Adrenergic-induced vascular adjustments--initial and escape reactions. II. Role of beta-adrenergic receptors within different sections of the isolated intestinal vascular bed

Simultaneous perfusion of rat isolated superior mesenteric arterial and venous beds: comparison of their vasoconstrictor and vasodilator responses to agonists

1. A new isolated perfused preparation is described that allows a direct comparison to be made of the responses of the perfused arterial and retrogradely perfused venous circulations of the rat superior mesenteric vascular bed. 2. In experiments comparing the responses of the intact arterially perfused mesentery and small intestine to those of the same preparation following removal of the intestine and division of the circulations, the increases in perfusion pressure produced by arginine-vasopressin (30 pmol) and noradrenaline (1 nmol) were retained by the arterial circulation and those induced by angiotensin II (30 pmol) by the venous circulation. Endothelin-1 (30 pmol) constricted both portions of the vasculature but the prolonged nature of its response was associated with only the venous vessels. 3. In the simultaneously perfused arterial and venous preparation arginine vasopressin (3-100 pmol) was a selective constrictor of the arterial circulation and angiotensin II (3-100 pmol) of the venous circulation. In addition, noradrenaline (0.3-10 nmol), 5-hydroxytryptamine (0.3-10 nmol) and KCl (1-60 micromol) were more active as constrictors of the arterial than the venous vessels, and U46619 (10-300 pmol) a more active constrictor of the venous than the arterial vessels. Endothelin-1 (3-100 pmol) constricted both the arterial and venous portions of the vasculature but was significantly longer acting as a venoconstrictor than an arterioconstrictor. 4. Angiotensin I (300 pmol) caused constrictions of the venous circulation which were dependent upon the presence of angiotensin converting enzyme for captopril (10 microM) abolished constrictions caused by angiotensin I but not by angiotensin II. 5. In preparations preconstricted by U46619 (0.3-3 microM), acetylcholine (0.01-100 nmol), bradykinin (0.001-nmol), sodium nitroprusside (0.01-lOnmol) or isoprenaline (1-l00pmol) produced dose-related dilatations of both the arterial and the venous vasculatures, whereas adenosine diphosphate (ADP, 0.01-lOOnmol) caused dose-dependent dilatations of the arterial circulation but principally constrictions of the venous circulation. The dilatations caused by acetylcholine and bradykinin in both portions of the circulation, and by ADP in the arterial circulation, were endothelium-dependent as they were inhibited by gossypol (3 microM), whereas dilatations to sodium nitroprusside were not. 6. This preparation allows the responses of the arteries and veins of a single perfused mesenteric bed to be compared. In addition, with this preparation it is possible to demonstrate that veins, as well as arteries, show significant endothelium-dependent relaxations. It is concluded that the venous portion of the vasculature is significantly involved in the responses of the intact circulation.

Direct observations of responses of mesenteric microcirculation of the rat to circulating noradrenaline

The responses of mesenteric microcirculation of the rat to circulating noradrenaline were studied by in vivo microscopy, using a photo-electric device placed on a television monitor to measure changes in diameter of individual vessels. Blood flow was measured in the anterior mesenteric artery with an electromagnetic flow transducer, mesenteric vascular conductance was computed on-line from mesenteric artery flow and arterial pressure. I.V. injection of noradrenaline induced a decrease in diameter of arterioles (less than 30 microns) which began simultaneously with the rise in arterial pressure and averaged 15% at 15-20 s. By contrast small and principal arteries (30-40 microns and 80-350 microns respectively) generally showed a diameter increase which averaged 10% and began at the peak of the pressor response. Venules and veins (12-560 microns) showed a diameter decrease which averaged 12-15% at 25-30 s. Meanwhile mesenteric vascular conductance fell by 65-75% in 10 s and rose to 30-50% above control in 25-30 s. The diameter increases of small and principal arteries were not reflex dilator responses initiated by the rise in systemic arterial pressure, nor due to beta-adrenoreceptor stimulation. However, local application of phentolamine abolished responses of all sections of the vascular tree indicating that they all depended on activation of alpha-adrenoreceptors. During I.V. infusion of noradrenaline small arteries showed a maintained increase in diameter which began at the peak of the pressor response, while arterioles initially decreased in diameter but then relaxed, often attaining resting diameter before infusion ceased. Meanwhile mesenteric flow and conductance decreased transiently, but then returned to near control levels, i.e. 'autoregulatory escape' occurred. It is argued that noradrenaline induced alpha-mediated contraction of all sections of the vascular tree; the tendency of arteries to constrict was counteracted by the rise in intravascular pressure caused by arteriolar constriction, active constriction of venous vessels may have been augmented by passive collapse secondary to arteriolar constriction. The secondary relaxation of arterial vessels reflects an inherent property of their smooth muscle to relax from the constrictor influence of noradrenaline and is more marked in proximal than distal vessels. It is proposed that the initial decrease in mesenteric vascular conductance in response to circulating noradrenaline may be attributed to active constriction of distal arterioles and the secondary increase in conductance ('escape') to secondary relaxation of more proximal arterial vessels.

Is there an active mechanism limiting the influence of the sympathetic system on the cerebral vascular bed? Evidence for vasomotor escape from sympathetic stimulation in the rabbit

The influence of the cervical sympathetic chain on cerebral circulation in the rabbit was studied by means of 3 complementary techniques. Two dynamic techniques involving chronically implanted probes were used: blood flow in the caudate nucleus (CN) was measured by thermal clearance; tissue PO2 and PCO2 in the same structure were measured by mass spectrometry. Other variables measured continuously and simultaneously included arterial blood pressure (BP), PaO2 and PaCO2. The third technique was a tissue sampling method based on the Fick principle and using 14C1 ethanol as tracer. Blood flow in 7 regions was measured at stable BP, PaO2 and PaCO2. Stimulation of the sympathetic chain at 15 Hz induced mean maximal decreases in CN blood flow of 23.9% (thermal clearance) and 24.4% (ethanol technique). Mean decrease of PO2 in the CN at 15 Hz was 16.6%. Significant falls in blood flow were observed with the ethanol technique in all 7 structures measured. During prolonged stimulation (greater than 1 min) CN blood flow and PO2 were found to escape towards the baseline level, which was sometimes even exceeded during the stimulation (blood flow). Stimulation frequency had only a very moderate influence on the rate of escape, and no evidence of a metabolic mechanism was found, although injection of barbiturate decreased the escape. These results are discussed with respect to the conflicting evidence on the effects of sympathetic stimulation in the brain, and to possible mechanisms for the escape phenomenon.

[Effect of urethane and penthobarbital on the behaviour of the renin-angiotensin-system in rat (author's transl)]

The behaviour of the renin-angiotensin-system was investigated after urethane and penthobarbital anaesthesia in rats. The recent results support the early observation that urethane increases the plasma-renin-activity and the haematocrit. It was further shown that urethane in the customary anaesthetic dose causes hypoproteinaemia and a decrease in the pressor response to angiotensin and in renin substrate concentration. Based on these results, its use as an anaesthetic in experiments connected with the renin-angiotensin-system does not appear to be appropriate.