Lack of effects of 1,25- and 24,25-dihydroxyvitamin D on parathyroid hormone response to hypocalcemia in cattle

Abstract

Effects of 1,25(OH)₂D₃ or 24,25(OH)₂D₃ on plasma PTH were examined following induced hypocalcemia with EGTA. EGTA infusions caused an elevation of plasma PTH within 10 min. Sixty min after the start of EGTA infusions, 1,25(OH)₂D₃ or 24,25(OH)₂D₃ were IV administered. Transient (within 5 min) elevations in plasma PTH were observed in two of five animals following the administration of 1,25(OH)₂D₃ or of 24,25(OH)₂D₃. Neither secosterol had an effect on the induced elevations in plasma PTH during the remaining 60 min of the EGTA infusions. Twenty-two hr following 24,25(OH)₂D₃ administration, plasma PTH, ionized and total calcium, inorganic phosphate, and magnesium were normal, while plasma 24,25(OH)₂D was elevated. The plasma PTH response to EGTA-induced hypocalcemia was not significantly altered from that observed prior to the administration of 24,25(OH)₂D₃. Animals, which were IV injected with 1,25(OH)₂D₃ received the same amount IM 60 min later. Twenty-two h following IM 1,25(OH)₂D₃, plasma 1,25(OH)₂D, ionized and total calcium, and plasma inorganic phosphate were elevated. Plasma PTH and magnesium were lowered. The PTH response to EGTA-induced hypocalcemia was significantly reduced in these animals. A similar reduction in the PTH response to induced hypocalcemia was observed in animals receiving 7 hr IV infusions of calcium chloride. The findings suggest that the blunted response was, in part, the consequence of the preceding hypercalcemia. These results indicate that 1,25(OH)₂D₃ does not directly regulate plasma PTH secretion and that 24,25(OH)₂D₃ has no effect on plasma PTH during induced hypocalcemia in the bovine species.