Actions of prostaglandin E2 on myocardial mechanics, coronary vascular resistance and oxygen consumption in the guinea-pig isolated heat preparation

Inotropic actions of eicosanoids

Eicosanoids (prostaglandins, leukotrienes, thromboxane A2 and other metabolites of C-20 polyunsaturated fatty acids) have numerous effects in the cardiovascular system. Direct inotropic actions have been repeatedly described, but appear in only very few cases to be due to direct modification of the inotropic state of the heart. Specific eicosanoid receptors have been identified on the surface of the sarcolemmal membrane. Signal transduction pathways in the cardiac myocyte involve the adenylate cyclase/cAMP system or stimulation of the phospholipase C/IP3 pathway. In general, concentrations of eicosanoids which affect myocardial contractility are higher as the response is less predictable than the effects on platelet function or vessel tone. Therefore, eicosanoid-induced extracardiac effects may be superimposed to more direct changes in the contractile state of the intact heart in vitro or in vivo. In contrast to non-failing hearts, there is a significant improvement of the contractile function in contractile failure ("stunning", ischemia, congestive heart failure) by vasodilating prostaglandins (e.g., PGI2). The mechanism of this action is still unknown.

The coronary vasodilator mediator released by hypoxia and isoprenaline is not affected by cyclo-oxygenase inhibition

Donor and recipient guinea-pig hearts were perfused in series, the recipient being perfused with regassed donor effluent. Coronary perfusion pressure and force and rate of contraction were measured. Exposure of donor hearts to hypoxia (1.5 min) and to isoprenaline (5 ng) caused the appearance of vasodilator material in recipient hearts, the direct beta-adrenoceptor effects of isoprenaline carried over in the effluent being antagonized in the recipient by propranolol. Cyclo-oxygenase was inhibited by infusion of meclofenamate (60 micrograms X min-1) which consistently abolished the vasodilator responses to arachidonic acid added to the donor. The vasodilator responses of the donor to hypoxia and isoprenaline were unaffected by meclofenamate. The falls in perfusion pressure of the recipient in response to material released by these procedures were also not significantly different before (hypoxia, 11.5 +/- 2.6mm Hg; isoprenaline, 10.3 +/- 1.3mm Hg) and during the infusion (hypoxia, 10.2 +/- 4.1; isoprenaline, 11.0 +/- 1.3mm Hg). The coronary vasodilator responses to hypoxia and isoprenaline and the vasodilator material released by these procedures do not therefore appear to be due to products of arachidonic acid via cyclo-oxygenase pathways. Furthermore, since there was also no potentiation of the responses, there does not appear to be a concomitant release of a prostanoid to inhibit the major vasodilator material. Adenosine, as the likely candidate for this predominant vasodilator mediator, is discussed.

Effects of prostaglandins on coronary flow rate and left ventricular work in isolated rat heart

In isolated rat hearts PGD2, PGE1, PGE2, PGF2 alpha and PGI2 increased coronary flow rate at concentrations of about 23, 1.4, 0.4, 2.5 and 0.6 nmol per l respectively. PGD2, PGE2, and PGI2 did not affect left ventricular work (W 1v). PGE1 (14 nmol/l) lowered W 1v (12% and PGF2 alpha (9 nmol/l) increased W 1v (10%). It is concluded that endogenously released PGI2 can probably affect coronary flow rate.

The action of the dihydro derivatives of prostacyclin--(6R)-PGI1 and (6S)-PGI1 on the heart and the coronary vasculature

