Mechanisms underlying pre- and postjunctional effects of neuropeptide Y in sympathetic vascular control

The effects of porcine neuropeptide Y (NPY) regarding sympathetic vascular control were studied in vitro on isolated rat blood vessels. The 10(-9)M NPY enhanced (about two-fold) the contractile responses to transmural nerve stimulation (TNS), noradrenaline (NA) and adrenaline (about two-fold) in the femoral artery. Higher concentrations of NPY (greater than 10(-8)M) caused an adrenoceptor-resistant contraction per se. The TNS-evoked [3H]NA efflux was significantly reduced by NPY in a concentration-dependent manner (threshold 10(-9)M). The calcium antagonist, nifedipine, abolished the contractile effects of NPY and the NPY-induced enhancement of NA contractions but did not influence the prejunctional inhibition of [3H]NA release. Receptor-binding studies showed that the ratio of alpha 1-to alpha 2-adrenoceptors in the femoral artery was 30:1. The NPY did not cause any detectable change in the number of alpha 1-or alpha 2-adrenoceptor binding sites or in the affinity of alpha 2-binding sites, as revealed by prazosin- and clonidine-binding, respectively. The NPY also inhibited the TNS-evoked [3H]NA release (by 42-86%) in the superior mesenteric and basilar arteries and in femoral and portal veins. The NPY still depressed TNS-evoked [3H]NA secretion from the portal vein in the presence of phentolamine. The NPY caused a clear-cut contraction in the basilar artery, increased the contractile force of spontaneous contractions in the portal vein, while only weak responses were observed in the superior mesenteric artery and femoral vein. The NA-induced contraction was markedly enhanced by NPY in the superior mesenteric artery, only slightly enhanced in the portal vein and uninfluenced in the femoral vein. In conclusion, in all blood vessels tested, NPY depresses the TNS-evoked [3H]NA secretion via a nifedipine-resistant action. Furthermore, NPY exerts a variable, Ca2+-dependent vasoconstrictor effect and enhancement of NA and TNS contractions.

Evidence for different pre-and post-junctional receptors for neuropeptide Y and related peptides

The effects of neuropeptide Y (NPY), peptide YY (PYY), desamido-NPY and five C-terminal fragments of NPY or PYY were tested on different smooth muscle preparations in vitro. The fragments were NPY 19-36, NPY 24-36, PYY 13-36, PYY 24-36 and PYY 27-36. NPY and PYY appear to exert three principally different effects at the level of the sympathetic neuroeffector junction. Firstly, they have a direct post-junctional effect, leading to constriction of certain blood vessels; this was studied on the guinea-pig iliac vein. Secondly, they potentiate the response to various vasoconstrictors; this was studied on the rabbit femoral artery and vein, using noradrenaline and histamine, respectively, as agonists. Thirdly, NPY and PYY act prejunctionally in that they suppress the release of noradrenaline from sympathetic nerve endings upon stimulation; this was studied in the rat vas deferens. NPY and PYY were approximately equipotent in constricting the guinea-pig iliac vein, while desamido-NPY and the fragments were without effect. Desamido-NPY and the fragments were ineffective also in potentiating the response to noradrenaline in the rabbit femoral artery, nor did they potentiate the response to histamine in the rabbit femoral vein. NPY and PYY potentiated the response to noradrenaline in the artery, as well as the response to histamine in the vein. The NPY- and PYY-induced suppression of noradrenaline release from the prostatic portion of the rat vas deferens was reproduced by PYY 13-36 but not by the shorter fragments nor by desamido-NPY. In conclusion, a C-terminal portion seems to be sufficient for exerting the prejunctional effect of NPY and PYY, while the whole sequence seems to be required for post-junctional (direct and modulatory) effects. An amidated C-terminal is crucial for maintaining the biological activity of NPY. Desamido-NPY and the fragments that were inactive as agonists also seemed inactive as antagonists.

