Dobutamine restores the reduced efficiency of energy transfer from total mechanical work to external mechanical work in stunned porcine myocardium

OBJECTIVE

In order to determine whether the relatively high oxygen consumption of stunned myocardium is related to decreased mechanical efficiency, myocardial oxygen consumption (MVO2) and its major determinants were studied in 10 open chest anaesthetised pigs.

METHODS

According to the time varying elastance concept, MVO2 is determined by contractility (Emax) and total mechanical work (PLA), which is the sum of the external work (EW) and potential energy (PE). Mechanical efficiency (EW/MVO2) equals the product of EW/PLA (= efficiency of energy transfer or EET) and PLA/MVO2. Emax is the slope of the end systolic pressure-segment length relationship, determined by gradually clamping the aorta. PLA is the area enclosed by the end systolic pressure-segment length relationship and the pressure-segment length trajectory. EW is the area of the pressure-segment length loop. Systemic haemodynamics, regional segment shortening, and MVO2 were determined at baseline, during stunning (two sequences of 10 min occlusion and 30 min of reperfusion), after a subsequent 50 beats.min-1 increase in heart rate by atrial pacing and additional infusion of 2 micrograms.kg-1.min-1 dobutamine.

RESULTS

Stunning decreased segment shortening from 18.2(SEM 1.9)% to 10.2(1.5)%, MVO2 from 4.16(0.27) x 10(-2) to 2.84(0.25) x 10(-2) mumol.beat-1.g-1, and Emax from 47(9) to 23(3) mm Hg.mm-1 (all p < 0.05). PLA decreased by 13(4)%, as EW decreased by 42(6)%, and PE tended to increase. Although EET decreased from 0.58(0.04) to 0.40(0.03) (p < 0.05), there was no decrease in the mechanical efficiency, as an increase in PE caused an increase in PLA/MVO2 which compensated for the decrease in EET. Dobutamine infusion increased Emax and EW per beat to 120(23)% and 67(8)% of baseline, respectively, while MVO2 [4.12(0.53) mumol.beat-1.g-1] and EET [0.57(0.04)] returned to baseline.

CONCLUSIONS

In stunned myocardium, mechanical efficiency is not decreased despite a decrease in EET. The increase in EET after dobutamine may explain the lack of the excessive increase in MVO2.

Coronary pressure-flow relation in left ventricular hypertrophy. Importance of changes in back pressure versus changes in minimum resistance

Perfusion abnormalities in the pressure-overloaded hypertrophied left ventricle could result from an increase in minimum coronary resistance or an increase in effective back pressure due to increased extravascular compressive forces. Since the pressure-flow relation of the maximally vasodilated coronary bed allows dissociation of minimum resistance (inverse slope [1/alpha PF]) and back pressure (pressure at zero flow [Pf = 0]), the present study was undertaken to examine the coronary pressure-flow relation in left ventricular hypertrophy (LVH). Ascending aortic banding in eight dogs at 6-8 weeks of age (LVH group) increased the left ventricular to body weight ratio to 8.7 +/- 0.6 g/kg as compared with 4.8 +/- 0.2 g/kg in nine normal dogs (p < 0.05). Maximum coronary vasodilation was produced by infusion of adenosine (1 mg/kg per minute i.v.). The slope of the coronary pressure-flow relation (alpha PF) was 5.8 +/- 0.5 10(-2) (ml/min per gram)/mm Hg in the LVH group and 9.3 +/- 0.6 10(-2) (ml/min per gram)/mm Hg in the normal group (p < 0.05). alpha PF was significantly correlated with the left ventricular to body weight ratio but not with coronary pressure, suggesting that the degree of hypertrophy and not exposure to high coronary pressure was responsible for the observed decrease in alpha PF. Pf = 0 was 24.1 +/- 2.6 mm Hg in the LVH group and 11.7 +/- 1.2 mm Hg in the normal group (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Increased activity of the sarcoplasmic reticular calcium pump in porcine stunned myocardium

OBJECTIVE

The aim was to determine whether changes in sarcoplasmic reticular Ca2+ transport activity and the degree of phosphorylation of phospholamban of "stunned" myocardium are involved in the reversible depression of contractile function.

METHODS

In anaesthetised open chest swine, stunning was induced by subjecting the myocardium perfused by the left anterior descending coronary artery to two cycles of 10 min of occlusion and 30 min of reperfusion. Before and after stunning, systemic haemodynamic variables and regional myocardial function and perfusion were determined, while biopsies were taken for determination of the content of high energy phosphate compounds. Sarcoplasmic reticular function (ATP dependent Ca2+ transport and phosphorylation of phospholamban) of the stunned and control myocardium was determined at the end of the stunning protocol.

