Effects of ATP-induced leg vasodilation on VO2 peak and leg O2 extraction during maximal exercise in humans

During maximal whole body exercise VO2 peak is limited by O2 delivery. In turn, it is though that blood flow at near-maximal exercise must be restrained by the sympathetic nervous system to maintain mean arterial pressure. To determine whether enhancing vasodilation across the leg results in higher O2 delivery and leg VO2 during near-maximal and maximal exercise in humans, seven men performed two maximal incremental exercise tests on the cycle ergometer. In random order, one test was performed with and one without (control exercise) infusion of ATP (8 mg in 1 ml of isotonic saline solution) into the right femoral artery at a rate of 80 microg.kg body mass-1.min-1. During near-maximal exercise (92% of VO2 peak), the infusion of ATP increased leg vascular conductance (+43%, P<0.05), leg blood flow (+20%, 1.7 l/min, P<0.05), and leg O2 delivery (+20%, 0.3 l/min, P<0.05). No effects were observed on leg or systemic VO2. Leg O2 fractional extraction was decreased from 85+/-3 (control) to 78+/-4% (ATP) in the infused leg (P<0.05), while it remained unchanged in the left leg (84+/-2 and 83+/-2%; control and ATP; n=3). ATP infusion at maximal exercise increased leg vascular conductance by 17% (P<0.05), while leg blood flow tended to be elevated by 0.8 l/min (P=0.08). However, neither systemic nor leg peak VO2 values where enhanced due to a reduction of O2 extraction from 84+/-4 to 76+/-4%, in the control and ATP conditions, respectively (P<0.05). In summary, the VO2 of the skeletal muscles of the lower extremities is not enhanced by limb vasodilation at near-maximal or maximal exercise in humans. The fact that ATP infusion resulted in a reduction of O2 extraction across the exercising leg suggests a vasodilating effect of ATP on less-active muscle fibers and other noncontracting tissues and that under normal conditions these regions are under high vasoconstrictor influence to ensure the most efficient flow distribution of the available cardiac output to the most active muscle fibers of the exercising limb.     

Opposing hemodynamic effects of substance P on pulmonary vasculature in rabbits

Substance P is a peptide implicated in the control of a variety of physiological processes. Although substance P-containing neurons impinge on the pulmonary vasculature, the effects of substance P on the pulmonary circulation have not been systematically investigated. Rabbits were anesthetized with methohexital sodium and paralyzed with pancuronium bromide. Injection of substance P (0.002-0.10 microgram/kg) in the vena cava produced dose-dependent pulmonary vasoconstriction and systemic vasodilation. Pulmonary arterial pressure reached a peak within 15-20 s and declined toward base line over 10 min. Aortic pressure fell rapidly, reaching minimum at 5-10 s. At higher doses cardiac output fell transiently, resulting in a 65% fall in pulmonary vascular conductance. If repeat substance P dosages were administered 15 min apart, there was no tachyphylaxis. Pulmonary vasoconstriction was inhibited by the cyclooxygenase blocker meclofenamate (10 mg/kg) and the thromboxane synthase inhibitor Dazmegrel (UK-38,485) (2 mg/kg). In contrast, vasoconstriction was enhanced by atropine (2 mg/kg). In Dazmegrel-treated animals in whom pulmonary vasoconstriction was established by epinephrine infusion, low doses of substance P produced vasodilation. Our findings indicate that substance P produces pulmonary vasoconstriction via prostaglandin (particularly thromboxane) generation and pulmonary vasodilation via activation of cholinergic pathways.     