Nitro compounds (isosorbide dinitrate, 5-isosorbide mononitrate and glyceryl trinitrate) on the femoral vein and femoral artery

In organ bath studies, the selectivity of isolated femoral vein and artery of rabbit to isosorbide dinitrate (ISDN), 5-isosorbide mononitrate (ISMN), major metabolite of ISDN, and glyceryl trinitrate (GTN) was compared. The femoral vein and artery contracted by norepinephrine were relaxed by all the nitro compounds dose-dependently. Potency order was GTN greater than ISDN greater than ISMN. The maximum inhibitory responses to the nitrocompounds and their pIC50 values (negative logarithms of doses to induce the 50% response) were greater in the femoral vein than in the femoral artery. For ISMN a 3 times greater sensitivity of femoral vein than of femoral artery was found.

Possible mechanism of the synergistic effect of heparin and dihydroergotamine

A combination of heparin and dihydroergotamine mesylate (Heparin-Dihydergot) has been shown to be more effective than either of these drugs alone in preventing postoperative venous thromboembolism. The possible mechanism for the synergistic effect of heparin and dihydroergotamine was assessed. Phlebograms obtained after administration of 0.5 mg of dihydroergotamine showed marked constriction of the tibial and soleal veins with a significant reduction in the mean diameter of the femoral vein. The effect on the coagulation mechanism was assessed in 28 patients. Blood samples were withdrawn from the femoral vein during varicose vein operations. A significant difference was observed in antithrombin III activity, kaolin-cephalin clotting time, beta-thromboglobulin, and fibrino-peptide levels between the control, dihydroergotamine, heparin, and the combination groups.

Endothelium-dependent contractions to arachidonic acid are mediated by products of cyclooxygenase

Arachidonic acid produces endothelium-dependent relaxation in canine arteries and endothelium-dependent contraction in veins. In canine femoral arteries, the relaxation is prevented by inhibitors of cyclooxygenase. To determine the role of cyclooxygenase in the contraction evoked by arachidonic acid in the veins, rings of canine femoral and intrapulmonary veins, with and without endothelium, were suspended in organ chambers and set at their optimum length for isometric tension measurements. In rings of femoral and pulmonary vein contracted with norepinephrine, arachidonic acid produced a concentration-dependent increase in tension that was eliminated by removal of the endothelium or by treatment with the inhibitors of cyclooxygenase (indomethacin, meclofenamate, or acetylsalicyclic acid). The contractions were not prevented by inhibitors of thromboxane synthetase or prostacyclin synthetase or lipoxygenase. Pulmonary and femoral veins with or without endothelium relaxed to low, but contracted to high concentrations of prostacyclin and prostaglandin E2. Prostaglandin F2 alpha caused endothelium-independent contractions in both blood vessels. The present study suggests that the endothelium-dependent contractions to arachidonic acid observed in canine veins are mediated by prostanoids other than thromboxane and prostacyclin.

[Experimental pharmacological studies of the venous tonus-modifying effect of dihydroergotamine]

In isolated human femoral veins, the dihydrogenated ergot alkaloid dihydroergotamine (DHE) produced an increase in tone at nanomolar concentrations. With regard to the onset and duration of action and reversibility, the DHE-induced contractions differed from those induced by noradrenaline and serotonin. In isolated femoral arteries examined for comparison DHE at comparatively high concentrations was much less effective. It is likely that the vasoconstrictor effect of DHE is due in part to its interaction with serotonin receptors and alpha-adrenoceptors. DHE as a partial agonist is able to antagonize the effects of serotonin and noradrenaline.

