Influence of 5- and 6-hydroxydopamine on adrenergic transmission and nerve terminal morphology in the canine pulmonary vascular bed

We studied the effects of 5- and 6-hydroxydopamine on adrenergic neurotransmission, fluorescence histochemistry, and nerve terminal ultrastructure in the canine pulmonary vascular bed. Fluorescence histochemistry on stretched preparations and sections of intrapulmonary artery and vein demonstrated that these vessels are well supplied with adrenergic nerves electron microscopy revealed adrenergic terminals in the adventitia and outer third of the media in the artery, but only in the adventitia in the vein. Adrenergic terminals in artery and vein contained many small and a few large dense-core vesicles. At least 20% of the terminals in the artery contained many small agranular vesicles and a few large opaque vesicles; this suggests that they were of the cholinergic type; Such terminals were not found in intrapulmonary veins. Under conditions of controlled blood flow, stimulation of the sympathetic nerves to the lung and intralobar injection of norepinephrine increased pressure in the perfused lobar artery and small intrapulmonary vein in a stimulus-related manner. The rise in pressure in the lobar artery and vein in response to nerve stimulation was blocked after administration of either 5- and 6-hydroxydopamine; Neither agent modified the response of the pulmonary vascular bed to norepinephrine; In contrast, the rise in pressure in the lobar artery and vein in response to both norepinephrine and to nerve stimulation was blocked by phenoxybenzamine, an alpha-receptor blocking agent. The attenuated neurogenic vasoconstrictor response in dogs treated with 5- and 6-hydroxydopamine was associated with a marked decrease in intensity of fluorescence of the abundant adrenergic innervation in both intrapulmonary artery and vein, and with the appearance of an osmiophilic material in dense-core vesicles of adrenergic terminals in artery and vein. We believe that these data suggest that 5- and 6-hydroxydopamine interfere with adrenergic transmission in intrapulmonary vessels by depleting norepinephrine from adrenergic terminals. Furthermore, we conclude from hemodynamic, histochemical, and ultrastructural studies that vasomotor tone in the pulmonary vascular bed can be regulated by the sympathetic nervous system.

The effect of indomethacin on renal function in pentobarbital-anesthetized dogs

The effects of indomethacin, an inhibitor of prostaglandin synthesis, on renal blood flow, glomerular filtration rate, urine flow and excretion of sodium and potassium were studied in the anesthetized dog. Indomethacin, 2.5 mg/kg i.v., decreased renal blood flow but increased aortic pressure and calculated renal vascular resistance. Glomerular filtration rate was not influenced by the synthetase inhibitor. Sodium excretion was decreased and para-aminohippurate extraction was increased after administration of indomethacin. Transient decreases in urine flow and potassium excretion were observed; however, both parameters returned to control value 75 minutes after administration of indomethacin. The early decrease in urine flow rate correlated closely with the decrease in sodium excretion. These data suggest that in the anesthetized dog, endogenous prostaglandins may serve to maintain renal blood flow but not glomerular filtration rate. Under the conditions of the present experiments, sodium excretion and to a lesser extent potassium excretion have been suggested as being dependent on prostaglandin synthesis.

Influence of inhibitors of prostaglandin synthesis on venoconstrictor responses to bradykinin

The interrelationship between prostaglandins (PGs) and bradykinin (BK) was studied in isolated canine saphenous veins. The hypothesis that PGs mediate the venoconstrictor effect of bradykinin was evaluated by determining the influence of low concentrations of indomethacin (Indo) (1 muM) or eicosa-5,8,11,14-tetraynoic acid (ETA) (3 muM), two inhibitors of PG synthesis, on cumulative concentration-response curves for BK or norepinephrine (NE). In the tissue bath, responses to BK improved with time while responses to NE did not vary. When strips were least responsive to BK, Indo and ETA enhanced these responses. When strips were most responsive to BK, neither inhibitor enhanced the responses. Neither Indo nor ETA altered responses to NE. Phentolamine (10(-8) M) did not alter responses to BK. These data suggest that endogenous PGs act to attenuate, rather than mediate, the venoconstrictor response to BK. Progressive enhancement of responses to BK of untreated saphenous vein strips is associated with a decreased ability of inhibitors of PG synthesis to enhance those responses also. Thus, there may be a time-related decrease in the ability of this preparation to synthesize PGs. From the present results, it cannot be determined whether saphenous veins in vivo are highly responsive or relatively unresponsive to the peptide. However, these data do suggest that PGs are a determinant of venous responsiveness to BK.

