Prior studies by our laboratory during the last grant cycle have demonstrated that the in vitro addition of 1,25(OH)2D3 (10-(8) to 10-(11) M) rapidly (sec to min) stimulated phosphoinositide turnover, activated PKC, increased [Ca2+](i) and decreased [pH](i), via inhibition of the Na+/H+ exchange, in the large intestine of vitamin D-sufficient rats and in CaCo-2 cells. Furthermore, 1,25(OH)2D3 and bethanechol chloride failed to elicit any of these aforementioned colonic biochemical responses in D-deficient rats. In vivo repletion of these D-deficient animals with 1,25(OH)2D3 for 5 to 7 days, however, restored the rapid biochemical effects of in vitro 1,25(OH)2D3 and bethanechol in this organ. Based on these prior findings, the three major specific aims of the present proposal are to: 1) further elucidate the mechanisms involved in the rapid, in vitro actions of 1,25(OH)2D3 on the phosphoinositide signal transduction system in the large intestine of vitamin D-sufficient rats and in CaCo-2 cells; 2) further characterize the physiological consequence of these rapid, in vitro biochemical actions of 1,25(OH)2D3 in the large intestine of vitamin D-sufficient rats and in CaCo-2 cells; and 3) determine the mechanism(s) responsible for the loss of these rapid, in vitro actions of 1,25(OH)2D3 in the large intestine of vitamin D-deficient animals and their correction by in vivo administration of 1,25(OH)2D3. It is strongly anticipated that the successful achievement of these specific aims should increase our knowledge with respect to the mechanisms of action and physiological effects of 1,25(OH)2D3 in these cells.