Partially phosphorylated phosphorylase in the rat heart after beta-receptor stimulation in vivo

The formation of the phosphorylase ab hybrid and its further transformation into phosphorylase a has been demonstrated in the rat heart after different periods of i.v. isoproterenol administration. Phosphorylase ab hybrid was determined in the presence of AMP and/or caffeine. Only the partially phosphorylated phosphorylase was found in the control rat hearts and its activity was 30% of the total phosphorylase. The phosphorylase ab hybrid was disclosed particularly after small isoproterenol doses (0.031-0.062 microgram.kg-1) and at short time interval (15 s) after its administration. Higher isoproterenol doses (0.25-0.5 microgram.kg-1) changed the partially phosphorylated phosphorylase to phosphorylase a (58%) after a longer time interval (40 s). The phosphorylase ab hybrid was revealed even at the maximal rate of stimulation. The formation of the phosphorylase ab hybrid in the rat heart in vivo appears to be of physiological significance. Our results confirmed the earlier suggestion that the -AMP/+AMP activity ratio reflects the percentage proportion of the phosphorylated subunits of phosphorylase but not of the activated phosphorylase molecules.