Trans- and paracellular calcium transport across the colonic mucosa after short- and long-term treatment with 1,25-dihydroxyvitamin D3.

The electrical parameters and the unidirectional fluxes of 45Ca and 3H-mannitol were measured in preparations of rat colon descendens freed from the muscularis externa and mounted in a modified Ussing-chamber. Two criteria were used to differentiate between changes in the trans- and the paracellular calcium transport after treatment with 1,25(OH)2D3: the fluxes of the simultaneously measured 3H-mannitol as a paracellular marker; the 45Ca fluxes in preparations with clamped potentials. After a short-time (6 h) pretreatment by s.c. administration of 1,25(OH)2D3 (250 ng kg-1) in normal rats the mucosa (m) to serosa (s) 45Ca flux under short circuit conditions increased about 65%, whereas the electrical parameters and the 3H-mannitol fluxes remained unchanged. In clamped epithelia the PD-independent m to s 45Ca flux was increased, whereas the PD-dependent flux remained unchanged. In contrast, after long-time (4 days) induction by 1,25(OH)2D3 the m to s 45Ca flux increased under short circuit conditions by about 100% and the m to s 3H-mannitol flux increased by 50%, PD and Isc decreased by more than 60%, whereas tissue resistance was the same, in clamped epithelia the calculated PD-independent, transcellular m to s 45Ca flux was 2.4 times and the PD-dependent, paracellular 45Ca-flux was 1.9 times higher than in controls, whereas the s to m 45Ca flux remained unchanged. On the basis of the relevant references the following conclusions were drawn: after short-time exposure to 1,25(OH)2D3 only the PD-dependent, transcellular m to s calcium transport is increased; this is probably due to a liponomic effect of 1,25(OH)2D3 at the brush border membrane.(ABSTRACT TRUNCATED AT 250 WORDS)