Unexpected maturation of PI3K and MAPK-ERK signaling in fetal ovine cardiomyocytes

In the first two-thirds of gestation, ovine fetal cardiomyocytes undergo mitosis to increase cardiac mass and accommodate fetal growth. Thereafter, some myocytes continue to proliferate while others mature and terminally differentiate into binucleated cells. At term (145 days gestational age; dGA) about 60% of cardiomyocytes become binucleated and exit the cell cycle under hormonal control. Rising thyroid hormone (T3) levels near term (135 dGA) inhibit proliferation and stimulate maturation. However, the degree to which intracellular signaling patterns change with age in response to T3 is unknown. We hypothesized that in vitro activation of ERK, Akt, and p70(S6K) by two regulators of cardiomyocyte cell cycle activity, T3 and insulin like growth factor-1 (IGF-1), would be similar in cardiomyocytes at gestational ages 100 and 135 dGA. IGF-1 and T3 each independently stimulated phosphorylation of ERK, Akt, and p70(S6K) in cells at both ages. In the younger mononucleated myocytes, the phosphorylation of ERK and Akt was reduced in the presence of IGF-1 and T3. However, the same hormone combination led to a dramatic twofold increase in the phosphorylation of these signaling proteins in the 135 dGA cardiomyocytes-even in cells that were not proliferating. In the older cells, both mono- and binucleated cells were affected. In conclusion, fetal ovine cardiomyocytes undergo profound maturation-related changes in signaling in response to T3 and IGF-1, but not to either factor alone. Differences in age-related response are likely to be related to milestones in fetal cardiac development as the myocardium prepares for ex utero life.

Insignificant response of the fetal placental circulation to arterial hypotension in sheep

Infusion of the angiotensin-converting enzyme inhibitor enalaprilat into fetal sheep caused a profound arterial hypotension within days. Five fetal lambs were infused with enalaprilat for 8 days starting at day 128 of gestation. Total accumulated dose was 0.30 ± 0.11 mg/kg. Arterial pressure decreased from 43.6 to 25.6 mmHg; venous pressure did not change. Biventricular output was not statistically significantly changed; placental blood flow decreased almost in proportion to the decrease in pressure but the increase in somatic flow was not statistically significant. There were no significant changes in pressure 30 min after the initial 50-μg loading dose of enalaprilat. However, the arterial pressure responses to test doses of ANG I were largely abolished. After 1 day, however, there was a significant decrease in somatic vascular resistance, which became stronger with time, but almost no decrease in the placental resistance. We conclude that the fetal somatic circulation exhibits a slow but strong decrease in resistance but that the response to hypotension is weak or absent in the fetal placenta, possibly because it is already fully relaxed.

Atrial natriuretic peptide inhibits angiotensin II-stimulated proliferation in fetal cardiomyocytes

The role of atrial natriuretic peptide (ANP) in regulating fetal cardiac growth is poorly understood. Angiotensin II (Ang II) stimulates proliferation in fetal sheep cardiomyocytes when growth is dependent on the activity of the mitogen-activated protein kinase (MAPK) and phosphoinositol-3-kinase (PI3K) pathways. We hypothesized that ANP would suppress near-term fetal cardiomyocyte proliferation in vitro and inhibit both the MAPK and PI3K pathways. Forty-eight hour 5-bromodeoxyuridine (BrdU) uptake (used as an index of proliferation) was measured in cardiomyocytes isolated from fetal sheep (135 day gestational age) in response to 100 nm Ang II with or without ANP (0.003-100 nm) or 1 microm 8-bromo-cGMP. The effects of these compounds on the MAPK and PI3K pathways were assessed by measuring extracellular signal-regulated kinase (ERK) and AKT phosphorylation following 10 min of treatment with Ang II, ANP or 8-bromo-cGMP. In right ventricular myocytes (RV), the lowest dose of ANP (0.003 nm) inhibited Ang II-stimulated BrdU uptake by 68%. Similarly, 8-bromo-cGMP suppressed Ang II-stimulated proliferation by 62%. The same effects were observed in left ventricular (LV) cardiomyocytes but the RV was more sensitive to the inhibitory effects of ANP than the LV (P < 0.0001). Intracellular cGMP was increased by 4-fold in the presence of 100 nm ANP. Ang II-stimulated ERK and Akt phosphorylation was inhibited by 100 nm ANP. The activity of ANP may in part be cGMP dependent, as 8-bromo-cGMP had similar effects on the cardiomyocytes.

Reduced systolic pressure load decreases cell-cycle activity in the fetal sheep heart

The fetal heart is highly sensitive to changes in mechanical load. We have previously demonstrated that increased cardiac load can stimulate cell cycle activity and maturation of immature cardiomyocytes, but the effects of reduced load are not known. Sixteen fetal sheep were given either continuous intravenous infusion of lactated Ringer solution (LR) or enalaprilat, an angiotensin-converting enzyme inhibitor beginning at 127 days gestational age. After 8 days, fetal arterial pressure in the enalaprilat-infused fetuses (23.8 +/- 2.8 mmHg) was lower than that of control fetuses (47.5 +/- 4.7 mmHg) (P < 0.0001). Although the body weights of the two groups of fetuses were similar, the heart weight-to-body weight ratios of the enalaprilat-infused fetuses were less than those of the LR-infused fetuses (5.6 +/- 0.5 g/kg vs. 7.0 +/- 0.6 g/kg, P < 0.0001). Dimensions of ventricular myocytes were not different between control and enalaprilat-infused fetuses. However, there was a significant decrease in cell cycle activity in both the right ventricle (P < 0.005) and the left ventricle (P < 0.002) of the enalaprilat-infused fetuses. Thus, we conclude a sustained reduction in systolic pressure load decreases hyperplastic growth in the fetal heart.

Effect of fetal anaemia on myocardial ischaemia-reperfusion injury and coronary vasoreactivity in adult sheep

AIMS

We investigated whether chronic fetal anaemia affects myocardial infarct in adulthood and elicits functional modifications in adult coronary vasoreactivity.

METHODS

Seven-month-old sheep that were made anaemic in utero and transfused to normal haematocrit before birth were studied. Infarct size was determined by tetrazolium after 1-h ischaemia (occlusion of the mid of left anterior descending artery) and 2-h reperfusion. The dose-response to vasoconstrictors and vasodilators was assessed in small resistance coronary arteries.

RESULTS

There were no significant differences between the animals previously subjected to in utero anaemia and the control animals regarding the percentage infarct size and the area-at-risk to the left ventricle. The ventricular function (dP/dt) was preserved. The percentage infarct size of the area-at-risk (70.7 +/- 3.5%) was larger than that in the controls (49.8 +/- 4.5%) (P = 0.006). The vascular responses were not altered. Endothelium-dependent relaxation to bradykinin (96.0 +/- 2.6% vs. 98.8 +/- 1.0%) was not affected by PGI(2) inhibitor (94.6 +/- 2.6% vs. 98.5 +/- 1.0%) but significantly reduced by the inhibition of nitric oxide (NO) in both anaemic (P < 0.05) and control (P < 0.001) groups with a significant right shift of EC(50) (P < 0.01). The non-NO-non-PGI(2)-mediated relaxation was slightly potentiated in anaemic animals.

CONCLUSIONS

Exposing fetal sheep to in utero anaemia in late gestation for 3 weeks may increase the susceptibility of adult hearts to ischaemia-reperfusion injury without major alterations in coronary vasomotor responsiveness. The impact of in utero anaemia at earlier period of pregnancy and on the earlier or later life of the adult is yet to be further investigated.