Studies on the nature of a prostaglandin receptor in canine and rabbit vascular smooth muscle

The contractile response of rabbit renal arteries and canine tibial arteries to prostaglandins A2, B2, F2alpha, E1, E2, D2, and B1 was associated with a reduction in total sulfhydryl group content of smooth muscle. The total sulfhydryl content of rabbit renal and canine tibial arteries and was not affected by norepinephrine or potassium chloride. Reduction of disulfide groups with dithiothreitol (DTT) selectively inhibited contractile responses to angiotensin and prostaglandins; 5,5'-Dithiobisnitrobenzoic acid (DTNB), a sulfhydryl group-oxidizing agent, reversed the inhibitory effect of DTT on the contractile responses to prostaglandins. Alkylation of free sulfhydryl groups with ethacrynic acid did not affect the contractile response of isolated canine tibial or rabbit renal arteries to any agonist studied. Ethacrynic acid added to muscle strips exposed to DTT resulted in alkylation of sulfhydryl groups produced by reduction of disulfide bonds and irreversibly prevented DTNB-induced reversal of DTT inhibition of contractile responses to prostaglandins. However, addition of ethacrynic acid to muscle strips contracted by prostaglandins did not inhibit subsequent responses to these acidic lipids. These findings support the hypothesis that contractile responses of rabbit renal and canine tibial arteries to prostaglandins are dependent on interactions between prostaglandins and disulfide groups located in or on the vascular smooth muscle cell, and the concept that membrane disulfide groups may be integral components of vascular smooth muscle receptors for prostaglandins.

Influence of prostaglandins E, A and F on vasoconstrictor responses to norepinephrine, renal nerve stimulation and angiotensin in the feline kidney

The effects of intrarenal infusion of prostaglandins (PGs) of the E, A and F series on renal vascular resistance and on vasoconstrictor responses to renal nerve stimulation (RNS), norepinephrine (NE) and angiotensin (A) were determined in the in situ feline kidney under conditions of controlled blood flow. Infusion of PGE2 (3 and 0.3 mug/min) and PGE1 (3 mug/min) resulted in a marked decrease in renal perfusion pressure and a reduction in responses to all vasoconstrictor stimuli. PGE2 (0.03 mug/min) did not alter perfusion pressure. However, responses to RNS and A but not to NE were attenuated. PGA2 (3 and 0.3 mug/min) had no significant effect on perfusion pressure. PGA1 (3 mug/min) resulted in a transient decrease in renal vascular resistance which was not maintained during the infusion period. PGA2 (3 mug/min) reduced the response to RNS at 10 and 30 cps and reduced the response to A, whereas responses to NE were not affected. PGA2 (0.3 mug/min) had no effect on responses to either of the pressor stimuli. PGA1 infusion resulted in an enhanced response to RNS at the highest stimulus frequency and decreased the response elicited by A. PGF2alpha (3 mug/min) had no significant effect on renal vascular resistance or on responses to NE and nerve stimulation. However, the response to angiotensin was decreased and responses to RNS at 10 and 30 cps were decreased 30 minutes after the PGF2alpha infusion. The present data demonstrate that, of the natural renal PGs, PGE2 and PGA2 possess the capacity to modulate the effects of the sympathetic nervous system on the feline kidney. In addition, the effects of PGE and PGA on responses to adrenergic stimuli and on vascular resistance could be separated.

