muscle cells. Compartmental analysis of 24Na influx indicated that 24Na exchange consisted of two kinetically distinguishable components in these cells: a large pool (approximately 45 mmol/l cell H2O) with a rapid rate of exchange [half time (t1/2) approximately 5.8 min] and a smaller pool (approximately 26 mmol/l cell H2O) that exchanged more slowly (t1/2 approximately 69 min). Only the latter appeared ouabain sensitive; thus we postulated that the rate of exchange of this pool might be determined by the inward leak of Na+. Based on this assumption, we derived estimates for Na+ influx rate (0.9 X 10(-12) mol . cm-2 . s-1) and Na+ permeability (0.3 X 10(-8) cm/s), which presumably represent upper limits for these parameters in smooth muscle. The location and significance of the rapidly exchanging pool were not resolved. The exchange of both cellular Na+ pools was increased by the ionophore monensin. Thus we speculated that the rapidly exchanging pool was associated with a small population of permeable (high Na+) smooth muscle cells, while the slowly exchanging pool was associated with smooth muscle cells with normal Na+ permeability (low-Na+ cells).