RESULTS

In the stunned myocardium the segment length shortening decreased from 17.4(SD 4.0)% to 3.5(4.4)%, while perfusion was 38% less than at baseline. ATP and total adenine nucleotide levels of the stunned myocardium were about 35% lower than in the control myocardium, but the energy charge was normal as creatine phosphate levels had increased by 66% over the content determined at baseline. Ca2+ uptake by the sarcoplasmic reticulum isolated from the stunned region was 17% (p < 0.05) higher than Ca2+ uptake from the control region [1240(303) and 1450(280) nmol.min-1.mg-1 protein, respectively]. In the presence of exogenous cyclic AMP dependent protein kinase the amount of 32P incorporated into phospholamban was similar for both myocardial regions.

CONCLUSIONS

In this model of stunned porcine myocardium, the phosphorylation state of phospholamban was unchanged, but Ca2+ uptake by the sarcoplasmic reticulum was slightly increased. The results indicate that a change in active Ca2+ transport by the sarcoplasmic reticulum is most likely not to be the principal cause of contractile dysfunction of stunned myocardium.

Altered coronary flow reserve in the hypertrophied heart: implications for therapy

Coronary flow reserve has been shown to be abnormally low in several models of left ventricular hypertrophy induced by long-standing pressure overload. Because the presence of hypertrophy is a risk factor for the development of subendocardial ischaemia and sudden death, efforts to restore alterations in flow reserve may prove beneficial. In the following review, we discuss potential mechanisms which might contribute to this abnormal vasodilator capacity in the hypertrophied heart, with particular emphasis on how chronic therapy may potentially reverse such abnormalities. In addition, we report how the acute administration of various classes of pharmacological agents can alter measurements of coronary flow reserve, as observed in our anaesthetised swine model. Such factors must be considered before interpreting any changes in coronary flow reserve in models of hypertrophy following chronic administration of drugs.

Proto-oncogene expression in porcine myocardium subjected to ischemia and reperfusion

The molecular basis of myocardial adaptation to ischemia and reperfusion is poorly understood. It is thought that nuclear proto-oncogenes act as third messengers, converting cytoplasmic signal transduction into long-term changes of gene expression. We studied the expression of six nuclear proto-oncogenes (Egr-1, c-fos, fosB, c-jun, junB, and c-myc) in myocardium subjected to ischemia and reperfusion in anesthetized pigs. Stunning was achieved by two 10-minute left anterior descending coronary artery occlusions separated by 30 minutes of reperfusion. Hearts were excised after the first occlusion, after the first reperfusion, and at 30, 120, 150, and 210 minutes of reperfusion after the second occlusion. Total RNA was prepared from stunned as well as normally perfused myocardial tissue and subjected to Northern blotting. The response of the six nuclear proto-oncogenes varied.fosB gene expression was never detected. The c-myc gene was expressed, but its level was unchanged by ischemia. c-jun expression was slightly increased by ischemia (3.1 +/- 0.6-fold). The c-fos, Egr-1, and junB genes were highly induced, being fivefold to sevenfold higher in experimental than in control tissue. In three animals pretreated with the beta 1-antagonist metoprolol and then subjected to the above experimental protocol, the induction of proto-oncogenes was similar to that in nonblocked controls. Our results show that the myocardial adaptive response to ischemic stress includes the induction of at least four transcription factors that may be further operative in repair processes and angiogenesis.

Recruitment of myocardial work and metabolism in regionally stunned porcine myocardium

We characterized postischemic changes in myocardial metabolism and regional external work, as measured by the integral of left ventricular pressure-segment-length loops. In 12 anesthetized swine, the left anterior descending coronary artery (LAD) was occluded for 10 min and reperfused for 30 min for two successive cycles. Before ischemia, regional work was 16,920 +/- 5,630 mmHg-mm/min and after stunning, work was reduced to 50 +/- 14% (P < 0.05). At baseline, oxygen and lactate consumption were 4.80 +/- 1.40 and 1.02 +/- 0.46 mumol.min-1 x g-1, respectively, and after stunning they were reduced to 3.24 +/- 0.80 (P < 0.05) and 0.16 +/- 0.21 mumol.min-1 x g-1 (P < 0.05), respectively. The atria were then paced 50 beats/min higher than the reperfusion heart rate, during and without an infusion of dobutamine (2 micrograms.kg-1 x min-1). During dobutamine, both regional external work and oxygen consumption returned to 98% of preischemic values, but lactate utilization remained depressed. We conclude that regional external work and oxygen consumption remain coupled during inotropic stimulation after stunning, with a preferential shift toward nonlactate substrates.