Cardiorespiratory responses to HCl vs. lactic acid infusion

Previous reports indicate that intravenous infusion of HCl can alter breathing and blood pressure even if reductions in systemic arterial pH are prevented. To extend these findings, as well as to determine whether other acids elicit comparable results, this report compares the cardiopulmonary response between right atrial infusion of lactic acid and HCl in awake ponies. Lactic acid, infused at a dose of 1.5 mmol/kg over 18 min, lowered systemic and pulmonary arterial pH 0.062 and 0.092 U, respectively, and increased pulmonary arterial pressure (delta Ppa, 4 mmHg), heart rate (HR, 4/min), and tidal volume (delta VT, 190 ml/m2). HCl, infused at a reduced dose of 0.5 mmol/kg over 18 min, lowered systemic and pulmonary arterial pH 0.024 and 0.047 U, respectively, but produced increases in Ppa (delta 23 mmHg), HR (delta 42/min), and VT (delta 321 ml/m2) that were significantly greater than from the larger dose of lactic acid. These results indicate that cardiopulmonary responses to infusion acidosis differ between the type of acid infused. It is suggested that, in the unanesthetized pony, HCl-induced infusion acidosis has a unique cardiopulmonary-stimulating action unrelated to the pH changes imparted to the circulating arterial blood and that this response is absent during the infusion of lactic acid.     

Intravenous acid infusion without lowering arterial pH stimulates breathing

The aim of this study was to determine whether increases in ventilation would occur during intravenous acid infusion even if systemic arterial pH was held constant. In six awake ponies, HCl (500 ml, approximately 0.312 M) was infused into the right atrium at a total dose of 1.0 meq/kg over 18 min while an equivalent dose of NaOH was infused into the left heart to restore systemic arterial pH to normal. Total ventilation increased at the onset of the infusion and remained elevated although systemic arterial pH was normal to slightly alkaline. The increase in ventilation during the initial 2 min of the infusion coincided with an increase in pulmonary arterial PCO2 and decrease in pulmonary arterial pH. As the infusion progressed, however, pulmonary arterial pH and PCO2 returned to near control values due to the recirculation of systemic arterial blood with an acid-base status that had been altered consequent to the hyperventilation. Pulmonary arterial blood pressure was increased significantly during the entire infusion. Infusion of equivalent doses of hypertonic saline led to only minor alterations in the variables that were measured. These experiments demonstrate that this dose of intravenous HCl can increase ventilation independent of reductions in systemic arterial pH. Because increases in ventilation and pulmonary arterial H+ were not well correlated throughout the entire infusion, and pulmonary arterial blood pressure was increased, it is not clear if the mechanism for this ventilatory response is due to stimulation of pulmonary chemoreceptors, pulmonary vascular mechanoreceptors, or some other mechanism unrelated to increases in systemic arterial H+ concentration.     

Influence of vasoactive intestinal polypeptide (VIP) on splanchnic and central hemodynamics in healthy subjects

The influence of VIP, a potent vasodilator, on central hemodynamics, splanchnic blood flow and glucose metabolism was studied in six healthy subjects. Teflon catheters were inserted into an artery, a femoral vein and a right-sided hepatic vein. A Swan-Ganz catheter was introduced percutaneously and its tip placed in the pulmonary artery. Determinations of cardiac output, systemic, pulmonary arterial and hepatic venous pressures as well as splanchnic blood flow were made in the basal state and at the end of two consecutive 45 min periods of VIP infusion at 5 and 10 ng/kg/min, respectively. Arterial blood samples for analysis of glucose, FFA, insulin and glucagon were drawn at timed intervals. VIP infusion at 5 ng/kg/min resulted in an increase in cardiac output (55%) and heart rate (25%) as well as a reduction in mean systemic arterial pressure (15%) and vascular resistance (45%). With the higher rate of VIP infusion heart rate tended to rise further while cardiac output and arterial pressure remained unchanged. At 15 min after the end of VIP infusion the above variables had returned to basal levels. Splanchnic blood flow and free hepatic venous pressure did not change significantly. Arterial concentrations of glucose, FFA, insulin and glucagon increased during VIP infusion. At 15 min after the end of infusion the glucose levels were still significantly higher than basal (20%). Net splanchnic glucose output did not change in response to VIP infusion. It is concluded that VIP exerts a potent vasodilatory effect resulting in augmented cardiac output and lowered systemic blood pressure and vascular resistance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of thromboxane A2 and platelet activating factor in early haemodynamic response to lipopolysaccharide in rats.

