PARTIALLY PHOSPHORYLATED PHOSPHORYLASE IN THE RAT-HEART AFTER BETA-RECEPTOR STIMULATION IN-VIVO

The formation of the phosphorylase ab hybrid and its further transform ation into phosphorylase a has been demonstrated in the rat heart afte r different periods of i.v. isoproterenol administration. Phosphorylas e ab hybrid was determined in the presence of AMP and/or caffeine. Onl y the partially phosphorylated phosphorylase was found in the control rat hearts and its activity was 30 % of the total phosphorylase. The p hosphorylase ab hybrid was disclosed particularly after small isoprote renol doses (0.031-0.062 mu g.kg(-1)) and at short time interval (15 s ) after its administration. Higher isoproterenol doses (0.25-0.5 mu g. kg(-1)) changed the partially phosphorylated phosphorylase to phosphor ylase a (58 %) after a longer time interval (40 s). The phosphorylase ab hybrid was revealed even at the maximal rate of stimulation. The fo rmation of the phosphorylase ab hybrid in the rat heart in vivo appear s to be of physiological significance. Our results confirmed the earli er suggestion that the -AMP/+AMP activity ratio reflects the percentag e proportion of the phosphorylated subunits of phosphorylase but not o f the activated phosphorylase molecules.