Vitamin D is essential for normal insulin secretion in vivo and in vitro. The differential effect of calcium and of the vitamin D endocrine system in the insulin response to secretagogues is still a subject of debate. To study the roles of calcium and the vitamin D system in the in vivo insulin response, GTT and insulin sensitivity tests were conducted in rats presenting vitamin D depletion and hypocalcemia or vitamin D depletion supplemented with calcium alone for 3, 7, or 14 days, vitamin D3 (6.5 nmol/day x 7 days), or 1,25(OH)2D3 (28 pmol/day x 7 days). Serum calcium was 1.28 +/- 0.04 mM in hypocalcemic vitamin D-depleted rats, 1.47 +/- 0.06 (NS), 1.8 +/- 0.2 (P < 0.0002), and 2.04 +/- 0.07 (P < 0.0001) mM after 3, 7, or 14 days, respectively, of calcium supplementation; vitamin D3- or 1,25(OH)2D3-supplemented animals had serum calcium of 2.61 +/- 0.04 or 2.48 +/- 0.05 mM (P < 0.0001 vs. hypocalcemic vitamin D-depleted rats). Rats with hypocalcemia and vitamin D depletion had significantly higher glucose concentrations (P < 0.0005) and lower insulin response during GTT than all other groups (P < 0.001). Differences in insulin sensitivity could not account for differences in response because exogenous insulin administration led to a similar drop in glucose concentrations in all groups, with the nadir averaging 51.7 +/- 2.6% of initial values. To distinguish between calcium and the vitamin D system in the GTT response, rats were treated with a nonhypercalcemic analogue of 1,25(OH)2D3, OCT (28 pmol/day x 4-7 days) with or without dietary calcium.(ABSTRACT TRUNCATED AT 250 WORDS)