INVERSE CONTROL OF GROWTH-HORMONE AND PROLACTIN SECRETION IN CLONIDINE-STIMULATED DAIRY-CATTLE

Clonidine is a specific alpha-2-adrenoreceptor agonist that stimulates growth hormone (GH) release in animals and humans. This drug was used to study the GH and prolactin (PRL) secretory response in dairy cows and heifers. An i.v. infusion of 10 mu g/kg body weight induced GH rel ease to a peak concentration after 30-60 min, while 2 mu g/kg had no e ffect on GH secretory patterns. Plasma PRL decreased significantly (P< 0.01) starting 15-60 min after both doses of clonidine, this effect la sting up to 6 h. Clonidine significantly lowered plasma insulin (P<0.0 1) and raised plasma glucose (P<0.01). The changes in plasma GH, PRL, insulin and glucose differed significantly between doses, the 10 mu g/ kg dose being more effective (P<0.01). The results of our investigatio n in dairy cattle provide evidence of (i) an increase in GH release af ter 10 mu g/kg clonidine; (ii) a concomitant decrease in PRL secretion , hence GH and PRL secretion in cattle appear inversely controlled; (i ii) a significant difference between the effects of the 2 and 10 mu g/ kg doses and (iv) no relationship between the changes in plasma GH and PRL after clonidine and plasma hormone levels before treatment.