Persistent changes in arterial blood gases in fetal sheep

Two anaesthetic protocols were compared using pregnant sheep. In both groups of animals, anaesthesia was induced using an intravenous (i.v.) injection of diazepam and ketamine. The ewes were then intubated for positive pressure ventilation using 0.8 L/min of nitrous oxide and 2 L/min oxygen with 1.1-1.8% halothane. If the ewe showed any signs of awakening, one of two protocols was followed. First, the halothane concentration was increased to 2-3% until the ewe was completely anaesthetized. Second, the halothane concentration was not altered, but the ewe was given doses of i.v. diazepam (0.1 mg/kg) and ketamine (1 mg/kg) until again completely anaesthetized. At the completion of surgery, maternal recovery was rapid and similar between the two groups. However, five days after surgery, the fetal arterial Po(2) and oxygen content of the fetuses receiving additional halothane (1.9 +/- 0.2 kPa and 4.4 +/- 1.0 mL/100 mL) were statistically significantly depressed when compared with the fetuses receiving additional diazepam and ketamine (2.9 +/- 0.1 kPa and 7.0 +/- 0.5 mL/100 mL). These results led us to conclude that certain anaesthetic protocols, in spite of good maternal recovery, can lead to deleterious effects upon the fetus that persist for at least five days after surgery.

Thyroid hormone inhibits proliferation of fetal cardiac myocytes in vitro

Thyroid hormone (T(3)) is a key regulator of fetal organ maturation. Premature elevations of thyroid hormone may lead to a 'mature' cardio-phenotype. Thyroid hormone will stimulate maturation of ovine fetal cardiomyocytes in culture by decreasing their proliferative capacity. Group 1 fetal cardiomyocytes (approximately 135 days gestation) were incubated with T3 (1.5, 3, 10, and 100 nM) and bromodeoxyuridine (BrdU; 10 microM) for 24 and 48 h. Group 2 cardiomyocytes were cultured with T3 alone for later protein analysis of cell cycle regulators. At all concentrations, T3 decreased BrdU uptake fourfold in serum media (P<0.001 versus serum, n=5). Following serum-free (SF) T3 treatment, BrdU uptake was inhibited when compared with serum (P<0.001 versus serum, n=5). p21 expression increased threefold (P<0.05 versus serum free, n=4) and cyclin D1 expression decreased twofold (P<0.05 versus serum, n=4) in T3-treated cardiomyocytes. (1) T3 inhibits fetal cardiomyocyte proliferation, while (2) p21 protein levels increase, and (3) cyclin D1 levels decrease. Thus, T3 may be a potent regulator of cardiomyocyte proliferation and maturation in the late gestation fetus.

Cortisol stimulates cell cycle activity in the cardiomyocyte of the sheep fetus

The role of cortisol in regulating cardiac myocyte growth in the near-term fetal sheep is unknown. We hypothesized that cortisol would suppress cardiomyocyte proliferation and stimulate cardiomyocyte binucleation and enlargement, signs of terminal differentiation. Cardiomyocyte dimensions and percent binucleation were determined in isolated cardiac myocytes from seven cortisol-treated and seven control fetuses; percentage of myocytes positive for Ki-67 was determined in an additional four cortisol-treated and four control hearts. Cortisol was infused into the circumflex coronary artery at subpressor rates (0.5 microg/kg.min, 7 d). Cortisol infusion had no hemodynamic effects, compared with controls or pretreatment conditions. Cortisol treatment increased heart weight (44.0 +/- 8.7 g vs. control, 34.9 +/- 9.1 g, P < 0.05). Heart to body weight ratio was greater in treated hearts, compared with controls (10.3 +/- 1.9 vs. 7.7 +/- 0.9 g/kg, P < 0.01). Ventricular myocyte length, width, and percent binucleation were not different between groups. The proportion of treated myocytes in the cell cycle staining for Ki-67 was higher in the left ventricle (5.5 +/- 0.1 vs. 2.7 +/- 0.4%, P < 0.005) and right ventricle (4.4 +/- 0.4 vs. 3.7 +/- 0.7%, P < 0.05), compared with controls. Wet weight to dry weight ratios from cortisol-treated and control hearts were not different. In conclusion, whereas cortisol infused into the fetal sheep heart has no effect on cardiomyocyte size or maturational state, it stimulates entry of cardiomyocytes in the cell cycle. Thus, increases in fetal heart mass associated with subpressor doses of cortisol are due to cardiomyocyte proliferation and not hypertrophic growth.

Fetal infusions of plasma cause an increase in umbilical vascular resistance in sheep

Earlier studies suggested that the fetal placental circulation is relatively inert with fetal placental flow increasing or decreasing with perfusion pressure. Subsequent studies have demonstrated that the placenta may not be an unreactive vascular bed. The present study was undertaken to determine if plasma infusion-induced hypertension increased fetal placental flow in proportion to the driving pressure across the fetal placental circulation. Six fetal sheep were operated on at 118-122 days to place intravascular catheters and a flow sensor on the common umbilical artery. Starting 6 days later, the fetuses were infused with adult sheep plasma. During the 7-day-long infusion period, they received a total of 1515+/-217 (SD) ml of fluid and 93.2+/-12.0 g of protein. Fetal plasma protein concentrations increased from 34.2+/-2.3 to 77.0+/-9.7 g/l (P<0.0001). Fetal arterial blood pressures rose from 42+/-3 to 59+/-4 mmHg (P<0.01) and venous pressures rose from 2.2+/-0.5 to 4.8+/-0.8 mmHg (P<0.01). In spite of the large increase in driving pressure, fetal placental blood flow remained (statistically) constant (627+/-299 ml/min and 552+/-221 ml/min) while fetal umbilical resistance increased from 0.077+/-0.038 to 0.115+/-0.053 mmHg min/ml (P<0.01). On day 7, plasma renin activity had fallen from 6.7+/-4.2 ng/(ml/h) at preinfusion control to 0.6+/-0.6 ng/(ml/h) (P<0.05) and plasma angiotensin-II concentration had fallen from 33.2+/-26.6 to 6.2+/-3.9 pg/ml, although this fall was not statistically significant (P=0.07). Fetal placental flow did not increase with increased driving pressure across the fetal placental circulation. The increase in fetal placental resistance may be a response to the increase in arterial pressure since there was no increase in flow.

The effects of anaemia as a programming agent in the fetal heart

The intrauterine environment plays a powerful role in determining the life-long risk of cardiovascular disease. A number of stressors are well known to affect the development of the cardiovascular system in utero including over/under maternal nutrition, excess glucocorticoid and chronic hypoxia. Chronic fetal anaemia in sheep is a complex stressor that alters cardiac loading conditions, causes hypoxic stress and stimulates large changes in flow to specific tissues, including large increases in resting coronary blood flow and conductance. Decreased viscosity can account for approximately half of the increased flow. It appears that immature hearts are 'plastic' in that increases in coronary conductance with fetal anaemia persist into adulthood even if the anaemia is corrected before birth. These large changes in conductance are possible only through extensive remodelling of the coronary tree. Adult hearts that were once anaemic in utero are more resistant to hypoxic stress as adults but it is not known whether such an adaptation would be deleterious in later life. These studies indicate the need for investigation into the basic mechanisms of coronary tree remodelling in the immature myocardium. New information on these mechanisms is likely to lead to better prevention of and therapies for adult-onset coronary disease.