Serotonin selectively aggravates subendocardial ischemia distal to a coronary artery stenosis during exercise

BACKGROUND

The coronary circulation has been shown to remain responsive to vasodilator and vasoconstrictor stimuli during myocardial ischemia. Because serotonin possesses both vasodilator and vasoconstrictor properties, we examined its effect in the coronary circulation distal to an arterial stenosis that resulted in myocardial hypoperfusion during exercise.

METHODS AND RESULTS

Seven chronically instrumented dogs were studied during treadmill exercise in the presence of a stenosis that reduced distal left circumflex coronary artery perfusion pressure to 42 +/- 1 mm Hg. Myocardial blood flow was assessed with radioactive microspheres during exercise before and during intracoronary infusion of 0.4 and 2.0 micrograms/kg-1.min-1 serotonin. The stenosis was adjusted to maintain distal coronary pressure constant during control exercise and with the two doses of serotonin. In seven dogs, the effect of serotonin (2.0 micrograms/kg-1.min-1) was also studied during exercise with normal arterial inflow. During control exercise, the stenosis decreased mean myocardial blood flow to 45% of flow in the normally perfused region. This decrease was most pronounced in the subendocardium (endocardial/epicardial ratio 0.36 +/- 0.06 versus 1.46 +/- 0.14 in the control region; p < 0.01). With no change in pressure distal to the stenosis, serotonin decreased subendocardial flow from 0.51 +/- 0.09 ml/min-1.g-1 to 0.41 +/- 0.12 (p < 0.05) and then to 0.35 +/- 0.08 ml/min-1.g-1 (p < 0.05) and tended to increase subepicardial flow from 1.47 +/- 0.17 to 1.91 +/- 0.23 and 1.85 +/- 0.21 ml/min-1.g-1 (p = 0.08) during infusions of 0.5 and 2.0 micrograms/kg-1.min-1, respectively, with no change in total arterial inflow. In contrast, in the absence of a stenosis, serotonin (2.0 micrograms/kg-1.min-1) increased subendocardial flow from 2.43 +/- 0.25 to 3.73 +/- 0.25 ml/min-1.g-1 (p < 0.01) and subepicardial flow from 1.88 +/- 0.20 to 5.29 +/- 0.38 ml/min-1.g-1 (p < 0.01).

CONCLUSIONS

During normal arterial inflow, serotonin dilated coronary resistance vessels and increased flow to all myocardial layers. During hypoperfusion, a vasodilator response was still present in the subepicardium, but vasoconstriction was then observed in the subendocardium. Our data suggest that serotonin constricts the intramural penetrating arteries, thereby selectively increasing resistance to subendocardial blood flow.

L-propionylcarnitine and myocardial performance in stunned porcine myocardium

Recently, we showed that L-propionylcarnitine did not affect recovery of regional contractile function of porcine myocardium subjected to 1 h of low-flow ischemia followed by 2 hr of reperfusion. In that study, ischemia may have been too severe and/or the duration of reperfusion too short to detect a beneficial effect of the compound. Therefore, in the present study we investigated the effects of saline (control group; n = 14) or pretreatment with L-propionyl-carnitine (3 days of 50 mg/kg p.o. b.i.d. + 50 mg/kg i.v. prior to the experiment; n = 13) on recovery of regional contractile function of the myocardium in open-chest anesthetized pigs, subjected to two cycles of 10 min of left anterior descending coronary artery (LADCA) occlusion, each followed by 30 min of reperfusion. In the control animals, at the end of the second reperfusion period, systemic vascular resistance had increased by 18%, which, however, was not observed in the L-propionylcarnitine-treated pigs. In the control group, during the first occlusion, systolic segment length shortening (SSLS) of the LADCA-perfused area decreased from 18.5 +/- 5.5% to -3.7 = 3.2%. After 30 min of reperfusion, SSLS of the LADCA-perfused area had only partially recovered to 6.2 +/- 5.9%. During the second occlusion-reperfusion cycle similar values for SSLS were observed. In the treated animals, SSLS of the LADCA-perfused area was slightly improved after the second occlusion-reperfusion cycle (p = 0.056). This effect did not result in an overall improvement in cardiac pump function.(ABSTRACT TRUNCATED AT 250 WORDS)

Pressure-maximal coronary flow relationship in regionally stunned porcine myocardium

