Enhanced Responsiveness of the Myocardial β-Adrenoceptor-Adenylate Cyclase System in the Perfused Rat Heart (I)

Abstract

Crude myocardial sarcolemmal membrane fractions were prepared from rat hearts subjected to total global ischemia with and without normoxic reperfusion, or global anoxic (N2) perfusion with and without normoxic reperfusion. The direct effects on β-adrenenoceptor number, G-protein levels and stimulation of the adenylate cyclase (AC) complex were assessed. In terms of AC activation, ischemia led to a marked increase (4-fold) in sensitivity to terbutaline (β2-agonist) and phorbol ester (tetradecanoyl phorbal acetate = TPA) stimulation, whereas the dobutamine (β1) responsiveness and Gpp(NH)p activation through GSα/Gi2α remained unaltered. However, forskolin-elicited holoenzyme activity fell markedly during normoxic reperfusion. Ischemia did not change the β1-adrenoceptor number, while β2-receptor population increased by approximately 45%. Western blots of myocardial GSA and Gi2α contents revealed that ischemia selectively diminished Gi2α levels only by some 50–70%. Contrastingly, anoxia selectively increased the AC sensitivity (2-fold) to β1-adrenergic stimulation. As subsequent to ischemia, anoxia also increased the sensitivy to TPA stimulation, however, only 2-fold. Gpp(NH)p activation was unchanged, while forskolin-enhanced activity gradually declined, also during ensuing normoxic reperfusion. Anoxia brought about a 75% enhancement in β1-receptor number, while β2-receptors remained unaffected. However, altered receptor number normalized on termination of normoxic reperfusion. Finally, anoxia led to a 50–60% decimation of myocardial Gi2α levels, while GSα was only marginally reduced. Despite the fact that the ischemia and anoxia effectuated a similar deterioration of physiological heart parameters, myocardial contents of energy rich phosphate moieties and loss of Gi2α, ischemia rendered the most profound increase in responsiveness of the sarcolemmal AC system.