Studies on 5-hydroxytryptamine receptors on isolated human femoral veins and arteries and the influence of dihydroergotamine

5-Hydroxytryptamine (5-HT) caused concentration-dependent contractions in human postmortem femoral venous and arterial strips. In veins, it was a more potent agonist than noradrenaline, while the reverse was true for arteries. Pizotifen and methysergide were competitive antagonists against 5-HT in femoral arteries with pA2 values of 8.18 and 8.37, while in femoral veins they exerted noncompetitive antagonism. Dihydroergotamine (DHE) antagonized the 5-HT effect in a manner which was not competitive both in veins and arteries. At concentrations employed in antagonist experiments, it increased the resting tone, particularly in veins. The contractile response of veins to DHE was inhibited when the vascular preparations were preincubated with pizotifen at nanomolar concentrations. The results suggest that 5-HT receptors on veins differ from those on arteries. DHE possesses comparatively high affinity for 5-HT receptors on veins. It is concluded that 5-HT receptors may be involved in the contractile response of veins to DHE.

Characterisation of postjunctional alpha-adrenoceptors in isolated human femoral veins and arteries

In order to characterise the pharmacological properties of postjunctional alpha-adrenoceptors, both the contractile effects of alpha-adrenoceptor agonists and the blocking potencies of selective alpha-adrenoceptor antagonists were studied in isolated human femoral veins and arteries. The veins were more sensitive to noradrenaline than the arteries. Guanfacine had a higher intrinsic activity in veins than in arteries, whereas the reverse was true for phenylephrine. The antagonists rauwolscine and yohimbine were more potent against noradrenaline in the veins than in arteries, while corynanthine was equally potent in either tissue. They antagonised the noradrenaline response in a competitive manner. Prazosin proved to be the most potent competitive antagonist in arteries, while in veins it exerted weak and non-competitive antagonism. The results suggest that the alpha-adrenoceptor population at the postjunctional site differs between human femoral veins and arteries. The veins seem to contain more alpha 2- than alpha 1-adrenoceptors postjunctionally, whereas in the arteries the alpha 1-subtype prevails. The results indicate the possibility of influencing selectively adrenergic reactions in the capacitance and resistance vessels.

Actions of co-dergocrine mesylate and its components at vascular smooth muscle

The mechanisms by which co-dergocrine mesylate (CODE) and its components modify vascular tone was investigated in vitro. Changes in tension were monitored isometrically on spiral strips of canine basilar arteries and femoral veins suspended in 10 ml organ baths and on spiral strips of canine saphenous veins superfused between two platinum electrodes for electric stimulation (150 mA, 0.1 ms, 2 Hz) and measurement of tritium overflow. Cumulative concentration-response curves were established for 5-hydroxytryptamine (5-HT), noradrenaline (NA), dihydroergocornine (DHCO), dihydroergocristine (DHEC), dihydro-alpha-ergokryptine (DH alpha E), dihydro-beta-ergokryptine (DH beta E) and CODE. CODE and its single components were about equally potent in antagonizing responses to 5-HT on basilar arteries. On femoral veins DHEC antagonized responses to NA noncompetitively whereas the other components caused competitive antagonism against NA. Compared to CODE, DH alpha E and DH beta E, the alpha-blocking potency of both DHCO and DHEC was about 10 times weaker. All 5 ergot compounds were about equally potent in antagonizing contractile responses to electric stimulation of saphenous veins whereas the stimulation-induced overflow of labeled NA was inhibited in the following order of potency: CODE greater than or equal to DHCO greater than DH alpha E greater than DH beta E greater than DHEC. The 5 ergot compounds stimulated arterial and venous smooth muscle at slightly higher concentrations than necessary for antagonism of 5-HT or NA. Evidence is presented that the mechanism of the stimulant action on arteries differs from that on veins. It is suggested that the diversity of actions of CODE is the base for its therapeutic effectiveness in patients suffering from regional vascular disorders.

