The role of alpha 1-adrenergic stimulation in inositol phosphate metabolism during post-ischaemic reperfusion

Myocardial alpha1-adrenoceptor: inotropic effect and physiologic and pathologic implications

Alpha1-adrenergic receptors have been found in myocardium of all mammalian species. Although the exact underlying mechanisms have not been conclusively determined, it would appear that the myocardial effects of alpha1-adrenoceptors may vary in importance according to the pathophysiologic process involved. In physiological conditions, this receptor system plays a role in cardiac growth, cardiac contraction, and has both an antiarrhythmic function as well as a role in cardiac adaptation to various situations. This system is also involved in some pathological processes such as ischemia/reperfusion, ischemic preconditioning, and cardiac hypertrophy. The role of alpha1-adrenoceptors in heart failure is somewhat controversial. Experimental evidence suggests that myocardial alpha1-adrenoceptors can have either beneficial or deleterious effects on the heart. It thus seems possible that the development of agents specific to certain subtypes of alpha1-adrenoceptor and a better understanding of their role in pathophysiologic states could be clinically relevant.

Intramyocardial infusion of tool drugs for the study of molecular mechanisms in ischemic preconditioning

Many of the new tool drugs useful for the study of molecular mechanisms of ischemic preconditioning (IP) are very valuable in in vitro systems but produce undesired side-effects after systemic injection in intact animals that limit their applicability. Our aim was to develop an experimental in vivo model that allows the use of said drugs in sufficiently high local concentrations, but avoiding at the same time the systemic side-effects. Several techniques were combined to study regional damage or protection as a result of local drug infusion such as nuclear staining, NADH fluorescence, fluorescent microspheres and tetrazolium salts. In open-chest pigs, the intramyocardial infusion (20 microliters/min) of the adenosine A1-receptor agonist N6-cyclohexyladenosine (0.3 mmol) for 10 min prior to a 60-min LAD-occlusion and 120-min reperfusion mimicked IP by exerting a local protection (n = 9, p < 0.001). Krebs-Henseleit buffer (negative control) was without protective effect. IP's cardioprotection was locally prevented by the intramyocardial application of the adenosine A1-receptor antagonist cyclopentyltheophylline (1 mmol, infused during IP; n = 6, p < 0.001) but not by KHB. The protein kinase C (PKC)-inhibitors staurosporine (100 nmol, n = 6) or bisindolylmaleimide (BIS, 25 mumol, n = 9) did not prevent IP locally. The PKC activator phorbol myristate acetate (PMA, 1 mumol, n = 6) was ineffective in preventing ischemic injury and increased the amount of necrosis in IP, whereas BIS exerted a local myocardial protection (n = 9, p < 0.001). In conclusion, the new model of intramyocardial infusion appears to be useful for the investigation of IP's signal transduction. Our data support the role of the adenosine A1-receptor in IP, but suggest that inhibition instead of activation of PKC may protect ischemic myocardium from infarction.

Signal transduction in myocardial ischaemia and reperfusion

Recent studies in the non-ischaemic myocardium indicated that drugs stimulating cAMP formation inhibit alpha 1-mediated inositol phosphate generation, while alpha 1-adrenergic stimulation lowered tissue cAMP levels, implicating cross-talk between alpha 1- and beta-adrenergic signalling pathways in normal physiological conditions. Massive amounts of endogenous catecholamines, predominantly noradrenaline, are released during myocardial ischaemia and reperfusion, causing stimulation of both alpha 1- and beta-adrenergic receptors which, in turn, may contribute to intracellular Ca2+ overload and subsequent cell damage. Since no information is available regarding cross-talk in pathophysiological conditions, the aim of this study was to evaluate the interactions between alpha 1- and beta-adrenergic signalling pathways during different periods of ischaemia and reperfusion. Isolated rat hearts were perfused retrogradely for 30 min before being subjected to (i) 5-25 min global ischaemia and (ii) 1-5 min of reperfusion after 20 min global ischaemia. Drugs (prazosin, 10(-7) M; propranolol, 10(-6) M; phenylephrine 3 x 10(-5) M; isoproterenol 10(-9) M) were added 10 min before the onset of ischaemia and were present during reperfusion. Increasing periods of ischaemia caused an immediate rise and progressive lowering in tissue cAMP and Ins(1,4,5)P3 levels respectively. In contrast, reperfusion caused an elevation in Ins(1,4,5)P3 levels and reduced cAMP. Prazosin elevated cAMP levels during both ischaemia and reperfusion, while propranolol had no effects on tissue Ins(1,4,5)P3. The activity of the alpha 1-adrenergic signal transduction pathway appears to have an inhibitory effect on the activity of the beta-adrenergic system during ischaemia and reperfusion.

A new method to measure cardiac inositol levels in intact animals

Inositol levels have been studied in cellular cultures and recently by perfusion of isolated hearts. The study was aimed to assess inositol turnover in rabbit hearts from intact animals. Thirty rabbits were injected i.v. three times (every 12 hr) with 25 microCi/kg of myo-3H-inositol. The rabbits 12 hr after the last injection were killed and the hearts perfused according to Langerdorff technique. Systolic and diastolic ventricular pressures (SVP, DVP), dp/dt, and coronary flow (CFl) were measured. The hearts (n = 14) were perfused under aerobic conditions and 16 hearts under ischemic conditions for 30 min. In addition, 5 hearts were perfused under aerobic conditions for 10 min, and 6 hearts were perfused under ischemic conditions for 10 min. Samples of myocardial tissue were taken from both groups at the end of 10-min and 30-min period of perfusion, and cAMP and inositol phosphates were assayed. The hearts subjected to ischaemia showed changes of cAMP and 3H-inositol. The cAMP was higher in the ischaemic (10 min and 30 min) than the control hearts, 0.22 +/- 0.09 and 0.21 +/- 0.08 versus 0.41 +/- 0.12 and 0.49 +/- 0.11 pmol 10(6) cells, respectively (p < .05, p < .001. The inositol trisphosphate was higher in control than ischemic hearts (10 min, 30 min), 0.42 +/- 0.02 and 0.39 +/- 0.01 versus 0.31 +/- 0.01 and 0.23 +/- 0.02 (percent of radioactivity) respectively, p < .001. Our data suggest that 3H-inositol may be studied by i.v. administration to intact animals. The ischemia was performed to verify the validity of this new technique.

Effects of ischaemia, reperfusion and alpha 1-adrenergic receptor stimulation on the inositoltrisphosphate receptor population in rat heart atria and ventricles

Binding sites specific for inositol 1,4,5-trisphosphate (InsP3) have been demonstrated in sarcoplasmic reticulum vesicles isolated from heart muscle. Scatchard analysis of a binding isotherm indicated a high as well as a low affinity binding site [1]. In this study a comparison was made between InsP3, binding to crude microsomal membranes prepared from rat heart atria and ventricles respectively. Results obtained showed a four-fold higher incidence of binding to atrial membranes. Furthermore, the receptor populations of the atria and ventricles behaved differently during conditions causing fluctuations in tissue InsP3, levels, viz. ischaemia, reperfusion and alpha 1-adrenergic stimulation. Reperfusion, as well as phenylephrine stimulation, caused an increase in InsP3 levels associated with down-regulation of the ventricular InsP3 receptor population while binding to atrial binding sites was elevated. In the ventricular population this down-regulation was the result of a reduction in Bmax alone with no changes in the Kd values of the high- or the low-affinity binding sites. The reason(s) for the differential response of the atrial and ventricular InsP3 receptor populations to changes in InsP3 levels, remains to be established.