Kinetics of Na+ uptake and transcellular transit by the pancreas

22Na uptake into strips of rabbit pancreas was measured for up to 10 min. The uptake curve was characterized by the presence of two plateaus separated by an inflexion point; a first "plateau" or an approximation of steady-state uptake was observed between 1 and 3 min; betwen 3 and 4 min the slope of the uptake curve increased again, finally decreasing to a new and higher steady-state uptake between 4 and 6 min. The data suggest that the first part of the uptake curve (from 0 to 3 min) represents uptake into most if not all cells, and the second part (from 3 to 10 min) represents the sum of "quasi" steady-state cellular uptake and of the equilibration of the ductal compartment in series with the cells. In this model a substantial delay (2.5-3.25 min) elapses between the filling of cellular and ductal compartments which is apparently of intracellular origin, implying restricted Na+ diffusion within the cytoplasm and an intracellular Na+ gradient. If this model is correct, then the mean transit time for Na+ across the whole organ should be approximately 3-4 min and be primarily the result of transcellular transit. The mean transit time for Na+ across the whole organ in vitro was measured and found to be 3.5 min on the average. The step that accounts for most of this time appears to be the transepithelial transit of Na+.