Sodium metabolism in rat resistance vessels

Using 22Na we have investigated the Na metabolism in rat mesenteric resistance vessels (internal diameter about 200 micron). The intracellular Na content was determined by washing the vessels at 0 degrees C in saline with Li substituted for Na, and was related to smooth muscle volume determined from measurements of media thickness and internal diameter using light microscopy. On this basis an internal Na concentration of 13.7 mmol l-1 cells was found. The efflux of Na into saline at 37 degrees C consists of two phases, a fast one and a slow one, where the slow phase was considered to be intracellular. It had a rate constant of 0.137 min-1, which was decreased by 50% if 1 mM-ouabain was added to or K withdrawn from the efflux medium. The gain of cell Na after addition of 1 mM-ouabain was much faster than would be expected from the decrease in efflux, indicating that 1 mM-ouabain increases the permeability to Na.