Abstract
The effects of changes in the steady level of diastolic blood pressure on fluid flux across the jejunum has been investigated in the anesthetized rat during perfusion with a nutrient-free and Na+-free solution. Diastolic blood pressure was manipulated by intravenous infusions, during the jejunal perfusions, of vasodilators (vasoactive intestinal polypeptide, acetyl-β-methylcholine, and phentolamine) and a vasoconstrictor (arginine vasopressin), each of which acts through a different cellular mechanism. The outcome was that fluid flux was related by a parabolic relationship with diastolic blood pressure in which net secretion occurred over the range 40–100 mmHg, whereas net absorption was recorded at diastolic pressures exceeding 100 mmHg and below 40 mmHg. Against a background of normal absorption promoted by perfusion with 145 mmol L−1 Na+/5 mmol L−1 glucose solution, reductions in diastolic blood pressure markedly reduced the mean rate of fluid absorption by 58% overall, whereas the rate of glucose absorption remained unchanged. Our results were explained on the basis that vasodilatation led to increased capillary pressure and then to net filtration of fluid from the mesenteric capillary bed. Experiments in which Escherichia coli heat-stable toxin was added to the jejunal perfusate confirmed the absence of a secretory response, which was consistent with the absence of effect of the toxin on diastolic blood pressure.
Highlights
The prevailing view of small intestinal secretion has been that the crypts of Lieberku€hn secrete a solution of Na+, ClÀ, and HCOÀ3, whereas absorption occurs at the villi, forming an external fluid circuit (Gregory 1962)
The diastolic blood pressures through I.V. infusion of vasoactive agents ranged from 128 mmHg during arginine vasopressin (AVP) infusion down to as low as 20 mmHg with continuous I.V infusion of vasoactive intestinal polypeptide (VIP) and MC and with a single bolus injection of phentolamine which caused sustained reductions (Fig. 1B–D)
Of special note was that the rate of glucose absorption remained constant when the diastolic blood pressure (DBP) was reduced (P = 0.78). These results demonstrate the effectiveness of reduced DBP in markedly reducing the rate of fluid absorption in the presence of normal glucose absorption. It has been shown for the perfused jejunal loop in vivo that, when the conditions for absorption are removed viz. absence of glucose and Na+ and inhibition of the Na+/H+ antiport by EIPA, net secretion was possible with rates as high as +60 lL cmÀ1 hÀ1
Summary
The prevailing view of small intestinal secretion has been that the crypts of Lieberku€hn secrete a solution of Na+, ClÀ, and HCOÀ3 , whereas absorption occurs at the villi, forming an external fluid circuit (Gregory 1962). The preparation used by Huott et al (1988) which consisted of monolayers derived from metastatic colonic carcinoma cells may not be entirely representative of normal function as perfused colonic crypts have been shown to be absorptive in function rather than secretory and that secretions were evoked only on stimulation by an agonist (Singh et al 1995)
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