Dual effect of dihydroergotamine and dihydroergotoxin in isolated human femoral veins and arteries

In human femoral vein strips, dihydroergotamine and dihydroergotoxin caused a slowly proceeding, long-lasting increase in tone. Dihydroergotamine had markedly higher affinity than noradrenaline, it possessed nearly the same intrinsic activity as noradrenaline. Dihydroergotoxin was less effective than dihydroergotamine. In isolated femoral artery strips, dihydroergotamine showed lower affinity and lower intrinsic activity in terms of noradrenaline. Dihydroergotoxin had negligible vasoconstrictor effects in arteries. In veins, dihydroergotoxin had a stronger alpha-adrenolytic effect than in arteries. In isolated arteries, dihydroergotoxin was equieffective to dihydroergotamine whose alpha-adrenoceptor blocking effect did not significantly differ in veins and arteries. Our results show that dihydroergotamine at very low concentrations exerted predominantly venoconstrictor effects and, therefore, it is more suited to increase the venous tone than dihydroergotoxin which at low concentrations showed alpha-adrenoceptor blocking activity.

Microvascular response to angiography

A controlled laboratory study using matched groups fo pure-bred Sprague-Dawley rats evaluated the histologic microvascular response to contrast angiography. The femoral vessels were studied by both light and transmission electron microscopy (TEM) following injection of the infrarenal aorta with Renografin-60 or saline at intervals from 2 hours to 56 days. None of the specimens studied showed significant cellular, interstitial, or intravascular injury. TEM demonstrated electron-dense particles which migrated from the vessel lumen into the endothelial cytoplasm and basal lamina and subsequently diffused back into the cytoplasm. This study demonstrates the absence of microvessel damage from a radiopaque contrast medium administered by techniques designed to parallel the clinical model.

[Comparative responsiveness of isolated saphenous, femoral and external carotid arteries and veins of dogs to dihydroergotamine mesylate (DEM) (author's transl)]

Changes in tension of spiral strips of saphenous, femoral and external carotid arteries and veins were measured isometrically. DEM stimulated 6 preparations in almost the same concentration ranges (pD2 values were 8.64 to 8.40). However, the dose-response curves for DEM indicted variations in responsiveness among the different arteries and veins. Compared with norepinephrine (NE) (1)( the intrinsic activity of DEM was 0.002 on saphenous arteries, 0.05 on saphenous veins, 0.03 on femoral arteries, 0.18 on femoral veins, 0.13 on external carotid arteries and 0.13 on external carotid jugular veins. Thus DEM contracted more potently the venous strips from the hind limbs. IN femoral and external carotid arteries, antagonism of serotonin by DEM or methysergide was investigated. DEM displaced the dose-contractile response curves for serotonin in a noncompetitive manner, and the antagonistic response of DEM to serotonin was about 8 times more effective in external carotid arteries (pD'2 value=6.96) than in femoral arteries (pD'2 value=6.05). Methysergide, unlike DEM, antagonized the response to serotonin in a competitive manner at low doses but in a noncompetitive manner in high doses, and was fairly equal in antagonizing response to serotonin in external carotid arteries (pA2 value=that the therapeutic value of DEM in the treatment of orthostatic hypotension is due to its selective anti-serotonin activity on the external carotid arteries.

Uneven distribution of postjunctional alpha 1-and alpha 2-like adrenoceptors in canine arterial and venous smooth muscle

We studied isolated canine arteries and veins to compare the pharmacological properties of their postjunctional alpha-adrenoceptors. Rings of femoral and splenic arteries and of femoral and saphenous veins were mounted for isometric tension recording in organ chambers filled with Krebs-Ringer bicarbonate solution. The four blood vessels contracted when exposed to methoxamine, norepinephrine, phenylephrine, and tramazoline; clonidine failed to induce contraction only in the splenic artery. The relative sensitivity to methoxamine was comparable in the arteries and veins, but that for phenylephrine was larger in the former. The veins, but not the arteries, were more sensitive to clonidine and tramazoline than to phenylephrine or methoxamine. Phentolamine was a competitive antagonist against norepinephrine in the arteries and the veins. Prazosin was a competitive antagonist arteries. The competitive antagonistic properties of yohimbine were more pronounced in the veins than in the arteries. Verapamil depressed to the same extent the contractile responses of saphenous veins to clonidine and norepinephrine, but reduced the contractions caused by methoxamine more than those due to norepinephrine. These results indicate the presence of both alpha 1- and alpha 2-like adrenoceptors on venous smooth muscle cells, whereas arterial smooth muscle cells contain mainly postjunctional alpha 1-adrenoceptors.