Interaction of prostaglandin A2 and prostaglandin B2 on vascular smooth muscle tone, vascular reactivity and electrolyte transport

The effects of prostaglandin A2 (PGA2) and prostaglandin B2 (PGB2) on vascular smooth muscle tone, electrolyte movements and responses to vasoactive stimuli were evaluated with superfused canine tibial arteries. PGA2 and PGB2 constricted superfused tibial arteries. PGB2 was 10.7 (8.3-14.1) times more potent as a constrictor than PGA2. PGA2 and PGB2-induced vasoconstriction was associated with a decrease in 22Na efflux and a tendency toward an increase in cellular sodium (expressed as micromoles per gram of wet weight). These prostaglandins also decreased the total potassium content of tibial arteries. 45Ca exchange was enhanced by PGA2 and PGB2. The time course of PG-induced increases in 45Ca efflux was similar to the temporal increase in force produced by PGA2 and PGB2. The duration of the contractile response to barium chloride was greatly prolonged during superfusion with both PGA2 and PGB2. These effects were probably not mediated by PG-induced alterations in the resting membrane potential of tibial arteries since presumed depolarization by both high potassium and zero-potassium physiologic saline solutions did not mimic the effects of these prostaglandins on vascular smooth muscle tone or responses to barium chloride. These data suggest that PGA2 and PGB2 may increase tone of vascular smooth muscle by inhibition of those processes involved in sequestration of calcium ion, i.e., the relaxation process, rather than acting on the contractile process.

Actions of prostaglandins E1 and F2alpha on isolated intrapulmonary vascular smooth muscle

The effects of PGE1 and PGF2alpha were studied on isolated strips of intrapulmonary arteries and veins from dog, sheep, swine and man. PGF2alpha contracted human arterial strips in a dose-dependent fashion, relaxed slightly sheep arteries and had no effect on dog arteries. Canine, sheep and human venous strips were contracted by PGF2alpha. PGE1 relaxed slightly both veins and arteries from dog and sheep. Human arteries usually contracted slightly and human veins usually relaxed slightly to PGE1. In a limited number of experiments, swine arteries and veins failed to respond to PGF2alpha or PGE1. All the vascular strips contracted well when exposed to NE. These results suggest that the responses of intrapulmonary vessels to PGF2alpha and PGE1 are species-dependent. PGF2alpha generally exhibits a contractile action, especially on veins. PGE1 usually relaxes intrapulmonary vessels. With regard to vessels from man, PGF2alpha is a powerful stimulant while PGE1 produces only small, variable effects.

Contractile responses of canine isolated pulmonary lobar arteries and veins to norepinephrine, serotonin, and tyramine

Isolated helical strips of canine intrapulmonary lobar arteries and veins (about 4 mm in diameter) undergo dose-related tension development when exposed to increasing concentrations (10(-8) - 10(-3) M) of norepinephrine (NE), serotonin or 5-hydroxytryptamine (5-HT) and tyramine (Tyr). Venous segments were generally more sensitive while the maximum tension development was greater in the arterial strips, probably owing to their greater thickness. Both strips were more sensitive to 5-HT than NE and only responded to Tyr at high concentrations. Norepinephrine and 5-HT were nearly equally efficacious, whereas Tyr was less so. Responses to the latter were slow to develop, exhibited tachyphylaxis, and were greatly inhibited by phentolamine (10(-8) M), an alpha-adrenergic blocker. Exposure to cocaine (10(-5) M) enhanced submaximal NE responses, inhibited Tyr contractions and had no consistent effect on 5-HT responses. Phentolamine (10(-8) M) was also found to inhibit NE responses without altering 5-HT probably acts on other receptors. Tyramine may, in part, act directly on alpha-adrenergic receptors but may also release NE from surviving adrenergic nerve terminals in the preparation. Cocaine inhibits this effect and potentiates responses to lower levels of NE, presumably by blocking NE uptake into nerve terminals although a post-junctional action cannot be excluded.

Influence of inhibitors of prostaglandin synthesis on the canine pulmonary vascular bed

