Direct modulation of 25-hydroxyvitamin D3 hydroxylation in kidney tubules by 1,25-dihydroxy-vitamin D3

Biphasic action of prostaglandin E2 on conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 in chick renal tubules

The effect of PGE2 on the conversion of 25-hydroxyvitamin D3 (25 OH D3) to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) by isolated renal tubules from vitamin D deficient chicks was studied under a variety of experimental conditions. In the absence of added vitamin D metabolites, PGE2 (2 x 10(-6)M) caused an immediate inhibition of formation of 1,25-(OH)2D3, followed by a delayed stimulation, apparent after 15 h exposure to PGE2. Pretreatment of the tubules with 1,25-(OH)-2D3 prevented the immediate inhibitory action of PGE2, and allowed the stimulation to be apparent after 4 h exposure to PGE2. The cyclic nucleotide phosphodiesterase inhibitor 3-isobutyl-1-methyl xanthine (IBMX) significantly stimulated the formation of 1,25-(OH)-2D3. PGE2 significantly inhibited 1,25-(OH)2D3 formation in tubules which had been stimulated by IBMX. PGE2 stimulated the adenylate cyclase activity in a crude particulate fraction from the chick kidney, and raised cyclic adenosine 3', 5'-monophosphate (cyclic AMP) levels in the renal tubules. It is concluded that PGE2 can either stimulate or inhibit 1,25-(OH)2D3 formation in chick renal tubules. The stimulatory effect may be partly due to elevation of cyclic AMP. The mechanism of the inhibitory effect requires further investigation.

In vitro modulation of renal 25-hydroxyvitamin D3 metabolism by vitamin D3 metabolites and calcium

It has been shown that 1,25-dihydroxyvitamin D3 (1,25-(OH)2-D3) and dietary Ca modulate renal metabolism of 25-hydroxyvitamin D3 (25-OH-D3) to 1,25-(OH)2-D3 and 24,25-dihydroxyvitamin D3 (24,25-(OH)2-D3) in the rat. However, it is not known if 1,25-(OH)2-D3 and Ca act directly on the kidney to modulate 25-OH-D3 metabolism or indirectly through other mechanisms, such as the modulation of parathyroid hormone secretion. Therefore, we have used isolated renal cortical slices from the rat to study the effect of 1,25-(OH)2-D3 and Ca in vitro on renal 25-OH-D3 metabolism. Incubation of renal slices from rats fed a vitamin D-deficient, low-Ca diet with 50 nM 1,25-(OH)2-D3 for 3 h resulted in a significant decrease in 1,25-(OH)2-D3 production and a significant increase in 24,25-(OH)2-D3 production. Increasing media Ca concentration from 0.5 to 2.5 mM resulted in a significant decrease in 1,25-(OH)2-D3 production but no change in 24,25-(OH)2-D3 production. The inhibitory effect of 1,25-(OH)2-D3 was blocked by cycloheximide, but the inhibitory effect of Ca was not blocked by cycloheximide. Renal 1,25-(OH)2-D3 production was inhibited to a greater extent by incubation with 1,25-(OH)2-D3 and Ca together than by incubation with 1,25-(OH)2-D3 and Ca separately. These studies indicate that 1,25-(OH)2-D3 and Ca act directly on the kidney to modulate renal 25-OH-D3 metabolism. They also suggest that the mechanism of modulation is different for each agent.