In view of variable results on maximal coronary blood flow in stunned myocardium, we studied the pressure-maximal coronary flow (PMCF) relationship in stunned myocardium in 12 anesthetized swine by using intracoronary adenosine (20 micrograms/kg). Subendocardial systolic segment shortening (SS) measured with sonomicrometry was 19 +/- 5% (means +/- SD) at baseline and 7 +/- 6% (P less than 0.01) at 30 min of reperfusion after 15 min of low-flow ischemia, at which time postsystolic shortening was present. Myocardial stunning increased the slope of the PMCF regression line (alpha PMCF) from 3.34 +/- 1.03 to 3.89 +/- 1.33 ml.min-1.mmHg-1 (P less than 0.01). Atrial pacing at 40 beats/min above spontaneous heart rate (n = 6) further reduced subendocardial SS to 6 +/- 6% (P less than 0.05). Dobutamine (4 micrograms.kg-1.min-1; n = 6) increased subendocardial SS to 13 +/- 5% (P less than 0.05) and abolished postsystolic shortening. Both interventions left alpha PMCF unchanged. In conclusion, myocardial stunning was associated with an increase in alpha PMCF that most likely resulted from the decreased contractile function. The absence of an effect of dobutamine may be due to its predominant action on diastolic function.

Catecholamines and regional hemodynamics during isovolemic hemodilution in anesthetized pigs

The effects of stepwise isovolemic hemodilution on systemic and regional hemodynamics, oxygen flux, and circulating catecholamines were studied in six pigs anesthetized with midazolam and fentanyl. Reduction of the hematocrit from 28 to 9% resulted in doubling of the cardiac output, mainly due to an increase in stroke volume. Regional blood flows, measured using the radioactive microsphere technique, showed an increase in blood flow to all organs except liver (hepatic artery fraction) and adrenals, with a redistribution of cardiac output in favor of heart and brain (increase in blood flow 420 and 170%, respectively). Oxygen flux to most organs did not decrease until hematocrit decreased to 9%, while total body oxygen consumption was well maintained. Left ventricular oxygen consumption increased, but because left ventricular blood flow also increased, left ventricular extraction ratio did not increase. Circulating catecholamines did not play any role in these regulatory mechanisms.

Effect of antiischemic therapy on coronary flow reserve and the pressure-maximal coronary flow relationship in anesthetized swine

The effect of nifedipine (0.5, 1.0, and 2.0 micrograms/kg/min), metoprolol (0.1, 0.5, and 1.0 mg/kg), the beta 1-selective adrenoceptor partial agonist epanolol (10, 50, and 200 micrograms/kg), or equivalent volumes of isotonic saline (n = 6, in each group), on coronary blood flow capacity were studied in anesthetized swine. Intracoronary bolus injections of adenosine (20 micrograms/kg/0.2 ml) were administered without and during three levels of coronary stenosis, prior to and following each dose of drug, to obtain maximal coronary blood flows at different perfusion pressures in the autoregulatory range. Coronary perfusion pressures were varied by partial inflation of a balloon around the left anterior descending coronary artery. Special care was taken that the stenoses not lead to myocardial ischemia. Three indices of coronary blood flow capacity were used: absolute coronary flow reserve (ACFR, the ratio of maximal to resting coronary blood flow), the slope and the extrapolated pressure at zero flow (Pzf) of the pressure-maximal coronary flow (PMCF) relationship, and relative coronary flow reserve (RCFR, the ratio of maximal coronary blood flow with a stenosis to maximal coronary blood flow without a stenosis) at two of the three levels of stenosis. Nifedipine decreased ACFR from 4.5 +/- 1.9 to 1.9 +/- 0.3 (mean +/- SD; p less than 0.05), reflecting in part the increase in resting coronary blood flow. The nifedipine-induced changes in maximal coronary blood flow were not only due to a drop in perfusion pressure, as the slope of the PMCF relationship decreased from 2.27 +/- 0.49 ml/(min.mm Hg) to 1.54 +/- 0.51 ml/(min.mm Hg) (p less than 0.05), and Pzf decreased from 30 +/- 4 mm Hg to 20 +/- 7 mm Hg (p less than 0.05). Consequently, calculated maximal coronary blood flow was attenuated from 114 +/- 31 ml/min to 93 +/- 37 ml/min at 80 mm Hg, but was enhanced from 23 +/- 13 to 37 +/- 24 ml/min at 40 mm Hg coronary perfusion pressure. In concert with the change in the PMCF relationship, RCFR at equivalent severe stenosis increased from 0.33 +/- 0.06 to 0.47 +/- 0.10 (p less than 0.05). No changes were observed with metoprolol, epanolol, or saline. The effect of nifedipine on the PMCF relationship not only provides a mechanism for the drug's antiischemic action, but should also be considered in the interpretation of coronary flow reserve measurements in patients on nifedipine treatment.