The mechanism of early pulmonary and systemic haemodynamic response to intravenous infusion of LPS from Escherichia coli was investigated in anesthetised Wistar rats. 10 mg of LPS given at a rate of 4 mg/kg/min but not at a rate of 1 mg/kg/min induced an increase in pulmonary arterial pressure (PAP) and a fall in systemic arterial pressure (SAP). Pretreatment with a PAF receptor antagonist; WEB 2170 (5 and 25 mg/kg) inhibited both PAP and SAP responses to LPS (4 mg/kg/min) while an inhibitor of thromboxane synthesis; Camonagrel (10 and 20 mg/kg) abolished PAP response without a major effect on SAP response to LPS. In conclusion, both PAF and TXA2 mediate LPS induced rise in pulmonary arterial pressure while LPS-induced fall in systemic arterial pressure is mediated by PAF.     

Effects of nitroglycerin and nitroprusside on the pulmonary hemorrhagic edema induced by cerebral compression and epinephrine

In vagotomized and pentobarbital-anesthetized rats, massive pulmonary hemorrhagic edema was produced by cerebral compression (CC) or an injection of epinephrine (EP) 0.25 mg/kg. The pulmonary changes were induced following severe Cushing reaction manifested by systemic hypertension. We determined the dose-effect relationship of nitroglycerin (NTG) and nitroprusside (NPS) on the changes in systemic arterial pressure and lung pathology. The drugs were given by iv infusion 5 min before CC or EP and continued throughout the experiment. NTG, 5 micrograms/kg/min, did not affect the pressor response and the pulmonary damage. In a dose of 10 micrograms/kg/min, the CC- and EP-induced changes were partially blocked. A dose of 20 micrograms/kg/min almost completely prevented the CC- and EP-induced pulmonary changes despite partial blockade of the pressor response. NPS exerted more potent effects than NTG on such changes. A dose of 5 micrograms/kg/min was capable of decreasing the pressor response by about 20% and the lung changes by about 50%. In a dose of 10 micrograms/kg/min, the pulmonary changes were almost completely prevented. The results suggest that vasodilators such as nitroglycerin and nitroprusside can block the pressor response and pulmonary pathology subsequent to CC or EP. The effects may be attributed to the ventricular unloading action of these vasodilators that decrease the preload and afterload of the heart. In this respect, nitroprusside is more potent than nitroglycerin.     

Bronchopulmonary and pressor effects of big-endothelin-1 in the guinea-pig

Injection of 1 nmol/kg Big-endothelin-1 (ET-1) into anaesthetized and ventilated guinea-pigs did not evoke significant changes in pulmonary inflation pressure and mean arterial blood pressure. In contrast, injection of 1 nmol/kg ET-1 induced marked and rapid bronchoconstrictor and pressor responses. When administered at a dose of 10 nmol/kg, Big-ET-1 induced marked long-lasting changes in pulmonary inflation pressure and mean arterial blood pressure developing slowly as compared to those evoked by ET-1. Furthermore, these increases reached maximal values by 20 min for pulmonary inflation pressure and 45 min for mean arterial blood pressure after injection of the peptide. When Big-ET-1 was incubated with -chymotrypsin [45 min at 37°C, enzyme : substrate ratio (wt/wt) : 0.5%] and injected into guinea-pigs at a dose of 1 nmol/kg, marked bronchoconstrictor and pressor responses were observed, developing with the same kinetics as those evoked by ET-1. The extent of the pressor response was similar and the bronchoconstriction was slightly lower than those evoked upon injection of 1 nmol/kg ET-1 treated or not with -chymotrypsin. The present results indicate that Big-ET exhibits moderate, if any, direct bronchoconstrictor and pressor activities in the guinea-pig. The slow metabolism of Big-ET-1 in an active form probably explains its long-lasting effects at a dose of 10 nmol/kg. This is indirectly confirmed by the in vitro treatment of Big-ET-1 with -chymotrypsin which converts the peptide into an active form.     

The influence of the inter-dose time interval on the cardiovascular response to methionine-enkephalin in the conscious dog

Intravenous injection of methionineenkephalin (10 μg/kg) into the conscious dog increased both heart rate and mean systemic arterial pressure. Progressive shortening of the inter-dose time interval from 5 min to 1 min and then to 30 sec did not alter the response, as the maximal mean systemic arterial pressure elevation was maintained and the maximal heart rate response increased slightly. In contrast to the results after discrete bolus dosing, continuous infusion of methionine-enkephalin at a constant rate of 10 μg/kg/min produced an initial elevation in heart rate and mean arterial pressure, but these parameters then began to return toward pretreatment levels despite continuous infusion at the same rate, indicating receptor desensitization. This desensitization pattern is most compatible with receptors of the nicotinic-cholinergic type. These data indicate the importance of dosing techniques in assessing cardiovascular responses to systemically administered enkephalins.     