Femoral vascular responses to purine and pyrimidine derivatives: release of 5-hydroxytryptamine by purine derivatives in isolated, cross-circulated rat hindlimb

The mode of actions of the purines, adenosine, adenosine-5'-triphosphate (ATP), guanosine and guanosine-5'-triphosphate (GTP), and the pyrimidines, cytidine, cytidine-5'-triphosphate (CTP), thymidine, thymidine-5'-triphosphate (TTP), uridine and uridine-5'-triphosphate (UTP) was investigated in the isolated hindlimb preparation of the rat. A single injection of adenosine, ATP, GTP or UTP into the femoral artery induced a biphasic response, a prominent vasoconstriction preceded by a transient vasodilatation, whereas guanosine and uridine caused only a vasoconstriction. Cytidine, CTP, thymidine and TTP were almost ineffective on the vascular bed. The vasoconstrictor responses to adenosine, ATP, guanosine and GTP were effectively antagonized by either methysergide or reserpine, whereas those to uridine and UTP were not modified by either methysergide or phentolamine. Adenine, D-(-)-ribose and hypoxanthine had no effect on the vascular bed. The 5-hydroxytryptamine (5-HT) release from the hindlimb was fluorometrically determined. The present results provide direct evidence that the vasoconstriction caused by ATP, adenosine, GTP and guanosine is attributed to the release of 5-HT from their stores and that purine nucleotides and nucleosides are capable of releasing 5-HT.

Tryptaminergic mechanism participating in induction of vasoconstriction by adenine nucleotides, adenosine, IMP and inosine in the isolated and blood-perfused hindlimb preparation of the rat

The isolated right hindlimb of the recipient rat was perfused at a constant flow rate through the femoral artery with heparinized blood from the carotid artery of a donor. The preparations were under a 99.0 +/- 0.8 mmHg of mean perfusion pressure (N = 63) and 3.3 +/- 0.1 ml/min of blood flow through the right femoral artery. The actions of adenosine, adenosine tri-, die- and monophosphate, inosine monophosphate and inosine on the femoral vascular bed were investigated, respectively. These substances injected into the femoral artery, with the exception of inosine, caused a dose-dependent vasoconstriction always preceded by a temporal vasodilatation. Inosine induced only a prompt vasoconstriction. The vasoconstrictor responses to these substances were diminished or reverted to vasodilator ones after repeated administrations and such were significantly prevented by pretreatment with either reserpine or methysergide. These results indicate that all the purines tested induce a vasoconstriction in the femoral vascular bed of the rat through a common (tryptaminergic) mechanism and that such seem to be potent releasers of 5-hydroxytryptamine from peripheral tryptaminergic storage sites.

Comparison of arteries with longitudinal and circular venous muscle from the rat

Rat portal, mesenteric, renal, and femoral veins possess functionally responsive circular and longitudinal smooth muscle layers in vitro. In contrast to the dominant rhythmically active longitudinal muscle of portal veins, rat longitudinal mesenteric veins lacked rhythmic activity and developed maximal force equivalent to that of mesenteric circular muscle. Renal and femoral veins exhibited predominantly circular smooth muscle responses. Rat veins must be subjected to between 1 g (renal and femoral) and 4 g (circular portal and mesenteric) of passive forse for optimal responsiveness. Contractile response to vasoactive agent including carbamylcholine, serotonin, and norepinephrine was quantitatively different among veins. Femoral veins developed greater maximal force in response to norepinephrine than to KCL, and the responses to norepinephrine were not altered by cocaine. In contrast, cocaine markedly potentiated responses to norepinephrine in portal, mesenteric, and renal veins and, to a lesser extent, in the mesenteric artery. These data demonstrate heterogeneity in rat venous tissue and suggest that neuronal innervation may markedly influence responses to norepinephrine in some, but not all, rat blood vessels.