Anaemia stimulates aquaporin 1 expression in the fetal sheep heart

Interstitial fluid fluxes are much greater in the fetus than in the adult, and filtration rates are increased over control in most tissues of the anaemic fetus. Increased capillary filtration may lead to cardiac oedema which, in turn, severely impacts cardiac function. Mechanisms that underlie these differences in flux are incompletely understood. One possible mechanism is an increase in capillary water permeability. Therefore, the goal of our study was to determine the level of expression of the water channel aquaporin 1 (AQP1) during cardiac development and in the anaemic fetal sheep heart. Hearts from chronically instrumented anaemic sheep fetuses and hearts from normal early fetal, late fetal, neonatal and adult sheep were used for Northern and Western analyses and immunohistochemistry. We found that AQP1 mRNA levels were lower in the young fetal left ventricle than in the adult left ventricle (P < 0.05). We also found that cardiac AQP1 expression was increased in anaemic fetuses compared to age-matched controls (P < 0.05). Expression of AQP1 in all groups was greatest in the microvascular endothelium. These data suggest that AQP1 plays an important role in the physiological accommodation to fetal anaemia.

Angiotensin II stimulates hyperplasia but not hypertrophy in immature ovine cardiomyocytes

Rat and sheep cardiac myocytes become binucleate as they complete the 'terminal differentiation' process soon after birth and are not able to divide thereafter. Angiotensin II (Ang II) is known to stimulate hypertrophic changes in rodent cardiomyocytes under both in vivo and in vitro conditions via the AT1 receptor and intracellular extracellular regulated kinase (ERK) signalling cascade. We sought to develop culture methods for immature sheep cardiomyocytes in order to test the hypothesis that Ang II is a hypertrophic agent in the immature myocardium of the sheep. We isolated fetal sheep cardiomyocytes and cultured them for 96 h, added Ang II and phenylephrine (PE) for 48 h, and measured footprint area and proliferation (5-bromo-2'-deoxyuridine (BrdU) uptake) separately in mono- vs. binucleate myocytes. We found that neither Ang II nor PE changed the footprint area of mononucleated cells. PE stimulated an increase in footprint area of binucleate cells but Ang II did not. Ang II increased myocyte BrdU uptake compared to serum free conditions, but PE did not affect BrdU uptake. The MAP kinase kinase (MEK) inhibitor UO126 prevented BrdU uptake in Ang II-stimulated cells and prevented cell hypertrophy in PE-stimulated cells. This paper establishes culture methods for immature sheep cardiomyocytes and reports that: (1) Ang II is not a hypertrophic agent; (2) Ang II stimulates hyperplastic growth among mononucleate myocytes; (3) PE is a hypertrophic agent in binucleate myocytes; and (4) the ERK cascade is required for the proliferation effect of Ang II and the hypertrophic effect of PE.

Augmentation of coronary conductance in adult sheep made anaemic during fetal life

Maximal coronary conductance with adenosine in anaemic fetal sheep is twice that of non-anaemic fetuses. To investigate whether this increase in conductance persists into adulthood we studied twin sheep as fetuses and again as adults. Nine anaemic fetuses (118 days gestation) underwent isovolaemic haemorrhage for 18.0 +/- 4.6 days (means +/- S.D.) during which time the haematocrit was reduced from 39.9 +/- 5.2 % to 16.3 +/- 3.4 % and oxygen content from 8.6 +/- 1.3 to 2.3 +/- 0.2 ml dl-1. At 138 days the anaemic fetuses were transfused; at delivery the haematocrit was 29.3 +/- 6.8 % compared to nine control fetuses in which the haematocrit was 38.5 +/- 4.3 %. The weight at delivery was 3.5 +/- 0.36 kg in the anaemic fetuses vs. 4.2 +/- 0.83 kg in controls. Twenty-eight weeks later, we placed an occluder on the descending thoracic aorta and inferior vena cava, a flow probe around the proximal left circumflex coronary artery, and catheters in the left atrial appendage, jugular and carotid vessels. Maximal coronary conductance was determined in the adults by recording coronary blood flow as driving pressure was altered by inflating the occluders while adenosine was infused into the left atrium. Right atrial, left atrial, systolic and mean arterial pressures, systemic vascular resistance and haematocrit were not different between 'in utero anaemic' and control adults. The adults that were anaemic in utero weighed less than the controls 39.4 +/- 4.6 kg vs. 45.0 +/- 5.6 kg. Maximal conductance was greater in the adults that were anaemic in utero: 11.2 +/- 4.0 ml min(-1) (100 g)(-1) mmHg-1 as compared to 6.1 +/- 1.8 ml min(-1) (100 g)(-1) mmHg(-1) in the controls. Vascular reactivity of the mesenteric arteries was not different. These data suggest that coronary conductance can be modified in utero by anaemia (high flow and hypoxaemia) and that the remodelled coronary tree persists to adulthood.

Does nitric oxide contribute to the basal vasodilation of pregnancy in conscious rabbits?

Pregnancy produces marked systemic vasodilation, but the mechanism is unknown. Experiments were performed in conscious rabbits to test the hypotheses that increased nitric oxide (NO) production contributes to the increased vascular conductance, but that the contribution varies among vascular beds. Rabbits were instrumented with aortic and vena caval catheters and ultrasonic flow probes implanted around the ascending aorta, superior mesenteric artery, terminal aorta, and/or a femoral artery. Hemodynamic responses to intravenous injection of N(omega)-nitro-L-arginine (L-NA; 20 mg/kg or increasing doses of 2, 5, 10, 15, and 20 mg/kg) were determined in rabbits first before pregnancy (NP) and then at the end of gestation (P). L-NA produced similar increases in arterial pressure between groups, but the following responses were larger (P < 0.05) when the rabbits were pregnant: 1) decreases in total peripheral conductance [-3.7 +/- 0.3 (NP), -5.0 +/- 0.5 (P) ml x min(-1) x mmHg(-1)], 2) decreases in mesenteric conductance [-0.47 +/- 0.05 (NP), -0.63 +/- 0.07 (P) ml x min(-1) x mmHg(-1)], 3) decreases in terminal aortic conductance [-0.43 +/- 0.05 (NP), -0.95 +/- 0.19 ml x min(-1) x mmHg(-1) (P)], and 4) decreases in heart rate [-41 +/- 4 (NP), -62 +/- 5 beats/min (P)]. Nevertheless, total peripheral and terminal aortic conductances remained elevated in the pregnant rabbits (P < 0.05) after L-NA. Furthermore, decreases in cardiac output and femoral conductance were not different between the reproductive states. We conclude that the contribution of NO to vascular tone increases during pregnancy, but only in some vascular beds. Moreover, the data support a role for NO in the pregnancy-induced increase in basal heart rate. Finally, unknown factors in addition to NO must also underlie the basal vasodilation observed during pregnancy.

Hemodynamic effects of S-nitrosocysteine, an intravenous regional vasodilator

BACKGROUND

S-nitrosocysteine is a carrier form of nitric oxide that can be delivered intravenously. S-nitrosocysteine is rapidly metabolized by plasma (half-life = 2-3 seconds), forming nitric oxide and cysteine. With its short half-life and potent vasodilatory properties, S-nitrosocysteine may be useful as a pulmonary vasodilating agent in cases of postoperative and chronic pulmonary hypertension.

OBJECTIVE

Our objective was to determine the hemodynamic properties of S-nitrosocysteine on the pulmonary and systemic circulations to assess its potential utility as a pulmonary vasodilatory agent.