Comparison of cardiopulmonary effects of dexmedetomidine administered as a constant rate infusion without loading dose in sheep and goats anaesthetised with sevoflurane

The purpose of this study was to compare the cardiopulmonary effects of dexmedetomidine in anaesthetised sheep and goats. The initial vasopressor effects were prevented by using a constant rate infusion (CRI), without a loading dose.In six sevoflurane anaesthetised goats and sheep dexmedetomidine was infused as CRI (2 μg/kg/h). Before, during and for 30 min after the infusion, cardiac output (CO), systemic and pulmonary arterial pressures, heart rate (HR), respiratory mechanics, inspired and expired gas concentrations, arterial and mixed venous blood gases and the plasma concentration of dexmedetomidine were measured.In goats, CO, HR and mean arterial pressure decreased below baseline values by 20, 40 and 20%, respectively. Systemic vascular resistance (SVR) only decreased after the infusion. Gas exchange and respiratory mechanics remained unchanged. In sheep, CO did not change and SVR decreased progressively up to 45% below baseline. Changes in gas exchange and respiratory mechanics were mild but highly variable. The plasma concentrations of dexmedetomidine were similar in sheep and goats.In conclusion, goats seem to be more sensitive to centrally mediated cardiovascular effects of dexmedetomidine than sheep, which is not related to dexmedetomidine disposition. Adverse pulmonary effects were completely prevented by CRI without loading dose in goats but not in sheep.     

Effects of calcitonin gene-related peptide on renal blood flow in the rat

The effects of alpha-rat calcitonin gene-related peptide (alpha-rCGRP) on systemic and renal hemodynamics and on renal electrolyte excretion were examined in normal anesthetized rats. In one group of rats (n = 7), infusions of alpha-rCGRP at doses of 10, 50, 100, and 500 ng/kg/min for 15 min each produced dose-related and significant decreases in mean arterial pressure from a control of 130 +/- 3 mm Hg to a maximal depressor response of 91 +/- 2 mm Hg. During the first three doses of alpha-rCGRP, renal blood flow progressively and significantly increased from a control of 5.0 +/- 0.3 ml/min to a peak level of 6.3 +/- 0.3 ml/min achieved during the 100 ng/kg/min infusion. With the highest infusion rate of 500 ng/kg/min, renal blood flow fell below the control level to 4.5 +/- 0.2 ml/min (P less than 0.05). The responses in renal blood flow and mean arterial pressure were associated with reductions in renal vascular resistance. After cessation of alpha-rCGRP infusions, arterial pressure, renal blood flow, and renal vascular resistance gradually returned toward the baseline values. In another group of rats (n = 9), infusion of alpha-rCGRP for 30 min at 100 ng/kg/min produced a significant reduction in urinary sodium excretion from 0.28 +/- 0.06 to 0.14 +/- 0.5 muEq/min (P less than 0.05). Urine flow and urinary potassium excretion also appeared to decrease, but the changes were not significantly different (P greater than 0.05) from their respective baselines. These results demonstrate that alpha-rCGRP is a potent and reversible hypotensive and renal vasodilatory agent in the anesthetized rat. The data also suggest that alpha-rCGRP may have significant effects on the excretory function of the kidney.     

Comparison of haemodynamic responses to dopamine and salbutamol in severe cardiogenic shock complicating acute myocardial infarction.

