Transient response of the calcium homeostatic system: effect of calcitonin.

With the use of an intravenous step load of calcium, a rapid (less than 30 min) process that helps regulate plasma calcium has been identified in pigs. The process appears to be calcitonin dependent, may not involve simple equilibration, and varies inversely with age, as it is virtually absent in old sheep compared with its intensity in young sheep. Based on these findings and a general model of calcium homeostasis, five classes of regulatory processes are proposed. Grade I (half time, t1/2, congruent to 15 min) is a transient and saturable response present only in young individuals and is calcitonin dependent. Grade II (1/2 congruent to 60-90 min) is parathyroid hormone and possibly calcitonin dependent. Grade III (t1/2 = 3-4 h) is parathyroid hormone dependent and probably renal in nature. Grade IV (t1/2, several hours) is vitamin D dependent and intestinal in nature. Grade V (t1/2, hours or days) is parathyroid hormone and calcitonin dependent and skeletal in nature.