METHODS

Eleven adult swine were anesthetized. Thermodilution (Swan-Ganz; Baxter International, Inc, Deerfield, Ill) and arterial catheters were inserted. Flow probes were placed around the coronary, renal, superior mesenteric, and iliac arteries. Incremental infusion doses of S-nitrosocysteine (5-80 nmol. kg(-1). min(-1)) were delivered into the right atrium. Cardiac output, right and left heart pressures, heart rate, Pao(2), and iliac, renal, coronary, and mesenteric blood flow rates were recorded at baseline and at each infusion dose of S-nitrosocysteine.

RESULTS

Low-dose S-nitrosocysteine infusion decreased mean pulmonary artery pressure (15%, P =.013) without a significant reduction in mean systemic artery pressure. Higher dose infusions produced further dose-dependent declines in pulmonary vascular resistance and measurable reductions in systemic vascular resistance (P =.01). At an S-nitrosocysteine dosage of 40 nmol. kg(-1). min(-1), there was a significant reduction in renal (P <.001) and mesenteric (P =.003) blood flow but no change in iliac (P >.2) or coronary (P >.2) blood flow. Cardiac output remained constant up to infusion rates of 40 nmol. kg(-1). min(-1) (P >.2). Doses higher than 5 nmol. kg(-1). min(-1) resulted in a substantial dose-dependent reduction in Pao(2) (P <.001), suggesting dilation of atelectatic areas of the lung.

CONCLUSION

S-nitrosocysteine is a potent vasodilatory agent capable of overcoming the hypoxic vasoconstrictive response of the lung. Our results suggest it may prove useful as a pulmonary vasodilatory agent at low doses. Higher dose infusions reduce mean systemic pressure and lead to compensatory reductions in renal and mesenteric blood flow without a decrease in cardiac output.

Right ventricular systolic pressure load alters myocyte maturation in fetal sheep

The effects of right ventricular (RV) systolic pressure (RVSP) load on fetal myocyte size and maturation were studied. Pulmonary artery (PA) pressure was increased by PA occlusion from mean 47.4 +/- 5.0 (+/-SD) to 71 +/- 13.6 mmHg (P < 0.0001) in eight RVSP-loaded near-term fetal sheep for 10 days. The maximal pressure generated by the RV with acute PA occlusion increased after RVSP load: 78 +/- 7 to 101 +/- 15 mmHg (P < 0.005). RVSP-load hearts were heavier (44.7 +/- 8.4 g) than five nonloaded hearts (31.8 +/- 0.2 g; P < 0.03); heart-to-body weight ratio (10.9 +/- 1.1 and 6.5 +/- 0.9 g/kg, respectively; P < 0.0001). RVSP-RV myocytes were longer (101.3 +/- 10.2 microm) than nonloaded RV myocytes (88.2 +/- 8.1 microm; P < 0. 02) and were more often binucleated (82 +/- 13%) than nonloaded myocytes (63 +/- 7%; P < 0.02). RVSP-loaded myocytes had less myofibrillar volume than did nonloaded hearts (44.1 +/- 4.4% and 56. 1 +/- 2.6%; P < 0.002). We conclude that RV systolic load 1) leads to RV myocyte enlargement, 2) has minor effects on left ventricular myocyte size, and 3) stimulates maturation (increased RV myocyte binucleation). Myocyte volume data suggest that RV systolic loading stimulates both hyperplastic and hypertrophic growth.

Hemodynamic changes in pregnancy

The basic mechanisms that underlie alterations in the physiology of pregnancy are virtually unknown. Basal oxygen consumption increases by some 50 mL/min in pregnant women at term. Blood volume increases gradually over gestation as does red cell mass. Cardiac output increases by some 50% by mid-third trimester. Stroke volume and heart rate increase over the course of pregnancy with heart rate increasing gradually until term. The heart of the pregnant woman remodels dramatically in the first few weeks of pregnancy; end diastolic volume increases. Stroke volume is augmented by the increase in end diastolic volume and maintenance of ejection fraction through a possible increase in contractile force. Systolic and diastolic blood pressures drop during normal pregnancy. There is evidence of blood vessel remodeling in all vessels. Venous compliance and venous blood volume are increased. Renal plasma flow increases by some 70% in pregnancy with glomerular filtration rate increasing by 50% by unknown mechanisms. The complex hormonal environment is changing throughout pregnancy. In summary, under the influence of circulating chemical mediators blood flow is redistributed to the uterus, breast, and kidney.

Temperature conditioning of nasal air: effects of vasoactive agents and involvement of nitric oxide

Nitric oxide (NO) is released into nasal air, but its function is unknown. We hypothesized that nasal vascular tone and/or flow influences temperature conditioning of nasal air and that NO participates in this process. We measured nasal air temperature (via a thermocouple) and exhaled nasal NO release (by chemiluminescence) in five humans and examined the effects of an aerosolized vasoconstrictor (oxymetazoline), a vasodilator (papaverine), N(G)-nitro-L-arginine methyl ester, an inhibitor of NO synthase, or saline (control). Compared with saline (which caused no changes in nasal air temperature or exhaled NO release), oxymetazoline (0.05%) reduced nasal air temperature and NO release (130.8 +/- 15.1 to 81.3 +/- 12.8 nl. min(-1). m(-2); P < 0.01). Papaverine (0.01 M) increased nasal air temperature and NO release (131.8 +/- 13.1 to 157.2 +/- 17.4 nl. min(-1). m(-2); P < 0.03). N(G)-nitro-L-arginine methyl ester reduced nasal air temperature and NO release (123.7 +/- 14.2 to 44.2 +/- 23.7 nl. min(-1). m(-2); P < 0.01). The results suggest that vascular tone and/or flow modulates temperature conditioning and that NO may participate in that function.

Nasal administration of the nitric oxide synthase inhibitor L-NAME induces daytime somnolence

In preliminary studies, human subjects complained of drowsiness after aerosolization of NG-nitro-L-arginine methyl ester (L-NAME) into the nasal passages. We compared the effects of a nasal aerosol of L-NAME (0.5 M, 4 ml) to those of saline on sleep onset latency and exhaled nasal nitric oxide (NO). L-NAME decreased sleep onset latency and exhaled nasal NO. Vasoconstriction and local effects of L-NAME on NO synthesis are unlikely to explain this effect since oxymetazoline, a vasoconstrictor, decreased exhaled NO but had no effect on sleep onset latency. We conclude that aerosolization of L-NAME to the nasal passages induces daytime sleepiness.

Angiotensin type 1 receptor antagonism with irbesartan inhibits ventricular hypertrophy and improves diastolic function in the remodeling post-myocardial infarction ventricle

To evaluate the role of angiotensin II (AII) on diastolic function during post-myocardial infarction (MI) ventricular remodeling, coronary ligation or sham operation was performed in male Sprague-Dawley rats. Experimental animals were maintained on either irbesartan, a selective AT1-receptor antagonist, or no treatment. Measurement of cardiac hypertrophy, diastolic function, and sarcoendoplasmic reticulum adenosine triphosphatase (ATPase; SERCA) and phospholamban (PLB) gene expression was assessed at 6 weeks after MI. Myocardial infarction caused a significant increase in myocardial mass and left ventricular (LV) filling pressure, whereas LV systolic pressure and +dP/dt were reduced. The time constant of isovolumic relaxation (tau) was markedly prolonged after MI. Post-MI hypertrophy was associated with substantial increases in the messenger RNA (mRNA) expression of atrial natriuretic peptide (ANP), but no significant changes in SERCA or PLB levels. Although irbesartan treatment did not significantly alter post-MI LV systolic or filling pressures, it nevertheless effectively decreased ventricular hypertrophy, improved tau, and normalized ANP expression. These results demonstrate that AT1-receptor antagonism has important effects on myocardial hypertrophy and ANP gene expression, which are independent of ventricular loading conditions. In addition, the improvement in diastolic function was not related to changes in SERCA and PLB gene expression, suggesting that enhanced myocardial relaxation was related to the blockade of AII effects on myocyte function or through a reduction of ventricular hypertrophy itself or both.