Twelve patients with severe persistent cardiogenic shock complicating acute myocardial infarction underwent single crossover treatment with intravenous dopamine and salbutamol to determine the more beneficial therapy. Salbutamol (10 to 40 microgram/min) reduced systemic vascular resistance and progressively increased both cardiac index and stroke index. Heart rate increased from 95 to 104 beats/min. Changes in mean arterial pressure and pulmonary artery end-diastolic pressure were small and insignificant. Dopamine infusion at rates of 200 and 400 micrograms/min also increased cardiac index and stroke index. Systemic vascular resistance fell slightly but mean arterial pressure rose from 57 to 65 mm Hg. Heart rate increased from 95 to 105 beats/min. Changes in pulmonary artery end-diastolic pressure were again small and insignificant. Dopamine infusion at 800 micrograms/min caused an appreciable increase in systemic vascular resistance; a further increment in mean arterial pressure was observed, though cardiac index fell slightly. Heart rate and pulmonary artery end-diastolic pressure rose steeply. Salbutamol, a vasodilator, increased cardiac output in patients with cardiogenic shock complicating acute myocardial infarction but did not influence blood pressure. If correction of hypotension is essential dopamine in low doses may be the preferred agent. Doses of 800 microgram/min, which is within the therapeutic range, worsen other manifestations of left ventricular dysfunction.     

Inhibition of phosphodiesterase 1 augments the pulmonary vasodilator response to inhaled nitric oxide in awake lambs with acute pulmonary hypertension

Phosphodiesterase 1 (PDE1) modulates vascular tone and the development of tolerance to nitric oxide (NO)-releasing drugs in the systemic circulation. Any role of PDE1 in the pulmonary circulation remains largely uncertain. We measured the expression of genes encoding PDE1 isozymes in the pulmonary vasculature and examined whether or not selective inhibition of PDE1 by vinpocetine attenuates pulmonary hypertension and augments the pulmonary vasodilator response to inhaled NO in lambs. Using RT-PCR, we detected PDE1A, PDE1B, and PDE1C mRNAs in pulmonary arteries and veins isolated from healthy lambs. In 13 lambs, the thromboxane A(2) analog U-46619 was infused intravenously to increase mean pulmonary arterial pressure to 35 mmHg. Four animals received an intravenous infusion of vinpocetine at incremental doses of 0.3, 1, and 3 mg.kg(-1).h(-1). In nine lambs, inhaled NO was administered in a random order at 2, 5, 10, and 20 ppm before and after an intravenous infusion of 1 mg.kg(-1).h(-1) vinpocetine. Administration of vinpocetine did not alter pulmonary and systemic hemodynamics or transpulmonary cGMP or cAMP release. Inhaled NO selectively reduced mean pulmonary arterial pressure, pulmonary capillary pressure, and pulmonary vascular resistance index, while increasing transpulmonary cGMP release. The addition of vinpocetine enhanced pulmonary vasodilation and transpulmonary cGMP release induced by NO breathing without causing systemic vasodilation but did not prolong the duration of pulmonary vasodilation after NO inhalation was discontinued. Our findings demonstrate that selective inhibition of PDE1 augments the therapeutic efficacy of inhaled NO in an ovine model of acute chemically induced pulmonary hypertension.     

Effects of substance P on renin release and renal function in anesthetized dogs.

Substance P (SP), a naturally occuring undecapeptide with hypotensive, vasodilatory and smooth muscle stimulating properties, was infused intravenously or intrarenally into anesthetized dogs. Infusions of SP intravenously suppressed renin secretion rate (RSR) from 204±45 to 52±18 ng/min (p < 0.02) at an infusion rate of 0.5 ng/kg/min, and to 50±22 ng/min (p < 0.05) at 5 ng/kg/min. When the concentration of SP was further increased to 50 ng/kg/min, RSR increased to a level above the control value (728±81, p < 0.01). Intrarenal infusion of SP produced similar changes in renin release. At infusion rates of 0.5 ng/kg/min and 5 ng/kg/min, RSR was suppressed from 145±18 to 56±18 ng/min (p < 0.05) and to 26±8 ng/min (p < 0.01) respectively. At 50 ng/kg/min, RSR increased to 251±59 (p > 0.1). Both intravenous and intrarenal administration of the peptide significantly lowered arterial blood pressure at the highest two doses. Intrarenal infusion of SP resulted in a significant dose-related increase in urine volume, sodium and potassium excretion, and renal blood flow. In contrast, intravenous infusions did not alter these parameters. Thus SP suppresses renin release in the presence and in the absence of diuresis, natriuresis, and vasodilation.     

