Strontium transport in the rat colon.

Concentration dependence of strontium (Sr) fluxes across the colon ascendens and descendens of the rat were measured in a modified Ussing-chamber. Mucosa (m) to serosa (s) and s to m Sr fluxes across both colonic segments were linearly related to the Sr concentration from 0.125 mmol/l to 10 mmol/l. In the colon ascendens m to s Sr fluxes were slightly higher than the fluxes in the opposite direction, resulting in net Sr absorption. In the colon descendens s to m fluxes were higher than the ms fluxes, resulting in net Sr secretion. Neither Sr nor calcium (Ca) showed a concentration dependent interaction with respect to their unidirectional fluxes in both parts of the colon. Only in the colon ascendens Sr at the highest concentration (10 mmol/l) inhibited m to s calcium transport. Experiments, in which the voltage dependence of the unidirectional Sr fluxes was measured confirmed the results obtained from the concentration dependence: The unidirectional fluxes of Sr across the colon ascendens and descendens were totally voltage dependent, i.e. diffusive. In the colon descendens the voltage dependence of the s to m flux was steeper than the flux from m to s. It is hypothesized that this prevalence is caused by an anomalous solvent drag effect. 1.25-Dihydroxyvitamin D3 [1.25 (OH)2D3] stimulated m to s calcium flux in the colon descendens but had no effect on Sr flux. The results demonstrate that Sr and Ca in the rat colon are transported by different mechanisms. In contrast to the Ca transport the Sr flux is only diffusive and insensitive to 1.25 (OH)2D3.