The action of the dihydro prostacyclins, (6R)-PGI1 and (6S)-PGI1, was studied on the isolated guinea pig heart and bovine coronary artery strips. PGE2 and PGI2 were used as standards. In the isolated guinea pig heart (6S)-PGI1 decreased the coronary perfusion pressure (CPP), myocardial force of contraction (MFC) and oxygen consumption (QO2). (6R)-PGI1 did not produce a significant change in these parameters. The ED50 (50% o maximum coronary dilation) was approximately 20 times higher for (6S)-PGI1 than for PGI2 or PGE2. Treatment of the hearts with reserpine + tyramine abolished the (6S)-PGI1-induced decrease in MFC but not the decrease in the CPP. The same pattern of responses was seen with PGE2. Bovine coronary artery strips were contracted by both (6S)-PGI1 and (6R)-PGI1, the ED50 (50% of maximum increase in tension) being 5 and 10 times higher than that for PGE2. The (6S)-PGI1-induced contraction was preceeded by a small relaxation, which, however, was much less than that seen after PGI2. It is concluded that the hydration of the 5,6 double bound in the PGI2 molecule results in an almost complete loss of PGI2-like activity and generates PGE-like activity. The same biological activity of both dihydro prostacyclins in the isolated coronary artery strip but not in the intact coronary vascular bed leads to suggest that the sites of action in these systems are different.

Effects of prostacyclin on coronary circulation, heart rate and myocardial contractile force in isolated hearts of guinea pig and rabbit - comparison with prostaglandin E2

Infusions of prostacyclin (PGI2) (3 x 10(-10) - 3 x 10(-7)M) into the coronary circulation of isolated hearts from ginea pigs or rabbits resulted in a concentration-dependent decrease in the coronary perfusion pressure (CPP). There was a slight decrease in left ventricular systolic pressure in the heart of the rabbit, whereas the heart rate remained unchanged. PGE2 was without effect on the heart of the rabbit but was as potent as PGI2 in decreasing the CPP in the guinea pig heart. 6-oxo-PGF1 alpha (up to 3 x 10(-6) M) did not affect any of the parameters measured.

The specificity of prostaglandin E2 (PGE2) in reducing coronary vascular resistance: A comparison with adenosine

Experiments were performed on the isolated, electrically driven guinea-pig heart, perfused at constant rate. All animals were pretreated with reserpine. Myocardial contractile force (MCF), coronary perfusion pressure (CPP) and myocardial oxygen consumption (QO2) were monitored continuously. Both adenosine (ADO) and PGE2 produced a concentration-dependent decrease in the CPP. The ED50 (50% of maximum response) was 2.1 +/- 0.6 X 10(-9)M for PGE2 but 40 +/- 7 X 10(-9)M for ADO (P less than 0.01) at 1.8 mM Ca(e). This coronary vasodilation was independent of the external Ca-concentration, which was varied between 0.55-9.0 mM. PGE2 had no effect on MCF or QO2 and the effect of ADO was only slight. There was no evidence that any action of ADO could be inhibited by simultaneously applied PGE2. The results provide evidence for the specific coronary vasodilating action of PGE2 which in this system is about 20 times as effective as adenosine.

Prostaglandin D2 (PGD2)--a potent coronary vasoconstrictor agent in the guinea pig isolated heart

The action of prostaglandin D2 (PGD2) on myocardial force of contraction (MFC) and coronary vascular resistance (CVR) was studied in the isovolumetrically perfused guinea pig heart at constant driving frequency (180 beats/min). PGD2 (2 . 10(-9) - 1 . 10(-6) M) produced a concentration-dependent increase in the CVR while the MFC remained unchanged. The ED50 (50% of maximum response) of the coronary vasomotor action amounted to 4.3 . 10(-8) M PGD2. The results give evidence for a potent coronary vasoconstrictor activity of PGD2.

Effect of fasting and streptozotocin-diabetes on the coronary flow in isolated rat hearts: A possible role of endogenous catecholamines and prostaglandins

The coronary flow rate of retrogradely perfused hearts from fasted (group I) and streptozotocin-diabetic rats (group II) is increased when compared with the flow rate of control, fed animals (group III). The enhanced coronary flow is absent when hearts of groups I and II are perfused in the presence of indomethacin (1 microgram/ml) in the perfusion fluid and the lowest flow rates are observed after depletion of the endogenous catecholamines by reserpin. Hearts from groups I and II showed a marked increase in prostaglandin-release which was counteracted both in the presence of indomethacin (1 microgram/ml) and by reserpin-pretreatment. The results suggest that the increased coronary flow rates in hearts from fasted and streptozotocin-diabetic rats are mediated by an effect of released endogenous catecholamines on coronary vascular smooth muscle and by a catecholamine-induced release of vasodilatory, prostaglandin-like substances.