The effects of two chemically dissimilar inhibitors of prostaglandin (PG) synthesis on vascular resistance and responses to pressor and depressor hormones were evaluated in the canine pulmonary vascular bed. Indomethacin or meclofenamate, 2.5-5 mg/kg iv, increased lobar arterial pressure. Since lobar blood flow was held constant and left atrial pressure did not change, the rise in pressure reflects an increase in vascular resistance. The rise in lobar pressure after indomethacin occurred in the absence of a change in lobar venous or translobar airway pressure. This agent enhanced the response to angiotensin but not to norepinephrine. Meclofenamate decreased responses to both agents. Indomethacin enhanced the dilator response to PGE1 and both indomethacin and meclofenamate increased the response to PGF2alpha. These data indicate that the rise in resistance after indomethacin or meclofenamate was the result of vasoconstriction in vessels upstream to the small veins, presumed to be small arteries. These data are consistent with the hypothesis that under resting conditions synthesis of a dilator prostaglandin may be important for the maintenance of the pulmonary vascular bed in a dilated state. However, results of the present study are not consistent with the postulate that prostaglandins modulate responses to norepinephrine but suggest that indomethacin and meclofenamate interfere with the inactivation of PGF2alpha and PGE1 in the lung.

Bronchopulmonary arterial shunting without anatomic anastomosis in the dog

The effects of bronchial arterial administration of vasoactive substances on the pulmonary circulation were studied by a new technique for selective catheterization of a bronchial artery in intact dogs. In most experiments, this technique permitted pressor agents to be distributed mainly to one lung with smaller amounts to the other lung. The intercostal arteries were avoided, and in all but 2 of 23 experiments only microscopic quantities of injected India ink could be identified in the distribution of the esophageal and mediastinal branches. These studies indicate that serotonin, angiotensin, histamine, and norepinephrine injected selectively into a bronchial artery increase lobar arterial pressure. Since blood flow was constant and left atrial pressure did not change, the increase in pressure suggests active pulmonary vasoconstriction. Additionally, the responses to bronchial and lobar arterial injections of pressor agents were similar. The contribution of bronchopulmonary shunt flow to pulmonary flow was small, since, under conditions of controlled lobar blood flow, changes in bronchial flow elicited by 65-75-mm Hg changes in bronchial arterial pressure produced little if any change in pressure in the perfused lobar artery or small vein. Bronchoconstriction contributed little to the response to bronchial administration of pressor agents, since responses were similar in the ventilated and the collapsed lobe. Injection of vasoflavine dyes into the bronchial artery showed the close proximity of bronchial and pulmonary arteries and confirmed the bronchial arterial origin of the vasa vasorum of pulmonary arteries. No vasa venorum were identified. Although no direct anatomic bronchial artery-pulmonary artery shunt was identified, ascorbic acid and 5-hydroxydopamine diffused rapidly into intrapulmonary arteries from the bronchial artery. These data suggest that the pulmonary pressor response results from passage of the vasoactive agents from the bronchial artery to the lobar artery through the vasa vasorum and by diffusion. Since no vasa venorum were found, pulmonary venoconstriction probably resulted from pressor agents reaching the veins by way of bronchopulmonary shunt flow. These results suggest a mechanism by which pressor substances present or liberated in the bronchial vascular bed can affect tone in the pulmonary vascular bed.

Influence of sympathetic stimulation and vasoactive substances on the canine pulmonary veins

The contribution of the intrapulmonary lobar veins to the increase in pulmonary vascular resistance in response to sympathetic stimulation was studied under conditions of controlled blood flow in the anesthetized dog in which vascular pressures were measured simultaneously in the perfused lobar artery, an intrapulmonary lobar vein 2-3 mm in diameter and in the left atrium. Stimulation of the stellate ganglia at 3, 10, and 30 cycles/s increased pressure in the lobar artery and small vein in a stimulus-related manner but decreased pressure in the left atrium. Injection of norepinephrine into the perfused lobar artery also increased pressure in the lobar artery and small vein but decreased pressure in the left atrium. The increase in lobar arterial and venous pressure in response to either injected norepinephrine or to nerve stimulation was antagonized by an alpha receptor blocking agent. The rise in pressure in both labor artery and small vein with nerve stimulation but not administered norepinephrine was inhibited by an adrenergic nerve terminal blocking agent. The results suggest that under conditions of steady flow, sympathetic nerve stimulation increases the resistance to flow in the lung by constricting pulmonary veins and vessels upstream to the small veins, and that at each stimulus-frequency studied approximately 50% of the total increase in resistance may be due to venoconstriction. It is concluded that the increase in resistance to flow in the lung in response to nerve stimulation is thre result of activation of alpha adrenergic receptors by norephinephrine liberated from adrenergic nerve terminals in venous segments and in vessels upstream to samll veins, presumed to be small arteries.