Physical characteristics and gas composition of nasal air affect nasal nitric oxide release

We studied modulation of release of nitric oxide (NO) into nasal passages by physical characteristics (airflow, temperature, humidity) or gases (oxygen, carbon dioxide) in nasal air of humans. Each characteristic or gas in nasal air was changed during voluntary soft palate elevation (to isolate nasal passages). Increasing airflow through the nose caused incremental increases in NO release from 211+/-23 nl/(min x m(-2)) at 1 L/min to 312+/-40 nl/(min x m(-2)) at 22 L/min (P<0.001, n = 6). Decreased humidity (dry airflow, 1-22 L/min) reduced NO release only at the highest airflow rate. Changing temperature (range 46 to 0 degrees C) had no effect on NO release. Hypoxia (below 4% O2) rapidly and reversibly decreased NO release (200+/-40 nl/(min x m(-2)) at 21% O2 versus 99+/-17 nl/(min x m(-2)) at 0% O2 for 3 min, (P<0.001, n = 5). Carbon dioxide (5%) reduced NO release slightly. We conclude that airflow, reduced humidity, carbon dioxide concentration, and oxygen concentration modulate NO release into nasal passages.

The sequential effects of estrogen administration and hypertension on cardiac function in ewes

OBJECTIVE

Our objective was to study the effect of estrogen administration and moderate hypertension on left ventricular size, pump function, and contractility in chronically instrumented ewes.

STUDY DESIGN

Ewes were either given 0.06 mg/kg 17beta-estradiol intramuscularly (n = 8) or were made hypertensive (n = 6) by inflation of an occluder around the aorta and were studied weekly. After 3 weeks each ewe received the opposite treatment.

RESULTS

Estrogen administration caused an increase in left ventricular chamber size at a given pressure, fractional shortening (21.9% +/- 2.9% to 28.5% +/- 3.7%), and stroke volume (1.4 +/- 0.3 mL/kg to 1.6 +/- 0.3 mL/kg). Subsequent hypertension further increased left ventricular size at a given pressure but decreased fractional shortening (20.0% +/- 4.4%) and stroke volume (1.3 +/- 0.3 mL/kg). With hypertension first, there was no left ventricular enlargement, even with subsequent estrogen administration, and there were no changes in left ventricular pump function. End-systolic pressure and stress-dimension relationships did not change with either treatment. The end-systolic wall stress-fractional shortening relationship was likewise unchanged, suggesting that neither treatment changed contractility.

CONCLUSIONS

The left ventricle previously exposed to hypertension does not remodel when exposed to estrogen, and cardiac pump function decreases when the estrogen enlarged heart is faced with moderate, subacute hypertension.

Alteration of ascending thoracic aorta compliance after treatment with menotropin

OBJECTIVE

Our purpose was to determine whether aortic size and compliance are altered by an exogenously induced rise in estrogen.

STUDY DESIGN

Magnetic resonance imaging was used to determine the aortic cross-sectional area/aortic pressure relationship in nine premenopausal women before and after menotropin therapy. Simultaneous electrocardiograms, carotid pulse tracings, phonocardiograms, and brachial artery pressures were obtained before each magnetic resonance imaging acquisition. Ascending thoracic aorta cross-sectional area was obtained every 32 msec and aligned with brachial artery pressures extrapolated from the carotid pulse tracing, allowing construction of the ascending thoracic aorta cross-sectional area/aortic pressure relationships. Aortic cross-sectional area was normalized to body surface area, and the shifts in the position for the ascending thoracic aorta cross-sectional area/aortic pressure relationship were determined with use of analysis of covariance.

RESULTS

Heart rate and aortic pressure were unchanged before and after menotropin treatment. Initial estradiol levels were < 20 pg/ml. After menotropin treatment (7.4 +/- 1.0 days) estradiol levels rose to 905 +/- 371 pg/ml (p < 0.0001). Ascending thoracic aorta cross-sectional area/body surface area was not significantly increased, adjusted y mean of 389 +/- 7 mm2/m2 before and 403 +/- 7 mm2/m2 after menotropin treatment (p < 0.24). The slope of the ascending aorta cross-sectional area/aortic pressure relationship, an index of aortic compliance, increased from 1.4 +/- 0.6 mm2/m2/mm Hg before to 1.7 +/- 0.6 mm2/m2/mm Hg after menotropin treatment (p < 0.001).

CONCLUSION

In premenopausal women a short-term rise in estrogen induced by menotropin treatment is associated with an increase in aortic compliance. Aorta size is not significantly increased within this time frame.

Decreased exhaled nitric oxide in subjects with HIV infection

BACKGROUND

Nitric oxide (NO) may be an important component of the host defence against infections. Endogenously produced NO is present in exhaled air and may be representative of respiratory tract production of NO. Since subjects infected with HIV are prone to develop respiratory infections, it was postulated that exhaled NO might be reduced in such individuals.

METHODS

The exhaled concentration of NO (nl/l) and minute ventilation (l/min) were measured and exhaled NO release (nl/min/m2) calculated in 36 subjects infected with HIV (20 non-smokers, 16 smokers) and 31 non-smoking subjects with no active medical conditions.

RESULTS

Exhaled NO from HIV positive individuals was less than from control subjects of similar age, height, and weight. Cigarette smoking did not account for the decreased exhaled NO in HIV positive individuals as both smoking and non-smoking HIV positive subjects had decreased exhaled NO compared with control subjects.

CONCLUSION

Exhaled NO is decreased in subjects infected with the HIV. Since NO functions in host defence against bacterial, viral, and fungal infections, reduced exhaled NO may indicate a mechanism of impaired host defence in HIV infection.

Myocardial imaging with a new transpulmonary lipid-fluorocarbon echo contrast agent: experimental studies in pigs

The development of echo contrast agents that can provide reliable opacification of the myocardium after intravenous injection is an important advancement for the clinical application of contrast echocardiography. In this study, the hemodynamic effects and echocardiographic characteristics of a new lipid-fluorocarbon echo contrast agent, Aerosomes MRX 115 (ImaRx Pharmaceutical Corp., Tucson, Ariz.) were studied in six anesthetized ventilated pigs. Intravenous injection of this new agent in doses ranging from 0.0005 to 0.01 ml/kg produced significant measurable and visible myocardial opacification without any effect on heart rate, systemic pressure, partial pressure of oxygen, or left ventricular systolic function. The two largest doses (0.005 and 0.01 ml/kg), however, resulted in mild reversible increases in mean pulmonary artery pressure of 12 and 16 mm Hg, respectively. In four animals, epicardial images were obtained before and during coronary artery occlusion. Intravenous contrast injection during coronary occlusion permitted delineation of the hypoperfused myocardial segment. This capability may further expand the utility of contrast echocardiography.

Toxic methemoglobinemia after topical anesthesia for transesophageal echocardiography

Topical benzocaine spray is an anesthetic agent that is commonly used during transesophageal echocardiography (TEE). This agent is believed to be relatively safe because of very low systemic absorption. We report a case of toxic methemoglobinemia induced by benzocaine use for pharyngeal anesthesia during TEE. Physicians who perform TEE and use benzocaine or other topical anesthetics need to be familiar with this potential complication and have the necessary treatment readily available.

