Abstract:The aim of the present study was to investigate the accumulation of inositol-l,4,5-trisphosphate (IP,) in isolated adult rat ventricular cardiomyocytes after a,-and P-adrenoceptor stimulation, separate and in combination, in order to elucidate a possible influence of concomitant P-adrenoceptor stirnulation on the al-adrenoceptor stimulated response. IP3 was measured by a radioligand binding assay based on an (1,4,5)IP3-specific binding protein from bovine adrenal cortex. The basal IP, content was 4.06?0.31 pmol/mg protein (N=56). a,-Adrenoceptor stimulation resulted in a rapid increase in the IP3 level, which reached a plateau, 50-80% above basal level, at 10-30 sec. The plateau lasted at least up to 120 sec., while at 300 sec. there was no significant difference between control values and values after a,-adrenoceptor stimulation. Li+ did not affect either the basal IP3 level, or the magnitude or time course of al-adrenoceptor-stimulated IP, accumulation. Combined adrenoceptor stimulation gave a similar response as separate al-adrenoceptor stimulation, whereas there was no significant change in the IP, level after P-adrenoceptor stimulation. No inhibitory influence of simultaneous P-adrenoceptor stimulation on the al-adrenoceptor-stimulated increase of IP3 mass was revealed.The existence of both a l -and P-adrenoceptors in rat myocardium is well established (Osnes et al. 1985;Terzic et al. 1993). al-Adrenoceptor stimulation activates phospholipase C, resulting in breakdown of phosphoinositide 4,5-bisphosphate to inositol-1,4,5-trisphosphate (IP3) and diacylglycer-01. IP, releases Ca2+ from internal stores in most cell types, and diacylglycerol activates protein kinase C (Terzic et al. 1993). Whereas protein kinase C has been implicated in the al-adrenergic inotropic effect (Terzic et al. 1993), and in the regulation of cardiac growth (Sugden & Bogoyevitch 1995), the physiological role of IP3 in the heart remains unclear, since a clearcut effect on Ca*+-release has not been found (Terzic et al. 1993).When al-adrenoceptors are stimulated under physiological conditions, P-adrenoceptors are activated simultaneously. An inhibitory influence of concomitant P-adrenoceptor stimulation has been reported for several q-adrenoceptor-mediated responses in the myocardium. At the functional level, the al-adrenoceptor-stimulated inotropic response was attenuated by 0-adrenoceptor stimulation (Skomedal et al. 1988; Osnes et af. 1989). The a,-adrenoceptor-stimulated increase in potassium uptake rate of isolated ventricular cardiomyocytes was also attenuated by concomitant P-adrenoceptor stimulation (Viko et al. 1996). However, when myocardial phospholipase C activity was determined after labelling of cells with radioactive inositol (Berridge & Irvine 1989), simultaneous P-adrenoceptor stimulation either inhibited (Guse et al. 1991) Changes in labelled inositol phosphates may not reflect regulation of the IP3 mass, and thus a possible mediator function of IP3. Generation of inositol phosphates may occur independently of...
