Enhanced responsiveness of the myocardial beta-adrenoceptor-adenylate cyclase system in the perfused rat heart (I)

Crude myocardial sarcolemmal membrane fractions were prepared from rat hear ts subjected to total global ischemia with and without normoxic reperfusion , or global anoxic (N-2) perfusion with and without normoxic reperfusion. T he direct effects on beta-adrenenoceptor number, G-protein levels and stimu lation of the adenylate cyclase (AC) complex were assessed. In terms of AC activation, ischemia led to a marked increase (4-fold) in se nsitivity to terbutaline (beta(2)-agonist) and phorbol ester (tetradecanoyl phorbal acetate = TPA) stimulation, whereas the dobutamine (beta(1)) respo nsiveness and Gpp(NH)p activation through G(s)alpha/G(i2)alpha remained una ltered. However, forskolin-elicited holoenzyme activity fell markedly durin g normoxic reperfusion. Ischemia did not change the beta(1)-adrenoceptor nu mber, while beta(2)-receptor population increased by approximately 45%. Wes tern blots of myocardial G(s)A and G(i2)alpha contents revealed that ischem ia selectively diminished G(i2)alpha levels only by some 50-70%. Contrastingly, anoxia selectively increased the AC sensitivity (2-fold) to beta(1)-adrenergic stimulation. As subsequent to ischemia, anoxia also incr eased the sensitivity to TPA stimulation, however, only 2-fold. Gpp(NH)p ac tivation was unchanged, while forskolin-enhanced activity gradually decline d, also during ensuing normoxic reperfusion. Anoxia brought about a 75% enh ancement in beta(1)-receptor number, while beta(2)-receptors remained unaff ected. However, altered receptor number normalized on termination of normox ic reperfusion. Finally, anoxia led to a 50-60% decimation of myocardial G( i2)alpha levels, while G(s)alpha was only marginally reduced. Despite the fact that the ischemia and anoxia effectuated a similar deterio ration of physiological heart parameters, myocardial contents of energy ric h phosphate moieties and loss of G(i2)alpha, ischemia rendered the most pro found increase in responsiveness of the sarcolemmal AC system.