Unusual venoconstrictor effects of angiotensin II

Cumulative dose-response curves to angiotensin II were performed on helical strips from canine lateral saphenous vein. Threshold concentrations were in the range of 10(-18)-10(-17)M. Increases in angiotensin from 10(-17)-10(-12)M failed to elicit further increases in tension. Subsequent increases in angiotensin concentration from 10(-11)-10(-7)M again produced dose-related increases in tension. Repeated dose-response curves in the same strips showed reduced maximal response. Responses to low concentrations of angiotensin were attenuated by low concentrations of phentolamine. These results suggest that, at extremely low concentrations angiotensin produced marked contractions in canine saphenous vein strips by releasing endogenous norepinephrine from sympathetic nerve terminals.

Comparison of the effects of prostaglandins F1alpha, F2alpha, F1beta, and F2beta on the canine pulmonary vascular bed

The effects of four F series prostaglandins on the pulmonary vascular bed were compared under conditions of controlled pulmonary blood flow in the intact spontaneously breathing dog. PGF1alpha and PGF2alpha increased lobar arterial pressure whereas PGF1beta and PGF2beta had little if any effect when infused into the lobar artery. The increase in lobar arterial pressure in response to PGF1alpha and PGF2alpha was associated with a significant increase in lobar venous pressure but no change in left atrial pressure. These data indicate that PGF1alpha and PGF2alpha increase pulmonary vascular resistance by constricting lobar veins and vessels upstream to small veins, presumed to be small arteries. It is concluded that in the pulmonary vascular bed the configuration of the hydroxyl group at carbon 9 is an important determinant of pressor activity.

NE and ACh responses of intrapulmonary vessels from dog, swine, sheep, and man

The effects of norepinephrine (NE) and acetylcholine (ACh) on isometric force generation were studied in isolated helical strips of intrapulmonary arteries (IPA) and veins (IPV) (3-5 mm diam) from lungs of dog, sheep, swine, and man. Cummalative dose-effect relationships (10-minus8-10-minus 4 M) were determined. All strips contracted in a dose-related fashion when exposed to NE but responses of sheep and swineIPV were weak. Acetylcholine relaxed canine IPA and contracted human IPA, CANINEIPV, and sheep IPV in a dose-related manner. Sheep IPA and swine IPA and IPV were unresponsive to ACh. Human IPV were relaxed by ACh but this effect wasreversed at the highest concentration (10 minus 4 M) tested. All vascular stripscontracted well in a potassium-rich (127 mM) bathing medium. It is concluded thatintrapulmonary lobar vessels from man, dog, swine, and sheep are responsive to autonomicneurohormones but that quantitative as well as qualitative differences exist.

Differential effects of prostaglandins A1 and A2 on pulmonary vascular resistance in the dog

The effects of PGA1 and PGA2 were studied in the canine pulmonary vascular bed. Infusion of PGA1 into the lobar artery decreased lobar arterial and venous pressure but did not change left atrial pressure. In contrast, PGA2 infusion increased lobar arterial and venous pressure and the effects of this substance were similar in experiments in which the lung was perfused with dextran or with blood. These data indicate that under conditions of controlled blood flow PGA1 decreases pulmonary vascular resistance by dilating intrapulmonary veins and to a lesser extent vessels upstream to the small veins, presumably small arteries. The present data show that PGA2 increases pulmonary vascular resistance by constricting intrapulmonary veins and upstream vessels. The predominant effect of PGA2 was on upstream vessels and the pressor effect was not due to interaction with formed elements in the blood or platelet aggregation.

