Bleeding-induced decrease in duodenal HCO3- secretion in the rat is mediated via alpha 2-adrenoceptors.

This study was performed on chloralosed rats in order to examine the influence of a minor blood loss on duodenal HCO3- secretion. The HCO3- output was measured by in situ titration in a duodenal segment. Blood loss of 0.6 ml per 100 g body wt (approximately 10% of total blood volume) reduced duodenal HCO3- secretion by about 30%. Pretreatment with guanethidine did not affect basal output of HCO3- but markedly reduced the bleeding-induced response. The alpha 2-adrenoceptor antagonist yohimbine did not affect the basal secretion but almost abolished the depression of duodenal HCO3- secretion due to blood loss. Pretreatment with the alpha 1-adrenoceptor antagonist prazosine or the beta-adrenoceptor antagonist propranolol did neither change basal duodenal HCO3- secretion nor the bleeding-induced decrease in duodenal HCO3- secretion. It is suggested that a small blood loss by means of a reflex activation of the sympathetic nervous system, and via the release of transmitter substances from adrenergic nerve endings, reduces the duodenal HCO3- secretion. The inhibitory effect is mainly due to activation of alpha 2-adrenoceptors.