Evidence against a role for dopamine D1 receptors in the myocardium of the pig

1. We investigated the presence of dopamine D1 receptors in the myocardium of anesthetized pigs using intravenous infusions of dopamine, alone and after alpha- and beta-adrenoceptor blockade and intracoronary infusions of the selective D1 receptor agonist, fenoldopam. 2. Intravenous infusion of dopamine (2.5, 5 and 10 micrograms kg-1 min-1 for 10 min, n = 6) caused dose-dependent changes in heart rate (from 94 +/- 6 to 132 +/- 10 beats min-1, P less than 0.05), the maximal rate of rise of left ventricular pressure (LVdP/dtmax; from 2280 +/- 170 to 4800 +/- 410 mmHgs-1, P less than 0.05), mean arterial blood pressure (from 87 +/- 5 to 62 +/- 3 mmHg) and systemic vascular resistance (from 40 +/- 4 to 28 +/- 2 mmHgl-1 min, P less than 0.05). The increases in heart rate and LVdP/dtmax were abolished when dopamine was infused after alpha- and beta-adrenoceptor blockade. The vasodilator response was, however, only minimally affected. 3. Intravenous infusions of dopamine decreased coronary vascular resistance from 0.90 +/- 0.06 to 0.53 +/- 0.07 mmHg ml-1 min 100 g (P less than 0.05). This action of dopamine was not observed when dopamine was infused after blockade of the alpha- and beta-adrenoceptors. 4. Pretreatment with alpha- and beta-adrenoceptor blockade had no effect or only slightly attenuated the dopamine-induced decrease in vascular resistance of the brain, kidneys, adrenals and small intestine. 5. In 7 animals, intracoronary doses of 0.04, 0.1, 0.2 and 0.4g kg1- min 1 of fenoldopam had no effect on coronary venous oxygen content, local myocardial oxygen consumption, coronary blood flow or coronary vascular resistance. However, systemic effects were observed at the highest two doses, as manifested by a drop in mean arterial blood pressure from 82 +/- 4 to 72 +/- 4mmHg (P < 0.05) due to peripheral vasodilatation (e.g. cerebral vascular bed). Heart rate, LVdP/dt,.,, regional myocardial segment length shortening and left ventricular end-diastolic pressure were not affected at these doses. In 2 animals the infusion rate was increased to 4jug kg1 min 1, but again there was no evidence for coronary vasodilatation. 6. We conclude that the intravenous infusion of dopamine after alpha- and beta-adrenoceptor blockade and the intracoronary infusion of fenoldopam provided no evidence for a major role of D1 receptors in the coronary circulation of pigs. The absence of any effect of the employed doses of fenoldopam on LVdP/dt.mx and on regional myocardial segment length shortening also indicates that fenoldopam does not exhibit any inotropic action in this species.

Endothelium dependent vasodilatation following brief ischaemia and reperfusion in anaesthetised swine

STUDY OBJECTIVE

The aim as to compare the responses of intracoronary infusions of ATP, an endothelium dependent vasodilator, with adenosine following brief ischaemia (10 min) and reperfusion in a model of myocardial stunning.

DESIGN

In group 1 (n = 6), coronary blood flow and endocardial (endo) and epicardial (epi) percent segment length shortening were measured in the distribution of the left anterior descending coronary artery before and during maximal intracoronary infusions of either adenosine or ATP (20 micrograms.kg-1.min-1). Measurements were obtained before and after myocardial stunning both at control heart rate and during atrial pacing (150 beats.min-1). In group 2 (n = 6), myocardial blood flows by microspheres and arterial-venous lactate and oxygen differences were determined following the same ischaemia-reperfusion protocol to characterise transmural changes in blood flow and metabolism in this model of stunning.

EXPERIMENTAL MATERIAL

The experiments were done on 12 anaesthetised swine, weight 25-39 kg.

MEASUREMENTS AND MAIN RESULTS

In group 1, baseline endo and epi segment length shortening were 16(SD 3)% and 14(6)% and following reperfusion were reduced to 10(4)% and 8(6)% respectively (p less than 0.05). Prior to stunning, minimal coronary resistances during adenosine and ATP were 0.81(0.40) and 0.76(0.25) mm Hg.min.ml-1 respectively and following reperfusion were 0.86(0.31) (NS) and 0.85(0.23) (NS) mm Hg.min.ml-1 respectively. Infusion of either vasodilator enhanced function by 30% following reperfusion whereas no such effect was observed prior to ischaemia. In group 2, no maldistribution of blood flow was observed following the same ischaemia-reperfusion protocol to account for this vasodilator enhancement in function. Percent lactate extraction values were 29(11)% and 25(14)% at preischaemic control and paced heart rates respectively, and following reperfusion were lowered to 0(12)% without pacing (p less than 0.05) and -1(34)% during pacing (p less than 0.05).

CONCLUSIONS

Brief ischaemia and reperfusion in swine induces myocardial stunning without altering the vasodilator responses of either ATP, an endothelium dependent vasodilator, or adenosine. Recruitment in postischaemic segment length shortening was observed during infusions of both vasodilators at a time when maldistribution of flow was not observed. Possible mechanisms include either enhanced washout of lactate from the reperfused myocardium or greater utilisation of substrates during higher blood flows.