Fetal pulmonary vasodilation after exogenous platelet-activating factor

To determine the fetal pulmonary vascular response to platelet-activating factor (PAF), we studied the hemodynamic effects of the infusion of PAF directly into the left pulmonary artery in 21 chronically catheterized fetal lambs. Left pulmonary arterial blood flow (Q) was measured with electromagnetic flow transducers. Ten-minute infusions of low-dose PAF (10-100 ng/min) produced increases in Q from a baseline of 71 +/- 5 to 207 +/- 20 ml/min (P less than 0.001) without changes in pulmonary arterial pressure. Pulmonary vasodilation with PAF was further confirmed through increases in Q with brief (15-s) infusions and increases in the slope of the pressure-flow relationship as assessed by rapid incremental compressions of the ductus arteriosus during PAF infusion. Infusion of Lyso-PAF had no effect on Q or pulmonary arterial pressure. Treatment with CV-3988, a selective PAF receptor antagonist, but not with meclofenamate, atropine, or diphenhydramine and cimetidine blocked the response to PAF infusion and did not affect baseline tone. Systemic infusion of high-dose PAF (300 ng/min) through the fetal inferior vena cava increased pulmonary arterial pressure (46.5 +/- 1.0 to 54.8 +/- 1.9 mmHg, P less than 0.01) and aorta pressure (44.3 +/- 1.0 to 52.7 +/- 2.2 mmHg, P less than 0.01) while also increasing Q. Neither PAF nor CV-3988 changed the gradient between pulmonary arterial and aorta pressures, suggesting that PAF does not affect ductal tone. We conclude that PAF is a potent fetal pulmonary vasodilator and that the effects are not mediated through cyclooxygenase products or by cholinergic or histaminergic effects.     

Atrial natriuretic peptide ameliorates hypoxic pulmonary vasoconstriction without influencing systemic circulation.

Hypoxic pulmonary vasoconstriction (HPV) is encountered during ascent to high altitude. Atrial natriuretic peptide (ANP) could be an option to treat HPV because of its natriuretic, diuretic, and vasodilatory properties. Data on effects of ANP on pulmonary and systemic circulation during HVP are conflicting, partly owing to anesthesia, surgical stress or uncontrolled dietary conditions. Therefore, ten conscious, chronically tracheotomized dogs were studied under standardized dietary conditions. The dogs were trained to breathe spontaneously at a ventilator circuit. Protocol: 30min of normoxia [inspiratory oxygen fraction (F(i)O(2))=0.21] were followed by 30min of hypoxia without ANP infusion (Hypoxia I, F(i)O(2)=0.1). While maintaining hypoxia an intravenous infusion of atrial natriuretic peptide was started with 50ng x kg body wt(-1) x min(-1) for 30min (Hypoxia+ANP1=low dose), followed by 1000ng x kg body wt(-1) x min(-1) for 30min (Hypoxia+ANP2=high dose). Thereafter, ANP infusion was stopped and hypoxia maintained for a final 30min (Hypoxia II). Compared to normoxia, mean pulmonary arterial pressure (MPAP) (16+/-0.7 vs. 26+/-1.3mmHg) and pulmonary vascular resistance (PVR) (448+/-28 vs. 764+/-89dyn x s(-1) x cm(-5)) increased during Hypoxia I and decreased during Hypoxia+ANP 1 (MPAP 20+/-1mmHg, PVR 542+/-55dyn x s(-1) x cm(-5)) (P<0.05). The higher dose of ANP did not further decrease MPAP or PVR, but started to have a tendency to decrease mean arterial pressure and cardiac output. We conclude that low dose ANP is able to reduce HPV without affecting systemic circulation during acute hypoxia.     

Cardiovascular and pulmonary response to oral administration of ATP in rabbits.

Extracellular purines such as ATP and adenosine participate in the regulation of cardiovascular and respiratory functions through specific P1 and P2 purine receptors. These properties have mainly been described after intravenous infusion. Experiments reported herein were designed to explore the possible effect of oral ATP administration (3 or 20 mg. kg(-1). day(-1)) on vascular, cardiac, and pulmonary functions in rabbits. Whereas a unique oral dose of ATP has no effect, chronic supplementation during 14 days reduces peripheral vascular resistance, pulmonary resistance, and respiratory frequency and increases arterial PO(2). No effect on central blood pressure and heart rate is observed, but an increase of the left ventricular work index is noticed subsequent to the diminution of vascular resistance. Rather similar cardiovascular modifications are observed in rabbits given 20 mg. kg(-1). day(-1) adenosine for 14 days but without variation of respiratory parameters. These original effects of repeated oral treatment with ATP may result from an adaptive metabolic response to nucleoside supplementation that might affect the turnover of extracellular purines leading to P1- and/or P2-receptor activation.     