Exhaled nitric oxide during exercise: site of release and modulation by ventilation and blood flow

To define the site of release and factors modulating exhaled nitric oxide (NO) during exercise in humans, we measured exhaled NO output during exercise, during exercise after balloon occlusion of the nasopharynx (to exclude nasal NO), and at rest with isocapneic hyperventilation or dobutamine infusion. Exhaled NO output increased from rest to exercise (57 +/- 10 to 171 +/- 30 nl.min-1.m-2; P < 0.003; n = 8). Exclusion of nasal NO reduced exhaled NO at rest and during exercise. Calculated nasal contribution at rest (53 +/- 5%) decreased during exercise (29 +/- 6%; P < 0.05), whereas nonnasal contribution increased (47 +/- 5 to 71 +/- 6%; P < 0.05). Isocapneic hyperventilation at rest increased exhaled NO output (51 +/- 8 to 94 +/- 22 nl.min-1.m-2; P = 0.05). Dobutamine infusion did not increase exhaled NO output. We conclude that nasal exhaled NO decreases (and nonnasal exhaled NO increases) with exercise. We also conclude that, under the conditions of this study, increased exhaled NO output during exercise is more closely related to increased ventilation than to increased blood flow.

Effects of estrogen and progestin on aortic size and compliance in postmenopausal women

OBJECTIVE

Our purpose was to determine whether sex steroids alter aortic size and compliance in postmenopausal women.

STUDY DESIGN

Twenty-six postmenopausal women were randomized to receive either conjugated estrogens 0.625 mg per day (group 1) or conjugated estrogens 0.625 mg per day and medroxyprogesterone 2.5 mg per day (group 2). Aortic cross-sectional area was measured by magnetic resonance imaging before and after 3 months of hormone therapy.

RESULTS

Estradiol levels increased in both group 1 and group 2 (p < 0.0001). Ascending aortic cross-sectional area increased from 439 +/- 7 mm2 to 466 +/- 7 mm2 in group 1 (p < 0.008) but was unchanged in group 2. Within the range of aortic pressures studied, no change in aortic compliance could be detected.

CONCLUSION

Estrogen therapy in postmenopausal women was associated with an increase in aortic size; but this effect was not detectable with the addition of progestin. The potential antagonistic effect of progestin on estrogen-induced aortic enlargement suggests that the favorable cardiovascular effects of postmenopausal estrogen therapy cannot be automatically extended to the combination estrogen-progestin.

Right ventricular function in chronically anemic fetal lambs

OBJECTIVE

Our purpose was to determine whether the increase in extravascular fluid in chronic fetal anemia occurs either because of heart failure or despite successful cardiac adaptation.

STUDY DESIGN

Right ventricular function curves were obtained in five ovine fetuses at the start, midpoint, and end of 5 to 8 days of anemia induced by isovolemic daily hemorrhage. Least-squares fit of the ascending and plateau lines of stroke volume versus right atrial pressure were used to establish breakpoints (intersection of the ascending and plateau lines), which were compared by analysis of variance for repeated measures. Myocardial blood flow was measured by microspheres.

RESULTS

Carotid arterial oxygen content was reduced from 7.0 +/- 0.3 to 2.1 +/- 0.1 ml/dl and the hematocrit from 29% +/- 1.8% to 13% +/- 0.6%. Breakpoint analysis of function curves showed that although right atrial pressure remained unchanged (3.4 +/- 0.7 and 3.6 +/- 0.6 mm Hg) stroke volume increased from 1.03 +/- 0.14 to 1.62 +/- 0.25 ml/kg. Both right and left ventricular coronary blood flow were increased, 1351 +/- 313 and 1166 +/- 264 ml/min per 100 gm. Excess fluid was present in abdomen and chest of most animals at autopsy.

CONCLUSION

Tissue edema during severe anemia occurs despite normal right atrial pressure, increased stroke volume, and markedly increased coronary blood flow, markers of successful cardiac adaptation.

Nasal contribution to exhaled nitric oxide at rest and during breathholding in humans

We characterized the nasal contribution to exhaled nitric oxide (NO) at rest and during breathholding in humans. Exhaled NO was greater during nose breathing (141 +/- 17 nl/min/M2, mean +/- SEM) compared with mouth breathing (68 +/- 6 nl/min/M2, n = 8, p < 0.001). After voluntary closure of the soft palate (VCSP) to eliminate nasal NO, exhaled NO from the mouth decreased further (30 +/- 4 nl/min/M2, p < 0.001). Release of NO into nasal passages during VCSP (217 +/- 19 nl/min/M2) was greater than exhaled NO during nasal breathing (141 +/- 17 nl/min/m2, p < 0.001), suggesting that nasal NO is taken up by the respiratory tract. During mouth breathing or nose breathing, NO concentrations sampled with a bronchoscope were higher in the nasopharynx than at the epiglottis or in the trachea in five subjects. Increased peak exhaled NO after a breathhold (33 +/- 7 ppb) was reduced (10 +/- 4 ppb, p < 0.001) after balloon occlusion of the nasopharynx. NO concentration during breathholding increased to a greater extent in the nasopharynx than in the pharynx or trachea. We conclude that the majority of exhaled NO at rest and during a breathhold originates in the nasopharynx.

Effects of ductus arteriosus occlusion on pulmonary artery pressure during in utero ventilation in fetal sheep

Seven fetal sheep were prepared to study the short-term effects of in utero ventilation and ductus arteriosus occlusion on pulmonary artery pressure and on fetal right ventricular function assessed using the right atrial pressure-right ventricular stroke volume relationship. Nine days post-surgery (140 days gestation), blood gas and haemodynamic values were obtained before and during in utero ventilation with 100% O2, and during ventilation with the ductus arteriosus occluded. Oxygen content increased significantly from 7.2 to 14.5 ml dl-1 with ventilation and remained elevated at 14.4 ml dl-1 with ventilation with the ductus arteriosus occluded. In utero ventilation produced a left to right atrial pressure gradient and depression of the right atrial pressure-right ventricular stroke volume relationship. Ductus arteriosus occlusion during in utero ventilation reduced the left to right atrial pressure gradient, and along with a decrease in pulmonary artery pressure, resulted in an upward shift of the right atrial pressure-right ventricular stroke volume relationship, but only to the preventilation level. This study indicates that the fetal right atrial pressure-right ventricular stroke volume relationship is significantly altered, both by changes in the left to right atrial pressure gradient and by changes in pulmonary artery pressure seen with in utero ventilation and subsequent ductus arteriosus occlusion.

Spontaneous echo contrast and hemorheologic abnormalities in cerebrovascular disease

BACKGROUND AND PURPOSE

Spontaneous echo contrast (SEC) is thought to represent a risk factor for cardioembolic stroke. In vitro studies suggest that SEC results from interaction between red cells and fibrinogen. To better understand the relation between SEC and stroke and to investigate the in vivo genesis of SEC, we examined the relation between SEC, the constituents of the blood, and plasma and serum viscosity in patients with acute stroke or chronic cerebrovascular disease.

METHODS

Fifty patients with acute stroke or chronic cerebrovascular disease referred for transesophageal echocardiogram (TEE) were studied by transthoracic echocardiography and TEE. Complete blood count, fibrinogen, albumin, gamma-globulin, and plasma and serum viscosity determinations were made. Left atrial SEC was graded as absent, mild, or marked by means of TEE.

