ROLE OF ADRENOCEPTORS IN THE HYPERTENSIVE RESPONSE TO FEEDING IN THE CONSCIOUS CALF

Ingestion of milk during suckling causes hypertension and tachycardia in young, unweaned animals of many species, but these responses are mo st pronounced in the calf. The present study was undertaken to assess the extent to which this phenomenon depends on activation of adrenocep tors in these animals. Mean basal heart rate was 100 +/- 8 beats/min a nd mean basal aortic blood pressure was 92 +/- 5 mmHg. The rise in hea rt rate during feeding was almost completely suppressed after proprano lol (2-4 mg/kg iv), which also significantly reduced the rise in blood pressure from 67 +/- 4 to 44 +/- 3 mmHg (P < 0.005). Additional pretr eatment with phentolamine (1.0 mg/kg and less than or equal to 0.1 mg . min(-1). kg(-1) iv) virtually eliminated the rise in blood pressure during feeding; it rose by only 8 +/- 4 mmHg (P < 0.001). Section of b oth splanchnic nerves also significantly reduced the rise in blood pre ssure during feeding, especially after pretreatment with propranolol. Neither section of the splanchnic nerves nor the administration of the blocking agents significantly affected the rises in plasma insulin an d pancreatic polypeptide that occurred after feeding. There was no det ectable rise in plasma neuropeptide Y concentration in response to fee ding. The hypertensive response to direct electrical stimulation of th e peripheral end of a splanchnic nerve and to intra-arterial injection s of norepinephrine were completely abolished after combined pretreatm ent with atropine, propranolol, and phentolamine after the ipsilateral adrenal vein had been tied off. It is concluded that the cardiovascul ar changes that occur during feeding in these animals are attributable very largely, if not entirely, to activation of adrenoceptors.