Haemodynamic profile of the potassium channel activator EMD 52692 in anaesthetized pigs

1. The systemic and regional haemodynamic effects of the potassium channel activator EMD 52692 or its solvent were investigated after intravenous and after intracoronary administration in anaesthetized pigs. 2. Consecutive intravenous 10 min infusions of EMD 52692 (0.15, 0.30, 0.60, 1.20 micrograms kg-1 min-1; n = 7) dose-dependently decreased mean arterial blood pressure by up to 50%. This was entirely due to peripheral vasodilatation, since cardiac output did not change. Heart rate increased by up to 50%, while left ventricular end diastolic pressure decreased dose-dependently from 6 +/- 1 mmHg to 3 +/- 1 mmHg (P less than 0.05), and stroke volume decreased from 30 +/- 2 ml to 21 +/- 2 ml (P less than 0.05). Left ventricular dP/dtmax was not affected. 3. Although cardiac output did not change, EMD 52692 caused a redistribution of blood flow from the arteriovenous anastomoses to the capillary channels. Blood flow to the adrenals, small intestine, stomach, bladder, spleen and brain increased, while renal blood flow decreased and blood flow to several muscle groups and skin were not altered. Vascular conductance was increased dose-dependently in all organs, except for the kidneys, where after the initial increase, vascular conductance returned to baseline with the highest dose. Particularly striking were the effects on the vasculature of the brain. With the highest dose of EMD 52692 blood flow more than doubled, while vascular conductance increased four fold. 4. Transmural myocardial blood flow increased slightly, which was entirely due to an increase in subepicardial blood flow. Myocardial O2-consumption and segment length shortening were not significantly affected. 5. After consecutive 10 min intracoronary infusions (0.0095, 0.019, 0.0375 and 0.075 microgram kg-1 min-1; n = 7) into the left anterior descending coronary artery (LADCA), mean arterial blood pressure was maintained with the lowest two doses, but decreased by up to 15% with the higher doses, whereas heart rate increased by up to 24%. Blood flow to the LADCA-perfused myocardium doubled with the highest dose, the subepicardium benefitting the most. Coronary venous O2-saturation increased dose-dependently from 23 +/- 2% to 60 +/- 4%, while myocardial O2-consumption of the LADCA-perfused myocardium was not affected by the drug. 6. It is concluded that EMD 52692 is a potent vasodilator, with particularly pronounced effects on vasculature of the brain. Its selectivity for vascular smooth muscle cells exceeds that for the myocytes, since with doses that are much higher than those of potential clinical interest no negative inotropic effects were observed. The compound primarily dilates arteries but some venodilatation may also occur.

Redistribution of cardiac output caused by opening of arteriovenous anastomoses by a combination of azaperone and metomidate

The effects of the butyrophenone, azaperone 5 mg kg-1 i.m. alone and after addition of the imidazole derivative metomidate 6 mg kg-1 i.v. were studied in eight conscious pigs. Fifteen minutes after administration of azaperone, systemic arterial pressure was reduced by 35% as a result of a 45% increase in systemic vascular conductance and 10% decrease in cardiac output (Q). After azaperone, 23% of the radioactive labelled microspheres (15 (SD 1) microns) injected into the left atrium were detected in the lungs as a result of opening of arteriovenous anastomoses (baseline 3%). The increase in arteriovenous anastomotic blood flow was at the expense of the nutritional (= capillary) channels. Flow to the brain was maintained, but that to the left ventricle decreased in parallel with the reduction in arterial pressure. Vascular conductance of most other organs, except the skin, increased or was maintained. The addition of metomidate had no effect on Q because an increase in stroke volume (by 30%) compensated for the decrease in heart rate. Systemic vascular conductance decreased, most noticeably in the brain, left ventricle and skeletal muscle. We conclude that azaperone alone and in combination with metomidate had only a moderate effect on Q, but caused a redistribution in favour of arteriovenous anastomoses.

Nimodipine has no effect on the cerebral circulation in conscious pigs, despite an increase in cardiac output

1. We studied the effects of four doses of nimodipine (0.5, 1, 2 and 4 micrograms kg-1 min-1) on systemic haemodynamics and on regional vascular beds, in particular the cerebral circulation, in conscious pigs. 2. Nimodipine caused dose-dependent, probably reflex-mediated, increases in heart rate (42% with the highest dose) and cardiac output (54%), while arterial blood pressure was only minimally affected. Left ventricular end-diastolic pressure and systemic vascular resistance decreased dose-dependently (35-40% at the highest dose) while stroke volume remained unchanged. 3. Total brain blood flow was not affected by the drug. Furthermore, we could not demonstrate any regional cerebral differences, as blood flows to both cerebral hemispheres as well as the diencephalon, cerebellum and brain stem remained unchanged. 4. Blood flow to the kidneys, liver, small intestine and skin also did not change. Nimodipine caused dose-dependent increases in blood flow to the stomach (95%), myocardium (97%) and adrenal glands (102%), while blood flow to skeletal muscles (267%) increased most. 5. It is concluded that in the conscious pig, nimodipine is an arterial vasodilator which shows some selectivity for the skeletal muscle vasculature but does not increase total or regional cerebral blood flow.