RESULTS

SEC was absent in 31 patients, mild in 10 patients, and marked in 9 patients. Higher grade of SEC was associated with a significantly greater percentage of patients with atrial fibrillation and larger left atrial dimension. Atrial fibrillation was present in 23% of the patients in the SEC absent group, 50% of the patients in the mild SEC group, and 78% of the patients in the marked SEC group (P < .01). Left atrial diameter averaged 3.8 +/- 0.6 cm in the SEC absent group, 4.3 +/- 1.1 in the mild SEC group, and 4.9 +/- 0.7 in the marked SEC group (P < .001). Hematocrit, white blood cell count, and platelet count did not differ among the three groups. Fibrinogen, gamma-globulin, plasma viscosity, and serum viscosity values were all significantly higher in the presence of SEC (P < .05). Fibrinogen values were 361 +/- 97 mg/dL in the SEC absent group and 427 +/- 135 mg/dL in the marked SEC group. gamma-Globulin levels were 0.75 +/- 0.23 g/dL in the SEC absent group and 1.06 +/- 0.48 g/dL in the marked SEC group. Both plasma viscosity (1.97 cp) and serum viscosity (1.64 cp) were higher in the marked SEC group than in the SEC absent group (1.77 and 1.50 cp, respectively).

CONCLUSIONS

In patients with acute stroke or chronic cerebrovascular disease, the severity of SEC was not related to albumin, hematocrit, white cell count, or platelet count but rather to elevated fibrinogen levels and concomitant increases in both plasma and serum viscosity. Moreover, increasing grade of SEC was associated with significantly increased left atrial diameter and a higher percentage of patients in atrial fibrillation.

Estrogen-induced left ventricular chamber enlargement in ewes

We studied the chronic effect of administration of a single large intramuscular dose of 17 beta-estradiol on left ventricular chamber size and output in the ewe. Fourteen oophorectomized ewes were successfully instrumented and studied, with measurements made of left ventricular, aortic, right and left atrial pressures, left ventricular stroke volume, and left ventricular minor axis dimension. Unanesthetized ewes were studied before and 1, 2, and 3 wk after intramuscular administration of 0.6 mg/kg 17 beta-estradiol (7 ewes) or 1.5 ml sesame oil placebo (7 ewes). Left ventricular end-diastolic pressure-end-diastolic dimension (LVEDP-EDD) and left ventricular end-diastolic pressure-stroke volume (LVEDP-SV) relationships were quantified during graded inferior vena caval occlusion and volume infusion. Left ventricular end-diastolic diameter was larger after estrogen but not after placebo administration. The LVEDP-EDD relationship shifted progressively rightward, indicating left ventricular chamber enlargement in the estrogen group but was unchanged in the placebo group. The plateau limb of the LVEDP-SV relationship in the estrogen group shifted up from a mean stroke volume of 77.1-89.5 ml/beat and did not change in the placebo group. We conclude that administration of a single large intramuscular dose of 17 beta-estradiol resulted in left ventricular chamber enlargement and increased stroke volume in the ewe.

Maximal myocardial blood flow is enhanced by chronic hypoxemia in late gestation fetal sheep

The measurement of maximal myocardial blood flow gives information about the total cross-sectional area of the coronary resistance vessels. During a continuous left atrial infusion of adenosine (60 micrograms.kg-1.min-1), maximal myocardial blood flow was measured in 4 fetuses hypoxemic for a minimum of 5-8 days (pH = 7.33 +/- 0.01, arterial PCO2 = 49.8 +/- 4.2 Torr, arterial PO2 = 16.1 +/- 1.3 Torr, and arterial concentration of O2 = 5.3 +/- 1.2 ml/dl). These data were compared with an identically instrumented group of normoxemic fetuses (n = 7) following the same study protocol (pH = 7.38 +/- 0.02, arterial PCO2 = 43.1 +/- 3.8 Torr, arterial PO2 = 19.8 +/- 2.0 Torr, and arterial concentration of O2 = 7.9 +/- 1.0 ml/dl) (P < 0.05). At comparable arterial pressures, the maximal myocardial flow (ml.min-1.100 g tissue-1) for hypoxemic vs. normoxemic fetuses was 974 +/- 273 and 630 +/- 181 for the total myocardium, 986 +/- 367 and 602 +/- 192 for the left ventricular free wall, 1,025 +/- 346 and 614 +/- 178 for the septum, and 1,231 +/- 274 and 757 +/- 269 for the right ventricular free wall, respectively (P < 0.01). These data suggest that hypoxemia in the fetus can significantly alter the coronary vascular bed, which, if confirmed, would represent an important adaptation in the developing fetus.

Severe right ventricular pressure loading in fetal sheep augments global myocardial blood flow to submaximal levels

BACKGROUND

It has previously been shown that the fetal right ventricle (RV) is sensitive to changes in arterial pressure and that its stroke volume is significantly reduced with acute increases in pulmonary arterial pressure. However, the myocardial blood flow (MBF) response to increases in pulmonary arterial pressure have not been investigated in the fetus.

METHODS AND RESULTS

To assess whether the RV afterload sensitivity to arterial pressure is associated with limitation in MBF, seven fetal lambs were instrumented at 130 days of gestation with a pulmonary arterial occluder and intravascular catheters. RV stroke volume was measured by an electromagnetic flow probe and MBF by 15-microns labeled microspheres. MBF was determined at baseline and during incremental increases in pulmonary arterial pressure. Maximal MBF was determined in seven additional fetuses during adenosine infusion. The highest tolerated pressure was associated with a 50% reduction in RV stroke volume. The highest pulmonary arterial occlusion pressure resulted in a doubling of MBF to all regions of the heart (266 +/- 99 to 504 +/- 158, 193 +/- 69 to 387 +/- 100, and 171 +/- 66 to 338 +/- 134 ml/min/100 g for the RV, septum, and left ventricle, respectively). The best correlation for increases in both RV and global MBF was the RV heart rate-systolic pulmonary pressure product. Adenosine infusion was associated with a threefold increase in global MBF that was significantly greater than the MBF achieved during pulmonary arterial occlusion.

CONCLUSIONS

The fetal RV sensitivity to acute pressure loading is not associated with limitation of MBF. The fetal myocardium has a remarkable flow reserve that allows for preservation of function during acute increases in arterial pressure.

Vascular pressure-volume relationships in pregnant and estrogen-treated guinea pigs

We investigated the relationship between mean circulatory filling pressure (MCFP) and blood volume in nonpregnant (NP), estrogen-treated (E), and pregnant (P) guinea pigs. Reversible circulatory arrest was produced by rapid ventricular pacing or acetylcholine in unanesthetized animals remote from surgery. MCFP (mmHg) was higher for E (7.1 +/- 0.3) than for NP (5.8 +/- 0.5) or P (5.3 +/- 0.4). The gradient for venous return, the difference between MCFP and right atrial pressure (mmHg), did not differ in NP- (6.0 +/- 0.5), P- (5.8 +/- 0.5), or E- (5.8 +/- 0.4) treated animals. Capacitance, the blood volume (ml/kg) at an MCFP of 6 mmHg, was increased in P (84 +/- 6) and E (89 +/- 7), compared with NP (64 +/- 5) animals. Compliance, the ratio of the change in volume to change in pressure in the range of 6-12 mmHg (ml.kg-1.mmHg-1), was greater in P (4.4 +/- 0.3) than NP (3.5 +/- 0.3) animals. Hexamethonium blockade did not affect MCFP, capacitance, or compliance. We conclude that the effect of blood volume expansion on the circulation in pregnancy cannot be predicted from knowledge of MCFP-blood volume relationships in the nonpregnant animal, because capacitance and compliance are altered. Estrogen administration to nonpregnant animals reproduces some of these effects.