Pharmacological basis of the treatment of orthostatic disorders with ergot alkaloids

Ergot alkaloids increase the tone of isolated canine vein strips dose-dependently in concentrations considerably lower than noradrenaline, but the maximal responses are only about one third of those to noradrenaline. This action can be blocked by prior administration of phentolamine. It is therefore concluded that the long-lasting stimulant action of ergot alkaloids on vascular smooth muscle is mediated mainly by alpha-adrenoceptors. Using the autoperfused hind limb of the cat, it has been shown that dihydroergotamine increases dose-dependently the tone of the capacitance vessels in a manner very similar to electrical stimulation of the sympathetic nerve with increasing frequencies. However, in contrast to sympathetic nerve stimulation, dihydroergotamine elicits only a very weak increase in arteriolar resistance.

16alpha-hydroxy-progesterone as a sex hormone antagonist: effect on venous diameters in the female mouse

Pellets of progesterone plus 1% 16alpha-hydroxy-progesterone (16alpha-OH), testosterone plus 1% 16alpha-OH, and estradiol-17beta plus 1% 16alpha-OH were implanted in the right uterine fat mass in ovariectomized (O) and ovariectomized-hysterectomized (OH) mice. Three weeks later they were killed and the diameters of the ovarian, uterine, and femoral veins and the inferior vena cava were measured and averaged. The averages were compared with averages for O and OH mice bearing pellets of glass and of progesterone, testosterone, and estradiol-17beta without 16alpha-OH. It is concluded that in castrate mice the size of ovarian and uterine veins is influenced not by the presence or absence of the uterus but by sex steroids, that progesterone usually causes decreases while testosterone and estradiol-17beta cause increases in vein diameter, and that both the presence of the uterus and the administration of 1% 16alpha-OH are associated with partial inhibition of increases in vein size induced by steroids. Uterine hypertrophy and urinary bladder distention resulting from the administration of testosterone and estradiol-17beta were not prevented by the addition of 1% 16alpha-OH.

Investigations on the mode of action of ergotamine in the isolated femoral vein of the dog

1 Experiments on spiral strips cut from the femoral vein of dogs suspended in Krebs-Henseleit solution were carried out.2 Ergotamine caused stimulation in concentrations about 350 times lower than noradrenaline (ED(50) of ergotamine = 2.2 x 10(-9) M; ED(50) of noradrenaline = 7.6 x 10(-7) M), but the maximal responses to ergotamine were only about one third those to noradrenaline.3 The pA(2) value of ergotamine against noradrenaline was 8.8.4 The effects of ergotamine can be blocked by prior administration of phentolamine. The pA(2) value for phentolamine against ergotamine was 6.8 and the pA(2) value for phentolamine against noradrenaline was 7.5.5 It is concluded that the stimulant action of ergotamine on smooth vascular muscle probably is mediated mainly via alpha-adrenoceptors.

Blockade of peripheral vascular responses to isoprenaline by three beta-adrenoceptor antagonists in the anaesthetized dog

1 A dog's hind limb was vascularly isolated by strong nylon snares and its sympathetic nerve supply interrupted. Blood was perfused at constant flow into the femoral artery and drained from the femoral vein. In some dogs the cardiac nerves were cut.2 Isoprenaline infused intravenously caused an increase in heart rate and a decrease in arterial resistance.3 Practolol (2 mg/kg) effectively blocked heart rate responses to isoprenaline infused at up to 10 mug/min but was relatively ineffective in blocking arterial responses. ICI 66082 (2 mg/kg) reduced vasomotor responses and propranolol (0.5 mg/kg) abolished vasomotor responses.4 Small cumulative doses of beta-adrenoceptor antagonists were given to some dogs. Practolol blocked heart rate responses in lower doses than were required to block vasomotor responses. Propranolol preferentially blocked vasomotor responses and ICI 66082 was intermediate between the other two in its effects.