Effects of the 15-methyl analogs of prostaglandins E2 and F2alpha on the pulmonary circulation in the intact dog

The effects of the 15-methul analogs of prostaglandins E2 (PGE2) and F2alpha (PGF2alpha) on the pulmonary circulation were studied in the intact dog under conditions of controlled blood flow. Infusions of either analog into the lobar artery increased lobar arterial pressure by more than 100 per cent. The rise in lobar arterial pressure was accompanied by a rise in lobar venous pressure and in pressure gradient from lobar artery to small vein but no change in pressure in the left atrium. The methyl analogs were about 10 times more potent than PGE2 and PGF2alpha in elevating pulmonary vascular resistance in the dog. The effects of the analogs on the pulmonary vascular bed were similar in experiments in which the lung was perfused with dextran or with blood. Both analogs contracted isolated helical segments of canine intrapulmonary artery and vein in a dose-related manner. In other experiments the effects of passive increases in venous pressure produced by distension of a balloon catheter in the lobar vein were contrasted with the action of the analogs on the pulmonary vascular bed. Balloon distension increased pressure in the lobar artery and small vein but had no effect on pressure in the left atrium. However, in contrast to the increase in gradient with the analogs, balloon distension decreased the pressure gradient from lobar artery to small vein. Results of the present study indicate that the prostaglandin analogs increase pulmonary vascular resistance by actively contricting pulmonary veins and vessels upstream to small veins, presumed to be small arteries. It is concluded that the analogs are potent pressor substances in the pulmonary circulation.

Effect of prostaglandin E2 on pulmonary vascular resistance in intact dog, swine and lamb

The effects of prostaglandin E2 (PGE2) on pulmonary vascular resistance in the intact dog, swine and lamb were studied using a right heart catheterization technique to isolate and perfuse the left lower lung lobe at controlled blood flow. Infusion of PGE2 into the lobar artery increased lobar arterial perfusion pressure but did not alter pressure in the left atrium in all 3 species. The increase in lobar arterial pressure was associated with a rise in pressure in the small intrapulmonary lobar vein in the dog but no change in pressure in these veins in the swine and lamb. Infusion of PGE2 into the iliac artery produced a marked decrease in perfusion pressure in the hindlimb of the dog. The effects of PGE2 on the canine lung occurred in the absence of any significant change in arterial blood gases, pH, hematocrit or rate and volume of respiration, and this substance increased pulmonary vascular resistance when the lung was perfused with dextran instead of blood. These results show that in dog, swine and lamb, PGE2 increases pulmonary vascular resistance; however, the site of vasoconstriction is different in the dog and swine or lamb. In the swine and lamb vasoconstriction occurred primarily in vessels upstream to the small veins, presumably small arteries, whereas in the dog lung, the pre- and postcapillary vessels were actively constricted by this naturally occurring substance.

Effects of indomethacin on venoconstrictor responses to bradykinin and norepinephrine

In order to determine whether the venoconstrictor response to BK was dependent on prostaglandin (PG) synthesis, effects of indomethacin (INDO) on responses to bradykinin (BK) and norepinephrine (NE) were studied in canine lateral saphenous vein. Cumlative dose-response curves (10-9-10-6M BK or NE) were done in the presence and absence of INDO (10-6M). In the presence of INDO, responses to BK were markedly enhanced while responses to NE were unchanged. After prolonged periods in the bath, responses to BK were enhanced in control strips while responses of strips which had been treated with INDO were depressed. These results suggest that BK does not normally cause venoconstriction by stimulating synthesis of a venoconstrictor PG and that the increase in response to BK after prolonged periods in the bath may be related to changes in PG synthesis.

Influence of prostaglandins E1 and F2alpha on pulmonary vascular resistance, isolated lobar vessels and cyclic nucleotide levels

The effects of prostaglandins E1 (PGE1) and F2alpha) on the pulmonary vascular bed were studied in the intact dog under conditions of controlled pulmonary blood flow. PGF2alpha increased lobar arterial and venous pressure when injected or infused into the lobar, artery. The pressor response was dose-related and doses as low as 0.03 and 0.1 mug, which established concentrations of 0.1 to 0.3 ng/ml in lobar arterial blood, increased pulmonary vascular resistance. PGF2alpha also increased airway resistance in the left lower lobe. However, the effects of this substance on the vascular bed were not related to its effects on bronchomotor tone since similar pressor responses were observed in normal and nonrespiring lobes, PGE1 decreased pressure in the lobar artery and vein when infused into the lobar artery and the effects of PGE1 and PGF2alpha on the pulmonary vascular bed were similar when the lung was perfused with dextran or with blood. PGF2alpha increased isometric tension in isolated helical segments of lobar vein 3 to 5 mm in diameter but was without effect on arterial segments of the same diameter. The increase in isometric tension in the venous segments with PGF2alpha was associated with a significant increase in intracellular levels of guanosine 3',5'-monophosphate (cGMP) but no change in adenosine 3',5'-monophosphate (cAMP) levels. PGE1 decreased isometric tension in both arterial and venous segments and the decrease in tension was accompanied by a significant elevation in smooth muscle cAMP levels and a small but significant reduction in vein cGMP. Results of the present study indicate that PGF2alpha increases pulmonary resistance by constricting lobar veins and to a lesser extent vessels upstream in the precapillary bed whereas PGE1 dilates lobar veins and upstream vessels. These results suggest that PGE1-induced vasodilation may be mediated by an increase in cAMP levels while PGF2alpha-induced venoconstriction may be related to increased smooth muscle levels of cGMP.