Acute hemodynamic effects of nisoldipine and pimobendan in conscious pigs with chronic heart failure

The acute systemic hemodynamic effects of the calcium antagonist nisoldipine and the pyridazinone-derivative pimobendan, a phosphodiesterase inhibitor with vasodilating as well as positive inotropic properties, were studied in conscious pigs with chronic heart failure. Left ventricular (LV) dysfunction, manifested by a 25% decrease in cardiac output (CO), a 35% increase in systemic vascular resistance (SVR), and a doubling of the left ventricular filling pressure, was induced by a proximal ligation of the left circumflex coronary artery. Two to three weeks after myocardial infarction, cumulative 10-min infusions of either nisoldipine (0.05, 0.1, 0.25, and 0.5 micrograms/kg/min), pimobendan (2.5, 5, 12.5, and 25 micrograms/kg/min) or the solvents were administered. Infusion of the solvents did not affect any of the hemodynamic variables. Both nisoldipine and pimobendan normalized CO and exhibited a similar cardiac profile [systemic vasodilatation, reduction in left ventricular filling pressure, and an increase in heart rate (HR)] except for the significantly (p less than 0.05) larger increase in LVdP/dtmax with pimobendan (85%) than with nisoldipine (45%). In animals with heart failure, lower doses of both nisoldipine (twice) and pimobendan (four times) were needed to elicit a 30% reduction in SVR than in animals with normal pump function. For both drugs, the slope of the line describing the vasodilatory and positive inotropic properties shifted more in favor of the vasodilatory actions during heart failure (p less than 0.05). We conclude that in swine with chronic LV dysfunction nisoldipine, despite its lack of inotropic properties, appeared to improve ventricular function to the same extent as the primary positive inotropic agent pimobendan.

Actions of nisoldipine in cardiovascular disease

The potent systemic and coronary vasodilatory properties of nisoldipine have been demonstrated in several studies in patients with cardiovascular disease. The increase in cardiac output results from enhancements of both stroke volume and heart rate, while coronary blood supply increases in excess of demand. There is evidence that nisoldipine favourably affects several indicators of myocardial ischemia during exercise such as time to onset of anginal complaints and ST segment changes. Furthermore, in patients with elevated left ventricular filling pressures and reduced cardiac pump function the drug tends to normalize these parameters, thus opening new avenues for the treatment of patients with heart failure. The effect of nisoldipine on the incidence of ventricular arrhythmias has not yet been studied in patients, but experimental evidence shows an antifibrillatory activity at doses which may, however, not be clinically applicable. Finally, in healthy men, nisoldipine partially inhibits platelet aggregation. This, together with a potential antiatherosclerotic effect, could be of additional benefit to patients with coronary artery disease.

Systemic haemodynamics of dihydropyridine derivatives in conscious pigs with or without propranolol

The systemic haemodynamic effects of a 10 min i.v. infusion of three dihydropyridine Ca2+ channel blockers, nifedipine (1, 2 and 4 micrograms.kg-1.min-1), nisoldipine (0.5, 1 and 2 micrograms.kg-1.min-1) and nimodipine (1, 2 and 4 micrograms.kg-1.min-1) were studied in instrumented conscious pigs with or without a propranolol-induced beta-adrenoceptor blockade (0.5 mg.kg-1 bolus followed by 0.5 mg.kg-1.h-1). Initial experiments showed that the solvent used for the Ca2+ channel blockers had no haemodynamic effects and that the effects of propranolol wer constant during a 30 min period. Nisoldipine, nimodipine and nifedipine elicited qualitatively similar effects, causing a dose-dependent decrease in blood pressure and systemic vascular resistance. These effects were accompanied by a reflex-mediated increase in heart rate, cardiac output and left ventricular rate of rise in pressure (LVdP/dtmax). Nisoldipine was about 2-3 times more potent than the other two drugs. Propranolol did not modify the vasodilation induced by the Ca2+ channel blockers but attenuated the increase in heart rate, cardiac output and LVdP/dtmax. In view of the reflex-tachycardia and the absence of negative inotropic actions, these Ca2+ channel blockers can be combined with beta-adrenoceptor antagonists without further compromising the left ventricular pump function.

