Adrenergic-induced vascular adjustments--initial and escape reactions. I. Influence of beta-adrenergic blocking agents on the intestinal circulation of the rat (in vivo)

Adrenoceptor-mediated modulation of Evans blue dye permeation of rat small intestine

In the present study, we addressed the problem whether sympathoadrenal mechanisms could influence the paracellular permeation of macromolecules from the lumen to the lamina propria of the small intestine. Experiments were conducted with rats that were anesthetized with ether for 10-20 min, during which time laparotomy was performed and six consecutive loops (each of 5 cm length) of the jejunum were prepared. A 3% solution of the azo dye, Evans blue (EB; MW 960.83) in phosphate-buffered saline, was instilled into each loop at a volume of 0.3 ml, this compound serving as a marker for tight junctional permeability. Thereafter, the abdomen was closed and the rats were allowed to wake up, but were killed after 60 min. The loops were dissected, opened, and rinsed with acetylcysteine in order to remove the adherent mucus layer. Each loop was weighed and incubated with formamide for 24 hr to elute the amount of EB absorbed, which was quantitated spectrophotometrically. In the control situation, the uptake was homogenous along the loops. beta-Adrenoceptor-blocking, or -stimulating agents could influence the uptake considerably. The data obtained could indicate that noradrenergic nerves, via an activation of beta 2-adrenoceptors, may cause an increase of tight junction permeability for macromolecules, but circulatory mechanisms also must be taken into account in order to explain the observed effects.

Adrenergic, purinergic, and endothelial mediators and modulators of norepinephrine-induced mesenteric autoregulatory escape

We evaluated the effects of potential factors in autoregulatory escape from norepinephrine-induced vasoconstriction in rat anterior mesenteric artery. We determined mesenteric artery blood flow velocity with a pulsed Doppler, sonic flowmeter, and systemic arterial blood pressure with a transducer. A 4-min norepinephrine infusion (0.125-1.0 x 10(-8) M/min) intravenously evoked a dose-dependent, initial vasoconstriction that was followed by rapid escape of blood flow toward or above the control value during sustained norepinephrine administration. Neonatal capsaicin treatment enhanced vasoconstrictor responses to norepinephrine but failed to affect escape parameters. Propranolol decreased norepinephrine-induced escape dose dependently. Adenosine deaminase attenuated escape, and the combination of this enzyme plus propranolol nearly abolished escape from norepinephrine-induced vasoconstriction. Methylene blue also diminished autoregulatory escape. These findings suggest that norepinephrine-induced autoregulatory escape involves simultaneous beta-adrenoceptor, purinergic, and endothelial mediation. Norepinephrine-evoked mesenteric vasoconstriction appears to involve predominantly alpha 2-adrenoceptors and is modulated by peptidergic sensory nerves and adenosine.

Effects of PAF antagonists on renal vascular escape and tachyphylaxis in perfused rabbit kidney

Renal vascular escape is a physiological phenomenon of adaptation that occurs in vascular smooth muscle. It has been described in many preparations subjected to electrical stimulation or treated with vasoactive agents, such as noreprinephrine, angiotensin and vasopressin. We have recently demonstrated that a naturally occurring ginkgolide (BN 52021), which is a PAF antagonist, was able to block norepinephrine-induced escape in perfused rabbit kidney. In the present work other PAF antagonists, such as the ginkgolides BN 52022 and BN 52024, and the synthetic compounds 48740 RP and WEB 2086, were tested. Their effects on renal vascular escape, perfusion pressure and tachyphylaxis were evaluated. They all were shown to block the escape. Among the ginkgolides, BN 52024 is generally recognized as one of the weaker PAF antagonists. However, in spite of this, BN 52024 was able to significantly and simultaneously block renal vascular escape and tachyphylaxis in perfused rabbit kidney infused with norepinephrine.

Beta-adrenergic blockade and the gastrointestinal system

As beta-adrenergic blockers grow increasingly popular for the therapy of a wide variety of disorders, it becomes increasingly important to appreciate the spectrum of physiologic effects on the gastrointestinal system and the potential hazards associated with use of these agents. This review details the effects of the beta-adrenergic blocking agents on the gastrointestinal tract.

On the contribution of prostaglandin-like substances to the action of bradykinin on intestinal motility and blood flow in canine jejunal loop in situ

Blood flow and motility were studied in a canine jejunal loop in situ with intact innervation. Bradykinin was administered into a side branch of the supplying artery and indomethacin was infused intravenously 1 h before experiments. In the control group without indomethacin infusion, bradykinin 1-10 nmol/l gradually increased blood flow without significantly altering motility. Higher concentrations of bradykinin (20-100 nmol/l) augmented rhythmic contractions of the intestine. Phasic blood flow decreased during contraction and increased after relaxation, and mean blood flow increased. Bradykinin (0.2-1.0 mumol/l) caused tonic intestinal contractions. Blood flow initially increased but was soon impeded in proportion to the amplitude and duration of the tonic contractions. With intestinal muscle relaxation, blood flow increased to values markedly higher than control. In the group pretreated with indomethacin, blood flow did not increase after bradykinin administration. However, administration of PGE2 produced significant increase in flow, similar to that observed after bradykinin. Acetylcholine or isoprenaline also markedly increased blood flow. Increased intestinal motility caused by bradykinin mechanically impeded blood flow through the intestine, thus masking its direct vasodilating action. The action of bradykinin on the intestinal vascular bed is probably mediated or modulated by endogenous prostaglandin-like substances.