Effects of alveolar and perfusion hypoxia and hypercapnia on pulmonary vascular resistance in the lamb

The effects of ventilatory hypoxia and hypercapnia and perfusion hypoxia and hypercapnia on pulmonary vascular resistance were studied in the intact lamb using right heart techniques to isolate and perfuse the left lower lobe. Ventilatory hypoxia increased vascular resistance in the left lower lobe by constricting predominantly vessels upstream from small lobar veins, presumably small arteries. The response to hypoxia was not blocked by phentolamine and diphenhydramine in doses that markedly decreased pressor responses to norepinephrine and histamine in the lung. Perfusion hypoxia did not alter vascular resistance in the perfused lobe. Ventilatory hypercapnia increased vascular resistance in the lung by constricting mainly upstream vessels, whereas perfusion hypercapnia decreased resistance by dilating upstream vessels. These data indicate that histamine and catecholamines are not involved in the response to alveolar hypoxia. These results suggest that the sensor site for ventilatory hypoxia is close to the alveolus since the response is unrelated to lobar arterial Po2. It is concluded that systemic reflexes are not necessarily involved in the response of the pulmonary vascular bed to ventilatory hypoxia or hypercapnia and that the magnitude and rapidity of this response suggest that it may represent an important local mechanism for the control of ventilation-perfusion relationships in this species.

Effects of prostaglandins B2 and B1 on the pulmonary circulation in the intact dog

The effects of prostaglandins (PG) B2 and B1 on the pulmonary circulation were studied in the intact spontaneously breathing dog under conditions of controlled blood flow using right heart and transseptal catheterization techniques to isolate and perfuse the left lower lung lobe. Infusion of PGB2 and PGB1 into the left lower lung lobe increased lobar arterial and lobar venous pressure but had no significant effect on left atrial pressure. PGB2 and PGB1 increased pressure gradients between the lobar artery and lobar small vein and between the lobar small vein and the left atrium. PGB2 and PGB1 increased isometric tension in isolated helical segments of lobar vein (3-5 mm diameter) but had little or no effect on lobar arterial segments of the same size. These results indicate that in the intact dog prostaglandins of the B series increase pulmonary vascular resistance by constricting lobar veins and to a lesser extent vessels upstream to lobar small veins, presumably small lobar arteries. PGB2 and PGB1 both produced large increases in pulmonary vascular resistance in the dog with PGB2 being about 10 times as potent as PGB1. It is concluded that PGB2 is one of the most potent pressor substances in the canine pulmonary vascular bed.

