Baroreflex control of jejunal blood flow and fluid transport in cats; effects of yohimbine, an α.‐adrenergic antagonist

The aim of the study was to test the hypothesis that baroreceptor unloading increases jejunal fluid absorption rate via an α 2 ‐adrenergic effect on electrogenic active transport. In 13 chloralose‐anaesthetized cats, the carotid sinus baroreceptors were isolated and perfused with arterial blood, and we studied the effects of a graded decrease in carotid sinus pressure on intestinal vascular resistance, net fluid absorption rate and the potential difference between the intestinal lumen and the peritoneal cavity (PD). Experiments were performed in seven control animals and in six animals pretreated with yohimbine, an α 2 ‐adrenergic antagonist, at a dose of 0.1 mg kg ‐I i.v. Yohimbine per se had no significant effects on systemic arterial pressure, intestinal vascular resistance, net fluid absorption rate or PD. In the control animals, baroreceptor unloading induced an increase in systemic arterial pressure, intestinal vascular resistance and net fluid absorption rate, and a decrease in the PD. Yohimbine pretreatment did not significantly affect the systemic blood pressure response to baroreceptor unloading, but abolished the effect on intestinal vascular resistance and PD. After yohimbine treatment, decreases in carotid sinus pressure still enhanced net fluid absorption rate, but this response was observed in a higher range of carotid sinus pressures than in control animals. We conclude that (1) a major component of the increase in jejunal absorption rate during baroreceptor unloading is due to a non‐electrogenic mechanism, which may be either active or passive; (2) this component of the response is not blocked by yohimbine at a dose sufficient for an effect on presynaptic α 2 ‐receptors; (3) the absorptive response to baroreceptor unloading is not a phenomenon secondary to the concomitant jejunal vasoconstriction.