The actions of a new beta-adrenoceptor blocking drug, ICI 66082, on the rabbit papillary muscle and on the dog heart

1 The actions of 4-(2-hydroxy-3-isopropylaminopropoxy) phenyl acetamide (ICI 66082), a new beta-adrenoceptor blocking drug, on the twitch response of the isolated papillary muscle of the rabbit and on dP/dt max and free heart rate of a denervated dog heart preparation, are described.2 ICI 66082 (up to 1 mg/ml) did not produce any depression of the twitch response of the rabbit papillary muscle. ICI 66082 antagonized the action of isoprenaline on this preparation at a concentration of 0.01 mug/ml.3 ICI 66082 (0.5-1.0 mg/kg intravenously) reduced the control value of dP/dt max in four dog preparations by a mean value of 529 mmHg/s (s.e. mean +/- 139 mm Hg/s), with no significant change in free heart rate. Antagonism of the effect of isoprenaline on dP/dt max and on free heart rate was demonstrated with ICI 66082 (0.1 mg/kg).4 ICI 66082 (1.0-1.5 mg/kg) produced no significant changes in dP/dt max or in free heart rate in four dogs pretreated with reserpine. A significant reduction (16% of the control value) in dP/dt max was observed with ICI 66082 at a high dose of 40-50 mg/kg.5 It is concluded that ICI 66082 is a competitive antagonist against the actions of isoprenaline on cardiac muscle, has no negative inotropic action (unless the dose exceeds 40 mg/kg) and lacks intrinsic sympathomimetic activity.

Response of intrahepatic vasculature to isoproterenol and epinephrine influsions

Response of hepatic arterial and intrahepatic portal venous vasculature to 10-min infusions of either isoproterenol or epinephrine was studied in 37 <i>in situ</i> dog liver preparations. Isoproterenol infusions at various dose rates (3.06–24.45 µg/min) were given by way of hepatic artery or portal vein. Hepatic arterial vasodilation, which was abolished by propranolol, was clearly evident while response of intrahepatic portal venous bed was incon sistent and not indicative of significant vasodilation. Arterial dilator response was attenuated later in the infusion period when isoproterenol was given at high dose rates. Femoral venous infusions of epinephrine (9.55 µg/min) resulted in a variable and slight hepatic arterial response while portal resistance was consistently increased. Following sotalol, epinephrine produced a marked increase in hepatic artery resistance in every case while intrahepatic portal venous resistance was increased to the same extent as before. It was concluded that both α-receptors (constrictors) and β-receptors (dilators) are present in canine hepatic arterial and mesenteric vessels while intrahepatic portal venous vasculature appears to possess few, if any, β-receptors.

Activity and thermosensitivity of canine cutaneous veins after inhibition of monoamine oxidase and catechol-O-methyl transferase

The lateral saphenous vein of the dog was perfused with homologous blood at constant flow. Changes in driving pressure were used to measure active changes in venous tone. Administration of tranylcypromine greatly reduced the activity of monoamine oxidase in the vein and increased its response to electric stimulation of the lumbar sympathetic chain and to norepinephrine infusions. Administration of pyrogallol (a catechol-O-methyl transferase inhibitor) also increased the response of the vein to adrenergic stimulation. These chemicals did not alter the increase in the sensitivity of the venous smooth muscle to sympathetic nerve impulses and infused norepinephrine produced by cooling the perfusate. Thus, the potentiating effect of local cooling on the response of the cutaneous veins to adrenergic stimulation cannot be explained by interference with the local enzymatic inactivation of catecholamines.