Adenosine causes a biphasic response in the ovine fetal placental vasculature

We have reported in a previous study that adenosine infusion causes fetal placental vascular resistance to increase after 2 min. To determine whether this action is followed by a more prolonged vasodilation, we studied 7 mature fetal lambs. At surgery, catheters were inserted into the fetal hindlimb arteries and veins. After a five day recovery period, control blood flow measurements were made by radiolabeled microsphere technique immediately after an infusion of 0.9% NaCl, (vehicle, 1.03 ml.min-1) into a fetal vein for 2 min. Within 5 min of the control blood flow measurement, adenosine (10 mg/min) was infused for 2 min. Blood flow measurements were repeated 5, 10, 15, 20 and 30 min after the end of the infusion period. Fetal arterial blood pressure dropped from 50 +/- 1 to 34 +/- 5 mmHg immediately after the adenosine infusion and returned to the control value within 5 min after the infusion. No further blood pressure response was detected. However, placental vascular resistance fell from 0.334 +/- 0.040 to 0.269 +/- 0.027 (P less than 0.05) at the 15 min measurement, remained low through the 20 min measurement (P less than 0.001) and was not different from control levels 30 min after the adenosine infusion. We conclude that the fetal placental vasculature responds to systemic adenosine infusion in a biphasic manner. The immediate reaction to adenosine is a transient vasoconstriction in the fetal placental vasculature followed by vasodilation 15 to 20 min after the initial exposure to adenosine.     

Failure of pulmonary acidosis to increase respiratory drive.

Experiments were performed to determine whether increases in acidity isolated to the pulmonary circulation would stimulate hypothesized pulmonary chemoreceptors and increase respiratory drive in the anesthetized paralyzed mechanically ventilated cat (n = 9). Respiratory drive was assessed by measuring the frequency and amplitude of the integrated phrenic neurogram. To create an isolated pulmonary acidosis, blood returning to the lung was acidified by infusion of 0.3 M lactic acid (1.91 ml/min) into the inferior vena cava, while systemic arterial pH was restored to near normal levels by simultaneous infusion of base (0.3 M NaOH) into the left atrium. Six minutes after the start of this dual infusion of acid and base, right ventricular (pulmonary) pH decreased from 7.286 to 7.179 and PCO2 increased 7 Torr. Systemic arterial pH and PCO2 were unchanged from measurements immediately before the infusion. This level of pulmonary acidosis failed to increase respiratory drive as assessed by phrenic activity. To test the sensitivity of the preparation to known systemic arterial chemical stimuli, a combined pulmonary and systemic acidosis was elicited by infusion of 0.3 M lactic acid into the inferior vena cava and 0.3 M NaCl into the left atrium. This infusion significantly lowered both systemic arterial and pulmonary arterial pH (7.343 to 7.155 for systemic arterial pH and 7.286 to 7.067 for pulmonary pH) and increased phrenic efferent activity 45%. We conclude that phrenic efferent activity is unaffected by moderate reductions in the pH of the pulmonary circulation in the absence of a significant systemic arterial acidosis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pulmonary and anti-platelet effects of intravenous and inhaled prostacyclin in man

Prostacyclin in aerosol was inhaled by three healthy volunteers (0.3 – 30 μg) and by twelve patients with bronchial asthma (200 or 400 μg). Essentially, no changes in pulmonary function were noticed. Intravenous infusion of prostacyclin (2 – 20 ng/kg/min) into six healthy volunteers also remained without effects on respiratory indices studied. Inhaled prostacyclin impaired platelet aggregability to ADP, dispersed circulating platelet aggregates and produced vasodilation comparable to those observed following intravenous administration of the hormone. We suggest that aerosols of prostacyclin might be used in future in treatment of arterial thrombo-embolism.