Increase in the coronary vascular resistance by indomethacin in the isolated guinea pig heart preparation in the absence of changes in mechanical performance and oxygen consumption

Experiments were performed in isolated guinea pig hearts, perfused at constant volume. Left ventricular pressure, left ventricular dp/dtmax, coronary vascular pressure and coronary venous pO2 were measured continuously. Indomethacin (1.4 X 10(-6) M) increased coronary vascular resistance by 15% over control (p less than 0.01) without altering the myocardial contractile force. When PGE2 (1.4 X10(-8) M) was subsequently added, it abolished the indomethacin action and lowered the coronary vascular resistance below the pre-indomethacin control value. This increase in coronary vascular resistance caused by indomethacin was also seen when the animals were pretreated with reserpine and/or the adrenoceptor blocking agents phenoxybenzamine and propranolol, but was prevented completely by previous addition of PGE2 (1.4 X 10(-7) M). The results provide evidence for possible involvement of endogenous prostaglandin-like substances in the maintenance of coronary vascular resistance in the isolated guinea pig heart.

The release of a coronary vasodilator metabolite from the guinea-pig isolated perfused heart stimulated by catecholamines, histamine and electrical pacing and by exposure to anoxia

1 A procedure involving two guinea-pig isolated hearts perfused in series is described for detecting in the recipient heart the release of a possible coronary vasodilator metabolite from the donor heart. 2 Adrenaline and isoprenaline stimulated the rate and force of contraction and produced a multiphasic coronary vascular response, the predominant phase of which was vasodilatation. When the beta-adrenoceptors of the recipient heart were blocked, stimulation of the donor heart by the catecholamines was associated with a coronary vasodilatation of the recipient heart. 3 Histamine stimulated rate and force of contraction and was predominantly coronary vasodilator. After blockade of histamine H1- and H2-receptors in the recipient heart, coronary vasodilatation followed increases in activity of the donor heart in response to histamine. 4 These vasodilator responses of the recipient heart were attributed to the release from the donor heart of a vasodilator metabolite by the increased activity. This is the proposed mechanism for the predominant coronary vasodilator response to catecholamines and histamine. 5 Periods of electrically-paced tachycardia and anoxia of the donor heart also led to the release of vasodilator activity. 6 The possible identity of the metabolite is discussed.

Interactions of isoprenaline and prostaglandin E2 with respect to myocardial contractile force, coronary vascular resistance and myocardial oxygen consumption in guinea-pig isolated hearts

Left ventricular pressure (LVP), left ventricular pressure derivative (LV dp/dtmax), coronary vascular resistance (CVR) and myocardial oxygen consumption (Qo2) were measured simultaneously in isolated, electrically driven hearts of guinea-pigs at constant perfusion rate. 2 LVP, LV dp/dtmax, CVR and Qo2 were greatly decreased by either the addition of prostaglandin E2 (50 ng/ml) to the perfusion fluid or pretreatment of the animals with reserpine. 3 Isoprenaline (0.5 nM to 100 nM) induced increases in LVP, LV dp/dtmax and Qo2. In the presence of prostaglandin E2, there was a parallel shift of the isoprenaline concentration-response curve for LVP and LV dp/dtmax. This effect was not seen, after the animals had been treated with reserpine. 4 Qo2 was also decreased by prostaglandin E2 only in non-reserpine treated animals. 5 CVR was diminished by isoprenaline in the untreated group. However, there was increase in CVR, when isoprenaline was added to either prostaglandin E2 or reserpine-pretreated hearts which was enhanced in the reserpine plus prostaglandin E2-treated group (P less than 0.01). 6 The results give evidence for different actions of prostaglandin E2 on isoprenaline concentration-response curves for LVP, LV dp/dtmax and CVR.