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 most pronounced in the calf. The present study was undertaken to assess the extent to which this phenomenon depends on activation of adrenoceptors in these animals. Mean basal heart rate was 100 +/- 8 beats/min and mean basal aortic blood pressure was 92 +/- 5 mmHg. The rise in heart rate during feeding was almost completely suppressed after propranolol (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 pretreatment with phentolamine (1.0 mg/kg and < or =0.1 mg x min(-1) x kg(-1) iv) virtually eliminated the rise in blood pressure during feeding; it rose by only 8 +/- 4 mmHg (P < 0.001). Section of both splanchnic nerves also significantly reduced the rise in blood pressure 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 and pancreatic polypeptide that occurred after feeding. There was no detectable rise in plasma neuropeptide Y concentration in response to feeding. The hypertensive response to direct electrical stimulation of the peripheral end of a splanchnic nerve and to intra-arterial injections of norepinephrine were completely abolished after combined pretreatment with atropine, propranolol, and phentolamine after the ipsilateral adrenal vein had been tied off. It is concluded that the cardiovascular changes that occur during feeding in these animals are attributable very largely, if not entirely, to activation of adrenoceptors.