Androgen-induced enhancement of vascular reactivity

Impairment of testosterone metabolism or excretion has been found in some patients with essential hypertension (Nowacynski et al. Can. J. Biochem. (1968), 46, 1031–1038). The effects of testosterone (10 mg/kg, intramuscular (i.m.)), methyltestosterone (10 mg/kg, i.m.), and vehicle on heart rate, arterial pressure, and responses of perfused canine hindpaws to intra-arterial norepinephrine (0.1–3.0 μg), tyramine (50 and 200 μg), angiotensin (0.1, 0.3, and 1.0 μg), and nitroglycerin (10 and 100 μg) were studied in three groups of dogs (five per group), 5 days after single doses of androgen or vehicle. Plasma and tibial artery electrolytes were also measured. After testosterone, heart rate (157 ± 5 beats/min) was higher than in control dogs after vehicle (132 ± 6 beats/min). Mean arterial pressure was similar in the two groups. Methyltestosterone-treated dogs had a significantly greater heart rate (160 ± 8 beats/min) than vehicle-treated animals. However, mean arterial pressures were similar in the two groups. A second series of dogs that received propranolol (1 mg/kg) 30 min before evaluation of heart rate showed no significant differences in heart rate among the treatment groups. Mean arterial pressures were similar in the three experimental groups. Pressor responses (expressed as the percentage change in perfusion pressure from the base line) to norepinephrine and tyramine were significantly enhanced after testosterone treatment. Pressor responses to angiotensin- and nitroglycerin-induced vasodilatation were unchanged after testosterone treatment when compared with vehicle controls. Pressor responses to tyramine, but not to norepinephrine, were also enhanced after methyltestosterone when compared with control responses after vehicle. Responses to angiotensin and nitroglycerin were similar in the two groups of animals. Plasma and tibial artery electrolytes were not different in the three experimental groups. These data suggest that single doses of androgen sensitized these animals to exogenously administered and endogenously released catecholamines. The effects were not mediated by sodium retention or altered plasma or tissue calcium concentrations.

Effect of sympathetic nerve stimulation on pulmonary vascular resistance in the dog

The effect of sympathetic nerve stimulation on the pulmonary circulation was studied in a hemodynamically separated dog lung in which blood flow was maintained constant with a pump. Electrical stimulation of the stellate ganglia at 3, 10, and 30 cps produced a significant increase in mean lobar arterial perfusion pressure. Since lobar blood flow and left atrial pressure did not change during nerve excitation, the increase in lobar arterial pressure reflected an increase in vascular resistance across the lung. The increase in pulmonary vascular resistance was dependent on the stimulus frequency, and the time from the onset of stimulation to the attainment of the peak response was inversely related to the stimulus frequency. The response to nerve stimulation was decreased by alpha-receptor blocking agents. Lobar perfusion with a roller pump had an effect similar to perfusion with a piston pump. The response to nerve stimulation was independent of changes in rate, rhythm, and volume of respiration, changes in aortic blood pressure, and changes in airway resistance. The effects of stellate stimulation and of injected norepinephrine on vascular resistance were similar in the hemodynamically separated lobe. In essence, sympathetic nerve stimulation produced an active increase in vascular resistance in the pulmonary circulation, and the contribution of passive factors such as changes in respiration, bronchomotor tone, and bronchial circulation was minimal. Since the response was blocked by phentolamine, the increase in pulmonary vascular resistance in response to nerve stimulation was attributed to activation of alpha receptors in the pulmonary vascular bed by neurally released norepinephrine. These results demonstrate that pulmonary vascular resistance can be increased at stimulus frequencies in the physiological range of discharge for the sympathetic nervous system.

Effect of prostaglandins E 1 , E 2 and A 2 on vascular resistance and responses to noradrenaline, nerve stimulation and angiotensin in the dog hindlimb

1. The relationship between the vasodilator and inhibitory effects of prostaglandin E(1) (PGE(1)), E(2) and A(2) on responses to nerve stimulation, noradrenaline and angiotensin was evaluated in the dog hindlimb preparation.2. PGE(1) and PGE(2) were equipotent as vasodilators in the hindlimb; however, PGE(1) was much more potent as an inhibitor of vasoconstrictor responses to nerve stimulation, noradrenaline and angiotensin.3. PGA(2) and PGE(2) were approximately equal as inhibitors of vasoconstrictor responses to noradrenaline, nerve stimulation and angiotensin; however, PGE(2) was far more potent as a vasodilator.4. Since there is no relationship between the vasodilator and inhibitory effects of PGE(1), E(2) and A(2), and since the inhibitory effect of PGA(2) was present at a time when hindlimb perfusion pressure had returned to control value, it is concluded that the inhibitory action is probably not the result of a physiological antagonism.5. Since each prostaglandin inhibited responses to nerve stimulation and noradrenaline to approximately the same extent and responses to angiotensin were also inhibited, it is suggested that these agents antagonize vasoconstrictor responses by a nonspecific depressant effect on smooth muscle cells.