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

IP3 type 1 receptors in the heart: Their predominance in atrial walls with ganglion cells

Previously we have shown that inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) are abundantly expressed in the atria of rat hearts. Since arrangement of atria is very heterogeneous, in this work we focused on the precise localization of IP3 receptors in the left atrium, where the gene expression of the type 1 IP3R was the highest. The mRNA levels of the IP3 type 1 receptors in the left atrium, left ventricle and myocytes were determined using real-time polymerase chain reaction and Taqman probe. For precise localization, immunohistochemistry with the antibody against type 1 IP3Rs was performed. The mRNA of type 1 IP3 receptor was more than three times higher in the left atrium than in the left ventricle, as determined by real-time PCR. Expression of the type 1 IP3 receptor mRNA was higher in the atria, especially in parts containing cardiac ganglion cells. The atrial auricles, which are particularly free of ganglion cells, and the ventricles (wall of the right and left ventricle and ventricular septum) contained four to five times less IP3 receptors than atrial samples with ganglia. IP3R type 1 immunoreactivity detected by a confocal microscope attributed the most condensed signal on ganglionic cells, although light immunoreactivity was also seen in cardiomyocytes. These results show that type 1IP3 receptors predominate in intrinsic neuronal ganglia of cardiac atria.

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.