Twenty-four-hour rhythmicity of growth hormone in captive adult mithuns (Bos frontalis)

A study was conducted in the month of May 2003 to characterize 24-h growth hormone (GH) rhythmicity in six adult female mithuns (Bos frontalis), a rare semi-wild ruminant. Six mithuns averaging 3.3 years of age and 282 kg body weight were maintained in semi-intensive system and fed twice daily. Blood samples collected at 30-minute intervals starting from 0600h for 24-h period were assayed for plasma GH. Growth hormone patterns consisted of frequent surges of varying amplitude. Growth hormone pulses occurred at an average frequency of 0.48/h. The rate did not differ significantly (P < 0.05) nor among mithuns or hour of the day. The magnitude of the GH secretory pulses varied significantly among mithuns and the 24-h period. Growth hormone peaks averaged 121.0 and 45.1 ng/ml in mithuns having the highest and lowest GH pulse amplitudes, respectively. During the 24-h period there were two to four 2- to 3-h periods in which GH pulses were reduced in amplitude. These trough periods occurred at approximately 6-h intervals in two mithuns, imparting rhythmicity to GH profiles to these animals. Periods of reduced pulse amplitudes and intervening secretory episodes were less periodic in the other mithuns. Mithuns were fed at 1100h and 1600h. Interestingly, unlike cattle, no synchronized postprandial fall of plasma GH was recorded. Peak and mean GH levels were associated positively (r = 0.95; P < 0.01). Results from the present study show that the GH pulses occur at frequent intervals throughout the day and night and alterations in GH levels and patterns are elicited more by pulse amplitude than frequency modulation.