Nisoldipine and perfusion of post-stenotic myocardium in conscious pigs with different degrees of concentric stenosis

1. The effects of oral nisoldipine on the perfusion and wall function of a myocardial segment distal to a fixed coronary artery stenosis were studied in 2 groups of conscious pigs with different degrees of stenosis. In group 1 (n = 8) systolic wall thickening (SWT) of the post-stenotic segment was more than 15% (27 +/- 4%); in group 2 (n = 7) SWT was less than 10% (7 +/- 1%). 2. The systemic haemodynamic profiles at baseline and during nisoldipine were similar in both groups. Dose-titrations of nisoldipine (0.24 +/- 0.02 mg kg-1 and 0.47 +/- 0.04 mg kg-1) were performed to obtain increases in heart rate of 25% and 50%, respectively. These increases were accompanied by increases in cardiac output (up to 50%) and left ventricular (LV)dP/dt max (60%), while systemic vascular resistance (35%) and mean arterial blood pressure (10%) were reduced. Left ventricular systolic and end-diastolic blood pressure and stroke volume were not affected. 3. In both groups, nisoldipine caused increases in blood flow to the non-stenotic area which favoured the subepicardium more than the subendocardium. Blood flow to the post-stenotic area of group 1 was normal at baseline and was only slightly enhanced (preferentially to the subepicardium) by nisoldipine. In the post-stenotic area of group 2 transmural and subendocardial blood flow were lower at baseline compared to the control area. Nisoldipine did not affect subepicardial blood flow but reduced subendocardial blood flow. 4. In spite of the reflex-mediated positive chronotropic actions of nisoldipine, the acute poststenotic systolic wall thickening was not affected by nisoldipine in either group. 5. We conclude that, under the experimental conditions employed (concentric stenosis, no coronary collaterals and acute drug administration), nisoldipine does not have a useful effect on post-stenotic myocardial blood flow, particularly in animals with severe stenosis. In view of a possible resetting of the baroreceptors (subsiding of the tachycardia) with chronic treatment and the presence of eccentric stenosis in many patients, additional studies are warranted.

5-Hydroxytryptamine-induced tachycardia in the pig: possible involvement of a new type of 5-hydroxytryptamine receptor

1. The mechanism of 5-hydroxytryptamine (5-HT)-induced tachycardia is species-dependent and is mediated directly or indirectly either by '5-HT1-like' (cat), 5-HT2 (rat, dog) or 5-HT3 (rabbit) receptors, or by an action similar to tyramine (guinea-pig). The present investigation is devoted to the analysis of the positive chronotropic effect of 5-HT in the pentobarbitone-anaesthetized pig. 2. Intravenous bolus injections of 5-HT (3, 10 and 30 micrograms kg-1) in pigs resulted in dose-dependent increases in heart rate of 24 +/- 2, 38 +/- 3 and 51 +/- 3 beats min-1, respectively (n = 39). Topical application of a high concentration of 5-HT (150 micrograms kg-1 in 5 ml) on the right atrium was also followed by tachycardia (38 +/- 6 beats min-1, n = 4). 3. A number of drugs which antagonize responses mediated by different 5-HT receptors--phenoxybenzamine, methiothepin, metergoline, methysergide and mesulergine ('5-HT1-like' and 5-HT2 receptors), ketanserin, cyproheptadine, pizotifen and mianserin (5-HT2 receptors), and MDL 72222 and ICS 205-930 (5-HT3 receptors)--did not attenuate the chronotropic responses to 5-HT. 4. The 5-HT-induced tachycardia was also not affected by antagonists at alpha- and beta-adrenoceptors, muscarinic, nicotinic, histamine and dopamine receptors, and calcium channels. 5. Selective inhibitors of 5-HT-uptake, indalpine and fluvoxamine, themselves increased porcine heart rate and facilitated 5-HT-induced tachycardia both in magnitude and in duration. 6. A number of putative selective agonists at '5-HT1-like' receptors or their possible subtypes (5- carboxamidotryptamine (5-CT), 8-hydroxy-24di-N,N-n-propylamino) tetralin (8-OH-DPAT), BEA 1654 and RU 24969), or at 5-HT3 receptors (2-methyl-5-HT), elicited no or only a weak tachycardiac response in the pig. RU 24969, but not 8-OH-DPAT, seemed to potentiate the responses to 5-HT, whereas 5-CT slightly inhibited these responses. 7. It was concluded that the tachycardia induced by 5-HT in the pig does not involve the receptors for some common neurotransmitter substances but may be mediated by a new 5-HT receptor type that is clearly different from '5-HT1-like', 5-HT2 or 5-HT3 receptors.