The effect of acute hypoxaemia on ventricular function during beta-adrenergic and cholinergic blockade in the fetal sheep

The effect of acute hypoxaemia on right and left ventricular function was investigated in 8 fetal sheep (137-140 days gestation). Fetuses were instrumented with electromagnetic flow sensors on the ascending aorta and the main pulmonary artery. After 8 days recovery, hypoxaemia was achieved by reducing the maternal ewe's inspired O2 concentration to 13.1 +/- 1.5%. Control and hypoxaemic arterial blood values were pH 7.37 +/- 0.04 (SD) and 7.35 +/- 0.06, PCO2 48.0 +/- 2.8 and 47.6 +/- 5.1 mmHg, PO2 19.9 +/- 2.2 and 11.4 +/- 1.5 mmHg, haematocrit 37.5 +/- 1.2 and 39.5 +/- 2.2, respectively. Arterial pressure increased insignificantly with acute hypoxaemia (50.2 +/- 3.9 to 53.6 +/- 8.1 mmHg). Left and right ventricular performance was assessed by generating biventricular function curves relating stroke volume to mean atrial pressure. All function curves were composed of steep ascending and plateau limbs that intersected at a breakpoint. Comparing control and hypoxaemia function curves, the left ventricular stroke volume breakpoints were 0.79 +/- 0.20 and 0.78 +/- 0.21 ml/kg, respectively, while the right ventricular stroke volume breakpoints were 0.99 +/- 0.11 and 0.88 +/- 0.21 ml/kg (n.s.). In 4 fetuses, acute hypoxaemia was associated with significant increases in arterial blood pressure (P less than 0.05). In these fetuses, the right ventricular function curve was shifted significantly downward compared to the control right ventricular curve. When nitroprusside was given to these hypertensive fetuses to return blood pressure to control levels, the right ventricular function curve returned to baseline. We conclude that even under conditions of extreme hypoxaemia, ventricular function is well preserved in the normotensive fetal sheep. However, when increases in arterial pressure also accompany hypoxaemia, detectable changes in right ventricular function can be accounted for by changes in arterial pressure.

Decreased nutrient blood flow during dopamine- and epinephrine-induced intestinal vasodilation

Mesenteric blood flow returned to control values promptly upon termination of i.a. infusions of vasodilator doses of acetylcholine (ACh) and isoproterenol (Iso) into the canine superior mesenteric artery (SMA). However, termination of vasodilator infusions of dopamine (DA) and epinephrine (Epi) into the SMA resulted in an additional increase in flow resembling the reactive hyperemic response observed upon termination of norepinephrine infusion into the SMA. In experiments that measured the effect of these agents on intestinal oxygen kinetics, i.a. ACh and Iso increased oxygen uptake but did not affect percent oxygen extraction. Equivasodilator i.a. infusions of DA and Epi decreased percent oxygen extraction. DA decreased oxygen uptake whereas Epi did not alter oxygen uptake. After administration of phenoxybenzamine, the effects of i.a. DA and Epi on oxygen kinetics became identical to those produced by equivasodilator infusions of ACh or Iso. Moreover, the reactive hyperemia-like response on cessation of DA and Epi infusions was abolished. In experiments that utilized 125I absorption from the jejunum as a measure of intestinal mucosal blood flow, i.a. ACh and Iso were found to increase mucosal blood flow in proportion to the increase in total SMA flow. In contrast, i.a. doses of DA and Epi that increased SMA flow nevertheless decreased mucosal flow. It was concluded that vasodilator doses of dopamine and Epi decrease oxygen delivery to the intestine at least in part due to a decrease in mucosal blood flow; this probably involves action on alpha adrenoreceptors.

Haemodynamic responses to dopamine and dobutamine infusions as a function of duration of infusion

In normal animals, dopamine and dobutamine produce similar haemodynamic effects. However, clinical observations on patients in heart failure suggest that dobutamine produces a more favourable and stable haemodynamic effect. One explanation is that the inotropic action of dopamine is largely an indirect one mediated through noradrenaline release, while that of dobutamine is direct. If this were true, one would predict that the haemodynamic effects of dopamine and dobutamine would be similar over short-term infusions, but that the response to dopamine would be attenuated with time as a result of noradrenaline depletion. Experiments are reported which are consistent with this explanation. 15-min infusions of graded doses of dopamine and dobutamine produced indistinguishable haemodynamic effects in a dog model in which the haemodynamic characteristics of heart failure were produced by a chronic aorta-left atrial shunt: dopamine also produced the expected increase in mesenteric and renal flows with this short-term exposure. With long-term infusions of dopamine (5 h), the specific mesenteric/renal vasodilatation was lost with time, and indices of myocardial contractility also were attenuated with time. In contrast, the haemodynamic actions of dobutamine were well maintained. Both amines lowered total peripheral resistance and ventricular filling pressures over the entire period. These results suggest that the haemodynamic responses to dopamine change with time, and that some of the discrepancies between animal observations (short term) and those in patients (invariably long term) can be explained on this basis rather than on the basis of species differences or underlying pathology. They also suggest that dopamine is a less suitable 'inotropic' drug than dobutamine.

Effect of a specific and a nonspecific vasodilator on regional blood flows in experimental heart failure

Urotensin I, a vasoactive peptide isolated from fish urophyses, has previously been demonstrated to cause specific mesenteric dilation in the dog. This mechanism of action should limit its maximal cardiovascular actions: no additional cardiovascular effects should ensure once maximal mesenteric vasodilation is achieved. Provided that the mesenteric vasodilatation does not result in decreased flow to other organs, the agent may, therefore, offer a theoretical advantage as an afterload reducing agent. In pentobarbital anesthetized dogs which were in heart failure as a result of chronic aortico -- left atrial shunt, the effects of urotensin I on cardiovascular hemodynamics were compared with the effects of a nonspecific vasodilator, sodium nitroprusside. At equidepressor doses (approximately 15% decrease in mean arterial pressure; sodium nitroprusside, 2 micrograms . kg-1 . min-1 i.v.; urotensin I, 10 mU . kg-1 . min-1 i.v.), both agents produced comparable falls in total peripheral resistance, left ventricular end diastolic pressure, and pulmonary capillary wedge pressure. Forward cardiac output was increased by both substances, although the increases were not statistically significant. Shunt flow, estimated by echocardiography, was reduced by both. In spite of the marked similarity in hemodynamic effects in this model, the two agents affected regional blood flows differently. Sodium nitroprusside did not significantly alter regional flows measured by radioactive microspheres, although calculated splanchnic, skin, and myocardial vascular resistances were reduced. In contrast, urotensin I, as in the normal dog, greatly increased mesenteric blood flow; this redistribution of cardiac output did not, however, result in underperfusion of other vital organs. These data suggest that urotensin I may be a useful agent in the reduction of afterload in heart failure, particularly since the unique mechanism of action appears to minimize the potential for adverse effects due to excessive dosage.

Measurement of mucosal blood flow by assay of absorption of 125I from the intestinal lumen

A method which utilized the absorption of 125I as a measure of intestinal mucosal blood flow was developed. In the pentobarbital anesthetized dog a segment of mid-jejunum was isolated from adjacent vasculature and perfused with 125I solution. Superior mesenteric artery flow, blood flow to the isolated segment of intestine and 125I absorption were measured. Changes in 125I absorption were found to correlated closely with changes in blood flow (r = 0.95). Using an autoradiographic technique 125I absorption was found to be localized to the intestinal mucosa.