EFFECT OF CONCOMITANT BETA-ADRENOCEPTOR STIMULATION ON ALPHA(1)-ADRENOCEPTOR-MEDIATED INCREASE OF INOSITOL-1,4,5-TRISPHOSPHATE MASS IN ADULT-RAT CARDIOMYOCYTES

The aim of the present study was to investigate the accumulation of in ositol-1,4,5-trisphosphate (IP3) in isolated adult rat ventricular car diomyocytes after alpha(1)- and beta-adrenoceptor stimulation, separat e and in combination, in order to elucidate a possible influence of co ncomitant beta-adrenoceptor stimulation on the cll-adrenoceptor stimul ated response. IP3 was measured by a radioligand binding assay based o n an (1,4,5)IP3-specific binding protein from bovine adrenal cortex. T he basal IP3 content was 4.06+/-0.31 pmol/mg protein (N=56), alpha(1)- 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 n o significant difference between control values and values after ai-ad renoceptor stimulation. Li+ did not affect either the basal IP3 level, or the magnitude or time course of alpha(1)-adrenoceptor-slimulated I P3 accumulation. Combined adrenoceptor stimulation gave a similar resp onse as separate al-adrenoceptor stimulation, whereas there was no sig nificant change in the IP3 level after beta-adrenoceptor stimulation. No inhibitory influence of simultaneous beta-adrenoceptor stimulation on the alpha(1)-adrenoceptor-stimulated increase of IP3 mass was revea led.